DYNAMICS OF BACTERIOPLANKTON ABUNDANCE AND PRODUCTION IN SEAGRASS COMMUNITIES OF A HYPERSALINE LAGOON

The significance of bacterioplankton in the flow of carbon and energy and in trophic dynamics of the upper Laguna Madre, Texas (USA), was estimated by measuring bacterioplankton abundance and production over an 18 mo period and over several diel cycles. Bacterioplankton production was estimated from incorporation rates of thymidine (DNA synthesis) and leucine (protein synthesis). These independent indices of bacterial growth were generally in agreement and yielded nearly identical annual estimates of bacterial production (25.24 g C m-2 yr-1 based on thymidine and 25.12 g C m-2 yr-1 based on leucine). Assuming a 30 % growth efficiency, the annual bacterioplankton growth could be supported by 15 % of the total primary production (seagrasses and phytoplankton), 17 % of the above-ground production of the dominant seagrass, Halodule wrightii, or 103 % of the phytoplankton production. Bacterial abundance was high throughout the year, often exceeding 1 x 10(10) cells l-1. Bacterioplankton production varied seasonally and over the diel cycle, with maximal values during warmer months and during daytime. Although changes in water temperature could account for some of this variation, shifts in the quantity and quality of the organic substrates supporting bacterial growth appeared to be the major factors regulating the variations in bacterioplankton production. Bacterioplankton in the Laguna Madre are a large and rapidly growing source of biomass potentially available for higher trophic levels. If this biomass is efficiently used by grazers, bacteria may be a major 'link' between seagrass production and secondary producers in the Laguna Madre ecosystem.