Analysis of biological and physical data taken during a cruise aboard NOAA Ship OREGON II in 1994 are furnishing the basis for a change in our understanding of physical regulation of biological processes within the coastal ecosystem adjacent to the Florida Keys National Marine Sanctuary (FKNMS). The work described represents a collaboration of University of Miami/RSMAS physical and biological oceanographers and AOML/OCD biological and chemical oceanographers.
Synoptic with a shipboard survey, an Ocean Surface Current Radar was used to map the surface current field in realtime. These revealed transient unstable baroclinic eddies ( Figure one). These appear to form in the wake of bottom topography off the Florida Keys depending upon the proximity of the Stream boundary and the shore. They move through the region along the frontal boundary between the north easterly flowing Gulf Stream and southwesterly flowing coastal counter current. Significantly the eddies appear to have distinct biochemical signatures as well. An integrated plankton sampler was towyo'ed across one of these features yielding fine-scale synoptic physical and biological information (Figures two, three and four. The eddy core is evident in temperature and salinity contours made during a transect of the eddy. The eddy core is to the right of the transect (Figure two). Using temperature as our reference, the core of the eddy appears to be associated with much higher than usual chlorophyll fluorescence (Figure three), a measure of phytoplankton abundance, particularly where cooler than expected surface waters were noted. This may be indicative of weak local upwelling. Multifrequency acoustic data indicates that very small particles in particular are concentrated on the convergent side of this feature (Figure four). Small particle abundance assessed by an in-situ optical particle counter mounted aboard the same tow vehicle also confirmed eddy enhancement. The V09 data was taken during passage of the convergent zone (Figure five). Last, microscope counts from samples taken at intervals along the transect from the ship's sea chest confirm that copepod eggs and nauplii are more abundant in near surface waters in the same zone (Figure six). This implies that the observed redistribution of the small particle field has trophodynamic (food-chain) significance.
That features so transient have so substantial a biological signature implies they can markedly affect offshore-inshore exchange processes. This represents a fundamental change in our understanding of biophysical interactions in this coastal environment and how its dynamics need to be monitored in the future. AOML and University of Miami/RSMAS cruise participants are completing a description of these results for the Journal of Geophysical Research.
