Ecological and biogeochemical processes in the Yeongsan River estuary, Yellow Sea

Abstract

1) Effects of a large–scale artificial dyke on sediment oxygen demand, sulfate reduction, benthic nutrient flux and benthic–pelagic coupling in the Yeongsan River estuary, Yellow Sea

We investigated the environmental and ecological impacts of an artificial dyke on sediment biogeochemistry, organic carbon oxidation, and benthic nutrient flux (BNF) and established the impact of its coupling to primary production in the water column via benthic–pelagic coupling in the Yeongsan River estuary (YE). The organic carbon (OC) and organic nitrogen (ON) contents near the dyke (YE1) were 3–7 times higher for OC and 3–9 times higher for ON than those far from the dyke (YE2 and YE3). The NH4+ and PO43- inventories at YE1 were 5–30 and 7–22 times higher than those at YE2 and YE3, respectively. The sediment oxygen demand (SOD) at YE1 was 5 times higher than at YE3. Sulfate respiration (SR) accounted for only 13% of the total C oxidation at YE3, whereas it was responsible for 73% of the carbon oxidation at YE1. The nitrogen and phosphate fluxes at YE1 accounted for 195% and 197%, respectively, of the N and P demands for primary production. Our results suggest that the artificial dyke reduced the tidal velocity causing massive organic matter deposition near the dyke, and the enrichment of organic matter in the surface sediment enhanced benthic respiration and the nutrient fluxes.

2) Effects of freshwater discharge on nutrients, primary production, bacteria production/respiration and net trophic system in the Yeongsan River estuary, Yellow Sea

We investigated the environmental and ecological impacts of freshwater discharge on nutrients, primary production, bacteria production, and respiration as related to the net ecosystem metabolism in the water column in the Yeongsan River estuary (YE). The concentrations of dissolved inorganic nitrogen (DIN; 2–130 µM) and phosphate (DIP; 0.3–1.6 µM) during freshwater discharge were higher than those 5–15 days post-freshwater discharge (DIN 30–32 µM, DIP 0.1–0.2 µM). The phytoplankton did not respond immediately to nutrient input via freshwater discharge due to the high turbidity and flushing rate. The phytoplankton bloomed when the salinity recovered to more than 27 psu. The bacteria production varied little (91–473 mg C m-2 d-1) with discharge and recovery but was significantly correlated with chlorophyll a (Chl.-a). The bacterial respiration was maximal 1–2 days post–freshwater discharge (4329–5245 mg C m-2 d-1) and then deceased by 5–15 days post–freshwater discharge (843–950 mg C m-2 d-1). The primary production (PP)–community respiration (CR) was negative 1–2 days post–freshwater discharge due to very low PP. The phytoplankton community was severely affected by turbidity and decreased salinity, whereas BR increased due to higher maintenance energy requirements. When the salinity recovered and Kd was lower 5–15 days post-freshwater discharge, the PP–CR in the surface water changed from negative to positive, resulting in low respiration. The autotrophic–heterotrophic switching on–off in YE was linked to the changes in carbon input, salinity, and light with freshwater discharge, suggesting that freshwater discharge is the key factor regulating trophic systems.

3) Effects of freshwater discharge on sediment oxygen demand, sulfate reduction, benthic nutrient flux, and benthic–pelagic coupling in the Yeongsan River estuary, Yellow Sea

We investigated the environmental and ecological impacts of freshwater discharge on sediment biogeochemistry, organic carbon oxidation, and benthic nutrient flux (BNF) and established its coupling to primary production in the water column via benthic–pelagic coupling in the high-nutrient Yeongsan River estuary (YE). The areal sediment oxygen demand (SOD) was always higher at YE1 (8–327 mmol m-2 d-1) than at YE3 (8–88 mmol m-2 d-1). The sulfate reduction rate (SRR) was the highest at YE1 (2.8–91.0 mmol m-2 d-1), followed by those at YE2 (5.1–20.4 mmol m-2 d-1) and YE3 (2.4–5.3 mmol m-2 d-1). Seasonal variations in SOD and SRR were related to the chlorophyll a (Chl.-a) of surface sediment and primary production (PP) in the water. The NH4+ flux in the sediment ranged from 0.08 to 2.16 µM h-1 and was always higher at YE1 than at YE3. The PO43- flux was the highest at YE1 in April 2011 (0.14 µM h-1) and the lowest at YE3 in September 2012 (0.002 µM h-1). The BNF was sufficient to supply 46–192% of the N demand and 63–199% of the P requirement for PP during the sampling period before the first massive discharge (4–32 tons × 106 d-1). However, the sediment nutrient released was not a major feature of phytoplankton production, contributing only 0.51–22.3% of the N demand and 0.54–31.3% of the P demand required for PP after a massive discharge (> 60 tons × 106 d-1). These results indicated that the first massive freshwater discharge (> 60 tons × 106 d-1) is the key factor regulating benthic–pelagic coupling.