Nutrient changes from aquaculture history and the environmental implications in a typical open coast of the East China Sea

Nutrient changes derived from high-resolution sediment records along a typical open coast of the East China Sea were analyzed to assess environmental variations associated with aquaculture practices over recent decades. Biogenic components and stable isotopes significantly fluctuated in the sediment profiles, suggesting substantial environmental changes since the 1930s. Slight increases in total nutrient contents and carbon stable isotopes (δ¹³C_org) were observed from the 1930s to the 1970s, likely due to increasing terrestrial nutrient inputs resulting from population growth within river watersheds. From the 1970s to the 1980s, δ¹⁵N values slightly decreased, suggesting an influx of anthropogenic nutrients primarily originating from chemical fertilizers. A minor decline was observed in total nitrogen and organic phosphorus, as well as total organic carbon contents during this period, coinciding with the expansion of kelp (Saccharina japonica) cultivation. Since the 1990s, nitrogen and phosphorus have substantially increased by over 31% and 22%, respectively, along with a notable shift toward heavier δ¹⁵N values attributed to feed inputs. Additionally, fluctuations in δ¹³C_org indicate that feed-based aquaculture has profoundly altered both nutrient structure and composition along the Lianjiang coast. The annual nitrogen and phosphorus loadings absorbed by aquaculture were calculated to be approximately 14,592 ± 3812 and 1848 ± 762 t in 2020, respectively. Meanwhile, annual nitrogen and phosphorus discharges from feed-based farming amounted to approximately 17,038 ± 4465 t and 3343 ± 612 t, respectively. Nevertheless, the net N loadings from aquaculture in Lianjiang were one order of magnitude lower than those for discharge or absorption. Kelp accounted for over 86% of nutrient removal in Lianjiang, despite its cultivation period lasting only 4–6 months each year. This suggests that significant net absorption occurs during the kelp cultivation season, which mitigates nutrient loads from adjacent coastal regions. Conversely, during non-cultivation seasons, prominent net discharge of nutrients was expected to affect adjacent coastal areas. Thus, seasonal variations in kelp cultivation practices appear to modify nutrient cycles in the adjacent marine environments.