Epilimnetic Zooplankton Research Articles

Effects of dissolved organic carbon gradient on epilimnetic zooplankton communities in lakes

Lake browning is expected to change aquatic ecosystems considerably. The changes that may occur in zooplankton communities with a high concentration of dissolved organic carbon (DOC) are little studied and not yet sufficiently understood. We studied zooplankton communities in Finland in 27 lakes with a wide DOC range. We explored how zooplankton diversity and biomass vary along a DOC gradient and how species belonging to different functional groups respond to varying DOC concentrations. The total biomass of zooplankton was not related to DOC concentration, but diversity decreased and the negative linear trend in diversity coincided with an increase in omnivorous zooplankton taxa, whereas several other herbivorous zooplankton taxa were displaced from the community along the gradient of DOC. The results of our study suggest that some well-adapted taxa, especially omnivorous taxa, can benefit from lake browning, but a larger number of taxa suffer from unfavourable conditions caused by high DOC concentration. DOC-induced changes in predation pressure and changes in phytoplankton community in terms of resource availability for zooplankton should be emphasized in future research to understand the effects of lake browning on zooplankton.

Epilimnetic zooplankton communities in soft-water lakes with isoetids in northern Poland

Soft-water lakes are valuable natural habitats and an important aquatic ecosystems. Lobelia lakes, which are classified based on the occurrence of isoetids, are the most characteristic of this group. The aim of our research was to assess the species richness and structure of pelagic zooplankton communities in the epilimnion of 45 Polish soft-water lakes. The lakes analyzed included a wide range of trophic states from oligotrophic to eutrophic. We noted 79 species of zooplankton, including 49 rotifer, 23 cladoceran, and seven copepod species. The species composition of epilimnetic zooplankton communities in these soft-water lakes was poor, and the species richness and zooplankton biomass were low. In these lakes, the species with the highest frequencies accounted for an important part of the total zooplankton biomass of the zooplankton communities, but the species occurring at lower frequencies differed among the lakes. The most common rotifer taxa were: Keratella cochlearis, Keratella cochlearis var. hispida, Polyarthra euryptera, Gastropus stylifer, and Trichocerca similis. The most important crustacean species in the soft-water lakes were: Macrocyclops albidus, Ceriodaphnia quadrangula, and Eudiaptomus sp. The major factors influencing rotifer species richness were total phosphorus, conductivity, and the trophic state of the lakes. Cladoceran species richness was positively related to lake area and pH, while the species richness of copepod communities was negatively related to chlorophyll a concentrations.

Fish-Zooplankton—A Predator-Prey Relations as a Key Factor for the Design of Zooplankton Distribution Sampling Program in Lake Kinneret, Israel

Long-term data record of Kinneret Epilimnetic Zooplankton biomass distribution and fish stock assessment was analyzed. The objective is aimed at defining the representativeness of the present sampling stations distribution for the measure of the entire Lake Kinneret zooplankton biomass. Previous studies documented the preeminence of temperature and fish predation impacts on zooplankton density: water temperature through growth rate trait and fish predation. Acoustic surveys indicated that fish shoals are mostly inhabiting the Peripheral region and much less the central part of the Kinneret pelagial. Due to fish zooplanktivory, densities of planktonic crustacean in the Pelagial periphery presented by 5 - 7 sampling stations are low. Nevertheless, large central lake area with higher densities of zooplankton biomass is presented by only one station. Therefore, stations average is incorrect as is total lake measure. The average result as indicated presently as total zooplankton biomass in lake Kinneret is, therefore, underestimated. Two optional corrective suggestions are concluded: 1) Additional sampling stations in the central part of the Pelagial region; 2) Usage of station value of aerial coefficient (promoter).

Comparing seasonal dynamics of the Lake Huron zooplankton community between 1983–1984 and 2007 and revisiting the impact of Bythotrephes planktivory

Zooplankton community composition can be influenced by lake productivity as well as planktivory by fish or invertebrates. Previous analyses based on long-term Lake Huron zooplankton data from August reported a shift in community composition between the 1980s and 2000s: proportional biomass of calanoid copepods increased while that of cyclopoid copepods and herbivorous cladocerans decreased. Herein, we used seasonally collected data from Lake Huron in 1983–1984 and 2007 and reported similar shifts in proportional biomass. We also used a series of generalized additive models to explore differences in seasonal abundance by species and found that all three cyclopoid copepod species (Diacyclops thomasi, Mesocylops edax, Tropocyclops prasinus mexicanus) exhibited higher abundance in 1983–1984 than in 2007. Surprisingly, only one (Epischura lacustris) of seven calanoid species exhibited higher abundance in 2007. The results for cladocerans were also mixed with Bosmina spp. exhibiting higher abundance in 1983–1984, while Daphnia galeata mendotae reached a higher level of abundance in 2007. We used a subset of the 2007 data to estimate not only the vertical distribution of Bythotrephes longimanus and their prey, but also the consumption by Bythotrephes in the top 20m of water. This epilimnetic layer was dominated by copepod copepodites and nauplii, and consumption either exceeded (Hammond Bay site) or equaled 65% (Detour site) of epilimnetic zooplankton production. The lack of spatial overlap between Bythotrephes and herbivorous cladocerans and cyclopoid copepod prey casts doubt on the hypothesis that Bythotrephes planktivory was the primary driver underlying the community composition changes in the 2000s.

Relative contribution of autochthonous and allochthonous carbon to limnetic zooplankton: A new cross-system approach

We measured the 13 C signature of epilimnetic CO 2 , particulate organic carbon (POC), and zooplankton in 27 lakes in late summer, and used the relationships between the POC and zooplankton signatures and the CO 2 signature to estimate the average autochthonous contribution to these fractions of the plankton and the average photosynthetic fractionation. Using this novel approach, average autochthonous contribution to epilimnetic POC was between 62 and 75 %. Epilimnetic zooplankton were between 77 and 91 % autochthonous, indicating that zooplankton bias their feeding towards the autochthonous fraction of POC. On average, zooplankton were 1.2‰ enriched in 13 C relative to POC, but their biased feeding on phytoplankton means that they can be depleted relative to POC in lakes where POC is highly depleted in 13 C. The relationship between 13 C-POC and 13 CO 2 allowed us to estimate average photosynthetic fraction as ―15.9‰. This estimate is independent of how much allochthonous C contributes to POC. Given the strength of the relationships between 13 C of POC or zooplankton with 13 CO 2 signature, we infer that variation in photosynthetic fractionation was not a major contributor to differences among lakes in POC and zooplankton 13 C signature. Allochthonous C is an important, although clearly secondary, source of C to zooplankton of these lakes in late summer.

Spatial and long-term temporal assessment of Lake Ontario water clarity, nutrients, chlorophyll a, and zooplankton

Secchi depth, total phosphorus, soluble reactive phosphorus, silica, chlorophyll a, and zooplankton (density, biomass, and average size) were sampled as part of a lake-wide, seasonal (spring, summer, and fall) assessment of Lake Ontario in 2003 to characterize the status of the lower aquatic food web. For each parameter, spatial comparisons were performed to test for differences between habitats (nearshore and offshore) and between regions (east and west) during each season. Significant differences between habitats were found only for silica and chlorophyll a; silica was higher nearshore in fall, and chlorophyll a was higher offshore in fall. Significant differences between regions were detected in Secchi depth, epilimnetic zooplankton mean length, total phosphorus and Cercopagis pengoi density and biomass; Secchi depth and zooplankton mean length were higher in the east in spring, total phosphorus and Cercopagis pengoi biomass were higher in the west in summer, and Cercopagis pengoi biomass was higher in the east in fall. Cercopagis pengoi was present lake-wide in summer and fall, but Bythotrephes longimanus was present only in fall in the Kingston basin. Mean spring total phosphorus, soluble reactive phosphorus, chlorophyll a, and epilimnetic zooplankton density and biomass are at or near record low levels. As we move into the future, persistent low levels of these lower food web elements will continue to stress alewife populations both through reduced food resources and food quality for zooplankton, and may force these fish to seek alternative food such as Mysis.

Invasive Predator, Bythotrephes, has Varied Effects on Ecosystem Function in Freshwater Lakes

Bythotrephes longimanus is an invertebrate predator that has invaded the North American Great Lakes and a number of inland lakes, where it preys on crustacean zooplankton. We examined the effect of Bythotrephes on two measures of ecosystem function during a four-month observational study of freshwater lakes on the boreal shield. Bythotrephes-invaded lakes had significantly lower epilimnetic zooplankton abundance and production compared to reference lakes. On average, Bythotrephes consumed 34% of zooplankton production when it was present in lakes. There was some evidence of changes in the timing of zooplankton production, as well as shifts to cooler, less productive habitats, which may lessen the overall effect of the invader on the transfer of energy to higher trophic levels. We experimentally demonstrated a weak trophic cascade where invader predation reduced zooplankton biomass, and subsequently increased phytoplankton growth. However, the response was small in magnitude and not biologically relevant at the whole lake-scale. The most conspicuous effect of Bythotrephes that we measured was a diversion of energy away from native predators at higher trophic levels.

Cyprinid predation on zooplankton along the longitudinal profile of a canyon-shaped reservoir

In the deep, elongated řimov Reservoir (South Bohemia, Czech Republic), the diet composition of the three most abundant cyprinid species, as well as the fish and zooplankton distribution. were investigated at three sites (dam, middle, tributary) located along the longitudinal transect of the reservoir, and during two periods of the year. The studied fish were roach (Rutilus rutilus) and bream (Abramis brama) of two length classes (10-18 cm SL and >18 cm SL), and bleak (Alburnus alburnus) of one length class (10-18 cm SL). The diets of all cyprinids along the entire longitudinal transect constituted almost exclusively crustacean zooplankton. In May, Daphnia galeata was a highly preferred prey item of all fish, except for those of the 10-18 cm length class in the tributary station, which consumed mainly bosminids. In August, bleak and roach of both length classes preyed on D. galeata and Leptodora kindtii. Larger bream (> 18 cm SL) consumed these prey items as well and, moreover, fed significantly on Diaphanosoma brachyurum. Proportions of L. kindtii in the cyprinid diets were particularly conspicuous in the dam and middle sites In the tributary site, cyclopoid copepods were an important prey item in the diets of both length classes of bream, in contrast to their infrequency in the diets of roach and bleak. Densities of epilimnetic zooplankton declined gradually downstream in the reservoir, they were about two-fold higher in the tributary site compared to the dam site in both months, respectively. A relatively higher total fish catch per unit effort was found in the tributary site in comparison to the dam or middle site, and a decreasing mean size of D. galeata in zooplankton was observed from the dam to the tributary site in both months. Such results indicated stronger fish predation pressure in the upstream part of the reservoir during the two periods of the year. Moreover, lower diet overlaps within the cyprinid assemblage of the tributary site than within the cyprinid assemblages of the dam and middle sites suggested intense competition for food at the upstream end of the reservoir.

Lake Ontario: food web dynamics in a changing ecosystem (1970–2000)

We examined stressors that have led to profound ecological changes in the Lake Ontario ecosystem and its fish community since 1970. The most notable changes have been reductions in phosphorus loading, invasion by Dreissena spp., fisheries management through stocking of exotic salmonids and control of sea lamprey (Petromyzon marinus), and fish harvest by anglers and double-crested cormorants (Phalacrocorax auritus). The response to these stressors has led to (i) declines in both algal photosynthesis and epilimnetic zooplankton production, (ii) decreases in alewife (Alosa pseudoharengus) abundance, (iii) declines in native Diporeia and lake whitefish (Coregonus clupeaformis), (iv) behavioral shifts in alewife spatial distribution benefitting native lake trout (Salvelinus namaycush), threespine stickleback (Gasterosteus aculeatus), and emerald shiner (Notropis atherinoides) populations, (v) dramatic increases in water clarity, (vi) predation impacts by cormorants on select fish species, and (vii) lake trout recruitment bottlenecks associated with alewife-induced thiamine deficiency. We expect stressor responses associated with anthropogenic forces like exotic species invasions and global climate warming to continue to impact the Lake Ontario ecosystem in the future and recommend continuous long-term ecological studies to enhance scientific understanding and management of this important resource.

Water outflow as a cause of changes in trophic conditions for zooplanktivorous fish in reservoirs

The seasonal changes and vertical distribution of zooplankton, the export of zooplankters by outflow through water management installations, and the feeding of zooplanktivorous fish were studied in two Bulgarian reservoirs. In a bottom‐draining reservoir, planktonic crustacea > 1.0 mm were found to be selectively exported as a result of their diurnal vertical migrations. Increases in water outflow for irrigation resulted in a decrease in total zooplankton abundance, as well as changes in zooplankton size and species composition at the beginning of the summer. At the same time, major changes occurred in the feeding patterns of bleak, Alburnus alburnus (L.), and juvenile pikeperch, Stizostedion lucioperca (L.), the principal zooplanktivores in this reservoir. It is easy for bleak to switch to feeding on an alternative food, but a lack of available large prey organisms is suggested as the main cause of the downstream movement of pikeperch juveniles. In a surface‐draining reservoir, the pattern of zooplankton export corresponded to those in natural lakes. Discharged water carried with it mainly small‐sized epilimnetic zooplankton from the most numerous groups (i.e. rotifers, juvenile copepods and cladocerans) in the reservoir. Furthermore, outflow exports represent a relatively small part of total zooplankton number, and thus, do not disturb the natural zooplankton succession, and changes in abundance and composition Coincidental changes in the trophic responses for zooplanktivores (mainly juvenile cyprinids and percids) were not found.

Experimental effects of temperature and prey assemblage on growth and lipid accumulation by Mysis relicta loven

The opossum shrimp (Mysis relicta) is a holarctic planktivore that undergoes diel vertical migration (DVM), ascending at night from deeper water or the lake bottom into the upper water column to feed. In deep lakes that thermally stratify, M. relicta spends most of the time on or near the lake bottom in cold water where food resources are low; but, M. relicta migrates daily into warmer layers to feed where food resources are more abundant. Hence, in deep lakes M. relicta traverses steep temperature and prey density gradients while migrating through the water column to forage. We conducted a laboratory growth experiment of factorial design that simulated the range of temperatures and densities of natural zooplankton forage encountered by M. relicta during DVM. Somatic growth, measured as change in total length and lipid-free biomass, was not significantly different among experimental treatments. Analysis of variance showed that M. relicta stored energy in the form of lipid instead of using the energy mainly to drive increased somatic growth, but only at the colder temperatures. Lipid storage was 24% (± 4.3 sd) in 4 °C-high forage treatment compared to 6% (± 1.1) at 14 °C-high forage treatment. Energy conversion efficiency (ECE) of M. relicta was highest (21.3% ± 5.8) at 4 °C and lowest (3.3% ± 0.9) at 14 °C, regardless of prey density. The experiment showed that Mysis relicta feeds at high food densities where ECE is low, but spends most of the time in colder water where ECE is high. These results support McLaren's metabolic efficiency hypothesis, which predicts that feeding in the more productive surface waters and then migrating to colder waters is reproductively advantageous. Moreover, owing to high metabolic efficiency in the coldest waters and rapid DVM, M. relicta is able to sequester much of the epilimnetic zooplankton production near or on the lake bottom, thereby forcing the lake food web toward a configuration dominated by mysids, benthic or deepwater fishes and small, agile zooplankton.

Age and Growth of Alewives in the Changing Pelagia of Lake Ontario, 1978–1992

We documented the age and growth of alewives Alosa pseudoharenqus in Lake Ontario during 1978–1992 and determined if growth was affected by intraspecific competition for epilimnetic zooplankton, lake temperature, or demand of salmonine piscivores for prey. Ages of juvenile alewives were determined from scales during 1978–1983, and ages of juvenile and adult alewives were determined from otoliths during 1984–1992. Indices of abundance for alewives were calculated from spring bottom trawl catches in 1978–1992; zooplankton density and epilimnetic temperature were monitored at two stations during 1981–1991; and salmonine demand each year during 1978–1992 was calculated with a simulation model. Although we encountered 11-year-old alewives, few fish lived longer than 7 years, and most fish in the population were younger than 6 years. Mean sizes at ages 1, 2, and 3 in spring averaged 93 mm (5.1g), 133 mm (17 g), and 149 mm (22 g), but from age 3 to age 8, mean size increased by only 5–7 mm and 2–3 g per year. Female alewives lived longer than male alewives and were always longer than male alewives at age 4 and older. Epilimnetic temperatures were suitable for rapid growth of juvenile alewives each year. Lake temperature had the potential to affect growth of adults but adult growth was not correlated with temperature suitability indices perhaps because temperature regimes differed among lake regions and alewives were mobile, Growth of alewives was not correlated with salmonine demand for prey. Competition for zooplankton among the two youngest alewife cohorts affected growth of age-1 alewives. Zooplankton density declined sharply in 1986, and should it decline again, growth of age-1 alewives will slow, unless numbers. of age-0 alewives fall. Whether growth of age-1 fish declines or numbers of age-0 fish fall, the result of another decline in zooplankton density will be a reduction in the production of alewives needed to support piscivores.

Thermal effects

Thermal effects

Roles of Predation, Food, and Temperature in Structuring the Epilimnetic Zooplankton Populations in Lake Ontario, 1981–1986

Abstract We sampled phytoplankton, zooplankton, and alewives Alosa pseudoharengus and measured water temperature in Lake Ontario during 1981–1986. Through the use of general linear regression models we then sought evidence of control of the eplimnetic zooplankton community (mid-July to mid-October) by producers, consumers, and temperature. Our measures of the zooplankton community were total biomass, cladoceran biomass, and the ratio of large to small Daphnia spp. (D. galeata mendotae and D. retrocurva). Zooplankton population variables assessed were abundance, egg ratio, and productivity. Through factor analysis, factors were created from the standardized, transformed independent variables for use in the regression analyses. Regression models showed significant inverse relationships (P < 0.05) between alewives and Bosmina longirostris (abundance, production, and egg ratio), Ceriodaphnia lacustris (egg ratio), and Daphnia retrocurva (egg ratio). Bosmina longirostris and D. retrocurva egg ratios were inver...

Phosphorus Flux through Epilimnetic Zooplankton from Lake Ontario: Relationship with Body Size and Significance to Phytoplankton

The flux of phosphate through epilimnetic zooplankton from Lake Ontario was measured by adding 32PO4 into whole lakewater, monitoring its accumulation by zooplankton over 48 h, and then measuring its release when animals were returned to unlabelled lake water. Phosphorus uptake was calculated as the sum of accumulation and release rates. The 10 species examined ranged in size from 36-μm Codonella cratera to 1-mm Daphnia retrocurva. Phosphorus uptake rates of herbivorous zooplankton ranged about 200-fold and increased with body size both within and among species, although not proportionally. Among species, there was a pronounced allometry, with smaller species having much higher rates per unit size. This allometry is similar to that described for other metabolic parameters and body size. There was no tendency for smaller species to release a greater fraction of the label they took up. These results suggest that the biomass of zooplankton consuming a given phytoplankton production will be much smaller if small zooplankton dominate, and therefore, this phosphorus sink will be much smaller. Further, the flow of phosphorus to higher trophic levels may be reduced. The result of changing the zooplankton size-distribution may be to change the amount of spring total phosphorus available to summer phytoplankton.