Biomass, Production and Grazing Rates of Pico- and Nanoplankton in Coral Reef Waters (Miyako Island, Japan)

Abstract

Concentrations of phytoplankton (cyanobacteria, autotrophic flagellates, and microphytoplankton) and planktonic microorganisms (bacteria, heterotrophic 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 using 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 showed that these microbial populations over the reef were in a very dynamic 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- and heterotrophic pico- and nanoplankton. This led to high production rates (10–25 μg C liter⁻¹ day⁻¹ for bacteria and flagellates). The microbial biomass was removed rapidly, since 60–70% of the bacterial production and 30–50% of the autotrophic production were grazed by the heterotrophic flagellates and ciliates, which themselves were grazed (50–70% of the production) by the higher trophic levels. These results suggest that dissolved organic matter was continuously channeled through the microbial loop to the higher trophic levels and the microbes had an important trophic role in terms of nutrient for the benthic organisms.