Variability In Zooplankton Abundance Research Articles

Comparative study between low and high-water quality and the zooplankton community of the Niger river, Niger

Seasonal variations in zooplankton abundance, diversity and the physicochemical properties of the Niger River were studied during 2018 and 2019.The aim of this study is to carry out a comparative study of water quality and the zooplankton community in the River Niger between the high-water period (February 2019) and the low-water period (April 2018). To do this, three water samples were taken at each period and at each station, in order to determine the physico-chemical parameters and then identify the zooplankton. Qualitative analysis of zooplankton revealed the presence of 48 taxa belonging to three main taxonomic groups: Rotifera, Cladocera and Copepoda. During low-water periods, the most frequent taxa were rotifers (74.89%), followed by copepods and copepodites (20.17%) and cladocera (4.93%). In contrast to high-water sampling, copepods and copepodites are the most important (53.2%), followed by rotifers (32.63%) and cladocerans (14.09%).Rotifers are the most abundant group and the most resistant to changes in environmental variables. The physicochemical parameters that significantly influence zooplankton populations are pH, dissolved oxygen, temperature, total phosphorus, silica and chlorophyll-a.

Joint effects of sea ice melt, freshwater discharge and tidal currents on zooplankton abundance in the Sea of Okhotsk: 2004 and 2013

The foremost flooding event of the past century happened in 2013 on the Amur River, which flows into the Sea of Okhotsk. Concurrently, the winter of 2012–2013 was a year of heavy sea ice and delayed onset of melting in this region. To examine the joint effects of these major, regional-scale hydrological freshening events on oceanographic processes, we compared physical (CTD, tides and currents) and biological (zooplankton) data measured in 2004 and 2013 in Academy Bay, Sea of Okhotsk. Our results indicate that the difference in sea water temperature between the two years played a primary role in shaping zooplankton variability. Data collected in 2013 showed that water temperature was colder and that the upper layer was substantially fresher (∼4–5 psμ) than in 2004. This decrease in water temperature and salinity reduction was accompanied by a significant decrease in the abundance of some key zooplankton species including Calanus glacialis, Pseudocalanus spp. and Sagitta elegans and a corresponding increase in the abundance of Limacina helicina. Delayed melting of sea ice in 2013 potentially triggered a mismatch in pelagic production that may have impacted the recruitment of calanoid copepods. Variation in tidal and subtidal advection of cold and highly saline Okhotsk Sea shelf water appears to be an important process influencing regional variation in zooplankton abundance. The occurrence of such hydrological events has the potential to trigger cascading effects through the food web.

A Study on Seasonal Variation in Zooplankton Abundance in Kadasgatti Minor Irrigation Tank of Bailhongal Taluk, Belagavi District, Karnataka State, INDIA.

Objectives: A study on seasonal variation in zooplankton community was undertaken to evaluate the present status of water quality at Kadasgatti minor irrigation tank (MIT) located in the northern transitional zone of Belagavi district. Methods and Statistical Analysis: Water samples were collected from January 2017 to December 2017. Physico-chemical parameters and qualitative and quantitative analysis of zooplankton were carried out. Pearson correlation was calculated using SPSS, IBM Version 21 software to find out interrelationships between water quality and zooplankton groups. Findings: A total of 52 species of zooplanktons were recorded during the study period with a total zooplankton abundance of 14327 individuals with a relative abundance of 35.42%. Rotifera was the dominant group with 27 species, followed by Cladocera, Copepoda and Ostracoda. The highest zooplankton abundance was observed in summer while minimum in post-monsoon season. Copepoda was highest abundant group during the entire study period with its maximum abundance during winter and gradually declined and reached to its minimum in post-monsoon. The presence of eutrophic indicator species like Brachionus calyciflorus, Brachionus angularis, Filina longiseta suggests eutrophication of the tank. Novelty: The study provides baseline data on the present status of the water body indicating that, anthropogenic activities, agricultural runoff are the main cause of eutrophication. Sustainable and holistic conservational strategies have to be adopted to protect the water body. Keywords: Cladocera; Copepoda; Eutrophication; Rotifera; Seasonal variation; Zooplankton abundance

Changes in water quality related to permafrost thaw may significantly impact zooplankton in small Arctic lakes.

Rising temperatures are leading to permafrost thaw over vast areas of the northern hemisphere. In the Canadian Arctic, permafrost degradation is causing significant changes in surface water quality due to the release of solutes that can alter conductivity, water clarity, and nutrient levels. For this study, we examined how changes in water quality associated with permafrost thaw might impact zooplankton, a group of organisms that play an important role in the food web of Arctic lakes. We conducted a biological and water quality survey of 37 lakes in the Mackenzie Delta region of Canada's Northwest Territories. We then used this data set to develop models linking variation in the abundance, diversity, and evenness of zooplankton communities to physicochemical, biological, and spatial variables. Subsequently, we used these models to predict how zooplankton communities might respond as water quality is altered by permafrost thaw. Our models explained 47%, 68%, and 69% of the variation in zooplankton abundance, diversity, and evenness, respectively. Importantly, the most parsimonious models always included variables affected by permafrost thaw, such as calcium and conductivity. Predictions based on our models suggest significant increases in zooplankton abundance (1.6-3.6 fold) and decreases in diversity (1.2-1.7 fold) and evenness (1.1-1.4 fold) in response to water quality changes associated with permafrost thaw. These changes are in line with those described for significant perturbations such as eutrophication, acidification, and the introduction of exotic species such as the spiny water flea (Bythotrephes). Given their important role in aquatic food webs, we expect these changes in zooplankton communities will have ramifications for organisms at higher (fish) and lower (phytoplankton) trophic positions in Arctic lakes.

Zooplankton summer composition in the southern Gulf of Mexico with emphasis on salp and hyperiid amphipod assemblages

AbstractMesoscale features within the Gulf of Mexico (GOM) are known to influence zooplankton dynamics. Here we describe the composition of the zooplankton assemblage off shelf during summer in relation to environmental conditions, with emphasis on hyperiid amphipods and salps. Zooplankton samples were collected in summer of 2015 and 2016 in the central and southern GOM and in the Yucatan Channel in 2015. Two anticyclonic gyres were present in the north and less intense coupled cyclonic-anticyclonic gyres in the south. Zooplankton abundances differed temporally and spatially. Copepods were the dominant group (>55% of total abundance), while several less abundant taxa contributed to inter-annual and spatial differences. Amphipods and salps comprised <3% and their abundances were positively correlated. Fifty-six hyperiid and 10 salp species were identified. The dominant amphipod species were:Lestrigonus bengalensis(summer 2015),Anchylomera blossevilleiandPrimnospp. juveniles (summer 2016). Dominant salp species wereIhlea punctata,Iasis cylindricaandThaliaspp. Lower salp and amphipod species richness and abundance were associated with anticyclonic structures. Spatial and temporal differences were partly associated with symbiotic relationships between the groups. This study supports previous evidence of high spatial and temporal variability in zooplankton abundance in off-shelf waters of the GOM.

Drone imaging of elasmobranchs: Whale sharks and golden cownose rays co-occurrence in a zooplankton hot-spot in southwestern Sea of Cortez

The golden cownose ray (Rhinoptera steindachneri) and the whale shark (Rhincodon typus) are two elasmobranch species commonly found in Bahía de La Paz, Baja California Sur, Mexico. There are no records of the species sharing the same sites, as their prey, ecology and migration sites normally deviate. Both are listed as endangered and vulnerable species, respectively, under Mexican and international regulations. We took images by a quad-copter near Mogote Beach and recorded whale sharks during late July, alongside a group of golden cownose rays, although earlier research suggests that the main whale shark feeding season starts in late August or September. These recordings showed for the first time, the co-occurrence of these two vulnerable elasmobranch species while feeding possibly simultaneously. This indicates that there might be a relation between the presence of the two species and the interannual variability in zooplankton abundance. The latter may have implications for possible resource overlapping, a current topic in elasmobranch research. Furthermore, these findings highlight the usefulness of drones when capturing biological data. This information is important in the understanding of the ecology and trophic interactions of threatened species and is a step forward towards their understanding and conservation.

Winter decrease of zooplankton abundance and biomass in subalpine oligotrophic Lake Atnsjøen (SE Norway)

Despite the rapidly changing winter conditions in temperate ecosystems, little attention has been devoted to the effects of these changes on lake ecology. Few studies on the seasonal changes in abundance and biomass of the major groups of the metazooplankton community (i.e. rotifers, cladocerans and copepods) in northern oligotrophic lakes include data from the ice-covered winter months. This study reports monthly variation in zooplankton abundance and biomass from June 2010 to October 2011, including winter, in an oligotrophic, subalpine lake in southeastern Norway (Lake Atnsjøen). Changes in rotifer, cladoceran, copepod, and total zooplankton abundances and biomass were related to seasonal variation in water temperature and phytoplankton biomass by means of ordination analysis. The zooplankton abundance and biomass were much lower in winter than during the open water season. However, an under-ice phytoplankton bloom occurred during the final winter months, when snow cover and ice thickness were reduced and (presumably) light penetration increased, leading to an increase in abundance of copepod nauplii. Winter zooplankton abundance was dominated by copepods and rotifers, while winter zooplankton biomass was dominated by copepods and cladocerans. Both phytoplankton and zooplankton had two biomass peaks in 2010 and one peak in 2011. Rotifers dominated zooplankton abundance with a peak in August and total zooplankton abundance followed a similar pattern. In contrast, cladocerans dominated zooplankton biomass with a peak in July and total zooplankton biomass also peaked at this time. Rotifer and total zooplankton abundance and rotifer biomass were most closely correlated to water temperature. However, cladoceran biomass and total biomass were most closely correlated with phytoplankton biomass, but also appeared to be dependent on other carbon sources. Estimates of non-phytoplankton particulate organic carbon indicated that this part of the carbon pool could be an additional food source for zooplankton particularly in early and mid-winter. The longer growing season in 2011 than in 2010, owing to earlier ice-off in 2011, may have contributed to higher phytoplankton and zooplankton biomass in 2011. With climate warming, this is an expected change in temperate lake ecosystems.

Effects of Climate, Limnological Features and Watershed Clearcut Logging on Long-Term Variation in Zooplankton Communities of Boreal Shield Lakes

In Canada, climate change and forest harvesting may both threaten the ecological integrity of boreal lakes. To disentangle the effects of natural variation in climate and lake environments from those of logging, we evaluated long-term variation (1991–2003) in zooplankton communities of six boreal lakes in Ontario. We monitored concomitantly changes in zooplankton abundance and composition in three undisturbed and three harvested lakes, five years prior and eight years after watershed clearcut logging. We tested the hypothesis that long-term natural variation in climate and lake environments will be more important drivers of zooplankton community changes than short-term impacts of logging. We used space/time interaction tests and asymmetric eigenvector maps to model zooplankton responses to environmental changes and logging. Year-to-year variation in zooplankton abundance and composition were almost an order of magnitude whereas among-lake variation was stable through time. Breakpoints in time series of zooplankton in each lake were not directly related to logging. Climatic and limnological features were the most important drivers of long-term variation in the zooplankton community, shading the effect of logging. These results highlight the need to better understand the pressures exerted by climate change on boreal lake ecosystems in the context of anthropogenic pressure, such as logging.

Temporal Variations in the Abundance and Diversity of Surface Zooplankton in the South Coast of Sri Lanka

Zooplankton have extensively studied in the Indian Ocean, but very few systematic studies have been carried out in the marine waters around Sri Lanka. This study aimed to understand the temporal variations in the abundance and diversity of surface zooplankton in the south coast of Sri Lanka. Surface zooplankton samples were collected at five different locations in the south coast of Sri Lanka in November 2015 (second inter monsoon), December 2015 (Northeast monsoon, NE) and May 2016 (Southwest monsoon SW) using R/V Samuddrika. The vertical hauls of zooplankton were collected from 5 m depth at each station using WP-2 net with 180 μm mesh size and samples were immediately preserved in 5% buffered formalin. At the laboratory, zooplankton were identified to the lowest possible taxon using existing guides, species diversity, abundance (ind/m3), evenness and richness were estimated with respect to three different sampling phases. The data were subjected to hierarchical cluster analysis to identify the similarity between temporal variation and zooplankton abundance. A total of 152 zooplankton species were identified during this study. Species belonging to phylum Arthropoda appeared be the most abundant group and their standing crop (whole time period and sites) was 1906±115 ind/m3. Arthropods occupied nearly 86% of total zooplankton community. Among arthropods, order Poecilostomatoida showed the highest abundance (858±8 ind/m3) followed by phylum Protochordata (~3% and average of 74 ind/m3) chetognaths (2.1%) and mollusks (~1.9%). The highest species abundance was recorded in Novemver (1392±5 ind/m3) and lowest was in May (360±2 ind/m3). Species diversity was also highest in November and lowest in May. Cluster analysis revealed two clearly visible abundance clusters with respect to time. The results indicated that there are significant temporal variations in zooplankton abundance (ANOVA, P<0.05) in the South coast of Sri Lanka. Keywords: Zooplankton, Abundance, Diversity, Cluster analysis

Deep-water zooplankton in the Mediterranean Sea: Results from a continuous, synchronous sampling over different regions using sediment traps

Information on the dynamics of deep-sea biota is extremely scant particularly for long-term time series on deep-sea zooplankton. Here, we present the results of a deep-sea zooplankton investigation over one annual cycle based on samples from sediment trap moorings in three sub-basins along the Mediterranean Sea. Deep-sea zooplankton assemblages were dominated by copepods, as in shallow waters, only in the Adriatic Sea (>60% of total abundance), but not in the deep Ionian Sea, where ostracods represented >80%, neither in the deep Alboran Sea, where polychaetes were >70%. We found that deep-sea zooplankton assemblages: i) are subjected to changes in their abundance and structure over time, ii) are characterized by different dominant taxa in different basins, and iii) display clear taxonomic segregation between shallow and near-bottom waters. Zooplankton biodiversity decreases with increasing water depth, but the equitability increases. We suggest here that variations of zooplankton abundance and assemblage structure are likely influenced by the trophic condition characterizing the basins. Our findings provide new insights on this largely unknown component of the deep ocean, and suggest that changes in the export of organic matter from the photic zone, such as those expected as a consequence of global change, can significantly influence zooplankton assemblages in the largest biome on Earth.

Community composition, abundance and biomass of zooplankton in Zhangzi Island waters, Northern Yellow Sea

Samples were collected monthly from the sea area around Zhangzi Island, northern Yellow Sea, from July 2009 to June 2010. Vertical net towing was used to examine spatial and temporal variability in zooplankton abundance and biomass. Overall, Calanus sinicus and Saggita crassa were the dominant species found during the study period, while the amphipod Themisto gracilipes was dominant in winter and spring. Vast numbers of the ctenophore species of the genus Beroe were found in October and November. It was not possible to count them, but they constituted a large portion of the total zooplankton biomass. Zooplankton species diversity was highest in October, and species evenness was highest in April. Zooplankton abundance (non-jellyfish) and biomass were highest in June and lowest in August, with annual averages of 131.3 ind./m³ and 217.5 mg/m³, respectively. Water temperature may be responsible for the variations in zooplankton abundance and biomass. Beroe biomass was negatively correlated with other zooplankton abundance. Longterm investigations will be carried out to learn more about the influence of the environment on zooplankton assemblages.

Zooplankton community of Keibul Lamjao National Park (KLNP) Manipur, India in relation to the physico-chemical variables of the water

Keibul Lamjao National Park (KLNP), a floating park in Loktak Lake, Manipur (India) was studied from Winter (WIN) to Post Monsoon (POM) for its zooplankton composition and some selected water parameters. The resultant data were subjected to multivariate techniques–Principal Component Analysis (PCA) and Canonical Correspondence Analysis (CCA). Analyses of water parameters with PCA revealed that the first PC axis (PC1) accounts for maximum variance in the seasonal data, explaining a variability of 91%. The PCA revealed that the seasonal variability in water parameters was due to the wet and dry cycle of seasons and the stations were distinguished on the basis of transparency and turbidity. Zooplankton abundance was dominated by copepods followed by cladocerans. Temporally, abundance of copepods reached a maximum during Post-monsoon (POM) (3 880 ind./L). Spatially, S6 was found to be most abundant of the other stations in zooplankton. Copepodites and nauplii larvae were the major components of zooplankton. The Rotifera were the least abundant among the three zooplankton groups. Brachionus formed the major component of Rotifera zooplankton at all the stations during the study period. In the Cladocera, Macrothrix was present during all the four seasons, while Pleuroxus, Oxyurella, Kurzia and, Diaphanosoma were rare. The CCA shows that maximal temporal variability in zooplankton abundance was explained by temperature and rainfall. ANOVA revealed no significant diff erence in mean zooplankton abundance among the seasons, but there was a statistically significant diff erence among the sites.

Seasonal Variations of Zooplankton Abundance in Shatt Al-Arab River, Southern Iraq

Zooplankton samples were collected seasonally from three stations in the Shatt Al-Arab River southern Iraq by Plankton net (0.085 mm mesh-size and 40 cm mouth aperture) during 2013. Physical and chemical parameters of the water were studied such as, water temperature, salinity, pH, dissolved oxygen, TDS and Turbidity. The population density of zooplankton ranged between 253 ind./m 3 at second station (Abu Al-Khasib) to 69223 ind./m 3 at third station (Al-Faw) in winter. The important groups of zooplankton in the study area were Copepoda (56.1%), then Cirripede larvae (19.5%) and Cladocera (12.4%).

Spatio-temporal variability of copepod abundance along the 20 °S monitoring transect in the Northern Benguela upwelling system from 2005 to 2011.

Long-term data sets are essential to understand climate-induced variability in marine ecosystems. This study provides the first comprehensive analysis of longer-term temporal and spatial variations in zooplankton abundance and copepod community structure in the northern Benguela upwelling system from 2005 to 2011. Samples were collected from the upper 200 m along a transect at 20°S perpendicular to the coast of Namibia to 70 nm offshore. Based on seasonal and interannual trends in surface temperature and salinity, three distinct time periods were discernible with stronger upwelling in spring and extensive warm-water intrusions in late summer, thus, high temperature amplitudes, in the years 2005/06 and 2010/11, and less intensive upwelling followed by weaker warm-water intrusions from 2008/09 to 2009/10. Zooplankton abundance reflected these changes with higher numbers in 2005/06 and 2010/11. In contrast, zooplankton density was lower in 2008/09 and 2009/10, when temperature gradients from spring to late summer were less pronounced. Spatially, copepod abundance tended to be highest between 30 and 60 nautical miles off the coast, coinciding with the shelf break and continental slope. The dominant larger calanoid copepods were Calanoides carinatus, Metridia lucens and Nannocalanus minor. On all three scales studied, i.e. spatially from the coast to offshore waters as well as temporally, both seasonally and interannually, maximum zooplankton abundance was not coupled to the coldest temperature regime, and hence strongest upwelling intensity. Pronounced temperature amplitudes, and therefore strong gradients within a year, were apparently important and resulted in higher zooplankton abundance.

Springtime Crustacean Zooplankton Abundance under Cold Water Conditions Is Determined Mainly by Temperature Regardless of Fish Predation

The seasonal succession of plankton communities is initiated as temperature starts to increase in early spring. Here, we examined the abundance of crustacean zooplankton in six large Finnish boreal lakes a few days after the melting of the ice for a period of ten years. We related zooplankton abundance to relevant abiotic and biotic factors, including temperature, wind and predation by fish and assessed the annual variability in zooplankton abundance at three spatial scales (lake, part of a lake, and habitat) and one temporal scale (year). Zooplankton abundance was positively associated with water temperature regardless of fish predation pressure and the spatial scale examined. This finding is significant as mean water temperature in the lakes concerned was very low (5.7 ° C), although the observed temperature range was actually quite wide (2-13 ° C). Predation by newly-hatched coregonids affected zooplankton density only in one lake. The effect of wind in one lake was significant in the littoral but not in the pelagic habitat. Temperature may be the ultimate factor determining zooplankton dynamics, particularly under cold water conditions.

Diet composition and food selectivity of sprat (Sprattus sprattus) in Hardangerfjord, Norway

The diet composition and prey selection of sprat (Sprattus sprattus) was studied in the Hardangerfjord, western Norway during four cruises in spring and autumn of 2009 and 2010. Feeding activity, indicated by stomach fullness, varied with sprat size, season and time of day. Small-sized sprat were found to feed both in spring and autumn, while the larger individuals had lower feeding activity late in the season. Diurnal variation in stomach fullness was observed in the autumn but not in spring. Sprat fed on a broad diversity of prey types, including copepods, phytoplankton and meroplankton. Multivariate statistical analysis suggested that the dietary composition of sprat differed significantly between years, seasons and among areas within the fjord. During spring, phytoplankton and invertebrate eggs dominated the diet, while copepods were most important in the autumn. Positive selection was found for Microsetella norvegica and euphausiid eggs, which were the most frequently occurring prey in stomachs, followed by Oithona and Acartia spp. Interannual variability in zooplankton abundance in the fjord, related to physical processes, may have a large impact on the feeding conditions for fjord populations of sprat.

Mesozooplankton community seasonal succession and its drivers: Insights from a British Columbia, Canada, fjord

The mesozooplankton dynamics in Rivers Inlet, a fjord in Central British Columbia, Canada, were studied from March to June of 2008, 2009 and 2010 to assess the interannual, seasonal, and spatial variability in zooplankton abundance and community structure under different physical environments and spring bloom scenarios. Samples were collected fortnightly during 2008–2009 and monthly in 2010 and provide one of the few multi-year zooplankton time series in the region. Two distinct zooplankton communities characterized the observed succession pattern. The winter–spring group was described by the presence of small, year-round omnivorous zooplankton: bryozoan cyphonautes, Microcalanus spp., Microsetella spp., Oithona spp., and Oncaea spp., as well as by large, diapausing copepods such as Eucalanus bungii, Neocalanus plumchrus, and Calanus marshallae and the euphausiids Thysanoessa spinifera. By contrast, the spring–summer community showed an increased abundance of Acartia longiremis, cladocerans, Limacina helicina, Metridia pacifica, Euphausia pacifica, appendicularians, Clione limacina, chaetognaths, polychaetes, Pseudocalanus spp., ostracods, and amphipods. The timing of zooplankton succession was consistently associated with the timing of the spring bloom, and was delayed in 2009 when the spring bloom occurred in May rather than April. The zooplankton succession dynamics are discussed in terms of dominant feeding guild structure to highlight the potential mechanisms of succession. Spatial variability in zooplankton distribution was mainly influenced by river flow and exchanges with adjacent coastal waters.

Changes in zooplankton community structure in Sanmen Bay, China

Seasonal variations in zooplankton abundance, biomass, species diversity and community structure were investigated in the Sanmen Bay, China. Samples were collected from 15 stations, on the seasonal basis, in April (spring), July (summer) and October 2005 (autumn) and January 2006 (winter), respectively. The results show that zooplankton species number, abundance and biomass varied widely and had distinct spatial heterogeneity in the Sanmen Bay. A total of 72 species of zooplankton belonging to 56 genera and 17 groups of pelagic larvae were identified. The zooplankton species richness was strongly related to salinity. Based on hierarchical cluster analysis, zooplankton in this study area were classified into three groups: coastal, neritic and pelagic groups, which corresponded to the upper, middle and lower portion of the Sanmen Bay, respectively. The coastal low-saline species were dominant in the study area. The zooplankton abundance and biomass reached a peak in summer, moderate in spring and autumn, and the lowest in winter. Zooplankton abundance decreased from the upper to lower portion of the bay in April, when the highest biomass occurred in the middle portion of the bay. There were the same spatial distribution patterns for the biomass and abundance in July, with the maximum in the middle of the bay. However, zooplankton abundance was the highest in the middle of the bay in October, when maximum biomass occurred near the lower of the bay. Zooplankton abundance and biomass were evenly distributed in the Sanmen Bay in January. Spatial and temporal variations in zooplankton and their relationship with environmental factors were also analyzed. The BIOENV results indicate that the combination of chlorophyll a (Chl-a), salinity, dissolved inorganic nitrogen (DIN), dissolved oxygen (DO) and silicate (SiO3) was responsible for the variations in zooplankton community structure in the Sanmen Bay. The environmental changes played an important role in changes in the zooplankton community structure in the Sanmen Bay.

Inter-decadal Variability of Zooplankton Abundance in the Middle Atlantic Bight

Interannual zooplankton abundance trends in the Middle Atlantic Bight region of the US Northeast continental shelf are described and related to variations in environmental variables for the years 1977 to 2009. Depth integrated plankton samples were collected on bimonthly broad-scale surveys of the region. Abundance information indicates that the region’s zooplankton population is flourishing. Total counts and biomass levels have been mostly above average in the second-half of the sampling period. Multivariate analysis of abundance data supported these findings by classifying the time series into three consecutive groups of years defined by varying abundance levels: 1) average in the late 1970s, 2) low in the 1980s, and 3) above average from 1993 to 2009. This analysis identified eleven taxa that had similar increasing interannual abundance patterns during the time series. Evidence is presented that these changes may be related to warming temperatures in the Northwest Atlantic. The increasing abundance levels of the eleven taxa was positively correlated (p<0.01) to surface temperature measurements and the recent trends of the Atlantic Multidecadal Oscillation index. A second, smaller group of six taxa had correlated abundance trends that were variable through the years, with peak production associated with low salinity in the late 1990s.

Zooplankton spatial and diurnal variations in the Changjiang River estuary before operation of the Three Gorges Dam

Estuarine plankton communities can serve as indicators of ecosystem modification in response to anthropogenic influences. The main objectives of this study were to describe the spatial distribution and diurnal variability in zooplankton abundance and biomass over almost entire salinity gradient of the Changjiang (Yangtze) River estuary and to provide a background reference for future studies. To accomplish this, data were collected from 29 stations in the estuary from May 19 to 26, 2003, including two anchor stations. The spatial and diurnal variations in zooplankton characteristics, i.e. abundance, biomass, and gross taxonomic composition, were examined. Generally, both the abundance and biomass gradually increased seaward and presented distinct spatial variations. In addition, the spatial data revealed a significant correlation between abundance and biomass; however, there was no significant correlation between abundance and biomass for the diurnal data. Although the zooplankton composition indicated distinct spatial differences in terms of dominant groups, copepods accounted for >50% of the total zooplankton abundance in most regions and times. Three zooplankton assemblages were recognized through hierarchical cluster analysis. These assemblages existed along the salinity gradient from fresh water to seawater, and their positions coincided with those of the three principal water masses in the estuary. The assemblages were classified as: (1) true estuarine, (2) estuarine and marine, and (3) euryhaline marine, which were characterized by the copepods Sinocalanus dorrii, Labidocera euchaeta, and Calanus sinicus, respectively. Both spatial and diurnal data indicated that there was no significant correlation between zooplankton abundance/biomass and depth-integrated phytoplankton abundance.