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 (sunlight- absorbing) dissolved organic matter (CDOM) from T. testudinum was examined under field and con- trolled laboratory conditions to obtain data that could be used to estimate the contribution of sea- grass-derived CDOM to the coastal pool. The laboratory studies measured the temperature depen- dence 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 Q10 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.