Structure and function in a model coastal ecosystem: silicon, the benthos and eutrophication

Abstract

Lowering the N:Si ratio (14:l to 1 : l ) in nutrient (N, P, Si) additions to large (13 m3) outdoor mesocosms with and without an intact benthic community generally resulted in higher standing stocks of diatoms. An enhanced &atom flora during the year-long experiment was not accompanied by a concomitant reduction in the extent of eutrophication caused by nutrient loading, as judged by oxygen concentration and metabolism or by chlorophyll a biomass. Enhanced growth of Atlantic menhaden Brevoortia tyrannusin the low N:Si ratio treatment suggested that diatoms promote transfer of energy to higher trophic levels. Removal of the benthic community and associated sediments had a dramatic effect on pelagic community structure. Zooplankton were holoplanktonic in character and higher trophic levels were dominated by ctenophores, medusae, chaetognaths and fish. In the presence of an intact benthos the zooplankton had a larger meroplanktonic component and higher trophic levels were dominated by larvae of benthic adults (anemones, shrimp) or temporary planktonic adults (mysids).