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 filiform e and Thalassia testudinum were enriched with a slow-release Osmocote(TM) ( N-P-K) fertilizer from August 1993 through September 1994. Primary producti on rates (as C-14 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. Popula tions of the brown alga Myriotrichia subcorymbosa and the red alga Acrochae tium flexuosum increased greatly in enriched plots of all 3 seagrass specie s. Multiple linear regression supported observational data in that pigment signatures selected for the dominant epiphytes (fucoxanthin, zeaxanthin, an d violaxanthin) explained 97 % of the variation in chi a. A strong correlat ion between measured and predicted chi a (r = 0.98) suggested that chi a is an excellent indicator of epiphytic biomass in this system. Production rat es of blades increased in enriched plots relative to controls but biomass o f blades was unaffected. The strong response of epiphytes to enrichment sug gests 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 i nto a seagrass monoculture.