Temporal Dynamics of Diatoms as Early Warning Indicators of Ecosystem Stress in Jakarta Bay, Indonesia

Jakarta Bay, one of Indonesia’s most heavily urbanized coastal systems, has experienced ongoing eutrophication caused by rapid urban growth and high human nutrient inputs. To evaluate how the ecosystem responds to nutrient enrichment, we analyzed long-term (2008–2019) phytoplankton data collected from the inner, middle, and outer zones of the bay during different monsoon seasons. Environmental parameters such as temperature, salinity, dissolved oxygen, nitrate, phosphate, and silicate were measured using standard oceanographic procedures, and phytoplankton composition, abundance, and diversity indices were used to assess trophic status. Results show a consistently diatom-dominated community, with Chaetoceros, Skeletonema, and Thalassiosira accounting for over 90% of total abundance. A notable bloom occurred between 2009 and 2010 during an intense La Niña phase, coinciding with increased nitrate levels and high N:P ratios (>16). Diversity (H′ = 0.7–1.6) and evenness (E = 0.3–0.6) remained low, indicating community simplification and ecological stress. Spatial analysis revealed that the inner bay was hypertrophic, while the outer bay remained mesotrophic. This dominance pattern correlates with consistently high inorganic nitrogen levels, elevated N:P ratios, and nutrient-loading hotspots near major river mouths. These findings demonstrate that diatom dominance is an effective bioindicator of eutrophication. The strong link between diatom blooms and altered nutrient ratios highlights that diatoms serve as “early warning indicators” of ecosystem degradation and imbalance in tropical megacity coasts. Urgent measures such as integrated nutrient management, improved wastewater treatment, and routine phytoplankton monitoring are essential to restore ecological resilience and prevent recurrent harmful algal blooms in Jakarta Bay.