Bay-scale patterns in the distribution, aggregation and spatial variability of larvae of benthic invertebrates

Abstract

This study aimed to investigate mechanisms of pattern formation in the larval distributions of benthic invertebrates by relating the spatial and temporal variability in the larval distributions to that of physical and biological variables, such as temperature, salinity, fluores- cence and current velocity. Larvae were sampled at 11 sites on Aug 7�8 and 11�12, 2008 and at 16 sites on Aug 2�4, 2009, with a 200 µm plankton ring net (0.75 m diameter) towed for 5 min at 3 m and 12 m depth (in and below the mixed layer, respectively) in St. George's Bay, Nova Scotia, Canada. In 2009, density, temperature, salinity, and fluorescence were measured with a conduc- tivity-temperature-density (CTD) cast at each station, and currents were quantified with an acoustic Doppler current profiler (ADCP) moored at 5 locations throughout the bay. In 2008, we only measured temperature. Gastropod, bivalve and, to a lesser extent, bryozoan larvae had very similar spatial distributions, but the distribution of decapod larvae followed a different pattern. These findings suggest that taxonomic groups that have functionally (i.e. swimming ability) simi- lar larvae (e.g. bivalves and gastropods) also show similar dispersion properties (distribution and spatial variability), while the opposite is true for groups with functionally dissimilar larvae (e.g. bivalves and decapods). We also found that larval distributions of all taxa were significantly aggregated, although the degree of aggregation varied among taxa. Using an aggregation-diffu- sion model, we demonstrated that horizontal swimming was not an effective means of forming aggregations even at modest levels of diffusion. We hypothesize that patterns in observed hori- zontal distribution at this scale (<40 km) are determined during the larval phase, and that the pri- mary mechanism for pattern formation is larval interaction with physical oceanographic structures (e.g. stratification and tidal currents).