Variation in top-down and bottom-up control of marine bivalves at differing spatial scales

At large spatial scales(> 1 km). the forces structuring marine communities are diverse and include oceanographic, geological. and human processes, as well as availability of regional species pools. Few studies in marine and e stuarine systems have examined the joint effects of predation (top-down) an d food availability (bottom-up) in controlling populations at these scales. We compared the relative influence of top-down and bottom-up factors to po pulation abundance of the infaunal clam Macomala balthican in two ecosystem s differing in spatial extent by an order of magnitude. the York River, 50 km long, and the Rhode River. 5 km long. In both rivers, two habitat types could be differentiated by either high or low density of Macoma. In the Yor k, Macoma abundance was tenfold greater upriver than downriver. predator ab undance was greater in the high clam density habitat (i.e. upriver), and pr edation was only evident where darns were dense (i.e. upriver, where food a vailability for predatory crabs was elevated). In this large-scale system, predators were apparently unable to travel between upriver and downriver ha bitats where food availability differed. Moreover, sedimentary carbon (i.e. food for deposit-Feeding clams) was significantly greater in the high clam -density habitat. this is consistent with the hypothesis that bottom-up fac tors principally dictate clam density, and moreover, that clam density driv es predator density. in the Rhode River, a smaller-scale system, predators could easily move among habitats differing in clam density. Macoma abundanc e and predator abundance were both threefold greater in mud than sand. In c ontrast, predation intensity was higher in sand than mud, consistent with a hypothesis of top-down control. As in the large-scale system, sedimentary carbon was significantly greater in the high clam-density habitat (i.e. mud ). In this smaller scale system, both top-down and bottom-up factors contri buted to habitat-specific patterns in clam population abundance. Hence, in this marine benthic ecosystem, bottom-up control was important at both larg e and small spatial scales, whereas top-down control was only important at the smaller spatial scale. (C) 2001 International Council for the Explorati on of the Sea.