Seasonal variation of physical and biogeochemical features in eastern Caribbean Surface Water

We describe vertical and temporal variation of water mass structure, dissol ved nutrients, phytoplankton chlorophyll a (Chl a), and dissolved organic m atter south of the island of Puerto Rico at 67 degreesW, 17 degrees 36'N wi th particular emphasis on variability of Caribbean Surface Water (CSW) feat ures. CSW at the Caribbean Time Series (CaTS) station experiences yearlong continental riverine influence as evidenced by surface salinity consistentl y below that of Tropical Atlantic Surface Water and elevated silicate conte nt. Salinity exhibits close correlation to average rainfall over the Orinoc o River basin with a 3-4 month offset, and salinity and silicate are in tur n closely correlated. Calculations using a simple mixing model and based on known end-members for salinity and silicate indicate that rainwater accoun ts for 0-3.1% of CSW, while river water accounts for 0.6-5.5% at CaTS, indi cating that surface waters of the NE Caribbean are under the influence of r iverine water throughout the year. No significant correlation was found bet ween salinity and phytoplankton Chi a, but the depth of the Chi a maximum w as statistically correlated to seasonal salinity fluctuations. This observa tion may account for the apparent seasonal Chi a increase in these waters d educed from remote-sensing estimates. Analysis of historical data shows tha t seasonal shoaling of the Chi a maximum results in a net increase in phyto plankton carbon biomass during the period of influence of the Orinoco River plume.