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 qua ntified seasonal changes in mat community composition, related these c hanges to diel and seasonal N-2 fixation rates, and evaluated communit y 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 mon thly intervals for 1 yr (1993-94) to determine changes in community co mposition. 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 cyan obacteria, diatoms, and photosynthetic bacteria. Mat biomass and commu nity 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 fixat ion. Nutrient (nitrate and phosphate) addition bioassays indicated tha t 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 m u M NaH2PO4) resulted in an increase (163%) in the relative abundance of diatoms. The addition of P alone more than doubled N-2 fixation rat es 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, togethe r with quantitative assessments of community composition based on chem otaxonomic pigments, can provide useful insights into the mechanisms t hat relate mat community structure and function to environmental const raints, including nutrient limitation and seasonal climatic changes.