Physical processes impacting Harmful Algal Blooms (HABs) in Lampung Bay, Indonesia​

Harmful algal blooms (HABs) have been considered as a worldwide issue as its occurrence devastates aquaculture industry through mass mortality of cultured fish in captivity. In addition, HABs also have the potential to harm human consuming marine products contaminated with toxins produced by harmful algae.  In Indonesia, HABs events had been recorded in twelve marine areas of its archipelagos where member of dinoflagelata, for example Cochlodinium sp, Alexandrium sp, Dinophysis sp, Gymnodinium sp and Ceratium sp were reported as the dominant causative of blooming. The long-term trend in the emergence and severity of these HABs is believed to be driven primarily by the anthropic pressure resulting in an increase of nutrient input, providing a biogeochemical “climate’ favoring these HABs. However, HABs occur over short period of time and the prevision of their occurrence is extremely dificult. Physical processes influencing vertical/horizontal transport, water temperature and salinity and access to light are a key factor in this high frequency dynamics. This study aims to analyse physical processing impacting Harmful Algae Blooms (HABs) in Lampung Bay, Indonesia, particularly in Hurun Bay where the HABs frequently occur.  Focus is notably made on turbulent water mixing which has a crucial impact on cysts resuspension, nutrients fluxes and access to light. With this aim,  the Palhadin/Detoxiphy project (CNRS, IRD, IFI)  was  built on data of phytoplankton community, concentration of nutrients, physical oceanography process and meteorology of study site. Integrated data collections were conducted by weekly phytoplankton and nutrient concentrations sampling; monthly spatial sampling of nutrients and hydrology in the full Hurun bay and high frequency measurements of physical processes from moored sensors at various depths and meteorological sensors. In addition, the project also relies on numerical modeling covering a wider area for better understanding water circulation in Lampung Bay, notably through Lagrangian analyses of water surface water circulation. Preliminary results for 2023, show that the El Niño event throughout 2023 resulted in low rates of rainfall and a more extended dry season resulted in low nutrients input from landmass to the study site and eventually caused only weak HABs that year. In addition, maximum concentrations of chlorophyll-a were observed during the transitional between the wet and dry season as well as during the dry season. Physical analysis show that the bottom layer is extremely dynamic, with intrusion of cold saline water resulting in strong stratification that interacts with tides. This bottom dynamic has a strong impact on water mixing.