Four agricultural streams were examined to determine if nitrogen or phosphorus was limiting the growth of the attached algal communities. Experiments were conducted in situ using nutrient-diffusing artificial substrata. Experiments conducted over a 2-year period demonstrated that the benthic algal communities in these streams were seldom limited by nitrogen or phosphorus. Nitrogen was, however, found limiting on two occasions when ambient nitrogen levels were reduced. These experiments were characterized by extended low flows and warm water temperatures. Large algal mats which proliferate during these periods may be responsible for reducing the available nitrogen to a limiting level. Major storm events are thought to interrupt the development of nitrogen limiting conditions by scouring the algae and increasing the nitrogen loading from the watershed. Water temperature was demonstrated as an important factor in controlling periphyton growth rates and may have influenced algal response to nutrient input. Ammonium additions often enhanced algal growth in the absence of nitrate stimulation.

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Repeated chronic toxicity experiments were conducted with the Chesapeake Bay zooplankter Acartia tonsa to evaluate its sensitivity to tributyltin (TBT). Experiments were initiated with A . tonsa nauplii <48 h old. All tests were conducted with continuous-flow conditions and measured TBT concentrations. Results from a 9-d range finding experiment and two 6-d chronic experiments are presented.TBT test concentrations were maintained with minimal fluctuations over time and toxicity results were similar in the repeated chronic experiments. The range-finding experiment demonstrated severe reductions in survival of A . tonsu at the lowest measured concentration (0.029 pg TBTIL).In the 6-d experiments, 0.023 and 0.024 p g TBT/L were the lowest measured TBT concentrations to significantly reduce survival relative to the controls (i.e., lowest observed effect concentration [LOEC]). No observed effect concentrations (NOEC) measured at 0.012 and 0.010 pg TBT/L were used to calculate chronic values of 0.016 and 0.017 pg TBT/L for these two experiments. A. tonsa nauplii are very sensitive to TBT relative to other estuarine organisms. These toxicity data are environmentally relevant since TBT concentrations exceeding the chronic value for A. tonsa have been reported from numerous locations in Chesapeake Bay.

In situ and mobile on-site striped bass prolarval and yearling survival studies were conducted in the Choptank River and in the Chesapeake and Delaware (C&D) Canal area of the Upper Chesapeake Bay. Extensive chemical analyses of both organic and inorganic contaminants in the habitat water were performed and water quality parameters were monitored during these experiments. Surviving yearling striped bass were subjected to histological examination.Prolarval survival data from both the in situ and mobile on-site studies were similar for each spawning area. Prolarval survival ranged from 30 to 45% after 96 h of exposure to Choptank River water in four separate experiments (two in situ, two on-site); control survival was greater than 74%.Based on a comparison of laboratory toxicity data and reported field concentrations in the Choptank River for various water quality and contaminant conditions, the following combination of conditions was potentially stressful to striped bass prolarvae: low hardness (36 to 48 mg/L CaCOd, monomeric aluminum (0.150 mg/L), cadmium (0.003 mg/L) and copper (0.040 mg/L). Yearling survival in both in siru and on-site tests ranged from 93 to 100% in both the control and Choptank River water after 9 d of exposure. Survival of striped bass prolarvae in C&D Canal water ranged from 52.5 to 70% after 96 h of exposure in four separate experiments. Control survival of prolarvae was not significantly different. Survival of yearling striped bass in on-site and in situ tests was 95% or greater in C&D Canal water and control water. Acute contaminant effects were not verified in the C&D Canal area.

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