California Sea Grant College Program

Phylogenetically diverse benthic producers (e.g., diatoms, green algae, cyanobacteria, and anoxygenic photobacteria) inhabit the sediments of coastal wetlands and mediate numerous ecosystem functions, including a substantial fraction of total ecosystem productivity. Despite their ubiquity and functional importance, spatia-temporal patterns in the distribution of major taxonomic groups and diversity­ related effects on function are poorly understood. Using a photopigment-based approach I investigated the composition of microproducer communities at two coastal sites in southern California and addressed (1) the temporal development of assemblages in a restored wetland, (2) the spatial distribution of major functional groups across various habitats, and (3) the influence of functional group diversity and dominance on primary productivity.

Within a large restoration site at Tijuana Estuary, pigment concentrations suggested rapid recovery of assemblage biomass (< 1yr) and at least partial recovery of taxonomic composition by 0.2 yr in restored mudflats. In Spartina-dominated salt marsh, however, composition and diversity took longer to mirror natural marsh communities (-1.5-2.2 yr). In Mission Bay, study of an older restoration site (6.3 yr) also suggested differences in rates of habitat development, with Spartina marsh and creek-banks probably preceding mixed species marsh. In a spatial analysis of communities, I found that natural sediments from vegetated marshes in Mission Bay were enriched in cyanobacteria and anoxygenic photobacteria, and had higher diversity, relative to unvegetated sediments, but no mudflat versus marsh difference was evident at Tijuana Estuary. In several independent studies of diversity and productivity, including a manipulation of functional richness in lab cultures, I found no effect of variation in functional diversity on rates of oxygenic photosynthesis and only minor increases in microproducer standing stocks in more diverse assemblages. However, results also tentatively suggest unique community roles for green algae (high oxygenic production rates) and anoxygenic photobacteria (enhancement of community biomass), supporting the notion that composition may be more important than absolute diversity to function.

These data re-enforce the idea that study of composition in microproducer assemblages is necessary in addition to simple measures of chlorophyll a. The functional consequences of altered diversity and composition should be further investigated in this cryptic but important flora.