Phytoplankton community distribution and light absorption properties in the northern Gulf of Mexico

Sumit Chakraborty, University of Southern Mississippi


The theme of this dissertation was to understand the spatio-temporal dynamics of the phytoplankton community, its light absorption properties, and its relationship to underlying physicochemical processes. Understanding these phenomena will benefit efforts to predict pathways of carbon transformation in the ocean, to estimate primary productivity (PP) and to characterize distributions of phytoplankton communities using ocean color remote sensing. This research entailed four different studies, which address different objectives. The first two studies dealt with phytoplankton community composition and its relationship to environmental variables. A chemotaxonomic approach was used, which was successful in revealing distinct phytoplankton assemblages in distinct water mass regimes. In the second study, a multivariate statistical analysis was used to examine community responses to seasonal variability in relation to different meteorological and environmental forcing. Clear differences in phytoplankton communities existed between stratified and non-stratified periods. Understanding the variability in the dominant light absorption constituents in the continental margin of northern Gulf of Mexico was the subject of the fourth study. Absorption budgets for the region revealed dominance of colored dissolved organic matter (CDOM). The presence of large contributions from the CDOM and non-algal particles (NAP) can lead to over- or underestimation of chlorophyll- a specific phytoplankton absorption ( a * [varphi] ) and such errors may then propagate to errors in PP estimates using current ocean color algorithms. Additionally, variations in the optical characteristics of phytoplankton also influence PP. Thus, the fourth study focused on describing the main sources of bio-optical variability affecting the spectral signatures of phytoplankton absorptions in the region. Results showed that changes in cell size, pigment composition and photoacclimation strongly affected phytoplankton spectral absorption. Values of a * [varphi] were largely influenced by the pigment package effect and cell size followed by pigment composition. The results from this study will benefit efforts to examine the critical role of phytoplankton in biogeochemical cycles of the northern Gulf of Mexico.