Nutrient Enrichment of Chesapeake Bay and Its Impact on the Habitat of Striped Bass: A Speculative Hypothesis

Abstract

Stocks of striped bass Morone saxatilis have declined in the Chesapeake Bay system over the last decade. We present evidence for the working hypothesis that the decline has resulted, in part, from loss of deep-water habitat for adults, caused by limiting concentrations of dissolved oxygen that are related, in turn, to nutrient enrichment and greater planktonic production. A related hypothesis is that changes in the near-shore habitat for juvenile striped bass, involving severe declines in submerged aquatic vegetation due to nutrient-driven planktonic shading, also have contributed to the decline of striped bass. Nutrients (nitrogen and phosphorns) and chlorophyll a, an indicator of phytoplankton biomass, have increased in many areas of the bay and tributaries over the past 20 to 30 years. These trends are qualitatively correlated with greater deoxygenation of the deep channel in the mid and upper bay. During the late 1970s, summer oxygen concentrations as low as 2 ml/liter approached to within 7-8 m of the surface, allowing water stressful to striped bass to intrude onto shoal areas of the bay. The volume of Chesapeake Bay bottom waters containing 0.5 ml O2/liter or less was about 15 times greater in July 1980 than in July 1950. The combination of the expanding hypoxic pool and summer temperatures above preferred levels for adult striped bass may contribute to an "oxygen-temperature squeeze" that forces adults onto shoal areas of the bay or out of the upper bay. Many of these shoal areas now lack suitable cover for juvenile striped bass and their prey. Strong intraspecific competition among striped bass may be occurring there. Striped bass Morone saxatilis populations havedeclined for the past decade over much of their east-coast range, especially in Chesapeake Bay (Fig. 1). The decline appears to be related to a failure in year classes during the entire period since the last dominant one in 1970, which brought peak landings in 1973. The importance of the striped bass fishery to Chesapeake Bay and the effects of its decline elsewhere along the east coast has led to increased attention to this species by the United States Congress, state resource managers, and the research community. As referenced in this issue byCoutant (1985), numerous factors may contribute to the decline of striped bass stocks, and this makes it difficult I Principal affiliation: College of Marine Studies,

Nitrogen Excess in North American Ecosystems: Predisposing Factors, Ecosystem Responses, and Management Strategies

“…Atmospheric N deposition, sewage, and agriculture each reportedly account for approximately one-third of the anthropogenic N loading to the bay Oppenheimer 1991, Hinga et al 1991). Eutrophication in Chesapeake Bay is resulting in an increasingly large volume of anoxic bottom waters (Officer et al 1984), and may be the cause for the loss of striped bass from the bay (Price et al 1985). Nitrogen deposition was also calculated to contribute from 10 to over 50% of the anthropogenic loading in six other coastal marine ecosystems in eastern North America (Fisher and Oppenheimer 1991, Hinga et al 1991, Paerl 1995.…”

Juvenile weakfish Cynoscion regalis distribution in relation to diel-cycling dissolved oxygen in an estuarine tributary

“…These hypoxia events range in intensity from moderate (2 to 5 mg O 2 l -1 ) to severe (< 2 mg O 2 l -1 ) and persist for durations that range from hours (Breitburg 1992, D'Avanzo & Kremer 1994, Tyler 2004) to days (Stanley & Nixon 1992, Eby & Crowder 2002 and from weeks to months (Officer et al 1984, Price et al 1985.…”

“…Most studies that address coastal hypoxia (Officer et al 1984, Price et al 1985, Stanley & Nixon 1992, Eby & Crowder 2002 and related effects on fishes (Garlo et al 1979, Pihl et al 1991, Lenihan et al 2001, Eby & Crowder 2002 have been conducted in stratified waters where severe hypoxia develops beneath the pycnocline and events range in duration from days to months. Our study was conducted in a system where DO fluctuates over the diel cycle (24 h) as a function of biological activity (photosynthesis and respiration), with the lowest and highest concentrations typically occurring in the early morning and late afternoon, respectively (D'Avanzo & Kremer 1994, Beck & Bruland 2000, Tyler 2004).…”

Ecosystem production and respiration in response to eutrophication in shallow temperate estuaries

“…Human activity on watersheds increases delivery of nutrients to estuaries via deforestation, agricultural activities, domestic use of fertilizers, and sewage treatment. Nutrient loading to coastal receiving waters worldwide has led to excessive growth of algae, anoxia and hypoxia, and loss of shellfish and finfish habitat (McComb et al 1981, Officer et al 1984, Price et al 1985, D'Avanzo & Kremer 1994. While ecosystem processes (e.g.…”