Response of a Detrital Foodweb to Eutrophication

The response of planktonic bacteria and micro-protozoa to an experimental eutrophication gradient in coastal marine waters was investigated. Bacterial and microflagellate numbers, as well as glutamic acid mineralization and ³H-thymidine incorporation, were measured in a series of 13 m³ mesocosms for 6 months. The systems received a daily addition of sea water (3.7% of total) and nutrients so that the nutrients added were 0, 2, 4, 16 and 32 times the anthropogenic nitrogen, phosphorus and silica loading of Narragansett Bay, Rhode Island.

Both the biomass and primary production of the planktonic algae increased in direct response to nutrient loading through the 16× treatment; the response in the 32× treatment equalled that of the 16× treatment. In general, bacterial numbers and growth (calculated from thymidine uptake) were higher in the more eutrophic tanks but there were lags in the microbial response to the peaks and valleys of the algal growth. These lags caused poor correlation between instantaneous measurements of microbial and algal parameters such as numbers of bacteria and algal production. However, when daily production was summed for the entire 6 months of the study there was excellent correlation between the total algal production and the total bacterial production (r = 0.89) for six tanks. It is difficult to convert from ³H-thymidine incorporation to absolute quantities of bacterial production. We used a conservative conversion factor derived from laboratory values for the thymidine content of bacterial cells to calculate bacterial production. Bacterial production (respiration plus growth) was nearly a constant fraction of planktonic primary production at all levels of eutrophication and averaged 31%. Our results are in good agreement with other estimates of the carbon flux through the detrital component of pelagic ecosystems.