Dinoflagellate planktonic-motile-stage and benthic-cyst assemblages from a monsoon-influenced tropical harbour: Elucidating the role of environmental conditions

Dinoflagellates are referred among potential bioinvaders and environmental proxies. Information from tropical regions on the coupling of cyst and motile stages of dinoflagellates characterized by high suspended-load, and high diversity but low abundance is limited. Here, the population dynamics of dinoflagellates (planktonic-motile-stages and benthic-cysts) from monsoon-influenced Cochin-harbour, southwest coast of India was investigated. A targeted sampling strategy is proposed over conventional methods to obtain better estimates of abundance and diversity. From the water column, the cyst-producing-dinoflagellates contribution was quantified for the first time, and it was up to 52% of total planktonic-motile-stages. Interestingly, the number of motile-stages versus the number of cysts for heterotrophs revealed an inverse trend and the explanation is not straightforward. This is because Cochin-harbour experiences discharges, i.e., freshwater (peak during monsoon), anthropogenic, and dredging activities throughout the year facilitating benthic-suspension, high sedimentation rate, and distinct seasonality in the availability of prey. In such an environment, motile-stages-diversity and cysts-species-richness formed potential environmental proxies. The presence of cysts of indicator taxa (Protoperidinium, Polykrikos, Lingulodinium) for nutrient-enrichment in relatively good proportions and low ranges of motile-stages diversity and cysts species-richness do signal eutrophic system. Autotrophic-cysts dominance illustrates that the heterotrophs need not be dominant in the eutrophic-system as reported in earlier studies. The dominance of euryhaline dinoflagellates (Gonyaulax spinifera, Protoperidinium pentagonum, P. leonis, Pyrophacus steinii) cysts provided additional information on cyst-producing-dinoflagellates as potential environmental indicators. The existence of 24 morphotypes belonging to potentially harmful-algal-bloom (HAB) species emphasizes HAB risks under favorable conditions.