Fate and effects of sewage sludge in the coastal marine environment: a mesocosm experiment

A mesocosm experiment to assess the fate and effects of sewage sludge in the coastal marine environment was conducted over 4 mo during the summer of 1984 with 9 different treatments of sewage sludge and nutrient addtions. Evidence from setthng rate studes, accumulahons of carbon and pollutant organics and rahos of in the sediment indicated up to 83 O/O of the sludge particulate inputs settled to the bottom and up to 51 % accumulated there. While no toxicity due to organics or metals was apparent, prolonged hypoxia and even anoxia occurred in upper treatment levels. At summer temperatures, sludge particulate inputs in excess of 1 g C m-2 d-' caused these hypoxic conditions. In contrast to low primary production in low and medium sludge treatments during the last 2 mo of the experiment, the highest sludge treatment had high production. We attribute these patterns to excessive grazing pressure from zooplankton and benthlc fauna in the lower 2 treatments and hypoxia-retarded grazing in the upper treatment. In the upper treatments, peaks of produchon stimulated peaks of excessive respiration suggesting that respiration of sludge particulates was enhanced or CO-metabolized with the production of fresh organic material.