Internal tidal bores generated by breaking internal waves cause drs.matic, high-frequency variation in temperature, salinity, water velocities, and concentration of chlorophyll .z on Conch Reef, Florida Keys. The arrival of bores on the reef slope is linked to a semidiurnal internal tide and is marked by temperature drops of up to 5.4"C and salinity increases of up to 0.6% in l-20 min. The:;e changes are accompanied by the sudden onset of upslope flow l-l 5 m above the bottom with speeds of lo-30 cm s-l. Cool, high-salinity water is transported from below the thermocline seaward of the reef and is resident on the reef slope for up to 4 h before it mixes with surface waters and recedes downslope. Compared with ambient surface water, this deep water can contain significantly elevated concentrations of di:;solved nitrate. Physical variability produced by this mechanism increases significantly with depth on the ree ?slope. Analysis of 3-yr temperature records indicates the arrival of internal bores is a consistent feature at this site from May through November, with peak activity in July-September. Pulsed delivery of subthermocline water appears to significantly affect the temperature, nutrient, and particle flux regimes on this coral reef.The sources, dynamics, and consequences of physical variability in coral reef ecosystems have interested biologist for at least 150 years. For example, Darwin's (1962 [ 18421) observation that reefs grow fastest near shelf edges, Goreau's ( 19 5 9) description of the effect of wave exposure on coral species distributions, Connell's (1978) formulation of the relationship between disturbance and species diversity, and many recent studies of physical disturbance in reef ecosystems (see Hughes 1993) all recognize the fundamental importance of temporal and spatial environmental variability.Single factors rarely explain the patterns and dynamics of coral reefs. However, the study of environmental variability provides an underlying context for understanding these complex ecosystems. BeAcknowledgments

We monitored settlement of sea urchins (Strongylocentrotus spp.) and crabs (primarily Cancer spp.) and concurrent physical variable:; at Bodega Marine Laboratory (BML) and Salt Point (northern California) fromApril through August 1992.lJpwelling favorable winds led to lower temperatures, higher salinities, and lower subsurface pressures at BMI,, while periods of relaxation from upwelling typically caused a lagged reversal of each of these trends. This response to relaxation was due to a combination of alongshore and onshore flow of near-surface water at this site. Crab settlement was significantly correlated with the rate of temperature increase, corresponding to relaxation events. In contrast, sea urchins settled primarily during an unusual relaxation event which poss,ibly involved remote forcing. Sea urchin and crab settlement were negatively correlated. Possible reasons include residence in different water masses, different biomechanical characteristics, and predation by crab larvae on sea urchin larvae. The time-course of settlement at Salt Point was similar to that at BML for bloth species groups.Although transport and nearshore oceanographic conditions have been a focus of settlement investigations since the early days of larval ecology (Young 1990), many questions remain regarding physical oceanographic influences on the transport and settlement of marine invertebrate larvae in the coastal ocean. 'The specific transport mechanisms underlying successful settlement have been identified for only a few species (e.g. Shanks 1986; Roughgarden et al. 1988; Farrell et al. 199 1). Several recent studies have focused specifically on the role of variable coastal winds in larval transport and settlement (e.g. Johnson et al. 1986; Hobbs et al. 1!)92). In this study, we Acknowledgments

Previous studies have shown that settlement of several crab species along the coast north of Point Reyes (38°00'N, 123°00'W) occurs primarily during relaxation from upwelling, when warm water flows poleward from the Gulf of the Farallones. During 1994 and 1995 we sampled planktonic larval distributions and hydrography both south and north of Point Reyes during upwelling to test whether high concentrations of crab and rockfish larvae were concentrated in the source of the relaxation flow to the south of Point Reyes. An upwelling plume off Point Reyes and an "upwelling shadow," indicated by warmer, less saline water in the northern Gulf, were evident in both years, as were frontal regions that marked the boundaries between water types of three different types: (1) newly upwelled, (2) oceanic, and (3) San Francisco Bay outflow. In addition, there was a fourth type, termed Gulf water, that was a mixture of these three types. Concentrations of larvae of cancrid, pinnotherid, and "coastal" crabs and rockfishes were high south of Point Reyes but were low or absent in the newly upwelled water north of the point. Within the upwelling shadow, these meroplankton taxa were associated with different water masses. Several intertidal crab species and early-stage cancrid crabs were concentrated in San Francisco Bay outflow water, and coastal Gulf water late-stage cancrid crabs, early-and late-stage pinnotherid crabs, and rockfishes were concentrated at the frontal region between newly upwelled and Gulf water. Of the taxa examined, only rockfishes were found offshore in oceanic water. The high concentrations of meroplankton observed suggest that the Gulf of the Farallones is an important retention area for larvae that settle into coastal populations in the Gulf and to the north via poleward transport during upwelling relaxation.

During spring and summer 1993, we monitored settlement of crabs (primarily Cancer spp.) and sea urchins Strongylocentrotus spp. in conjunction with physical variables associated with coastal circulation to investigate how physical conditions influence the spatial distribution of recruitment along the coast. Observations were made along a 100 km stretch of the northern California coast (USA), from the Gulf of the Farallones north to Point Arena. Temperature, salinity and wind stress data indicated fluctuations in upwelling and provided evidence for the alongshore, northward flow of warm, low salinity water during upwelling relaxation events which typically lasted several days. On a weekly time scale, crab settlement was positively correlated with temperature, and negatively correlated with salinity, indicating that settlement occurred during relaxation events. Correlations were higher north of Point Reyes, where high settlement occurred only during relaxation, than south of Point Reyes, where settlement occurred both during relaxation events and to a lesser degree during upwelling. Overall crab settlement was higher south of Point Reyes. On a daily time scale, crab settlement north of Point Reyes was associated with the sharp increase in temperature observed as the relaxation current reached that point on the coast. This association suggested that crabs were transported northward alongshore In the thermal front which propagated northward during each relaxation event. This alongshore transport mechanism may be responsible for the predictable pattern of settlement variability within this system: with continuous, event-modulated settlement south of Point Reyes and episodic, event-dependent settlement to the north. Similar physical/biological interactions may occur at other points along this coast and along the midlatitude boundaries of other oceans.

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.