Elucidating the Hidden Nonconservative Behavior of DOM in Large River‐Dominated Estuarine and Coastal Environments

Abstract

Understanding the mixing behavior and biogeochemical cycling of heterogeneous dissolved organic matter (DOM) at the land‐ocean interface has been a major challenge, especially in large river‐dominated estuarine and coastal environments with multiple and complex end‐members of riverwater and seawater. Significant correlations and thus an “apparent” conservative behavior are generally observed for the bulk DOM when plotting concentration‐salinity relationships, but the relatively small deviations from the apparent conservative behavior are difficult to be quantified. In July 2016, the flood in the Changjiang River basin caused the Changjiang Diluted Water extended widely to the western East China Sea, making it a natural laboratory to examine the river‐sea mixing behavior of riverine DOM and other optical properties. Here a new biogeochemical approach was established to elucidate their hidden nonconservative behavior in the study area. Comparisons in the bulk dissolved organic carbon and the three chromophoric DOM properties among 13 pairs of same‐salinity patches (with similar salinities but quite different distances from the river mouth) revealed that the East China Sea shelf was a significant sink for all the three chromophoric DOM properties. The Changjiang Diluted Water surface branches transporting southward tended to serve as a sink for the bulk dissolved organic carbon while the ones transporting northward were less a sink or even a source. The different removal extents of various biogeochemical constituents can also be calculated from the same‐salinity patch comparisons. Our results obtained by the new approach provide detailed magnitudes of sink or source for different DOM properties and their in situ removal/production extents, giving new insights into biogeochemical processes in large river‐dominated estuarine and coastal environments.

Seasonal Variations in Molecular Size of Chromophoric Dissolved Organic Matter From the Lower Changjiang (Yangtze) River

“…Note that only data for natural freshwaters were included in Figures 8 and 9, and it has been well known that the S 275-295 values would be greatly elevated during DOM photodegradation (Del Vecchio & Blough, 2002;Helms et al, 2008) or during estuarine mixing with seawater (Gao et al, 2019;Xie et al, 2012). For example, Zhou et al (2018) Helms et al (2008), and that between colloidal percentage and S R is based on the correlation established in Figure 7b.…”

New Insights Into the Non‐Conservative Behaviors of Nutrients Triggering Phytoplankton Blooms in the Changjiang (Yangtze) River Estuary

“…Changing end‐member nutrient concentrations in the study area are common (Gao et al., 2015; Zhang et al., 2007), hence a non‐linear relationship between nutrient concentrations and salinity does not necessarily indicate a non‐conservative nature of nutrients. On the other hand, high correlation coefficient values ( R 2 ) do not necessary mean that nutrients are strictly conservative (Gao & Zong, 2021; Gao et al., 2019). Considering these factors, we must be very careful when drawing conclusions based on these correlations.…”

“…Thus, in the lower Changjiang River Estuary and the adjacent ECS shelf, the concentrations of these two nutrients consistently showed a negative correlation with salinity in the dry seasons and a positive correlation in the flood seasons (Figures S3 and S4 in Supporting Information ). It should be noted that the concentration of NH 4 + and NO 2 − were greatly elevated at the stations near the river mouth, which might be due to anthropogenic pollution from the nearby coasts (Gao et al., 2019).…”

“…The Changjiang River Estuary and the adjacent ECS shelf have quite different physical and chemical regimes, which largely result in seasonal variations in concentrations and properties of nutrients and particulate and dissolved organic matter in this area. The distribution and variation patterns of concentrations of dissolved nutrients, dissolved organic carbon, chromophoric dissolved organic matter, and other dissolved carbohydrate compounds also change greatly with season (Gao et al, 2019(Gao et al, , 2020Liu et al, 2016;Song et al, 2017;Zhang et al, 2007).…”

Terrestrial exports of dissolved and particulate organic carbon affect nearshore ecosystems of the Pacific coastal temperate rainforest

“…The apparent conservative mixing of DOC in estuaries has been reported across many systems, but in reality, DOC within these dynamic waters is subject to numerous small, concurrent changes in its sources and sinks (Cifuentes and Eldridge 1998; Abril et al 2002; Gao et al 2019). In contrast, the concentrations of POC across the plumes were much higher than the monthly mean concentrations in both fresh and marine waters.…”