IMPACTS OF SEASONALITY AND NUTRIENTS ON MICROBIAL MAT COMMUNITY STRUCTURE AND FUNCTION

To understand the mechanisms responsible for seasonal fluctuations in growth and N-2 fixation in intertidal microbial mat communties, we quantified seasonal changes in mat community composition, related these changes to diel and seasonal N-2 fixation rates, and evaluated community responses (growth, N(?)2 fixation, composition) to long-term (22 d) nutrient addition bioassays. A temperate intertidal cyanobacterial mat community, located in coastal North Carolina, USA, was sampled at monthly intervals for 1 yr (1993-94) to determine changes in community composition. The abundances of major phototrophic groups were quantified based on the relative concentrations of taxa-specific photopigments (chlorophylls and carotenoids). The most abundant phototrophs were cyanobacteria, diatoms, and photosynthetic bacteria. Mat biomass and community composition underwent marked changes on both monthly and seasonal scales and corresponded with seasonal shifts in the diel patterns of N-2 fixation. Diatom biomass increased during periods of low N-2 fixation. Nutrient (nitrate and phosphate) addition bioassays indicated that both cyanobacterial and diatom growth were N limited. Cyanobacteria were able to circumvent N limitation by N-2 fixation. The addition of high concentrations of N (100 mu M NaNO3) in combination with P (100 mu M NaH2PO4) resulted in an increase (163%) in the relative abundance of diatoms. The addition of P alone more than doubled N-2 fixation rates and cyanobacterial abundance increased (+34%) relative to diatoms. However, N and NP additions significantly lowered (by more than 75%) N-2 fixation rates. Here we show that manipulative experiments, together with quantitative assessments of community composition based on chemotaxonomic pigments, can provide useful insights into the mechanisms that relate mat community structure and function to environmental constraints, including nutrient limitation and seasonal climatic changes.