MICROPHYTOBENTHOS - THE ECOLOGICAL ROLE OF THE SECRET GARDEN OF UNVEGETATED, SHALLOW-WATER MARINE HABITATS .1. DISTRIBUTION, ABUNDANCE AND PRIMARY PRODUCTION

The microphytobenthos consists of unicellular eukaryotic alg-ae and cy anobacteria that grow within the upper several millimeters of illumina ted sediments, typically appearing only as a subtle brownish or greeni sh shading. The surficial layer of the sediment is a zone of intense m icrobial and geochemical activity and of considerable physical reworki ng. In many shallow ecosystems, the biomass of benthic microalgae ofte n exceeds that of the phytoplankton in the overlying waters. Direct co mparison of the abundance of benthic and suspended microalgae is compl icated by the means used to measure biomass and by the vertical and ho rizontal distribution of the microphytobenthos in the sediment. Where biomass has been estimated as chlorophyll a, there may be negligible t o large (40%) error due to interference by degradation products, excep t where chlorophyll is measured by high-performance liquid chromatogra phy, The vertical distribution of microphytobenthos, aside from mat-fo rming species, is determined by the opposing effects of their vertical migration, which tends to concentrate them near the surface, and phys ical mixing by overlying currents, which tends to cause an even vertic al distribution through the mixed layer of sediment. Uncertainties in vertical distribution are compounded by frequently patchy horizontal d istribution. Under-sampling on small (<1 m) scales can lead to errors in the estimate that are comparable to the ranges of seasonal and geog raphic variation. These uncertainties are compounded by biases in the techniques used to estimate production by the microphytobenthos. In mo st environments studied, biomass (as chlorophyll a) and light availabi lity appear to be the principal determinants of benthic primary produc tion. The effect of variable light intensities on integral production can be described by a functional response curve. When normalized to th e chlorophyll content of the surficial sediment, the residual variatio n in the data described by the functional response curve is due to cha nges in the chlorophyll-specific response to irradiance. Production by the benthos is often a significant fraction of production in the wate r column and microphytobenthos may contribute directly to water column production when they are resuspended, Thus on both the basis of bioma ss and biogeochemical reactivity, benthic microalgae play significant roles in system productivity and trophic dynamics, as well as such hab itat characteristics as sediment stability.