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.     

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.     

U.S. EPA Great Lakes National Program Office monitoring of the Laurentian Great Lakes: Insights from 40 years of data collection

The U.S. EPA Great Lakes National Program Office (GLNPO) implements long-term monitoring programs to assess Great Lakes ecosystem status and trends for many interrelated ecosystem components, including offshore water quality as well as offshore phytoplankton, zooplankton and benthos; chemical contaminants in air, sediments, and predator fish; hypoxia in Lake Erie's central basin; and coastal wetland health. These programs are conducted in fulfillment of Clean Water Act mandates and Great Lakes Water Quality Agreement commitments. This special issue presents findings from GLNPO's Great Lakes Biology Monitoring Program, Great Lakes Water Quality Monitoring Program, Lake Erie Dissolved Oxygen Monitoring Program, Integrated Atmospheric Deposition Network, Great Lakes Fish Monitoring and Surveillance Program, and Great Lakes Sediment Surveillance Program. These GLNPO programs have generated temporal and spatial datasets for all five Great Lakes that form the basis for assessment of the state of these lakes, including trends in nutrients, key biological indicators, and contaminants in air, sediments and fish. These datasets are used by researchers and managers across the Great Lakes basin for investigating physical, chemical and biological drivers of ongoing ecosystem changes; some of these analyses are presented in this special issue, along with discussion of new methods and approaches for monitoring.     

Spatio-temporal variation of microcystins and its relationship to biotic and abiotic factors in Hongze Lake,China

An investigation of three particle-associated microcystin (MC) congeners (MC-LR, MC-RR, and MC-YR) was performed in 2013 from August to November in Hongze Lake, China. All MCs congeners exhibited significant spatial and temporal variability. Particle-associated MCs were commonly found in this lake, but MC-LR was present at low levels (0.075–1.252 μg/L). All three MC congeners were found to be significantly correlated with non-toxic phytoplankton (e.g., chlorophyte, cryptophyte and diatoms), while only MC-YR and total MC were correlated with cyanobacteria biomass. Copepods, rotifers and three genera of Cladocera were positively correlated with MC concentrations, but feeding habits of these zooplankton species might have different effects on different MC congeners and total MC distributions. Linear mixed effects (LME) model results showed that the interactions between cyanobacteria and other non-toxic phytoplankton or zooplankton species may drive MCs dynamics. Physiochemical parameters also may affect MC congener variability individually or through their effects on toxic cyanobacteria. The results of this study suggested that MCs distribution in Hongze Lake was site-specific and could be influenced by biotic and abiotic factors both individually and through their interactions.     

Brachionus leydigii (Monogononta: Ploima) reported from the western basin of Lake Erie

Several species of non-indigenous planktonic invertebrates have historically been introduced to the Laurentian Great Lakes. Previous introductions of non-indigenous planktonic invertebrates to the Great Lakes have been crustacean zooplankton, specifically Cladocera and Copepoda. This report documents the first known occurrence of Brachionus leydigii var. tridentatus (Zernov, 1901) in Lake Erie and possibly the first detection of a non-indigenous rotifer species in the Laurentian Great Lakes. The specimen was collected from a U.S. EPA monitoring station in the western basin of Lake Erie on April 4, 2016.     

Unusually High Levels of n-6 Polyunsaturated Fatty Acids in Whale Sharks and Reef Manta Rays

Fatty acid (FA) signature analysis has been increasingly used to assess dietary preferences and trophodynamics in marine animals. We investigated FA signatures of connective tissue of the whale shark Rhincodon typus and muscle tissue of the reef manta ray Manta alfredi. We found high levels of n-6 polyunsaturated fatty acids (PUFA), dominated by arachidonic acid (20:4n-6; 12–17 % of total FA), and comparatively lower levels of the essential n-3 PUFA—eicosapentaenoic acid (20:5n-3; ~1 %) and docosahexaenoic acid (22:6n-3; 3–10 %). Whale sharks and reef manta rays are regularly observed feeding on surface aggregations of coastal crustacean zooplankton during the day, which generally have FA profiles dominated by n-3 PUFA. The high levels of n-6 PUFA in both giant elasmobranchs raise new questions about the origin of their main food source.     

The burning question: does burning before flooding lower methyl mercury production and bioaccumulation?

Production of methyl mercury (MeHg) is elevated in new hydroelectric reservoirs because organic carbon stimulates methylation of inorganic mercury (Hg) stored in the terrestrial system. This can cause adverse health in fish and in organisms that eat fish. We expected that burning vegetation before flooding would decrease the amount of Hg and organic carbon and thereby lower MeHg production. We conducted a replicated field experiment to investigate the effects of burning vegetation and soil before flooding on MeHg production and bioaccumulation. Vegetation and soil were added to mesocosms in the following combinations: unburned vegetation and unburned soil (Fresh treatments), burned vegetation and unburned soil (Partial Burn treatments), and burned vegetation and burned soil (Complete Burn treatments). Controls had no added vegetation or soil. During combustion with propane torches, a large percentage of the total Hg (THg) and MeHg was lost from vegetation and soil. THg and MeHg concentrations were highest in the surface water of Fresh treatments, lower in Partial Burn treatments and lowest in Complete Burn treatments and controls. Differences in concentrations of MeHg in biota were consistent among treatments, but did not follow aqueous concentrations. On the final sample date, MeHg concentrations in biota of Controls and Partial Burn treatments were greater than in Complete Burn and Fresh treatments. The lack of relationship between MeHg in biota and MeHg in water may have been due to modification of the bioavailability of MeHg by dissolved organic matter as the ratios of MeHg in biota to water were inversely correlated with concentrations of dissolved organic carbon. Although burning before flooding decreased MeHg concentrations in the water, it did not lower MeHg accumulation in the lower food web.     

Edge effects on abiotic conditions,zooplankton, macroinvertebrates,and larval fishes in Great Lakes fringing marshes

Fragmentation and edge creation is common in many freshwater coastal wetlands, though relatively little is known about edge effects on abiotic conditions and faunal communities within these habitats. We investigated edge effects associated with anthropogenic fragmentation in 16 fringing coastal marshes of Lake Michigan and Lake Huron. Environmental data, zooplankton, macroinvertebrates, and larval fish were collected along transects extending into each marsh from reference (i.e., where the wetland naturally interfaced with open water) and anthropogenic edges (i.e., where the wetland interfaced with open water habitats created by vegetation removal). Physical and chemical gradients were apparent from marsh edges toward marsh interiors regardless of edge type. Faunal communities appeared to respond to these gradients. Zooplankton biomass, macroinvertebrate richness and macroinvertebrate Shannon diversity were depressed at edges and increased toward marsh interiors. Larval fish catch per unit effort, taxon richness, and Shannon diversity increased from reference edges toward marsh interiors. Larvae of individual fish species displayed varying patterns across edges. Our results suggest that because of edge effects, fragmentation of coastal marshes causes impacts that exceed the area of marsh habitat that is actually lost. For example, as a marsh's protected inner core area is reduced, the marsh fragment may cease to function as a viable refuge from hydrologic energy and open water predators. Therefore, fragmentation should be viewed as a significant impact to freshwater coastal marsh ecosystems similar to how it is regarded in terrestrial ecosystem management.     

Daphnia lumholtzi Sars a new alien species in Lake St. Clair

Daphnia lumholtzi Sars, an exotic tropical/subtropical cladoceran from Australia, southeast Asia, and Africa, was newly found in Lake St. Clair in a vertical tow sample taken at 3 m depth on 25 July 2007. The species was previously found in 1990/1991 in some reservoirs in the southern United States from where it colonized many waters north to the Great Lakes. In 1999, it was found in Lake Erie. This cladoceran had a density of 117 individuals/m³ when we collected it from Lake St. Clair and it was represented by both females (95.12 %) and males (4.88 %). It seems that D. lumholtzi will continue to expand its distribution area in the Great Lakes.     

Predation by Limnocalanus as a Potentially Major Source of Winter Naupliar Mortality in Lake Michigan

Predation by adult Limnocalanus macrurus may have a significant impact on naupliar mortality in Lake Michigan. Naupliar mortality rates were estimated from plankton samples from winter and early spring of 1981, and compared with estimates of potential mortality due to predation by adult Limnocalanus. Naupliar mortality rates ranged from. 028 to .149 per day. Predation by Limnocalanus could have accounted for between 3 and 100 percent of the naupliar mortality. These percentages were highest in winter, and decreased with declining Limnocalanus densities in spring. Naupliar mortality rates increased from January through April, during the time of the spring “bloom” in the lake. Predation, rather than competition for food, is probably, the main influence on naupliar mortality rates.