Effects of microalgal exudates and intact cells on subtropical marine zooplankton

Harmful algal blooms (HABs) affect coastal waters worldwide and very often lead to the disruption of seafood harvesting and commercial activities, because of potential hazards to human health associated with the consumption of contaminated mussels, crustaceans and fish. HAB events are frequently caused by outbreaks of toxin-producing dinoflagellates, which are subject to top-down control by zooplankton. The aim of this study was to analyze the effects of dinoflagellate exudates and intact cells on the survivorship and mobility of zooplankton taxa from a subtropical location (Ubatuba, Brazil). Lethal effects were observed in five out of six taxa investigated, three of which (copepod nauplii, tintinnids and gastropod larvae) when exposed to dinoflagellate exudates and two (rotifers and brachyuran zoeae) when exposed to intact cells. In addition, gastropod larvae displayed mobility impairment during exposure to dinoflagellate exudates. Only polychaete larvae were not apparently affected during the course of the experiments. Zooplankton responses usually varied according to the dinoflagellate species tested. For instance, exudates from Alexandrium tamiyavanichii, Gonyaulax sp. and Gymnodinium sp. decreased survivorship of planktonic copepod nauplii but did not affect bottom-dwelling harpacticoid nauplii, which were in turn killed by exudates from Prorocentrum lima, a epibenthic dinoflagellate. These results suggest that HAB events do not cause indiscriminate zooplankton mortality, but may instead generate community shifts and complex cascading effects through the pelagic and benthic food web. Species-specific monitoring of zooplankton responses to HABs is therefore an important step to understand the ecological implications of dinoflagellate outbreaks in coastal waters, and their impact on marine farming activities.