Dissolved organic matter from tropical peatlands impacts shelf sea light availability on coral reefs 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 recognized as a serious threat to coastal habitats. The flux of strongly 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. Here, 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 5–10-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 in the underwater irradiance spectrum towards longer wavelengths. The seasonal presence of terrigenous CDOM also causes the depth of 10% light penetration to shoal by 1–5 m, or 10–45%. We further estimate that on average 0.6 m, or 25%, of this terrigenous CDOM-mediated shoaling might be attributable to the enhanced loss of dissolved organic matter caused by peatland disturbance. We show that the seasonal change in the light environment is correlated with photo-acclimation by phytoplankton, and infer that terrigenous CDOM likely contributes to limiting the depth distribution of photosynthetic corals. Our results thus reveal an ecologically important but largely overlooked impact of human modifications to carbon fluxes that is likely becoming increasingly important in coastal seas.

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.…”

Quantifying photodegradation of peatland-derived dissolved organic carbon in the coastal ocean of Southeast Asia

“…Third, measurements of the water column inherent optical properties (i.e., particulate absorption and backscattering) are available for both regions Martin et al, 2021).…”

“…Despite these limitations, it is important to use a spectral AQY rather than applying a broadband AQY when simulating photodegradation in an optically thick water layer because the irradiance spectrum shifts strongly to longer wavelengths (with lower AQY) within just 1-2 m below the surface in these optically complex waters (Martin et al, 2021). However, the broadband AQY in our experiments was necessarily determined by exposing optically thin tDOC solutions to full-spectrum irradiance.…”

“…Regarding the IOPs in the water, the particulate absorption and backscattering are lower than the CDOM absorption for both modeled regions, partly because of the low chlorophyll-a concentrations in the study region . The light attenuation coefficient, Kd, is therefore dominated by CDOM absorption (Martin et al, 2021), especially at ultraviolet wavelengths. Given the multiple controlling factors of in-situ photodegradation rates, we recommend performing spectrally resolved optical modeling to obtain more accurate estimates of the photochemical decay rate constant that can then be used by simplified models across much larger scales.…”

“…This is important in the context of the global carbon cycle, because the riverine input and the biogeochemical processing of tDOC can have significant impacts on coastal marine environments. Intact tDOC can absorb sunlight and lead to ecologically harmful "coastal darkening" by shoaling the euphotic zone and altering the spectral quality of light underwater Martin et al, 2021;. Moreover, a significant fraction of tDOC may undergo remineralization in shelf seas, as shown for the Eurasian Shelf , the North Sea , the Louisiana Shelf , and the Sunda Shelf .…”