Anoxia occurs annually in deeper waters of the central portion of the Chesapeake Bay and presently extends from Baltimore to the mouth of the Potomac estuary. This condition, which encompasses some 5 billion cubic meters of water and lasts from May to September, is the result of increased stratification of the water column in early spring, with consequent curtailment of reoxygenation of the bottom waters across the halocline, and benthic decay of organic detritus accumulated from plankton blooms of the previous summer and fall. The Chesapeake Bay anoxia appears to have had significant ecological effects on many marine species, including several of economic importance.

Eight stations in the main body of Chesapeake Bay and one on the continental shelf were sampled seven times over a period of 13 months to investigate the nitrogenous nutrition of the phytoplankton.The rates at which the phytoplankton were utilizing NO,, NO,, NHr+, and urea N were determined.The data demonstrate that for a large portion of the year there is inadequate N nutrient available to permit a single doubling of the particulate N. Over temperatures from 4" -28°C and salinities from 2-32&, there was a universally high phytoplankton preference for NH4+ and urea N over NO,-and NO,.A relative preference index indicated that NHa' concentrations in excess of O-5-1.0 pg-atom N liter-l almost totally suppressed NO, utilization.Urea N was used after NH,+ in order of prcfcrence, and when the sum of available NH4+ and urea N was insufficient to meet the phytoplankton N nutrient demand, NO; was used. When the sum of all available N nutrients was less than that required to satiate the phytoplankton demand, NIL', urea N, NO,-, and NO,-were all utilized at rates proportional to their availability. For the midbay region in October 1973, NO, was the dominant N nutrient present both in the water and in the diet of the phytoplankton.

The spring freshet increases density stratification in Chesapeake Bay and minimizes oxygen tr111nsfer from the surface to the deep layer so that waters below 10m depth experience oxygen depletion which may lead to anoxia during June to September. Respiration in the water of the deep layer is the major factor contributing to oxygen depletion. Benthic respiration seems secondary. Organic matter from the previous year which bas settled into the deep layer during winter provides moot of the oxygen demand but some new production in the surface layer may sink and thus supplement the organic matter accumulated in the deep layer.

Phosphomonoester concentrations were 0 to 0.09 E.cg-atom liter-l in Chesapeake Bay from December 1972 to December 1973. Alkaline phosphatase activity associated with natural phytoplankton assemblages indicated the cells' potential to utilize the monoesters as a phosphorus source. However, ecological interpretation of alkaline phosphatase activity data is complicated by the necessity to increase the monoester concentration in order to measure enzyme activity fluorometrically.The half-saturation constant ( K, ) was 0.31 for 3-0-methyl fluorescein hydrolysis by a natural phytoplankton assemblage and 0.75 PM for glucose-6-POp by a nanoplankter in culture, and maximum velocities (V,) were 3.2 and 6.4 nm (pg Chl a h)".In one experiment with a natural phytoplankton assemblage, organisms in the 0.8-5-pm size range comprised 78% of the plant biomass and were responsible for 70% of the phosphorus uptake from glucose-6-POs when size fractionation preceded experimental incubations. Phosphomonocsters may contribute to phytoplankton phosphorus nutrition during much of the year, but are in greatest demand in spring in Chesapeake Bay.

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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,

Phosphorus availability in estuaries may have a seasonal cycle with a maximum usually occurring in the summer when orthophosphate is released into oxygen depleted deep water and transported to the euphotic zone by turbulent mixing. Superimposed on the annual fluctuation of total phosphorus is the rapid turnover of orthophosphate and phosphorus monoesters in the euphotic zone. The concentrations of these materials in surface waters are similar and phosphate uptake kinetics from each type by natural phytoplankton assemblages are similar which suggests that phosphorus monoesters may be significant in phytoplankton phosphorus nutrition.