A study of nutrients, underwater light dynamics, and their correlation with phytoplankton biomass was conducted in the tropical estuary of Jakarta Bay, Indonesia, in the dry season during the period from 2001 to 2019. This study analyzed the spatial and temporal dynamics of phytoplankton biomass and its correlation with phytoplankton biomass. There was significant increase in nutrient concentration in Jakarta Bay, with annual means of 27.97 µM dissolved inorganic nitrogen (DIN) and 11.31 µM phosphates in 2001, increasing to 88.99 µM DIN and 25.92 µM phosphates in 2019. Increased mean nutrient concentrations were accompanied by increased mean phytoplankton biomass, from 15.81 µg Chl-a L−1 in 2001 to 21.31 µg Chl-a L−1 in 2019. The eutrophication status of Jakarta Bay waters was calculated using the Tropical Index for Marine Systems eutrophication index, which showed increased areas of hyper-eutrophic and eutrophic zones, while the mesotrophic area decreased. The hyper-eutrophic zone dominated the areas around river mouths and the inner part of the bay, while eutrophic status was observed in the middle part of the bay and mesotrophic status was found in the outer part of the bay. The area of hyper-eutrophic water increased 1.5-fold, from 75.1 km2 in 2001 to 114.0 km2 in 2019. Increasing eutrophication of the bay has had negative ecological consequences including algal blooms, hypoxic conditions, and mass mortality of marine biota, and it urgently requires remediation.

Algae bloom is one of the symptoms of eutrophication process in an estuary, causing in some ecological and societal problems for the aquatic environments and human activities. Algae blooms observation was conducted in the dry season period between 2014 to 2016, in an eutrophic embayment of Jakarta Bay, Indonesia. Observations were made by conducting transect-boat survey and plankton sampling once the bloom was observed. Samples of chlorophyll-a and net plankton were taken and measured in the laboratory for phytoplankton biomass analysis, cell counting, and identification of bloom-forming species. In the 3 consecutive years of observation, the most frequent bloom-forming species observed were Skeletonema costatum, Pseudonitzschia Spp., and Noctiluca scintillans, The most frequent location of the blooms observed was in the inner part of the bay, in the area in front of Marunda, Ancol and Angke. Phytoplankton biomass during the bloom events were ranged between 126.3 to 232.3 μg Chl-a L−1, with the highest was in 2015 of during the Skeletonema costatum bloom. Blooms were also observed but in low frequency and low biomass at the middle part of the bay off Marunda coastline area. Cell phytoplankton counting were ranged from 1.139 ϗ 106 cell L−1 to 7.893 ϗ 106 cell L−1. During period of surveys, there was no mass mortality of fishes were detected.

Coastal eutrophication is defined as the increase of rate of organic supply into an estuary. In the recent global discussion, eutrophication is seen as a negative situation, in fact, it has positive impacts such as increase in primary production which lead to the increase of fisheries production and hence, community income. This paper promotes a simple diagram called eutrophication-benefit curve which correlates between levels of eutrophication and its benefits to both ecological and economical aspects. In the beginning of eutrophication process, environment gets benefit from the increase of the nutritional state of the water which leads to the increase of primary production and fisheries production. This is obvious in the comparison between northern and southern coast waters of Java Island, showing a higher primary production and phytoplankton biomass and hence fisheries production in the northern coast waters of Java compared to those of the southern coast waters of Java. However, the rapid increase of eutrophication level of the northern coast waters of Java Island causes eutrophication negative effects such as oxygen depletion, algae bloom and disturbance of coral reef ecosystems. This paper shows the optimum level of eutrophication level that can give benefit to both ecological and social environment.