Brian E. Lapointe
Director, Florida Keys Programs
Division of Marine Science
Harbor Branch Oceanographic Institution
Limnology & Oceanography 42(5, part 2) 1997 p.1119-1131. 1997
ABSTRACT
Some scientists have speculated that recent dramatic macroalgal overgrowth of fringing coral reefs on the north coast of Jamaica resulted primarily from reduced grazing due to overfishing and die-off of the sea urchin Diadema antillarum and dismissed entirely the possible role of eutrophication.
My study tested the alternative hypothesis that nutrient enrichment was the primary factor causing the spectacular macroalgal blooms that recently developed on reefs at Discovery Bay, Jamaica, and Palm Beach County, FL.
In both locations, groundwater discharges contributed to elevated water column DIN and SRP concentrations that exceeded nutrient thresholds (-uM DIN, O.1 uM SRP) for eutrophication on coral reefs.
At Discovery Bay, DIN and SRP concentrations ranged from 5 uM and 0.12 uM on the fore reef to 28 uM and 0.33 uM around the near shore springs, respectively.
High seawater DIN:SRP ratios (33.1 – 100:1), and macroalgal alkaline phosphatase activity [APA; 20 to 90 uM SRP released (g dry wt)-1 h-1] and tissue C:P (956:1) and N:P ratios (45:1) indicated P-limited productivity at Discovery Bay, which was corroborated by experimental studies where P-enrichment significantly increased Pmax of the shallow water opportunistic chlorophyte Chaetomorpha linum.
Recent increases in SRP concentrations of the fore reef at Discovery Bay above 0.1 uM, combined with the physical disturbance of Hurricane Allen in 1980, explains the increased standing crop biomass of Sargassum polyceratium and other macroalgae that now dominate this habitat.
In Florida, DIN and SRP concentrations ranged from 0.75 uM and 0.13 uM to 3.44 and 0.33 um on deep reefs (20 to 30 m) around blooms of Codium isthmocladum,, respectively.
Lower seawater DIN:SRP ratios (<15:1) and macroalgae APA [<20 uM SRP released (g dry wt)-1 h-1] and tissue N:P ratios suggested N-limited productivity, which was also confirmed experimentally where effects of N-enrichment significantly increased a, the photosynthetic efficiency under low irradiance.
Tissue d 15N ratios of C. isthmocladum ranged from +10.0 to 12.0 0/00 during the summer bloom, indicating waste-water-contaminated groundwaters as the Nitrogen source supporting algal growth.
These results are consistent with other case studies of eutrophication on coral reefs and refute recent speculations that reduced herbivory was the primary factor causing dramatic macroalgal overgrowth of coral reefs in Jamaica.