Dissolved organic matter from tropical peatlands reduces shelf sea light availability in the Singapore Strait, Southeast Asia

Abstract

Shelf seas provide valuable ecosystem services, but their productivity and ecological functioning depend critically on sunlight transmitted through the water column. Anthropogenic reductions in underwater light availability are thus a serious threat to coastal habitats. The flux of light-absorbing coloured dissolved organic matter (CDOM) from land to sea may have increased world-wide, but how this has altered the availability and spectral quality of light in shelf seas remains poorly known. We present time-series data from the Sunda Shelf in Southeast Asia, where the monsoon-driven reversal in ocean currents supplies water enriched in CDOM from tropical peatlands for part of the year, resulting in 5- to 10-fold seasonal variation in light absorption by CDOM. We show that this terrigenous CDOM can dominate underwater light absorption at wavelengths up to 500 nm, and shift the underwater irradiance spectrum towards longer wavelengths. The seasonal presence of terrigenous CDOM also reduces the 10% light penetration depth by 1-5 m, or 10-45%. We estimate that on average 0.6 m, or 25%, of this terrigenous CDOM-mediated shoaling might be attributable to the enhanced input of dissolved organic matter following peatland disturbance. The seasonal change in the light environment is correlated with changes in phytoplankton absorption spectra that suggest a photo-acclimation response, and we infer that terrigenous CDOM likely contributes to limiting the depth distribution of photosynthetic corals. The results reveal an ecologically important but largely overlooked impact of human modifications to carbon fluxes that is likely increasingly important in coastal seas.

Rising dissolved organic carbon concentrations in coastal waters of northwestern Borneo related to tropical peatland conversion

“…Since TSM concentration did not show a temporal trend, the increase in CDOM that we found implies that the euphotic zone depth across the coastal waters likely showed a long-term decrease. Such coastal darkening is thought to have affected marine productivity and ecological functioning elsewhere by reducing productivity and light availability to benthic photosynthetic communities ( 9 , 10 ). The overall increase in CDOM absorption that we have estimated (0.0352 m −1 at 440 nm) is similar in magnitude to the long-term increase reported off Norway (0.012 m −1 at 500 nm), which was linked to ecological regime shifts ( 9 ).…”

“…However, across most of the world, we lack observational data of such changes. tDOC is not only quantitatively important in the global carbon cycle but can also affect coastal ecosystems by reducing the amount of light available for photosynthesis ( 9 , 10 ) and acidifying seawater if it decomposes to CO 2 ( 11 , 12 ). It is therefore important to quantify long-term tDOC dynamics in coastal waters around the world and understand their drivers.…”

Extensive Remineralization of Peatland‐Derived Dissolved Organic Carbon and Ocean Acidification in the Sunda Shelf Sea, Southeast Asia

“…The relatively low chlorophyll‐ a concentrations year‐round suggest that autochthonous organic matter production rates are moderate and, importantly, do not show strong seasonality. This is likely due to persistent light limitation caused by strong vertical light attenuation (Chow et al., 2019; Martin et al., in press; Morgan et al., 2020) and tidal mixing of the water column, which prevents vertical stratification (Mayer & Pohlmann, 2014). Moreover, the very small annual range in seawater temperature means that microbial physiological rates will not vary strongly.…”

Inherent Optical Properties based Vulnerability Assessment of Euphotic Zone Compression in peatland influenced Southeast Asian coastal waters

“…The southwest monsoon exhibits south-westerly winds with relatively higher temperatures and less rainfall than the early northeast monsoon from June to September. The sampling and analytical methods for CDOM and particulate absorption, backscattering coefficients, and radiometric measurements were summarized in (Martin et al, 2021) and (Zhou et al, 2021). Seawater samples were collected at 5m depth with Niskin bottles to analyze absorption by CDOM a g (l) and DOC since October 2017.…”

“…Several semi-analytical algorithms for K d retrieval have been developed based on the relationship between total absorption (at) and backscattering (b b ) coefficients to light attenuation (Gordon et al, 1975;Kirk, 1984;Lee et al, 2005;Lee et al, 2014). Radiometer measured and semi-analytically calculated K d and Z d -10% (Lee et al, 2005) for this region were compared by (Martin et al, 2021) with a mean absolute percent error for euphotic zone depth of 9%. Hence, we adopted the same approach for K d retrieval developed by (Lee et al, 2014) to account for a shift in the phase function between pure water molecular backscattering and particle backscattering.…”