Factors affecting rotifer assemblages along a nearshore to offshore transect in southeastern Lake Michigan

Rotifer assemblages have been widely used as indicators of trophic conditions. Our goal was to evaluate whether differences in rotifer assemblages along a nearshore to offshore transect in southeastern Lake Michigan were consistent with productivity differences among sites. Sampling took place in March-December during 2013–2019 at a nearshore (17-m deep), mid-depth (45-m deep), and offshore site (110-m deep). Conochilus and Keratella were present in most samples (84–100%) and were the most abundant taxa overall at each site, combining for around 64–69% by density of the rotifers. Although the productivity gradient was not large between sites, there was a progression with the highest contributions of Keratella, Conochilus, and Kellicottia occurring at the nearshore, mid-depth, and offshore sites, respectively, which is consistent with a gradient of decreasing productivity from nearshore to offshore. The rotifer community also demonstrated strong seasonal patterns. In particular, there was a progression from dominance by illoricate forms, particularly Synchaeta, early in the season, to loricate and colonial forms in the summer and fall. Redundancy analysis indicated that in addition to productivity (i.e., total phosphorus), predatory factors (e.g., cyclopoid copepods, Cercopagis, Bythotrephes) had a strong influence on the rotifer community and seasonal patterns. Although productivity was important in structuring rotifer assemblages along our nearshore to offshore gradient, other factors, especially those related to predation, are difficult to separate from productivity effects. This complexity somewhat limits the usefulness of rotifers as an indicator group for productivity among sites that do not have extreme differences in trophy within a lake.     

Spatial and temporal distributions of Dreissena spp. veligers in Lake Huron: Does calcium limit settling success?

The larval stage of invasive Dreissena spp. mussels (i.e., veligers) are understudied despite their seasonal numerical dominance among plankton. We report the spring and summer veliger densities and size structure across the main basin, North Channel, and Georgian Bay of Lake Huron, and seek to explain spatiotemporal variation. Monthly sampling was conducted at 9 transects and up to 3 sites per transect from spring through summer 2017. Veliger densities peaked in June and July, and we found comparable densities and biomasses of veligers between basins, despite differences in density of juvenile and adult mussels across these regions. Using a generalized additive model to explain variations in veliger density, we found that temperature, chlorophyll a, and nitrates/nitrites were most important. We generated an index of veliger attrition based on size distributions that revealed a higher rate of attrition in the North Channel than the rest of the lake. A logistic model indicated a threshold calcium concentration of around 22 mg/L was necessary for veligers to survive to larger sizes and recruit to their juvenile and benthic adult life stages. Improved understanding of factors that regulate the production and survival of Dreissena veligers could improve the ability of managers to assess future invasion threats as well as explore potential control options.     

Stomach contents and stable isotopes analysis indicate Hemimysis anomala in Lake Ontario are broadly omnivorous

Hemimysis anomala is a recent invader to North American aquatic ecosystems and is spreading rapidly throughout the Great Lakes region. This is the first littoral mysid in the North American Great Lakes; and, as such, the ecosystem effects are unknown and could be substantial. These effects depend on the role of Hemimysis in the food web and, therefore, on its diet. We examined the stomach contents of two life stages of Hemimysis from two sites in Lake Ontario during the growing season (May–November 2010). We also report the relationship between zooplankton hard parts and size for a number of potential prey species to allow the back-calculation of prey lengths from stomach contents. Both juvenile (2–5 mm) and adult Hemimysis (5–11 mm) were omnivorous, consuming phytoplankton, zooplankton, and benthos when available. However, adults appeared slightly more carnivorous and incorporated larger prey in their diets. Hemimysis were able to consume zooplankton prey up to 30% of their own length, including Bythotrephes longimanus and Cercopagis pengoi. Daphnia and Bosmina were selected over other prey by both juvenile and adult Hemimysis and were most common in stomachs during July and September when their abundances in the zooplankton were highest. Measurements of δ¹³C and δ¹⁵N corroborated stomach content materials, indicating an omnivorous diet which included benthic and pelagic sources. Omnivory by Hemimysis is typical of mysids in general and makes them less sensitive to seasonal dynamics of preferred prey items.     

Major shift in the phenology of crustacean biomass in western Lake Superior associated with temperature anomaly

Variable weather patterns during the early months of 2014 and 2015 resulted in differences between years in spring and summer surface water temperatures in the offshore areas of western Lake Superior. Zooplankton were collected in western Lake Superior during several cruises from late spring to early fall in 2014 and 2015 to test the hypotheses that colder summer water temperatures in 2014 were correlated with reduced zooplankton biomass, later peaks in zooplankton biomass, and a smaller contribution of warm-water taxa to the zooplankton assemblage. The total amount of zooplankton biomass from early June through early October did not differ greatly between years. Of the taxonomic subcategories (large-bodied calanoids, small-bodied calanoids, cyclopoids, nauplii, and herbivorous cladocerans) however, cyclopoid and cladoceran biomass was somewhat smaller in 2014 compared to 2015, providing some support for the hypothesis that warm-water taxa contribute less to offshore zooplankton biomass in colder years. The timing of peak biomass for cladocerans, small-bodied calanoids, and cyclopoids did not differ between years, but peaks in the biomass of copepod nauplii and large-bodied calanoids (primarily Limnocalanus macrurus) occurred several weeks later in 2014 than 2015 which was evidence for phenological delay during a year with unusually cold spring and summer surface water temperatures. Though this study only evaluates the role of temperature in driving zooplankton biomass and phenology in Lake Superior, it does provide insight into the potential effects of climate variability on the Lake Superior food web.     

Submerged aquatic vegetation cover and complexity drive crustacean zooplankton community structure in a large fluvial lake: An in situ approach

Submerged aquatic vegetation (SAV) is considered an important driver of the zooplankton community, owing to the refuge offered by its structural complexity. However, non-destructive in situ approaches to assess quantitatively the features of SAV beds and their effects on zooplankton community are still lacking. This study aims to: 1) estimate SAV cover and complexity using subaquatic upward photographs (SUP) and 2) analyse the relationship between SAV variables and crustacean zooplankton composition and functional diversity (FD). SAV biomass and zooplankton were sampled at 52 stations in a large fluvial lake. Two metrics based on SUP were used to quantify SAV cover and complexity: 1) the percentage of the underwater landscape occupied by SAV (PLAND: SAV cover), and 2) its perimeter-area fractal dimension (PAFRAC: SAV complexity). Zooplankton composition and functional diversity were estimated based on crustacean species assemblages and functional traits. We used linear models to describe the relationships between SAV indices, environmental variables and zooplankton abundance, biomass and FD. SUP was an effective method to estimate in situ SAV cover and complexity. PLAND and PAFRAC were positively related to SAV biomass and zooplankton abundance, yielding a non-destructive assessment of the SAV-zooplankton relationships. Our study partially supports the habitat complexity-ecological niche availability hypothesis, as SAV cover and complexity influenced zooplankton functional groups and diversity indices. However, water transparency and depth also had important effects in interaction with PLAND and PAFRAC, and it remains difficult to fully disentangle the effects of SAV complexity from SAV cover on crustacean zooplankton community structure.     

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 20 m 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.     

A comparative examination of recent changes in nutrients and lower food web structure in Lake Michigan and Lake Huron

The lower food webs of Lake Huron and Lake Michigan have experienced similar reductions in the spring phytoplankton bloom and summer populations of Diporeia and cladocerans since the early 2000s. At the same time phosphorus concentrations have decreased and water clarity and silica concentrations have increased. Key periods of change, identified by using a method based on sequential t-tests, were 2003–2005 (Huron) and 2004–2006 (Michigan). Estimated filtration capacity suggests that dreissenid grazing would have been insufficient to directly impact phytoplankton in the deeper waters of either lake by this time (mid 2000s). Despite some evidence of decreased chlorophyll:TP ratios, consistent with grazing limitation of phytoplankton, the main impact of dreissenids on the offshore waters was probably remote, e.g., through interception of nutrients by nearshore populations. A mass balance model indicates that decreased phosphorus loading could not account for observed in-lake phosphorus declines. However, model-inferred internal phosphorus dynamics were strongly correlated between the lakes, with periods of increased internal loading in the 1990s, and increased phosphorus loss starting in 2000 in Lake Michigan and 2003 in Lake Huron, prior to dreissenid expansion into deep water of both lakes. This suggests a limited role for deep populations of dreissenids in the initial phosphorus declines in the lakes, and also suggests a role for meteorological influence on phosphorus dynamics. The high synchrony in lower trophic level changes between Lake Michigan and Lake Huron suggests that both lakes should be considered when investigating underlying causal factors of these changes.     

Spatio-temporal trends in the density and condition of a secondary consumer,Bythotrephes, in southern Lake Michigan

Natural gradients in temperature, nutrient loading, and primary productivity contribute to broad scale regional differences in the food web structure of large lakes, such as Lake Michigan. These factors influence spatial patterns of primary production and resource reliance of high trophic level consumers. Secondary consumers, such as larval fish and predatory zooplankton represent an important intermediate link within large lake food webs, but spatial patterns in their density and physiological condition have not been as thoroughly assessed. To analyze the spatial and temporal condition of secondary consumers in Lake Michigan, we sampled the ubiquitous spiny water flea, Bythotrephes cederströmii during the 2015 Cooperative Science and Monitoring field year in Lake Michigan. Monthly estimates of density, instar frequency, length-at-age, and RNA content of Bythotrephes were compared between the eastern and western shores of southern Lake Michigan. Condition indices differed seasonally between the eastern and western shores. During June-August, Bythotrephes were more abundant and in better condition along the eastern shore of Lake Michigan, but in September, all indices were comparable between transects. While this study focused on a single year, other studies across multiple years have demonstrated consistent upwellings and cooler temperatures along the western shore of southern Lake Michigan relative to the eastern shore. A temporal lag in preferable environmental conditions along the western shore may be common and lead to delayed growth and reduced physiological condition of secondary consumers, such as predatory zooplankton and late spring emerging larval fish.     

Invertebrate communities of former southern moravian floodplains (Czechoslovakia) and impacts of regulation

The natural hydrobiological conditions of southern Moravian floodplains have been drastically altered by regulation of the lower Dyje river and its tributaries. Natural floodplains were characterized by temporary pools and swamps, each inhabited by a typical community of aquatic invertebrates, and permanent water bodies (rivers and abandoned channels). Some aquatic taxa migrated onto ‘terrestrial’ localities during flood periods. Following regulation, floods were eliminated and most of the wet floodplain habitats disappeared, together with their typical species. The results of faunistic studies carried out in different permanent and temporary water bodies of the floodplains are reviewed. Altogether 188 taxa of planktonic and 206 taxa of benthic and semiplanktonic animals have been recorded. Of the planktonic community, rotifers (96 taxa) and copepods (41 taxa) have very favourable conditions for their development, especially in the spring. The main season for cladocera (53 taxa) development (summer) is less favourable due to the short duration of flooding (in flooded meadows) or poor oxygen conditions (in temporary pools with a high content of decaying organic matter). Among the zoobenthos, mainly facultative aquatic macroinvertebrates (larvae of insects) appear in these localities.     

Microbiological and chemical study of the nile river water quality

This study aims to Integrate the biological and chemical characteristics of Nile river to evaluate the trophic and autotrophic state of the river. The use of biological indices especially phytoplankton as an indicator of pollution, and the relation between some chemical variables revealed that the Nile river has poor quality water and light pollution conditions. Regarding the bacteriological examination, the results showed seasonal fluctuation of the faecal indicator bacteria, new indicators of pollution and pathogenic bacteria with highly significant correlation between the faecal bacterial indicators. The use of total yeasts as indicators of pollution revealed significant negative correlation with classical bacterial indicators. The statistical analysis between some bacterial and chemical variables (algal groups and chlorophyll “a") indicated significant relation between each other. The variation of zooplankton counts in the four seasons was positively correlated with the chlorophyll “a” concentration, both zooplankton and chlorophyll “a” content peak in the winter. The virological examination showed only one sample which induced CPE in BGM cells, and the virus count was 12 plaque forming unit per liter (PFU/L).