Effects of water-column enrichment on the production dynamics of three seagrass species and their epiphytic algae

Monospecific beds of the seagrasses Halodule wrightii, Syringodium filiforme and Thalassia testudinum were enriched with a slow-release Osmocote(TM) (N-P-K) fertilizer from August 1993 through September 1994. Primary production rates (as ¹⁴C uptake), biomass (dry weight), and chlorophyll a (chl a) (measured by HPLC) of epiphytes in enriched beds were significantly greater than those of epiphytes in control beds. Based on microscopic observations, the dominant epiphytic algae were diatoms and red and brown algae. Populations of the brown alga Myriotrichia subcorymbosa and the red alga Acrochaetium flexuosum increased greatly in enriched plots of all 3 seagrass species. Multiple linear regression supported observational data in that pigment signatures selected for the dominant epiphytes (fucoxanthin, zeaxanthin, and violaxanthin) explained 97% of the variation in chl a. A strong correlation between measured and predicted chl a (r = 0.98) suggested that chl a is an excellent indicator of epiphytic biomass in this system. Production rates of blades increased in enriched plots relative to controls but biomass of blades was unaffected. The strong response of epiphytes to enrichment suggests that cultural eutrophication could pose a threat to seagrass beds of Big Lagoon, Perdido Key, Florida, USA. Negative effects could be manifested as a reduction in the coverage of shallow-water sediments by seagrass beds and/or the elimination of 1 or 2 species, possibly converting Big Lagoon into a seagrass monoculture.