Records of biogeochemical variables for Semarang Bay, Indonesia, facing potential coastal deoxygenation

Coastal areas worldwide, including the Indonesian seas, are experiencing a decline in dissolved oxygen (DO) concentration, leading to deoxygenation. Semarang Bay, due to its semi-enclosed nature and significant terrestrial input, is particularly vulnerable to this phenomenon. We analyzed multi-annual records of biogeochemical variables, including nutrient concentrations, chlorophyll-a (Chl-a), and both surface and bottom dissolved oxygen (DO), to assess the possibility of coastal deoxygenation. The study focuses specifically on bottom DO to understand its variability and potential impact on the sediment-water interface. It aims to evaluate the potential for coastal deoxygenation in the bay region by examining sediment stable isotope signatures and biogeochemical variable records in connection with their potential influence on DO at the sediment-water interface. The analysis reveals contrasting trends in sea surface temperature, bottom temperature, and DO concentrations. Spatial analysis uncovers distinct patterns, emphasizing the influence of monsoon seasons on temperature, DO, and Chl-a concentrations. The study notes a declining trend in DO and bottom DO (i.e., −0.055 and −0.048 mmol/m³ per year, respectively), underscoring the need to monitor dissolved oxygen dynamics at the sediment-water interface. Isotope analysis of surface sediment suggests potential sediment deoxygenation at specific sampling sites, irrespective of proximity to the shoreline or bathymetric depth, with similar indications in flood channels. This research offers valuable insights into the complex dynamics of coastal biogeochemistry in Semarang Bay. The findings underscore the need for further research to refine models and explore alternative approaches to address the identified limitations, thereby contributing to enhanced environmental monitoring and assessment in the study site.