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

Abstract

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 ∼7 × 106 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 (x̄ = 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-μm and >5-μm 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 (x̄ = 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-μm Chl a size fraction, and comparable to grazing rates on the >5-μm 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. © 2008, by the American Society of Limnology and Oceanography, Inc.