The effects of seagrass patch size and energy regime on growth of a suspension-feeding bivalve

Abstract

An investigation of how the presence of seagrass and seagrass patch size in high- and low-energy environments affected growth of an infaunal bivalve (hard clam, Mercenaria mercenaria) was carried out. Two general size classes of clams were used, mean length ca. 40.0 mm (41.9 mm) and mean length ca. 20.0 mm (22.3 mm and 19.0 mm), to see if growth responses were the same for large- and small-sized clams. Seagrass density, length and species composition as well as sediment characteristics were measured at high- and low-energy sites to determine if changes in seagrass and/or sediment dynamics could explain observed growth patterns. The incidence of siphon cropping was also evaluated as a potential influence on clam growth. Large clams grew faster inside than outside vegetation at both high- and low-energy sites. Within the vegetation, growth of large clams was faster in medium-sized patches of seagrass (2-3 m across) than in small patches (1 m across) and was intermediate in large patches (> 4-5 m across). There was no apparent correlation between seagrass shoot density, blade length, or species composition and growth of clams in patches of different size. Growth of small clams (22.3 mm) within seagrass was independent of the two seagrass patch sizes tested (large vs. small), but did vary among sites. Growth of clams was not correlated with differences in shoot density, blade length or species composition among sites. Growth of small clams (19.0 mm) also varied with energy regime and with presence/absence of vegetation. Small clams grew significantly more within seagrass under both high- and low-energy conditions, but the effect was more pronounced at high-energy sites than at low. Seagrass shoot density, blade length, and species composition did not vary between high and low energy regimes, but did vary among sites from the beginning to the end of the experiment. Long-term averages of sediment stability based on grain size characteristics suggest that the baffling effect of seagrass varies greatly with energy regime. Vegetated sediments at high-energy sites contained significantly more fine material than the unvegetated sediments while there was no difference in the fine fraction between vegetated and unvegetated sediments at low-energy sites. The difference in sediment stability between seagrass cover under high- and low-energy conditions may contribute to the magnitude of the difference in the growth response of small clams to the presence of vegetation at exposed and protected sites. Other factors also contributed to the increased growth of clams in seagrass beds at the protected sites where grain size analysis suggests similar sedimentary dynamics both within and outside of the vegetation. Mean adjusted siphon weights, however, for both large and small clams were independent of seagrass cover and energy regime implying that siphon nipping differences did not influence growth patterns of clams. Based on this study, and previous investigations, the effect of seagrass cover on growth of clams appears to be the result of a complex interaction among food supply, predation disturbance, and sediment stability with the relative importance of these processes varying with size of the clam, hydrographic regime, and local site differences.