MICROPHYTOBENTHOS COMMUNITY PRODUCTION AT A NEAR-SHORE CORAL-REEF - SEASONAL-VARIATION AND RESPONSE TO AMMONIUM RECYCLED BY HOLOTHURIANS

Production of the microphytobenthos community of a near-shore reef in the Great Barrier Reef system was measured on 7 occasions over 13 mo u sing in situ respirometry with dome chambers. In addition, underwater light levels were recorded, which allowed the construction of in situ production-irradiance (P-I) curves for these sediment communities. Dur ing all months the community was distinctly autotrophic, with 24 h pro duction to respiration (P/R) ratios ranging from 2.9 to 4.4. We estima ted an annual community net-production of 168 g C m(-2). hll parameter s expressing the overall magnitude of production (community net and gr oss production; photosynthetic capacity, P-max) showed distinctly lowe r values in winter, indicating a strong seasonality. Between 63 and 75 % of the variation of these parameters was explained by seasonal chang es in temperature. Most other P-I curve parameters (light compensation point, I-c; light saturation point, I-k; photosynthetic efficiency, a lpha) showed less obvious long term temporal patterns and were subject to large variation between single measuring days. A considerable prop ortion of the variation in I-c, I-k and cc (46 to 68%)could be attribu ted to daily differences in underwater light conditions,indicating rap id photoadaptation by the microalgal communities to variation in light conditions due to turbidity and/or cloud cover. In in situ enrichment experiments the excretion of holothurians enhanced community net prod uction and P-max by about 1.2%. The enhancement most likely resulted f rom a rise of NH4+ concentration (0.1 mu mol l(-1) above the backgroun d level). The enhancement due to holothurian excretion products, in co njunction with the low quantum yield efficiency exhibited by the benth ic community, indicate that production of benthic microalgae in our st udy area was limited by the availability of inorganic nitrogen.