Temporal Change in Crayfish Communities and links to a Changing Environment
by
Brie Anna Edwards
A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy
Department of Ecology and Evolutionary Biology University of Toronto
© Copyright by Brie A. Edwards 2013
ii
Temporal Change in Crayfish Communities and links to a
Changing Environment
Brie A. Edwards
Doctor of Philosophy
Department of Ecology and Evolution University of Toronto
2013
Abstract
Community ecology and conservation are complementary disciplines under the umbrella of
ecology, providing insight into the factors that determine where and how communities exist, and
informing efforts aimed at sustaining the diversity and persistence of the organisms that comprise
them. Conservation ecologists apply the principles of ecology and other disciplines, to the
maintenance of biodiversity. This thesis uses this approach to assess the status of freshwater
crayfish in south-central Ontario and investigate potential anthropogenic drivers of crayfish
community change. I start with a temporal analysis of crayfish relative abundance over a period
of 18 years and find that all species have experienced significant population declines across the
sampled range, resulting in reduced species distributions and crayfish community diversity.
Next I employ multivariate statistical techniques to relate changes in crayfish communities
between the two time periods to ecological and anthropogenic changes. I identify a number of
threats in the region that are correlated with crayfish decline and are likely to pose a threat to
aquatic ecosystems more broadly in the region, including calcium (Ca) decline, metal pollution,
human development, and species introductions. In the latter two chapters I look more closely at
Ca decline as a mechanism driving crayfish declines. First, laboratory analysis of the effect of Ca
availability on juvenile Orconectes virilis (a Shield native) reveals that survival is significantly
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reduced below 0.5-0.9 mg·L-1, which is one of the lowest ever reported Ca requirement
thresholds for a crustacean. Second, a correlative study using adult inter-moult crayfish collected
from lakes that range broadly in their Ca concentrations, indicates that for O. virilis, carapace Ca
content is significantly related to lake Ca concentration, and is under-saturating below 8 mg·L-1.
This collective body of work identifies significant anthropogenic threats to crayfish and their
aquatic ecosystems in south-central Ontario.
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Acknowledgments Before anyone else, I have to thank Ron Reid, my first real science mentor and a lasting friend,
for sharing his contagious love of benthic critters and crayfish, and encouraging me to become a
researcher. Next, I thank my co-supervisors Don Jackson and Keith Somers, without whom none
of this would have been possible. Keith saw potential in me that I didn’t know was there,
introduced me to Don, and got the whole project off the ground. He has been a fantastic mentor,
teacher, promoter and constructive critic, for which I am truly grateful. From Don I have learned
more about the world, ecology, and myself than I have from any other influence in my life. He
has mentored me with patience and generosity, and challenged me to exceed even my own
expectations. I will be forever thankful for his support, within and beyond academia. I further
thank my committee members Nick Collins and Harold Harvey for great discussions, sage
advice, and unwavering positivity. This work wouldn’t have been possible without an excellent
team of people to assist in the lab and field, so many thanks go out to Ellen Fanning, Amir
Lewin, Vern Lewis, Simona Tencaliuc, Jesse Elders, and Kraig Picken.
I consider myself extremely lucky to have had the opportunity to work with and be guided by
Helen Rodd. She has encouraged and bolstered me through many highs and lows, has always
been there to listen or to laugh, and has become a true friend and inspiration. I am also forever
grateful for the lasting friendships and many things I have learned from and shared with lab
mates and fellow candidates over the years, including Maggie Neff, Monica Granados, Meg
Eizenman, Bronwyn Rayfield, Maria Bennell, Cam Weadick, Anna Price, Caren Scott, Steve
Walker, Sapna Sharma, Mark Poesch, Andrew Drake, Angela Streker, Karen Alofs, Dak de
Kerckhove, Aleks Polakowska, Jonathan Ruppert, Crystal Vincent, and Alex De Serrano.
My extended families and dear friends outside academia are too numerous to name, but I owe
each and every one of you endless thanks for supporting and encouraging me in your unique
way. I would like to thank my parents Craig Edwards and Nepher Lewis for immersing me in
the outdoors since birth and instilling in me an appreciation for the natural world that started me
on the path toward a career in ecology. I thank my brother Colgan Edwards for sharing that
enchanted path with me as we grew, and for being my best friend and biggest fan for as long as I
can remember. Finally, I thank my incredible partner Kraig Picken, for helping me in every way
imaginable throughout this journey. He sacrificed, collaborated, constructed, brainstormed, and
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problem solved right along with me, and even proved himself a superior crayfish diver, as hard
as it is to admit. His fierce adoration and pride in my work was a driving force from the start to
the finish of this degree and over every bump along the way. Special mention goes out to my
son, Fisher Picken, whose arrival revived my curious mind and gave me the renewed perspective
and balance that carried me through the final year.
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Table of Contents Acknowledgments.......................................................................................................................... iv
Table of Contents........................................................................................................................... vi
List of Tables .................................................................................................................................. x
List of Figures ................................................................................................................................ xi
List of Appendices ....................................................................................................................... xiv
Chapter 1 General Introduction ...................................................................................................... 1
References....................................................................................................................................... 8
Chapter 2 Multispecies crayfish declines in lakes: implications for species distributions and richness..................................................................................................................................... 14
2.1 Abstract ............................................................................................................................. 14
2.2 Introduction....................................................................................................................... 14
2.2.1 Study Lakes and Study Region ............................................................................. 16
2.3 Methods............................................................................................................................. 18
2.3.1 Site selection and crayfish sampling..................................................................... 18
2.3.2 Chemistry sampling .............................................................................................. 19
2.3.3 Data analysis ......................................................................................................... 19
2.4 Results............................................................................................................................... 21
2.4.1 Crayfish................................................................................................................. 21
2.4.2 Water chemistry .................................................................................................... 26
2.5 Discussion ......................................................................................................................... 29
2.5.1 Crayfish trends ...................................................................................................... 29
2.5.2 Potential environmental links ............................................................................... 31
2.5.3 General implications ............................................................................................. 34
2.6 Acknowledgements........................................................................................................... 34
References..................................................................................................................................... 35
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Supplementary Tables and Figures............................................................................................... 40
Copyright Acknowledgements...................................................................................................... 41
Chapter 3 Temporal changes in crayfish communities and environmental change...................... 42
3.1 Abstract ............................................................................................................................. 42
3.2 Introduction....................................................................................................................... 42
3.3 Materials and Methods...................................................................................................... 44
3.3.1 Crayfish assemblage data...................................................................................... 44
3.3.2 Environmental matrices ........................................................................................ 44
3.3.3 Statistical Analyses ............................................................................................... 47
3.4 Results............................................................................................................................... 50
3.5 Discussion ......................................................................................................................... 55
3.5.1 Assemblage patterns ............................................................................................. 55
3.5.2 Ecological Drivers ................................................................................................ 57
3.5.3 Implications for Lake Ecosystems ........................................................................ 59
3.6 Acknowledgements........................................................................................................... 61
References..................................................................................................................................... 62
Supplementary Tables and Figures............................................................................................... 67
Copyright Acknowledgements...................................................................................................... 71
Chapter 4 Effects of calcium (Ca) availability on the survival, growth, and Ca content of freshwater crayfish, Orconectes virilis, originating from waters experiencing [Ca] decline .. 72
4.1 Abstract ............................................................................................................................. 72
4.2 Introduction....................................................................................................................... 72
4.3 Methods............................................................................................................................. 74
4.3.1 Crayfish................................................................................................................. 74
4.3.2 Experimental Design............................................................................................. 75
4.3.3 Statistical Analyses ............................................................................................... 76
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4.4 Results............................................................................................................................... 77
4.4.1 Survival ................................................................................................................. 77
4.4.2 Ca Content ............................................................................................................ 78
4.4.3 Growth and Moulting............................................................................................ 79
4.5 Discussion ......................................................................................................................... 81
4.6 Acknowledgements........................................................................................................... 84
References..................................................................................................................................... 85
Chapter 5 Evaluating the effect of lake calcium concentration on the acquisition of carapace calcium by freshwater crayfish ................................................................................................ 88
5.1 Abstract ............................................................................................................................. 88
5.2 Introduction....................................................................................................................... 88
5.3 Methods............................................................................................................................. 90
5.3.1 Crayfish Collection ............................................................................................... 90
5.3.2 Analytical Techniques .......................................................................................... 92
5.3.3 Statistical Analyses ............................................................................................... 93
5.4 Results............................................................................................................................... 93
5.5 Discussion ......................................................................................................................... 97
5.6 Acknowledgements......................................................................................................... 100
References................................................................................................................................... 101
Chapter 6 General Conclusion .................................................................................................... 103
References................................................................................................................................... 107
Appendix 1. Lake Information.................................................................................................... 110
Appendix 2. Historical Lake Chemistry ..................................................................................... 114
Appendix 3. Current Lake Chemistry......................................................................................... 116
Appendix 4. Presence/Absence of Predatory Fish...................................................................... 118
Appendix 5. Historical Crayfish Survey Data ............................................................................ 120
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Appendix 6. Current Crayfish Survey Data................................................................................ 124
Appendix 7. 2009 Juvenile Experiment Data ............................................................................. 127
Appendix 8. 2010 Juvenile Experiment Data ............................................................................. 129
Appendix 9. Carapace Broad Survey Sample Data .................................................................... 131
Appendix 10. Red Chalk East Carapace Sample Data ............................................................... 135
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List of Tables Table 2-1. Results of the model II regression analyses comparing current and historical relative
abundances (Catch Per Unit Effort [CPUE]) for all lakes in which a species was found. n = the
number of lakes in which a species was found, p = probability associated with the difference
from the null expectation (1:1), % historical = the percentage of lakes in which the species
occurred historically that have current populations of the species. .............................................. 20
Table 2-2. Chemical variables that changed significantly in all study lakes or in the Shield lakes
between the historical survey and the current study. Δ = % change, Alkti = alkalinity, DIC =
dissolve inorganic C, p = probability associated with paired t-tests comparing current and
historical values, * indicates differences were significant only in Shield lakes and means and Δ
apply only to Shield lakes, ns = not significant (p > 0.05). .......................................................... 26
Table 3-1. Summary information for all environmental variables in the crayfish-containing lakes
(66 historical and 57 current) considered in the analyses. ............................................................ 46
Table 3-2. Pearson correlations and significance for indirect analyses of the association of
environmental variables with the principal component axes from the ordinations of community
data (Fig. 2) as well as unique community variation constrained by each environmental variable
in the RDA ordinations (Fig. 3). ................................................................................................... 49
Table 4-1. Percentage of replicate individuals in each treatment in both years that were observed
to moult ≥ 1 or ≥ 2 times over the course of the experiment, and the mean ± SE moult increment
(in days) from individuals who moulted two or more times......................................................... 80
Table 5-1. General characteristics of each lake sampled (south-central Ontario) and
corresponding crayfish specimen information, ordered by lake [Ca]........................................... 91
Table 5-2. Variables considered in the step-wise multiple regression model predicting individual
O. virilis carapace Ca content (%) using (a) the full multi-lake survey samples and (b) the Red
Chalk East population sample, with the percentage of the explained variation in carapace Ca
content associated with each variable in the final model.............................................................. 97
xi
List of Figures Figure 2-1. Study region in Ontario, Canada, showing the 100 lakes surveyed. Tertiary
watersheds are delineated and identified by the following codes: 2EA = Georgian Bay tributaries,
2EB = Moon and Go Home rivers, 2EC = Severn River, 2HF = Cameron Lake drainage, 2CF =
Sudbury Region, 2KD = Upper Madawaska River, 2KB = Deep River, 2HH = Kawartha Lakes
region, 2HG = Scugog River. ....................................................................................................... 17
Figure 2-2. Model II regression plots for current vs historical abundance (as catch per unit effort
[CPUE]) of Orconectes virilis (A), O. propinquus (B), O. immunis (C), O. obscurus (D),
Cambarus bartonii (E), and C. robustus (F), in relation to the 1:1 null expectation (dashed line).
Insets show details for species with many low abundances. ........................................................ 22
Figure 2-3. Frequency distributions of crayfish species richness in the historical and current
surveys. ......................................................................................................................................... 23
Figure 2-4. Changes in the number of lakes with populations of Orconectes virilis (O.v.), O.
propinquus (O.p.), O. immunis (O.i.), O. obscurus (O.o.), O. rusticus (O.r.), Cambarus bartonii
(C.b.), and C. robustus (C.r.), between the historical and current surveys in relation to the 1:1
null expectation (dashed line). ...................................................................................................... 24
Figure 2-5. Distributions of Orconectes propinquus (A), O. virilis (B), Cambarus bartonii (C),
and C. robustus (D), indicating populations that appear stable, have declined by ≥50%, appear to
have been lost, or were newly detected. ....................................................................................... 25
Figure 2-6. Model II regression plots for log(x) transformed current vs historical alkalinity
(Alkti) (A), Al (B), Ca* (C), Cl (D), dissolved organic C (DIC*) (E), Mg* (F), Na (G), NO3 (H),
and SO4 (I), in relation to the 1:1 null expectation (dashed line). Off-Shield lakes are designated
by open circles. A constant of 1 (2 in the case of alkalinity) was added before transformation of
Al, Cl, DIC, Mg, Na, and NO3. * indicates differences were significant only in Shield lakes and
so off-Shield lakes were excluded from analysis.......................................................................... 28
Figure 3-1. Map of south-central Ontario indicating the survey range in dark shading, with an
inset of the location in the broader Great Lakes region. ............................................................... 45
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Figure 3-2. Flow diagram to illustrate the organization of data matrices and which matrices were
involved in the various ordination techniques employed to answer the basic research questions.
The ‘H’ and grey fill indicate historical data, and the C and white fill indicate current data....... 48
Figure 3-3. Principal component analysis ordination of community data from the historical (a)
and current (b) surveys. Lakes are identified by a two letter code (see Appendix 1)................... 51
Figure 3-4. Redundancy analysis ordination of community data constrained by environmental
predictors from the historical (a) and current (b) surveys............................................................. 53
Figure 3-5. Redundancy analysis ordination of community data from the combined historical and
current surveys constrained by environmental predictors including time period. Historical sites
are white and current sites are grey............................................................................................... 55
Figure 4-1. Percent of replicate individuals in each treatment surviving on each day over the 12
week experiment in 2009. ............................................................................................................. 78
Figure 4-2. Percent of replicate individuals in each treatment surviving on each day over the 12
week experiment in 2010. ............................................................................................................. 78
Figure 4-3. Box plots indicating the median (heavy line) and distribution of calcium contents (%)
of replicate individuals in each treatment in both years. Box edges indicate the first and third
quartiles, and whiskers represent the nearest value within [quartile values ± 1.5 x Inter Quartile
Range]. Outliers are represented by open circles.......................................................................... 79
Figure 4-4. Box plots indicating the median (heavy line) and distribution of growth rates (mg
day-1) of replicate individuals in each treatment in both years. Box edges indicate the first and
third quartiles, and whiskers represent the nearest value within [quartile values ± 1.5 x Inter
Quartile Range]. Outliers are represented by open circles. .......................................................... 80
Figure 5-1. Carapace of an O. rusticus specimen after removal (a), followed by rinsing with
deionized water and obtaining a hole punch from the posterior region (b). ................................. 92
Figure 5-2. Relationships between carapace Ca content (mean % ± SE) and lake [Ca] (mg·L-1)
for O. virilis (black fill), O. rusticus (white fill), and C. bartonii (grey fill). Points represent mean
±SE for each sampled population, and simple linear regression lines are indicated for O. virilis
xiii
(solid) and O. rusticus (dashed). Inset provides an expanded view of the cluster of lakes with
[Ca] < 4 mg·L-1. ............................................................................................................................ 94
Figure 5-3. Relationships between carapace Ca content (mean % ± SE) and lake [Ca] (mg·L-1)
for O. virilis (black fill), O. rusticus (white fill) in their shared range of [Ca] > 8 mg·L-1. Points
represent mean ± SE for each sampled population, and simple linear regression lines are
indicated for O. virilis (solid) and O. rusticus (dashed). .............................................................. 95
Figure 5-4. Logarithmic relationship (solid black line) between carapace Ca content (mean % ±
SE) and lake [Ca] (mg·L-1) for collected O. virilis (filled circles) across the broad survey range,
indicating that conditions may be sub-saturating below 8 mg·L-1. The three reference lake
samples from France (1987) have been added to the plot (open squares) for comparative
purposes. ....................................................................................................................................... 96
Figure 5-5. Relationships between carapace Ca content (%) and categories of carapace hardness
(HS: hard-soft, H: hard, VH: very hard) for O. virilis. Bars represent mean ± SE of individuals
for each category........................................................................................................................... 96
Figure 5-6. Relationships between carapace Ca content (%) and (a) carapace length (mm) and (b)
carapace hardness category (SH: soft-hard, HS: hard-soft, H: hard, VH: very hard) in the O.
virilis individuals sampled from Red Chalk East Lake. Panel (a) differentiates male individuals
(open circles) from female individuals (filled circles). Bars on panel (b) represent mean ± SE of
individuals for each category. ....................................................................................................... 97
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List of Appendices Appendix 1. Lake Information.................................................................................................... 110
Appendix 2. Historical Lake Chemistry ..................................................................................... 114
Appendix 3. Current Lake Chemistry......................................................................................... 116
Appendix 4. Presence/Absence of Predatory Fish...................................................................... 118
Appendix 5. Historical Crayfish Survey Data ............................................................................ 120
Appendix 6. Current Crayfish Survey Data................................................................................ 124
Appendix 7. 2009 Juvenile Experiment Data ............................................................................. 127
Appendix 8. 2010 Juvenile Experiment Data ............................................................................. 129
Appendix 9. Carapace Broad Survey Sample Data .................................................................... 131
Appendix 10. Red Chalk East Carapace Sample Data ............................................................... 135
Chapter 1 General Introduction
Two important sub-disciplines under the umbrella of ecology are community ecology and
conservation. In many ways, these two sub-disciplines are complements to one another,
providing insight into the factors that determine where and how communities exist, as well as
informing efforts aimed at sustaining the diversity and persistence of the organisms that
comprise them (Smith and Smith 2006). A fundamental goal in community ecology is to
understand the environmental constraints on species distributions and the factors affecting
assemblage patterns, and the primary applications of this field of study are related to global
change and conservation (Simberloff 2004, Smith and Smith 2006). Fueled by a growing
appreciation for the impacts of anthropogenic activities on natural ecosystems, conservation
ecologists apply the principles of population and community ecology, as well as many additional
disciplines to the maintenance of biodiversity (Smith and Smith 2006). One of the most pressing
issues in the field today is bridging the gap between conservation assessment and the
implementation of conservation action (Knight et al. 2008, Lysne et al. 2008). Despite a number
of notable instances where good models of populations and communities have enabled successful
predictions and management actions in relation to conservation and invasive species,
improvements in our ability to detect and quantify threats, their interactions, and predicted
outcomes are necessary in order to inform effective conservation measures (Simberloff 2004,
Venter et al. 2006, Rands et al. 2010, Amano 2012).
Freshwater ecosystems are more threatened than any other system on the planet (Master 1991,
Jelks et al. 2008, Magurran 2009, Abell et al. 2011) and freshwater species are disappearing at a
rate five times that of terrestrial animals (Ricciardi & Rasmussen 1999). Despite this situation,
freshwater conservation has continually lagged behind that of terrestrial and marine systems
(Amis et al. 2009). Aquatic communities, such as fish or benthic invertebrates, are thought to be
structured by biotic and abiotic factors that operate on multiple temporal and spatial scales (Tonn
1990, Smith and Powell 1971, Jackson et al. 2001, Mykra et al. 2007). Given the imperiled state
of aquatic biota, it is important to understand how these factors structure communities, as well as
how community membership can change over time and under anthropogenic influence. Aquatic
systems face an ever-growing list of both local and regional anthropogenic impacts, including
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climatic warming, physical alteration and disturbance, various forms of chemical pollution,
introductions of non-native species, and overexploitation for human uses (Allan and Flecker
1993, Wilcove et al. 1998), which have collectively led to widespread habitat loss, water quality
degradation, reductions in ecosystem function and stability, and the loss of biodiversity
(Williams et al. 1989, Revenga et al. 2000, Jelks et al. 2008, Magurran 2009). There is a growing
body of literature documenting the extent to which aquatic species have been negatively affected
by environmental changes in North America (e.g. Williams et al. 1989, Stuart et al. 2004, Taylor
et al. 2007, Jelks et al. 2008). However, beyond some well studied fish populations, researchers
in general have yet to quantitatively relate declines in particular community members to specific
environmental or anthropogenic influences.
Communities that are sensitive to biotic and abiotic characteristics of environmental change and
that influence ecosystem structure or function are important for identifying the most influential
stressors on aquatic ecosystems. Crayfish are a good choice by these criteria because their
relative abundance is driven by the specific chemical and physical conditions of their aquatic
environments (Capelli and Magnusson 1983, France and Collins 1993, Larson and Olden 2013).
Crayfish have historically been used as biological indicators of environmental condition in
relation to anthropogenic stressors such as contamination by heavy metals (Alikhan et al. 1990)
and lake acidification (Reid and David 1990, Somers et al. 1996, David et al. 1997). Crayfish
also greatly influence the structure and function of the ecosystems they inhabit by physically
modifying aquatic habitats and altering the flow of energy through the food chain (Momot 1995,
Creed and Reed 2004).
Despite their importance in aquatic ecosystems and potential for use as biological indicators,
North American crayfish have received less conservation attention than other freshwater taxa
(Taylor et al. 1996, 2007). Information on the status of crayfish in Canada is even more limited
(Hamr 1998, 2006, Guiasu 2007), however several native populations in lakes in south-central
Ontario were shown to have experienced significant declines (David et al. 1994), potentially
indicating broader ecological issues in the region. Ontario is the crayfish biodiversity hotspot in
Canada, and is home to 9 of the 11 species that can be found nationwide, including natives
Fallicambarus fodiens, Cambarus diogenes, C. bartonii, C. robustus, Orconectes immunis, O.
propinquus, and O. virilis, as well as non-natives O. obscurus and O. rusticus. There is evidence
that effective conservation initiatives should focus on biodiversity hotspots, where efforts are
2
likely to have a greater overall impact (Dobson et al. 1997, Myers et al. 2000). Conservation
assessment and then local (state, provincial) or national protection is needed where endangered
or threatened species of crayfish are identified (Taylor et al. 2007, Crandal and Buhay 2008).
The longer a species is recognized as threatened or at risk, the greater its chances of recovery
(Taylor et al. 2005). Therefore, the driving objective of this thesis was to ascertain the status of
crayfish in south-central Ontario lakes, and to identify any contributing environmental threats.
Very little is known about the environmental factors that shape the membership and relative
abundances of crayfish communities. However, research has shown that the occurrence and
relative abundance of crayfish in lakes may be related to a wide variety of factors including pH
(France and Collins 1983, Davies 1989, France 1993, Seiler and Turner 2004), water temperature
(Somers and Green 1993, Jiravanichpaisal et al. 2004, Hammond et al. 2006), calcium (Ca)
availability (France 1987, Keller et al. 2001, Rukke 2002, Hammond et al. 2006), lake
morphology (Jones and Momot 1981), productivity (Jones and Momot 1981, France 1985),
predators (Collins et al. 1983, Hill and Lodge 1995, Nystrom et al. 2006), other crayfish species
(Hill and Lodge 1999), and anthropogenic habitat alteration (Schultz et al. 2006).
The inland lakes of central Ontario, once considered pristine, increasingly face the full spectrum
of top-ranked anthropogenic impacts to aquatic ecosystems world wide (Allan and Flecker 1993,
Wilcove et al. 1998, Venter et al. 2006), including: the introduction of new fish predators such as
smallmouth bass; new crayfish competitors such as the rusty crayfish; habitat degradation related
to shore development; pollution and chemical change related to acidification and metal
contamination; and the depletion of cations such as Ca.
Physical habitat alteration and increased recreational development of some lakes in the region
has been noted (e.g., Molot and Dillon 2008, Yan et al. 2008) but not directly quantified.
Crayfish preferentially inhabit the near-shore littoral zone of lakes (Jones and Momot 1981).
Depending on the species, crayfish use rocks, macrophyte beds, and coarse woody debris as
refugia (Nystrom et al. 2006) and, in some cases, construct depressions and burrows as long- or
short-term dwellings (Crocker and Barr 1968). As the number of cottages and recreational homes
around the lakes increase, natural shorelines are transformed, and undesired (by homeowners)
substrates, plants, and materials are removed (Christensen et al. 1996) and often are replaced
with sand. Such alterations result in less suitable habitat for crayfish and could lead to crowding
3
and increased competition, cannibalism, and predation. Nutrient inputs also increase with the
number of dwellings present in a watershed, often with eutrophying effects (Carpenter et al.
1998, Moore et al. 2003). Species range in their tolerance to trophic conditions in lakes, but all of
Ontario’s open-water crayfish, except O. immunis, are thought to prefer, cool, clear, well-
oxygenated conditions (Crocker and Barr 1968, Capelli 1975, Berrill 1978).
Introductions of smallmouth bass (Micropterus dolomieui) have occurred across the study
region, and the range of smallmouth bass is predicted to expand because of continued illegal
angler introductions and climate change (MacRae and Jackson 2001,Vander Zanden et al. 2004,
Sharma and Jackson 2008). Smallmouth bass are voracious predators of crayfish (Crocker and
Barr 1968, Collins et al. 1983, Somers and Green 1993, Taylor et al. 2005). Crayfish abundance
(as CPUE) often decreases in the presence of smallmouth bass because of predation-induced
mortality or reduced catchability caused by predator-avoidance behavior (Collins et al. 1983, Hill
and Lodge 1995, Nystrom et al. 2006).
At the outset of this study, it was unclear whether the presence of nonnative crayfish was
potentially negatively impacting native species. Orconectes rusticus is thought to have very
similar environmental preferences to, and competitive dominance over O. virilis and O.
propinquus, frequently leading to their replacement (Lodge et al. 1986, Olden et al. 2006, Olden
et al. 2011). Abundances of native crayfish species might have declined when lakes were first
invaded by the two nonnative crayfish species, O. rusticus and O. obscurus, particularly in lakes
south of the Shield where O. rusticus was introduced (Berrill 1978, David et al. 1997, Wilson et
al. 2004). However, crayfish can no longer legally be sold as bait or be transported between
water bodies in Ontario because of concern that O. rusticus could invade new habitats.
Moreover, water-chemistry issues, such as low Ca concentrations, might prevent the
establishment of O. rusticus in the hundreds of thousands of lakes on the Canadian Shield
(Edwards et al. 2013). However, the realized current ranges of these two species were unknown
previous to this dissertation research.
One particularly promising hypothesis regarding drivers of crayfish declines in the region relates
to historical acidification and metal contamination caused by the long-range transport of
sulphuric and nitric acid precursors, which have significantly altered water chemistry throughout
the inland Canadian Precabrian Shield (hereafter referred to as ‘Shield’) lakes of south-central
4
Ontario (Stoddard et al. 1999, Keller et al 2003). A side effect of the subsequent acidification
recovery in lakes and reduced cation leaching from the surrounding soils has been the reduction
of major cations, including Ca (Whatmough and Dillon 2003, Jeziorski et al. 2008, Molot and
Dillon 2008, Watmough and Aherne 2008). Calcium concentrations in watershed soil pools and
surface waters are now well below pre-acidification levels (Stoddard et al. 1999, Watmough and
Dillon 2003, Jeziorski et al. 2008), and lake concentrations are predicted to continue to drop 10
to 40% from their current levels (Watmough and Aherne 2008).
Calcium is an essential element for most biota. In addition to many physiological roles, crayfish
must acquire Ca for the formation of exoskeletons and might experience increased metabolic
costs in low-Ca environments (Greenaway 1974, Cairns and Yan 2009). Changes in Ca
acquisition could make crayfish more vulnerable to predation, cannibalism, or competition, and
impact their growth, survival, and reproductive success (Stein 1977, France 1987, Keller et al.
2001, Rukke 2002, Hammond et al. 2006, Cairns and Yan 2009). Calcium concentrations in
many of south-central Ontario’s Shield lakes are now
Research Overview
My first two thesis chapters investigate the status and ecology of freshwater crayfish in south-
central Ontario in the context of temporal change. In Chapter 2 I quantify temporal change in
crayfish populations from 100 lakes in south-central Ontario using two sampling periods
separated by approximately 20 years. I further assess the spatial and taxonomic breadth of these
changes, and relate trends to quantified environmental changes over the same period. I found
that the majority of populations of all species detected in the study region have declined severely
between sampling periods, and that widespread population losses have resulted in reduced
crayfish community diversity and shrunken or patchy species distributions. Several significant
environmental changes were identified in the region, which may be implicated as contributing to
the declines observed in crayfish. These include physiochemical changes, such as calcium
decline, inputs of aluminum, chloride and nutrients, shore development, and near-shore warming,
as well as the introduction of centrarchid predatory fish.
In Chapter 3 I use multivariate ordination techniques to identify important environmental drivers
of crayfish relative abundance and community composition, both historically and currently using
the south-central Ontario survey data. I further identify changes in species-environment
relationships and temporal changes in crayfish assemblages between the historical and current
surveys, in order to elucidate emerging threats to aquatic systems in the region. I found that
community composition had changed significantly between the two time periods, and that the
importance of anthropogenic stressors in driving crayfish abundance had increased relative to
natural drivers. In particular, my results indicate that nutrient inputs and shore development from
increasing human activities, the invasion of warm-water centrarchid fishes, and chemical
changes such as acidification and calcium decline, as well as increases in aluminum, appear to be
driving crayfish declines.
My last two thesis chapters focus on the relationship between environmental calcium availability
and various aspects of freshwater crayfish biology, in order to assess the potential for regional Ca
declines to negatively impact crayfish populations in south-central Ontario.
6
In Chapter 4 I conduct laboratory experiments to determine the effects of [Ca] on the growth, Ca
acquisition, and survival of juvenile O. virilis originating from a region experiencing Ca decline.
I found that neither growth nor whole-body calcium content was affected by Ca availability.
Survivorship, however, was significantly reduced at 0.5 – 0.9 mgL-1. This acute tolerance
threshold is lower than any other studied freshwater crayfish species, and might indicate that the
species will be able to persist under forecast [Ca] of lakes in regions experiencing [Ca] decline.
However, the additional pressures imposed in a natural setting will likely increase this
requirement above our lab based determination, making actual outcomes difficult to predict.
In Chapter 5 I use field collections of O. virilis, O. rusticus, and C. bartonii from lakes over a
broad range of [Ca], in order to describe the relationship between the availability of aquatic Ca
and the incorporation of Ca into adult carapaces at intermolt. I find that carapace Ca content is
significantly positively related to lake [Ca] in O. virilis, and that lake [Ca] appears to become
saturating for this species at approximately 8 mgL-1. The results further indicate that the
carapace Ca content of O. virilis is also significantly positively related to carapace hardness
(rigidity) and the size of the individual, but is not significantly related to sex. No significant
relationship was detected between carapace Ca and lake [Ca] in either C. bartonii or O. rusticus,
most likely because they were sampled over a comparatively small range of lake [Ca]. Overall
this study indicates that despite some individual level variability, O. virilis is experiencing low
[Ca] stress in the majority of soft-water Boreal Shield lakes undergoing Ca declines throughout
its range.
Chapters 2 and 3 are published or in press, and have been included in this thesis with permission
from the publishers:
Edwards, B.A., Jackson, D.A., and Somers, K.M. 2009. Multispecies crayfish declines in lakes:
implications for species distributions and richness. Journal of the North American Benthological
Society 28: 719-732.
Edwards, B.A., Jackson, D.A., and Somers, K.M. In Press. Linking temporal changes in crayfish
communities to environmental changes in boreal Shield lakes in south-central Ontario. Canadian
Journal of Fisheries and Aquatic Sciences.
7
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13
Chapter 2 Multispecies crayfish declines in lakes: implications for species
distributions and richness
2.1 Abstract Aquatic communities are highly threatened by anthropogenic and climate changes. However,
despite their importance in these communities, information regarding temporal changes in
populations and assemblages of North American crayfish is scarce. Long-term monitoring of
crayfish populations in south-central Ontario, Canada, indicates that the populations are in a
significant state of decline. We sought to determine whether these population declines are
spatially and taxonomically broad, and if so, what factors might be associated with the declines.
We sampled crayfish abundance (catch per unit effort) in 100 lakes, and compared current
abundances to survey results from the early 1990s. Abundances of all species (natives and
nonnatives) declined significantly during this interval. Declines were both severe (63– 96% loss
of abundance) and geographically widespread for all species. Previous studies have documented
native species declines caused by the invasive crayfish Orconectes rusticus, but this species was
absent from almost all lakes and was not a factor in the declines. We hypothesize that the
introduction of predatory smallmouth bass (Micropterus dolomieui), increases in Al
concentrations, and reduced Ca concentrations in these lakes are negatively affecting crayfish
populations.
2.2 Introduction Aquatic ecosystems are severely threatened by environmental change and loss of biodiversity. In
North America, the number of aquatic taxa in need of conservation or monitoring vastly
outnumbers the number of terrestrial taxa of the same status (Master 1991). Literature on the
status of North American freshwater fishes (e.g., Williams et al. 1989, Jelks et al. 2008) and
amphibians (reviewed in Stuart et al. 2004) has mounted during the past 20y, but North
American crayfish have received less attention (Taylor et al. 1996, 2007) despite their
importance in aquatic ecosystems (Momot et al. 1978). Information on the status of crayfish in
Canada is even more limited (Hamr 1998, 2006).
14
Ontario has the richest crayfish biodiversity in Canada and is home to 9 of the 11 species that can
be found nationwide. These species include natives (Fallicambarus fodiens, Cambarus diogenes,
C. bartonii, C. robustus, Orconectes immunis, O. propinquus, and O. virilis) and nonnatives (O.
obscurus and O. rusticus). Work has begun on determining the status of crayfish in southern
Ontario, particularly in relation to the burrowing crayfishes C. diogenes and F. fodiens (Hamr
2006, Guiasu 2007), but crayfish status elsewhere in Canada has not been directly evaluated. The
most recent status report of the Canadian Endangered Species Conservation Council (CESCC
2006; www. wildspecies.ca) lists no native species as ‘‘At Risk’’ or ‘‘May be At Risk.’’ The
burrowing crayfishes C. diogenes and F. fodiens are listed as ‘‘Sensitive,’’ whereas the
remaining native species are listed as ‘‘Secure.’’ However, this ranking has been based on very
sparse monitoring efforts, particularly in central and northern Ontario. In fact, several native
populations in lakes in south-central Ontario appear to have experienced significant declines
(David et al. 1994) that might indicate broader ecological issues.
The general consensus is that 4 major causes of aquatic imperilment exist: 1) loss or degradation
of appropriate habitat, 2) various forms of chemical pollution, 3) introductions of nonnative
species, and 4) overexploitation for human uses (Allan and Flecker 1993, Wilcove et al. 1998).
In the case of crayfish, overexploitation for human uses is not a likely factor. Reason exists to
hypothesize that any of the other 3 factors might be affecting crayfish, but to date, no studies
have examined the broader status of crayfish populations and communities. In general, crayfish
community structure is not well understood, and few data exist with which to assess particular
trends in crayfish communities.
Crayfish are the largest freshwater crustaceans in North America and where present, they often
comprise a large portion of the total biomass in the ecosystem (Momot et al. 1978). Crayfish are
omnivores, and can act as primary consumers, carnivores, and decomposers (Momot 1995, Dorn
and Wojdak 2004). Thus, they are an important component of aquatic food webs and transfer
energy from lower to higher trophic levels when they are consumed by predatory fishes,
mammals, and birds. Systems without crayfish have diminished energy cycling, community
productivity, and food availability at the top of the food chain. Thus, crayfish might be keystone
species where they are present (Momot 1995). Crayfish are ecosystem engineers. They modify
the physical environments in which they live and alter breakdown and availability of basal food
15
sources (Creed and Reed 2004). Thus, changes to crayfish communities could have broad effects
in aquatic ecosystems.
Our objective was to determine the current status of crayfish populations across south-central
Ontario (>22,000 km2), and if changes were detected, to determine whether particular
environmental factors might be contributing to these changes. We compared crayfish abundances
and water chemistry data from 100 lakes to survey results from the early 1990s (David et al.
1997, Ontario Ministry of Environment [MOE], unpublished data) to assess changes. We
evaluated the community-level and distributional implications of any population changes
detected and determined environmental factors that might be related to the patterns observed in
the abundances of crayfish in these lakes.
2.2.1 Study Lakes and Study Region
Our survey included 100 inland lakes from 9 tertiary watersheds across south-central Ontario
(Fig. 1), based on the availability of historical records for crayfish and water chemistry. The
lakes range in longitude from 78.239°W (Kawartha region) to 81.039°W (Georgian Bay region),
range in latitude from 44.109°N (Scugog region) to 46.269°N (Sudbury area), and cover a wide
geographical range (minimum convex polygon area of 22,300 km2).
Most of the lakes were in 4 tertiary watersheds: Georgian Bay Tributaries (2EA), Moon and Go
Home rivers (2EB), Severn River (2EC), and the Cameron Lake Drainage (2HF). The geology of
this region is typical of the Canadian Precambrian Shield, with a predominance of granitic
bedrock and pockets of thin, acidic, and nutrient-limited soils (Jeffries and Snyder 1983,
Chapman and Putnam 1984). Secondary-growth, mixed forest covers much of the study area,
with patches of coniferous- and deciduous-dominated stands distributed throughout the forest
(David et al. 1997). These watersheds are characterized by different intensities of cottage
development and recreational use and include a number of small communities.
Eight more northern Shield lakes were from the Sudbury region (2CF) and from Algonquin Park
(Upper Madawaska River [2KD] and Deep [2KB] watersheds and the northeastern corner of the
Moon and Go Home River watershed [2EB]). The Sudbury region is sparsely vegetated with
secondary-growth mixed forest (Girard et al. 2006). This area of Algonquin Park is dominated by
coniferous forest and is completely undeveloped (except for walking trails and camp sites) where
16
the study lakes were situated. Four lakes were south of the Shield and enabled us to compare and
contrast crayfish populations and physicochemical variables between the 2 geological regions.
The 4 off-Shield lakes (1 in the southern tip of tertiary watershed 2HF and 3 in watersheds 2HH
and 2HG) are in heavily urbanized and deforested areas, experience high levels of recreational
use year-round, and are hard-water lakes that are rich in Ca. Collectively, the Shield lakes and
the southerly off-Shield lakes represented a long gradient of anthropogenic impacts, including
atmospheric acidification and metal deposition, and human development intensity.
Figure 2-1. Study region in Ontario, Canada, showing the 100 lakes surveyed. Tertiary watersheds are delineated and identified by the following codes: 2EA = Georgian Bay tributaries, 2EB = Moon and Go Home rivers, 2EC = Severn River, 2HF = Cameron Lake drainage, 2CF = Sudbury Region, 2KD = Upper Madawaska River, 2KB = Deep River, 2HH = Kawartha Lakes region, 2HG = Scugog River.
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2.3 Methods
2.3.1 Site selection and crayfish sampling
Site selection and sampling methods were developed by the MOE (David et al. 1994). Recently
molted individuals, berried females, or females carrying young rarely enter traps (Somers and
Green 1993, Richards et al. 1996). Moreover, crayfish are most active, and thus, most likely to
enter traps, when surface water temperatures are >20°C (Somers and Green 1993, Richards et al.
1996, Hein et al. 2007). Therefore, each lake was sampled once between late June and the end of
August to coincide with the period of highest crayfish catchability when both cambarid and
orconectid crayfishes should be in an intermolt stage, females should not be carrying young
(Crocker and Barr 1968, Somers and Green 1993, David et al. 1994, Richards et al. 1996, but see
Hamr and Berrill 1985), and when water temperatures had reached 20°C. This limited
catchability window constrained sampling such that most lakes were sampled over 3 summers
(2005–2007). Five lakes were part of MOE’s regular crayfish population monitoring program
and had been sampled in 2002 to 2004 (MOE, unpublished data). The historical data were
collected over 6 summers (1989–1992, 1994, 1995).
Crayfish were collected with standard, commercially available, wire-mesh Gee minnow traps.
Each trap was baited with a single perforated plastic film canister (5cm tall, 3cm diameter) filled
with fish-flavored canned cat food. Funnel entrances were enlarged to 3.5-cm diameter to
accommodate larger crayfish. Traps were set and then retrieved the following day (18–26 h later)
so that traps were in place at night when crayfish are most active.
Three site types (rock, macrophyte, or woody debris dominated) were sampled in each lake
because species vary in terms of their preferred habitats (Crocker and Barr 1968, Nystrom et al.
2006). Fifty- four traps were set in each lake (18 traps at each of the 3 site types). Three trap
lines, each consisting of 6 traps secured at 3-m intervals, were set perpendicular to the shore 2 to
5 m apart at locations with a minimum depth of 0.5 m for the first trap in a line and a place on
the shoreline where the line could be tied. Trap depths ranged from 0.5 to 8 m depending on the
slope of the littoral zone. This design is similar to that used to collect the historical data, in which
trap depths ranged from 1 to 6 m in depth at 1-m depth intervals (David et al. 1997). Single
baited traps effectively sample a radius of 4.2 m (56.3 m2, Acosta and Perry 2000). Therefore,
the total area sampled by the historical and current sampling designs is equivalent. Crayfish were
18
identified using the taxonomic keys in Crocker and Barr (1968). The number of crayfish/species
in each trap was used to calculate a catch per unit effort (CPUE) estimate of relative abundance.
2.3.2 Chemistry sampling
Water chemistry was sampled in the same year that crayfish were sampled in early spring before
lakes became stratified. Epilimnetic water was collected using a 5-m vertical composite sampler
(Girard et al. 2007). Access to a small number of lakes was difficult in spring because of road
washouts, so water samples from these lakes were collected with the same apparatus but in
summer when crayfish were sampled. In deeper lakes, this sampling approach represents the
chemical conditions of only the epilimnion, but this approach is appropriate because it matches
that method used by David et al. (1997) and because crayfish preferentially inhabit this depth
(Jones and Momot 1981).
A total of 18 chemical variables (pH, alkalinity [mgL-1], conductivity [mScm-1], Al, Mn, and
Fe [μgL-1], dissolved inorganic and organic C [DIC and DOC, respectively; mgL-1], Mg, Ca,
K, Na, Cl, and SO4 [mgL-1], and NH4, NO3, total Kjeldahl N, and total P [μgL-1]) were
analyzed from these samples by the Ontario Ministry of the Environment, using standard
analytical methods and quality assurance/quality control procedures (Anonymous 1983).
Accidental freezing in the laboratory resulted in the loss of 35 DIC samples in the current survey,
and 6 DIC samples were lost for unreported reasons in the historical study. These losses reduced
the number of lakes for which the comparison of DIC was possible to 60 (Scugog Lake was the
only off-Shield lake retained). A linear regression model relating Cl to Na was used to estimate
the historical Na concentration for Scugog Lake.
2.3.3 Data analysis
Model II major axis regression was used to examine the congruence between current and
historical CPUE for each species. Model II regressions are appropriate when both variables are
subject to error (from natural inherent variability or measurement error; Chen and Jackson 2000),
and allowed us to determine the slope and y-intercept of the interrelationship between 2 variables
when we were not interested in the independent and dependent (i.e., predictor and response)
relationship between them (Sokal and Rohlf 1995). If no changes had occurred between the 2
survey periods, one would expect a strong regression relationship with slope = 1. This
19
expectation was evaluated with t-tests to determine whether the modeled slopes differed
significantly from the null expectation of a 1:1 relationship. The t-tests were based on the
formula, t = (b1 - 1)/SEb1, where b1 is the slope of the major (Model II) axis and SEb1 is its
standard error (see Yang et al. 2004).
To evaluate changes at the community level, a frequency distribution of the number of lakes with
0, 1, 2, 3, and 4 species was generated for both the historical and current data sets. Fisher’s exact
test was used to determine whether these species richness relationships had changed significantly
between the 2 survey periods.
Paired t-tests were used to screen chemical variables for significant changes between current and
historical conditions in all lakes and then in only Shield lakes. Tests were corrected for multiple
comparisons with the False Discovery Rate approach (Benjamini and Hochberg 1995). This
correction technique is particularly appropriate when conducting exploratory analyses or when
the variables under consideration covary (reviewed in de Castro and Singer 2006). Each variable
that showed a significant change was further analyzed with Model II regression to obtain a visual
assessment of the trends between the 2 surveys, again with the 1:1 line as the baseline
expectation.
Table 2-1. Results of the model II regression analyses comparing current and historical relative abundances (Catch Per Unit Effort [CPUE]) for all lakes in which a species was found. n = the number of lakes in which a species was found, p = probability associated with the difference from the null expectation (1:1), % historical = the percentage of lakes in which the species occurred historically that have current populations of the species.
Species n r SE Slope p % historical
O. virilis 57 0.61 0.044 0.28
2.4 Results
2.4.1 Crayfish
2.4.1.1 Abundance
Seven crayfish species (O. immunis, O. obscurus, O. propinquus, O. rusticus, O. virilis, C.
bartonii, and C. robustus) were encountered during both studies. Crayfish were not found in 25
lakes during the historical survey and in 33 lakes during our survey. Where crayfish were found,
total crayfish abundance (as CPUE) including all species present ranged from 0.01 to 12.8 in the
historical survey and from 0.02 (in 8 lakes) to 2.89 in our survey.
Abundances of all species decreased between the 2 surveys. Slopes of all lines generated by
Model II regression analyses were significantly
Figure 2-2. Model II regression plots for current vs historical abundance (as catch per unit effort [CPUE]) of Orconectes virilis (A), O. propinquus (B), O. immunis (C), O. obscurus (D), Cambarus bartonii (E), and C. robustus (F), in relation to the 1:1 null expectation (dashed line). Insets show details for species with many low abundances.
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2.4.1.2 Species richness
Species richness was distinctly lower in our survey than in the historical survey, but the shift was
not statistically significant (Fisher’s exact test, p = 0.069; Fig. 3). The percentage of lakes with
no crayfish increased from 25 to 33%, and the modal species richness decreased from 2 (32% of
lakes) to 1 (38% of lakes) (Fig. 3). In the historical survey, 53% of lakes contained ≤1 species
and numerous lakes contained 3 or 4 species, whereas in our survey, 71% of lakes contained ≤1
species and none were found to continue supporting 4 species.
0
5
10
15
20
25
30
35
40
0 1 2 3 4Number of species
Num
ber o
f lak
es
Historical
Current
Figure 2-3. Frequency distributions of crayfish species richness in the historical and current surveys.
2.4.1.3 Distribution
The number of lakes inhabited by each species has decreased for all species except O. immunis
(Fig. 4). The orconectids tended to have fewer apparent losses and less serious declines than did
the cambarids. Orconectes immunis appears to have been lost from 2 lakes (40.0% of historical
occurrences) and was newly detected in 4 lakes (80.0%). Orconectes propinquus appears to have
been lost from 10 lakes (28.6%) and has declined by ≥50% in 10 lakes (28.6%) (Fig. 5A). In
total, 57.1% of O. propinquus populations are currently at risk or have been lost, but these losses
were partially offset by 4 new detections (11.4%). No obvious spatial patterns in O. propinquus
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losses or detections were observed (Fig. 5A), although 3 losses occurred in the lakes in the Black
River system at the northern tip of the Severn River Watershed (2EC). Orconectes virilis appears
to have been lost from 16 lakes (34.0%) and has declined ≥50% in 15 lakes (31.9%) (Fig. 5B). In
total, 66.0% of O. virilis populations are currently at risk or have been lost, but these losses were
offset by 10 new detections (21.3%). A large number of O. virilis population losses or reductions
occurred in one set of connected lakes in the Black River system, whereas O. virilis was newly
detected in each of a string of 4 connected lakes in the northwestern portion of the Cameron
Lake Drainage (2HF) (Fig. 5B).
Figure 2-4. Changes in the number of lakes with populations of Orconectes virilis (O.v.), O. propinquus (O.p.), O. immunis (O.i.), O. obscurus (O.o.), O. rusticus (O.r.), Cambarus bartonii (C.b.), and C. robustus (C.r.), between the historical and current surveys in relation to the 1:1 null expectation (dashed line).
The apparent loss of populations was most marked for the 2 cambarid species (Fig. 4). Cambarus
bartonii appears to have been lost from 19 lakes (59.4%) and has declined by ≥50% in 7 lakes
(21.9%) (Fig. 5C). In total, 81.3% of C. bartonii populations have been lost or are at risk, and
these losses were offset by only 2 new detections (6.3%). Cambarus bartonii losses and declines
have occurred across the northern and southern portions of its historical range, but a narrow band
of currently stable populations was found across the center of the range. Cambarus robustus
24
appears to have been lost from 6 lakes (54.5%) and has declined substantially in 1 lake (9.1%)
(Fig. 5D). These results indicate that 63.6% of C. robustus populations are currently at risk or
have been lost, and these losses were not offset by new detections. Cambarus robustus appears
to have been lost in all of its southern and eastern historical range and is now limited to a small
segment of the northern tip of a single watershed, the Georgian Bay tributaries (2EA).
Figure 2-5. Distributions of Orconectes propinquus (A), O. virilis (B), Cambarus bartonii (C), and C. robustus (D), indicating populations that appear stable, have declined by ≥50%, appear to have been lost, or were newly detected.
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2.4.2 Water chemistry
Significant changes between the historical and current chemical conditions were detected for 9
variables (Table 2). Mean alkalinity, Al, Cl, Na, and NO3 increased significantly (by ~16, 76, 33,
45, and 132%, respectively), whereas mean SO4 decreased significantly (~25%) between
surveys. However, alkalinity did not differ significantly between surveys when off-Shield lakes
were omitted from the analysis. Temporal trends in the remaining variables differed between
lakes on and off the Canadian Shield. Mean Ca and Mg decreased significantly (10%) between
surveys in Shield lakes but increased in off-Shield lakes. In contrast, DIC increased significantly
(16%) in most Shield lakes for which data were available but decreased in off-Shield Lake
Scugog. Comparisons at the watershed level on the Shield were generally consistent with these
findings, however not all watersheds showed a significant change in every variable (Table S1).
Table 2-2. Chemical variables that changed significantly in all study lakes or in the Shield lakes between the historical survey and the current study. Δ = % change, Alkti = alkalinity, DIC = dissolve inorganic C, p = probability associated with paired t-tests comparing current and historical values, * indicates differences were significant only in Shield lakes and means and Δ apply only to Shield lakes, ns = not significant (p > 0.05).
Parameter Current Historical Δ % All Lakes Shield Lakes
Alkti (mg·L-1) 8.22 7.09 16.03 0.0102 NS
Al (μg·L-1) 52.30 29.70 76.09
Model II regression results (Fig. 6A–I) provide support for the differences detected by paired t-
tests; however, 3 cases warrant cautious interpretation. Present and historical values of alkalinity
were strongly correlated (r = 0.91; Fig 6A). However, values declined slightly in a large number
of lakes and increased strongly in a few lakes with historically low concentrations. These few
lakes with large increases might have influenced the results of the paired t-test. The increase in
Al was well supported (r = 0.63; Fig. 6B), but Al decreased in a large number of lakes. NO3
increased dramatically in many lakes, but the pattern of increases was highly variable (r = 0.25;
Fig. 6H) and the regression was driven by increases in lakes with historically low NO3
concentrations.
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Figure 2-6. Model II regression plots for log(x) transformed current vs historic