Copepod Egg Production Rates Research Articles

Ingestion and impact of microplastics on arctic Calanus copepods

Microplastics (MPs) are contaminants of emerging concern in the Arctic, but knowledge of their potential effects on Arctic plankton food webs remains scarce. We experimentally investigated ingestion and effects of MPs (20 μm polyethylene spheres) on the arctic copepods Calanus finmarchicus, C. glacialis and C. hyperboreus. These species dominate arctic zooplankton biomass and are relevant target species to investigate the potential impacts of MPs on the Arctic marine ecosystem. Females of each species were exposed to two concentrations of MPs (200 and 20,000 MPs L−1) in combination with different food (diatom) concentrations, reflecting high (3000–5000 cells mL−1, spring phytoplankton bloom) and low (50–500 cells mL−1, pre/post bloom) food conditions. MPs did not affect negatively fecal pellet production rates in any of the species at the studied exposure concentrations. However, egg production rates of copepods exposed to MPs were 8 times higher compared with the controls, which suggests that MP exposure can cause stress-induced spawning in arctic copepods. Microscopic examination of the fecal pellets confirmed ingested MPs in the three species (up to aprox. 1000 MPs cop−1 d−1). The number of MPs per pellet decreased exponentially with increasing food concentration. The daily ingestion of MPs per copepod was higher at low- food concentrations (250–500 cells mL−1). At our exposure conditions, the presence of MPs inside C. hyperboreus fecal pellets did not affect their sinking rates. Overall, our experimental research show that 1) acute exposure to virgin polyethylene MPs has a low impact on arctic Calanus species at environmentally relevant MP concentrations, independent of food availability, and 2) arctic copepods influence the environmental fate of plankton-sized MPs by exporting buoyant MPs from the surface layer to the sea floor via fecal pellets.

Salinity regulation of copepod egg production in a large microtidal estuary

ABSTRACT Salinity is a key variable for ecological processes in estuaries. Acartia tonsa is a typical estuarine copepod whose responses to salinity have been thoroughly studied in the laboratory. However, results cannot be extrapolated to the field, and formal comparisons between lab and field responses to salinity were not attempted. Here we compare lab-based with field copepod egg production rates (EPR) from the Rio de la Plata estuary (RPE), with focus on A. tonsa. Field work was conducted between 2009 and 2011 in the mixing zone of the RPE. Water temperature, salinity and chlorophyll-a varied over ample ranges (temperature: 10.54 - 24.56oC, salinity: 2.83 - 32.99, chlorophyll-a: 0.62 - 7.27 mg m-3). A. tonsa was the strongly dominant species. EPR ranged between 6.7 and 95.7 eggs female-1 day-1, and correlated to salinity, temperature (weakly), but not to chlorophyll. The relationship between A. tonsa’s EPR (EPRAT) and salinity was consistent with that obtained under laboratory conditions: a humped pattern with a maximum at intermediate salinities. However, differences were also evident, e.g., higher EPRAT was measured in the field. We speculate that discrepancies derive from nutritional differences between field and reference (laboratory) data sets. Besides salinity, food quality and quantity may be first order drivers of A. tonsa’s productivity in the RPE.

CCEIII: Persistent functional relationships between copepod egg production rates and food concentration through anomalously warm conditions in the California Current Ecosystem

We hypothesized that copepod egg production rates would be suppressed relative to their normal functional relationships with food availability during the NE Pacific warm anomalies of 2014–2015 and the El Niño event of 2015–2016. We also hypothesized that copepods would show increased rates of egg production and recruitment in association with ocean fronts. Three species of copepods (Calanus pacificus, Metridia pacifica, and Eucalanus californicus) were incubated at sea in simulated in situ conditions to ascertain egg production rates and egg hatching success during a sequence of 3–5 California Current Ecosystem-Long Term Ecological Research (CCE-LTER) cruises across diverse ocean conditions. Several indices of food availability were measured. The warm events had no significant effect on egg production rates measured in the experiments conducted here. Egg production and net naupliar production rates varied markedly among copepod species and across spatial gradients, but relationships with food concentration during warm events did not consistently diverge from functional relationships during El Niño-neutral conditions. Egg production was most commonly elevated inshore compared to offshore, but was not elevated within fronts. The egg production of C. pacificus was best explained by total Chl-a, while E. californicus was best explained by the > 20 size fraction of Chl-a. M. pacifica showed no relationship between egg production and any food metric tested. C. pacificus showed higher EPR than the other two species in virtually all conditions encountered, which may contribute to its numerical dominance throughout the region.

Annual egg production rates of calanoid copepod species on the continental shelf of the Eastern Tropical Pacific off Mexico

We provide the first estimations of calanoid copepod egg production rates (EPR) in the Eastern Tropical Pacific over an annual cycle (January–December 2011). Gravid females were collected twice monthly and incubated for 12 h without food to estimate EPR, weight-specific fecundity (Gf), spawning success (SS, percentage of females to spawn out of the total species incubated per month and season) and egg hatching success (EHS). This study reports the average EPR of 10 species and the monthly EPR and Gf of four planktonic calanoid copepods (Centropages furcatus, Temora discaudata, Pontellina sobrina, and Nannocalanus minor) that spawned with enough frequency to infer their seasonal reproductive patterns. These species showed distinct seasonal reproductive strategies. Most copepod species spawned sporadically with large EPR variability, while three copepod species reproduced throughout the year (C. furcatus, T. discaudata and P. sobrina) and N. minor spawned only during the mixed period (Feb–May). The four species had relatively similar average EPR (C. furcatus 16, T. discaudata 18, P. sobrina 13, and N. minor 12 eggs fem−1 day−1). These are the first EPR estimations of P. sobrina and its previously known reproductive period is expanded. A Canonical Correspondence Analysis (CCA) was used to analyze EPR and species abundance of all calanoid copepods (40 spp.) collected throughout the time series in relation to temperature, salinity, mixed layer depth (MLD), dissolved oxygen, and chlorophyll a (Chl-a) concentrations to identify the variables that best explained the copepod abundance variability. Temperature, Chl-a, and salinity had the strongest effect on the biological variables, linked to seasonal and episodic upwelling-downwelling processes in the surveyed area. As a result of moderate upwelling events and seasonal variation of environmental conditions, it appears relatively few species are capable of maintaining continuous reproduction under the relatively higher temperatures and strong fluctuations of food availability that exist in this coastal habitat of the Eastern Tropical Pacific.

Toxicity and nutritional inadequacy of Karenia brevis: synergistic mechanisms disrupt top-down grazer control

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 444:15-30 (2012) - DOI: https://doi.org/10.3354/meps09401 Toxicity and nutritional inadequacy of Karenia ­brevis: synergistic mechanisms disrupt top-down grazer control Rebecca J. Waggett1,2,*, D. Ransom Hardison1, Patricia A. Tester1 1National Ocean Service, National Oceanic and Atmospheric Administration, 101 Pivers Island Road, Beaufort, North Carolina 28516-9722, USA 2Present address: The University of Tampa, 401 W. Kennedy Blvd., Tampa, Florida 33606, USA *Email: rwaggett@ut.edu ABSTRACT: Zooplankton grazers are capable of influencing food-web dynamics by exerting top-down control over phytoplankton prey populations. Certain toxic or unpalatable algal species have evolved mechanisms to disrupt grazer control, thereby facilitating the formation of massive, monospecific blooms. The harmful algal bloom (HAB)-forming dinoflagellate Karenia brevis has been associated with lethal and sublethal effects on zooplankton that may offer both direct and indirect support of bloom formation and maintenance. Reductions in copepod grazing on K. brevis have been attributed to acute physiological incapacitation and nutritional inadequacy. To evaluate the potential toxicity or nutritional inadequacy of K. brevis, food removal and egg production experiments were conducted using the copepod Acartia tonsa and K. brevis strains CCMP 2228, Wilson, and SP-1, characterized using liquid chromatography-mass spectrometry (LC-MS) as having high, low, and no brevetoxin levels, respectively. Variable grazing rates were found in experiments involving mixtures of toxic CCMP 2228 and Wilson strains. However, in experiments with toxic CCMP 2228 and non-toxic SP-1 strains, A. tonsa grazed SP-1 at significantly higher rates than the toxic alternative. Additionally, A. tonsa experienced significantly greater mortality when exposed to toxic K. brevis strains, particularly after prolonged exposure. Egg production rates of copepods fed toxic K. brevis strains were similar to those of starved copepods, while those of copepods fed non-toxic SP-1 and the nutritious Rhodomonas salina were significantly higher. Analysis indicates that K. brevis impacts grazer populations via multiple synergistic mechanisms: (1) decreased ingestion rates, (2) decreased egg production, and (3) increased mortality of copepods through a combination of toxicity and nutritional inadequacy. KEY WORDS: Copepod grazing · Harmful algae · Toxic dinoflagellate · Chemical deterrent · Karenia brevis · Brevetoxin Full text in pdf format PreviousNextCite this article as: Waggett RJ, Hardison DR, Tester PA (2012) Toxicity and nutritional inadequacy of Karenia ­brevis: synergistic mechanisms disrupt top-down grazer control. Mar Ecol Prog Ser 444:15-30. https://doi.org/10.3354/meps09401 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 444. Online publication date: January 10, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Interannual variations in vital rates of copepods and euphausiids during the RISE study 2004–2006

The River Influences on Shelf Ecosystems (RISE) program investigated the role of the Columbia River plume in enhancing productivity in the upwelling zone off Washington during four cruises from 2004 to 2006. Measurements of growth rates and brood sizes of euphausiids and egg production rates of copepods were used as indices of secondary production to determine whether these rates differed (1) among cruises as a function of differences in upwelling strength and (2) with latitude, both within the RISE study area and between the coastal waters of Washington and Oregon. Euphausia pacifica growth rates were significantly higher during June 2006 than during July 2004 and June 2005 but not significantly different between the RISE study area and Newport Hydrographic (NH) Line, Oregon. Euphausiid brood sizes were significantly higher during August 2005 than during any other cruise for both E. pacifica and Thysanoessa spinifera; our experiments did not indicate that brood sizes were higher in the northern part of the RISE study region. E. pacifica broods were larger for NH than RISE, but T. spinifera broods were not. Significant differences in egg production rates (EPRs) were found among cruises for both Calanus pacificus and C. marshallae, with higher EPRs during August 2005. EPRs on other cruises were less than half the maximum rates known for these species. EPRs of C. marshallae were similar between RISE and NH; C. pacificus EPRs were significantly higher (lower) in the RISE region in 2005 (2006). Interannual differences in ocean conditions affected zooplankton production more strongly than differences in latitude.

Supplemental effects of diet mixing on absorption of ingested organic carbon in the marine copepod Acartia tonsa

We investigated increased carbon absorption efficiencies (AEs) as a possible cause for positive effects of diet mixing on copepod egg production rates (EPRs) and hatching success (EHS). Female Acartia tonsa were fed 14 C/ 51 Cr dual-labelled Dunaliella tertiolecta (Dun), Amphidinium carterae (Amp), Phaeocystis globosa (Pha), and 3 pairwise 1:1 mixes of the 3 diets. AEs, derived from the ratios of labels in algae and copepod faecal pellets, were 44% on Dun, 37% on Amp, and 49% on Pha, but increased significantly to 61% on Dun + Amp. As a result, EPRs remained low in all tested diets except for Dun + Amp, where it was twice that in the individual diets. Linear multiple regres- sion analysis revealed that EPRs were strongly dependent on the ingestion and absorption of the fatty acids 18:3(n-3) and 22:6(n-3) so that the simultaneous ingestion and absorption of 18:3(n-3) from Dun and 22:6(n-3) from Amp enhanced EPR in the Dun + Amp diet. EHS was low with the Dun diet, which was devoid of 20:5(n-3) and 22:6(n-3). Multiple regression analysis showed that EHS depended on 16:1(n-7) and any or all of 22:6(n-3), 20:5(n-3), or 18:5(n-3).

Sub-Lethal Effects of Elevated Concentration of CO2 on Planktonic Copepods and Sea Urchins

Data concerning the effects of high CO2 concentrations on marine organisms are essential for both predicting future impacts of the increasing atmospheric CO2 concentration and assessing the effects of deep-sea CO2sequestration. Here we review our recent studies evaluating the effects of elevated CO2 concentrations in seawater on the mortality and egg production of the marine planktonic copepod, Acartia steueri, and on the fertilization rate and larval morphology of sea urchin embryos, Hemicentrotus pulcherrimus and Echinometra mathaei. Under conditions of +10,000 ppm CO2 in seawater (pH 6.8), the egg production rates of copepods decreased significantly. The survival rates of adult copepods were not affected when reared under increased CO2 for 8 days, however longer exposure times could have revealed toxic effects of elevated CO2 concentrations. The fertilization rate of sea urchin eggs of both species decreased with increasing CO2 concentration. Furthermore, the size of pluteus larvae decreased with increasing CO2 concentration and malformed skeletogenesis was observed in both larvae. This suggests that calcification is affected by elevated CO2 in the seawater. From these results, we conclude that increased CO2 concentration in seawater will chronically affect several marine organisms and we discuss the effects of increased CO2 on the marine carbon cycle and marine ecosystem.

Supplementation of a diatom diet with cholesterol can enhance copepod egg‐production rates

Cholesterol is required for animal growth, yet it cannot be biosynthesized de novo by crustaceans. Dietary sterols are thus necessary for the growth and reproduction of copepods. Sterols vary widely in both composition and concentration in phytoplankton. To explore the potential for sterol limitation of copepod growth, I did experiments with adult copepods feeding on diatoms with and without cholesterol supplementation. Cholesterol was added to the diet by (1) homogenizing the insoluble cholesterol with an ultrasonic disintegrator or (2) preparing gelatin/acacia microcapsules with and without cholesterol using triolein as a carrier. egg‐production rates were measured after 2–4 d on the experimental diets, except in one experiment, in which an 18‐d feeding period was used. Eggproduction rates increased 1.5–2‐fold for Acartia hudsonica, Acartia tonsa, and Calanus finamrchicus when the diatom Thalassiosira weissflogii was supplemented with 5–20 µg L−1 cholesterol; supplementation at 1 µg L−1 had no effect. Nearly three‐fold higher rates of egg production were observed with C. finmarchicus after feeding for 18 d on the supplemented diet. Cholesterol had a positive effect on egg‐production rates when added by either method. Egg hatch rates for A. hudsonica were also higher when they were fed T. weissflogii supplemented with cholesterol (91% hatched) than when they were fed an unsupplemented diet (40% hatched). Egg production of Centropages hamatus feeding on T. weissflogii was unaffected by supplementation. egg‐production rates of A. hudsonica were unaffected when Chaetoceros affinis, Phaeodactylum tricornutum, or Thalassiosira rotula were supplemented. Higher egg‐production rates in supplemented treatments were caused by a decrease in the frequency of small clutches during spawning events rather than an increase in maximum clutch size. These experiments demonstrate the potential for limitation of egg production by dietary sterols when copepods feed exclusively on diatoms.

Feeding, respiration and egg production rates of copepods during austral spring in the Indian sector of the Antarctic Ocean: role of the zooplankton community in carbon transformation

During the austral spring period of 1996, the composition, age structure and physiological activity of zooplankton were studied in the Indian sector of the Southern Ocean. Zooplankton biomass ranged from less than 1 g m −2 in the Northern Polar Front Zone (PFZ) to 16 g m −2 near the ice edge in the Seasonal Ice Zone (SIZ). Zooplankton communities were dominated by copepods associated with euphausiid larvae. At all stations, species composition of copepods was dominated in number by small species ( Oithona spp, Ctenocalanus citer). Northern stations were characterized by Calanus simillimus and Metridia lucens. Southern stations showed high abundance of Calanoides acutus, Calanus propinquus and Rhincalanus gigas. Stage distribution was analyzed for the four main contributors to the copepod biomass ( Calanus simillimus, Calanoides acutus, Calanus propinquus and Rhincalanus gigas). Gut pigment content and gut transit time showed a strong day–night periodicity. Gut transit times were usually high with values ranging from 1 h ( Calanus propinquus) to 1 h 30 min ( Rhincalanus gigas). Maximum ingestion rates were recorded for Calanus propinquus and Pleuromamma robusta. Respiration rates were measured for 13 species of copepods and varied from 0.5–0.6 μl O 2 ind −1 day −1 for smaller species to 20–62 μl O 2 ind −1 day −1 for the larger ones. The impact of the copepod population was estimated from the CO 2 produced per m −2 and per day, which showed a release of 4.2–4.5 mmol. It corresponded to a minimum ingestion of 41.4% in the Permanent Open Ocean Zone (POOZ) and 22.6% in the SIZ of the daily primary production. The budget between carbon ingestion and respiratory requirements appears to be nearly balanced, but with the exception of Calanus propinquus, cannot accommodate the addition of the cost of egg production, which only partially relies on food intake. During austral spring, the population studied appeared to rely mostly on phytoplankton as food, though additional use of internal energy reserves and intake of protozoan cells was likely needed to fully balance the energy budget.

Copepod egg production in the NW Mediterranean: effects of winter environmental conditions

In the NW Mediterranean the highest abundance of phytoplankton is found during late winter, when temperatures are low. During summer, on the other hand, temperatures are high and phytoplankton abundance low, resulting in low copepod egg production rates (EPR). Despite the apparently better food conditions for copepod growth during winter, we lack information on copepod EPR during the cold season and on the possible effect temperature has on inhibiting this metabolic rate. To solve this uncertainty, a study was carried out to assess the effects exerted by the winter envi- ronmental conditions on the EPR of 3 representative planktonic copepod species of the NW Mediter- ranean (Centropages typicus, Clausocalanus lividus, Acartia clausi). The data were collected during 2 oceanographic cruises in March 1999 (Hivern-99) and January-February 2000 (Hivern-00) and the results compared to those of typical summer conditions. During the Hivern-99 cruise the phytoplank- ton abundance was very high (ranging from 1 to 1.5 µg > 5 µm chlorophyll a (chl a) l -1 ), indicating bloom conditions. During the Hivern-00, the phytoplankton bloom had not yet developed, evident by the chl a concentration of ~0.5 µg l -1 . The recorded EPR during Hivern-99 were amongst the highest for the studied species, and the clearance rates deduced from them indicate a shorter (phytoplankton - copepods) food web during the winter phytoplankton bloom than for the rest of the year, when the planktonic trophic interactions take place through a longer microbial food web.

Latitudinal changes of copepod egg production rates in Atlantic waters:temperature and food availability as the main driving factors

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 181:155-162 (1999) - doi:10.3354/meps181155 Latitudinal changes of copepod egg production rates in Atlantic waters: temperature and food availability as the main driving factors Albert Calbet1,*, Susana Agustí2 1Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, Honolulu, Hawaii 96822, USA 2Instituto Mediterraneo de Estudios Avanzados (CSIC-UIB), c/ Miquel Marqués 21, E-07190 Esporles, Mallorca (Islas Baleares), Spain *E-mail: acalbet@iniki.soest.hawaii.edu ABSTRACT: Planktonic copepod abundance and egg production rates were determined in relation to temperature and chlorophyll concentration across a latitudinal transect in the Atlantic Ocean, ranging from 52°S to 24°N. Copepod distribution followed a pattern similar to that of chlorophyll a, with higher values at the southern latitudes. In contrast, egg production rates were significantly higher in the Atlantic intertropical zone, in the vicinity of the Equator. Copepod abundance was determined mainly by phytoplankton concentration, while rate processes (egg production rates) depended on chlorophyll a concentration and temperature. We propose multiple regression models including these 2 factors that explain up to 70% of the carbon specific egg production rates. The accuracy of these models versus previous ones including only temperature is also discussed. KEY WORDS: Atlantic Ocean · Copepod · Egg production · Chlorophyll · Temperature Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 181. Publication date: May 18, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Copepod egg production rates in the Pomeranian Bay (Southern Baltic Sea) as a function of phytoplankton abundance and taxonomic composition

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 174:183-195 (1998) - doi:10.3354/meps174183 Copepod egg production rates in the Pomeranian Bay (Southern Baltic Sea) as a function of phytoplankton abundance and taxonomic composition Katrin Schmidt*, Paul Kähler, Bodo von Bodungen Institut für Ostseeforschung an der Universität Rostock, Seestraße 15, D-18119 Rostock, Germany *E-mail: katrin.schmidt@io-warnemuende.de ABSTRACT: The copepod community of the Pomeranian Bay, in the outer part of the Oder Estuary (Southern Baltic Sea), is dominated by Acartia bifilosa Giesbrecht in winter and spring and by A. tonsa Dana in autumn. During summer both species co-occur. The egg production of these species was studied in 1994/1995. Rates (eggs female-1 d-1) were found to vary both seasonally and spatially. In winter, egg production rates of A. bifilosa were low, but showed a positive response to increasing chlorophyll a concentrations. During the late phase of the spring bloom (dominated by the diatom Skeletonema costatum), egg production of A. bifilosa was clearly higher than in winter, while water temperature remained at a low level. Phytoplankton concentrations during the spring bloom varied, but egg production rates were uniform all over the bay, suggesting production was not food limited. Length-specific weight and lipid content indicate that females were in good physiological condition. During summer and autumn, both high and very low egg production rates were observed for both species. The relation between egg production rate and chlorophyll a concentration was negative. The analysis of the phytoplankton composition in summer suggested that a high proportion of dinoflagellates was beneficial for egg production whereas high proportions of cyanobacterial colonies and filaments had negative effects. Except during the spring bloom period, the egg production of Acartia spp. in the Pomeranian Bay was limited by the amount and quality of phytoplankton. KEY WORDS: Acartia spp. · Egg production · Phytoplankton · Food quality · Pomeranian Bay Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 174. Publication date: November 26, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Aureococcus anophagefferens: Causes and ecological consequences of brown tides in U.S. mid‐Atlantic coastal waters

Aureococcus anophagefferens is a picoplankionic alga that since 1985 has bloomed in coastal embayments of the western mid‐Atlantic, ranging from Narragansett Bay, Rhode Island, to Barnegat Bay, New Jersey, with greatest incidence of recurrence in Long Island bays, New York. Blooms of this small alga, referred to as “brown tide,” can persist for several months during late spring and summer at densities in excess of 1.0×106 cells ml−1. They are not associated with anomalous chlorophyll a, dissolved oxygen, or inorganic macronutrient (N, P) levels. Meterologically induced reduced flushing rates, elevated salinities, and delivery of micronutrients (e.g. iron) from the watershed have been implicated in bloom initiation. Brown tides have had severe detrimental effects on the benthos, especially eelgrass (Zostera marina) and suspension‐feeding bivalves, including bay scallops (Argopecten irradians) and blue mussels (Mytilus edulis). Adult bivalves experience sublethal effects (e.g. inhibition of clearance rates) at Aureococcus concentrations as low as ∼2×105 cells ml−1 and mortalities at ∼106 cells ml−1, attributed to toxicity of this microalga. Impacts of brown tide on zooplankton are less clear, but reduced egg production rates of copepods and reduced population growth rates of ciliates are documented at higher brown tide concentrations (≥1.0×106 cells ml−1). We summarize the state of knowledge about the physical, chemical, and biological factors that may contribute to brown tide initiation, maintenance, and decline and assess its ecological effects.

Copepod egg production and food resources in Exmouth Gulf, Western Australia

Measurements of plankton community structure, copepod egg production and potential copepod trophic resources were made in Exmouth Gulf, north-westem Australia. Egg production rates by four of the dominant copepod species-Acartia fossae, Parvocalanus crassirostris, Oithona attenuata and O. simplex-were measured by bottle incubations and the egg-ratio technique. Plankton abundance and biomass did not differ greatly within the gulf; however, highest values of chlorophyll a, particulate carbon and nitrogen, and copepod egg production rates occurred in the south-east of the gulf. Though egg production rates were low and apparently severely food-limited, resuspension of bottom sediments or export of material from adjacent salt flats may fuel production in shallow inshore areas of the gulf. P. crassirostris appeared to be omnivorous and O. attenuata primarily herbivorous, but the trophic resources used by O. simplex and A. fossae could not be identified. From the egg production data, it was calculated that adult females of the four dominant copepod species graze 12% of the total particulate carbon each day.

Copepod egg production, moulting and growth rates, and secondary production, in the Skagerrak in August 1988

Measurements of hydrography, chlorophyll, moulting rates of juvenile copepods and egg production rates of adult female copepods were made at eight stations along a transect across the Skagerrak. The goals of the study were to determine (i) if there were correlations between spatial variations in hydrography, phytoplankton and copepod production rates, (ii) if copepod egg production rates were correlated with juvenile growth rates, and (iii) if there was evidence of food-niche separation among co-occumng female copepods The 200 km wide Skagerrak had a stratified water column in the center and a mixed water column along the margins. Such spatial variations should lead to a dominance of small phytoplankton cells in the center and large cells along the margins; however, during our study blooms of Gyrodinium aureolum and Ceratium (three species) masked any locally driven differences in cell size: 50% of chla was >11 μm, 5% in the 11–50 μm fraction and 45% <50 μm. averaged for all stations. Chlorophyll ranged from 0.2 to 2.5 μg l −1 at most depths and stations. Specific growth rates of copepods averaged 0.10 day −1 for adult females and 0.27 day −1 for juveniles The latter is similar to maximum rates known from laboratory studies, thus were probably not food-limited. Egg production rates were food-limited with the degree of limitation varying among species: 75% of maximum for Centropages typicus , 50% for Calanus finmarchicus , 30% for Paracalanus parvus and 15% for Acartia longiremis and Temora longicornis . The degree of limitation was unrelated to female body size suggesting food-niche separation among adults. Copepod production, summed over all species, ranged from 3 to 8 mg carbon m −3 day −1 and averaged 4.6 mg carbon m −1 day −1 . Egg production accounted for 25% of the total.