LONG-TERM CHANGES IN THE CONCENTRATION OF ZOOPLANKTON AND PARTICULATE MATERIAL OVER A FRINGING REEF IN ST. JOHN, US VIRGIN ISLANDS

Seawater quality on fringing reefs in St. John was quantified in August 2008, and a comparison with a previous analysis completed in the same location in 1970 was used to detect changes over the last 40 yrs; a compilation of equivalent results obtained throughout the Caribbean over a similar period strengthened the historic contrast. The study was designed to use a sampling regime similar to that employed previously and, therefore, seawater quality was assessed from the concentrations of particulate matter (PM), particulate organic matter (POM), and zooplankton; samples were collected throughout the day at the surface and at 10-m depth. In 2008, seawater quality was characterized by 9.3 mg L-1 PM, 3.9 mg L-1 POM, and 4.3 zooplankters L-1, of which 42% were copepods (values averaged across all samples). Zooplankton density differed significantly from those recorded in 1970, and was 88% lower, whereas PM was 21-fold greater in 2008 compared to 1970. The broader historic contrast suggested that the concentration of PM has increased exponentially since 1960, although zooplankton densities are statistically indistinguishable. These results are consistent with the hypothesis that seawater quality over Caribbean reefs has changed over the last four decades.