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While recent research has shed insight into how bottom hypoxia affects pelagic food webs in coastal marine ecosystems and natural lakes, its effects on man-made lake (reservoir) food webs remains more incomplete. To address this gap, we conducted a study in two midwestern USA reservoirs to examine how the spatial overlap and vertical distributions of dominant zooplanktivores (i.e., pelagic fish, bentho-pelagic Chaoborus spp.) and their prey vary between periods of normoxia and hypoxia. Surprisingly, we found high levels of spatial overlap between zooplankton and both intermediate consumers (pelagic fish and Chaoborus) during both normoxia and hypoxia, though the extent of spatial overlap was higher during hypoxia at night relative to day. As expected, pelagic fish and zooplankton avoided hypoxic waters, and Chaoborus moved from hypoxic waters during the day to the well-oxygenated surface waters at night. Using our findings, we discuss the potential influence of bottom hypoxia and Chaoborus on the function and structure of north-temperate reservoir food webs. Si des travaux de recherche recents ont ameliore la comprehension de l'incidence de l'hypoxie de fond sur les reseaux trophiques pelagiques dans les ecosystemes marins cotiers et les lacs naturels, la comprehension des effets de cette hypoxie sur les reseaux trophiques de lacs artificiels (reservoirs) demeure moins complete. Pour combler cette lacune, nous avons mene une etude dans deux reservoirs du Midwest americain afin d'examiner les variations du chevauchement dans l'espace et de la repartition verticale de zooplanctivores dominants (c.-a-d., poissons pelagiques et benthopelagiques du genre Chaoborus) et de leurs proies entre des periodes de normoxie et d'hypoxie. Etonnement, nous avons releve des degres eleves de chevauchement dans l'espace entre le zooplancton et les deux types de consommateurs intermediaires (poissons pelagiques et Chaoborus) tant durant des conditions de normoxie que d'hypoxie, l'ampleur de ce chevauchement etant toutefois plus grande durant l'hypoxie la nuit que le jour. Comme prevu, les poissons pelagiques et le zooplancton evitent les eaux hypoxiques, et les Chaoborus se deplacent entre des eaux hypoxiques durant le jour et des eaux de surface bien oxygenees la nuit. A la lumiere de nos observations, nous discutons de l'influence potentielle de l'hypoxie de fond et des Chaoborus sur la fonction et la structure des reseaux trophiques de reservoirs de regions nordiques temperees. [Traduit par la Redaction], Introduction Bottom hypoxia ( In general, organisms have been shown to move vertically (up) or horizontally (nearshore) to avoid bottom hypoxia, which in turn could alter their spatial overlap and [...]

ABSTRACT. Municipal water managers use copper sulfate (CuS[O.sub.4]) to control algae, predominantly phytoplankton, in water supply reservoirs. In multiple-purpose upground reservoirs in northwestern Ohio, CuS[O.sub.4] application regimens vary from no [...]

Seasonal degradation of aquatic habitats from hypoxia occurs in numerous freshwater and coastal marine systems and can result in direct mortality or displacement of fish. Yet, fishery landings from these systems are frequently unresponsive to changes in the severity and extent of hypoxia, and population-scale effects have been difficult to measure except in extreme hypoxic conditions with hypoxia-sensitive species. We investigated fine-scale temporal and spatial variability in dissolved oxygen in Lake Erie as it related to fish distribution and catch efficiencies of both active (bottom trawls) and passive (trap nets) fishing gears. Temperature and dissolved oxygen loggers placed near the edge of the hypolimnion exhibited much higher than expected variability. Hypoxic episodes of variable durations were frequently punctuated by periods of normoxia, consistent with high-frequency internal waves. High-resolution interpolations of water quality and hydroacoustic surveys suggest that fish habitat is compressed during hypoxic episodes, resulting in higher fish densities near the edges of hypoxia. At fixed locations with passive commercial fishing gear, catches with the highest values occurred when bottom waters were hypoxic for intermediate proportions of time. Proximity to hypoxia explained significant variation in bottom trawl catches, with higher catch rates near the edge of hypoxia. These results emphasize how hypoxia may elevate catch rates in various types of fishing gears, leading to a lack of association between indices of hypoxia and fishery landings. Increased catch rates of fish at the edges of hypoxia have important implications for stock assessment models that assume catchability is spatially homogeneous. Une degradation saisonniere des habitats aquatiques decoulant de l'hypoxie caracterise de nombreux systemes marins cotiers et d'eau douce et peut entrainer la mortalite directe ou le deplacement de poissons. Cela dit, les debarquements provenant de ces systemes sont bien souvent peu sensibles aux changements de l'intensite et de l'etendue de l'hypoxie, et les effets a l'echelle de la population sont difficiles a mesurer sauf pour les especes sensibles a l'hypoxie dans des conditions d'hypoxie extreme. Nous avons etudie la variabilite temporelle et spatiale a petite echelle de l'oxygene dissous dans le lac Erie et son lien avec la repartition de poissons et l'efficacite d'engins actifs (chaluts de fond) et passifs (filets-pieges). Des enregistreurs de temperature et d'oxygene dissous places pres de la bordure de l'hypolimnion temoignaient d'une variabilite beaucoup plus grande que prevu. Des episodes hypoxiques de duree variable etaient frequemment ponctues de periodes de normoxie, ce qui concorde avec la presence de vagues internes de haute frequence. Des interpolations de haute resolution des resultats de leves hydroacoustiques et de la qualite de l'eau donnent a penser que l'habitat des poissons est comprime durant les periodes hypoxiques, produisant des densites de poissons plus elevees en bordure de zones d'hypoxie. Aux emplacements fixes d'engins de peche commerciale passifs, les plus grandes prises se produisaient quand les eaux de fond etaient hypoxiques pour des proportions de temps intermediaires. La proximite de l'hypoxie explique des variations significatives des prises au chalut de fond, dont des taux de prises plus grands en bordure des zones d'hypoxie. Ces resultats soulignent comment l'hypoxie peut rehausser les taux de prises de divers types d'engins de peche, se traduisant par l'absence de lien entre les indices d'hypoxie et les debarquements. Des taux de prises accrus en bordure de zones hypoxiques ont d'importantes consequences pour les modeles d'evaluation des stocks reposant sur l'hypothese de l'homogeneite spatiale de la capturabilite. [Traduit par la Redaction], Introduction Hypoxia in aquatic systems is a worldwide phenomenon often linked to excessive nutrient loadings from human activities (Diaz 2001; Diaz and Rosenberg 2008). Numerous experimental laboratory and field studies [...]

Rising temperatures caused by climate change are likely to affect cool‐water and warm‐water fishes differently. Yet, forecasts of anticipated temperature effects on fishes of different thermal guilds are lacking, especially in freshwater ecosystems. Towards this end, we used spatially explicit, growth rate potential (GRP) models to project changes in seasonal habitat quality for a warm‐water piscivore (largemouth bass Micropterus salmoides), a cool‐water piscivore (walleye Sander vitreus) and a hybrid piscivore (saugeye S. vitreus × S. canadensis) in two Midwestern reservoirs. We assessed habitat quality for two periods (early and middle 21st century) under two realistic greenhouse gas emission scenarios (a mid‐century emissions peak and a rapid continuous increase in emissions). Largemouth bass were projected to experience enhanced or slightly reduced habitat during all seasons, and throughout the mid‐21st century. By contrast, walleye habitat was projected to decline with anticipated warming, except during the spring in the smaller of our two study reservoirs and during the fall in the larger of our two study reservoirs. Saugeye habitat was projected to either increase modestly or decline slightly during the spring and fall and declines in habitat quality and quantity that were smaller than those for walleye were identified during summer. Collectively, our findings indicate that climate warming will differentially alter habitat suitability for reservoir piscivores, favouring warm‐water species over cool‐water species. We expect these changes in habitat quality to impact the dynamics of reservoir fish populations to varying degrees necessitating the consideration of climate when making future management decisions. [ABSTRACT FROM AUTHOR]

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Both abiotic and biotic explanations have been proposed to explain recent recurrent nuisance/harmful algal blooms in the western basin and central basin of Lake Erie. We used two long-term (>10 years) dataseis to test ( 1 ) whether Lake Erie total phytoplankton biomass and cyanobacterial biomass changed over time and (2) whether phytoplankton abundance was influenced by soluble reactive phosphorus or nitrate loading from agriculturally-dominated tributaries (Maumee and Sandusky rivers). We found that whereas total phytoplankton biomass decreased in Lake Erie's western basin from 1970 to 1987, it increased starting in the mid-1990s. Total phytoplankton and cyanobacterial seasonal (May-October) arithmetic mean wet-weight biomasses each significantly increased with increased water-year total soluble reactive phosphorus load from the Maumee River and the sum of soluble reactive phosphorus load from the Maumee and Sandusky rivers, but not for the Sandusky River alone during 1996-2006. During this same time period, neither total phytoplankton nor cyanobacterial biomass was correlated with nitrate load. Consequently, recently increased tributary soluble reactive phosphorus loads from the Maumee River likely contributed greatly to increased western basin and (central basin) cyanobacterial biomass and more frequent occurrence of harmful algal blooms. Managers thus must incorporate the form of and source location from which nutrients are delivered to lakes into their management plans, rather than solely considering total (both in terms of form and amount) nutrient load to the whole lake. Further, future studies need to address the relative contributions of not only external loads, but also sources of internal loading. [ABSTRACT FROM AUTHOR]