Effects of microscale water level fluctuations and altered ultraviolet radiation on periphytic microbiota

Microscale fluctuations in water level (1-20 mm) are common on a diurnal ba sis in shallow (<5-10 cm) wetlands, coupled to evapotranspiration losses du ring the daytime in excess of groundwater resupply. These depth variations alter the intensity of UV irradiance reaching attached periphytic algal and bacterial microbial communities. Effects of alterations of UV irradiance b y micro-changes in water level on periphytic microbiota were examined exper imentally. Attached microbial communities, grown on glass fiber filters in situ in a natural wetland, were exposed experimentally to near-natural leve ls of UV irradiance of differing spectral quality. UV intensity was altered by varying the distance of the communities from the light source, changes in UV-attenuating natural dissolved organic matter (DOM), and small changes in water level (2 or 4 mm). Algal productivity and photosynthetic oxygen p roduction were significantly reduced by small enhancements of UV-B radiatio n, by decreased water levels of only 2 mm, and by reductions in concentrati ons of DOM. UV-B had only small short-term effects on chlorophyll a, althou gh small increases in water depth and DOM concentration reduced pigment dam age. Experimental removal of UV-B during in situ growth indicated that alga e could adapt to UV radiation during growth in natural environments. Microb ial oxygen consumption and bacterial productivity and biomass were also low ered significantly by UV-B exposure, and damage decreased with small (2 mm) increases in water depth or in DOM concentration. Selective inhibitors of algal photosynthesis and production of released extracellular organic subst rates caused a concomitant reduction in bacterial productivity and a signif icant increase in magnitude of UV-B damage to bacterial biomass. These effe cts suggested that metabolic interactions between the periphytic autotrophs and heterotrophs altered community responses to UV-B radiation. Microscale water level reductions, common on a diurnal basis in shallow wetlands, and associated increased UV intensity can result in rapid alterations in perip hytic metabolism.