Benthic Food Web Flows in the Santa Monica Basin Estimated Using Inverse Methodology

Abstract

Inverse methods were used with measurements of sediment (POC) fluxes and total benthic metabolism to infer the roles of the different food-web components in the benthos of the sub-oxic Santa Monica basin off Southern California. High nitrate and sulfate fluxes into the sediments observed there guaranteed that anaerobic bacteria would have important roles. We were able to estimate the relative roles of the bacteria and the grazers that prey on the bacteria for several alternative food-web flow structures.

Our analysis showed high carbon consumption by anaerobic bacteria; an inverse relationship between grazer size and consumption rate; and more consumption of carbon by anaerobic than by aerobic grazers. Low production efficiency of anaerobic metabolism caused much of the carbon entering the benthos to be respired by the bacteria, rather than being used in bacterial growth. Bacteria consumed material poor in nitrogen while the grazers consumed material rich in nitrogen. Although bacteria consumed more carbon than did the grazers, the bacteria released equal amount of ammonia. Small protozoan microfauna consumed and mineralized most of the material available to the grazer populations.

The assumed mode of grazer feeding affected not only their bacterial consumption but also bacteria production. When bacteria were the only food for the grazers, the increased bacterial consumption was accompanied by an increase in bacterial production efficiency such that highly grazed anaerobic bacteria rivaled the efficiencies found in moderately grazed aerobic bacteria. Because of high bacterial production efficiencies, the additional link in the food-web when bacteria were the only grazer food source caused only a small absolute difference in the transfer efficiency (4% when detrital feeding was allowed and 2% when it was not).