Community Structure and Functional Dynamics of Benthic Microalgae in Salt Marshes

Abstract

Benthic microalgae are a ubiquitous feature in sediments directly exposed to full sunlight or shaded by a vascular plant canopy in coastal salt marshes. Diatoms, cyanobacteria, and green algae are the dominant groups. Of these, diatoms are universally present and abundant, exhibit migratory rhythms driven mainly by light, and are by far the taxonomically most diverse group. Dense mats of cyanobacteria and secondarily green algae frequently develop where light levels are high. The more abundant species of all three algal groups are widely distributed within and among salt marshes of the United States and Europe. Standing crops of benthic microalgae beneath various vascular plant canopies exhibit mean annual values of 60 to 160 mg chl a m⁻². Annual benthic microalgal production (BMP) has been shown to range from 28 g C m⁻² y⁻¹ beneath Juncus roemerianus to 314 g C m⁻² y⁻¹ beneath Jaumea carnosa. In general, BMP increases in a southerly direction in Atlantic coast marshes but is lowest in Gulf Coast marshes. In Atlantic and southern California marshes a significant portion of benthic microalgal production occurs when the overstory vascular plants are dormant. Experimental manipulations have shown that BMP and biomass beneath Spartina alterniflora are limited by nitrogen supplies and grazing activities. Manipulation of light appears to primarily affect the relative dominance of diatoms and cyanobacteria in the benthic microalgal assemblage. The ratio of annual BMP to net aerial production of the overstory vascular plant canopy is 10 to 60% in Atlantic and Gulf Coast marshes and 75 to 140% in a southern California marsh. The benthic microalgal portion of this two component productivity system has been shown by multiple stable isotope studies to be a major component of salt marsh food webs. Diatoms, in particular, are the preferred food item of a diverse array of invertebrate and fish species.