Primary production dynamics in seagrass beds of Mississippi Sound: the contributions of seagrass, epiphytic algae, sand microflora, and phytoplankton

Primary production rates of 4 autotrophic components in Halodule wrightii Aschers. beds off Horn Island in Mississippi Sound (USA) were measured over an annual cycle. Hourly production rates varied from as little as 0.9 mg C m-' for H, wrightii leaves in winter to as high as 1143 mg C m-' for eplphytic algae during summer. Stepwise multiple regression showed that only 15 % of the variation in hourly epiphytic algal production could be related to a single environmental variable (i.e. light energy). Variations in hourly production rates for the other productivity components were best explained by light energy, water temperature, tidal range and blade density; R2 for these regressions was 0.66 and 0.80 for phytoplankton and seagrass blades respectively. The epiphytic algal assemblage on H. wrightii leaves was dominated by the red alga Acrochaetium flexuosum Vickers and 12 taxa of araphid, monoraphid and biraphid diatoms. Phytoplankton over the beds were virtually all centric diatoms, whereas the microflora associated with the sandy sediments in which H. wrightii was rooted was dominated by small pennate diatoms. Annual production rates (g C were estimated as follows: epiphytic algae (905), phytoplankton (468), sand microflora (339) and Halodule wriyhtij (256). System production was dominated by the microalgae, with the contribution of H. rvrightji blades being only 13 % of the total. Epiphytic algae were the single most important productivity con~ponent, being responsible for 46 and 60 % of total system and benthic production respectively. The high standing crops and production rates of the epiphytic and benthic microalgae are of interest in that recent studies have indicated these algae can be the principal source of organic matter in seagrass food webs.