Seasonal, spatial, and niche-driven patterns of species succession in Gulf of Maine communities of the toxic marine diatom Pseudo-nitzschia

Background/Question/Methods

Species in the toxic marine diatom genus Pseudo-nitzschia are commonly observed in the Gulf of Maine (GOM). A few GOM species produce high levels of the neurotoxin domoic acid (DA) in culture, however little is known about which environmental processes influence Pseudo-nitzschia species assembly and growth in the GOM. Toxic and non-toxic Pseudo-nitzschia species share common morphological characteristics, and molecular approaches and/or electron microscopy are required for definitive identification. Here, a rapid and high-throughput DNA fingerprinting method, automated ribosomal intergenic spacer analysis (ARISA), was combined with environmental sequencing to quantitatively describe Pseudo-nitzschia species distributions in the Gulf of Maine. Field samples for characterization of Pseudo-nitzschia species diversity were collected during monthly large-scale hydrographic surveys from May to August in 2008 and 2010. Beta diversity was estimated using the Bray-Curtis similarity index. Nutrient and chlorophyll a concentrations were discretely sampled, and Spearman’s correlation analysis and canonical correspondence analysis were used to correlate environmental variables to observed Pseudo-nitzschia community structure and composition.

Results/Conclusions

Pseudo-nitzschia species were detected on all cruises, in each of 209 environmental samples analyzed to date. A total of eight Pseudo-nitzschia species were identified, and three additional unidentified ARISA fragments were detected during both years. Nearshore GOM communities were primarily dominated by the nontoxic species P. delicatissima, although the toxic species P. seriata was sometimes detected in Massachusetts coastal waters. Approximately 100 km offshore at Georges Bank, more spatially and temporally diverse assemblages were observed. Transitions in species composition occurred seasonally at Georges Bank during both years, and between hydrographically distinct water masses during the same cruise. More than six species demonstrated positive or negative associations with salinity and/or temperature (p<0.05 for Spearman’s R). The toxic species P. seriata dominated May and June communities in waters ranging from 5-10°C, at salinities from 32-33. At higher temperatures (>15°C) and salinities (32-34) observed later in the season, a separate toxic species within the P. pseudodelicatissima species-complex dominated Pseudo-nitzschia communities and more diverse species assemblages were observed at Georges Bank. A majority of species were observed in a temperature window from 10°C to 15°C, and a few species were only detected at ~20°C. We are in the process of analyzing additional surface and subsurface samples to test the hypothesis that temperature/salinity preferences of Pseudo-nitzschia species reflect the different source waters mixing in the temperate Gulf of Maine region.