No abstract

Water samples collected during three sampling trips to Lake Erie displayed oxygen isotopic values of dissolved phosphate (delta18Op) that were largely out of equilibrium with ambient conditions, indicating that source signatures may be discerned. delta18Op, values in the Lake ranged from +10% per hundred to +17% per hundred, whereas the equilibrium value was expected to be around +14% per hundred. The riverine weighted average delta18Op, value was +11% per hundred and may represent one source of phosphate to the Lake. The lake delta18Op, values indicated that there must be one or more as yet uncharacterized source(s) of phosphate with a high delta18Op value. Potential sources other than rivers are not yet well-characterized with respectto delta18Op of phosphate, but we speculate that a likely source may be the release of phosphate from sediments under reducing conditions created during anoxic events in the hypolimnion of the central basin of Lake Erie. Identifying potential phosphorus sources to the Lake is vital for designing effective management plans for reducing nutrient inputs and associated eutrophication.

Water clarity in North America's Laurentian Great Lakes has undergone considerable change over the last several decades as a consequence of invasive species, eutrophication, and implemented nutrient management practices. Satellite observations from the CZCS, SeaWiFS, and MODIS-Aqua sensors have been used in tandem with long term records of Secchi disk depth (Z SD ) to provide a retrospective analysis of spatial and temporal variations in water clarity over the Great Lakes. A simple empirical algorithm is presented, relating Z SD to remote-sensing reflectance at 550 nm (R rs550 ). Results suggest remarkable and complex changes in water clarity over the Great Lakes. Lakes Ontario, Huron, and Michigan have seen increases in average Z SD over the three sensor periods of 58%, 49%, and 62%, respectively. Lake Erie shows highly variable Z SD with no consistent long term trends, while Lake Superior has remained fairly consistent in its lake-wide water clarity conditions. Temporal trends document the decrease in whiting events on Lake Michigan while capturing the ongoing occurrence of these seasonal bright-water events on Lake Ontario. Results indicate a divergence in Z SD trends between nearshore and offshore environments; with larger increases in offshore than nearshore Z SD and some nearshore areas suggesting a decrease in Z SD . Offshore regions of Lakes Huron, Michigan, and Superior show diminished Z SD seasonality in contrast to increasing seasonal variance in the nearshore. Spatial and temporal variations in Z SD are in agreement with documented reductions in Great Lakes bioproductivity, degrading nearshore water quality, and changing biogeochemical processes influencing whiting events and sediment resuspension.

No abstract

Little is known about the molecular and physiological function of co-occurring microbes within freshwater cyanobacterial harmful algal blooms (cHABs). To address this, community metatranscriptomes collected from the western basin of Lake Erie during August 2012 were examined. Using sequence data, we tested the hypothesis that the activity of the microbial community members is independent of community structure. Predicted metabolic and physiological functional profiles from spatially distinct metatranscriptomes were determined to be >90% similar between sites. Targeted analysis of Microcystis aeruginosa, the historical causative agent of cyanobacterial harmful algal blooms over the past ϳ20 years, as well as analysis of Planktothrix agardhii and Anabaena cylindrica, revealed ongoing transcription of genes involved in microcystin toxin synthesis as well as the acquisition of both nitrogen and phosphorus, nutrients often implicated as independent bottom-up drivers of eutrophication in aquatic systems. Transcription of genes involved in carbon dioxide (CO 2 ) concentration and metabolism also provided support for the alternate hypothesis that high-pH conditions and dense algal biomass result in CO 2 -limiting conditions that further favor cyanobacterial dominance. Additionally, the presence of Microcystis-specific cyanophage sequences provided preliminary evidence of possible top-down virus-mediated control of cHAB populations. Overall, these data provide insight into the complex series of constraints associated with Microcystis blooms that dominate the western basin of Lake Erie during summer months, demonstrating that multiple environmental factors work to shape the microbial community. F reshwater ecosystems are considered among the most endangered in the biosphere (1). One threat to fresh waters around the world is the now nearly annual occurrence of blooms of toxic cyanobacteria. Molecular signatures of cyanobacterial harmful algal blooms (cHABs) have been identified on all seven continents, and these blooms have been occurring with increased frequency and duration in recent years (2-4). The accumulation of bloom biomass has been associated with fish, avian, and mammal intoxication (5, 6); the formation of hypoxic zones (7); the production of taste and odor compounds (8, 9); and even human liver failure in extreme cases (10).The Laurentian Great Lakes are an important freshwater resource, holding ϳ18% of the world's potable water and ϳ84% of the surface waters in North America (11). cHABs have had a persistent presence in Lake Erie and other Laurentian Great Lakes for several decades (3, 12, 13). Nuisance biomass and toxin production associated with cHABs in the Great Lakes have had a detrimental effect not only on ecosystem health but also on the economic health of the surrounding communities; indeed, in August 2014, a bloom event led to the shutdown of the water supply for some ϳ500,000 residents in the region of the city of Toledo, OH (14). To date, research efforts have broadly identified nutrient inputs an...