BIOMASS, PRODUCTION AND GRAZING RATES OF PICO-PLANKTON AND NANOPLANKTON IN CORAL-REEF WATERS (MIYAKO ISLAND, JAPAN)

Concentrations of phytoplankton (cyanobacteria, autotrophic flagellate s, and microphytoplankton) and planktonic microorganisms (bacteria, he terotrophic flagellates, and ciliates) were measured in the water over a fringing coral reef at Miyako Island (Japan). Their in situ growth and production rates, as well as their grazing rates, were estimated u sing diffusion chambers. Bacteria dominated the heterotrophic biomass (37-73% of total C), whereas nanoflagellates dominated the autotrophic biomass (65-75% of total autotropic C). Growth and production rates s howed that these microbial populations over the reef were in a very dy namic state: growth rates ranged between 2 and 4 doublings day(-1) for the bacteria, and between 1.5 and 3 doublings day(-1) for the auto- a nd heterotrophic pico- and nanoplankton. This led to high production r ates (10-25 mu g C liter(-1) day(-1) for bacteria and flagellates). Th e microbial biomass was removed rapidly, since 60-70% of the bacterial production and 30-50% of the autotrophic production were grazed by th e heterotrophic flagellates and ciliates, which themselves were grazed (50-70% of the production) by the higher trophic levels. These result s suggest that dissolved organic matter was continuously channeled thr ough the microbial loop to the higher trophic levels and the microbes had an important trophic role in terms of nutrient for the benthic org anisms.