Phytoplankton food quality determines time windows for successful zooplankton reproductive pulses

Recruitment success at the early life stages is a critical process for zooplankton demography. Copepods often dominate the zooplankton in marine coastal zones and are prey of the majority of fish larvae. Hypotheses interpreting variations of copepod recruitment are based on the concepts of "naupliar predation," "nutritional deficiency," and "toxic effect" of diatom diets. Contradictory laboratory and field studies have reached opposite conclusions on the effects of diatoms on copepod reproductive success, blurring our view of marine food-web energy flow from diatoms to higher consumers by means of copepods. Here we report estimates of copepod feeding selectivity and reproduction in response to seasonally changing phytoplankton characteristics measured in a highly productive coastal upwelling area off the coast of central Chile. The variable phytoplankton diversity and changing food quality had a strong and highly significant impact on the feeding selectivity, reproduction, and larval survival of three indigenous copepod species. Seasonal changes in copepod feeding behavior were related to the alternating protozoan-diatom diets, mostly based on dinoflagellates and ciliates during winter and autumn (low highly unsaturated fatty acids [HUFA]/polyunsaturated fatty acids [PUFA] availability), but switched to a diet of centric and chain-forming diatoms (high HUFA/PUFA availability) during the spring/summer upwelling period. Ingestion of diatom cells induced a positive effect on egg production. However, a negative relationship was found between egg hatching success, naupliar survival, and diatom ingestion. Depending on the phytoplankton species, diets had different effects on copepod reproduction and recruitment. In consequence, it seems that the classical marine food web model does not apply to some coastal upwelling systems.     

Microbial decomposition of proteins and lipids in copepod versus rotifer carcasses

Zooplankton carcasses are common within aquatic systems and potentially serve as organic-rich substrates for bacteria. We compared the microbial decomposition of representative crustacean (copepod) and non-crustacean (rotifer) zooplankton carcasses and monitored changes in carcass protein and lipid contents. Our results showed that carcass decomposition was mainly driven by bacteria colonizing from the surrounding water. Carcass-associated bacteria displayed higher protease and lipase activities than free-living bacteria. Protein content of copepod carcasses decreased by 70% within the first 8 h and shifted from larger to smaller sized proteins, while protein loss in rotifer carcasses was insignificant. Carcass lipid content did not change significantly over 24 h in either zooplankton type, although polar branched fatty acids increased on copepod carcasses indicating an increase in viable microbial biomass. Our results suggest differential turnover of protein versus lipid within a zooplankton carcass and that carcasses from different zooplankton groups would affect water column microbial processes differently.     

Microsetella norvegica: a rare report of a potentially abundant copepod on the Scotian Shelf

Fine mesh (0.080 mm) zooplankton samples were collected along an onshore-offshore transect during three cruises (February to May, 1983) off southwest Nova Scotia, Canada. The small harpacticoid copepod Microsetella norvegica was numerically the dominant copepod during the winter (February) cruise, where it composed up to 50% of the total zooplankton, and was consistently among the three most abundant copepods during the spring cruises. Due to its small size (<0.45 mm in length), M. norvegica has rarely been reported from plankton surveys of the northwest Atlantic. However, existing reports indicated it varies interannually from being a rare to an abundant member of the zooplankton community in the northwest Atlantic.     

The life cycles of two coexisting copepods, <Emphasis Type="Italic">Calanus chilensis</Emphasis> and <Emphasis Type="Italic">Centropages brachiatu</Emphasis>s,in the upwelling zone off northern Chile (23°S)

A time series study of the copepods Calanus chilensis and Centropages brachiatus was carried out at the coastal upwelling zone of Mejillones (23°S, northern Chile), to analyze their annual life cycles in association with upwelling variation. These species co-exist in the upwelling zone. Weekly sampling of zooplankton and oceanographic variables including Chlorophyll-a and phytoplankton composition were obtained during January–December 2002 at a fixed station (ca. 90 m depth). Stages of abundances, their proportions, changes in body length of adult females, sex ratio and egg production rate (EPR), were used as proxies to examine copepods’ demography. Upwelling, assessed by weekly Ekman transport and oceanographic conditions, was intermittent throughout the year with lack of periodicity components. Populations of both copepod species did not correlate with these non-predictable upwelling events. C. chilensis reproduced year-round and the population showed ca. 15 peaks of adults with an average time interval between peaks of 20 days. C. brachiatus showed a similar life cycle, also having 15 peaks of adults at about 22 days of time intervals. Cross-correlation functions and spectral analysis showed that both populations correlated positively through time, but not in phase, evidencing a time lag for their reproductive cycles. The lag was also evident in their population abundances. Both species differ in their development rates and this may result in non-in phase life cycles. Our findings suggest that species-dependent attributes, such as development rates, modulated by adaptations to temperature, might impose constraints in the species life cycles determining the population cycles. Such attributes must be considered when modeling and understanding population dynamics and secondary production of copepods.     

A simple plankton model for the Oregon upwelling ecosystem: Sensitivity and validation against time-series ocean data

Simple plankton models serve as useful platforms for testing our understanding of the mechanisms underlying ecosystem dynamics. A simple, one-dimensional plankton model was developed to describe the dynamics of nitrate, ammonium, two phytoplankton size-classes, meso-zooplankton, and detritus in the Oregon upwelling ecosystem. Computational simplicity was maintained by linking the biological model to a one-dimensional, cross-shelf physical model driven by the daily coastal upwelling index. The model sacrificed resolution of regional-scale and along-shore (north to south) processes and assumed that seasonal productivity is primarily driven by local cross-shelf Ekman transport of surface waters and upwelling of nutrient-rich water from depth.Our goals were to see how well a simple plankton model could capture the general temporal and spatial dynamics of the system, test system sensitivity to alternate parameter set values, and observe system response to the effective scale of potential retention mechanisms. Model performance across the central Oregon shelf was evaluated against two years (2000-2001) of chlorophyll and copepod time-series observations. While the modeled meso-zooplankton biomass was close in scale to the observed copepod biomass, phytoplankton was overestimated relative to that inferred from the observed surface chlorophyll concentration. Inshore, the system was most sensitive to the nutrient uptake kinetics of diatom-size phytoplankton and to the functional grazing response of meso-zooplankton. Meso-zooplankton was more sensitive to alternate parameter values than was phytoplankton. Reduction of meso-zooplankton cross-shelf advection rates (crudely representing behavioral retention mechanisms) reduced the scale of model error relative to the observed seasonal mean inshore copepod biomass but had little effect of the modeled meso-zooplankton biomass offshore nor upon phytoplankton biomass across the entire shelf.     

Nodularin concentrations in Baltic Sea zooplankton and fish during a cyanobacterial bloom

Toxic cyanobacterial blooms, dominated by Nodularia spumigena, are a recurrent phenomenon in the Baltic Sea during late summer. Nodularin, a potent hepatotoxin, has been previously observed to accumulate on different trophic levels, in zooplankton, mysid shrimps, fish as well as benthic organisms, even in waterfowl. While the largest concentrations of nodularin have been measured from the benthic organisms and the food web originating from them, the concentrations in the pelagic organisms are not negligible. The observations on concentrations in zooplankton and planktivorous fish are sporadic, however. A field study in the Gulf of Finland, northern Baltic Sea, was conducted during cyanobacterial bloom season where zooplankton (copepod Eurytemora affinis, cladoceran Pleopsis polyphemoides) and fish (herring, sprat, three-spined stickleback) samples for toxin analyses were collected from the same sampling areas, concurrently with phytoplankton community samples. N. spumigena was most abundant in the eastern Gulf of Finland. In this same sampling area, cladoceran P. polyphemoides contained more nodularin than in the other areas, suggesting that this species has a low capacity to avoid cyanobacterial exposure when the abundance of cyanobacterial filaments is high. In copepod E. affinis nodularin concentrations were high in all of the sampling areas, irrespective of the N. spumigena cell numbers. Furthermore, nodularin concentrations in herring samples were highest in the eastern Gulf of Finland. Three-spined stickleback contained the highest concentrations of nodularin of all the three fish species included in this study, probably because it prefers upper water layers where also the risk of nodularin accumulation in zooplankton is the highest. No linear relationship was found between N. spumigena abundance and nodularin concentration in zooplankton and fish, but in the eastern area where the most dense surface-floating bloom was observed, the nodularin concentrations in zooplankton were high. The maximum concentrations in zooplankton and fish samples in this study were higher than measured before, suggesting that the temporal variation of nodularin concentrations in pelagic communities can be large, and vary from negligible to potentially harmful.     

Zooplankton community structure across an eddy-generated upwelling band close to a tropical bay-mangrove ecosystem

Mesoscale eddies in the world’s oceans are ubiquitous and bring about episodic pulses of nutrients into the photic zone. Transient in nature, the role of eddy pumping in coastal enrichment via plankton production, and subsequent organic flux is not yet fully realised. In the context of a cyclonic cold-core eddy that propagates annually under the influence of the East India Coastal Current and enriches coastal waters in the western Bay of Bengal north of 16°N, a detailed study on zooplankton community structure along with phytoplankton composition and associated water quality was undertaken during April–May 2002 coinciding with the spring intermonsoon. Zooplankton samples were collected at 32 hydrographically different (salinity 24.5–35.6 PSU) GPS fixed locations representing bay-mangrove areas and nearshore waters (30 m) close to the River Godavari, which is one of India’s largest estuarine systems. During the study, the bay-coastal waters were typified by elevated nutrient levels (nitrate 10.73–22.04 μM), high salinity (27.98–35.52 PSU), and relatively low temperatures (30.63–31.40°C). Altogether, 95 zooplankton taxa were encountered with copepods forming the predominant population. Agglomerative Hierarchical Cluster Analysis (AHCA) and Non-metric Multidimensional Scaling (NMDS) based on Bray–Curtis similarity (PRIMER) analysis revealed appreciable alterations in zooplankton structure across bay-mangrove locations and coastal waters (Stress 0.11; ANOSIM test Global R: 0.94, P = 0.1%). Similarity Percentage (SIMPER) analysis revealed zooplankton associations through “discriminating species” for each location (Kakinada Bay, Cluster I, 27.9 ± 3.0 PSU; upwelling band, cluster II, 35.5 ± 0.2 PSU; offshore waters, cluster III, 34.2 ± 0.4 PSU; mangrove outlets, cluster IV, 32.7 ± 1.3 PSU and mangrove creeks, cluster V, 33.5 ± 0.6 PSU). The index of multivariate dispersion (IMD) illustrated high variability in zooplankton standing stock for mangrove/sea locations relative to the bay. Concurrent observations on phytoplankton revealed the importance of diatoms (r: 0.640, P ≤ 0.05). Within the eddy-generated band of upwelled water, a significant top-down control of diatoms by herbivorous zooplankton resulted in a comparative increase in abundance of dinophyceans. Based on zooplankton abundance data and species association patterns, it was possible to distinguish different zooplankton/copepod communities in accordance with mesoscale variability in physical, chemical and biological processes under tropical conditions. This was confirmed through canonical correspondence analysis (CCA) that represented coastal-offshore waters and the Bay environment in this area.     

Weight and chemical composition of some important oceanic zooplankton in the North Pacific Ocean

Wet and dry weight, total carbon, nitrogen, hydrogen, and ash contents were determined on 33 species of zooplankton distributed predominantly in the open sea region of the North Pacific. Sampling covered the waters from 44°N to the equator. Average percentage of dry weight to wet weight was about 19% of all samples from the whole area. Percentage dry weight of carbon in copepods was on an average 51.5%. The highest value, 66.6%, was obtained in eggs of the copepod Pareuchaeta sarsi. Mixed zooplankton was assumed to contain carbon comprising about 35 to 45% of the dry weight. Carbon contained in the zooplankton biomass existing in the upper 200 m in the western parts of the northern North Pacific and Bering Sea during spring and summer was estimated to range from 20 to 85 mg C/m³. Nitrogen content varied considerably with localities. Average ratio of carbon to nitrogen was 8.5 in subarctic copepods, and 4.1 in subtropic-tropic copepods. This ratio also varied with season. In the copepod Calanus cristatus the ratio was highest (10.0) in May, immediately after the spring bloom of phytoplankton, when the animals contained much fat. The ratio fell to 5.1 in December. There seemed to be a large seasonal variation in boreal zooplankton due to great fluctuations of environmental conditions, especially the amount of food available; in tropical species the range was small because of environmental uniformity. Average hydrogen content was about 6 to 10%. The percentage of ash to dry weight amounted to 39.3% in pteropods and 3.4% in copepods.     

Quantification of copepod gut content by differential length amplification quantitative PCR (dla-qPCR)

Quantification of feeding rates and selectivity of zooplankton is vital for understanding the mechanisms structuring marine ecosystems. However, methodological limitations have made many of these studies difficult. Recently, molecular based methods have demonstrated that DNA from prey species can be used to identify zooplankton gut contents, and further, quantitative gut content estimates by quantitative PCR (qPCR) assays targeted to the 18S rRNA gene have been used to estimate feeding rates in appendicularians and copepods. However, while standard single primer based qPCR assays were quantitative for the filter feeding appendicularian Oikopleura dioica, feeding rates were consistently underestimated in the copepod Calanus finmarchicus. In this study, we test the hypothesis that prey DNA is rapidly digested after ingestion by copepods and describe a qPCR-based assay, differential length amplification qPCR (dla-qPCR), to account for DNA digestion. The assay utilizes multiple primer sets that amplify different sized fragments of the prey 18S rRNA gene and, based on the differential amplification of these fragments, the degree of digestion is estimated and corrected for. Application of this approach to C. finmarchicus fed Rhodomonas marina significantly improved quantitative feeding estimates compared to standard qPCR. The development of dla-qPCR represents a significant advancement towards a quantitative method for assessing in situ copepod feeding rates without involving cultivation-based manipulation.     

Carbon substrate usage by zooplankton-associated bacteria,phytoplankton-associated bacteria,and free-living bacteria under aerobic and anaerobic conditions

Mesozooplankton provide oxic and anoxic microhabitats for associated bacteria, whose carbon substrate usage activities complement those of the ambient bacteria. The metabolic profiles of bacterial communities associated with the calanoid copepod Acartia tonsa under aerobic and anaerobic conditions were examined in comparison with phytoplankton-associated bacteria. Carbon substrate usage by phytoplankton-associated bacteria was significantly different than that of copepod-associated bacteria in both aerobic and anaerobic conditions. Substrate utilization by copepod-associated bacteria was more dependent upon oxygen condition than whether the bacteria were located on the copepod exoskeleton or within the gut. Results suggest that gut bacteria were responsible for a large portion of anaerobic substrate usage by copepod-associated bacteria. The metabolic profiles of bacteria associated with six common zooplankton groups and free-living bacteria collected in July 2012 from the York River estuary, Virginia, (37°14′50.36″N, 76°29′58.03W) were also compared, and there were significant differences in their substrate utilization patterns between aerobic and anaerobic incubations, and among the different zooplankton groups. Through trophic interactions, phytoplankton-associated or free-living bacteria may be introduced to the anoxic zooplankton gut and its associated bacterial community. Inclusion of these anaerobic microenvironments and their microbial inhabitants increased the total number of substrates used by 57 % over what was used by aerobic phytoplankton-associated bacteria alone, and by 50 % over what was used by aerobic free-living bacteria in the York River. Therefore, the presence of zooplankton-associated microhabitats and their bacteria expanded the functionality of aquatic microbial communities and led to a more comprehensive substrate usage.     

椒江口春、秋季浮游动物分布特征及与主要环境因子的关系

2009年5月和10月对椒江口(121.35°E～121.85°E,28.50°N～28.80°N)浮游动物进行调查,分析其群落结构、生物量和丰度的时空分布特征及与主要环境因子的关系.结果表明,该海域浮游动物有明显的季节变化,春季鉴定到14大类50种,卡玛拉水母(Malagazzia carolinae)为绝对优势种,秋季鉴定到14大类73种,优势种分别为百陶箭虫(Sagitta bedoti)、双生水母(Diphyes chamissonis)、亚强真哲水蚤(Eucalanus subcrassus)、微刺哲水蚤(Canthocalanus pauper)、中华胸刺水蚤(Centropages sinensis)和肥胖箭虫(Sagitta enflata);多样性指数为秋季(2.59)高于春季(1.82),生物量和丰度为春季(972.66 mg/m3和1 743.54 ind/m3)远高于秋季(65.30 mg/m3和31.94 ind/m3).总生物量和丰度的空间分布由优势种决定,春季高值区出现在咸淡水交汇的出海口处;秋季有沿河口向外递增的趋势.典范对应分析(CCA)表明,营养盐、盐度和溶解氧为影响春秋季椒江口浮游动物分布的环境因子;浮游动物群落存在明显的季节和空间异质性;各物种适宜的生态环境不同.与类似河口的现状相比,椒江口的浮游动物种类丰富,可能与影响该河口的水团多样有关;与历史资料相比,椒江口4、10月份浮游动物的生物量、丰度及优势类群保持相对稳定.图9表6参44     

Zooplankton feeding ecology: contents of fecal pellets of the copepods Temora turbinata and T. stylifera from continental shelf and slope waters near the mouth of the Mississippi River

In-situ feeding habits of the copepods Temora turbinata and T. stylifera were investigated by scanning electron microscope examination of fecal pellets, the contents of which reflected copepod gut contents upon capture. Pellet contents were compared with assemblages of available phytoplankton in the water column at the times of zooplankton sampling. Samples were collected in continental shelf and slope waters of the Gulf of Mexico near the mouth of the Mississippi River. Both species ingested a wide size range and taxonomic array of phytoplankters, and to a lesser extent, other crustaceans. Fecal pellets contained primarily the remains of the phytoplankters that were most abundant in the water at times of collection. There was considerable overlap in the food items ingested by adult females of both copepod species, or two stages of T. turbinata copepodites. Thus, T. turbinata and T. stylifera are omnivores, but primarily opportunistic herbivores.     

Pelagic-benthic coupling in a subtidal system of the North-Western Mediterranean

Suspended particulate matter, zooplankton, and macrobenthos dynamics were investigated in a shallow area of the Ligurian Sea (north-west Mediterranean) characterized by wide temporal variability over an annual cycle. As indicated by multivariate analyses, the seasonal dynamics can be summarized as follows: (1) a late winter-early spring phytoplankton bloom followed by high zooplankton and macrobenthos densities during the spring months; (2) low-quality particulate suspended matter in summer, and an increase in the importance of zooplankton taxa with a wide range of feeding strategies, a decrease in macrofaunal abundance, and an increase in deposit-feeders and predators; and (3) a second phytoplankton bloom in autumn, followed by an increase in copepod density and a low macrofaunal abundance. In conclusion, pelagic and benthic communities in the coastal area of the Ligurian Sea mainly seem to be controlled bottom-up. Our results suggest that the quality of the particulate organic matter may play an important role in determining the temporal changes of both plankton and benthic assemblages, while the direct influence of other environmental features (such as sediment grain size) is relevant only for some macrobenthic taxa (e.g. crustaceans).     

Stable C and N isotope analysis elucidated the importance of zooplankton in a tropical seagrass bed of Santiago Island,Northwestern Philippines

The role of zooplankton in a tropical seagrass ecosystem was investigated in milkfish farms pollution-impacted and -unimpacted seagrass beds in Santiago Island coral reefs, Northwestern Philippines. The aim was to compare between the two sites: (1) abiotic factors and zooplankton community parameters, and (2) the trophic structure using C and N stable isotopes. Low water (98–119?mV) and sediment (–121 to ?138?mV) Oxidation Reduction Potential values indicated a reducing environment in the impacted site. Zooplankton in the impacted site showed the typical community response to eutrophication (low diversity, but high total abundance due to the dominance of the cyclopoid copepod Oithona oculata), generally few elevated δ¹⁵N values, but a significant shift towards depleted ¹³C due to the organic enrichment of fish-farm feeds. Apart from suggesting a highly complex food web with POM and zooplankton as main food sources in the unimpacted site, the Bayesian mixing model simulation generated reduced complexity in feeding interactions between basal sources, zooplankton, and fish including adults of a key fish species, Siganus fuscescens, in the impacted sites. In this study, C and N stable isotope analysis has clarified the importance of zooplankton as fish prey in a seagrass bed food web.     

Ecological aspects of plankton production

The study was carried out in the neritic and estuarine waters of Porto Novo, Coromandel Coast, Bay of Bengal, India during the period January, 1960 to December, 1967. The average displacement volume of plankton usually varied between 2 and 4 c²/m². During summer, with a season of high plankton productivity, the average plankton displacement volume rose to 8 c²/m³. Generally speaking, the average zooplankton density (standing crop) was usually between 80,000 and 100,000 organisms/m³, of which copepods alone comprised usually between 70,000 and 90,000 organisms/m³. The average copepod density per sample varied from 30,000 to 50,000 organisms/m³. However, in the summer months, the copepod density was usually not less than 100,000 organisms/m³; in some years this was even higher (from 125,000 to 170,000 organisms/m³). Copepods comprised between 80 and 95% of the zooplankton population. The maximum non-copepod population in the zooplankton seldom reached 30%, was ofter below 25%, and usually less than 20%. During the period March to October (in some years as early as February, and in some years up to November), either an increasing or a steady trend of plankton production was evident. It would appear that salinity and rainfall determine the occurrence and distribution of plankton in Porto Novo.Contribution No. 189 from the Centre of Advanced Study in Marine Biology, Marine Biological Station of Annamalai University, Porto Novo, Tamilnadu, India.     

Composite forces shape population dynamics of copepod crustaceans

Understanding the processes that control species abundance and distribution is a major challenge in ecology, yet for a large number of potentially important organisms, we know little about the biotic and abiotic factors that influence population size. One group of aquatic organisms that defies traditional demographic analyses is the Crustacea, particularly those with complex life cycles. We used likelihood techniques and information theoretics to evaluate a suite of models representing alternative hypotheses on factors controlling the abundance of two copepod crustaceans in a small, tropical floodplain lake. Quantitative zooplankton samples were collected at three stations in a Venezuelan floodplain lake from June through December 1984; the average sampling interval was two days. We constructed a series of models with stage structure that incorporated six biotic and abiotic covariates in various combinations to account for temporal changes in abundance of these target species and in their population growth rates. Our analysis produced several novel insights into copepod population dynamics. We found that multiple forces affected the abundance of particular stages, that these factors differed between species as well as among stages within each species, and that biotic processes had the largest effects on copepod population dynamics. Density dependence had a large effect on the survival of Oithona amazonica copepodites and on population growth rate of Diaptomus negrensis.     

Food selection by copepods: discrimination on the basis of food quality

The copepod Acartia tonsa displayed nearly two-fold higher ingestion rates on faster-growing cells of the diatom Thalassiosira weissflogii compared to ingestion rates on slower-growing cells of that species at the same cell concentration. Ingestion rates on slow-growing cells were also enhanced by the addition of cell-free aliquots of algal exudate to the experimental feeding chambers. In addition, the faster-growing algal cells were selectively ingested by the copepod when the two cell types were mixed together in different proportions, indicating that physiological differences between growing cells are a critical factor in the food detection/selection process of zooplankton. Consideration of cell carbon, nitrogen, and protein composition suggests that the copepods are maximizing nitrogenous ingestion (total protein and/or nitrogen). Selectivity for cells with higher protein content results in a higher daily protein ration, even if the selection process results in a decreased rate of ingestion in mixtures of cell types.     

Modèle de détection rapide des effets sublétaux des polluants. II. Un exemple in situ: anomalie métabolique du copépode hyponeustonique Anomalocera patersoni en coïncidence avec une marée noire

An unusual correlation has been observed between the amylase and trypsin content of a natural zooplankton population and of the hyponeustonic copepod Anomalocera patersoni and the oil spill of the Amoco Cadiz in 1978. This anomaly has also been observed in Artemia sp. cultures (San Francisco Bay strain) poisoned by copper. This phenomenon could originate from a general physiological perturbation. Such a non-specific perturbation could explain the anormal correlation between parameters which are usually independent and specifically regulated. The determination of such anomalous regulation could lead to detection of the sublethal effect of pollutants in the field. The method requires no prior knowledge of the area and can be applied for detection of a new perturbation; in fact, if organisms adapt to the new conditions, the anomaly disappears. The method has enabled physiological perturbations of zooplankton to be recorded 1 yr after the Amoco Cadiz oil spill. The hyponeustonic copepod Anomalocera patersoni was particularly perturbed, even at residual hydrocarbon concentrations usually considered by chemists as approaching normal values.     

ATCase activity, RNA:DNA ratio, gonad development stage, and egg production in the female copepod Calanus helgolandicus

Some investigators have proposed the measurement of aspartate transcarbamylase (ATCase) activity as a suitable technique for estimating zooplankton productivity. However, this technique has never been comprehensively evaluated under controlled laboratory conditions nor compared extensively with other productivity techniques in the field. This paper describes such an evaluation, using the reproduction of the calanoid copepod Calanus helgolandicus as an index of germinal growth, an important aspect of zooplankton productivity. ATCase activity and RNA:DNA ratio both increased significantly with increasing gonad maturation. In addition, ATCase activity in females responded immediately to changing food conditions, whilst changes in egg production lagged behind by ≃24 h. Shifting the ATCase data by 24 h revealed a significant and positive correlation of ATCase activity with egg production. However, in well-fed females, ATCase activity assayed at constant temperature was apparently independent of environmental temperature conditions, whilst egg production was temperature-dependent. In the field, ATCase activity was significantly correlated with egg production only in autumn and winter; when a complete seasonal cycle was considered, no correlation was found between the two measurements. These findings suggest that ATCase is involved partially in either germinal growth and/or in other biosynthetic processes of female C. helgolandicus; thus, ATCase activity does not reflect copepod egg-production with any certainty. Received: 4 June 1998 / 3 June 1999     

Persistent “upwelling shadows” and their influence on zooplankton distributions

Physical (temperature and salinity) and biological (zooplankton) properties were sampled over a 15 mo period in 1988 and 1989 to monitor nearshore environmental variability in northern Monterey Bay, California, USA. During the upwelling seasons of 1988 and 1989, there were two distinct bodies of water along the sampling transect. The offshore water body was characterized by recently upwelled water (typically <12°C) while the nearshore body was significantly warmer (approaching 16°C). A sharp thermal gradient (=front) separated the two water bodies. This front persisted 6.5 km from shore throughout the upwelling season of 1988 and was present again in 1989. Zooplankton-assemblage analysis confirmed the presence of two distinct regions in northern Monterey Bay. We refer to this phenomenon as an upwelling shadow, which is the result of the advection of recently upwelled water bearing nutrients and larvae past coastal recesses which are equatorward of upwelling centers.