Variation In Zooplankton Biomass Research Articles

Seasonal and spatial dynamics of mesozooplankton community in a subtropical embayment

Mesozooplankton abundance and community composition are influenced by hydrography and biological interactions. In subtropical systems influenced by seasonal monsoons, seasonal cycle of plankton communities could be disrupted and restructured by short but intense weather events. Our survey of a shallow subtropical embayment to the northern part of the South China Sea demonstrated temporal variations in zooplankton biomass and composition between monsoon periods: not only were the wet (summer monsoon) and dry (winter monsoon) periods distinctive from each other, but also the transition phase between the two. The meroplankton communities were distinct between the monsoon periods with fish, decapods and echinoderm peaked during the wet season and cirripedia bloomed in dry season. This pattern can be attributed to both changes in growth conditions and influx of individuals through monsoon-driven water currents. The community shift during the transitional phase, defined by the rapid changes in salinity and temperature, corresponded to the reported reproductive cycles of marine organisms, including commercially important fishes, crustaceans, and echinoderms. The interannual variability observed during the wet period reflected changes in rainfall between consecutive years, highlighting the importance of having a long time series with which to establish a baseline information to better manage nursery habitats like the one surveyed, particularly in light of increasing coastal development. Subtropical coastal zooplankton communities are highly dynamic with multiple pulses and interruptions driven by weather events.

Long‐Term Variation in Mesozooplankton Biomass Caused by Top‐Down Effects: A Case Study in the Coastal Sea of Japan

AbstractMesozooplankton is a key group for the recruitment of fish and global biogeochemical cycles. Four decadal observations in the coastal Sea of Japan, the marginal sea of the North Pacific, indicate that wet weight‐based mesozooplankton biomass is controlled by both environment‐induced bottom‐up and predatory‐induced top‐down processes. Interannual variations in mesozooplankton biomass using a generalized linear model approach showed a decrease in biomass during the 1980s, followed by a rapid increase in the early 1990s, and a gradual decrease in the 2010s. These interannual variations were the mirror image of those seen small pelagic planktivorous fish biomass. The difference in zooplankton biomass from the previous year was positively correlated with that of the Pacific decadal oscillation index and negatively with that of fish biomass. The latter was supported by a Granger causality analysis. Therefore, top‐down control is one of the main causes of long‐term variations of zooplankton biomass in the ocean.

Widespread agrochemicals differentially affect zooplankton biomass and community structure.

Anthropogenic environmental change is causing habitat deterioration at unprecedented rates in freshwater ecosystems. Despite increasing more rapidly than many other agents of global change, synthetic chemical pollution-including agrochemicals such as pesticides-has received relatively little attention in freshwater community and ecosystem ecology. Determining the combined effects of multiple agrochemicals on complex biological systems remains a major challenge, requiring a cross-field integration of ecology and ecotoxicology. Using a large-scale array of experimental ponds, we investigated the response of zooplankton community properties (biomass, composition, and diversity metrics) to the individual and joint presence of three globally widespread agrochemicals: the herbicide glyphosate, the neonicotinoid insecticide imidacloprid, and nutrient fertilizers. We tracked temporal variation in zooplankton biomass and community structure along single and combined pesticide gradients (each spanning eight levels), under low (mesotrophic) and high (eutrophic) nutrient-enriched conditions, and quantified (1) response threshold concentrations, (2) agrochemical interactions, and (3) community resistance and recovery. We found that the biomass of major zooplankton groups differed in their sensitivity to pesticides: ≥0.3mg/L glyphosate elicited long-lasting declines in rotifer communities, both pesticides impaired copepods (≥3µg/L imidacloprid and ≥5.5mg/L glyphosate), whereas some cladocerans were highly tolerant to pesticide contamination. Strong interactive effects of pesticides were only recorded in ponds treated with the combination of the highest doses. Overall, glyphosate was the most influential driver of aggregate community properties of zooplankton, with biomass and community structure responding rapidly but recovering unequally over time. Total community biomass showed little resistance when first exposed to glyphosate, but rapidly recovered and even increased with glyphosate concentration over time; in contrast, taxon richness decreased in more contaminated ponds but failed to recover. Our results indicate that the biomass of tolerant taxa compensated for the loss of sensitive species after the first exposure, conferring greater community resistance upon a subsequent contamination event; a case of pollution-induced community tolerance in freshwater animals. These findings suggest that zooplankton biomass may be more resilient to agrochemical pollution than community structure; yetall community properties measured in this study were affected at glyphosate concentrations below common water quality guidelines in North America.

The ecology of the whale shark in Djibouti

Within the Gulf of Aden, Djibouti is an important site where whale sharks (Rhincodon typus Smith 1828) aggregate seasonally; however, up to now, only few studies have been carried out in the area (Rowat et al. in Environ Biol Fishes, 2007. https://doi.org/10.1007/s10641-006-9148-7; Mar Freshw Res, 2011. https://doi.org/10.1071/MF10135; Rezzolla and Storai in Cybium 34:195–206, 2010). From 2015 to 2018, whale shark photo-id, skin biopsies, and zooplankton samples were collected to investigate the ecology of this species in Djibouti. Of the 190 photo-identified individuals, the majority were juvenile males, confirming a significant male-based aggregation (X2 = 65, df = 1, p < 0.0001). Within each season, the maximum number of times an individual was re-sighted was 11, while the inter-annual re-sighting rates varied between 11 and 32%. Animals were feeding on surface zooplankton, consisting primarily of copepods (85%); however, Chaetognatha and Ctenophora appeared to be important taxa as well, since the areas most visited were also those with higher concentration of these organisms. Feeding zooplankton patches had a mean biomass of 42.2 ± 31.9 mg m−3, almost 1.5 higher than control stations, indicating that the abundance of this species was positively related to food availability. Furthermore, whale sharks exhibited a clear temporal distribution associated with the variation in zooplankton biomass due to the monsoon winds. Sharks aggregated in October–February, with a peak in sightings in November, and dispersed outside the upwelling season. δ13C values suggested that whale sharks might migrate to other habitats or shift their activities to deeper areas outside the aggregation phase. Little intra-specific variation in δ15N and δ13C was found between animals, suggesting a similar habitat use.

River discharge as a major driving force on spatial and temporal variations in zooplankton biomass and community structure in the Godavari estuary India.

Variability in horizontal zooplankton biomass distribution was investigated over 13months in the Godavari estuary, along with physical (river discharge, temperature, salinity), chemical (nutrients, particulate organic matter), biological (phytoplankton biomass), and geological (suspended matter) properties to examine the influencing factors on their spatial and temporal variabilities. The entire estuary was filled with freshwater during peak discharge period and salinity near zero, increased to ~34psu during dry period with relatively high nutrient levels during former than the latter period. Due to low flushing time (<1day) and high suspended load (>500mgL-1) during peak discharge period, picoplankton (cyanophyceae) contributed significantly to the phytoplankton biomass (Chl-a) whereas microplankton and nanoplankton (bacillariophyceae, and chlorophyceae) during moderate and mostly microplankton during dry period. Zooplankton biomass was the lowest during peak discharge period and increased during moderate followed by dry period. The zooplankton abundance was controlled by dead organic matter during peak discharge period, while both phytoplankton biomass and dead organic matter during moderate discharge and mostly phytoplankton biomass during dry period. This study suggests that significant modification of physico-chemical properties by river discharge led to changes in phytoplankton composition and dead organic matter concentrations that alters biomass, abundance, and composition of zooplankton in the Godavari estuary.

Mesozooplankton biomass and copepod estimated production in a temperate estuary (Mondego estuary): effects of processes operating at different timescales.

A fundamental step in characterizing marine planktonic communities is the knowledge of the variability of their biomass and production, in order to assess the flux of matter through the food web and estimate the fate of this energy. The zooplankton assemblage's dynamics of the Mondego estuary (Portugal) have been extensively characterized, but none have focused on biomass and secondary production. In the present study, zooplankton biomass and secondary production were investigated with a seasonal basis in a fixed station, located at the mouth of the Mondego estuary (Portugal), with emphasis on the copepod community. In order to study the influence of physical processes operating at different temporal scales, sub-surface and near-bottom samples were collected every 2 h in diel and tidal cycles, over neap tides. Copepodsdominated the biomass during spring (0.585 ± 0.671 mg C m-3),summer (0.287 ± 0.383 mg C m-3), andwinter (0.221 ± 0.128 mg C m-3).Lower observed biomass values were probably due to the higher mesh size used in comparison to previous studies in other estuarine systems. No differences were detected between seasons in total zooplankton biomass. Concerning diel cycle, significant variation occurred mainly in spring and winter, with substantially higher biomass values being registered at night for both the surface and bottom. Moreover, copepod biomass was consistently higher at the bottom during winter. In this season, bottom samples presented higher copepod biomass both at ebb and flood tides. From the GLM analyses run for each season, chlorophyll a, diel phase, and depth were significant factors. Theconsiderable complexity in the processes structuring the dynamics and composition of biomass and production rates has been shown. Our results demonstrated that along spring and winter, secondary production varied according to diel phase and depth. In winter, night and bottom samples revealed consistently higher daily secondary production rates. From this, it is possible to conclude that local factors (river flow, predation, food limitation) regulate the seasonal variations in zooplankton biomass and production in the Mondego estuary.

The variations of zooplankton biomass and their migration associated with the Yellow Sea Warm Current

Multidisciplinary oceanographic investigations were carried out in the southern Yellow Sea (YS) from December 2006 to February 2007. The ship-based investigations provided spatial distributions of temperature, salinity, and zooplankton. The horizontal distributions of temperature and salinity indicate the location of the Yellow Sea Warm Current (YSWC), and the net-sampling results display low biomass of zooplankton in the YSWC area. The mooring observations show the temporal variability of the current, bottom temperature, and volume backscattering strength (Sv) at the mooring sites. The current and bottom temperature records illustrate the process of the arrival of the warm water advected by the YSWC. The time series of Sv measured with acoustic Doppler current profilers (ADCPs) indicate a distinct change in the diel vertical migration (DVM) pattern of the zooplankton. This change is associated with the arrival of the warm water. Our investigation suggests that the transport of low-biomass water by the YSWC causes the zooplankton biomass in the study area to decrease. The YSWC also causes the DVM to change due to enhanced vertical mixing.

Late-summer zooplankton community structure, abundance, and distribution in the Hudson Bay system (Canada) and their relationships with environmental conditions, 2003–2006

Zooplankton communities were examined for the first time in three different hydrographic regions of the Hudson Bay system (HBS) in early August to early September from 2003 to 2006. Sampling was conducted at 50 stations distributed along different transects located in Hudson Bay (HB), Hudson Strait (HS), and Foxe Basin (FB). Variations in zooplankton biomass, abundance, taxonomic composition, and diversity in relation to environmental variables were studied using multivariate techniques. During all sampling years, the total zooplankton biomass was on average four times lower in HB than in HS and FB. Clustering samples by their relative species compositions revealed no interannual variation in zooplankton community but showed a marked interregional variability between the three regions. Water column stratification explained the greatest proportion (25%) of this spatial variability. According to redundancy analysis (RDA), the zooplankton taxa that contribute most to the separation of the three regions are Microcalanus spp., Oithona similis, Oncaea borealis, Aeginopsis laurentii, Sagitta elegans, Fritillaria sp., and larvae of cnidaria, chaetognatha, and pteropoda in HB; hyperiid amphipods in FB; and Pseudocalanus spp. CI–CV, Calanus glacialis CI–CVI, Calanus finmarchicus CI–CVI, Calanus hyperboreus CV–CVI, Acartia longiremis CI–CV, Metridia longa N3-N6 CI-CIII CVIf, Eukrohnia hamata, larvae of echinodermata, mollusca, cirripedia, appendicularia, and polychaeta in the northwestern and southeastern HS transects. For the HB transect, the RDA analyzed allowed us to distinguish three regions (HB west, central, and east) with different environmental gradients and zooplankton assemblages, in particular higher concentration of Pseudocalanus spp. nauplii and CI–CVI, as well as benthic macrozooplankton and meroplankton larvae in western HB. In HS, Calanoid species (mainly C. finmarchicus and C. glacialis) were mostly observed at the north shore stations associated with the weakly stratified Arctic–North Atlantic waters coming from southwestern Davis Strait (inflow). In general, the RDA models tested among the HBS regions were very consistent with its general surface circulation pattern for summer conditions in terms of environmental variables and distinct zooplankton assemblages. Overall, zooplankton biomass and diversity indices (H′, J′, and S) were lower in the most stratified environment (i.e., HB) than in the deeper (FB) and more dynamic (HS) regions. The results of this work clearly show that the spatial differentiation and structure of the zooplankton communities are strongly influenced by the hydrodynamic conditions in the HBS that, trough their actions on temperature, salinity, stratification, mixing conditions and depth strata, lead to the spatial differentiation of these communities.

Seasonal and spatial variation of mesozooplankton biomass in Ushuaia and Golondrina bays (Beagle Channel, Argentina))

Fil: Biancalana, Florencia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Bahia Blanca. Instituto Argentino de Oceanografia. Universidad Nacional del Sur. Instituto Argentino de Oceanografia; Argentina

The utility of body size indices derived from optical plankton counter data for the characterization of marine zooplankton assemblages

To evaluate the utility of size-based indices derived from an Optical Plankton Counter (OPC) through detection of spatial and temporal trends in zooplankton biomass, zooplankton size data were collected with an OPC across the Scotian Shelf region of the northwest Atlantic Ocean in April and October of 1997, 1998, and 1999. Eight size-based indices were computed – three simple size metrics (arithmetic mean, geometric mean, coefficient of variation) and metrics derived from the Normalized Biomass Size Spectrum (NBSS; X- and Y-coordinates and curvature of a fitted quadratic function) and the Pareto distribution (Y-intercept and slope). Results indicate that the simple size indices and those derived from the Pareto distribution consistently accounted for the greatest portion of annual variation in zooplankton biomass whereas indices derived from the NBSS accounted only for some secondary patterns. Simple indices also accounted for the greatest portion of spatial variance in zooplankton biomass whereas the NBSS and Pareto accounted for secondary patterns. Patterns in zooplankton communities based on these indices reflected broad taxonomic trends and were related to independent observations on atmospheric and hydrographic conditions in the study area. Size-based zooplankton data from continuous survey instruments can provide powerful adjuncts to both freshwater and marine aquatic monitoring.

Climatic forcing of zooplankton dynamics is stronger during low densities of planktivorous fish

We analyze interannual variation in zooplankton biomass in the southwestern Barents Sea in spring (May) and summer (June‐July) 1959–1993. Using a threshold modeling approach, we quantify spatial, climatic, and across‐season autoregressive effects under contrasting regimes of low and high densities of planktivorous fish (capelin and herring). Main findings: (1) zooplankton biomass is mainly influenced by capelin feeding, first in the offshore parts of the western Barents Sea in spring and subsequently farther east and north in summer, whereas direct effects of herring are quantitatively less important; (2) effects of climate are stronger during the zooplankton increase phase in spring than in summer and are better predicted using the North Atlantic Oscillation index than using reconstructed sea surface temperature and Atlantic water influx; (3) regional anomalies in zooplankton biomass persist from spring to summer but not from summer to the subsequent spring, indicating that the observed fluctuations have little effect on next year's dynamics; and (4) effects of climatic variation and planktivorous fish interact: climatic (and autoregressive) effects are mainly evident when and where the effect of feeding is weak. Zooplankton dynamics thus appear to be under shifting top‐down and climatic control, the relative importance of the two processes varying spatially, seasonally, and interannually.

Impact of climate change on long-term zooplankton biomass in the upwelling region of the Gulf of Guinea

Abstract Wiafe, G., Yaqub, H. B., Mensah, M. A., and Frid, C. L. J. 2008. Impact of climate change on long-term zooplankton biomass in the upwelling region of the Gulf of Guinea. – ICES Journal of Marine Science, 65: 318–324. We investigated long-term changes in coastal zooplankton in the upwelling region in the Gulf of Guinea, 1969–1992, in relation to climatic and biotic factors. We considered the role of hydrographic and climatic factors, i.e. sea surface temperature (SST), salinity, sea level pressure, windfield, and Southern Oscillation Index (SOI), in the long-term variation of zooplankton in a multiple regression analysis, along with the abundance of Sardinella. Annual variation in zooplankton biomass was cyclical, with the annual peak occurring during the major upwelling season, July–September. Over the 24-year period, there was a downward trend in zooplankton biomass (equivalent to 6.33 ml per 1000 m3 per year). The decomposed trend in SST during the major upwelling revealed gradual warming of surface waters. This trend was believed to be the main influence on the abundance of the large copepod Calanoides carinatus (sensitive to temperatures above 23°C), which appears in the coastal waters only during the major upwelling season. The warming trend associated with global climate change could affect zooplankton community structure, especially during the major upwelling season. Global warming coupled with “top–down” (predation) control by Sardinella might be responsible for the long-term decline in zooplankton biomass in the upwelling region of the Gulf of Guinea.

Seasonal and inter-annual variation of mesozooplankton in the coastal upwelling zone off central-southern Chile

Zooplankton sampling at Station 18 off Concepción (36°30′S and 73°07′W), on an average frequency of 30 days (August 2002 to December 2005), allowed the assessment of seasonal and inter-annual variation in zooplankton biomass, its C and N content, and the community structure in relation to upwelling variability. Copepods contributed 79% of the total zooplankton community and were mostly represented by Paracalanus parvus, Oithona similis, Oithona nana, Calanus chilensis, and Rhincalanus nasutus. Other copepod species, euphausiids (mainly Euphausia mucronata), gelatinous zooplankton, and crustacean larvae comprised the rest of the community. Changes in the depth of the upper boundary of the oxygen minimum zone indicated the strongly seasonal upwelling pattern. The bulk of zooplankton biomass and total copepod abundance were both strongly and positively associated with a shallow (<20 m) oxygen minimum zone; these values increased in spring/summer, when upwelling prevailed. Gelatinous zooplankton showed positive abundance anomalies in the spring and winter, whereas euphausiids had no seasonal pattern and a positive anomaly in the fall. The C content and the C/N ratio of zooplankton biomass significantly increased during the spring when chlorophyll- a was high (>5 mg m −3). No major changes in zooplankton biomass and species were found from one year to the next. We concluded that upwelling is the key process modulating variability in zooplankton biomass and its community structure in this zone. The spring/summer increase in zooplankton may be largely the result of the aggregation of dominant copepods within the upwelling region; these may reproduce throughout the year, increasing their C content and C/N ratios given high diatom concentrations.

Variability and predictability in a zooplankton community: The roles of disturbance and dispersal

Disturbances are expected to increase variability and/or decrease predictability in ecosystems, while dispersal (or immigration) may have positive or negative effects on ecosystem stability. We conducted a mesocosm experiment with pelagic zooplankton to examine the independent and interactive effects of a nutrient pulse and immigration from neighbouring lakes on (a) temporal variation in the mesocosm communities and (b) variation among spatial replicates. Surprisingly, nutrient enrichment had no significant effect on among-replicate variability in algal or zooplankton biomass, and it decreased both among-replicate variability in zooplankton community composition and temporal variability of algal biomass (measured as the coefficient of variation). The nutrient pulse increased temporal variation in zooplankton biomass but decreased temporal change in relative species abundances. However, both these effects depended on dispersal, suggesting a context-dependent role of dispersal in community stability. We discuss the importance of considering multiple measures and aspects of community variability.

Seasonal dynamics and grazing rate of zooplankton in Yueqing Bay, China

The species composition, biomass, abundance, and species diversity of zooplankton were determined for samples collected from August 2002 to May 2003 from 14 stations in Yueqing Bay, China. Phytoplankton growth rate and microzooplankton grazing rate were obtained by using the dilution method developed by Landry and Hassett. The spatial and temporal variations of zooplankton and its relationship with environmental factors were also analyzed. The results showed that the zooplankton in the Yueqing Bay could be divided into four ecotypes, namely coastal low saline species, estuary brackish water species, offshore warm water species, and eurytopic species. A total of 75 species of zooplankton belonging to 56 genera and 17 groups of pelagic larva were identified in the Yueqing Bay. The coastal low saline species was the dominant ecotype in the study area, and the dominant species were Labidocera euchaeta, Acartia pacifica, Acrocalanus gibber, Pseudeuphausia sinica, and Sagitta bedoti among others. There was considerable seasonal variation in zooplankton biomass and abundance in the surveyed areas. The peak biomass appeared in August, descending in November and in May, and the lowest biomass appeared in February. Similarly, the highest abundance of zooplankton was observed in August, with the abundance descending in the following months: May, November, and February. There were similar horizontal distribution patterns for the biomass and the abundance of zooplankton. They both increased from the upper to the lower bay in February and May, but decreased from the upper to the lower bay in August. Biomass and abundance were evenly distributed in the Yueqing Bay in November. Moreover, there was marked seasonal variation in the species diversity of zooplankton, which conformed to the abundance of zooplankton. Results of the dilution experiments indicated that there was grazing pressure of microzooplankton on phytoplankton in the Yueqing Bay throughout the year though the rate of microzooplankton grazing on phytoplankton varied seasonally. Phytoplanktons were growing at 0.26–2.07/d and grazed by microzooplankton at a rate of 0.15–0.48/d in different seasons.

Zooplankton and the oceanography of the eastern tropical Pacific: A review

We review the spatial and temporal patterns of zooplankton in the eastern tropical Pacific Ocean and relationships with oceanographic factors that affect zooplankton distribution, abundance and trophic relationships. Large-scale spatial patterns of some zooplankton groups show broad coincidence with surface water masses, circulation, and upwelling regions, in agreement with an ecological and dynamic partitioning of the pelagic ecosystem. The papers reviewed and a new compilation of zooplankton volume data at large-scale show that abundance patterns of zooplankton biomass have their highest values in the upwelling regions, including the Gulf of Tehuantepec, the Costa Rica Dome, the equatorial cold tongue, and the coast of Peru. Some of the first studies of zooplankton vertical distribution were done in this region, and a general review of the topic is presented. The possible physiological implications of vertical migration in zooplankton and the main hypotheses are described, with remarks on the importance of the oxygen minimum zone (OMZ) as a barrier to both the vertical distribution and migration of zooplankton in the region. Recent results, using multiple-net gear, show that vertical distribution is more complex than previously thought. There are some well-adapted species that do live and migrate within the OMZ. Temporal patterns are reviewed and summarized with historical data. Seasonal variations in zooplankton biomass follow productivity cycles in upwelling areas. No zooplankton time series exist to resolve ENSO effects in oceanic regions, but some El Niño events have had effects in the Peru Current ecosystem. Multidecadal periods of up to 50 years show a shift from a warm sardine regime with a low zooplankton biomass to a cool anchovy regime in the eastern Pacific with higher zooplankton biomasses. However, zooplankton volume off Peru has remained at low values since the 1972 El Niño, a trend opposite to that of anchoveta biomass since 1984. Studies of trophic relations emphasize the difference in the productivity cycle in the eastern tropical Pacific compared to temperate or polar ecosystems, with no particular peaks in the stocks of either zooplankton or phytoplankton. Productivity is more dependent on local events like coastal upwelling or water circulation, especially in the equatorial countercurrent and around the equatorial cool-tongue. Micrograzers are very important in the tropics as are predatory mesozooplankton. Up to 70% of the daily primary productivity is consumed by microzooplankton, which thus regulates the phytoplankton stocks. Micrograzers are an important link between primary producers, including bacteria, and mesozooplankton, constituting up to 80% of mesozooplankton food. Oceanography affects zooplankton trophic relationships through spatial–temporal effects on primary productivity and on the distributions of metabolic factors, food organisms, and predators. This paper is part of a comprehensive review of the oceanography of the eastern tropical Pacific.

Spatial/temporal variations in zooplankton biomass and ecological characteristics of major species in the southern part of the Japan Sea: a review

During the 1990s many studies on zooplankton in the Japan Sea have been carried out. In this review, I have synthesized the study of horizontal distribution, seasonal and annual variations of zooplankton biomass, and ecological characteristics of major component species in the southern Japan Sea, which area is influenced by the warm Tsushima Current. The zooplankton biomass (annual mean) in the southern Japan Sea was lower than in the subarctic Pacific, including the northern Japan Sea, and similar to biomass levels in Kuroshio waters. Temporal variations in zooplankton biomass showed both seasonal and year-to-year components. Seasonal biomass increases to a maximum in spring with a weak secondary peak in autumn. As for long-term changes, 3–6 year cycles were identified, with the dynamics of the surface warm Tsushima Current and the subsurface cold water playing important roles in determining the yearly zooplankton community structure and biomass. Cold water species in the southern Japan Sea had extensive diel vertical migrations whose range is restricted in summer by the development of a thermocline. Among these species, the herbivores Euphausia pacifica and Metridia pacifica encounter a lower food supply, resulting in lower growth rates. The vertical dispersal of epipelagic carnivorous zooplankton such as Sagitta elegans and Themisto japonica to the deep-sea is probably facilitated by reduced interspecific competition. Their interaction with Japan Sea Proper Water, characterized by near-zero temperatures in the meso- and bathypelagic zones suppresses growth rates of the mesopelagic zooplankton. The lack of micronektonic predators in the mesopelagic zone may allow the persistence of slow growing populations.

Intercalibrating SCOR, NORPAC and bongo nets and the consequences for interpreting decadal‐scale variation in zooplankton biomass in the Gulf of Alaska

AbstractThe Canadian weathership time series of zooplankton wet weight biomass, collected at Ocean Station Papa in the Gulf of Alaska from 1956 to 1981, is one of the most comprehensive of its kind in marine science. In 1966, the sampling gear changed from a white North Pacific (NORPAC) net to a black Scientific Committee on Oceanic Research (SCOR) net and it was recently discovered that insufficient intercalibration samples were collected to understand how their sampling properties differed. A Canada‐GLOBEC project to redo the intercalibration of these net types and to understand how they relate to current sampling gear (bongo net) started in 1997. Seventy replicates of the three net types were collected in deep water in the Gulf of Alaska. The major finding is that all nets have similar sampling characteristics, whereas earlier reports indicated that NORPAC biomass values should be multiplied by a factor of 1.538 to be equivalent to the SCOR net. It now appears that this factor arose because flowmeters were not used in the original 1956–81 sampling (volume filtered was estimated from tow length × mouth area). A positive bias was introduced into the SCOR values because relatively more water passed through the SCOR net (undetected without a flow meter) than through the NORPAC net. This means that the unmetered NORPAC samples from 1956 to 1966 should not be adjusted and the unmetered SCOR values should be reduced by a factor that is related to wire angle. The general effect on the entire series is to lower the average biomass estimates, but more so for the early portion of the series than the later years.

Predation impact of juvenile Gilchristella aestuaria (Clupeidae) and Atherina breviceps (Atherinidae) on the zooplankton in the temperate Kariega estuary, South Africa

Seasonal changes in the predation impact of juvenile (SL 18–25mm) Atherina breviceps and Gilchristella aestuaria on zooplankton was investigated at three stations in the middle reaches of the permanently open Kariega estuary during 1999 and 2000. Daily rations were estimated using the gut fullness index approach. Variations in zooplankton biomass demonstrated a seasonal pattern with maximum values (generally >40mg dwt m−3) recorded in summer and minimum values during winter (generally <25mg dwt m−3). Total biomass of G. aestuaria during the study ranged between 0.04g dwt m−2 and 0.2g dwt m−2 and between 0.01g dwt m−2 and 0.04g dwt m−2 for A. breviceps. There were no seasonal patterns in biomass for either species. Results of the gut content analysis indicated that both species consume mainly copepods, amphipods and mysids. There were no seasonal patterns in the dominant prey in the guts of the juveniles. Peaks in feeding activity for G. aestuaria were recorded during daylight hours while maximum feeding activity of A. breviceps occurred at night. Gut evacuation rates (k, h−1) of G. aestuaria and A. breviceps during summer were estimated at 0.41h−1 and 0.68h−1 respectively. During winter, evacuation rates were lower and were estimated at 0.37h−1 for A. breviceps and 0.28h−1 for G. aestuaria. Daily rations of A. breviceps during summer and winter were equivalent to 8.3% and 4% body dwt d−1, while for G. aestuaria daily ration was estimated at 7.9% body dwt during summer and 4.3% body dwt d−1 during winter. Results indicate that the predation impact of G. aestuaria ranged between 2.5% and 33.6% of the zooplankton secondary production while A. breviceps impact was equivalent to between 3.3% and 11.1% of the zooplankton secondary production.

Hydrodynamic Control of Late Summer Species Composition and Abundance of Zooplankton in Hudson Bay and Hudson Strait (Canada)

The spatial variation in zooplankton biomass, abundance and species composition in relation to hydrography and chlorophyll a (Chl a) was studied in the subarctic waters of Hudson Bay and Hudson Strait. Sampling was carried out in early September 1993 at 21 stations arranged along a transect following the Quebec coast from James Bay, in Hudson Bay, to the vicinity of Ungava Bay in Hudson Strait. Both the biomass and the abundance of total zooplankton were low along the lower part of Hudson Bay (averaging 1. 6g DM m -2 and 9432 ind. m -2 ) and increased sharply toward the upper end of the Bay and in Hudson Strait (averaging 6.0 g DM m -2 and 40 583 ind. m -2 ). A total of 80 zooplankton taxa was identified in the samples. Copepods were clearly numerically dominant at all sampling stations, accounting for more than 85% and 93% of the zooplankton community in the Bay and the Strait, respectively. Clustering samples by their relative species composition revealed four groups distributed along well defined environmental gradients characterizing the distribution of physical variables and Chl a. The first group, located in the most southern region of Hudson Bay and farther offshore, northwest of the Belcher and Sleeper Islands, was strongly influenced by freshwater run-off from James Bay and other major rivers around the Bay, and was characterized by the presence of two euryhaline copepod species (Acartia longiremis and Centropages hamatus). The second and the third groups occupied the largest region along the sampling transect, from the middle of Hudson Bay to the western region of Hudson Strait, and were characterized by a typical arctic zooplankton fauna related to the cyclonic circulation in central Hudson Bay. The fourth group was located in the easternmost part of the sampling transect in Hudson Strait where the highest phytoplankton biomass values were observed (Chl a ∼220 mg m -2 ). The zooplankton assemblage there showed an important increase in the abundance of the large herbivorous copepod Calanus glacialis/finmarchicus, which were numerically four times more abundant in the central Strait region (averaging 15 251 ind. m -2 ) than in the western side of Hudson Strait and in Hudson Bay (3629 ind. m -2 ). These results support the hypothesis that the structure in the local biological community is influenced by the local hydrodynamic features which, through their action on surface water temperature, salinity, stratification and mixing conditions, lead to spatial differentiation of the phytoplankton and zooplankton communities.