No abstract

Anthropogenic activities in estuarine watersheds alter physical and chemical characteristics of these ecosystems. Increased nutrient loading and changes in hydrology affect oxygen budgets and subsequently alter the spatial and temporal extent of hypoxia. We used classification and regression tree analyses to examine behavioral avoidance thresholds over a range of dissolved oxygen concentrations. In an examination of 10 fish species, all avoided areas with dissolved oxygen concentrations of <2.0 mg·L–1. Our data suggest that the dissolved oxygen avoidance threshold expressed by fish may be context-dependent. Specifically, the spatial extent of hypoxia significantly affected avoidance thresholds. Fish may occupy poorer quality habitat (areas with lower dissolved oxygen levels) as conditions worsen and the size of the oxygenated refuge shrinks. When the entire estuary is oxygenated, fish utilize all depths. During hypoxic episodes, fish are restricted to oxygenated, shallow, warmer areas. This habitat compression may result in higher densities and greater overlap with potential competitors and predators. Hypoxic zones alter habitat use and may increase bioenergetic costs, potentially resulting in sublethal effects, i.e., decreased growth and condition, on the estuarine fish community.

Climate is a critical driver of many fish populations, assemblages, and aquatic communities. However, direct observational studies of climate change impacts on North American inland fishes are rare. In this synthesis, we (1) summarize climate trends that may influence North American inland fish populations and assemblages, (2) compile 31 peer‐reviewed studies of documented climate change effects on North American inland fish populations and assemblages, and (3) highlight four case studies representing a variety of observed responses ranging from warmwater systems in the southwestern and southeastern United States to coldwater systems along the Pacific Coast and Canadian Shield. We conclude by identifying key data gaps and research needs to inform adaptive, ecosystem‐based approaches to managing North American inland fishes and fisheries in a changing climate.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Fishes are important pools of nitrogen (N) and phosphorus (P), and they have important direct and indirect effects on cycling of these potentially limiting nutrients in lakes. We used a model of fish bioenergetics coupled to a mass balance model of fish N and P budgets to investigate the stoichiometric interactions among fishes, their food, and the nutrient recycling that results from this predator-prey interaction. We tested the hypothesis that the N:P ratios in fish excretion (supply ratios) are high but potentially variable because of fishes' high P requirement and the likelihood of P limitation of fish growth. A survey of 186 cases representing 18 fish species demonstrated that limitation of fish growth due to availability of P in food was exceedingly rare in natural ecosystems. Fish growth rates were energy limited in almost all systems. Fish grew at an average of 26% of the maximum potential rate determined from bioenergetics. As a result, the predicted N:P supply ratio from fish excretion was low and relatively constant (the median mass ratio for piscivores was 6.1, and 13.1 for all other fishes). Excretion rates for N and P by fishes were predictable from body size, food nutrient concentration, and environmental temperature. Nutrient recycling by fishes will tend to alleviate P limitation of phytoplankton growth, but the importance of this effect will be directly proportional to the magnitude of nutrient regeneration rates from fishes relative to other sources available to phytoplankton. RECYCLING STOICHIOMETRY OF FISHES 1817 webs (Peters and Rigler 1973, Elser et al. 1988). Fish also excrete nutrients that can be important to phytoplankton, especially in lakes where fishes feed on benthos, which results in translocation of nutrients from benthic to pelagic habitats (Lamarra 1975, Brabrand et al. 1990, Reinertsen et al. 1990, Boers et al. 1991, Carpenter et al. 1992a, b, Schindler et al. 1993, 1995, Leavitt et al. 1994, Vanni 1995). Although most studies have concentrated on P fluxes from fish excretion, N excretion is also a potentially important nutrient source from fish communities (Kraft 1992, Vanni 1995). However, our understanding of the effects of fishes on nutrient cycling in lakes remains incomplete. Because of the implications for phytoplankton community interactions and allocation patterns of N and P among ecosystem components, a new "stoichiometric approach" (Sterner 1990, Sterner et al. 1992) has been introduced that stresses the implications of elemental ratios of nutrients recycled by consumers. This approach evaluates a consumer-food inter...

Three sequential hurricanes, Dennis, Floyd, and Irene, affected coastal North Carolina in September and October 1999. These hurricanes inundated the region with up to 1 m of rainfall, causing 50-to 500-year flooding in the watershed of the Pamlico Sound, the largest lagoonal estuary in the United States and a key West Atlantic fisheries nursery. We investigated the ecosystem-level impacts on and responses of the Sound to the floodwater discharge. Floodwaters displaced three-fourths of the volume of the Sound, depressed salinity by a similar amount, and delivered at least half of the typical annual nitrogen load to this nitrogen-sensitive ecosystem. Organic carbon concentrations in floodwaters entering Pamlico Sound via a major tributary (the Neuse River Estuary) were at least 2-fold higher than concentrations under prefloodwater conditions. A cascading set of physical, chemical, and ecological impacts followed, including strong vertical stratification, bottom water hypoxia, a sustained increase in algal biomass, displacement of many marine organisms, and a rise in fish disease. Because of the Sound's long residence time (Ϸ1 year), we hypothesize that the effects of the short-term nutrient enrichment could prove to be multiannual. A predicted increase in the frequency of hurricane activity over the next few decades may cause longer-term biogeochemical and trophic changes in this and other estuarine and coastal habitats.