Abstract
The sediment in Laguna de Términos, the largest and shallowest system in the Southwest portion of the Gulf of Mexico features a broad range of ecological and hydrobiological characteristics driven by annual weather cycles (dry and wet seasons), causing large salinity gradients during the wet season due to large river discharges. Four sampling campaigns were carried out during the wet and the dry seasons in 2009 and 2010 on a selection of 13 out of 35 stations. Measurements of Sediment Oxygen Demand (SOD) and nutrient fluxes at the sediment-water interface were performed using lab incubations with 15 cm diameter sediment cores. SOD fluctuated between 1327 ± 161 and 2248 ± 359 μmol m−2 h−1 for dry and wet seasons respectively. Silicate effluxes were also significantly higher during the wet seasons (89.4 ± 15.9 μmol m−2 h−1) than during the dry season (46.5 ± 11.4 μmol m−2 h−1). PO4 fluxes were low all over the study period without seasonal trend. No significant difference was measured for DIN fluxes but there was a tendency for DIN uptake during the wet season (−2.9 ± 18.8 μmol m−2 h−1) and conversely an efflux during the dry season (24.3 ± 7.3 μmol m−2 h−1). SOD correlated to organic matter and chloropigment content of the sediments while silicate fluxes responded to enhanced chloropigments in the sediments. During both seasons, total benthic nutrient fluxes overwhelmed largely riverine inputs and benthic carbon mineralization rates approximated a significant proportion of the pelagic organic carbon production. We conclude that benthic processes in Laguna de Términos are largely driven by weather variability and that they contribute substantially to carbon and nutrient budgets in this shallow sub-tropical system.
Highlights
Due to human activities, large concentrations of contaminants and nutrients have accumulated in sediments of natural water bodies, changing environmental conditions (Li and Cai, 2015; Birch, 2017)
We describe exchange rates measured at the sediment-water interface in a tropical lagoon, to assess the importance of sediments in total system metabolism and to investigate how much of the variability can be ascribed to the alternations of rainy and dry seasons
A salinity distribution close to the one observed during the dry season was observed in October 2009, a period of strong positive salinity anomaly related to an El Niño Modoki driven drought episode (Fichez et al, 2017)
Summary
Large concentrations of contaminants and nutrients have accumulated in sediments of natural water bodies, changing environmental conditions (Li and Cai, 2015; Birch, 2017). The processes that govern the fate of these substances in the sediment result from the complex interactions with the biogeochemical cycles of major redox and biogenic elements such as C, N, O, P and Si (Middelburg and Soetaert, 2004). The pelagic biogeochemical cycles are strongly linked to the sediment compartments where organic matter mineralization preferentially occurs, resulting in enhanced nutrient fluxes and oxygen uptake rates at the water-sediment interface (Archer and Devol, 1992; Cowan and Boynton, 1996; Grenz et al, 2003). The transformation of biologically available forms of N into inert gas, together with different external sources such as river inputs, surface runoff and atmospheric precipitation, controls the variation of pelagic nutrient concentrations over inter-annual time scales (Soetaert and Middelburg, 2009). The soluble nitrogenous compounds released from the sediments during the decomposition of organic matter can supply 30–100% of dissolved N utilized by phytoplankton in overlying water (Grenz et al, 2010)
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