HCNSO Student Theses and Dissertations

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Defense Date


Document Type

Thesis - NSU Access Only

Degree Name

M.S. Marine Biology


Oceanographic Center

First Advisor

Richard E. Dodge

Second Advisor

Julian P. McCreary

Third Advisor

Gary L. Hitchcock

Fourth Advisor

Bart Baca


The optical front of the Gulf Stream off southeast Florida is an abrupt and distinct visual boundary between green coastal water and deep blue Gulf Stream water. The goal of the research presented in this thesis was to describe phytoplankton biomass and composition in relation to the position and strength of this optical front. Twenty-seven cruises were conducted off the coast of Fort Lauderdale, Florida between July 1990 and March 1992. On each cruise, a transect of vertical CTD casts was conducted across the front to a depth of 100 m. Estimates of vertical frontal strength (buoyancy frequency) and horizontal frontal strength (salinity and density gradients across the front) were calculated from the CTD data. Biological samples were collected at three stations, one at the front and one on each side of the front, for the estimation of chlorophyll concentration by fluorometry and for the microscopic enumeration of the microplankton.

Results showed that chlorophyll concentration estimates were not correlated with the estimates of vertical frontal strength, horizontal frontal strength or the distance of the front from shore (via the Spearman rank-order correlation). Chlorophyll concentrations at the Gulf Stream station were significantly lower than those at both the inshore and front stations, but chlorophyll concentrations at the front were not significantly different from those inshore (via the Kruskal-Wallis H test) Chlorophyll concentrations at the front, however, were elevated above average during the late summer/early fall when the mixed layer depth was shallower than average.

Total microplankton abundance did not differ significantly between the three stations. However, when the microplankton were divided into major taxonomic categories, significant differences in abundances between the three stations were detected. In general, abundances were highest near shore and declined offshore. Except for a weak correlation between Pyrrhophyta and the salinity gradient across. the front, abundances of microplankton groups did not correlate with the physical variables measured. Scenarios that might explain these results are discussed.

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