The impact of predator-prey relationships on the formation of harmful algal blooms in Heterosigma akashiwo

Heterotrophic protists are the primary grazers of phytoplankton in the marine environment, and changes in the rates of consumption will influence phytoplankton abundance, community composition, and ecological function. The objective of this research was to investigate the influence of predators on the formation and promotion of harmful algal blooms (HABs), specifically blooms of the toxic raphidophyte alga, Heterosigma akashiwo. To observe microscopic predator-prey interactions and their macroscopic distributions, video and image analysis were used to simultaneously quantify population distributions and individual 3D movements of both protistan predators and prey cells in laboratory tanks with realistic salinity structures. In behavioral experiments, the ciliate predator, Favella ehrenbergii did not avoid a layer of H. akashiwo, leading to high ciliate mortality (Harvey and Menden-Deuer, 2011). Contrary to our hypotheses, the presence of Favella sp. caused H. akashiwo to significantly alter its swimming behavior, resulting in avoidance of the predator (Harvey and Menden-Deuer, 2012). This is the first report of predator-induced shifts in the population distribution of a phytoplankton species. Chemical cues were shown to be important in driving predator-prey behavioral shifts. Both predators and prey were shown to shift their movement behavior in response to the presence of only chemical cues, however there was a high level of species-specific responses of both predator and prey (Harvey et al. accepted). Additionally, predator-induced avoidance behaviors in H. akashiwo were also observed in these experiments. The results presented here suggests that H. akashiwo has multiple mechanisms, including toxicity, predator-induced avoidance behaviors, and the use of chemical cues that will result in a reduction in grazing pressure, leading to increased survival and potential for accumulation in this HAB alga. These findings highlight the importance of quantifying the mechanistic basis of organism interactions in addition to bulk growth/loss rates in understanding phytoplankton population dynamics and ultimately the fate of material and energy in the marine food web