Bacteria, flagellates and nutrients in island fringing coral reef waters: Influence of the ocean, the reef and eutrophication

We sampled subsurface (2 m) eutrophied and non-eutrophied coral reef waters , the adjacent ocean and Harbour bay along the southern coast of Curacao, S outhern Caribbean, and measured nutrient concentrations and microbial varia bles. In Harbour bay very high ammonium concentrations (up to 35 mu M) and bacterial production (up to 11 mu gC L-1 h(-1)) were found. Runoff into the bay increased bacterial production as well as the numbers of bacteria and flagellates. The effects of out flowing Harbour water were limited to the w aters in front of the town. Ammonium, nitrite, nitrate and phosphate concen trations were higher close to the urbanized area than at non-eutrophied ree f sites over 10 km away. The presence of coprostanol relates this eutrophic ation to sewage discharge. At the eutrophied site bacterial production was occasionally increased from 0.1-0.5 mu gC L-1 h(-1) to about 1.2 mu gC L-1 h(-1) by runoff or because it originated from the Harbour bay, but no stimu lation of sewage discharge on bacteria was observed due to rapid dilution o f the input. Nitrate concentration were consistently higher in noneutrophie d reef water (0.2-0.6 mu M) than in the adjacent oceanic water (0.1-0.3 mu M). Bacterial numbers were lower in water over the reefs than in oceanic wa ter. Flagellate bacterivory was similar in both waters. Bacterial productio n was 2-3 times higher in non-eutrophied reef water than in oceanic water i n periods of reduced reef currents and mixing. The coastal reef ecosystem o f Curacao is subjected to short- and long-term fluctuations of nutrients an d micro-organisms in the oceanic water mass surrounding the island. We obse rved such changes at all stations in phosphate concentration, N:P ratio, ba cterial and flagellate numbers and bacterial production over a I-yr period.