Influence of seagrass habitat patch size on growth and survival of juvenile bay scallops, Argopecten irradians concentricus (Say)

Abstract

Patchy seagrass meadows are common features on high energy sand shoals in temperate coastal waters. We tested the effect of seagrass patch size within patchy seagrass meadows on growth and survival of juvenile bay scallops, Argopecten irradians concentricus (Say), during spring (two sites) and fall (one site) settlement periods in Back Sound, North Carolina, USA. Because plant characteristics such as density and species composition (proportion of Zostera marina L. and Halodule wrightii Aschers) may influence growth and predation rates, we also quantified these variables during each time period. Weekly growth rates of scallops in the spring were more than twice that of the fall (∼1.5 vs. 0.6 mm per week). Spring growth rates were statistically greater at Cape Lookout than at Oscar Shoal. The actual difference in growth, however, was <1 mm (7.34±0.13 mm and 6.45±0.19 mm over 5 weeks, respectively), and most likely would not have produced significant biological effects such as differential predation rates. Average growth of juvenile scallops placed in the center (0.75×0.75 m area) of replicate small (∼1 m across) patches was faster than in large (>8 m across) patches over 5–7 week periods during both seasons. The difference in growth, however, was small (∼1 mm) and would probably have little biological significance. In the spring, scallop survivorship after 2 weeks did not differ between sites or among seagrass patches of varying size. After an additional 3 weeks however, scallop survivorship was approximately 30% greater at the site dominated by Zostera marina with lower shoot density, but there was no difference in survivorship among seagrass patch sizes. The same pattern was observed over the cumulative 5-week period. In the fall, predation rates on juvenile scallops were significantly higher in small patches than in large over very short time intervals (e.g., 24 h). Over 3-, 4- and 7-week time intervals, however, there was no effect of patch size on scallop survival, even though shoot densities were significantly greater in medium- and large-sized patches than in small. Lack of a long-term patch size and/or seagrass density effect on scallop survivorship may be due to differential responses to patch size by the many potential predators that prey on juvenile scallops.