Temporal variability and vertical structure in larval abundance : the potential roles of biological and physical processes

Abstract

Recruitment variability in benthic invertebrate populations results from variability in planktonic larval supply and from processes occurring during and after larval settlement onto the seafloor. The focus of this thesis is on the temporal and spatial variability in larval supply, the extent to which planktonic larval distributions are determined by larval behaviors and physical processes, and how differentially distributed larvae are advected to potential adult habitats by inner-shelf circulation such as wind-driven upwelling and downwelling. This research capitalized on two sets of larval concentration time series, collected by moored zooplankton pumps, and complemented by synoptic hydrographic time-series data. High variability was observed in larval concentration time series, yet the variations were nonrandom. Within the context of the sampling regime, two dominant modes of variability existed. One source of variation was associated with the synoptic meteorological time scale, and the other with the diurnal time scale. Over relatively long time scales, larvae were associated with particular water masses, defined by temperature-salinity characteristics. Within a particular water mass, group-specific vertical patterns were observed over both long and short time scales. "Low frequency" temporal variations resulted primarily from wind-driven cross-shelf transport of water masses in which larvae were differentially distributed relative to the thermocline. "Higher frequency" variations were attributed to diel vertical migrations. These findings suggest that larvae were passive to the degree that they were horizontally advected with certain water masses, but active to the degree that they could alter their vertical position in the water column. Local hydrodynamics, larval associations with specific water masses, and the vertical structure of larvae resulted in differential larval transport to potential adult habitats. Larval data indicate the times and places of possible coupling between water-column organisms and the benthos, leading to certain predictions regarding when and where larval settlement should be greatest. Time-series measurements of larval concentration yield a new perspective on the temporal and spatial variability in larval distributions at an inner-shelf site. Determining how processes operating at the synoptic and diurnal time scales are coupled, and to what extent they influence recruitment variability, represents a challenging extension of this work.