Patterns of productivity during eutrophication: a mesocosm experiment

In a 28 mo mesocosm experiment, levels and patterns of productivity and respiration were observed for a range of nutrient additions selected to provide a gradation from conditions in lower Narragansett Bay, Rhode Island, USA, to maximum impact for a n urban estuary receiving present day levels of sewage effluents.For a 32-fold increase in nutrients, system apparent production increased by only a factor of 3 5.Seasonal patterns of autotrophy during the winter-spring diatom bloom a n d heterotrophy during summer and early fall occurred at all treatment levels.With the exception of the 8 X treatment, all treatments above 2 X had a greater respiratory demand in the water column than the benthos.The highest treatment mesocosm (32 X ) went briefly anoxic during the second summer of the experiment during a period when little productivity was occurring in the water column.