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Eutrophication and Algal Proliferation in Florida’s Springs
Forest HydrologySpring 2014
Water Quality and Aquatic Health
• Tenet #1: Contaminants from land end up in the water– Industrial, urban, agricultural chemicals
• Tenet #2: Aquatic systems may respond, often in undesirable ways
• Habitat viability• Aesthetics (color, aroma, clarity)• Function (support C storage, N removal, flow)• Human use potential (e.g., drinking or irrigation water)
Eutrophication
• Def: Excess C fixation– Primary production is
stimulated. Can be a good thing (e.g., more fish)
– Can induce changes in dominant primary producers (e.g., algae vs. rooted plants)
– Can alter dissolved oxygen dynamics (nighttime lows)• Fish and invertebrate impacts• Changes in color, clarity, aroma
More P
Less P
http://www.sjrwmd.com/publications/pdfs/fs_lapopka.pdf
Reduction in Water Clarity = Changes
in Bottom Habitats
Eutrophication may stimulate the growth of algae that produce harmful toxins
Red Tide
Dead Zone in the Gulf of Mexico
http://serc.carleton.edu/microbelife/topics/deadzone/
Scope of the Problem in Florida
Source: USEPA (http://iaspub.epa.gov/waters10/state_rept.control?p_state=FL&p_cycle=2002)
What Causes Eutrophication?
• Leibig’s “Law of the Minimum”– Some element (or light
or water) limits primary production
– Adding that thing will increase yields (GPP)
– What is limiting in forests? Crops? Lakes? Pelagic ocean?
Justus von Liebig
What Limits Aquatic Production?
Typical Symptoms: Alleviation of Nutrient Limitation
(GPP)
• Phosphorus limitation in shallow temperate lakes
• Nitrogen limitation in estuarine systems
V. Smith, L&O 2006V. Smith, L&O 1982
Global Nitrogen Enrichment
• Humans have massively amplified global N cycle– Terrestrial Inputs
• 1890: ~ 150 Tg N yr-1
• 2005: ~ 290+ Tg N yr-1
– River Outputs• 1890: ~ 30 Tg N yr-1
• 2005: ~ 60+ Tg N yr-1
• N frequently limits terrestrial and aquatic primary production– Eutrophication
Gruber and Galloway 2008
Local Nitrogen Enrichment• The Floridan Aquifer (our
primary water source) is:– Vulnerable to nitrate
contamination– Locally enriched as much as
30,000% over background (~ 50-100 ppb as N)
• Springs are sentinels of aquifer pollution– Florida has world’s highest
density of 1st magnitude springs (> 100 cfs)
Arthur et al. 2006
Weeki Wachee20011950’s
Mission Springs Chassowitzka (T. Frazer)
Weeki Wachee
Mill Pond Spring
GROW FASTER LOST MORE SLOWLY
Core Question:
What Causes Algae to Reach Nuisance Levels?
• Hnull: N loading alleviated GPP limitation, algae exploded (conventional wisdom)
• Evidence generally runs counter to this hypothesis– Springs were light limited even at low concentrations (Odum 1957)– Algal cover/AFDM is uncorrelated with [NO3] (Stevenson et al. 2004)
– Flowing water mesocosms show algal growth saturation at ~ 110 ppb (Albertin et al. 2007)
– Nuisance algae exists principally near the spring vents, high nitrate persists downstream (Stevenson et al. 2004)
N Enrichment in Springs
From Stevenson et al. 2004 Ecological condition of algae and nutrients in Florida Springs DEP Contract #WM858
Fall 2002 (closed circles) and Spring 2003 (open triangles)
No correlation between algae and N
N Enrichment and Primary Production[No Significant Association]
• More N does not mean more GPP
(GPP)
Alexander Springs (50 ppb N-NO3)
Visualizing the Problem
Silver Springs (1,400 ppb N-NO3)
Qualitative Insight: Comparing Assimilatory Demand vs. Load
• Primary Production is very high– 8-20 g O2/m2/d (ca. 1,500 g C/m2/yr)
• N demand is proportional– 0.05 – 0.15 g N/m2/day
• N flux (over 5,000 m reach) is large– Now: ca. 30 g N/m2/d (240 x Ua)
– Before: ca. 2.5 g N/m2/d (20 x Ua)
• In rivers, the salient measure of availability may be flux (not concentration)
• Because of light limitation, this is best indexed to demand
• When does flux:demand become critical?
Back to First Principles:Controls on Algal Biomass
bottom up effects
top down effects
Algae Biomass
Grazers Flow RatesDissolved Oxygen
Nutrients Light
mediating factors
Algal Loss Rates - Scouring
• Flow has widely declined, in areas a lot– Silver Springs– White Springs– Kissingen Spring
• Lower discharge means lower scour
• Algal cover varies with flow velocity (King 2014)
Algal Loss Rates - Grazing
• Algal cover is predicted by:– Dissolved oxygen (DO)– Grazer density
• DO is keystone variable for aquatic animal health– Proxy for groundwater age?
Observational Evidence:Grazers and Algae are Correlated
Liebowitz et al. (in review)
Threshold effect?
~ 20 g m-2
Combined model (snails, flow, light) explains over 50% of algae variation
Snail Biomass (g m-2)
Alg
ae B
iom
ass
(g m
-2)
Experimental Evidence: Snails Control Algae
• Enclosed & excluded snails
Liebowitz et al. (in prep)
0 50 100 150 200 250 300 3500
5
10
15
20
25
30
35
40
45
f(x) = 2.46086606987551 exp( − 0.00313473225145049 x )R² = 0.640491892532922
f(x) = 12.8376995134515 exp( − 0.00459539280306784 x )R² = 0.55239167058656
f(x) = 14.9535593057998 exp( − 0.00782615105298781 x )R² = 0.847684879293451
f(x) = 38.1271888766727 exp( − 0.00923800694915261 x )R² = 0.934313949655305
HS
Expo-nential (HS)
GF
Expo-nential (GF)
MP
Expo-nential (MP)
ST
Gastropod wet weight (g m-2)
Alg
ae
AF
DM
(g
m-2
)
Observational Evidence:What Controls Snails?
• Changes in DO– Flow varying?
• Changes in salinity & [Ca++]• Human disturbance• Snail density model r2 > 0.6
– Dissolved oxygen– Salinity– pH– Light– SAV
Liebowitz et al. (in review)
Strong (2004)
Evidence of Alternative States
Experiment 1 – Low Initial Algae: Intermediate density of snails able to control algal accumulation.
Experiment 2 – High Initial Algae: No density of snails capable of controlling accumulation.
Shape of hysteresis is site dependent.
Summary
• Nitrate is a poor predictor of algal abundance– Load >> Demand (N is, and may have always been
sufficient to satisfy all ecosystem demand)• Grazers exert a dominant control on algae
– Evidence of “escape density” thresholds that are really important for management
• Dissolved oxygen (among other things) impacts grazers– Even short term stress has lasting impacts, in part
because algal biomass can escape control
Complex Ecological Causes