Effects of green algal mats on infaunal colonization of a New England mud flat - long-lasting but highly localized effects

The response of benthic infauna to green algal mats is commonly examined in either small-scale short-term experiments or large-scale long-term data sets that are averaged over large areas. In this study, we used a small-scale approach to study long-term effects caused by the recurring appearance of green algal mats in the early 1990's on a New England mud flat. Algal mats frequently covered the inner parts of the mud flat, where algae persisted for 6 months between July and December, and were incorporated into the sediment after the growth season. These inner parts of the mud hat differed from the outer parts, where green algae never occurred. Between 1979 and 1996, infaunal numbers increased ten-fold at the 37 stations in the inner part, but not at the 19 stations in the outer part of the mud flat. Detritus-feeding annelids primarily contributed to the numerical increase of infaunal colonizers; grazing gastropods and amphipods, and suspension- and filter-feeding bivalves showed no change. The recurring development and deposition of green algae in the inner part of the mud flat resulted in a localized yet persistent change in the infaunal community. The results suggest that it is necessary to follow the fate of algal mars after the growth season. We propose that effects on infaunal colonizers are most severe and long-lasting where decaying algal mats finally become incorporated into the sediment. Brood protecting annelids are most likely to benefit from detrital material provided by moderately dense algal mats.