Nutrient surplus and higher temperatures promote Gonyaulax polygramma blooms during the downwelling period in a semi-enclosed bay, in eastern Indonesia: a report on associated blooms and fish mortality

Some dinoflagellate species are known to produce toxins, and their high abundances are commonly associated with water discoloration and hypoxia. Blooms of dinoflagellate Gonyaulax polygramma are known to attribute adverse effects on marine environment as bloom decays reduce dissolved oxygen levels and lead to fish mortalities. This present study conveys a first report on G. polygramma dynamics and blooms in relation with environmental drivers, as well as related fish kill events in Ambon Bay, an Indonesian semi-enclosed bay. Cell abundances, dissolved nutrient concentrations and water physical properties were observed on a biweekly campaign between 2018 and 2021. Cells of G. polygramma were persistently found in plankton samples, and they flourished during warm months in the upwelling-relaxation and downwelling period (encompassing second transition period and the dry season, between September and February), resulting in surface water discolorations and fish kills. Persistent anthropogenic discharges during these periods maintain essential nutrient surplus for fuelling G. polygramma blooms. In addition, high water temperature and salinity levels were found to correspond to rapid growth of G. polygramma cells and subsequently formed blooms. This study crucially reveals the important link between anthropogenic activities and algal blooms in Ambon Bay during the absence of upwelling period, which would be helpful in mitigating environmental problems that lead to economic losses.