Effects of solar radiation on dissolved organic matter cycling in a subtropical seagrass meadow

The influence of sunlight on bacterioplankton production was investigated over a 12-month period in Laguna Madre, a shallow, subtropical, seagrass-dominated lagoon in south Texas. Two types of experiments were conducted every 6 to 8 weeks from June 1996 through June 1997 to assess the effect of photochemical transformations of dissolved organic matter (DOM) on bacterial production and the net effect of sunlight exposure on bacterioplankton and phytoplankton production in surface waters. Photobleaching of DOM was observed; however, estimates of the photomineralization of DOM in May and July were very low and indicated that this process could represent at most 3% of water column respiration. Photochemical transformations of DOM did not have any significant effect on bacterioplankton activity. However, bacterial production measured during exposure of water samples to natural sunlight in the summer months, with and without ultraviolet light, was enhanced relative to dark controls. Phytoplankton release of bioavailable substrates appeared to be responsible for light-mediated increases in bacterioplankton production. Bacterio- and phytoplankton were not photoinhibited by exposure to ultraviolet radiation in surface waters. Overall, we observed no indication of photoinhibition of microbial processes in Laguna Madre, and photochemical transformations appeared to play a minor role in the cycling of DOM. The low photoreactivity of DOM in combination with its relatively high bioreactivity appeared responsible for the minor role of photochemical transformations in the cycling of DOM in Laguna Madre.