Colored dissolved organic matter from tropical peatlands strongly impacts euphotic zone depth in a shelf sea

Abstract:

The flux of organic carbon from land to the ocean is an important yet enigmatic part of the global carbon cycle. While this flux is thought to have increased significantly world-wide as a result of human activities (such as land-use change), the biogeochemical fate of this carbon is poorly known. Consequently, we know little about the environmental impacts of this flux on coastal waters. We will present data from a biogeochemical time-series station on the Sunda Shelf in Southeast Asia, which demonstrate an important but largely neglected impact of these fluxes: because terrigenous dissolved organic carbon is very rich in colored dissolved organic matter (CDOM), it is responsible for substantial underwater light absorption and thereby significantly reduces the depth of the euphotic zone in this shelf sea. Our data reveal a large input of terrigenous CDOM during the southwest monsoon season, with CDOM absorption at 440 nm increasing five-fold and the slope of the CDOM absorption spectrum from 275–295 nm decreasing to below 0.020 nm^-1. This input is sustained for several months at our site and originates from the extensive tropical peatlands found in parts of Southeast Asia. Together with accompanying bio-optical data, we show that this terrigenous CDOM is responsible for reducing the euphotic zone depth by at least 10–30%. To our knowledge, these data are the first to directly quantify the effect specifically of terrigenous CDOM on the euphotic zone depth in a shelf sea. Because the extensive land conversion of Southeast Asia’s peatlands has likely increased the flux of dissolved organic carbon to sea, a significant proportion of this terrigenous CDOM-mediated “coastal darkening” may be anthropogenic. We suggest that anthropogenic increases in terrigenous organic carbon fluxes have likely had similarly significant impacts on euphotic zone depths in shelf seas world-wide. These impacts must be quantified urgently to develop a complete understanding of the environmental consequences of land–ocean carbon fluxes.