Effect of green macroalgal mats on burial depth of soft-shelled clams Mya arenaria

Abstract

Green macroalgal mats are becoming prevalent in many parts of the world, including on important clam-harvesting beaches in SW New Brunswick, Canada. Such mats (Enteromorpha sp. and Cladophora sp.) may be affecting populations of soft-shelled clams Mya arenaria (L.). We inves- tigated the effect of these mats on burial depth of soft-shelled clams in the field (2 impacted sites with high algal cover and 2 reference sites with no algal mats) and laboratory. At impacted sites, burial depth was significantly shallower for clams under macroalgal mats than for those in areas clear of algae. In the comparison of areas clear of algae at impacted sites to the reference sites, clam burial depth was not significantly different; rather, burial depth varied between sites independent of site type. Field measurements of benthic respiration and total sulfides were significantly higher in areas in which algal mats were present than in areas in which they were absent. In an 8 d laboratory exper- iment, clams (4 per aquarium) were placed in sand (10 cm deep) and covered with 0, 2 or 6 cm of macroalgae. Clam burial depth quickly decreased under algae and remained significantly shallower under 2 and 6 cm of algae than in the control (no algae). Near the end of the experiment, we removed the algae, and burial depth quickly increased. At the end of the experiment, clam body mass and dis- solved oxygen at the sediment surface did not differ significantly between treatments, although both variables showed a decreasing trend with increasing algal mat cover. Dissolved organic carbon in pore water at the end of the experiment was significantly higher in the 6 cm algal treatment than in the control and 2 cm algal aquaria. The presence of algal mats clearly affects burial depth of soft- shelled clams. This in turn may have impacts on predator-clam interactions and on the surrounding environment.