Crustacean Zooplankton Biomass Research Articles

Spatial Density, Biomass, and Composition of Crustacean Zooplankton on Lake Michigan Beaches

Ecosystem alterations to Lake Michigan (LM), such as the invasion of dreissenid mussels, have decreased the abundance of phytoplankton and, subsequently, zooplankton, which has implications for Great Lake fisheries. The community composition of zooplankton has also been altered over the past 20 years in LM, shifting the summer dominance from cladoceran to calanoid taxa. However, most of our information on zooplankton community dynamics is from deeper pelagic zones. The nearshore beach (≤1 m) habitat, which serves as a critical nursery for some larvae and juvenile fish, is composed of different zooplankton taxa than deeper zones, but limited data are available for comparison. We conducted a standardized summer sampling campaign to characterize the zooplankton community across 32 beaches in LM. We found the lowest crustacean zooplankton density and biomass on northern LM beaches and a higher zooplankton density and biomass on Greater Green Bay and central LM beaches, which had warmer water temperatures. Smaller-bodied cladocera (mainly Bosmina) and cyclopoid taxa were the most abundant groups on beaches. Our results provide the first comprehensive characterization of zooplankton on beaches in the Great Lakes, with applications for other lake systems. These results may help identify potential “hot spots” of fish recruitment for management.

Importance of long-term intensive monitoring programs for understanding multiple drivers influencing Lake Ontario zooplankton communities

Drivers of lower food web composition and productivity in Lake Ontario have undergone extensive changes in the last 40 years, including nutrient abatement, fluctuations in planktivores (Alewife), and invasion by dreissenid mussels and predatory cladocerans. Temporally intensive long-term index stations are critical for understanding these drivers and interpreting the results of periodic lake-wide spatially intensive surveys such as Cooperative Science and Monitoring Initiative (CSMI). We compare epilimnetic physical–chemical parameters and zooplankton metrics at a Kingston Basin biomonitoring site (Station 81) over three time stanzas (1981–1986, 1987–1995 and 2007–2017). In the most recent stanza, mean May-October temperature increased by 2.5 °C, and despite static total phosphorus levels, chlorophyll has significantly decreased and Secchi depth has increased. Between Stanzas 2 and 3, epilimnetic density, biomass and production of crustacean zooplankton have declined by 88%, 79% and 67%, respectively. Bosminids, Daphnia retrocurva, Diacyclops and juvenile cyclopoids are most impacted, whereas larger taxa (calanoids, Daphnia galeata, Holopedium and predatory cladocerans) have remained stable or increased. While some taxa have increased in size over time, zooplankton egg ratios have remained stable. Dreissenid veligers are now numerically dominant and have replaced some of the lost crustacean production. Redundancy Analysis showed environmental drivers (Secchi and temperature) significantly influenced zooplankton during the 1981–1995 period but not in the recent stanza. Alewife were not a significant driver despite substantial declines since the 1990s. Resource competition by Dreissena for the strongly reduced phytoplankton productivity, combined with predation by invasive cladocerans Cercopagis and Bythotrephes have also likely influenced Kingston Basin zooplankton.

Strong Behavioral Effects of Omnivorous Fish on Amphibian Oviposition Habitat Selection: Potential Consequences for Ecosystem Shifts

Perceived predation risk to offspring may have similar ultimate community-level impacts to those of consumptive trophic interactions. The present study investigated the behavioral effects of common carpCyprinus carpio–an omnivorous fish capable of triggering an ecosystem shift to an algae-dominated state–on anurans, using a natural experiment conducted in a system of fish-stocked ponds. We compared oviposition patterns and larval densities of anurans and abundance of zooplankton and phytoplankton in ponds where fish were virtually absent and ponds where common carp was dominant. All studied anuran species bred in fish-poor ponds, while in ponds with high fish densities most of them oviposited infrequently or virtually did not breed. Oviposition habitat selection coupled with fish trophic pressure resulted in diametrically different tadpole densities between fish-poor and fish-dominated ponds. The alimentary tract contents of tadpoles of three locally common anurans,Pelobates fuscus,Hyla orientalis, andRana temporaria, contained large numbers of unicellular algae, but also significant numbers of zooplanktonic grazers. According to stable nitrogen isotope analyses, tadpoles occupied a trophic level similar to a primary consumer, snailLymnaea stagnalis, indicating that they fed mainly on algae. While total biomass of crustacean zooplankton did not differ between pond types, chlorophyllaconcentrations were low in fish-poor ponds compared to fish-dominated ponds and negatively related to total tadpole biomass. Our findings indicate that scarcity of anuran larvae, resulting mainly from the behavioral responses of breeding anurans to fish predation risk, may facilitate algal production, although ecosystem shifts in the presence of fish occur along more pathways than a top-down cascade.

Seven Years Study of the Seasonal Dynamics of Zooplankton Communities in a Large Subtropical Floodplain Ecosystem: A Test of the PEG Model.

Irregular hydrological events, according to a classic plankton ecology group (PEG) study, can generate major deviations from the standard PEG model. However, little is known about the function of hydrological factors in influencing the seasonal dynamics of plankton. We used multivariate and Partial Least Squares Path Modeling to analyze the seasonal variation in crustacean zooplankton and related environmental factors from winter 2009 to winter 2016 in Lake Poyang, the largest freshwater lake in China. We found a distinct seasonal pattern in zooplankton development, which deviated, in part, from the PEG model, as we found indications of (1) a weaker degree of food limitation in winter and spring, likely due to high concentrations of allochthonous sources caused by decomposition of seasonally flooded hygrophytes, also affecting sediment dynamics; (2) a peak in crustacean zooplankton biomass in summer when the water level was high (and predation was lower), and where horizontal transport of zooplankton from the littoral zone to the pelagic was possibleand (3) a higher predation pressure in autumn, likely due to a shrinking water volume that left the fish concentrated in less water. The majority of these differences can be attributed to the direct or indirect impacts of physical factor variation.

Drivers of variation in crustacean zooplankton production rates differ across regions off the west coast of Vancouver Island and in the subarctic NE Pacific

Abstract The subarctic NE Pacific is comprised of several oceanographic regimes, in which regional variability in sea surface temperature (SST), satellite chlorophyll a, and crustacean zooplankton biomass influence the production rates of crustacean zooplankton. Traditional methods for estimating zooplankton production rates are labour/time intensive and restricted to select copepod species. A practical field alternative is the ‘chitobiase method’, which yields community-level biomass production rates (BPR) analogous to traditional moulting rate methods. BPR was measured along the west coast of Vancouver Island and in the subarctic NE Pacific during 2005, 2009–2011, and 2015–2018. Generalized additive modelling identified SST and the developing crustacean zooplankton biomass as key drivers of BPR variability. BPR varied positively with the proportion of developing copepod biomass relative to that of non-copepod crustacean zooplankton biomass. Our analysis indicates that BPR variation is associated with zooplankton community composition, but that high zooplankton biomass is not necessarily predictive of high BPR. Specifically, higher BPR is associated with a higher relative biomass of large-bodied, cold-water indicator species, and a lower biomass of non-copepod crustaceans.

Spatial and temporal trends of Minnesota River phytoplankton and zooplankton

AbstractPlankton communities have important roles in aquatic ecosystems, but studies of plankton in lotic systems are infrequent. We collected over 100 water, phytoplankton, and zooplankton samples during 2016–2018 to explore spatiotemporal trends in Minnesota River plankton communities and evaluate relationships with physico‐chemical factors. Phytoplankton and zooplankton community structure exhibited temporal patterns but only the zooplankton community differed spatially. Cyanobacteria (M ± SE; 11.27 ± 1.43 mm3/L) and diatoms (8.12 ± 1.08 mm3/L) dominated phytoplankton biovolume with seasonal peaks in Cyanobacteria occurring during July–September and peaks in diatoms occurring during May, August, and September. All phytoplankton taxa except Cryptophyta exhibited a negative relationship with relative discharge. Crustacean zooplankton biomass was greatest at two upstream sites (146.7 ± 32.6 μg/L) where cladocerans and copepods were likely exported from upstream of dams where water residence time is greater. Within the lower free‐flowing reach rotifers dominated the zooplankton community (207.9 ± 40.9 individuals/L and 6.5 ± 1.0 μg/L). Thus, spatial differences in zooplankton community structure were primarily attributed to the influence of dams. Seasonal patterns in zooplankton community structure included peaks in Chydoridae, cyclopoid, immature copepod, and rotifer biomass during May and Bosminidae biomass during October. Excluding the influence of dams on zooplankton, the cumulative effects of month and relative discharge were the most important for explaining variability in plankton community structure. Baseline understanding of plankton community dynamics provides the ability to quantify responses to future perturbations such as climate change and establishment of invasive planktivores.

Разработка индексов трофности для Онежского озера с помощью экологической информационной системы

GIS technologies were used to study the spatial distribution of zones with different trophic conditions in the water area of Lake Onego. Empirical data on the trophic conditions of waters sampled in 1982–2020 were as the basis for the map development. Two periods, before and after 2000, were considered separately to assess the impact of climate change. As indicators of trophic conditions, the number of saprophytic bacterioplankton, biomass of crustacean zooplankton, concentration of chlorophyll “a” and water transparency were used. All characteristics were converted to a normal distribution, and the values at individual points were interpolated for the entire water area of the lake, which was presented as a network of 544 squares. Each cell of the water area was characterized by four values of indicators. To assess trophic conditions, multivariate statistics (principal component analysis) and an expert system (Bayesian classifier) were used. The first principal component can be considered as the integral value of trophic conditions in a given part of the water area. The Bayesian classification method allows to get an estimate of the probability of classifying each cell in the water area as “high trophic conditions” and “low trophic conditions” and assign a significance threshold (p = 0.05). The cartograms built on the basis of both methods are basically the same. Most of Lake Onego area is characterized as low trophic conditions. The vast area with high trophic conditions was identified in the northwestern part of Lake Onego where on the coast large cities are located (Petrozavodsk and Kondopoga), which is associated with active anthropogenic pollution in the last century. In recent years, due to industry recession, trophic conditions in this area has sharply decreased, with the exception of Kondopozhskaya Bay, where there is an increase in water pollution as a result of trout farms.

A lake-wide approach for large lake zooplankton monitoring: Results from the 2006–2016 Lake Superior Cooperative Science and Monitoring Initiative surveys

Whole-lake surveys of Lake Superior were completed during late summer in 2006, 2011, and 2016 to assess lower food web conditions under the Cooperative Science and Monitoring Initiative (CSMI). These surveys used a spatially stratified probability approach based on depth to assess food web conditions within different depth zones. We evaluated differences in crustacean zooplankton biomass, rotifer density, and the community structure of both groups in nearshore (<30 m), midshore (30–100 m), and offshore (>100 m) depth zones and investigated changes in these parameters within zones over time. Although nearshore crustacean biomasses and rotifer densities were highly variable, the depth zones differed from each other based on these parameters and should be considered separately. Crustacean biomass, community structure, and vertical position were consistent over time across depth zones. The differences that did occur were within the range of known annual variability. Total rotifer densities were lower in 2016 than in 2006 in all depth zones, but the genera that contributed to the lower values were not the same across zones. Further studies are needed to know whether these differences reflect annual variability or long-term trends. Finally, we show how the depth zones used in this study can facilitate comparisons between monitoring programs. This is important because most zooplankton studies are limited to certain depth zones and changes in zooplankton parameters may not occur uniformly across zones. The high variability in nearshore zooplankton parameters suggests that additional research may be needed to effectively track changes there.

Hypolimnetic Hypoxia Increases the Biomass Variability and Compositional Variability of Crustacean Zooplankton Communities

In freshwater lakes and reservoirs, climate change and eutrophication are increasing the occurrence of low-dissolved oxygen concentrations (hypoxia), which has the potential to alter the variability of zooplankton seasonal dynamics. We sampled zooplankton and physical, chemical and biological variables (e.g., temperature, dissolved oxygen, and chlorophyll a) in four reservoirs during the summer stratified period for three consecutive years. The hypolimnion (bottom waters) of two reservoirs remained oxic throughout the entire stratified period, whereas the hypolimnion of the other two reservoirs became hypoxic during the stratified period. Biomass variability (measured as the coefficient of the variation of zooplankton biomass) and compositional variability (measured as the community composition of zooplankton) of crustacean zooplankton communities were similar throughout the summer in the oxic reservoirs; however, biomass variability and compositional variability significantly increased after the onset of hypoxia in the two seasonally-hypoxic reservoirs. The increase in biomass variability in the seasonally-hypoxic reservoirs was driven largely by an increase in the variability of copepod biomass, while the increase in compositional variability was driven by increased variability in the dominance (proportion of total crustacean zooplankton biomass) of copepod taxa. Our results suggest that hypoxia may increase the seasonal variability of crustacean zooplankton communities.

Unclear associations between small pelagic fish and jellyfish in several major marine ecosystems

During the last 20 years, a series of studies has suggested trends of increasing jellyfish (Cnidaria and Ctenophora) biomass in several major ecosystems worldwide. Some of these systems have been heavily fished, causing a decline among their historically dominant small pelagic fish stocks, or have experienced environmental shifts favouring jellyfish proliferation. Apparent reduction in fish abundance alongside increasing jellyfish abundance has led to hypotheses suggesting that jellyfish in these areas could be replacing small planktivorous fish through resource competition and/or through predation on early life stages of fish. In this study, we test these hypotheses using extended and published data of jellyfish, small pelagic fish and crustacean zooplankton biomass from four major ecosystems within the period of 1960 to 2014: the Southeastern Bering Sea, the Black Sea, the Northern California Current and the Northern Benguela. Except for a negative association between jellyfish and crustacean zooplankton in the Black Sea, we found no evidence of jellyfish biomass being related to the biomass of small pelagic fish nor to a common crustacean zooplankton resource. Calculations of the energy requirements of small pelagic fish and jellyfish stocks in the most recent years suggest that fish predation on crustacean zooplankton is 2–30 times higher than jellyfish predation, depending on ecosystem. However, compared with available historical data in the Southeastern Bering Sea and the Black Sea, it is evident that jellyfish have increased their share of the common resource, and that jellyfish can account for up to 30% of the combined fish-jellyfish energy consumption. We conclude that the best available time-series data do not suggest that jellyfish are outcompeting, or have replaced, small pelagic fish on a regional scale in any of the four investigated ecosystems. However, further clarification of the role of jellyfish requires higher-resolution spatial, temporal and taxonomic sampling of the pelagic community.

Effects of sediment dredging on crustacean zooplankton community of Shuangqiao River in Chaohu Basin

为研究河流底泥疏浚工程对富营养化河流水生生态系统的影响，选取巢湖双桥河开展为期一年的河流生态监测与调查，对比分析了巢湖双桥河疏浚前和疏浚后的水质指标、浮游甲壳动物群落结构及生物多样性指数.研究发现：底泥疏浚后双桥河水质得到明显改善，其中氮、磷营养盐分别削减48.09%和19.09%；底泥疏浚对浮游甲壳动物群落结构影响显著，疏浚后浮游甲壳动物生物量从1.86 mg/L下降到0.25 mg/L，下降了86.40%；疏浚后生物多样性指数高于疏浚前，且受季节影响较大；浮游甲壳动物与水环境因子相关性研究发现：pH、温度、铵态氮、硝态氮、可溶性氮、总氮和氮磷比对浮游甲壳动物总生物量均有极显著影响.研究结果表明，河流底泥疏浚工程能显著削减水体中氮、磷营养盐，并从一定程度上改善河流浮游甲壳动物群落结构，促进河流水生生态系统向健康、安全的方向发展.因此，对于双桥河这种受城市外源污染影响较大的城市污染型河流，在削减外源营养盐特别是氮输入的条件下，实施底泥疏浚工程可以促进其水生生态系统的修复.;The effects of sediment dredging events on the aquatic ecosystem of Shuangqiao River in Chaohu Basin were researched in the present study, a one-year investigation on the aquatic ecosystem operated in the river. The water quality, the crustacean zooplankton community and the biodiversity index were compared and analyzed pre-with post-dredging. Our results indicated that:The sediment dredging events improved the water quality of Shuangqiao River significantly, and the concentration of nitrogen (N)and phosphorus (P)decreased by 48.09% and 19.09%, respectively. The sediment dredging events had a significant influence on the crustacean zooplankton community, the biomass of crustacean zooplankton decreased from 1.86 mg/L to 0.25 mg/L, which declined by 86.40%. The biodiversity index was mainly affected by sediment dredging and season. There's significant relationships between crustacean zooplankton and environment factors:pH, temperature, ammonium, nitrate, total dissolved nitrogen, total nitrogen and nitrogen:phosphorus ratio were positive to the biomass of crustacean zooplankton. Our results suggested that the sediment dredging would reduce the internal nutrients of river significantly. Furthermore, the sediment dredging events had a significant effect on the crustacean zooplankton community. The aquatic ecosystems of river would be healthy and safety by the operation of sediment dredging. Therefore, the sediment dredging would be beneficial for the aquatic ecosystem, while the eutrophication management of river was not only attribute to the removal of urban external pollution, but also to the exogenous pollution, especially for the reduction of N input.

Preliminary evaluation of ecological effects of silver and bighead carps to control cyanobacterial blooms in the early eutrophication lakes

放养鲢（Hypophthalmichthys molitrix）、鳙（Aristichthys mobilis）控制富营养化湖泊蓝藻水华暴发是当前最有效的手段之一.为评估富营养化初期湖泊鲢、鳙控藻的生态效果，2016年在洱海红山湾开展原位围栏鲢、鳙控藻实验，对围栏内外的浮游生物群落结构及水环境因子的变化进行对比分析，同时测定了围栏内鲢、鳙的生长及摄食节律.结果表明：围栏内鲢、鳙呈现匀速生长，围栏生存环境适宜.根据Egger's模型估算鲢、鳙的日摄食量分别为24.96%和18.18%，鲢的日粮高于鳙；滤食率结果表明，鲢对浮游植物的摄食率高于鳙（分别为3.01和2.19 L/（g·h），而鳙对浮游动物的滤食率高于鲢（分别为18.61和13.54 L/（g·h））.研究期间，围栏内外水体理化因子无显著差异；但围栏内外微囊藻生物量差异明显，并且在7月达到生物量峰值（围栏内为1.7 mg/L，围栏外为2.4 mg/L）.鲢、鳙对浮游植物的优势种类——微囊藻和隐藻有明显的削减效果，6-8月逐渐上升并在8月达到最高（64%）；当浮游植物优势种由微囊藻替换为硅藻门和绿藻门的种类时，鲢、鳙的削减效率不明显（10-11月为负值）.此外，鲢、鳙放养显著降低了浮游甲壳动物的总生物量和枝角类生物量.因此，结合实验结果得出，鲢具有更高的控藻能力，尤其对微囊藻水华，但同时具备了较高对浮游甲壳动物的削减能力，需要进一步分析富营养化初期湖泊——洱海中实施非经典生物操纵的适宜性.;It is one of the most effective ways to control the outbreak of cyanobacteria blooms in eutrophic lakes by stocking silver and bighead carps. In order to evaluate the ecological effects of controlling algae by silver and bighead carps in early eutrophication lakes. In 2016, a little biomanipulation in-situ pen with the stock of silver and bighead carps was built to control cyanobacterial blooms in Hongshan Bay of Lake Erhai. The changes of plankton community structure and water environmental factors were compared and analyzed within and outside the pen. Meanwhile, the growth and feeding rhythm of silver and bighead carps were measured. The pen-cultured silver and bighead carp all displayed normal growth, the survival environment of the pen is suitable. Daily rations of silver and bighead carp were estimated by Egger's model in the main growing season. Filtration rate was calculated from the daily ration and the density plankton in the lake. Daily rations of silver and bighead carp was 24.96% and 18.18%. The daily rations silver carp is higher than that of bighead carp. Filtration rates of silver and bighead carp for phytoplankton were 3.01 L/(g·h) and 2.19 L/(g·h), respectively, and filtration rates for zooplankton were 13.54 L/(g·h) and 18.61 L/(g·h), respectively. Silver carp had a stronger ability of eliminating phytoplankton than bighead carp. Bighead carp had a stronger ability of eliminating zooplankton than silver carp. During the study, there was no significant difference in the environmental parameters within and outside the pen. The biomass of Microcystis reached its peak in July (1.7 mg/L in the fish pen and 2.4 mg/L in the surrounding lake water). The Microcystis biomass was significantly lower in the pen. The present stocking density of silver and bighead carp(about 20 g/m³ in June)was effective to control Microcystis. From June to August, the reduction rate of the phytoplankton in the silver and bighead carp gradually increased, and the rate of reduction was up to 64% in August. When phytoplankton's dominant species Chlorophyta and diatom replace Microcystis, the rate of reduction decreased gradually, with negative values in October and November. Furthermore, the biomass of crustacean zooplankton biomass and cladocerans biomass were significantly lower in the pen. Therefore, in combination with several experimental results, silver carp had a stronger ability of eliminating phytoplankton than bighead carp. Especially for Microcystis blooms, silver carp at the same time have the higher on the reduction ability of crustacean zooplankton. We need to further analyze the suitability of non-traditional biological manipulation in the early stage of eutrophication Lake Erhai.

Warming Strengthens the Ecological Role of Intraspecific Variation in a Predator

Recent work shows communities and ecosystems can be shaped by predator intraspecific variation, but it is unclear whether the magnitude and direction of these influences are context-dependent. Temperature is an environmental context of strong ecological influence and widespread relevance given global warming trends. Warming should increase per capita predator effects on prey through increases in predator metabolic rate, potentially exacerbating intraspecific differences in ecological effects. Here, we used two populations of the potent pelagic freshwater predator, Western Mosquitofish (Gambusia affinis), to test how experimental pond temperature mediates the differences between their ecological impacts. Mosquitofish introduction induced a strong pelagic trophic cascade, causing a large reduction of crustacean zooplankton biomass, an increase in phytoplankton biomass, and changes to ecosystem-level response variables. Warming (+2°C above unwarmed treatments) exacerbated fish-induced reduction of zooplankto...

Physicochemical Process, Crustacean, and Microcystis Biomass Changes In Situ Enclosure after Introduction of Silver Carp at Meiliang Bay, Lake Taihu.

In order to control cyanobacteria blooms with silver carp in Lake Taihu, an in situ experiment was carried out by stocking silver carp at a biomass of 35, 70, and 150 g m−3 and no carp control in waterproof enclosures. Physicochemical water parameters and biomass of plankton were measured in enclosures to evaluate the suitable stocking density of silver carp for relieving internal nutrients and constraining cyanobacteria growth in Lake Taihu. It is found that the 35 g m−3 silver carp group and 70 g m−3 silver carp group presented lower total phosphorus, lower chlorophyll-a, and higher water transparency. Increased nitrogen to phosphorus ratio, which indicated the result of algae decline in fish presence enclosures, was attributed to decline of phosphorus. Phosphorus decline also exerted limitation on reestablish of cyanobacteria bloom. Crustacean zooplankton biomass and Microcystis biomass decreased significantly in fish presence enclosures. Silver carp could be more effective to regulate algae bloom in enclosures with dense cyanobacteria. Therefore, nonclassic manipulation is supposed to be appropriate method to get rid of cyanobacteria blooms in Lake Taihu by stocking 35 to 70 g m−3 silver carp in application.

Stuck between a rock and a hard place: zooplankton vertical distribution and hypoxia in the Gulf of Finland, Baltic Sea

Zooplankton often migrate vertically to deeper dark water during the day to avoid visual predators such as fish, a process which can strengthen benthic–pelagic coupling. In the Gulf of Finland, Baltic Sea, a pronounced hypoxic layer develops when there is an inflow of anoxic bottom water from the Central Baltic Sea, which could be a barrier for vertical migrants. Here, we report an acoustic study of the distributions of crustacean zooplankton (mysid shrimp and the copepod Limnocalanus macrurus), gelatinous zooplankton (Aurelia aurita) and fish. Zooplankton trawl nets were used to ground-truth acoustic data. Vertical profiles of oxygen concentration were taken, and the physiological impact of hypoxia on mysids was investigated using biochemical assays. We hypothesised that the vertical distribution of zooplankton and fish would be significantly affected by vertical heterogeneity of oxygen concentrations because anoxia and hypoxia are known to affect physiology and swimming behaviour. In addition, we hypothesised that mysids present in areas with hypoxia would exhibit a preparatory antioxidant response, protecting them from oxidative damage during migrations. The acoustic data showed peaks of crustacean zooplankton biomass in hypoxic ( 75 m), whereas fish shoals and A. aurita medusae were found in normoxic (5–6 mL L−1) upper water layers (<40 m), with individual fish in deeper water excepting that rule. Mysid shrimp from areas with hypoxia had significantly enhanced antioxidant potential compared with conspecifics from areas with no hypoxia and had no significant indications of oxidative damage. We conclude that mysids can protect themselves from oxidative damage, enabling them to inhabit hypoxic water. Our data suggest that hypoxic and low oxygen zones (up to 4 mL L−1) may provide some zooplankton species with a refuge from visual predators such as fish.

Abundance and spatial distribution of Mysis diluviana in Lake Ontario in 2008 estimated with 120 kHz hydroacoustic surveys and net tows

Mysids are an important component of Great Lakes foodwebs, both as a prey for fish and as a predator on zooplankton. We monitored mysid abundance in Lake Ontario using lake-wide hydroacoustics data and vertical net hauls collected 1–5 August 2008 during the Ontario Ministry of Natural Resources and New York State Department of Conservation forage fish survey. Acoustic volume backscattering strength was highly correlated with both density and biomass of mysids although the correlation with biomass was stronger. The slopes of these relationships were not significantly different from theory (0.10) indicating a linear relationship between abundance and backscattering strength. Size structure significantly affected the relationship between backscattering strength and density but not between backscattering strength and biomass. Average target strength for areas with small mysids was −91.9 dB per mysid, and in areas with both small and large mysids it was −88.2 dB per animal. Acoustic estimates for Lake Ontario calculated with these regressions provided a lake-wide estimate weighted by lake area within depth intervals, of 228 mysids m−2 (CV 17.6%) and 0.93 g dw m−2 (CV 12.9%). Density and biomass in 14 net hauls averaged 316 mysids m−2 (range 0 to 1113) and 1.35 g dw m−2 (range 0 to 3.89). Mysid biomass was 33% of the biomass of other crustacean zooplankton in the summer of 2008. As in 2005, areas of low mysid abundance were detected in the middle of the transects and along the north shore. Lakewide mysid abundance in the summer of 2008 was higher than in a similar acoustic survey in the summer of 2005 and in whole-lake autumn 2000–2007 net surveys, suggesting that mysids are persisting in Lake Ontario and abundant relative to other Great Lakes.

Development of crustacean plankton in a shallow, polyhumic reservoir in the first 20 years after impoundment (northeast Poland)

Multiannual changes of structure and biomass of crustacean zooplankton were studied in the shallow, lowland, polyhumic Siemianówka Reservoir on the upper Narew River in northeast Poland. High ammonium and soluble reactive phosphorus ion availability for phytoplankton, low water transparency, and high dissolved oxygen concentration resulted in humoeutrophication and caused an intensive development of summer cyanobacteria. We compiled and analyzed long-term (1993–2011) crustacean zooplankton community data and chlorophyll a concentrations. Zooplankton biomass and structure in the summer were related to the intensity of the cyanoprokaryota blooms. As chlorophyll a concentration increased, crustacean zooplankton biomass decreased and smaller crustacean species dominated the zooplankton structure. Large species of crustaceans (Daphnia longispina, D. magna, and Eudiaptomus graciloides) disappeared from the reservoir after filamentous cyanobacteria dominated the phytoplankton. Long-term studies suggest that phytoplankton with strong dominance of filamentous cyanobacteria shape the crustacean community but are probably an insignificant food source for the crustacean zooplankton in the Siemianówka Reservoir.

Biomass and productivity responses of zooplankton communities to experimental thermocline deepening

Lake thermocline depth is likely to be altered with climate change. We assessed the response of crustacean zooplankton biomass and productivity to an experimental whole‐lake manipulation of thermocline depth. Weekly sampling occurred in Experimental and Reference years and responses were assessed with a before—after‐control—impact design. While one of three lake basins remained un‐manipulated (control), two others experienced thermocline deepening via: active mixing simulating increased wind‐stress (‘mixing with thermocline deepening’); and increased overall heat content, simulating greater water clarity (‘thermocline deepening’). Thus, we simultaneously identified seasonal responses to three treatments: mixing with thermocline deepening, thermocline deepening alone, and an isolated mixing effect. Total crustacean zooplankton biomass was enhanced by all treatment effects, owing to positive responses to thermocline deepening in Bosmina and Daphnia and a positive cyclopoid copepod response to mixing. Enhancement of biomass production rates (BP) occurred only with thermocline deepening, with no effect of mixing alone. The lack of BP response to active mixing was likely due to compositional shifts from fast growing Daphnia to slower growing cylopoids. BP variation was explained by positive and negative relationships with mean water‐column temperature and hypolimnetic temperature, respectively. Hypolimnetic warming, occurring naturally with the seasonal thermocline descent was more dramatic for the active mixing manipulation. This loss of hypolimnetic refuge for Daphnia appears to have had a negative effect on the community daily production to biomass ratio (P : Bdaily). Our study indicates a greater likelihood for reduced food‐web transfer efficiency under scenarios when mixing and hypolimnetic warming accompany thermocline deepening.

Annual variation in biomass and the community structure of crustacean zooplankton over 5 years in Lake Toya, Japan

The biomass and population dynamics of crustacean zooplankton were determined in oligotrophic Lake Toya in Japan over 5 years from May 1992 to May 1997. In 1992 and 1993, zooplankton biomass was up to 4.3 g dry weight m−2, whereas it decreased to <1 g dry weight m−2 after 1994. This extreme change in biomass was associated with the succession of dominant species from larger ones, such as Daphnia longispina and Cyclops strenuus (s. lat.), to smaller ones, such as Eubosmina tanakai and Bosmina longirostris. Consequently, this biomass change seemed to cause an increase in the chlorophyll a concentration in the euphotic zone and a decline in lake transparency. Because the birth rates of the dominant species were somewhat higher after 1994, the decline in the populations of larger crustaceans seemed to depend more on their rate of death rather than rate of birth, and this higher death rate is not considered to be attributed to food shortage. Although these results strongly suggest a top-down cascading effect of fish predation upon crustaceans, annual catches of two commercially important planktivorous fish species have also decreased in the lake, coincidentally with decreases in zooplankton biomass. This may be attributable to fishing regulations that prohibit catching smaller fish, implying that such smaller fish affect zooplankton and phytoplankton, as well as lake transparency.

Testing the light : nutrient hypothesis in an oligotrophic boreal lake

Anthropogenic changes in the nitrogen (N), phosphorus (P), and carbon (C) cycles have altered nutrient concentrations and the light climate in freshwaters globally. These factors affect phytoplankton (PPr) and bacterial production (BP), which constitute the basal energy resource for higher trophic levels in the pelagic zone of lakes. The light : nutrient hypothesis (LNH) predicts that although basal production decreases at low light, seston C : nutrient ratios also decrease, thus increasing food quality for crustacean zooplankton and potentially offsetting the negative effects of reduced food availability. We tested the LNH in an oligotrophic boreal lake by manipulating N, P, C and reducing light, and measuring PPr, BP, seston C : nutrient ratios and zooplankton biomass in 32 mesocosms. Low light strongly reduced zooplankton biomass in contrast to LNH predictions. PPr did not decrease with low light as predicted by the LNH, however, the phytoplankton community shifted towards low light adapted, but potentially less edible phytoplankton species, such as colony forming Dinobryon (Chrysophyta) and gymnoid (Dinoflagellata) taxa, which were negatively correlated with zooplankton biomass. Seston C : nutrient ratios did not decrease with reduced light, possibly due to the high abundance of mixotrophic phytoplankton across treatments. BP decreased with low light and correlations between BP, bacterial biomass, ciliates and zooplankton suggest that bacteria may be coupled with zooplankton biomass. Thus, the LNH was inadequate when predicting changes in crustacean zooplankton biomass in this typical oligotrophic boreal system, where Daphnia is rare and mixotrophic phytoplankton are abundant. Instead, alternative explanations, such as changes in phytoplankton edibility and energy transferred through the microbial food chain may need investigation to explain reduced zooplankton biomass in low light treatments.