Convergence of trophic state and the lower food web in Lakes Huron,Michigan and Superior

Signs of increasing oligotrophication have been apparent in the open waters of both Lake Huron and Lake Michigan in recent years. Spring total phosphorus (TP) and the relative percentage of particulate phosphorus have declined in both lakes; spring TP concentrations in Lake Huron are now slightly lower than those in Lake Superior, while those in Lake Michigan are higher by only about 1 μg P/L. Furthermore, spring soluble silica concentrations have increased significantly in both lakes, consistent with decreases in productivity. Transparencies in Lakes Huron and Michigan have increased, and in most regions are currently roughly equivalent to those seen in Lake Superior. Seasonality of chlorophyll, as estimated by SeaWiFS satellite imagery, has been dramatically reduced in Lake Huron and Lake Michigan, with the spring bloom largely absent from both lakes and instead a seasonal maximum occurring in autumn, as is the case in Lake Superior. As of 2006, the loss of cladocerans and the increased importance of calanoids, in particular Limnocalanus, have resulted in crustacean zooplankton communities in Lake Huron and Lake Michigan closely resembling that in Lake Superior in size and structure. Decreases in Diporeia in offshore waters have resulted in abundances of non-dreissenid benthos communities in these lakes that approach those of Lake Superior. These changes have resulted in a distinct convergence of the trophic state and lower food web in the three lakes, with Lake Huron more oligotrophic than Lake Superior by some measures.     

Seasonal zooplankton dynamics in Lake Michigan: Disentangling impacts of resource limitation,ecosystem engineering,and predation during a critical ecosystem transition

We examined seasonal dynamics of zooplankton at an offshore station in Lake Michigan from 1994 to 2003 and 2007 to 2008. This period saw variable weather, declines in planktivorous fish abundance, the introduction and expansion of dreissenid mussels, and a slow decline in total phosphorus concentrations. After the major expansion of mussels into deep water (2007–2008), chlorophyll in spring declined sharply, Secchi depth increased markedly in all seasons, and planktivorous fish biomass declined to record-low levels. Overlaying these dramatic ecosystem-level changes, the zooplankton community exhibited complex seasonal dynamics between 1994–2003 and 2007–2008. Phenology of the zooplankton maximum was affected by onset of thermal stratification, but there was no other discernable effect due to temperature. Interannual variability in zooplankton biomass during 1994 and 2003 was strongly driven by planktivorous fish abundance, particularly age-0 and age-1 alewives. In 2007–2008, there were large decreases in Diacyclops thomasi and Daphnia mendotae possibly caused by food limitation as well as increased predation and indirect negative effects from increases in Bythotrephes longimanus abundance and in foraging efficiency associated with increased light penetration. The Bythotrephes increase was likely driven in part by decreased predation from yearling and older alewife. While there was a major decrease in epilimnetic–metalimnetic herbivorous cladocerans in 2007–2008, there was an increase in large omnivorous and predacious calanoid copepods, especially those in the hypolimnion. Thus, changes to the zooplankton community are the result of cascading, synergistic interactions, including a shift from vertebrate to invertebrate planktivory and mussel ecosystem impacts on light climate and chlorophyll.     

Contrasting zooplankton communities of two bays of the large,shallow, eutrophic Lake Taihu,China: Their relationship to environmental factors

Zooplankton are an important link in aquatic food webs of lakes serving as consumers of algae, bacteria, and other microorganisms and as prey for fish and invertebrates. Despite their importance, little is known about the structure of the zooplankton communities of subtropic, large, shallow, eutrophic freshwater lakes. Our investigation of zooplankton communities in Lake Taihu, a subtropic, shallow, eutrophic lake and the third largest lake in China provides new information on this subject. Zooplankton, phytoplankton, and water chemistry samples were collected monthly from July 2006 to June 2007 in Meiliang and Gonghu Bays of Lake Taihu. Thirty zooplankton species were identified in Meiliang Bay with small-bodied cladocerans Bosmina coregoni and Ceriodaphnia cornuta contributing 21% and 11%, respectively to total zooplankton abundance which averaged 459 ind/L. Thirty-five species were identified in Gonghu Bay with the rotifers Polyarthra trigla and Brachionus calyciflorus the dominant species, contributing 21% and 11% respectively to total zooplankton abundance which averaged 467 ind/L. Predation by lake anchovy (Coilia ectenes taihuensis) and ice fish (Neosalanx tangkahkeii taihuensis) likely accounted for the dominance of both bays by small-bodied species. Community structure and community patterns were correlated with differences in Microcystis blooms and organic matter levels (chemical oxygen demand) in the two bays. Based on canonical correspondence analyses dissolved total nitrogen, orthophosphate, Cyclotella and Pinnularia also contributed to variability in zooplankton community composition.     

Lake Superior zooplankton biomass: Alternate estimates from a probability-based net survey and spatially extensive LOPC surveys

We conducted a probability-based net tow sampling of Lake Superior in 2006 and compared the zooplankton biomass estimate with an estimate from laser optical plankton counter (LOPC) surveys. The net survey consisted of 52 sites stratified across three depth zones (0–30, 30–150, > 150 m). The LOPC tow surveys were extensive and spatially covered much of Lake Superior (> 1300 km of towing). The LOPC was field calibrated to Lake Superior zooplankton samples collected across the years of 2004 to 2006. The volume-weighted lake-wide zooplankton biomass determined by traditional net tows to 100-m sample depth was 20.1 (± 7.8 SD n = 52) mg dry-weight m^− 3. The estimates varied by depth zones within the lake, where nearshore (0–30 m) estimates were highest and highly variable. Net sites for the LOPC field calibration were removed to allow for LOPC validation with independent nets; the resulting net-based estimate 20.0 (± 9.3 SD n = 38) mg dry-weight m^− 3 and LOPC lake-wide estimate 19.1 (± 3.3 SD) mg dry-weight m^− 3 agreed well. Consistency across survey methods for lake-wide estimates suggested that LOPC survey data provides a comparable assessment tool to traditional nets for collecting zooplankton biomass data. We briefly compare our results with some observed historical patterns. Onshore–offshore trends in zooplankton biomass concentrations were similar to the last major lake-wide survey in 1973. The LOPC provided high resolution data on zooplankton biomass distribution. Using simultaneously collected in situ sensor data, the LOPC zooplankton biomass distributions over horizontal and vertical space can be modeled as a function of temperature and fluorescence.     

Historical and recent biomass and food web relations of Limnocalanus in Lake Huron

Reported population biomass of Limnocalanus macrurus in Lake Huron has been underestimated by several-fold owing to application of an inappropriate length–weight regression model. During August and September the underestimates can exceed three-fold, suggesting that secondary production of the species in the lake is greater than previously thought. Increased representation of the species in the plankton community in recent years is associated with a decrease in its trophic level deduced through nitrogen stable isotope analysis of specimens from opportunistic samples obtained in 1993, 1995, and 2009. Limnocalanus¹⁵N enrichment relative to primary herbivores appears to have decreased by 1.5‰ over this time period. Similarly, Diaptomus¹⁵N enrichment relative to primary herbivores appeared to decrease by 1.1‰ from 1993 to 2009. Our data offer at least two possible explanations for this shift. (1) Limnocalanus has adopted a more herbivorous diet in response to the ecological changes in Lake Huron or (2) hypolimnetic Diaptomus have adopted a more herbivorous diet and Limnocalanus is relying in part on Diaptomus as a food source.     

Impact of industrial discharges on the zooplankton community in the Messaieed industrial area,Qatar (Persian Gulf)

Zooplankton samples were collected from the Messaieed Marine Area that is subject to industrial and sewage discharge, thermal pollution, as well as oil loading and unloading activities. Copepods formed 74% of the zooplankton community followed by bivalves (7.6%) and Gastropod veligers (5.5%), Ostracods (4.7%) and Cladocera (3.4%). Industrial discharge not only impacted the total number of zooplankton but also species diversity. Diversity indices were higher for inner coastal waters. Pollution levels more than reduction in food source impacted the distribution of main zooplankton groups. The pollution stress changed the structure of zooplankton community rather than impacting the total population. Copepods showed resistance to oil pollution but were reduced in numbers at areas affected by oxygen demanding wastes. Polychaete larvae and Chaetognatha withstand elevated temperature, ammonia and pH levels.     

Short‐term effects of planktonic rotifers and cladocerans on phytoplankton of the River Nile

The responses of River Nile phytoplankton to different zooplankton communities have been analyzed. Different communities have been made up of natural Nile water of rotifers and small cladocerans, rotifers after exclusion of macrozooplankton by size fraction and large cladocerans plus small cladocerans and rotifers. A set of experiments in enclosures were designed to determine the short‐term effects produced by these zooplankton assemblages on phytoplankton structure. The phytoplankton community are made up of Bacillariophyceae, Cyanophyceae and Chlorophyceae, as the main algal groups. It seems that, grazing does not play a leading role in controlling phytoplankton production and its standing stock in the River Nile.     

Rotifer communities in the Laurentian Great Lakes, 1983–2006 and factors affecting their composition

Rotifer communities were examined from the open waters of the Laurentian Great Lakes between 1983 and 2006 in the first inter-lake comparison involving all five lakes. Seventy rotifer taxa were found in summer samples, although most (> 80%) individuals were drawn from the following taxa: Conochilus unicornis, Polyarthra vulgaris, Keratella cochlearis, Kellicottia longispina, Keratella crassa, Synchaeta spp., Polyarthra major, Ploesoma truncatum, Keratella earlinae, and Keratella quadrata. Distributions of Filinia longiseta and several species of Brachionus and Trichocera were restricted to regions of higher trophy, notably the western basin of Lake Erie, but most taxa were widely distributed, differing in relative dominance rather than presence or absence. Rotifer community structure appeared to respond in tractable ways to differences in trophic state, with the four most common genera exhibited the following preferences, in order of decreasing trophy: Polyarthra, Keratella, Conochilus, and Kellicottia. Recent oligotrophication in Lake Huron was reflected in decreases in Keratella dominance and increased dominance of Conochilus, while in Lake Michigan oligotrophication was associated with decreases in Polyarthra and Keratella and increases in Conochilus, and Kellicottia. In Lake Ontario, the appearance of the exotic predatory Cercopagis pengoi was strongly associated with dramatic reductions in Polyarthra, while more gradual sequential increases in the dominance of Keratella, Conochilus and Kellicottia were probably the result of decreasing trophic state. Bythotrephes was identified in multivariate analyses as correlating strongly with rotifer composition, although only in Lake Michigan was a clear relationship between the predatory cladoceran and rotifer species shifts observed.     

A New Record of Pinaciophora Fluviatilis Greeff Sensu Penard from the Laurentian Great Lakes

This report clarifies the presence of Pinaciophora fluviatilis Greeff sensu Penard in the Laurentian Great Lakes. Transmission electron microscopic observations were made of a shadowcast carbon replica of a single scale. Taxonomic problems of this organism are discussed.     

Differences between Arctic and Atlantic fjord systems on bioaccumulation of persistent organic pollutants in zooplankton from Svalbard

Differences in bioaccumulation of persistent organic pollutants (POPs) between fjords characterized by different water masses were investigated by comparing POP concentrations, patterns and bioaccumulation factors (BAFs) in seven species of zooplankton from Liefdefjorden (Arctic water mass) and Kongsfjorden (Atlantic water mass), Svalbard, Norway. No difference in concentrations and patterns of POPs was observed in seawater and POM; however higher concentrations and BAFs for certain POPs were found in species of zooplankton from Kongsfjorden. The same species were sampled in both fjords and the differences in concentrations of POPs and BAFs were most likely due to fjord specific characteristics, such as ice cover and timing of snow/glacier melt. These confounding factors make it difficult to conclude on water mass (Arctic vs. Atlantic) specific differences and further to extrapolate these results to possible climate change effects on accumulation of POPs in zooplankton. The present study suggests that zooplankton do biomagnify POPs, which is important for understanding contaminant uptake and flux in zooplankton, though consciousness regarding the method of evaluation is important.