Isotope measurements were performed on dissolved nitrate (NO₃⁻) and ammonium (NH₄⁺) in the coastal waters of the northeastern Beibu Gulf, China, to investigate the seasonal nitrate sources and their biogeochemical processes, which are due to the rapid development of local industrialisation and urbanisation. The high N/P ratio observed in the coastal bay during both fall and spring suggests that P is a limiting nutrient, which in turn indicates that increasing P causes conditions favourable for algal blooms. Higher nutrient concentrations and δ¹⁵N-NO₃⁻ and δ¹⁵N-NH₄⁺ values were found in the nearshore area in the fall, suggesting that nutrients originated mainly from land-based pollution. A Bayesian isotope mixing model was used to calculate the contribution of potential NO₃⁻ sources and the results showed that in the nearshore area, NO₃⁻ originated mainly from manure and sewage (58%). In the spring, however, in addition to the impact of urban sewage effluents, the exchange of sediment and water was another important factor causing higher nutrient concentrations and positive NO₃⁻ isotopes in the nearshore area. There were lower concentrations of nutrients and an increase in δ¹⁵N-NO₃⁻ and δ¹⁵N-NH₄⁺ values in the offshore area in the fall, and the NO₃⁻ loss in the surface water was mainly caused by the process of assimilation. However, the exchange of sediment and water was the dominant factor causing higher nutrient concentrations (except for NO₃⁻) and positive dual nitrate isotopes but lower NO₃⁻ concentration in the offshore area during the spring. Overall, isotope analysis of NO₃⁻ and NH₄⁺ helps to illustrate the major sources of the former and their biological transformation in the northeastern Beibu Gulf.

In this study, we present the first complete mitochondrial genome sequence of the giant clam Lambis chiragra. The total length of the mitogenome is 16,404 bp. It contains the typical mitochondrial genomic structure, including 13 protein-coding genes, 22 transfer RNA genes, 2 ribosomal RNA genes, and 1 control region (D-loop). Mitogenome base composition is biased toward A + T content, at 66.4%. A phylogenetic tree based on complete mitogenome sequences revealed that, L.chiragra is the closest extant relative of Conomurex luhuanus.