A comparison of PCB bioaccumulation factors between an arctic and a temperate marine food web

To test how environmental conditions in the Arctic and the resulting ecological adaptations affect accumulation of persistent organic pollutants (POPs) in the marine food web, bioaccumulation of four polychlorinated biphenyls (PCBs) in an arctic (Barents Sea 77 °N-82 °N) and a temperate marine (Baltic Sea 54 °N-62 °N) food web were compared. Three different trophic levels were studied (zooplankton, fish, and seal), representing the span from first-level consumer to top predator. Previously published high-quality data on PCB water concentrations in the two areas were used for calculation of bioaccumulation factors (BAF). BAF was calculated as the ratio of the PCB concentration in the organism ([PCB]_org; pg/kg lipid) to the dissolved water concentration (C_w; pg/L). The BAF_Arctic:BAF_Temperate ratios were above 1 for all four PCB congeners in zooplankton (6.4-13.8) and planktivorous fish (2.9-5.0)), whereas the ratios were below 1 in seal. The mean ratio between arctic and temperate BAFs for all trophic levels and congeners (BAF_Arcti:BAF_Temperate) was 4.8. When the data were corrected for the seawater temperature difference between the two ecosystems, the ratio was 2.0. We conclude that bioaccumulation differences caused by ecological or physiological adaptations of organisms between the two ecosystems were well within a water concentration variability of 50%. Further, our data support the hypothesis that lower seawater temperature lead to a thermodynamically favoured passive partitioning to organic matrices and thus elevated ambient BAFs in the Arctic compared to the Baltic Sea. This would imply that bioaccumulation in the Arctic may be described in the same way as bioaccumulation in temperate regions, e.g. by the use of mechanistic models parameterised for the Arctic.     

Role of predation by zooplankton in transport and fate of protozoan (oo)cysts in granular activated carbon filtration

The significance of zooplankton in the transport and fate of pathogenic organisms in drinking water is poorly understood, although many hints of the role of predation in the persistence of microorganisms through water treatment processes can be found in literature. The objective of this study was to assess the impact of predation by natural zooplankton on the transport and fate of protozoan (oo)cysts in granular activated carbon (GAC) filtration process. UV-irradiated unlabelled Cryptosporidium parvum and Giardia lamblia (oo)cysts were seeded into two pilot-scale GAC filtration columns operated under full-scale conditions. In a two-week period after seeding, a reduction of free (oo)cysts retained in the filter bed was observed. Zooplankton was isolated from the filter bed and effluent water on a 30 μm net before and during the two-week period after seeding; it was enumerated and identified. Rotifers, which are potential predators of (oo)cysts, accounted for the major part of the isolated zooplankton. Analytical methods were developed to detect (oo)cysts internalized in natural zooplankton isolated from the filter bed and effluent water. Sample sonication was optimized to disrupt zooplankton organisms and release internalized microorganisms. (Oo)cysts released from zooplankton after sonication were isolated by IMS and stained (EasyStain™) for microscopic counting. Both Cryptosporidium and Giardia (oo)cysts were detected in association with zooplankton in the filter bed samples as well as in the effluent of GAC filters. The results of this study suggest that predation by zooplankton can play a role in the remobilization of persistent pathogens such as Cryptosporidium and Giardia (oo)cysts retained in GAC filter beds, and consequently in the transmission of these pathogens in drinking water.     

Mercury distribution and speciation in Lake Balaton, Hungary

The distribution and speciation of mercury in air, rain, lake water, sediment, and zooplankton in Lake Balaton (Hungary) were investigated between 1999 and 2002. In air, total gaseous mercury (TGM) ranged from 0.4 to 5.9 ng m(-3) and particulate phase mercury (PPM) from 0.01 to 0.39 ng m(-3). Higher concentrations of both TGM and PPM occurred during daytime. Higher concentrations of PPM occurred in winter. In rain and snow, total mercury ranged from 10.8 to 36.7 ng L(-1) in summer but levels up to 191 ng L(-1) in winter. Monomethylmercury (MMHg) concentrations ranged from 0.09 to 1.26 ng L(-1) and showed no seasonal variations. Total Hg in the unfiltered lake water varied spatially, with concentrations ranging from 1.4 to 6.5 ng L(-1). Approximately 70% of the total Hg is dissolved. MMHg levels ranged from 0.08 to 0.44 ng L(-1) as total and from 0.05 to 0.37 ng L(-1) in the dissolved form. Lower Hg concentrations in the water column occurred in winter. In suspended particulate matter and in sediment, total mercury ranged from 9 to 160 ng g(-1) dw, and MMHg ranged from 0.07 to 0.84 ng g(-1) dw. In zooplankton, an average mercury level of 31.0+/-6.8 ng g(-1) dw occurred, with MMHg accounting for approximately 17%. In sediments, suspended-matter- and zooplankton-high Hg and MMHg levels occurred at the mouth of the River Zala, but, in the lake, higher concentrations occurred on the Northern side, and an increasing trend from north-west to north-east was observed. In general, regarding Hg, Lake Balaton can be considered as a relatively uncontaminated site. The high-pH and well-oxygenated water as well as the low organic matter content of the sediment does not favour the methylation of Hg. In addition, bioconcentration and bioaccumulation factors are relatively low compared to other aquatic systems.     

An abrupt decline in springtime zooplankton diel vertical migration due to a shift in stratification regime

Returned signal strength intensity measurements from an acoustic Doppler current profiler deployed at an open-water site in Lake Superior from May-July 2019 were used to estimate the scattering intensity over the top 50 m of the water column. It displayed a strong diel signal, with higher scattering intensity at night, consistent with the normal diel vertical migration of planktonic organisms. However, the intensity of the nocturnal scattering signal decreased abruptly on 10 June, coincident with the end of winter stratification (cold water on top of warmer water) and the beginning of the neutrally stratified period. We hypothesize that the sudden change in zooplankton behavior is due the availability of phytoplankton deep in the water column after the transition, made possible by radiatively driven convection mixing the entire water column on time scales of hours.     

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.     

Temperature gradients,not food resource gradients,affect growth rate of migrating Daphnia mendotae in Lake Michigan

Zooplankton production plays a critical role in the Great Lakes ecosystem, and vertical migration, which is exhibited by many zooplankton species, could affect production. We examined the effects of water temperature and food resource gradients on the growth rate of zooplankton undergoing vertical migration in Lake Michigan. In three laboratory experiments, juvenile Daphnia mendotae, native herbivorous cladocerans, were incubated for 5 days at water temperatures associated with the epilimnion and deep chlorophyll maxima (DCM) of Lake Michigan and were fed food resources collected directly from these regions. Growth rate strongly depended on water temperature, as Daphnia incubated at the epilimnetic temperature (21 °C) grew 42% faster than those at the DCM temperature (8 °C). Growth rate of Daphnia that alternated between the two temperatures every 12 h (0.108 day^− 1) was similar to the arithmetic average growth rate of the two water temperature treatment extremes (0.110 day^− 1), suggesting fluctuating temperatures alone do not substantially influence Daphnia growth. In contrast, food resources derived from different depths did not affect growth rate, nor was there a significant interaction between food resource origin and water temperature effects. Our results indicate that vertical migration will reduce growth rate, and hence zooplankton production, through reduced temperature, not from changes in resources. Consideration of the effects of vertical migration, especially given the known variability in this behavior, may substantially improve zooplankton production estimates in the Great Lakes.     

Crustacean zooplankton available for larval walleyes in a Lake Michigan embayment

Adequate densities of zooplankton prey are critical for growth and survival of larvae of many fish species. Little information exists on the density of zooplankton in Great Lakes inshore areas during early spring, when larvae of important fishes rely on zooplankton. Reduced age-0 walleye recruitment and the absence of data on zooplankton availability for larval walleyes in northern Green Bay, Lake Michigan, led us to assess zooplankton densities during this critical spring period. We conducted biweekly vertical plankton tows in 2014–2016 near reefs and river plumes used by spawning walleyes for periods when larval walleyes were expected to be relying on zooplankton prey. Densities of zooplankton were well below literature values identified for good growth and survival of larval walleyes, averaging 1.5 individuals L⁻¹ for all taxa and 0.12 individuals L⁻¹ for large-bodied taxa across all sites and sampling dates. Various factors could contribute to the low density of zooplankton observed. We found low but significantly higher densities of cyclopoid copepods, nauplii, Bosmina, and total zooplankton at river mouth sites compared to open water sites. These results suggest that food availability for larval walleye in our study area was severely limiting which is consistent with the paucity of strong year classes observed since 2000. We suspect northern Green Bay has limited potential for producing strong year classes of walleyes under such conditions. Fishery managers working in unproductive waters should consider assessing the zooplankton community during critical periods to identify potential bottlenecks to reproductive success and larval fish survival.     

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.     

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.     

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.