First report on an unusual bloom of the potentially toxic epibenthic dinoflagellate Prorocentrum rhathymum from Bangaram Lagoon of the Lakshadweep archipelago: Arabian Sea

Abstract

Benthic dinoflagellates contribute significantly to the primary production and thereby the sustenance of shallow marine environments. However, some of them are considered potentially toxic or harmful species posing a serious threat to this marine ecosystem. The paper describes an extensive bloom of the dinoflagellate Prorocentrum rhathymum for the first time from the Lakshadweep waters in the eastern Arabian Sea. The dinoflagellates were observed as an intense bloom in the Bangaram Lagoon of Lakshadweep archipelago. The cells of P. rhathymum (7.6 x105 cells L−1) were observed in mucilaginous aggregations with macroalgal debris and filaments of the cyanobacterium, Trichodesmium erythraeum. The bloom area was devoid of other microalgal species, and P. rhathymum was also observed to be attached to fresh macroalgal thalli in the nearshore areas (4.67 x104cells g−1 wet weight of macroalgae). These observations suggest that the debris of macroalgae washed into the water column act as a substrate for the transport of P. rhathymum towards the water column. Warm sea surface temperature (SST, 29.7 °C) and stable conditions of the water column favoured the bloom of P. rhathymum in the Bangaram Lagoon during the spring inter-monsoon season. Coral reef ecosystems along the Indian EEZ are the least surveyed regarding the prevalence of harmful or toxic species, and most of the harmful algal bloom (HAB) events in these systems remain overlooked. Routine monitoring and meticulous bloom physiology studies can provide insights into the prevalence of HAB and prediction of such events in these diverse ecosystems.