Abstract
Optical characterization of particle size distributions, particle volumes, and particle number concentrations was conducted and compared to phytoplankton size classes as inferred from phytoplankton pigment composition in the region of the northern Gulf of Mexico (nGOM) influenced by the Mississippi and Atchafalaya River systems. Using a Laser In-Situ Scattering and Transmissometer (LISST 100X), we acquired vertical profiles of particle concentration and size from nearshore estuarine regions to open ocean waters. Profiles of spectral beam attenuation using an underwater absorption and attenuation spectrometer (AC-9) were acquired alongside the LISST measurements. These optical observations were compared to phytoplankton size classes as inferred from phytoplankton pigment composition. Strong spatial gradients in particle volume and number concentration were observed in conjunction with variations in the magnitude of beam attenuation, with higher values in the estuary and inner shelf waters. The observed relationship between the spectral slope of the beam attenuation, γ, corrected for colored dissolved organic matter (CDOM) absorption, and the slope of the LISST-derived particle size distribution, ξ, followed general theoretical expectations, although there were notable deviations, particularly at estuary and inner shelf stations. The near-surface particle number and volume concentrations were examined in relation to water mass type as well as in relation to phytoplankton composition and size class as inferred from high-pressure liquid chromatography (HPLC) pigment analyses. Near-surface particle volume and number concentration were highest in estuary waters as compared to outer shelf waters. The median fraction of total chlorophyll associated with the microphytoplankton size fraction (20 – 200 μm) was lower for mid-shelf stations compared to estuary and inner shelf stations. The median total volume fraction of larger particles in the size range (20 – 200 μm) for inner and mid-shelf stations was significantly lower than for estuary stations. Outer shelf stations demonstrated relatively large variability in chlorophyll fractions associated with the different size classes. Subsurface peaks were evident in vertical profiles of particle volume and number concentration and coincided with distinct in phytoplankton pigment composition, providing insight about vertical variations in community composition in relation to physical gradients. These findings provide novel information about particle characteristics and their relationship to phytoplankton composition in the nGOM.