Deep-Sea Research II 53 (2006) 3078–3098 Cross-shelf distribution of copepods and the role of event-scale winds in a northern California upwelling zone Katherine M. Papastephanou a , Stephen M. Bollens a,b, , Anne M. Slaughter a a Department of Biology and the Romberg Tiburon Center for Environmental Studies, San Francisco State University, 1600 Holloway Ave., San Francisco, CA 94132, USA b School of Biological Sciences, Washington State University Vancouver, 14204 NE Salmon Creek Avenue, Vancouver, WA 98686, USA Accepted 9 July 2006 Abstract Cross-shelf distribution and abundance of copepod nauplii and copepodids were measured during three summer upwelling seasons (2000–2002) in a coastal upwelling zone off northern California. These 3 years varied considerably in the intensity of winds, abundance of chlorophyll, and water temperature. The cruises in 2000 were characterized by relaxation conditions, with generally high levels of chlorophyll and high water temperature. The cruises in 2001 and 2002 were dominated by strong and persistent upwelling events, leading to lower chlorophyll and water temperatures. The copepod assemblage was dominated by Oithona spp., Acartia spp. and Pseudocalanus spp., with Metridia pacifica (lucens), Microsetella rosea, Oncaea spp. and Tortanus discaudatus also common during all 3 years. The cross-shelf distribution of copepods was generally shifted offshore during upwelling and onshore during relaxation events, although some variability between species occurred. Abundance of all life stages generally exhibited a negative correlation with cross-shelf transport averaged over at least 1–4 days and lagged by 0–3 days, indicating lower abundances during and immediately after active upwelling. However, copepod nauplii seemed to respond positively to wind events lasting 1–5 days followed by a period of relaxation lasting 6 or 7 days. These rapid rates of change in abundance are probably too great to be due to in situ growth and reproduction alone; physical processes must also play a role. These results suggest a highly dynamic relationship between copepods and upwelling events off northern California, with species-specific responses to upwelling to be expected. r 2006 Elsevier Ltd. All rights reserved. Keywords: Copepods; Nauplii; Cross-shelf transport; Oithona spp.; Microsetella rosea; Oncaea spp. 1. Introduction The continental shelf off central and northern California is characterized by a strong eastern boundary current (the California Current) and seasonal winds. In the spring and summer, the area is dominated by equatorward winds that result in coastal upwelling and offshore movement of surface waters via Ekman transport, bringing colder and ARTICLE IN PRESS www.elsevier.com/locate/dsr2 0967-0645/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.07.014 Corresponding author. School of Biological Sciences, Wa- shington State University Vancouver, 14204 NE Salmon Creek Avenue, Vancouver, WA, 98686, USA. Tel.: +1 360 546 9116; fax: +1 360 546 9064. E-mail address: [email protected] (S.M. Bollens).
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Deep-Sea Research II 53 (2006) 3078–3098
www.elsevier.com/locate/dsr2
Cross-shelf distribution of copepods and the role of event-scalewinds in a northern California upwelling zone
Katherine M. Papastephanoua, Stephen M. Bollensa,b,�, Anne M. Slaughtera
aDepartment of Biology and the Romberg Tiburon Center for Environmental Studies, San Francisco State University, 1600 Holloway Ave.,
San Francisco, CA 94132, USAbSchool of Biological Sciences, Washington State University Vancouver, 14204 NE Salmon Creek Avenue, Vancouver, WA 98686, USA
Accepted 9 July 2006
Abstract
Cross-shelf distribution and abundance of copepod nauplii and copepodids were measured during three summer
upwelling seasons (2000–2002) in a coastal upwelling zone off northern California. These 3 years varied considerably in the
intensity of winds, abundance of chlorophyll, and water temperature. The cruises in 2000 were characterized by relaxation
conditions, with generally high levels of chlorophyll and high water temperature. The cruises in 2001 and 2002 were
dominated by strong and persistent upwelling events, leading to lower chlorophyll and water temperatures. The copepod
assemblage was dominated by Oithona spp., Acartia spp. and Pseudocalanus spp., with Metridia pacifica (lucens),
Microsetella rosea, Oncaea spp. and Tortanus discaudatus also common during all 3 years. The cross-shelf distribution of
copepods was generally shifted offshore during upwelling and onshore during relaxation events, although some variability
between species occurred. Abundance of all life stages generally exhibited a negative correlation with cross-shelf transport
averaged over at least 1–4 days and lagged by 0–3 days, indicating lower abundances during and immediately after active
upwelling. However, copepod nauplii seemed to respond positively to wind events lasting 1–5 days followed by a period of
relaxation lasting 6 or 7 days. These rapid rates of change in abundance are probably too great to be due to in situ growth
and reproduction alone; physical processes must also play a role. These results suggest a highly dynamic relationship
between copepods and upwelling events off northern California, with species-specific responses to upwelling to be
The continental shelf off central and northernCalifornia is characterized by a strong easternboundary current (the California Current) andseasonal winds. In the spring and summer, the areais dominated by equatorward winds that result incoastal upwelling and offshore movement of surfacewaters via Ekman transport, bringing colder and
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nutrient-rich waters into the system. Periodicreversals of these winds can lead to downwellingconditions. The interplay of currents, reversingwinds, offshore and onshore Ekman transport andvarying coastline geometry creates complex patternsof water movement and ephemeral physical condi-tions, which in turn affect the organisms in thesystem.
The California Current system exhibits the highproductivity at lower trophic levels that is typical ofupwelling systems; it is rich in phytoplankton andzooplankton, especially during the active upwellingseason. Copepods are an abundant component ofthe system biota and it is generally accepted that theevents set in motion by coastal upwelling (additionof nutrients, increase in phytoplankton) result inincreased copepod abundance. Planktonic organ-isms in the system are subject to alongshore andcross-shelf advection and their populations might beexpected to move across the shelf according to theonshore–offshore water movement associated withcoastal upwelling.
Numerous studies have examined the cross-shelfdistribution of copepods during general upwellingconditions (Peterson et al., 1979; Verheye andHutchings, 1988; Escribano and Hidalgo, 2000;Keister and Peterson, 2003; Morgan et al., 2003;Lamb and Peterson, 2005) with varying results.Some reported that upwelling had no effect oncross-shelf copepod distribution (Bernal and McGo-wan, 1981; Cross and Small, 1967). Others haveinvestigated the behavioral mechanisms copepodsmight employ in order to avoid being advectedbeyond the shelf and out of the rich system, such asdiel vertical migration (Smith et al., 1981a; Wro-blewski, 1982; Batchelder et al., 2002), ontogeneticvertical migration (Verheye et al., 1992; Slagstadand Tande, 1996; Peterson, 1998) or both (Verheyeet al., 1991; Verheye and Field, 1992). However,these studies focused on the overall pattern ofoffshore movement of surface water associated withupwelling. Models have shown that constantupwelling winds cannot explain the high productiv-ity of upwelling systems or correlate to shelfretention of plankton (Wroblewski, 1980; Botsfordet al., 2003). However, not much is known about thecross-shelf distributions of copepods as affected bywater movement induced by much shorter-scale(several days) wind events. Such events maysignificantly influence copepod population distribu-tions and abundances over the shelf. Smith et al.(1981b), for example, demonstrated that patterns of
zooplankton composition and biomass can besignificantly affected by cross-shelf advection fluc-tuations on a smaller (i.e. daily) time scale, andDorman et al. (2005) recently examined thisphenomenon in relation to euphausiids.
Additionally, it may be important to examine therole of not only those wind events that induceupwelling, but also relaxation from upwelling andeven those wind events that lead to downwelling.For instance, despite high nutrients in the upwellingzone off northwest Africa, Jones and Halpern(1981) found that primary productivity remainedlow during upwelling winds until wind strengthdecreased. Similarly, Smith et al. (1986) reportedthat numbers of zooplankton taxa and individualsactually decreased as upwelling intensity increased,and found that copepod nauplii abundance in-creased during active downwelling. Peterson et al.(1988) found that several copepod taxa off the coastof Chile were low in abundance during activeupwelling and higher during relaxation. Similarissues have been addressed with respect to mer-oplankton off the coast of northern California byWing et al. (1995).
In the California Current system, periods ofstrong, equatorward alongshore winds that resultin active upwelling are interspersed with winds ofvariable direction, strength and duration, leading torelaxation or downwelling. The interaction ofupwelling favorable events and relaxation or down-welling conditions on very short time scales (i.e.event scales defined by individual wind ‘‘events’’)may have profound impacts on the biota in theupwelling zone. Our objective was to understand therelationship between event-scale winds and thedistribution and numerical response of copepodpopulations. To achieve this, we recorded hourlywind data and repeatedly collected copepods from across-shelf transect during three upwelling seasons.By considering specific wind events in conjunctionwith the cross-shelf distribution and abundance ofvarious life stages of several taxonomic groupsof copepods, we examined not only the movementof copepod populations across the shelf, butalso their relationship and numerical response toindividual upwelling and relaxation events.
2. Materials and methods
Wind and sea-surface temperature data wereobtained from the National Data Buoy Center(NDBC). Chlorophyll, copepod nauplii and
ARTICLE IN PRESSK.M. Papastephanou et al. / Deep-Sea Research II 53 (2006) 3078–30983080
copepodid samples were collected from set locationsalong cross-shelf transects off Bodega Bay, CA,during three cruises in the spring/summer season of2000, 2001 and 2002 on the R.V. Point Sur (Fig. 1).Not all sample types were collected at the same timeor at all stations due to weather constraints and/orlimited equipment availability time (Table 1).
2.1. Hydrographic data collection
Hourly values of sea-surface temperature, windspeed, and wind direction were downloaded fromthe NDBC website for station 46013 (Bodega Baystation at 3811303700N, 12311904800W) for spring2000–2002. Wind direction was rotated 351 accord-ing to the orientation of the central Californiacoastline, and adjusted wind direction data wereused to calculate alongshore and cross-shelf com-ponents of wind speed data. Expected hourly cross-shelf Ekman transport values based on the along-shore wind vector and Coriolis were calculatedaccording to Bakun (1973) and a 15-h movingaverage was applied. Additional temperature datawere also collected from CTD casts using a Seabird911 Plus CTD.
2.2. Chlorophyll data collection
Chlorophyll data were kindly provided by R.Dugdale and F. Wilkerson and methods are briefly
Fig. 1. Location of CoOP/WEST sampling stations and NDBC Station
m isobaths are represented by dotted lines (?).
outlined here (Wilkerson et al., 2006). Watersamples of 280 or 50ml (during blooms) werecollected from a depth of 5m in a 10-L Niskin bottlemounted on a Seabird 911 Plus CTD rosetteapparatus. These samples were filtered onto 47-mm, 0.7-mm pore size Whatman GF/F filters, whichwere then stored at 4 1C until extraction andanalysis in the lab (usually less than 2 weeks).Chlorophyll retained on the filters was extracted atroom temperature in 90% acetone in the darkfor 24 h and read before and after acidificationwith three drops 10% HCl on a Turner 10fluorometer according to Strickland and Parsons(1972).
2.3. Zooplankton collection
Microzooplankton samples were collected from 10-L Niskin bottles mounted on the Seabird 911 CTD.Samples were collected from 5, 15 and 25m waterdepths, filtered separately or together (if equal involume) over 30-mm mesh and preserved in a borate-buffered 5–10% formalin-seawater solution. Meso-zooplankton samples were collected using a 1.0m(2000) or 0.5m (2001 and 2002) diameter ring net witha 73-mmmesh. The ring net was hauled vertically froma depth of 200m or from 5m above the seafloor,whichever was shallower, to the surface at a rate of10mmin�1. Exact depth range of sampling wasdetermined using a Vemco Minilog-TDR depth
46013 (’) in the study area off central California. 200 and 1000-
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Table 1
Summary of stations sampled for copepod nauplii and copepodids; n ¼ number of samples
Samples dates (times) Stations n Sample dates (times) Stations n
1–30 June 2000 3 June (03:05–08:00) D1–D4 4 4 June (11:40–17:35) D1–D5 5
7 June (13:35) D2 1 12 June (09:05–11:50) D1–D3 3
9 June (14:32–19:33) D1–D3 3 24 June (05:58–22:23) D1–D5 5
11 June (08:33–21:21) D1–D5 5 28–29 June (20:13–02:18) D1–D5 5
12 June (04:13–08:38) D1–D5 5 Total 18
13 June (09:05–11:10) D2, D3 2
18–19 June (21:45–04:00) D1, D2, D4 3
20 June (10:25, 23:19) D2, D2 2
24 June (05:55–22:22) D1–D5 5
26 June (10:25, 23:52) D1, D1 2
28–29 June (19:52–02:16) D1, D3–D5 4
Total 36
17 May–15 June 2001 19 May (18:50–23:40) D1–D4 4 23 May (04:40–09:54) D1–D5 5
23 May (06:05–10:29) D2–D5 4 24 May (07:49–14:45) D1–D4 4
24 May (08:19–12:20) D2–D4 3 27 May (09:58–16:50) D1–D5 5
27 May (09:38–15:28) D2–D5 4 30 May (12:04–18:18) D1–D5 5
28 May (09:48) D2 1 5 June (12:25–20:03) D1, D3, D4 3
30 May (11:51–18:10) D1–D5 5 6 June (01:00) D5 1
4 June (23:20) D2 1 Total 23
5–6 June (04:15–00:31) D2, D3, D1–D5 7
8 June (01:14, 14:20) D2, D2 2
10 June (00:53) D1 1
13 June (09:12–14:09) D1–D3 3
14 June (08:51) D4 1
Total 36
30 May–28 June 2002 31 May–1 June (22:08–02:50) D1–D3 3 3 June (05:05, 06:51) D1, D2 2
3 June (04:34–13:34) D1–D5 5 16 June (04:12–12:37) D2–D5 4
4 June (08:38) D1 1 18 June (14:46, 13:06) D4, D5 2
5 June (00:50, 10:02) D1, D1 2 24–25 June (23:35–07:52) D1–D5 5
6 June (08:31) D1 1
7 June (08:32) D1 1
8 June (09:08) D1 1
9 June (08:59–11:59) D1–D3 3
16 June (03:45–14:38) D1–D5 5
19 June (05:50, 07:39) D3–D5 3
19 June (05:50, 07:39) D1, D2 2
20 June (08:13) D1 1
21 June (07:17, 13:40) D1, D2 2
22 June (01:17) D2 1
24–25 June (23:09–07:49) D1, D2, D4, D5 4
Total 35 Total 13
K.M. Papastephanou et al. / Deep-Sea Research II 53 (2006) 3078–3098 3081
recorder in 2001 and 2002. The net was fitted with aGeneral Oceanics flowmeter to determine the volumeof water filtered. Mesozooplankton samples werepreserved in a borate-buffered 7–10% formalin-sea-water solution.
Samples from the D-line, or central line of thesampling area, were chosen for analysis because this
transect was most frequently sampled. Additionally,moorings collecting physical data were locatedalong this transect. Those stations with bottomdepth less than 150m (D1 and D2) are referred to asnearshore, and those stations with bottom depthsgreater than 150m (D3, D4 and D5) are consideredoffshore.
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2.4. Zooplankton enumeration and identification
In the laboratory, microzooplankton sampleswere analyzed for copepod nauplii by enumerationand identification of individuals under a Leica MZ6stereomicroscope at 25.6–40� total magnification.Nauplii were identified to genus when possible, orcategorized according to size, developmental stageand general taxonomic group using Gibbons andOgilvie (1933), Johnson (1934), Ogilvie (1953),Hirakawa (1974), Gibson and Grice (1978), Malt(1982) and Trujillo-Ortiz (1986). Calanoid naupliithat we were unable to identify to genus level weregrouped according to size; individuals with bodylength measuring less than 150 mm during stagesNI–III or less than 200 mm during stages NIII–VIwere counted as ‘‘unidentified small calanoid’’ andlarger individuals were counted as ‘‘unidentifiedlarge calanoid’’. Each of these groupings likelycontained several taxa. Dense samples (containing4500 nauplii per sample) were split to 50% using a1L Folsom splitter to achieve �200–300 individualsfor enumeration and identification. Raw countswere divided by the volume of water filtered (anddivided by Folsom split percentage, if applicable) todetermine naupliim�3.
Mesozooplankton samples were analyzed forcopepodids by enumeration and identification ofindividuals under a Leica MZ6 stereomicroscope at16–40� total magnification. A subsample of eachsample (typically 0.02–0.1%) was taken with a 10-ml Stempel pipette to obtain 200–400 individuals forenumeration and identification to lowest possibletaxon using Olson (1949), Dawson and Knatz(1980), Gardner and Szabo (1982) and Frost(1989). To achieve similar taxonomic groupingsfor direct comparison with nauplii data (e.g.,plotting and statistical analyses), we combined allidentified species of Acartia (Acartia longiremis,Acartia tonsa and Acartia danae) and Oithona
iremis and Acartia tonsa were more common thanAcartia danae. In terms of Oithona spp., Oithona
similis was the predominant species present, withOithona spinirostris occurring rarely and in muchlower abundances. Both Pseudocalanus newmani
and Pseudocalanus mimus have been found inMOCNESS samples from the CoOP/WEST study,but as we were unable to distinguish the two speciesin our samples all individuals from the genus weregrouped together as Pseudocalanus spp. Similarly,we were unable to distinguish between Calanus
marshallae and Calanus pacificus and thereforegrouped all individuals found into Calanus spp.We did not identify to species individuals in thegrouping Oncaea spp. Those individuals from taxanot listed above, as well as some unidentified earlycopepodids, were grouped as ‘‘other’’. Raw countswere divided by the volume of water filtered duringthe net tow and subsample fraction to calculateindividualsm�3.
2.5. Correlation analysis
Hourly estimated Ekman offshore transportvalues (based on wind speed and direction) wereaveraged over 1–10 days starting 0 (no lag) and 19days before each copepod sample was collected. Weused this data to create a matrix of 200 Ekmantransport values to compare to each copepodabundance estimate. Absolute abundance (in-dividualsm�3) of copepodids, nauplii, and totalcopepods were correlated with corresponding Ek-man transport values using Matlab 5.3.
3. Results
3.1. Wind, upwelling, temperature and chlorophyll
data
Wind conditions, upwelling indices, sea-surfacetemperature and station D2 chlorophyll concentra-tions varied inter-annually (Table 2). Each of thethree spring/summer periods is described below.
May–June 2000 was characterized by prolongedperiods (44 days) of relaxation from upwellingwinds, with episodes of low Ekman transportoccurring in early June (around Year Day 155),preceded by a strong upwelling event, and again inmid and late June (YD 170, 180), each preceded bya moderate upwelling event (Fig. 2A). Sea-surfacetemperatures varied between 8.6 and 13.1 1C, with amean of 10.6 1C. The value of 5m chlorophyll a atstation D2 during the sampling period variedbetween 0.77 and 21.9 mgL�1, with a mean of10.3 mgL�1. The mean upwelling index was122m�3 s�1 per 100m coastline.
In contrast, May–June 2001 was characterized bystronger and more persistent upwelling winds, withmuch shorter periods of relaxation (o2 days). Themean hourly upwelling index was 201m�3 s�1 per100m in 2001, compared with 122m�3 s�1 per 100min 2000 (Fig. 2B and A). Sea-surface temperaturesvaried between 8.6 and 11.9 1C, with a mean of
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Table 2
Summary of physical data collected at station D2 during CoOP-WEST sampling: SST and UI means for 1 May –30 June; chlorophyll
means from cruise periods; SE ¼ standard error
Sampling period Sea surface temperature (1C)
mean (SE)
Upwelling index (m3 s�1 per
100m coastline) mean (SE)
5m chlorophyll a (mgL�1)mean (SE)
1 May–30 June 2000 10.6 (0.0) 122 (4.0) 10.3 (1.7)
n ¼ 1413 n ¼ 1413 n ¼ 28
1 May–30 June 2001 9.6 (0.0) 201 (4.4) 4.6 (1.1)
n ¼ 1453 n ¼ 1454 n ¼ 19
1 May–30 June 2002 9.3 (0.0) 228 (2.6) 3.0 (1.0)
n ¼ 1460 n ¼ 1460 n ¼ 13
K.M. Papastephanou et al. / Deep-Sea Research II 53 (2006) 3078–3098 3083
9.6 1C. Chlorophyll a (5m water depth) at stationD2 during the sampling period was much lowerthan in 2000, varying between 0.2 and 7.2 mgL�1
with a mean value of 4.6 mgL�1.May–June 2002 was characterized by three
prolonged periods (410 days) of strong upwellingand a few moderate episodes (2–3 days) of relaxa-tion. In this and other respects, upwelling condi-tions during spring/summer 2002 were fairly similarto those in spring/summer 2001. Mean hourlyupwelling index values was 228m�3 s�1 per 100m(Fig. 2C). Sea-surface temperatures varied between7.7 and 12.0 1C, with a mean of 9.3 1C. Five meterchlorophyll a at station D2 during the samplingperiod varied between 0.4 and 12.2 mgL�1, with amean of 3.0 mgL�1.
3.2. Cross-shelf copepod distributions
The continental shelf copepod community wasdominated by about seven taxa, with other groupsrepresented in smaller numbers (Table 3). In orderof decreasing abundance, copepod nauplii wereprimarily comprised of Oithona spp., Acartia spp.,unidentified large calanoids, unidentified smallcalanoids, Microsetella rosea, Oncaea spp., andTortanus discaudatus (Fig. 3). Similarly, copepodidswere primarily comprised of Oithona spp. (e.g., O.
similis, O. spinirostris), Acartia spp. (e.g., A. long-
iremis, A. tonsa, A. danae), Pseudocalanus spp.,Metridia pacifica (lucens), Microsetella rosea, On-
caea spp. and Tortanus discaudatus (Fig. 4). Otherspecies occasionally found in low abundanceincluded Corycaeus spp., Calanus spp. (e.g., C.
marshallae, C. pacificus), and Eucalanus spp. Oitho-
na spp. were found at every station on every sampledate. Acartia spp. and Tortanus discaudatus (aprimarily estuarine species) were usually limited tothe nearshore stations (D1 and D2). However,
caea spp. and Tortanus discaudatus were all found atleast once at each of the five stations over the courseof this study. Copepod nauplii were generally muchmore abundant than copepodids, with maximummean total abundances of nauplii approximatelydouble those of copepodids (although the limitednumber of copepodid samples may affect theabsolute values and interpretation of these data).
Considering the station-specific abundances inMay–June 2000, copepod nauplii were more abun-dant at inshore stations (D1 and D2) than stationsfurther offshore (Fig. 3A). Oithona spp. weredominant at the offshore stations (D3–D5), makingup 57–72% of the total copepod nauplii present,and were also abundant at D1 and D2, but made upa smaller percentage of the assemblage (17.9% and30.3%, respectively). Acartia spp. comprised 22.8%of the assemblage at D1, but was much lessabundant (o2% of the assemblage) at all the otherstations. Unidentified large and small calanoidcopepod nauplii were present at all stations, butwere a proportionally larger constituent of thenearshore copepod nauplii assemblage (unidentifiedlarge calanoids ¼ 30.3% at D1, unidentified smallcalanoids ¼ 43.2% at D2). Tortanus discaudatus
was found only at D1 and D2 and made up a verysmall proportion of the assemblage (0.5% and0.1%, respectively). Microsetella rosea also wasfound in small numbers, but increased steadilyacross the shelf, from 0.1% of the assemblage at D1to 1.1% at D5, while Oncaea spp. varied from 0.1%to 0.9% of the assemblage with no apparent cross-shelf pattern.
In 2001, mean copepod nauplii abundances weresomewhat lower than in 2000 and did not exhibitthe general cross-shelf gradient seen in 2000(Fig. 3B). Compared to 2000, Oithona spp. made
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-300
-100
100
300
500
700
130 140 150 160 170 180Year Day
(B)
(C)
(A)
Cro
ss-s
helf
Ekm
an tr
ansp
ort (
m3 s-1
per
100m
coa
stlin
e)
-300
-100
100
300
500
700
-300
-100
100
300
500
700
130 140 150 160 170 180
130 140 150 160 170 180
Fig. 2. Ekman upwelling index (15-h moving average calculated from alongshore vector of winds measured at NDBC buoy 46013
according to Bakun (1973)) during May–June of 2000 (A), 2001 (B), and 2002 (C). May–June average upwelling index represented by
dashed line (- - - -).
K.M. Papastephanou et al. / Deep-Sea Research II 53 (2006) 3078–30983084
up a smaller percentage of the assemblage in 2001,ranging from 6.0% at D1 to 54.0% at D5. Acartia
spp. were most abundant at D1, making up 42.4%of the assemblage. Unidentified large and smallcalanoids were common across the shelf andrepresented about 20–35% of the assemblage.
Tortanus discaudatus was present across the shelfand was particularly abundant at D1 (4.8%).Microsetella rosea varied from 0.2% to 1.1%, withno discernable cross-shelf pattern apparent. Theportion of the assemblage comprised by Oncaea
spp. in 2001 was somewhat higher than in 2000,
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Table 3
Mean copepodid abundance (individualsm�3) and standard error (SE) from ring net D-line surveys
Taxon Spring 2000 Spring 2001 Spring 2002
Mean SE Mean SE Mean SE
Acartia longiremis 4 4 1344 928 31 24
Acartia spp. 3 3 3724 3392 358 358
Acartia tonsa 1957 1162 168 125 351 337
Aetidius armatus 1 1 2 2 1 1
Calanus spp. 50 18 11 5 6 2
Candacia bipinnata 0 0 0 0 1 1
Clausocalanus spp. 21 12 1 1 1 1
Corycaeus spp. 2 2 0 0 1 1
Ctenocalanus spp. 4 2 3 2 2 1
Epilabidocera spp. 2 2 0 0 4 4
Euaetideous spp. 0 0 1 0 0 0
Eucalanus californicus 2 1 2 1 22 10
Eucalanus hylinas 0 0 o1 o1 o1 o1
Euchaeta elongata 1 1 1 1 3 2
Euchirella curticauda 1 0 1 1 0 0
Euterpina acutifrons 0 0 0 0 71 71
Gaetanus spp. 0 0 0 0 o1 o1
Limnoithona spp. 2 2 0 0 0 0
Lubbockia spp. 1 1 30 29 0 0
Lucicutia spp. 0 0 1 1 5 3
Metridia pacifica (lucens) 131 44 106 27 181 42
Microsetella rosea 286 53 455 156 1087 529
Oithona similis 1515 159 903 112 766 118
Oncea spp. 195 34 258 43 753 286
Paracalanus spp. 4 4 1 1 0 0
Pseudocalanus spp. 2069 533 2165 508 622 294
Tortanus discaudatus 45 29 77 50 51 45
Unidentified copepodids 1934 1031 1450 684 74 28
Unidentified CI-CV 37 31 7 3 4 2
Ctenocalanus+
Paracalanus+
Clausocalanus spp.
K.M. Papastephanou et al. / Deep-Sea Research II 53 (2006) 3078–3098 3085
ranging from 1.4% at D1 to 0.4% at D5 (comparedto 0.1–0.9% in 2000).
Mean copepod nauplii abundance in 2002 wasgenerally lower than in 2000 and 2001, and showedno significant cross-shelf gradient (Fig. 3C). Oneconsistent pattern observed during all three sam-pling periods was the cross-shelf gradient of Oithona
spp., which, in 2002, increased from 11.5% at D1 to49.3% at D5. Acartia spp. comprised 28.1% of theassemblage at D1, 7.9% to 10.1% from D2 to D4,and 7.5% at D5. Unidentified large calanoids madeup between 9.4% (D4) and 36.2% (D1) of theassemblage, while small calanoids ranged between9.4% (D1) and 40.0% (D3). Tortanus discaudatus
ranged from 4.3% at D1 to 0.1% at D5. Micro-
setella rosea increased steadily from 0.6% of theassemblage at D1 to 1.7% at D5, while Oncaea spp.
comprised more than 6.2% of the assemblage at D1,D4 and D5.
Copepodid abundance during May–June 2000exhibited a striking decline with distance from shore(Fig. 4A). Oithona spp. were present at relativelysimilar abundances at all stations across the shelf,but its relative proportion to the entire assemblagevaried from 7.2% at D1 to 51.2% at D5. Acartia
spp. were only found near shore, making up 35.5%of the assemblage at D1 and 17.6% at D2. Tortanus
discaudatus, similarly found only at nearshorestations, comprised 0.6% and 0.9% at D1 and D2,respectively. Pseudocalanus spp. were mostabundant closer to shore, accounting for 47.7% oftotal copepodids at D2. Metridia pacifica (lucens),Microsetella rosea and Oncaea spp. all exhibitedfairly similar abundances across the shelf, but
Fig. 3. Mean (7SE) cross-shelf abundance and distribution of copepod nauplii along the D-line, averaged over all sampling dates in June
2000 (A), May–June 2001 (B), and May–June 2002 (C). n ¼ number of samples analyzed from a station and individuals counted (italics),
e.g., n ¼ 4 (samples); 1841 (individuals).
K.M. Papastephanou et al. / Deep-Sea Research II 53 (2006) 3078–30983086
represented greater proportions of the copepodassemblage with increasing distance from shore.Metridia pacifica (lucens) increased from1.1% to
5.0%, Microstella rosea from 2.0% to 9.8%, andOncaea spp. from 1.1% to 12.6% (D1–D5). Otherspecies (‘‘Other’’) made up a considerable portion of
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D5 D4 D3 D2 D1
Station onshoreoffshore
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epod
id a
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(#
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)
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(C)
(A)
Oithona spp.
Acartia spp.
Pseudocalanusspp.
Metridia pacifica(lucens)
Microsetellarosea
Oncaea spp.
Tortanusdiscaudatus
Other
n=3;1036 n=3; 710 n=4; 1020 n=4; 786 n=4; 786
n=4; 1848 n=5; 1736 n=5; 827 n=4; 582 n=6; 1312
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Fig. 4. Mean (7SE) cross-shelf abundance and distribution of copepodids along the D-line, averaged over all sampling dates in June 2000
(A), May–June 2001 (B), and May–June 2002 (C). n ¼ number of samples analyzed from a station and individuals counted (italics), e.g.,
n ¼ 3 (samples); 1036 (individuals).
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the assemblage (34.7%) at D1 and decreased withdistance from shore.
Copepodid abundances in 2001 showed manypatterns similar to 2000, although with slightly lowerabundances (Fig. 4B). Mean copepodid abundanceswere much higher (ca. 7 fold) at D1 than at any otherstation on the D line. As in 2000, Acartia spp. madeup a large portion of the assemblage at D1 (42.8%),Pseudocalanus spp. were highly abundant nearshore(48.0% at D2), and Metridia pacifica (lucens),Microsetella rosea, and Oncaea spp. abundances allincreased with increasing distance from shore(D1–D5). Metridia pacifica (lucens) and Oncaea
spp. were present at proportions similar to 2000, butthe proportion of Microsetella rosea in 2001 washigher than 2000 (4.6% at D1, 15.7% at D5).Tortanus discaudatus was only present at D1 (1.6%).A marked difference between 2000 and 2001 is thenear-absence of Oithona spp. at D1 (only 0.1%).
In 2002, copepodids again showed highest abun-dance at D1 and much lower abundances at allother D stations (Fig. 4C). Acartia spp. was againdominant at D1 (26.9%). Pseudocalanus spp. wereagain most abundant closer to shore, making up23.9% of the assemblage at D2. Oithona spp.increased from 4.7% at D1 to 37.8% at D5.Microsetella rosea varied from 14.5% to 19.8%between D2 and D5, with a maximum of 29.9% atD1. Tortanus discaudatus, found only at D1 and D2during previous years, was found further from shore(at D3) in 2002. Both Oncaea spp. and Microsetella
rosea made up larger portions of the assemblages atall stations in 2002 than in 2000 and 2001; Oncaea
spp. ranged between 13.2% and 18.4% with nodiscernable cross-shelf pattern.
3.3. Copepod abundance in relation to temporal
patterns of winds
Copepod nauplii and copepodid abundances weregenerally lower during active upwelling and higherduring periods of relaxation (Fig. 5). We rancorrelations of copepod nauplii and copepodidabundances with Ekman transport values integratedover various time periods (1–10 days) and lagged bydifferent time periods (0–19 days). No apparentpatterns were evident when we grouped nearshoreand midshelf station (D1–D3) data and comparedthem to grouped data from outer shelf and offshorestations (D4 and D5). We also examined totalnauplii abundances between stations and years andfound them to be highly variable.
Total nauplii abundance (all taxa combined) wasconsistently negatively correlated with Ekmantransport (i.e. low abundances corresponded tohigh upwelling index values). Specifically, naupliiabundance at D2 was negatively correlated(po0:05) with Ekman transport averaged over 1–8days and lagged by 0–2 days, depending on the year(Table 4). In 2000 only, naupliar abundance waspositively correlated with transport when transportwas averaged over 1–5 days and lagged by 6 or 7days. In 2001 only, additional negative correlationswere found with transport averaged over 1–5 daysand lagged 9–11 days. Otherwise, patterns of totalnauplii between stations and years were highlyvariable.
Considering individual taxa separately yieldedadditional patterns (results not shown). For in-stance, significant negative correlations betweenOithona spp. naupliar abundance at D2 andtransport were apparent when transport was aver-aged over 1–8 days and lagged by 0–1 days (allyears). Several significant correlations were foundbetween Microsetella rosea nauplii and transport,although no consistent patterns emerged. Interest-ingly, Microsetella rosea nauplii abundance wasgenerally positively correlated with transport whenlagged by 0–10 days at D3 and D5 in 2001, whereasthe previous taxa tended to be negatively correlatedwith transport when the lag time was short. Oncaea
spp. nauplii similarly exhibited a positive correla-tion to transport when lag periods were short; thiscorrelation was found at stations D1–D3 for all 3years. Correlations between abundance of early(NI–III) unidentified calanoid nauplii and transportwere highly variable, but were generally negative atstations D2–D5 when transport was lagged by lessthan 13 days and positive when transport waslagged by more than 13 days. Late stage (NIV–VI)unidentified calanoid nauplii showed a consistentnegative correlation pattern only in 2000, whentransport was averaged over 5–10 days and laggedby 5–16 days.
Total copepodids (all taxa combined) also yieldedvariable correlations between abundance at D2 andEkman transport, although correlations were con-sistently negative (Table 5). In 2000, significantnegative correlations occurred when transport wasaveraged over 1–2 days and lagged for 2–3 days,and again when averaged over 5–10 days and lagged10–12 days. Correlation patterns in 2001 weresimilar to those in 2000: negative significantcorrelations were apparent when averaged over
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136 146 156 166 176
Fig. 5. Ekman upwelling index (15-h moving average) and time series of abundances of copepod nauplii (divided by 10) and copepodids at
station D2 in 2000 (A), 2001 (B), and 2002 (C).
K.M. Papastephanou et al. / Deep-Sea Research II 53 (2006) 3078–3098 3089
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Table 4
Correlation coefficients (R) for copepod nauplii abundance at D2 and upwelling index
No. of days over
which upwelling
values were averaged
Lag (days) between upwelling value and sampling date
Rows indicate number of days over which upwelling values were averaged; columns correspond to duration (number of days) of lag.
Negative values are in italics; bold numbers have p values o0.05; underlined numbers have p values o0.01.
K.M. Papastephanou et al. / Deep-Sea Research II 53 (2006) 3078–30983090
1–2 days and lagged 2–3 days, generally positivewhen averaged over 1–6 days and lagged by 2–6days, and negative again when averaged over 5–8days and lagged by 7–8 days. In 2002, totalcopepodid abundance was negatively correlated totransport when transport was averaged over 1–8days and lagged 0–2 days, but generally positivewhen lagged by 5–12 days.
As with copepod nauplii data, correlation pat-terns became clearer when copepodid taxa wereconsidered individually (results not shown). Corre-lations between Oithona spp. copepodids andtransport were most significant in 2002: negative
when transport was lagged 0–4 days, positive whenlagged by 5–12 days, and negative when lagged bymore than 13 days. Results from Acartia spp.copepodids were more mixed, with significantlynegative correlations to transport averaged over 5–9days and lagged by 9 days in 2000, and positivecorrelations with transport lagged by 0–4 days in2001.
Pseudocalanus spp. copepodid abundance wasconsistently negatively correlated with Ekmantransport when lagged by 2–5 days and positivelycorrelated when lagged by 6–10 or more days during2000 and 2001. This pattern, interestingly, was
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Table 5
Correlation coefficients (R) for copepodid abundance at D2 and upwelling index
No. of days over
which upwelling
values were averaged
Lag (days) between upwelling value and sampling date
Rows indicate number of days over which upwelling values were averaged; columns correspond to duration (number of days) of lag.
Negative values are in italics, bold numbers have p values o0.05; underlined numbers have p values o0.01.
K.M. Papastephanou et al. / Deep-Sea Research II 53 (2006) 3078–3098 3091
reversed (positive–negative) at all stations in 2002.There was a significant positive correlation betweenMicrosetella rosea and Ekman transport whentransport was averaged over 1–4 days and laggedby about 5 days. This general pattern of positivecorrelations when transport was lagged only a shorttime (0–5 days) was consistent across all stationsand years for Microsetella rosea. This pattern ofpositive correlations also occurred between abun-dance of Oncaea spp. and transport at all stations in2000 and was apparent after a slightly longer lag(6–8 days) at the outer shelf stations in 2001 and2002.
3.4. Copepod distribution and abundance: upwelling
versus relaxation conditions
In an effort to directly compare the copepodresponse to upwelling vs. relaxation conditions, wechose data collected along a cross-shelf transectduring a time span that included one activeupwelling period and one relaxation period (May–-June 2000). This was the only cruise that providedsamples where nauplii and copepodids weresampled at all cross-shelf stations (D1–D5) duringboth upwelling and relaxation conditions. Duringactive upwelling, copepod nauplii abundances were
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UnidentifiedLarge Calanoid
UnidentifiedSmall Calanoid
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Tortanusdiscaudatus
Other
Fig. 6. Cross-shelf abundance and distribution of copepod nauplii along D-line during active upwelling on 12 June 2000 (A) and during
relaxation from upwelling on 24 June 2000 (B).
K.M. Papastephanou et al. / Deep-Sea Research II 53 (2006) 3078–30983092
low (less than 20,000m�3) at all stations andincreased slightly with distance from shore(D1–D5) (Fig. 6A). During relaxation, naupliiabundances were generally much higher than duringactive upwelling, particularly inshore, with decreas-ing abundance with distance from shore (D1–D5)(Fig. 6B). During both conditions, Oithona spp.made up a larger portion of the assemblage atoffshore stations than nearshore stations, but wasrelatively more abundant at nearshore stationsduring relaxation. Acartia spp. and unidentifiedcalanoids were more abundant inshore duringrelaxation than upwelling.
Copepodid stages also exhibited different pat-terns of distribution and abundance during activeupwelling and relaxation; they were generally moreabundant during relaxation, especially at nearshorestations (Fig. 7A and B). They exhibited a differentcross-shelf pattern than nauplii during upwelling.During relaxation, the cross-shelf gradient wasmore pronounced for copepodids than for nauplii.Pseudocalanus spp. represented a large portion ofthe assemblage inshore during both active upwellingand relaxation conditions, but was proportionately
more abundant than other copepodid taxa at D3.Oithona spp. were more abundant at D2 duringrelaxation, as were Acartia spp. at D1. A cross-shelfgradient of decreasing copepodid abundance withincreasing distance from shore (D1–D5) was appar-ent during both upwelling and relaxation, but thepattern was much more pronounced during relaxa-tion. In addition, a larger proportion of ‘‘other’’(identified and unidentified) taxa were presentinshore during relaxation.
4. Discussion
Among the primary differences between theupwelling seasons of 2000, 2001 and 2002 were thedurations of wind-driven upwelling and the inter-vening periods of relaxation, which resulted indifferences in water temperature and chlorophylla. Mean water temperatures and chlorophyllabundances were lower during periods dominatedby active upwelling (2001 and 2002) compared toextended periods of relaxation from upwelling(2000).
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)Oithona spp.
Acartia spp.
Pseudocalanusspp.
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Fig. 7. Cross-shelf abundance and distribution of copepodids along D-line during active upwelling on 4 June 2000 (A) and during
relaxation from upwelling on 24 June 2000 (B).
K.M. Papastephanou et al. / Deep-Sea Research II 53 (2006) 3078–3098 3093
4.1. Copepod community composition, abundance,
and cross-shelf distribution
The copepod community composition was consis-tent over our 3-year course of sampling (all majortaxa were found at least once at every station duringall three sampling periods). Copepod assemblageswere dominated by Oithona spp. and Acartia spp.,and included a substantial proportion of Pseudocala-
nus spp. and Metridia pacifica (lucens). Unidentifiedcalanoids made up the largest percentage of thenauplii assemblage, with Microsetella rosea, Oncaea
spp. and Tortanus discaudatus also present. Theseresults are consistent with Peterson and Keister(2002), who found all of these taxa except Tortanus
discaudatus in at least 20% of their samples collectednear the California-Oregon border. The consistentlyhigh abundance of Oithona spp. in our samples wasnot surprising; Bigelow (1926) described Oithona
similis as the most abundant and ubiquitous copepodin the world. In addition, Gallienne and Robins(2001) suggested that Oithona spp. have beenhistorically undersampled by large mesh sizes andunderrepresented in the literature. Our mesh sizes of30mm (nauplii) and 73mm (copepodids) were smallenough to retain Oithona spp.
Copepodid abundances were much lower thannauplii abundances, as expected. We found gener-ally higher copepodid abundances inshore, espe-cially Pseudocalanus spp. and Acartia spp., withOncaea spp. and Microsetella rosea representing thehighest proportions of the offshore assemblages.These findings are consistent with Morgan et al.(2003), who reported an on-shelf copepod biomass2.8 times greater than off-shelf biomass (based on ashelf break at 180m) and identified Calanus
marshallae, Pseudocalanus mimus, and Acartia long-
iremis as indicators of continental shelf waters inOregon.
4.2. Effects of upwelling
Our correlation analysis attempted to relateabundance of copepods to calculated Ekmancross-shelf transport. As such, we examined onlyone of many factors potentially affecting copepodabundance and distribution within this dynamicsystem; these calculations did not include informa-tion about other physical factors (advection ofcopepods out of the system, offshore wind-stresscurl, coastal topography, alongshore transport) orbiological processes (birth, development and growth
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rates, vertical migration). We can, however, com-pare patterns of changes in abundance to generalpatterns of upwelling.
We found a positive correlation between totalcopepod nauplii abundance and cross-shelf trans-port averaged over 1–5 days and lagged by 6–7 daysin 2000, a year characterized by prolonged periodsof relaxation and higher chlorophyll. In the simplestmodel of the biological response to upwelling,upwelled water brings nutrients into surface waters,where they become available to phytoplankton,resulting in an increase in chlorophyll, and zoo-plankton (e.g., copepods) respond in turn to thisabundant food source by reproducing. In reality,however, it appears that the biological componentsof the system require not only the introduction ofnutrients into the system during coastal upwelling,but also an opportunity to use those nutrientsbefore they are advected out of the system. Thegenerally negative correlations in 2001 and 2002,years dominated by active upwelling, imply that thecopepods simply were not able to utilize thenutrients and phytoplankton before they werequickly advected off the shelf or sank out of thesystem.
One of the most striking taxon-specific patternswe observed was a general negative correlationbetween Oithona spp. nauplii abundance and Ek-man transport; Oithona spp. were a significantcomponent of the assemblage at every station, butwere found in lowest numbers during and just afterhigh values of Ekman transport. This may be due toadvection out of the system, as we suspect, oralternatively, from either mortality or verticalmigration out of surface waters to avoid increasedturbulence (Incze et al., 2001) (both of which werebeyond our means of measuring).
On the other hand, we found a consistentlypositive correlation between Ekman transport andabundance of all stages of both Microsetella rosea
and Oncaea spp. These taxa made up largerpercentages of the assemblage at the offshorestations, but exhibited this positive numericalresponse to upwelling at all stations. The lag timefor this positive response was brief (0–6 days), so thehigh abundance of these taxa could not have beenthe result of a reproductive response to theparticular upwelling event to which their abundancewas being correlated. Also, this positive correlationwas seen concurrently in both naupliar and cope-podid stages, implying the existence of a standingpopulation prior to any given upwelling event.
Keister and Peterson (2003) identified Microsetella
spp. and Oncaea spp. as indicators of El Ninoconditions in the upwelling zone off Oregon,implying origins to the south (e.g., off California)for these populations. Additionally, Morgan et al.(2003) identified both of these taxa as indicators ofoff-shelf copepod populations. It is possible thatthese two taxa are adapted to offshore conditions,such as lower food (chlorophyll) availability, andthus seem to dominate the system when other,nearshore taxa move out of the system during activeupwelling. The vertical distribution of these twogenera, about which very little is known, might beanother possible explanation for the differentnumerical response of these taxa to upwellingcompared to that of other copepods in our study.
Previous research on the effects of upwelling oncopepods has yielded mixed results: Smith et al.(1986) found the fewest taxa and lowest abundanceof copepods during the strongest upwelling off Pt.Conception, CA. Similarly, Peterson et al. (1988)reported that off Chile, Acartia tonsa, Calanoides
patagoniensis, and Paracalanus parvus were found inlow abundance during active upwelling and highabundance during relaxation, but Oithona spp.,Oncaea spp., Centropages brachiatus, and Calanus
chilensis exhibited no consistent relationship withupwelling. Our correlations indicate there are someregular patterns of change in copepod abundance inresponse to wind events off northern California,although species-specific responses are to be ex-pected.
With respect to possible distributional changes,Pillar (1986) reported that persistent mild upwellingoff the southern and southwest coasts of Africaresulted in displacement of copepod populationssuch that peaks in abundance were found offshore.Blackburn (1979) reported similar findings in theupwelling zone off northwest Africa. We did not seethis pattern in copepodids, but copepod nauplii, onthe other hand, were indeed less abundant near-shore during years with persistent upwelling (2001and 2002). Copepod nauplii abundance was higherat inshore stations during periods of relaxed winds(2000) and less abundant during strong persistentupwelling-favorable winds (2001 and 2002). Theirmore uniform cross-shelf distribution in 2001 and2002 is most easily explained by cross-shelf advec-tion of naupliar stages during upwelling. Theincreased presence of Acartia spp., usually a coastaltaxon, at offshore stations in 2002 also might beexplained, at least in part, by cross-shelf advection.
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In our one direct comparison of an individualupwelling event followed by an individual relaxa-tion event (Fig. 7), we observed abundances ofoffshore copepodids and nearshore nauplii thatwere lower during active upwelling and higherduring relaxation. This general decrease in zoo-plankton abundance during active upwelling isconsistent with Dorman et al. (2005), who foundlower abundances of Euphausia pacifica duringactive upwelling in this same system. Similarly,Smith et al. (1986) reported a decrease in copepodabundance during upwelling, but noted that thepercentage of the assemblage comprised by C.
pacificus, P. parvus and Oithona spp. increased asupwelling strength increased. Our data support thisfinding for Oithona spp. and Pseudocalanus spp.copepodids, which made up a larger percentage ofthe nearshore assemblage during upwelling thanduring periods of relaxation. Smith et al. (1986) alsofound that peaks of nauplii did not occur near thecoast, but always coincided with areas of elevatedchlorophyll a, and were most abundant and farthestfrom the coast during upwelling. We also foundhigher abundances of nauplii at offshore stationsduring upwelling, perhaps due to cross-shelfadvection.
Why would different copepod taxa have suchdissimilar responses to upwelling? Different beha-viors may have evolved as a way to avoidcompetition for space and resources. For instance,Williams and Conway (1980) reported differingvertical migration behavior patterns which mini-mized competition between species (Calanus fin-
marchicus and Calanus helgolandicus) withsympatric distributions. These different behaviorsand distributional patterns lead to different feedingregimes and strategies, and may simplify resourcepartitioning for taxa co-occurring in an upwellingzone. Such differential vertical distributions may inturn have different implications for each taxon’stransport and/or retention on the shelf, and thustheir relationship to wind events.
The general increase in abundance of bothcopepod nauplii and copepodids between upwellingand relaxation events in June 2000 (Figs. 6 and 7)may be due to several factors, as their numbers areaffected by both physical factors (advection) andbiological factors (reproduction, predation, etc.).While it is not possible with our data to determinethe effect of each of these factors, we can examinetheir relative importance. For instance, copepodnauplii abundance at station D1 increased drama-
tically over this 12-day period, from 4890m�3
during upwelling to 103,826m�3 during relaxation(421-fold increase) (Fig. 6). Based on exponentialpopulation growth, an increase of this magnitudewould require a growth rate of 0.254 d�1. However,adjusted to the mean temperature of 10 1C measuredduring our sampling, maximum intrinsic rates ofincrease for dominant taxa are generally much lessthan this: approximately 0.12 d�1 for Oithona spp.(Sabatini and Kiørboe, 1994; Huntley and Lopez,1992), 0.14 d�1 for Acartia spp. (Wroblewski, 1980;Huntley and Lopez, 1992), and 0.15 d�1 forPseudocalanus spp. (Huntley and Lopez, 1992).Estimates of maximum intrinsic rates of increaseusually assume a stable age distribution; even higherrates are theoretically possible in a situation where apopulation is dominated by adults that reproduceand yields an age distribution that is skewedtowards young stages (Bollens, 1988). With respectto copepodids, abundance increased at D1 betweenupwelling and relaxation events 20 days apart from5114 to 32,000m�3, a 6-fold increase (Fig. 7). Thisincrease would require a growth rate of at least0.092 d�1, which is possible based on maximumgrowth rates in the literature, but unlikely since suchestimates assume no predation or other mortality.Therefore, it is unlikely that our observed changesin abundance were solely due to biological pro-cesses; advective processes must have had animpact.
In addition to cross-shelf flow, equatorwardalongshore water movement occurs during upwel-ling-favorable wind events in northern California;this also affects plankton distribution. Huyer (1983)described a southward coastal jet off the coast ofOregon with maximum displacement occurring15–20 km offshore. Bernal and McGowan (1981)reported that changes in zooplankton biomassduring upwelling were uncorrelated with upwellingand were better predicted by transport from thenorth. Wing et al. (1995) found abundance ofmeroplankton off northern California to be posi-tively affected by reversal of alongshore currents(i.e. from southerly to northerly) associated withrelaxation of upwelling conditions. Thus, thedistribution and abundance patterns of copepodswe observed are probably associated with along-shore flow patterns in addition to cross-shelfadvection.
General offshore movement of upwelled water atthe surface is not the only physical oceanographicfactor affecting cross-shelf distribution to consider;
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an undercurrent of reverse direction also may occur(Huyer, 1983). During active upwelling, copepodsmay vertically migrate into this layer to betransported back towards shore and avoid beingswept out of the upwelling system; this may explainthe pronounced pattern of high inshore copepodidabundance seen in 2002. Peterson (1998) presented amodel of ontogenetic migration of Calanus mar-
shallae in an upwelling zone in which copepod egg-laying occurred near shore, development occurredas individuals were advected offshore, and sinkingof stage CV into the deep onshore current resultedin transport back inshore for egg-laying. This modelpredicted a broad distribution of life stages acrossthe shelf, yet we consistently found all copepodidtaxa except Oithona spp. to be more abundant closeto the shore; perhaps our sampling to a maximumof 200m was not deep enough to capture allindividuals employing diel vertical migration indeeper water.
In addition to alongshore currents and deepcountercurrents, other large-scale circulation pat-terns may affect copepod distribution in this region,such as jets, eddies and flow patterns affected bycoastal topography. Huntley et al. (2000) reporteddistinctly different zooplankton populations in a jetfrom the California Current and a cyclonic eddythat resulted from it. Lamb and Peterson (2005)suggested that a recirculating gyre off the coast ofOregon may be responsible for nearshore retentionof plankton during active upwelling. In and aroundour (WEST) study site off northern California,Kuebel Cervantes and Allen (2006) modeled re-circulation of water displaced offshore by Pt. Reyesand back onshore to the south. Similarly, VanderWoude et al. (2006) used satellite observations todescribe two retentive embayments, one north of PtReyes and one south of Pt. Reyes (Drakes Bay). Thedegree to which these recirculation features affectzooplankton populations, however, is currentlyunknown.
In interpreting our results, it is also necessary toconsider possible confounding factors. In a systemas dynamic as the northern California coastalupwelling zone, periods of upwelling and relaxationvary in duration and can alternate rapidly. There-fore when calculating our correlations, we cannotalways isolate the long-term effects of an individualupwelling event. Correlations occurring at longerlag times (e.g., the significant negative correlationbetween Oithona spp. copepodid abundance andtransport lagged over more than 13 days in 2002)
may include the effects of several individual windevents.
In summary, we observed that the communitycomposition of copepods in the upwelling region offnorthern California was quite stable during threeconsecutive upwelling seasons. Many of these taxaexhibited consistent numerical and/or distributionalresponses to upwelling transport, but others did not.The cross-shelf distribution of copepods was gen-erally shifted offshore during upwelling and inshoreduring relaxation events. Copepod nauplii andcopepodid abundances were generally much higherin this upwelling zone during relaxation than duringactive upwelling. Abundances of all life stagesgenerally exhibited a negative correlation withcross-shelf transport averaged over at least 1–4days and lagged by 0–3 days (with Microsetella
rosea and Oncaea spp. being notable exceptions).Copepod nauplii, on the other hand, seemed torespond positively to wind events lasting 1–5 daysfollowed by a period of relaxation lasting 6 or 7days. Understanding the exact timescales fordifferent copepod species to respond to individualwind events will require combining field data suchas ours with coupled physical–biological models ofupwelling ecosystems (e.g., Batchelder et al., 2002;Botsford et al., 2003; Botsford et al., 2006), as wellas additional information on the life histories ofthese particular taxa.
Acknowledgements
We thank Sean Avent, Jeff Dorman, TessaJohnson and the officers and crew of the R.V. Point
Sur for assistance in the field. We would also like tothank the Frances Wilkerson and Dick Dugdalelabs for providing chlorophyll data and members ofToby Garfield’s lab for assistance in writingcomputer programs. Gratitude is extended to theother members of the CoOP/WEST group for theirinput and guidance. This project was funded by theNational Science Foundation (NSF Grant # OCE-9908072 to S.M. Bollens).
References
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