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

Abstract

Seagrasses play a variety of important ecological roles in coastal ecosystems. Here we present evidence that seagrass detritus from the widespread species Thalassia testudinum is an important source of ocean color and a UV-protective substance in a low latitude coastal shelf region of the United States. The production and light-induced degradation of chromophoric (sunlightabsorbing) dissolved organic matter (CDOM) from T. testudinum was examined under field and controlled laboratory conditions to obtain data that could be used to estimate the contribution of seagrass-derived CDOM to the coastal pool. The laboratory studies measured the temperature dependence and photodegradation of the spectral (UV-visible, fluorescence) and molecular mass properties of CDOM produced during the degradation of T. testudinum detritus. The rate of CDOM production is temperature-dependent with rates doubling when temperature increases from 21.4 to 32.6°C. The magnitude of this increase is close to the widely observed Q 10 factor for microbial decomposition, indicating that the CDOM production was likely microbially mediated. The absorption coefficients and fluorescence of CDOM from T. testudinum decreased on exposure to solar UV radiation (UVR) and the wavelength dependence was determined for this photobleaching process.KEY WORDS: Thalassia testudinum · Seagrass · CDOM production · Coastal ocean color · South Florida · UVR · Photobleaching · Mass spectra Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 282: [59][60][61][62][63][64][65][66][67][68][69][70][71][72] 2004 Green & , Hoge et al. 1995, Coble 1996, Vodacek et al. 1997, Del Castillo et al. 2000, Markager & Vincent 2000, Moran et al. 2000, Stedmon & Markager 2001, Twardowski & Donaghay 2001, Hu et al. 2002. CDOM is also an important photochemical source of reactive oxygen species in the sea (Kieber et al. 2003) and it is primarily responsible for the attenuation of solar UV radiation (UVR) in coastal environments (Vodacek et al. 1997. UV exposure has a wide range of potential impacts on coastal ecosystems (Haeder et al. 2003, including inhibition of the photosynthetic activity of corals (Lesser 2000) and of seagrasses (Larkum & Wood 1993, Detres et al. 2001, Figueroa et al. 2002, Durako et al. 2003.A few field studies have suggested that seagrasses may be an important source of CDOM in subtropical and tropical coastal ecosystems (Burdige et al. 2002. UV protective substances in seagrasses (Detres et al. 2001, Brandt & Koch 2003, Durako et al. 2003 could provide one source of CDOM to the overlying ocean water. One class of UV-absorbing compounds, the alkanones, is a component of the water-soluble substances released by decomposing seagrass detritus (Hernandez et al. 2001). In this paper, we use the term 'detritus' to refer to senescent blades of seagrass that are frequently found on the ocean bottom and intertidal zones in coastal regions. CDOM is often susceptible to light-induced loss of its UV and visible absorptio...

Saltmarshes as a source of chromophoric dissolved organic matter (CDOM) to Southern California coastal waters

“…Our S values ranged from 0.0100 to 0.0112 nm 21 with no statistical difference between ebb vs. flood tides and sites, consistent with a single source of similarly bleached CDOM in the marsh and surf zone waters. Our S values are on the low end of the range (0.012-0.018 nm 21 ) previously reported for estuarine waters (Vodacek et al 1997;Stabenau et al 2004;Tzortziou et al 2007). Lower S values have been attributed to a CDOM fraction that is more humic or more terrestrial in nature (Green and Blough 1994; Vodacek Whitehead et al 2000).…”

“…Lower S values have been attributed to a CDOM fraction that is more humic or more terrestrial in nature (Green and Blough 1994; Vodacek Whitehead et al 2000). S values increase as CDOM is photobleached and decrease with aging in soils and sediments (Stabenau et al 2004;Tzortziou et al 2007). …”

“…Unexpectedly, a protein-like peak and terrestrial humic-like peaks were produced by the brown seaweed growing on the tide gates. Submerged seagrass beds in coastal South Florida have previously been shown to be significant contributors of CDOM with terrestrial characteristics to coastal waters (Stabenau et al 2004), making up 40-50% of the coastal CDOM pool. There is no seagrass offshore at this site, but it does occur in parts of the down coast Upper Newport Back Bay (Pednekar et al 2005).…”

“…This is on the order of the residence time for the sloughs. In a recent study of the photobleaching of CDOM from seagrass leachates, the half-life for absorbance loss ranged from 20 to 30 h depending on temperature, with the fluorescence decay of the humic peak reported as being faster (Stabenau et al 2004). Half-lives of 18-27 h have been reported for mangrove and wetland leachates' fluorescence decay by photochemical degradation (Scully et al 2004).…”

Planktonic and benthic microalgal community composition as indicators of terrestrial influence on a fringing reef in Ishigaki Island, Southwest Japan

“…However, this is not the case and CDOM must have come from sources within 23 Shiraho Reef such as seagrass meadows, macroalgal colonies, and bottom sediments. CDOM 24 can be derived from benthic habitats such as seagrass meadows (Stabenau et al, 2004). It can typhoons.…”