ABSTRACT: 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 situ production-irradiance (P-I) curves for these sediment communities. During all months the community was distinctly autotrophic, with 24 h production to respiration (P/R) ratios ranging from 2.9 to 4.4. We estimated an annual community net-production of 168 g C m^-2. All parameters expressing the overall magnitude of production (community net and gross production; photosynthetic capacity, P_max) showed distinctly lower values in winter, indicating a strong seasonality. 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, I_c; light saturation point, I_k; photosynthetic efficiency, 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_c, I_k 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 situ enrichment experiments the excretion of holothurians enhanced community net production and P_max by about 12%. The enhancement most likely resulted from a rise of NH₄⁺ concentration (0.1 µmol l^-1 above the background 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 nitrogen.

KEY WORDS: Coral reef ecology · Benthic production · Nutrient cycling · Sediments · Holothurians · Near-shore reefs

Published in MEPS Vol. 169 (1998) on August 6
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998