While it is widely believed that oceanographic conditions influence benthic filter-feeding organisms, linkages between water column and population processes on rocky shores have been difficult to document. Here we combine laboratory and field studies to explore the relationship between water column conditions and the feeding, growth and population dynamics of the northern acorn barnacle Semibalanus balanoides. S. balanoides feeding behavior in a flow tank was strongly dictated by flow speeds, food concentrations, and temperatures. The percentage of barnacles feeding increased with both flow speed and food concentrations, but was depressed at high temperatures. These patterns were accurately reflected in S. balanoides feeding in the field. Moreover, within and among populat i o n~, variation in barnacle growth over a wide geographic area directly reflected corresponding variation in flow speed, food abundance, and water temperature. Flow effects on barnacles also had conspicuous population consequences. Since barnacle recruitment and growth are both enhanced at high flow speeds, barnacle density-dependent dynamics may often be strongly affected by flow conditions. Our results suggest that water column processes can powerfully affect passive filter feeding organisms on rocky shores and that oceanographic conditions may often leave a strong signature on the distribution, abundance, and dynamics of assemblages of sessile marine organisms.

Geographic vanahon in secondary production can profoundly influence populahon and community processes yet is rarely studied in benthic manne organisms In this paper w e document stnlung vanahon withln and among locations in the growth and reproducbve output of the acorn barnacle Sernlbalanus balanoides and discuss the potential consequences of our results on understanding recruitment and other population processes Within Narragansett Bay (Rhode Island, USA) barnacle growth and reproduction is far greater than at nearby open coast locations At comparable tidal heights recruits in the Bay grew to nearly twice the basal diameter and had almost 10 times the reproductive output compared to recruits on the open coast Transplant experiments suggest that the high secondary production of Bay barnacles is largely a phenotypic response to high primary production Within-and among-site patterns in barnacle secondary production associated with enhanced flow velocities were also conspicuous Barnacles at low tidal heights, in habitats exposed to large tidal currents and in waveexposed habitats had higher growth and reproduction than barnacles at hlgher tidal heights or habltats with lower velocities Water column food concentrations and fluid velocities both strongly influence barnacle growth and reproduction through their combined effects on the flux of food to indi\~lduals Our results imply that predictable variation in food supply rates may lead to important and equally predictable consequences on regional larval production and the density-dependent dynamics of adult benthic populations

We quantified daily larval settlement of the acorn barnacle Semibalan~is balanoides in a small embayment within Narragansett Bay, Rhode Island, USA, to examine the hypothesis that local wind patterns influence shoreline settlement. Daily larval settlement and the accumulation of barnacle recruits were both strongly correlated with local wind patterns within and among years. When prevailing winds were out of the south, larval settlement was enhanced on the northern side of the bay, whereas when winds were out of the north, larval settlement was enhanced on the southern side of the bay. These patterns were observed over 2 settlement seasons and led to daily as well as interannual spatial patterns in both larval settlement and the accumulation of barnacle recruits. The patterns of shoreline settlement appear to be caused by wind-induced changes in larval concentrations on different sides of the bay. Water column larval densities were strongly correlated with daily wind patterns. Larval densities were always highest on the down-wind side of the bay. Our data show that spatial and temporal patterns in the settlement of benthic invertebrates can be strongly linked to local weather conditions through the transport of larvae by wind-dr~ven currents.

It has recently been proposed that many communities are structured by a hierarchy of interactions in which facilitation by foundation species is of primary importance. We conducted the first explicit experimental test of this hypothesis by investigating the organization of positive interactions on New England cobblestone beaches. In this midintertidal community, wave-generated substrate instability and solar stress largely limit marine organisms to the shelter of cordgrass beds. Cordgrass, which can establish and persist without the aid of other foundation species, facilitates a dense assemblage of inhabitants (e.g., mussels, snails, seaweeds) with roots/rhizomes that stabilize substrate and a dense canopy that baffles waves and provides shade. Within the cordgrass bed community, ribbed mussels further enhance physical conditions and densities of other species (e.g., amphipods, barnacles) by providing crevice space and hard substrate. We conclude that cordgrass bed communities are hierarchically organized: secondary interactions (e.g., facilitation by ribbed mussels) play a key role within an assemblage dependent on primary facilitation by the independently successful foundation species cordgrass. Our results identify emergent indirect positive interactions in the form of facilitation cascades, have broad implications for conservation, and help unify existing models of community organization that were developed without considering the fundamental role of positive interactions.