Sedimentary geochemical records and their indications for environmental variations in Bohai Bay

In order to understand the long−term variations in the nutritional environment and the key influencing factors of Bohai Bay, sediment cores from the mouth of the bay (BH15) and the outer area of the bay (BH47) were collected in April and June 2018 separately. The contents of total organic carbon (TOC), total nitrogen (TN), phosphorus (P), biogenic silica (BSi), and stable isotopes of carbon and nitrogen (δ¹³ C, δ¹⁵ N) were analyzed, and the causes of the changes of the above parameters were discussed in combination with the changes of climate and sur-rounding human activities. The results showed that the TN content in both sediment cores exhibited an increasing trend since the 1980s, which is consistent with the long-term variations in dissolved inorganic nitrogen (DIN) in the sea water. Sewage discharge, fertilizer application and mariculture are the main sources of nitrogen in the Bohai Bay, among which the input of agricultural fertilizer has decreased since 2007, while the discharge of mariculture and domestic sewage into the sea has shown a continuous upward trend. Inorganic Phosphorus (IP) was the main form of phosphorus in sediments. The IP in BH15 in the bay has shown a decreasing trend since the mid-1990s, while the IP in BH47 from the mouth of the bay has shown a decreasing trend after 1970s and a slow increasing trend since 1990s. All of them show obvious terrigenous input characteristics. The results of TOC/TN ratio、δ¹³ C value and δ¹³ C two-endmember mixing model indicated that the sediment organic matter in the mouth of Bohai Bay and adjacent sea area was influenced by both terrestrial input and marine autochthonous sources, with marine organic matter being the dominant source, the contribution of marine organic matter has decreased Since the 1990s. Compared with the BH47 core from the mouth of the bay, the BH15 core is closer to the coastal area, more significantly affected by terrestrial input, with higher TN content and contribution of terrestrial organic matter. This study shows that in order to effectively manage the water environment of Bohai Bay, it is necessary to strengthen the control of mariculture and domestic sewage discharge into the sea while controlling the use of agricultural fertilizers in the fu-ture.