Phytoplankton dynamics within a discrete water mass off Cape Hatteras, North Carolina: the Lagrangian experiment

As part of the Department of Energy Ocean Margins Program, we examined the spatial and temporal variability in primary production and phytoplankton pigments during two cruises to the shelf waters between the Chesapeake Bay and Cape Hatteras. The first cruise was conducted in March 1996, reflecting well-mixed conditions just prior to the spring transition, while the second cruise was conducted during July 1996 when the water column was more stratified. During each cruise, primary production and high performance liquid chromatography (HPLC) pigments for the whole community and <8-μm size fraction were determined for several successive days within a discrete water mass by following a near-surface tracking drogue. ch, production ranged from 0.50 to 0.65 g C m−2 d−1, with 52–62% of the production attributed to the smaller size fraction. About 50% of the total chlorophyll a (chl a) was found in the >8-μm size fraction. Pigment composition was dominated by chlorophylls a, c1 and c2, and fucoxanthin, indicating the importance of diatoms. In July, production was more variable, ranging from 0.38 to 2.09 g C m−2 d−1, with 41–83% of production attributed to the <8-μm size fraction. Rates increased over the 4-day study. Most of the chl a was found in the <8-μm size fraction. The phytoplankton pigments were dominated by chl a and fucoxanthin with chlorophylls c1 and c2, again indicating the importance of diatoms during this cruise.CHEMTAX (Mackey et al. CHEMTAX Userʹs Manual: a program for estimating class abundances from chemical markers—application to HPLC measurements of phytoplankton pigments. CSIRO Marine Laboratories, Report 229, Hobart, 42 pp.), a factor analysis computer program, was used to examine phytoplankton community structure using marker pigments from our HPLC analyses to determine the relative importance of various taxa. In March, diatoms dominated whole water samples, with consistent contributions from dinoflagellates and cryptophytes. The <8-μm fraction was dominated by small diatoms, chrysophytes, cryptophytes and dinoflagellates. In July, diatoms were still present and important, but prymnesiophytes, cryptophytes and cyanobacteria contributed in both size classes. Correlation analyses indicated that primary production was positively correlated with light and temperature. Chl a biomass was positively correlated with the concentrations of NO2+NO3 and negatively correlated with temperature. These correlations support the observation that temperature played a major role in the phytoplankton dynamics in this shelf ecosystem.