Abstract
Estuaries modify the concentration and composition of riverine nutrients and organic matter (OM), which eventually determine the riverine flux effect to coasts. Nutrients, organic carbon (OC), pigments, and amino acids (AAs) from the samples collected in the eutrophic Wenchang River Estuary (WRE) in China and the oligotrophic Krka River Estuary (KRE) in Croatia were analyzed in order to have a better understanding of how estuaries regulate terrestrial materials. We found a clear increase of dissolved inorganic nitrogen (DIN) concentration and its subsequent decrease due to the removal of DIN (over 100 μM) in the WRE, whereas DIN showed minor variation lengthwise in the KRE, i.e., with the salinity changes, ranging between 1.0 and 5.8 μM. The elevated algae-derived OC, dissolved organic nitrogen, and particulate AAs nitrogen suggest that the OM assimilation may explain approximately one-third of the DIN removal in the WRE, whereas in the oligotrophic KRE, such inorganic to organic transformation is not likely to be significant. Due to the prominent estuarine nitrogen removal/assimilation process, DIN/dissolved inorganic phosphorus (DIP) ratio was as high as 425 in the upper WRE under strong riverine influence, but it declined to as low as 5.4 at the mouth of WRE, that is even lower than DIN/DIP ratio at the mouth of oligotrophic KRE (12). When compared with other rivers worldwide, the Wenchang River showed high nutrients and organic carbon yields. Given the contrasting estuarine process (e.g., DIN removal) between the KRE and the WRE, apparent high nutrient yield from eutrophic rivers should be viewed with caution in ocean studies as the final impact to coastal zone could be similar to the oligotrophic rivers like the KRE.
Highlights
Estuarine systems serve as active transition zones connecting the rivers and the seas, and are among the most active interfaces in the biosphere and the geosphere, playing important roles in biogeochemical cycles under the framework of land-ocean interaction (Jickells, 1998; Durr et al, 2011)
Different types of river basins of the Wenchang River Estuary (WRE) and the Krka River Estuary (KRE) make the biogeochemical processes of their estuaries differ from each other, resulting in different modifications of the net biogenic elements fluxes
The eutrophic WRE showed a sharp removal of dissolved inorganic nitrogen (DIN), accompanied by increasing organic nitrogen content
Summary
Estuarine systems serve as active transition zones connecting the rivers and the seas, and are among the most active interfaces in the biosphere and the geosphere, playing important roles in biogeochemical cycles under the framework of land-ocean interaction (Jickells, 1998; Durr et al, 2011). Given the highly spatial and temporal heterogeneity of medium and small rivers, and lack of routine monitoring systems, their roles in terrestrial material export remain of interest, and they are important questions to be answered (Hilton et al, 2008; Wheatcroft et al, 2010; Wu et al, 2017). Terrestrial materials undergo a longer time of alternation and/or cycling process within the estuaries, likely enhancing the estuarine filtering or alternation effect (Durr et al, 2011). The estuarine processing of terrestrial materials strongly modifies and determines the net river flux, affecting its amount and impact on coastal seas (Unger et al, 2013). Eutrophic rivers may even have a counteracting process in the estuaries that regulates the final output nutrient concentrations and compositions to the sea. It is hard to draw a simple conclusion that oligotrophic rivers have a smaller impact on coastal zone relative to eutrophic rivers
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