On the population ecology of the toxigenic marine diatom genus Pseudo-nitzschia: Perspectives from the growth and mortality environments

Diatoms in the toxigenic genus Pseudo-nitzschia are ubiquitous in the ocean. Blooms of Pseudo-nitzschia spp. have been seen in coastal and open-ocean ecosystems when other diatoms are restricted. Pseudo-nitzschia's ability to proliferate across such wide-ranging ecosystems suggests that through natural selection intrinsic properties are expressed in these toxic diatoms making them a highly competitive phytoplankter in the world's oceans. Here, the population ecology of Pseudo-nitzschia spp. was studied under multiple agents of selection operating from both the growth and mortality environment in the coastal Pacific Northwest. The goal of this thesis was to understand whether growth advantages or mortality reduction contributes most to Pseudo-nitzschia spp. abundance, distribution and persistence. I present data showing the results of 36 Pseudo-nitzschia-specific growth and microzooplankton grazing experiments. These data showed consistently moderate to high growth rates for Pseudo-nitzschia spp., similar to phytoplankton < 5 mum, across wide-ranging environmental conditions. Simultaneously, Pseudo-nitzschia spp. microzooplankton grazing mortality rates, while variable, were always lower than Pseudo-nitzschia spp. growth. Pseudo-nitzschia spp. grazing rates were statistically similar to grazing rates on phytoplankton > 5 mum. In field and laboratory experiments the Pseudo-nitzschia-produced toxin, domoic acid, does not appear to affect microzooplankton grazers in the intracellular or dissolved phase. Laboratory experiments showed that grazing on toxigenic Pseudo-nitzschia spp. was species-specific. When the protist, Protoperidinium pellucidium, was offered both the non-toxic Pseudo-nitzschia pungens and the toxic Pseudo-nitzschia multiseries, it ingested and grew only on the toxic P.multiseries. Protoperidinium excentricum did not ingest either Pseudo-nitzschia spp. Pseudo-nitzschia spp. also appeared to escape intense grazing by the dominant calanoid copepod species from the study region. This was confirmed using electivity indices. Pseudo-nitzschia spp. in natural assemblages were either neutrally or negatively selected for by copepods, and this selection was not related to particulate domoic acid concentration. Copepods grazed heavily on microzooplankton, potentially further liberating Pseudo-nitzschia spp. from microzooplankton grazing. Given that Pseudo-nitzschia spp. growth rates were comparable to, and often higher than that of small phytoplankton, but grazing mortality was similar to large phytoplankton, I conclude that Pseudo-nitzschia spp. possess uniquely combined adaptive strategies, consistent with a continuum of both r- and K-selection.