Variations in phytoplankton pigments, size structure and community composition related to wind forcing and water mass properties on the North Carolina inner shelf

Abstract

Variations in phytoplankton pigments, size structure and community composition were examined in inner shelf waters off North Carolina in relationship to water mass properties subject to the influence of low salinity outflow from the Chesapeake Bay, alternations in wind forcing and interactions with shelf water masses. Observations were made aboard the R/V Edwin Link during May 1997. Episodes of upwelling-favorable conditions were accompanied by detachment of the low salinity outflow plume from the coast, and enhanced dispersion and mixing with shelf waters. Distinct water masses were identified using cluster analysis of temperature–salinity ( T–S) properties. Two major clusters, distinguished on the basis of relatively high and low salinities, were identified as ambient coastal water (A) and modified bay water (B). Each of these could be further separated into two water mass types characterized by relatively high and low temperatures. The pigment and taxonomic composition of these water masses were examined. The carotenoid fucoxanthin was generally the most abundant accessory pigment. Zeaxanthin and chlorophyll b were also relatively abundant at most stations. Ratios to chlorophyll a of 19′-hexanoyloxyfucoxanthin and peridinin were higher for the high salinity clusters. A shift to downwelling-favorable conditions toward the latter part of the cruise gave rise to a narrow, southward flowing jet of low salinity Chesapeake Bay water corresponding to the low salinity, relatively high temperature water mass identified by cluster analysis. Fucoxanthin dominated the accessory pigments in this low salinity feature. Proportions of chlorophyll a associated with different phytoplankton classes, as estimated using CHEMTAX software (Mar. Ecol. Prog. Ser. 144 (1996) 265), varied among groups in a manner consistent with pigment composition. A diverse assemblage of diatoms, cyanobacteria, cryptophytes and prasinophytes accounted for the majority of chlorophyll a at most stations. Haptophytes and dinoflagellates were relatively more abundant in warmer, higher salinity stations. The low salinity jet exhibited high biomass and low diversity compared to other water masses. Its phytoplankton population was dominated by diatoms larger than 8 μm in diameter. The results demonstrate that phytoplankton community composition varies substantially among the different water masses in this coastal region and is subject to event scale (i.e., days to weeks) changes in relation to wind forcing and shelf circulation processes.