Nutrient variations across multiple timescales and their driving factors in the Laizhou Bay, China

Over the past decade, due to anthropogenic regulation, the water quality in Laizhou Bay (LZB) has generally been stable, but nitrogen remains high, and the bay environment is still in a state of mild eutrophication. With the influence of climate change and human activities, the spatiotemporal variation of nutrients in the Laizhou Bay (LZB) has undergone significant changes, resulting in a series of environmental problems such as eutrophication and red tides. To better understand the spatiotemporal dynamics of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) in the LZB, this study conducted 27 field surveys from 2015 to 2023, along with a continuous observation over a tidal cycle on August 22, 2024, to assess the current distribution of nutrients in the bay. The results showed that at the tidal-cycle scale, nutrient concentrations (DIN and DIP) were lowest during high tide and highest during low tide. In terms of seasonal variation, nutrient concentrations were highest in autumn. From an interannual perspective, nutrient levels remained generally stable from 2015 to 2023 in the LZB. Medium and small rivers in the southwestern of the LZB were identified as major contributors to the bay's nutrient variability in both space and time.