Impact of a flood event on the zooplankton of an estuarine lake

Shallow coastal lakes are prone to large fluctuations in physico-chemical variables such as salinity and turbidity. This is now escalating in response to global change. A flood event in March 2014 resulted in a silt plume spreading through part of Lake St Lucia (South Africa). To determine the impact of this event on zooplankton, the Narrows region of St Lucia was sampled on a monthly basis from March to September 2014. For comparative purposes, data from samples collected prior to the flood event were included in the analyses. Analysis of similarity (ANOSIM) revealed dissimilarities in zooplankton community structure among the sampling occasions. The March 2014–May 2014 period was characterized by the highest abundance of freshwater species. Conversely, the abundance of the resident St Lucia copepods Acartiella natalensis and Oithona brevicornis was lowest during this time, and highest in September 2014. The other dominant copepod Pseudodiaptomus stuhlmanni prevailed in March 2014, but declined markedly in April. As of September 2014, P. stuhlmanni had yet to regain its pre-flood densities. The BIOENV procedure, which relates biological and environmental data, revealed that turbidity, salinity and dissolved oxygen were responsible for the observed changes in zooplankton community structure during the study period. Careful management of turbidity and salinity is stressed, as both factors are major drivers of the biota of St Lucia and similar systems worldwide.     

鄱阳湖丰水期水位波动对浮游动物群落演替的影响

为了解鄱阳湖夏季丰水期水位剧烈波动过程中浮游动物的群落演替特征,2012年夏季鄱阳湖水位剧烈波动期间,于6月24日、7月7日和8月27日当水位下降且接近17.6 m时,在江西鄱阳湖国家级自然保护区内的1个浅水碟形湖泊设置4个采样点进行采样调查。共发现浮游动物65种,其中轮虫52种,枝角类7种,桡足类6种,多为营浮游生活的广温性和嗜温性种类。单因子方差分析(one-way ANOVA)显示3个月之间浮游动物的密度和生物量均具有显著差异(P0.05),7月份浮游动物密度(1030.17±68.18个/L)显著高于6月份(325.16±41.60个/L)和8月份(203.79±24.91个/L);6月份浮游动物的生物量(0.56±0.04 mg/L)显著低于7月份(1.22±0.11 mg/L)和8月份(0.99±0.11 mg/L)。基于浮游动物多度的聚类分析和自组织映射神经网络图均揭示夏季3个月份的浮游动物可区分为明显的3个群落:6月群落、7月群落和8月群落。蒙特卡罗检验发现水温、电导率、浊度和溶氧与浮游动物群落结构变化显著相关(P0.05)。典型相关分析显示,6月份浮游动物群落与叶绿素a含量呈显著正相关关系,7月份浮游动物群落与水体温度呈显著正相关关系,8月份浮游动物群落与水深和电导率、浊度和溶氧呈显著正相关关系(P0.05)。在3个月均为优势物种的盖氏精囊轮虫与叶绿素a含量呈正相关关系(P0.05),与水温、pH、溶氧呈负相关(P0.05)。夏季水位波动过程中浮游动物的群落结构在时间上(月份之间)发生明显演替,呈现轮虫密度逐渐降低,枝角类和桡足类密度逐渐增加的变化规律。水位波动引起环境因子的改变,从而对浮游动物的群落演替产生了重要影响。     

Beneficial and detrimental interactive effects of dissolved organic matter and ultraviolet radiation on Zooplankton in a transparent lake

While changes in dissolved organic matter (DOM) concentrations are expected to affect zooplankton species through attenuation of potentially damaging ultraviolet (UV) radiation, generation of potentially beneficial or harmful photoproducts, pH alteration, and microbial food web stimulation, the combined effects of such changes on zooplankton community structure have not been studied previously. Our purpose was to determine how an increase in allochthonous DOM and associated changes in pH in an initially transparent lake may affect zooplankton community structure, and how exposure to solar UV may alter these DOM and pH effects. We ran microcosm experiments manipulating UV, DOM, and pH near the surface of Lake Giles in northeastern Pennsylvania. We found that when DOM was added in the presence of ambient UV, Daphnia and copepod UV-mortality was reduced by approximately three and two times compared to UV exposure without extra DOM. When DOM was added in the absence of UV, adult Daphnia and copepods were reduced compared to no DOM addition in the absence of UV. Daphnia and cyclopoid egg production and rotifer abundance were generally higher in the presence of DOM, regardless of UV treatment. The lower abundance yet high egg production in the presence of DOM and absence of UV may be explained by higher abundance of egg-bearing adults compared to non-egg-bearers. We conclude that allochthonous DOM benefits some zooplankton in a high-UV environment, but may be detrimental under low-UV conditions. Overall, Daphnia abundance and egg production were higher than that of calanoid copepods in the DOM additions, indicating that in some lakes an increase in allochthonous DOM may lead to a zooplankton community shift favoring Daphnia over calanoid copepods.     

Basin scale distribution of zooplankton in the Ligurian Sea (north-western Mediterranean) in late autumn

Mesozooplankton biomass and abundance were evaluated in epipelagic waters at 59 stations covering the Italian sector of the Ligurian Sea (north-western Mediterranean) in December 1990. This region is characterised by a cyclonic circulation which encloses a central divergence zone and is associated with a main thermohaline front offshore the western Ligurian coast. At the end of autumn, mesozooplankton biomass (range: 0.80–4.24 mg DW m⁻³) and the abundance (range: 83.8–932 ind. m⁻³) were lower in the divergence zone. On the contrary, in the Ligurian frontal zone at the periphery of the divergence and on the eastern continental shelf the greatest values of biomass and abundance were recorded. Copepods and appendicularians dominated the mesozooplankton community, the main taxa being the copepods Clausocalanus spp. (46% of total zooplankton) and Oithona spp. (15%) and the appendicularian Fritillaria spp. (12%). Three hydrological sub-regions, i.e. the divergence, the eastern continental shelf and the periphery of the divergence, were characterised by different zooplankton communities and characteristic species. Environmental differences between the three zones were mainly related to changes in bottom topography, sea surface temperatures and quantity of particulate organic matter. Vertical mesozooplankton abundance and taxa distribution from the surface to 1,900 m depth were also examined in one station. The results showed that the bulk of the community was concentrated in the upper 200 m, small copepods being dominant particularly in the upper 50 m. The copepod community was more diversified in sub-superficial waters, with a maximum observed in the 200–400 m layer. The distributions of main zooplankton taxa described in epipelagic waters in the eastern Ligurian Sea in autumn were compared with their distribution at surface in the north-western Mediterranean obtained by sampling performed with the Continuous Plankton Recorder in 1997–1999. The analysis of the zooplankton community in CPR samples confirms the dominance of small copepods (Paracalanus spp., Clausocalanus spp., Oithona spp.) and appendicularians in the north-western Mediterranean in late autumn-winter and shows that their distribution is mainly related to the main mesoscale hydrographic features characterising this basin. Guest editors: S. Souissi & G. A. Boxshall Copepoda in the Mediterranean: Papers from the 9th International Conference on Copepoda, Hammamet, Tunisia     

The longitudinal distribution of copepods in Corumbá Reservoir,State of Goiás,Brazil

The assessment of factors that determine the spatial distribution of zooplankton abundance is a central issue in zooplankton ecology. Since these factors are, in the most cases, spatially autocorrelated, especially in reservoirs, it is frequently difficult to isolate the primary causes that explain the variation in zooplankton abundance. In this paper, the spatial variation of copepods in Corumbá Reservoir (Goiás, Brazil) was studied. Sets of variables were used to model copepod abundance variation: limnology (abiotic variables), phytoplankton and spatial position of sampling sites. The copepod assemblage was composed of Thermocyclops minutus, Thermocyclops decipiens, Mesocyclops longisetus and Notodiaptomus iheringi. Along the reservoir's main axis, abundance increases, albeit not linearly, towards the lacustrine zone. High abundance, similar to that found in the lacustrine region, was also observed in the lateral arms. These patterns were very similar for different years and seasons. Copepods were significantly correlated with phytoplankton and geography. After controlling for the effect of space, the relationship between copepods and phytoplankton was not significant (partial Mantel test). This result indicates that hydrology is the primary process that controls not only the abundance of copepods, but also limnological features and food resources, which are commonly used to explain variations in zooplankton abundance.     

Inter-annual variation in summer zooplankton community structure in Prydz Bay, Antarctica, from 1999 to 2006

Inter-annual variations in zooplankton community structure in Prydz Bay were investigated using multivariate analysis based on samples collected with a 330-μm mesh, 0.5-m² Norpac net during the austral summer from 1999 to 2006. Two distinct communities, an oceanic and a neritic community, were consistently identified in all surveys. Oceanic communities had higher diversity and were indicated by species such as Haloptilus ocellatus, Heterorhabdus austrinus, Thysanoessa macrura, Rhincalanus gigas, Scolecithricella minor and Oikopleura sp.. Neritic communities were indicated by Euphausia crystallorophias and Stephos longipes and were characterized by fewer but more abundant species. In 1999 and 2006, a transitional community was also distinguished near the continental shelf edge, where ice coverage was more extensive than either the oceanic or neritic regions. Significant inter-annual variations in community structure (mainly involving species abundance rather than species composition) were found in both oceanic and neritic communities, being more obvious in the latter. The timing and amplitude of sea ice retreat (polynya appearance), and its effect on food availability, had strong influences on zooplankton community structure. In oceanic communities during years with earlier ice retreat, the extra time available for phytoplankton blooms to accumulate resulted in a higher proportion of large copepods (Calanoides acutus, Calanus propinquus, Metridia gerlachei) (especially the younger copepodites) in the zooplankton assemblage. In neritic communities, zooplankton such as the ice krill E. crystallorophias, and large copepods (C. acutus, C. propinquus, M. gerlachei), also showed higher abundance and earlier developmental stages in years with larger polynya. On the other hand, in years with later ice retreat, smaller polynya, and less time for phytoplankton blooms to form, the abundance of large copepods was lower and older age classes were more common.     

Recurrent response patterns of a zooplankton community to whole-lake fish manipulation

1. In a series of whole-lake manipulations conducted from 1984 to 1991, planktivorous fishes were alternately removed and restocked in a small mesotrophic lake, resulting in dramatic changes in the zooplankton community. 2. Response patterns in the zooplankton community, which include species and size structure, and within-year community variability, were examined. Variation in the zooplankton community in unmanipulated years was much lower than that in manipulated years, regardless of direction of the manipulation (i.e. decreasing or increasing planktivory). 3. The succession of zooplankton species abundance was repeated in the second removal of planktivorous fishes. The community shifted from small-bodied cladocerans, copepods and rotifers, through an intermediate state with high abundance of Holopedium, to an assemblage dominated by large-bodied daphnids.     

Zooplankton dynamics in a gypsum karst lake and interrelation with the abiotic environment

Zooplankton species composition and abundance variation was studied in Lake Amvrakia, which is a deep, temperate, gypsum karst lake situated in the western Greece. The two year survey of zooplankton revealed 33 species (23 rotifers, five cladocerans, four copepods and one mollusc larva). The mean integrated abundance of the total zooplankton ranged between 83.6 and 442.7 ind. L⁻¹, with the higher density to be recorded in the surface 0–20 m layer. Small numbers of specimens of almost all species were found also in the hypoxic or anoxic hypolimnion. Copepods and especially the calanoid Eudiaptomus drieschi dominated the zooplankton community throughout the sampling period, followed by Dreissena polymorpha larvae, rotifers and cladocerans. Seasonal succession among the cladocerans and the most abundant rotifer species was observed. The concentration of chlorophyll-a was the most important factor for the variation of total zooplankton, as well as for the rotifers’ community. Dissolved oxygen affected copepods and cladocerans, water level correlated mainly with the molluscs larvae of D. polymorpha, while temperature influenced the variation of several rotifers, the cladoceran Diaphanosoma orghidani and the mollusc larvae. Negative correlation of conductivity with the cladoceran Daphnia cucullata and the copepods E. drieschi and Macrocyclops albidus was found. The differences in species composition found in Lake Amvrakia in comparison to the nearby lakes are probably ought to the geographical isolation and perhaps to its particular chemistry (e.g., elevated conductivity).     

Temporal variations and pond age effect on plankton communities in semi-intensive freshwater marron (Cherax cainii,Austin and Ryan, 2002) earthen aquaculture ponds in Western Australia

The abundance and diversity of the plankton community represents the health of the aquatic ecosystem, and plays an important role in the growth of cultured animals under aquaculture conditions. The temporal variations of plankton abundance, taxonomic composition, diversity, evenness and species richness were studied in three old and three new semi-intensive marron (Cherax cainii, Austin and Ryan, 2002) ponds. Water parameters such as temperature, dissolved oxygen, pH, turbidity, TAN, nitrite, nitrate and reactive phosphate were recorded, and plankton samples were collected every two months, for one year of juvenile production cycle. A total of twenty-six phytoplankton and seven zooplankton genera were recorded. Chlorophyceae was the dominant class of phytoplankton throughout the year, followed by Trebouxiophyceae. Rotifera comprised 49.8% of the total zooplankton community (individuals L^?1), the largest proportion of any group. Temporal variations impacted the plankton abundance and community structure, and plankton abundance were more abundant during summer. The pond age did not influence the phytoplankton abundance, whereas zooplankton abundance was higher in older ponds.     

Mesozooplankton of the Kosi Bay lakes,South Africa

The Kosi coastal lake system, a chain of four interconnected basins, is located in the subtropical north-eastern corner of South Africa. Little information is available on zooplankton of the system and the main aim of this study is to report on zooplankton samples collected during 2002 and 2003. The set of samples consists of seasonal, subsurface mesozooplankton samples that were collected during nighttime in each of the lakes. A well-developed salinity gradient was evident along the interconnected lakes in the subsurface water during all seasons, ranging from freshwater in the upper lake Amanzamnyama to a maximum of 22 recorded in Lake Makhawulani. The zooplankton community structures of the lakes reflected the salinity gradient of the system, with some coastal marine taxa recorded in the lakes closer to the mouth and only freshwater taxa recorded in Lake Amanzamnyama. Mesozooplankton diversity and abundance were relatively low compared to other estuarine systems along the eastern coast of South Africa. The dominant taxa were calanoid copepods Acartiella natalensis and Pseudodiaptomus stuhlmanni and the mysid Mesopodopsis africana in the lower lakes, whereas cyclopoids Mesocyclops sp. and Thermocyclops sp. dominated the freshwater lake Amanzamnyama.     

An experimental study of the effects of nutrient supply and Chaoborus predation on zooplankton communities of a shallow tropical reservoir (Lake Brobo, Côte d'Ivoire)

1. Based on two mesocosm experiments and 10 in vitro predation experiments, this work aimed to evaluate the impact of nutrient supply and Chaoborus predation on the structure of the zooplankton community in a small reservoir in Côte d'Ivoire. 2. During the first mesocosm experiment (M1), P enrichment had no effect on phytoplankton biomass (chlorophyll a) but significantly increased the biomass of some herbivorous zooplankton species (Filinia sp, Ceriodaphnia affinis). During the second experiment (M2), N and P enrichment greatly increased phytoplankton biomass, rotifers and cladocerans (C. affinis, C. cornuta, Moina micrura and Diaphanosoma excisum). In both experiments, nutrient addition had a negative impact on cyclopoid copepods. 3. Larger zooplankton, such as cladocerans or copepodites and adults of Thermocyclops sp., were significantly reduced in enclosures with Chaoborus in both mesocosm experiments, whereas there was no significant reduction of rotifers and copepod nauplii. This selective predation by Chaoborus shaped the zooplankton community and modified its size structure. In addition, a significant Chaoborus effect on chlorophyll a was shown in both experiments. 4. The preference of Chaoborus for larger prey was confirmed in the predation experiments. Cladocerans D. excisum and M. micrura were the most selected prey. Rotifer abundance was not significantly reduced in any of the 10 experiments performed. 5. In conclusion, both bottom‐up and top‐down factors may exert a structuring control on the zooplankton community. Nutrients favoured more strictly herbivorous taxa and disadvantaged the cyclopoid copepods. Chaoborus predation had a strong direct negative impact on larger crustaceans, favoured small herbivores (rotifer, nauplii) and seemed to cascade down to phytoplankton.     

Zooplankton dynamics in the upstream part of Stratos reservoir (Greece)

The investigation of the zooplankton community in the upstream part of Stratos reservoir during a 24 months survey (September 2004–August 2006) revealed 26 invertebrate species (14 rotifers, 6 cladocerans, 5 copepods and one mollusk larva). The mean abundance of the total zooplankton was higher in the first sampling period (2004–2005) and ranged between 8.81 and 47.74 ind. L⁻¹, than the second period (2005–2006) when fluctuated between 1.91 and 43.09 ind. L⁻¹. The seasonal variation was strongly influenced by the presence of rotifers, which accounting on average for 68.4% in total. Among them Keratella cochlearis and the order Bdelloidea were numerically the most important, while Macrocyclops albidus prevailed among the copepods and Bosmina longirostris among the cladocerans. Dreissena polymorpha was the only mollusk found in the zooplankton community. Rotifers, copepods and cladocerans showed a seasonal succession with the former preceding in the abundance having their first maximum in spring, while copepods and cladocerans followed, having peaks of abundance in early summer and in autumn, respectively. No seasonal succession among the cladoceran species was observed. The intense water flow in the upstream part of the reservoir, as well as temperature, conductivity, DO, pH, phosphates and silicates, were significant parameters controlling abiotic and biotic elements of the ecosystem and consequently influencing the seasonal variation and the dynamics of the zooplankton community.     

Differential zooplankton feeding behaviors,selectivities, and community impacts of two planktivorous fishes

Synopsis We examined the feeding behaviors and selectives of two common planktivorous fishes, pumpkinseeds Lepomis gibbosa and fathead minnows Pimephales promelas in the laboratory. Ingestion rates for both pumpkinseeds and fathead minnows feeding on zooplankton increased as a function of fish length. Pumpkinseeds fed on zooplankton strictly as particulate feeders, with preferences increasing as a function of zooplankton body size regardless of taxonomic identity. Preferences were highest for large Daphnia, intermediate for intermediate-sized copepods, and lowest for small Ceriodaphnia. Fathead minnows displayed the ability to use both particulate-feeding and filter-feeding behaviors. Differential preferences tended to reflect both zooplankton size and taxon, being highest for large, slow-swimming Daphnia, intermediate for small Ceriodaphnia, and lowest for faster-swimming copepods. These differences in prey capture behaviors and preferences of the two fishes are reflected in the zooplankton taxonomic composition of small ponds containing each fish type. The crustacean zooplankton assemblages in ponds containing both pumpkinseeds and fathead minnows were dominated by copepods. Cladocerans were rare. In ponds containing pumpkinseeds, but no fathead minnows, cladocerans were abundant, generally accounting for up to 80% of total crustacean zooplankton biomass. These results suggest that the type of planktivore, and not simply the presence or abundance of planktivores in a system, can determine zooplankton community structure.     

Zooplankton distribution in four ponds of different salinity: a seasonal study in the solar salterns of Sfax (Tunisia)

The seasonal distribution of metazooplankton and large-sized ciliates was studied in four ponds of different salinity in the solar salterns of Sfax (Tunisia). Total zooplankton abundance varied from 1 × 10³ to 4.7 × 10⁶ ind m^–3. Salinity had a negative effect on the abundance of copepods and rotifers which were absent in the pond with the highest salt concentration (180) in which the number of taxa was low and Artemia or the ciliate Fabrea largely dominated the zooplankton community. Temperature and the presence of Dunaliella salina as prey appeared as key factors in controlling the abundance of Artemia, while organic detritus appeared as important in the diet of Fabrea. Change in zooplankton species composition along the hypersaline gradient (40–90) was primarily related to salinity. However, our data suggest the importance of both the abundance and composition of food in the spatial and temporal variations of some zooplankton species.     

Distribution patterns of micro- and meso-zooplankton communities in sea ice regions of Lützow-Holm Bay, East Antarctica

In the Southern Ocean, zooplankton research has focused on krill and macro-zooplankton despite the high densities of micro- and meso-zooplankton. We investigated their community structure in relation to different sea ice conditions around Japan’s Syowa Station in Lützow-Holm Bay, in the summers of 2011 and 2012. Zooplankton samples were collected using vertical hauls (0–150 m), with a closing net of 100-μm mesh size. The results of cluster analysis showed that the communities in this region were separated into fast ice, pack ice, and open ocean fauna. The fast ice fauna had lower zooplankton abundance (393.8–958.9 inds. m^?3) and was dominated by cyclopoid copepods of Oncaea spp. (54.9–74.8 %) and Oithona similis (6.6–19.9 %). Deep-water calanoid copepods were also found at the fast ice stations. Pack ice and open ocean fauna had higher zooplankton abundance (943.6–2,639.8 inds. m^?3) and were characterized by a high density of foraminiferans in both years (6.6–61.9 %). Their test size distribution indicated that these organisms were possibly released from melting sea ice. The pteropod Limacina spp. was a major contributor to total abundance of zooplankton in the open ocean zone in 2012 (26.4 %). The physical and/or biological changes between 2 years may affect the abundance and distribution of the dominant zooplankton taxa such as cyclopoid copepods, foraminiferans, and pteropods. Information on the relationships between the different species associated with sea ice will help to infer the possible future impacts of climate change on the sea ice regions.     

The effects of flood pulse on multiple aquatic organisms in a seasonal shallow lake

Different aquatic organisms have different reactions to environmental variations due to their different ways of reaction traits. Understanding the effect of hydrological disturbance on lake from the perspective of multiple aquatic organisms is important for lake management. Poyang Lake (PYL) experiences severe hydrological disturbance under the effects of flood pulse during the rainy season. In this study, we analyzed the responses of phytoplankton, zooplankton, and macroinvertebrates to hydrological disturbance during the rainy season in PYL. Flood pulse determined the spatial variation in turbidity (Turb), transparency (Trans), dissolved oxygen (DO), total phosphorus (TP), and soluble reactive phosphorus. Physical factors (Turb, Trans, conductivity) explained phytoplankton (9.6%) and zooplankton (15%) community variation, and the combination of physical and nutrient factors explained macroinvertebrate (3.8%) community variation. Cyclotella sp., Attheya zachariasi, and Melosira ambigua were the keystone taxa of phytoplankton, and these taxa were driven by Turb and Trans. Brachionus angularia, Polyarthra vulgaris, Filinia longiseta, and Diffugia globulosa were the keystone taxa of zooplankton and entire aquatic organisms, which contributed a lot to maintain the biological community stability possibly through the food web. These keystone taxa were stimulated by high Trans, DO, dissolved organic carbon (DOC) and nitrate (NO₃^?). We found habitat conditions (Turb, Trans and DO) were the prominent factors influencing the aquatic organism structure in strong hydrological disturbance environment. For the keystone taxa, both habitat conditions (i.e., DO) and exogenous nutrient inputs (i.e., DOC and NO₃^?) caused significant effect. This study provides new insights into the holistic response of multiple aquatic communities to flood pulse as well as the role of keystone species in maintaining community stability, which could guide the conservation and management of seasonal lake ecosystems.

     

Identifying biotic drivers of population dynamics in a benthic–pelagic community

Benthic species and communities are linked to pelagic zooplankton through life‐stages encompassing both benthic and pelagic habitats and through a mutual dependency on primary producers as a food source. Many zooplankton taxa contribute to the sedimentary system as benthic eggs. Our main aim was to investigate the nature of the population level biotic interactions between and within these two seemingly independent communities, both dependent on the pelagic primary production, while simultaneously accounting for environmental drivers (salinity, temperature, and oxygen conditions). To this end, we applied multivariate autoregressive state‐space models to long (1966–2007) time series of annual abundance data, comparing models with and without interspecific interactions, and models with and without environmental variables included. We were not able to detect any direct coupling between sediment‐dwelling benthic taxa and pelagic copepods and cladocerans on the annual scale, but the most parsimonious model indicated that interactions within the benthic community are important. There were also positive residual correlations between the copepods and cladocerans potentially reflecting the availability of a shared resource or similar seasonal dependence, whereas both groups tended to correlate negatively with the zoobenthic taxa. The most notable single interaction within the benthic community was a tendency for a negative effect of Limecola balthica on the amphipods Monoporeia affinis and Pontoporeia femorata which can help explain the observed decrease in amphipods due to increased competitive interference.     

Strategies of zooplanktivory shape the dynamics and diversity of littoral plankton communities: a mesocosm approach

Planktivorous fish can exert strong top‐down control on zooplankton communities. By incorporating different feeding strategies, from selective particulate feeding to cruising filter feeding, fish species target distinct prey. In this study, we investigated the effects of two species with different feeding strategies, the three‐spined stickleback (Gasterosteus aculeatus (L.)) and roach (Rutilus rutilus (L.)), on a low‐diversity brackish water zooplankton community using a 16‐day mesocosm experiment. The experiment was conducted on a small‐bodied spring zooplankton community in high‐nutrient conditions, as well as a large‐bodied summer community in low‐nutrient conditions. Effects were highly dependent on the initial zooplankton community structure and hence seasonal variation. In a small‐bodied community with high predation pressure and no dispersal or migration, the selective particulate‐feeding stickleback depleted the zooplankton community and decreased its diversity more radically than the cruising filter‐feeding roach. Cladocerans rather than copepods were efficiently removed by predation, and their removal caused altered patterns in rotifer abundance. In a large‐bodied summer community with initial high taxonomic and functional diversity, predation pressure was lower and resource availability was high for omnivorous crustaceans preying on other zooplankton. In this community, predation maintained diversity, regardless of predator species. During both experimental periods, predation influenced the competitive relationship between the dominant calanoid copepods, and altered species composition and size structure of the zooplankton community. Changes also occurred to an extent at the level of nontarget prey, such as microzooplankton and rotifers, emphasizing the importance of subtle predation effects. We discuss our results in the context of the adaptive foraging mechanism and relate them to the natural littoral community.     

Near-bottom depletion of zooplankton over a coral reef II: relationships with zooplankton swimming ability

Intense predation by corals and associated fauna can generate vertical gradients of their zooplankton prey. The goal of this study was to characterize the small-scale distribution of zooplankton above the coral reef of Eilat, Israel. Four vertical arrays, each consisting five underwater pumps attached 0.5–3 m apart on a taut mooring, were deployed for 10 days at two sites over the reef slope. A distinct layer of depleted zooplankton was repeatedly found in the 1.5 m high benthic boundary layer. The gradient was sharpest for strong swimmers (copepods and polychaeta), intermediate for weak swimmers (nauplii, mollusks and appendicularia), and lacking for passive taxa (eggs and foraminifera). Spatio-temporal changes in the abundance of copepods and polychaetes were highly correlated, more so in the water aloft (Pearson r>0.9) than near the bottom (r>0.8). The spatio-temporal correlations between weak swimmers and between passive taxa were much weaker (r<0.8 and r<0.4, respectively). Flow and shear stress did not affect the distribution patterns. The correspondence between the zooplankton distribution and their swimming ability indicates that bottom avoidance contribute to the formation of depleted layer over the reef.     

Growth of pike larvae: effects of prey, turbidity and food quality

We studied experimentally the effects of turbidity and prey composition on pike larval growth and hypothesized that pike larval growth varies with turbidity and food quality. We reared the first-feeding pike larvae (Esox lucius) in laboratory tanks with (1) clear or (2) turbid water provided with zooplankton rations from (3) an inner and (4) an outer archipelago site. The sites differ in physical features, salinity, eutrophication status, zooplankton community structure and density. Pike larvae showed the highest weight increase in clear water with zooplankton from the outer site and the poorest weight increase in turbid water with zooplankton as prey from the inner site. Our fatty acid analysis revealed that unsaturated fatty acid levels were highest in the outer site. The relative percentage of copepods was also higher in the outer site. This study supports the hypothesis that turbidity weakens the ability of pike larvae to capture certain prey. Further, zooplankton community composition matters in turbid water, but is not a primary factor in clear water.