The role of epiphytic and epibenthic algal productivity in a tropical seagrass, Syringodium isoetifolium (Aschers.) Dandy, community

Abstract

The overall aim of this study, which was part of a multidisciplinary investigation, was to evaluate the role of epiphytic and epibenthic algae in a tropical seagrass, Syringodium isoetifoliurn (Aschers.) Dandy, community. Algal biomass (chlorophyll a) and productivity were determined over 20 days. The rates of photosynthesis were measured with both gas (oxygen) release and H¹⁴CO₃ incorporation techniques. The photosynthetic characteristics of the algal community are described by the relationship between the rates of photosynthesis and irradiance (PI curve). The saturating light intensity (I_k), determined in the laboratory, was 240 µE m^-2 s^-1 (1 µE= 1 µmol). However, the maximum rate of net epibenthic production (P_max) in situ of 2 g C g chla^-1.h^-1 was reached only at light intensities above the leaf canopy of more than 480 µE m^-2 s^-1 because of shading. The photoperiod (the period when algae were at P_max was determined from the daily light profiles and used to calculate the mean net productivity of the epibenthic (4.2 g C m^-2 day^-1) and epiphytic (11.5 g C m^-2 day^-1.) algae during the study. The net epiphytic and seagrass productivities were equal and positively correlated (slope = 1.0; r² = 0.92). However, the net seagrass leaf production was only 25% of the total seagrass net production. Most of the seagrass production appeared to be directed elsewhere, possibly into the below-ground tissue. Epiphytic algae accounted for more than four times the above-ground primary production. Compared with the bare sediment surface, the dense seagrass stand afforded 27±4 (s.d.) times more surface area on which the epiphyton could colonize. Thus, seagrasses provided the surfaces essential for maintaining the substantial epiphytic primary productivity, a passive but nonetheless important role.

We compare our findings with other results of the multidisciplinary study and construct a carbon budget from which we infer a model of carbon transfer through this ecosystem. The algae were quantitatively the most important of the primary producers, seeming to provide most of the organic carbon to the higher trophic groups, at least for the period of this study.