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Stable isotope ratios (513C) from samples of water, sediments, and biota traced the behavior of organic carbon for 3 summer months in estuarine mesocosms (three controls, three with added sewage sludge, three with added inorganic nutrients). Isotope ratios proved to be a useful quantitative tracer for sewage carbon as well as for the fresh phytoplanktonic carbon produced during nutrient fertilization. Sewage sludge sedimented within hours of its addition, and approximately 50% remained in sediments after 99 days. The sludge was not inert, but was biologically oxidized at rates similar to those of phytoplankton carbon. Its residence time in the water column was too short for uptake by zooplankton, but it was readily assimilated by some benthic organisms. Fresh phytoplanktonic carbon from nutrient-induced blooms was isotopically heavy and thus distinguishable from old primary production (fixed before the experiment). It flowed through the pelagic and benthic food webs more extensively and more uniformly than did sludge carbon.

zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Wastewater solids were added to a series of marine mesocosms at the Marine Ecosystem Research Laboratory (MERL), University of Rhode Island. There was a range of sludge addition simulating 1, 4, and 8 times the average nitrogen enrichment of Narragansett Bay. Three other mesocosms received inorganic nutrients at similar levels, and three mesocosms were maintained as controls. In systems receiving nutrients or sludge, primary production was initially increased. Respiration of the increased loading in the sludge treatment generally resulted in decreased dissolved oxygen concentrations. In the highest level of sludge treatment, the benthic respiration resulted in anoxic conditions in the fourth month of the experiment. The benthos seemed to be the area primarily affected by wastewater disposal. Species feeding at or near the sediment-water interface showed a strong positive density response to sludge addition because of the increase in available food. Based on the findings of this experiment and comparison with marine areas of wastewater disposal, the condition of the benthic community can be generally defined based on organic loading rates. Organic loading rates less than approximately 0.1 g/m 2 • d C had little effect on the sediment community. Loading rates between 0.1 and 1.0 g/m 2 «d C resulted in an enriched sediment community, while loading over 1.5 g/m 2 -d C produced degraded conditions. The quantification of the relationship between organic loading and benthic response presented here can be useful in evaluating wastewater treatment and disposal options.zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Water Environ. Res., 65, 879 (1993).

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