Monitoring the dissolved organic carbon budget in Taihu Lake from space

Dissolved organic matter (DOM) refers to various organic molecules (sugars, fatty acids, and alkanes), is ubiquitous in global lakes, provides food/energy for heterotrophic bacterial growth, and serves as a carbon pool of the global carbon cycle and climate change. Moreover, colored DOM can influence phytoplankton photosynthesis through light absorption, and DOM decomposition consumes dissolved oxygen and deteriorates water quality. 

Dissolved organic matter content is usually determined by dissolved organic carbon (DOC). With the advantage of having a high spatiotemporal resolution, satellite-based data are optimal for dynamically monitoring DOC dynamics. However, only some empirical band combination algorithms have been proposed for regional lakes, and long-term series DOC satellite products for eutrophic lakes have not been published. 

Lake DOC has two source types. Autochthonous DOC is mainly derived from phytoplankton through extracellular release and cell degradation. Allochthonous DOC is primarily transported from the terrestrial ecosystem by input rivers. To understand the carbon cycle in a eutrophic lake, it is indeed necessary to know its DOC storage and the DOC exchange fluxes between the lake and surrounding rivers. 

Globally, lake eutrophication is a common problem, and approximately 63% of lakes with areas > 25 km2 are characterized by eutrophication. Machine learning has achieved exceptionally satisfactory results in remotely retrieving aquatic environmental variables in complex inland waters. Taking the typical eutrophic Lake Taihu as a study area, we realize the observation of lake DOC storage and exchange flux from space. 

For Lake Taihu, the DOC distributions were spatially consistent with those of phytoplankton content (Chl-a), and high DOC concentrations were observed in the northwestern lake bays. The satellite-derived monthly climatology DOC level displayed spatial distributions similar to those of the annual mean results, with high values in the northwest lake bays. What is noteworthy is that low DOC was observed in the Tiaoxi River estuary in the South Taihu. 

In Lake Taihu, the annual-monthly mean DOC level increased from 2003–2018. A positive relationship was found between the annual mean DOC level and Chl-a. As with the in-situ data, satellite-derived DOC levels were high in summer July and September, when Chl-a levels were also high. In contrast, the DOC level was low in June, when high basin precipitation diluted riverine input DOC. 

Annual input and output DOC fluxes by surrounding rivers varied greatly from year to year during 2008-2018. By dividing the lake DOC storage by the total DOC input flux, we obtained that the turnover time for riverine input DOC in Lake Taihu was approximately 24–43 days during 2008–2018. 

Although the mean net riverine DOC input was approximately 5.2 times the lake DOC storage volume during 2008-2018, DOC variations in Lake Taihu were controlled by phytoplankton growth. Therefore, terrigenous DOC was quickly transformed into other carbon forms after entering Lake Taihu. These indicate that the low dissolved oxygen level in Lake Taihu might be related to the decomposition of the large amount of terrigenous DOC.

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