Proceedings of the 2019 Mississippi Water Resources Conference

Year: 2019 Authors: Boyette A.D., Cruz V.J., Graham W.M.

Improved understanding of microplankton (< 200 µm) community dynamics and trophic connectivity between primary producers and heterotrophic protists is central to plankton ecology and water quality studies. Despite their ecological significance in structuring aquatic ecosystems, there is limited knowledge on phytoplankton-microzooplankton trophic interactions in the northern Gulf of Mexico (nGOM). Here we describe the microplankton community structure using a morphological based functional group (MBFG) approach to classify microplankton images obtained from imaging in-flow technology (FlowCAM®), primary production (photosynthesis-irradiance curves, P-E), and phytoplankton apparent growth and mortality (dilution experiments) within inner shelf surface waters of the nGOM during two seasons (fall, spring). Additionally, we evaluated the importance of protist grazers, particularly ciliates, as predators on phytoplankton. Phytoplankton biomass was dominated by diatoms in both seasons, with average chlorophyll a (3.8 mg m^-3) more than twice that of fall (1.7 mg m^-3). Needle-like (Pseudo-nitzschia sp.) and cells with setae (Chaetoceros sp.) were the predominant diatoms, whereas small (<20 µm) cryptophytes, prymnesiophytes, and Heterocapsa sp. dinoflagellates comprised more than 80% of the flagellate community in both seasons. Aloricate choreotrichid ciliates (Strombidium sp., Strombilidium sp.) were the primary microzooplankton grazers, though >50% of the ciliates were mixotrophic the Mesodinium rubrum. Phytoplankton potential production (0.01 x 0.38 gC m^-3d^-1) and apparent growth (0.01 x 2.53 d^-1) were greater in spring, although highest maximum photosynthetic rates (PBmax = 34.07 g C g Chl^-1 h^-1) were measured in fall. Microzooplankton consumed ~ 40% phytoplankton biomass and > 65% of the daily primary production. The ratio of microzooplankton grazing to phytoplankton growth (m:µ) averaged 1.14, suggesting that microzooplankton grazing is an important top-down control on phytoplankton biomass in this system.