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Production of the microphytobenthos community of a near-shore reef in the Great Barrier Reef system was measured on 7 occasions over 13 mo using in situ respirometry with dome chambers. In addition, underwater light levels were recorded, which allowed the construction of in s ~ t u production-irradiance (P-I) curves for these sediment communities. During all months the community was distlnctly autotrophic, with 24 h production torespirat~on (P/R) ratios ranging from 2.9 to 4.4 . We estimated an annual community net-production of 168 g C m-"11 parameters expressing the overall magnitude of production (community net and gross production; photosynthetic capacity, P,.) showed distinctly lower values in winter, indicating a strong seasonaIity. Between 63 and 75% of the variation of these parameters was explained by seasonal changes in temperature. Most other P-I curve parameters (light compensation point, 1; light saturation point, Ik; photosynthetic effiaency, a) showed less obvious long term temporal patterns and were subject to large variation between single measuring days. A considerable proportion of the variation in I,, Ik and a (46 to 68%) could be attributed to daily differences in underwater light conditions, indicating rapid photoadaptation by the microalgal communities to variation in light conditions due to turbidity and/or cloud cover. In in ntu enrichment experiments the excretion of holothurians enhanced community net production and P,, by about 12%. The enhancement most l~kely resulted from a rise of NH4+ concentration (0 l pm01 I-' above the ba.ckground level). The enhancement due to holothurian excretion products, in conjunction with the low quantum yield efficiency exhibited by the benthic community, indicate that production of benthic microalgae in our study area was limited by the availability of inorganic nltrogen

Microphytobenthos community production at a near-shore coral reef:seasonal variation and response to ammonium recycled by holothurians