Limnol. Oceanogr., 44(5), 1999, 1259–1267

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 mM NO3), P-enriched (10 mM PO4), 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 areaand 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 Nand Penriched 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 N1P-enriched treatments where nutrient values dropped to near-ambient levels. Results from these experiments point to N and P colimitation in tropical carbonate sediments. Unconsolidated carbonate substrates, although apparently barren, are acknowledged to be biologically active components of coral reef systems (Kinsey 1977). In a tropical sandy reef flat studied by Yap et al. (1994), the same area investigated in this paper, gross primary production by the sand substrate constitutes about 10% of the aggregate per unit area production of the autotrophic components of the system. Yet this seemingly small contribution becomes substantial when the extent of the areal coverage of the substrate is considered (;85% of total reef flat area). The microphytobenthic communities responsible for this production have been studied extensively in various systems such as estuaries (Pinckney and Zingmark 1993a; MacIntyre et al. 1996), temperate shallow-water sediment systems (Sundbäck and Snoeijs 1991; MacIntyre and Cullen 1995), and coral reef lagoons (Charpy and Charpy-Roubaud 1990; Johnstone et al. 1990). Environmental factors influencing the production of these communities include irradiance (Daehnick et al. 1992; Pinckney and Zingmark 1993b), tidal stage and sun angles (Pinckney and Zingmark 1991), the abrasive effect of sand movement (Delgado et al. 1991), and nutrient input (Nilsson and Sundbäck 1991; Pinckney et al. 1995). In tropical coral reef environments, dissolved nutrient concentrations in the overlying water column are generally lower than in temperate regions (Crossland 1983) except in cases where nutrients are supplied externally by groundwater (Johannes 1980; D’Elia et al. 1981), river runoff (Matson 1990), or direct anthropogenic input (Smith et al. 1981). Nutrient-limited responses have been confirmed in various coral