Seagrass patch size and survivorship of an infaunal bivalve

In nature, seagrass beds can occur as a mosaic of differently sized patches in a matrix of unvegetated sediments. While continuous, extensive seagrass meadows are recognized for their importance in providing refuge for a variety of organisms, the refuge potential of smaller (1-10 m across) vegetated patches has not been widely investigated. The degree of protection afforded by small seagrass patches may vary with patch size due to changes in edge to interior ratios and/or changes in plant characteristics. In mark-recapture experiments conducted at three separate sites, survivorship (i.e., those not lost to predation) of juvenile hard clams (Mercenaria mercenaria) varied among sites and increased with the areal extent of natural seagrass cover at all sites. At least 40% more clams were recovered live from one site than from the other two sites. Twice as many clams were recovered live from large patches of seagrass (5-10 m across) than from small (ca 1 m across), 18 times more were recovered live from large patches than from unvegetated sediments, and approximately eight times as many were recovered live from small patches than from unvegetated sediments. These patterns in clam survivorship were positively, but not significantly, correlated to patterns in below-ground biomass and shoot density among sites and seagrass patch-size treatments. When below-ground biomass and overall shoot density were held constant by using artificial seagrass mats there was no significant difference in the proportion of clams recovered live between 1 x 2 m and 4 x 4 m patches of seagrass. These results suggest that the refuge potential of natural patches of seagrass for infaunal prey species changes with patch size due to differences in the structural characteristics of the vegetation, not changes in the areal extent of the habitat.

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