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

Abstract

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.Dissolved organic matter (DOM), the largest reservoir of organic matter in most aquatic ecosystems, is the primary substrate fueling bacterioplankton activity (Pomeroy 1974;Azam et al. 1983;Wetzel 1992). DOM is also responsible for the absorption of most of the ultraviolet radiation (UVR) in natural waters (Armstrong and Boalch 1961;Zika 1981). Solar UVR has been found to inhibit both phytoplankton and bacterioplankton activity (Lorenzen 1979;Herndl et al. 1993;Holm-Hansen et al. 1993) and therefore has important 1 To whom correspondence should be addressed. Present address: Department of Biological Sciences, 629 Science-Engineering Building, University of Arkansas, Fayetteville, Arkansas.2 Present address: Department of Biological Sciences, 700 Sumter Street, Room 006, University of South Carolina, Columbia, South Carolina 29208. AcknowledgmentsThe authors especially thank Beau Hardegree for extensive and essential field and laboratory assistance. We also appreciate the assistance of Don Hockaday at the UT-PanAm Coastal Studies Laboratory, South Padre Island, Texas, who provided needed laboratory space and facilities during field trips to LLM. The manuscript was improved by comments of three anonymous reviewers.

Emergence in pelagic communities

“…There is abundant evidence for the photolytic breakdown of refractory DOC into assimilable forms in surface waters under high irradiance and for positive responses of bacterial biomass thereto (reviews of Thomas, 1997;Cole, 1999). Equally, the numbers of free-living bacteria increase in response to the production of organic carbon produced by phytoplankton (Nakano et al, 1998;Ziegler and Benner, 2000). The implied "leakiness" of organic products from algal cells seems, at first sight, unduly profligate and unhelpful to primary anabolism.…”

Role of the seagrass Thalassia testudinum as a source of chromophoric dissolved organic matter in coastal south Florida

“…Seagrasses are well known as a major source of biomass in the coastal ocean (Odum 1971, Harrison 1989, Hemminga & Duarte 2000 and have been investigated for their role in the production of bioavailable DOC (Ziegler & Benner 1999a. Ziegler et al (2000) mentioned that compounds in the water of a subtropical seagrass meadow absorbed UV radiation and that exposure to solar radiation resulted in photobleaching of water samples from this system. Both Burdige et al (2002) and Zepp et al (2002) mentioned in oral presentations that seagrasses may be an important source of CDOM in oligotrophic ocean water of the Bahamas or around the Florida Keys, respectively.…”

“…Past studies have indicated that the polymeric substances that make up CDOM are not readily decomposed by microorganisms, but CDOM bioavailability changes on exposure to solar radiation (Mopper & Kieber 2000, Moran et al 2000, Ziegler & Benner 2000, Miller et al 2002, Zepp 67 Mar Ecol Prog Ser 282: 59-72, 2004 2003a). It has been known for years that absorption of solar UV radiation by terrestrially derived humic substances and CDOM results in a reduction in its light absorption and fluorescence (i.e.…”

“…The difference in these results may reflect differences in the sensitivity of these 2 methods. Also, as noted by Ziegler et al (2000), competing uptake of tritiated leucine by phytoplankton could have resulted in underestimates of the production of bioavailable photoproducts by the bacteria.Our studies indicated that seagrass-derived CDOM is readily photobleached by solar radiation ( Table 2). The decrease in absorption coefficients in the UV spectral region with increasing exposure time was exponential and first-order kinetic expressions were used to describe the photobleaching kinetics.…”

The decomposition characteristics of the seagrass <i>Thalassia hemprichii</i> in Okinawa, Japan

“…This is most likely due to additional consumption by benthic animals, such as; crabs, gammarids and sphaeromatids. These small animals ap- Zepp et al 1998;Chin-Leo and Benner 1991;Mans et al 1998;Ziegler and Benner 2000), substrate composition (Boer 2000), water depth, and waves Romaro 1997, 1996) impact the process of plant litter decom position. In this study, the large variation of organic carbon and nitrogen contents at St.1 may be caused by the unstable environmental condition without seagrasses.…”