Nutritional environment and influencing factors of seawater and surface sediments in the Jiaozhou Bay

Seawater and surface sediment samples at 30 stations in the Jiaozhou Bay (JZB) and river water and coastal water samples at 18 stations along the JZB were collected in August 2019 and May 2021, respectively. Dissolved inorganic nutrient concentrations and structure in the water and the contents of total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), biogenic silica (BSi), and stable isotopes of carbon and nitrogen (δ¹³ C, δ¹⁵ N) in surface sediments were analyzed to clarify the present nutritional environment of seawater and surface sediment and its main influencing factors in the JZB. The results show that the spatial distribution of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicate (DSi) in seawater is consistent with that in the coastal water of JZB. The high values that occur northeast of the bay are mainly caused by river input and sewage discharge, and the low values mainly occur at the center and bay mouth. Combined with the analysis of historical data, we found that the nutrient concentrations of JZB in summer continued to rise from 1990 to 2008 and then declined since 2006 because of the declining nutrient loadings related to the implementation of total pollutant load control management and freshwater loadings from surrounding rivers. The decreased nutrient loadings were mainly found in rivers such as the Dagu River, causing the disappearance of high concentrations in the western re-gion after 2010. The phosphorus limitation accelerated after 2000 as a result of the imbalanced input of nitrogen and phosphorus. The high values of TOC, TN, and TP in the surface sediments of JZB are concentrated along the northeast and east coasts, combined with BSi and water nutrient analysis, which is mainly caused by the river input and sewage discharge and the high primary productivity they bring. Sediment biogenic elements are well coupled with water column nutrients in spatial distribution. The adverse effect of coarse sediment grain size on the preservation of organic matter and the strong hydrodynamic effect of the bay mouth jointly lead to the low content of biogen-ic elements in the west, middle, and mouth of the bay. The δ¹³ C and two-endmember mixing model show that the source of organic matter in the surface sediments of JZB is mainly marine-derived, accounting for an average of 64%. The eastern coast is obviously affected by terrestrial input. The spatial distribution of δ¹⁵ N shows that the nitrogen in the surface sediment of JZB is jointly affected by mariculture and sewage discharge. The current situation of the water body and sediment nutrient environments shows that the control of sewage discharge from the northeast river and the coastal area is the key to the later pollution control of JZB.