Temperature, salinity, flow speeds, and pldnklvn concentrations can be highly variable on the slope of Conch Reef, Florida Keys (USA), as warm surface water is mixed with cool, subsurface water forced onshore by broken internal waves. In August 1995 the water column seaward of the reef exhibited strong temperature and density stratification with a sharp pycnocline and associated subsurface chlorophyll a maximum layer at 45 to 60 m depth. On the reef slope, near-bottom zooplankton sampling at 22 to 28 m showed high concentrations of calanoid copepods, crab zoea, and fish larvae associated with upslope flow of cool, chlorophyll-rich water. In contrast to these periods of high concentrations, zooplankton concentrations were low during periods of long-shore and offshore flow of warm surface waters. Both the frequency of internal bore arrival and the mean durat~on of cool water events increase with increasing depth on the reef slope. Delivery of zooplankton to the reef is, thereforit, d s o erpec:ed :G incic;sc ' .:i?h dcpth. 2 sh?r!-term se!!!emen! experiment showed increased settlement of serpulid worms at 20 and 30 m depth compared with 15 m, and a 15.5 mo transplant experiment showed significantly enhanced growth rates of the suspens~on-feeding coral Madracis mirabilis (Scleractinia: Pocilloporidae) at 30 m depth relative to growth at 15 or 20 m. Internal tidal bores appear to be a predictable, periodic source of cross-shelf transport to Florida coral reefs and an important influence on the spatial and temporal heterogeneity of suspended food particles and larval delivery to the benthos.

Callyspongia vaginalis, a common reef sponge in the Florida Keys, USA, exhibits depthspecific differences in bioenergetics and growth that are a function of food availability. We measured several physiological parameters in situ to construct the bioenergetic budgets of sponges living in deep and shallow waters. Respiration rates were measured in a recirculating flow respirometer and pumping rates were measured by filming dye ejected from sponge oscula. In addition, inhalent and exhalent water sampled from around sponge colonies at both depths was analyzed using flow cytometry to quantify the concentration and clearance rates of picoplankton. These parameters were used to construct an energetic budget for sponges from each depth and revealed that the scope for growth was substantially greater for deep sponges compared to shallow sponges. The greater scope for growth of deep sponges is likely due to the greater abundance of picoplankton in the deep versus shallow habitat. Both naturally occurring sponges and those used in a reciprocal transplant experiment between 12 and 25 m exhibited significantly greater growth in the deep than the shallow habitat. Hence, bottom-up forcing in the form of increased food availability may be of principal importance to the growth and physiological ecology of suspension-feeding sponges.

What drives community change on large spatial scales? An opportunity to address this fundamental question was provided by a massive subtidal recruitment of the mussel, Mytilus edulis, across the southwest Gulf of Maine (GOM), USA, in 1995. Since M. edulis is consumed by many predator species, we hypothesized that the episodic increase in food resources had a bottom-up effect on populations of mussel consumers. The mussel recruitment event was unprecedented in its large spatial extent, high coverage of the sea floor, and depth of penetration into the rocky subtidal zone. During 1995-1997 we tracked mussels and their consumer populations in the area to document what happened. Surveys across a 120-km marine landscape revealed that juvenile mussels covered 33-91% of the bottom (8-12 m depth) at 13 of 17 sites between October 1995 and June 1996. The cover of mussel prey in June 1996 explained significant variation in the density and biomass of mussel predators, the sea stars Asterias spp. and rock crabs Cancer spp., at 16-17 sites 10, 13, and 23 months after mussel recruitment. These results are consistent with the hypothesis that the extensive mussel recruitment had a bottom-up effect on the subtidal food web. Levels of Asterias spp. recruitment, biomass, and density following mussel recruitment were the highest recorded at two monitored sites in 16-18 years. One year after the large pulse of mussel food resources, Asterias spp. recruitment was positively related to the percent cover of mussels, suggesting a feedback between food supply and consumer recruitment on a large spatial scale. Crab size-structure data supported the interpretation that recruitment contributed to the high densities of rock crabs recorded in July 1997. We suggest that the bottom-up effects observed in consumer populations were produced by increased growth, elevated reproduction and recruitment, and possibly by immigration.Coupling between bottom-up and top-down control was observed after a lag of 12-14 months post-mussel recruitment when predation by sea stars, rock crabs, and sea urchins eliminated extensive beds of juvenile mussels across the study region. The elimination of Mytilus prey triggered cannibalism in the sea star Asterias vulgaris, which contributed to density-dependent population declines in sea stars between 1996 and 1997. A key assumption of a consumer reproduction and recruitment response-that some of the larvae produced by local adult stocks of consumers could be retained in the study region-was tested by investigating the potential larval dispersal of sea stars and crabs in a simulated three-dimensional flow field of the southwest GOM. The simulation supported the assumption as 15-75% of particles released below 1-m depth were retained in the region over 2-5 week periods approximating the larval life-span of the consumers. Furthermore, the degree of retention increased dramatically between the surface (1 m) and 15-m depth, implying that the extent of large-scale connectivity by passive larval dispersal is highly dependent o...

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Hydrographic measurements indicate that the thermocline and the phytoplankton-rich chlorophyll maximum layer are vertically displaced over a rocky pinnacle in the central Gulf of Maine by internal waves with maximum amplitudes of 27 m. Such predictable downwelling events are linked to rapid, 2-to 3-fold increases in chlorophyll a, an indicator of phytoplankton concentration, in pulses of warm water recorded 4 cm above the bottom (29-m depth). The 1.5-5.6'C temperature fluctuations had an average period of 10.6 min and were generated on both ebb and flood tides. Local lee waves and the arrival of solitons propagated from Georges Bank are hypothesized to explain the timing of the internal waves. Because internal waves and chlorophyll mxima are pervasive features of stratified temperate seas, this mechanism of food coupling should be common in other rocky subtidal habitats.Despite the long-acknowledged dependence of bottomdwelling populations on pelagic processes (1), relatively little is known about how events in the water column influence organisms inhabiting rocky subtidal habitats along the underwater coastline of continents at high latitudes and on offshore ledges, banks, and seamounts. Recent benthicpelagic coupling research has linked the impingement of an oxygen minimum layer (2) and topographically enhanced currents (3) to the vertical zonation of invertebrate communities on Pacific seamounts. The importance of understanding the process of food supply to the sea floor is underscored by the finding that the input of particulate organic carbon can regulate the size and distribution of populations in soft bottom habitats (4, 5). Moreover, the timing ofphytoplankton supply is thought to influence the evolution of benthic invertebrate reproductive strategies (6).Internal waves are generated when the ebb tide forces a shallow thermocline over the edge of a steep bottom feature such as a bank, ledge, or continental shelf (7). A prominent depression ofthe thermocline is produced on the downstream side and held over the abrupt bottom until the ebbing currents slacken. The released depression propagates away from the topographic feature as a packet of internal waves (8, 9). Because of their ability to move phytoplankton-rich chlorophyll maximum layers downward (10-12), internal waves are a likely mechanism of benthic-pelagic food coupling. To date, investigations of internal wave effects on phytoplankton distribution have been restricted to the water column, and it is not evident that downwelled chlorophyll maximum layers could reach the rough surface of rocky bottoms where topographically induced upwelling could redirect downward flow (13-15).We studied benthic-pelagic coupling at Ammen Rock Pinnacle (ARP), a rocky ledge located 105 km offshore in the central Gulf of Maine characterized by steep bottom topography, swift currents (averages, 12.7-25.5 cm/sec) and abundant populations of suspension-feeding sponges, anemones, bryozoans, and ascidians ( Fig. 1 and refs. 16 and 17). Observations of rapid vertical...

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We tested whether larval black flies actively control the positioning of their feeding appendages (labral fans), and if so, whether their posture represents a balance between the conflicting demands of drag and feeding. We compared the postures of live larvae with the postures of larvae killed by heat-shock in three different flow regimes in a laboratory experiment; we assumed that the postures of heat-killed larvae approximated a passive response to drag. The average height of the labral fans above the bed declined significantly in faster flows, and was significantly greater in live than dead larvae. There was also a significant interaction effect, since the difference between the fan heights of live and dead larvae was greater in slower flows. Two mechanisms may contribute to this result. Larvae in slower flows have to increase their fan heights more than larvae living in faster flows to achieve comparable increases in velocity and thus particle flux. In addition, muscular strength may limit the feeding postures larvae can assume. The fan heights of live larvae also varied depending on the concentration of food particles: larvae exposed to low food concentrations held their fans higher above the bed than did larvae exposed to high food concentrations in the same flow regime. This change in posture is due neither to an uneven particle concentration in the boundary layer nor to added drag from particles trapped in the labral fans. Collectively, our results indicate that these suspension feeders actively control their feeding posture, and suggest that these varying postures represent a dynamic balance between the conflicting needs of minimizing drag and maximizing feeding.