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A mechanistic understanding of the effects of nutrient enrichment in lotic systems has been advanced over the last two decades such that identification of management thresholds for the prevention of eutrophication is now possible. This study describes relationships among primary nutrients (phosphorus and nitrogen), benthic chlorophyll a concentrations, daily dissolved oxygen (DO) concentrations, and the condition of macroinvertebrate and fish communities in small rivers and streams in Ohio, USA. Clear associations between nutrients, secondary response indicators (i.e., benthic chlorophyll and DO), and biological condition were found, and change points between the various indicators were identified for use in water quality criteria for nutrients in small rivers and streams (<1300 km(2)). A change point in benthic chlorophyll a density was detected at an inorganic nitrogen concentration of 0.435 mg/l (+/-0.599 SD), and a total phosphorus (TP) concentration of 0.038 mg/l (+/-0.085 SD). Daily variation in DO concentration was significantly related to benthic chlorophyll concentration and canopy cover, and a change point in 24-h DO concentration range was detected at a benthic chlorophyll level of 182 mg/m(2). The condition of macroinvertebrate communities was related to benthic chlorophyll concentration and both minimum and 24-h range of DO concentration. The condition of fish communities was best explained by habitat quality. The thresholds found in relationships between the stressor and the response variables, when interpreted in light of the uncertainty surrounding individual change points, may now serve as a framework for nutrient criteria in water quality standards.

Relative abundances of young-of-the-year (YOY) spot (Leiostomus xanthurus) sampled by trawling in two tidal creeks on Masonboro Island, North Carolina, were highest in the shallow creek margins. Abundances of benthic invertebrates sampled by cores in one of the study creeks, were also maximal at the creek margin. Polychaetes associated with the creek margin composed the largest fraction of spot diets, indicating spot located in the creek margins were in the area of highest food availability. Spot predators sampled by trawling and seining were uncommon and were primarily juvenile (75–200 mm total length) southern flounder (Paralichthys lethostigma), hake (Urophycis spp.), and bluefish (Pomotomus saltatrix). Large YOY spot experienced a size refuge from predation, as only small YOY spot were found in predator stomachs. In laboratory experiments testing the effects of the presence of a predator, food, or both food and predators on the depth distribution of YOY spot, food had a stronger effect on spot distributions than predators The results of field observations and laboratory experiments suggest food has the strongest effect on spot distribution in the field, and that food is relatively more important than refuge from predation to the nursery function of shallow estuarine habitats.

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