Short-Term Responses of Coral Reef Microphytobenthic Communities to Inorganic Nutrient Loading

The responses of coral reef flat microphytobenthos to short-term exposure to elevated levels of inorganic nitrogen (N) and phosphorus (P) were investigated in 1994 and 1995. Sand samples collected from the reef flat were maintained over 7 d in triplicate cultures with N-enriched (100 μM NO3), P-enriched (10 μM P04), and ambient seawater. A fourth experiment used a treatment of combined N and P enrichment. The sediment samples were assessed for chlorophyll a (Chl a) content and photosynthesis-irradiance (P-I) responses. P-I curves, constructed from area-and Chl a-specific metabolic rates, showed consistently higher maximal rates in the nutrient-enriched samples. Sediments exposed to enhanced levels of N exhibited the highest Chl a content while both N- and P-enriched samples showed increased photosynthetic yield. Very little depletion of nutrients in the water column was detected over time in the batch cultures except in the N+P-enriched treatments where nutrient values dropped to near-ambient levels. Results from these experiments point to N and P colimitation in tropical carbonate sediments.