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.

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Ocean warming trends are observed and coincide with the increase in concentrations of greenhouse gases in the atmosphere resulting from human activities. At the ocean surface, most of the incoming infrared (IR) radiation is absorbed within the top micrometers of the ocean's surface where the thermal skin layer (TSL) exists. Thus, the incident IR radiation does not directly heat the upper few meters of the ocean. This paper investigates the physical mechanism between the absorption of IR radiation and its effect on heat transfer at the air‐sea boundary. The hypothesis is that given the heat lost through the air‐sea interface is controlled by the TSL, the TSL adjusts in response to variations in incident IR radiation to maintain the surface heat loss. This modulates the flow of heat from below and hence controls upper ocean heat content. This hypothesis is tested using the increase in incoming longwave radiation from clouds and analyzing vertical temperature profiles in the TSL retrieved from sea‐surface emission spectra. The additional energy from the absorption of increasing IR radiation adjusts the curvature of the TSL such that the upward conduction of heat from the bulk of the ocean into the TSL is reduced. The additional energy absorbed within the TSL supports more of the surface heat loss. Thus, more heat beneath the TSL is retained leading to the observed increase in upper ocean heat content.

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.

No abstract