Dynamics and Structure of Marine Populations and Communities

Behavioral-Physical Regulation of Larval Release, Transport and Recruitment-

BMLs shore-based coastal ocean observation network is being coupled with intensive boat-based behavioral studies and large-scale, long-term recruitment studies to provide insights into how larvae migrate away and back to nearshore communities. BML's new high-speed research vessel, the R/V Mussel Point, now enables us to readily conduct studies between the Gulf of Farallones and Point Arena to the edge of the continental shelf. A coastal ocean observation network is being expanded to monitor the flux of water masses and accompanying nutrients, phytoplankton and larvae through our region. Existing monitoring capabilities at BML, including CODAR, an instrumented mooring (ADCP, CTD) just offshore of the laboratory and a meteorological station (See BOON), were expanded to include monitoring of temperature, salinity, sea level, chlorophyll fluorescence, turbidity, dissolved nitrate concentration and meteorological variables at six sites between BML and Scripps Institute of Oceanography. We are working with the Gulf of Farallones and Cordell Bank National Marine Sanctuaries to deploy moorings across the region.

Because most marine organisms produce microscopic, long lived, widely dispersing larvae that are very difficult to study, major advances in our understanding of marine population dynamics and community structure requires the coupling of larval and adult biology and oceanography and ecology. To address this problem, my colleagues and I obtained funding from the California Coastal Conservancy and State Water Resources Control Board to enhance our Network for Environmental Observations of the Coastal Ocean and integrate it with a new more elaborate coastal observing network being developed by the Coastal Ocean Currents Monitoring Program. This network is essential for determining the flux of water masses and accompanying nutrients, phytoplankton and larvae to local populations and communities, and in tandem with intensive boat and shore based sampling, is providing insights into the coupling of oceanographic processes with the regulation and dynamics of coastal communities.

A prevailing paradigm is that strong upwelling winds overwhelm larvae sweeping them far offshore so that few of them return to replenish coastal communities in northern California and in other upwelling systems on the western margins of continents. Moreover, upwelling winds are strongest near BML along the West Coast so the recruitment limitation is supposedly most pronounced here and postsettlement density-dependent processes, such as competition, facilitation and predation, regulate communities in the Pacific Northwest. However, NSF-funded research conducted in my laboratory during the review period does not support this paradigm. The vast majority of the larvae of all 46 species of crustaceans studied are retained very close to shore (<1-5 km depending on locale) off BML throughout the height of the upwelling season. Moreover, larvae of some species undertake true migrations between adult habitats nearshore and nursery grounds over the middle or outer shelf, as I have demonstrated for species in the very different oceanographic regime along the East Coast. Species-specific differences in larval migration patterns along both coasts result from diel and ontogenetic vertical migrations around mean depth preferences of larvae.

We also demonstrated that larvae are retained throughout development and recruitment is enhanced in a small embayment (Bodega Bay) and estuary (Bodega Harbor) along an upwelling coast. Previously, larval retention was known to occur only in the upwelling shadows in the lee of major headlands, such as Point Reyes.A persistent recirculation cell occurs in the bottom waters of northern Bodega Bay during the upwelling season, and larvae remain in the bay throughout development by either staying deep in the water column or undertaking diel vertical migrations. Postlarvaereturning to settle along the coast also are entrained in this recirculation cell resulting in much higher larval supply to Bodega Bay than the open coast for most species studied. Peak settlement occurred during the height of the upwelling season during spring and summer, and most species recruited in surface waters during upwelling rather than relaxation events in two years of strong upwelling; other species recruited in bottom waters or during relaxation events. Thus, upwelling does not appear to limit recruitment and interspecific differences in larval delivery mechanisms were apparent.  High frequency sampling subsequently was conducted to further link variability in oceanographic conditions with larval delivery to settlement sites. Finally, we demonstrated that high larval supply in the deposition zones of headlands did not translate into high settlement; rather, larvae delayed settling until they reached more favorable adult habitat along exposed shores.

Mace, A. J. and S. G. Morgan. 2006. Biological and physical coupling in the lee of a small headland: contrasting larval transport mechanisms in an upwelling region. Marine Ecology Progress Series 324:185-196

Roughan, M., A. J. Mace, J. L. Largier, S. G. Morgan, J. L. Fisher, M. L. Carter. 2005. Density driven headland retention in a strong upwelling system: Implications for larval transport. Journal of Geophysical Research 110, C10027, 1-18.

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