High-resolution observations of plankton spatial distributions correlated with hydrography in the Great South Channel, Georges Bank

Abstract

During a cruise to Georges Bank in May 1992, the Video Plankton Recorder (VPR) was towyoed while non-invasively obtaining images of the plankton and environmental (CTD) data. Data from an 8 h transect across the Great South Channel (GSC) were analyzed on a continuum of spatial scales from coarse-scale (100 km) to micro-scale (mm). Abundance was determined for 12 taxonomic groups including: invertebrate larvae (ophiopluteus larvae, anthozoa larvae: Cerianthus sp.), hydroids, copepods (Calanus sp., Pseudocalanus sp.), pteropods (Limacina retroversa, Clione sp.), ctenophores (Mnemiopsis sp., Pleurobrachia sp.), larvacea (Oikopleura sp.), chaetognatha (Sagitta sp.), and diatom colonies (Chaetoceros socialis).Species-specific plots of the positions of individual plankton in the water column and plots of the temperature and salinity at which the plankton were observed (temperature-salinity-plankton plots) showed that major taxonomic groups were patchy at coarse scales because of their association with specific water masses of different origin and associated temperature/density discontinuities (pycnocline and fronts). Analysis of the T-S characteristics of water types indicated that diatom colonies and ophiopluteus larvae of echinoderms were transported to GSC in a band of cold water originating on the south flank of Georges Bank. Within this band, diatom colonies formed an intense patch at a front reaching a density of 5 ml−1. Within each water mass, fine-scale (10s of meters) plankton patchiness was associated with regions of vertical stability as indicated by the association of plankton with regions of high gradient Richardson number. Aggregation of plankton at the microscale (<1 m) occurred significantly only for plankton capable of active swimming, suggesting a dynamic interaction between biological and physical variables at this spatial scale. On occasion, veliger larvae of Limacina retroversa were found in spawning patches at concentrations exceeding 600 mk−1 within a few centimeters of the air-sea interface. The ability to observe and quantify local concentrations of plankton together with micro-scale physics, but over broad spatial scales, will help provide information on the coupling between spatial scales necessary to understand how individuals interact to form populations and communities in the world oceans.