The role of sinking phytodetritus in structuring shallow-water benthic communities

Sinking phytodetritus from surface waters represents a prime source of high-quality food supply for marine benthic organisms. Despite its obvious relevance, the structuring role it plays in shallow sedimentary communities is surprisingly poorly studied. In this article we review the contribution of natural sources of organic food supply to abundance, composition, and diversity of shallow-water macrobenthic communities. We then present a case study that tests how the addition of spring bloom-levels of diatom phytodetritus to a 20-m deep sedimentary habitat contributes to variation in benthic community structure in coastal Newfoundland. In situ patches of otherwise undisturbed sediment were enriched by syringing high and low concentrations of phytodetritus onto their surface. After one week and five weeks, macrofauna in the patches were sampled and compared with ambient sediments where no phytodetritus was added. The experiment was conducted during the summer and then repeated in the fall in order to evaluate how seasonal variation in colonizers might influence infaunal response. Despite significant temporal changes in macrofaunal abundance and composition within and between experiments, the only response that could be attributed to the phytodetritus addition was a rapid response during the first week of the summer experiment. Multiple measures of diversity (species richness, rarefaction, Margalef's index) indicated reduced diversity with phytodetritus addition. These responses did not persist through the five weeks of the summer experiment and were not observed during the fall experiment, suggesting that the effect of food supply is short term and strongly dependent on seasonal timing. In both experiments, the organic material was largely undetectable even after one week. The rapid utilization of phytodetrital patches in shallow-water environments, in concert with higher background levels of phytodetrital flux, may represent a key difference in structuring of shallow-water and deep-sea sedimentary communities.