Intrinsic growth and microzooplankton grazing on toxigenic Pseudo-nitzschia spp. diatoms from the coastal northeast Pacific

We investigated the population ecology of toxigenic diatoms within the genus Pseudo-nitzschia on the Pacific Northwest coast during 2003, 2004, and 2005. Pseudo-nitzschia spp. were widespread and abundant across the region, and the maximum density reached at our experimental stations was similar to 7 x 10(6) cells L-1. Pseudo-nitzschia spp. biomass did not correlate with total phytoplankton biomass, indicating that the growth response and/or mortality rate of Pseudo-nitzschia spp. were dissimilar to other phytoplankton. In dilution experiments across wide-ranging ocean conditions, Pseudo-nitzschia spp. intrinsic growth rates were moderate to high ((x) over bar = 0.97 d(-1), range 0.24 d(-1) to 2.30 d(-1), n = 36), and they were consistently higher than the corresponding growth rates of the aggregate <5-mu m and >5-mu m chlorophyll a (Chl a) communities. Pseudo-nitzschia spp. growth was predicted by irradiance and temperature but not ambient nitrate concentration, whereas both Chl a size fractions showed dependence upon nitrate for growth. Microzooplankton grazing rates on Pseudo-nitzschia spp. were moderate ((x) over bar = 0.32 d(-1), range 0.00 d(-1) to 1.70 d(-1), n = 36); they were nearly always lower than corresponding Pseudo-nitzschia spp. intrinsic growth, significantly lower than the grazing rates on the <5-mu m Chl a size fraction, and comparable to grazing rates on the >5-mu m Chl a size fraction. Our results show that the strong competitive fitness of Pseudo-nitzschia spp. results more from expression of characteristics that enable sustained high growth during variable and unfavorable conditions than from intrinsic adaptations that reduce grazing mortality.