Lisa A. Eby

Ecosystem impacts of three sequential hurricanes (Dennis, Floyd, and Irene) on the United States' largest lagoonal estuary, Pamlico Sound, NC

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 Sounds long residence time (≈1 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.

STOICHIOMETRY OF FISHES AND THEIR PREY: IMPLICATIONS FOR NUTRIENT RECYCLING

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 a...

Ecological Response to Hurricane Events in the Pamlico Sound System, North Carolina, and Implications for Assessment and Management in a Regime of Increased Frequency

Since the mid 1990s, the Atlantic and Gulf Coast regions have experienced a dramatic increase in the number of hurricane landfalls. In eastern North Carolina alone, eight hurricances have affected the coast in the past 9 years. These storms have exhibited individualistic hydrologic, nutrient, and sediment loading effects and represent a formidable challenge to nutrient management aimed at reducing eutrophication in the Pamlico Sound and its estuarine tributaries. Different rainfall amounts among hurricanes lead to variable freshwater and nutrient discharge and variable nutrient, organic matter, and sediment enrichment. These enrichments differentially affected physical and chemical properties (salinity, water residence time, transparency, stratification, dissolved oxygen), phytoplankton primary production, and phytoplankton community composition. Contrasting ecological responses were accompanied, by changes in nutrient and oxygen cycling, habitat, and higher trophic levels, including different direct effects on fish populations. Floodwaters from the two largest hurricances, Fran (1996) and Floyd (1999), exerted, multi-month to multi-annual effects on hydrology, nutrient loads, productivity, and biotic composition. Relatively low rainfall coastal hurricanes like Isabel (2003) and Ophelia (2005) caused strong vertical mixing and storm surges, but relatively minor hydrologic and nutrient effects. Both hydrologic loading and wind forcing are important drivers and must be integrated with nutrient loading in assessing short-term and long-term ecological effects of these storms. These climatic forcings cannot be managed but should be considered in the development of water quality management strategies for these and other large estuarine ecosystems faced with increasing frequencies and intensities of hurricane activity.

Climate Change Effects on North American Inland Fish Populations and Assemblages

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.

VARIABILITY AND DYNAMICS OF A DESERT STREAM COMMUNITY

Communities can be highly variable over a few years, but remain fairly constant over the long term. Evaluating measures and examining the variability associated with long-term change is useful because it increases our understanding of and our ability to predict responses to disturbances. We used long-term fish community data from Aravaipa Creek, Arizona, USA, a Sonoran Desert stream, to determine whether there had been long-term changes in the community composition, what measures best describe these shifts, and what environmental factors are correlated with the changes. Aravaipa Creek is an intrinsically variable system and one of the few remaining desert streams to support its complete historical assemblage of native fishes. Multivariate analyses illustrated important changes in composition of the native fish community that were not described by traditional measures of persistence and stability. In the early 1980s, changes in community composition were correlated with alterations in base flow, while more recent changes are likely associated with the presence of exotic species. Changes in stream morphology, hydrology, and climate have decreased flow variability, thereby increasing the likelihood of exotic establishment, and may have increased the downstream connection between Aravaipa Creek and the San Pedro River thus increasing the likelihood of repeated invasions by nonnative species. These results support previous research in intrinsically variable desert systems, which conclude that retention of high flow variability is important to conservation of the native fish community. In addition, although connectivity in aquatic lotic systems is important, isolation from large river systems teaming with exotics may be important in preserving these remnant native fish assemblages.

Hypoxia in the Neuse River Estuary: Responses of Blue Crabs and Crabbers

Abstract The fishery for blue crab Callinectes sapidus is the most valuable commercial fishery in North Carolina. In 1998, hard blue crab landings totaled more than 27 metric tons and were worth US

Effects of Hypoxic Disturbances on an Estuarine Nekton Assemblage across Multiple Scales

40.5 million. This fishery depends on a healthy estuarine habitat for crab growth and survival. In addition, nearly all the landings are taken in shallow estuaries and sounds. Like estuaries all over the United States, the Neuse River estuary, North Carolina, receives substantial nutrient loading. A shallow, wind-driven system, it experiences only intermittent density stratification and subsequent hypoxia. Blue crabs respond to this spatially and temporally dynamic hypoxia by moving to the oxygenated shallow edges of the river. When the system is well oxygenated, blue crabs also can occupy the deeper habitats in the river. Categorical and regression tree analyses show crabs occupied habitat with oxygen concentrations above 2.4 mg/L. We estimated average temporary habitat loss attributable to low oxygen in our s...

Fisheries management to reduce contaminant consumption

Disturbances influence ecological communities over a wide range of scales. We investigated the effects of localized hypoxic disturbances on an estuarine fish assemblage at several spatial (m2 and 10s km2) and temporal (days, seasons, years) scales in a multivariate framework (temperature, salinity, depth, dissolved oxygen). We examined whether seasons, years) scales in a multivariate framework (temperature, salinity, depth, dissolved oxygen). We examined whether there were consistent changes in fish and crustacean estuarine assemblage characteristics along environmental gradients and whether these relationships were altered by hypoxic disturbances. We also investigated at what scale dissolved oxygen concentration may be influencing the structure of motile estuarine assemblages and whether the size of the hypoxic zone altered its effects on the estuarine assemblage. Hypoxic disturbances altered fish and crustacean assemblages along the depth gradients that were present during well-oxygenated periods. Species diversity, richness, and catch rates were lower in hypoxic patches than in oxygenated areas. Dissolved oxygen concentration remained an important explanatory variable for patch-level assemblage dissimilarity, species richness, and diversity when data were aggregated across seasons. When we examined the data at a larger scale, by aggregating information across the study area, we did not detect influences of hypoxia on assemblage structure. Fish moved out of local hypoxic zones, but remained within the estuary even in years with extensive hypoxia. There was no effect of size of the hypoxic distrubance on whether organisms responded to hypoxia or on diversity or richness of the study site. These results suggest that these local disturbances play an important role in structuring motile species assemblages at a patch-level within an estuary, but regional factors such as recruitment and migration are important in influencing species assemblages for the entire estuary over months and years.

Evidence of Climate-Induced Range Contractions in Bull Trout Salvelinus confluentus in a Rocky Mountain Watershed, U.S.A.

Lake Michigan is a microcosm of global environmental issues. A history of problems has plagued the lake, arising from the wide range of human activities the basin supports. Much of Lake Michigan`s watershed is agriculturally developed, and the shoreline is dotted with major urban, industrial centers. The lake has supported important commercial shipping and fishing industries for more than a century. In the 1960s and 1970s eutrophication was a concern. More recently toxic contaminants, particularly PCBs, and invasions by exotic species, such as the zebra mussel (Dreissena polymorpha), have captured headlines. More than 200 years of development and exploitation have taken Lake Michigan far from a pristine state. The Lake Michigan fishery in intensively managed, and food web manipulation may more effectively reduce PCB exposure than cleanup activities do. Four management options are discussed in this article: trophic cascade; growth maximization; size of stocked fish; and selective species stocking. The most promising option, well supported by data is in many ways the simplist: selective stocking of species that accumulate contaminants at the lowest levels. 51 refs., 6 figs., 1 tab.

Effects of Road Decommissioning on Stream Habitat Characteristics in the South Fork Flathead River, Montana

Many freshwater fish species are considered vulnerable to stream temperature warming associated with climate change because they are ectothermic, yet there are surprisingly few studies documenting changes in distributions. Streams and rivers in the U.S. Rocky Mountains have been warming for several decades. At the same time these systems have been experiencing an increase in the severity and frequency of wildfires, which often results in habitat changes including increased water temperatures. We resampled 74 sites across a Rocky Mountain watershed 17 to 20 years after initial samples to determine whether there were trends in bull trout occurrence associated with temperature, wildfire, or other habitat variables. We found that site abandonment probabilities (0.36) were significantly higher than colonization probabilities (0.13), which indicated a reduction in the number of occupied sites. Site abandonment probabilities were greater at low elevations with warm temperatures. Other covariates, such as the presence of wildfire, nonnative brook trout, proximity to areas with many adults, and various stream habitat descriptors, were not associated with changes in probability of occupancy. Higher abandonment probabilities at low elevation for bull trout provide initial evidence validating the predictions made by bioclimatic models that bull trout populations will retreat to higher, cooler thermal refuges as water temperatures increase. The geographic breadth of these declines across the region is unknown but the approach of revisiting historical sites using an occupancy framework provides a useful template for additional assessments.