Near bottom dinoflagellate populations on the northwest Florida Shelf
Katy Grabowski1, Daniel Kamykowski1, John M. Morrison2, Anita McCulloch1, Geoff
Sinclair3
North Carolina State University1, University of North Carolina Wilmington2, Louisiana
Universities Marine Consortium3
Harmful Algal Bloom dinoflagellate populations are most commonly sought near the sea
surface in the Gulf of Mexico. Seed populations of harmful dinoflagellates usually exist
18-74 km from the coastline on the shelf in the Gulf. Succession models and computer
models have suggested that dinoflagellates populations could exist near the sediment
interface, separate from surface populations. While the water column is extremely
oligotrophic in the spring and fall, the sediments on the shelf contain higher nutrient
levels, and larger diatoms will grow near bottom in depths where the euphotic zone
reaches the sediments. In deeper stratified columns, as the euphotic zone separates from
the nutrient rich sediments, dinoflagellates potentially have the ability to dominate over
diatoms as they can migrate vertically through the water column between the nutrient rich
sediments to through the base of the euphotic zone higher in the water column. Three
cruises (May 2008, October 2008, and July 2009) occurred on the North West Florida
shelf to investigate the spatial and temporal distributions of these near bottom
dinoflagellate populations. Cross shelf transects between the 20-60m depth contours
were taken on each cruise. Samples focused on the sediment interface were analyzed on
the FlowCAM for dinoflagellate populations. Hydrographic, nutrient, and chlorophyll a
concentration data were also collected in conjunction with near bottom samples. HPLC
samples were also taken to compare to FlowCAM results of the diatom vs. dinoflagellate
contribution to chlorophyll a. Nitrate samples showed low concentration throughout the
water column, with higher concentrations at the sediment interface. Bottom oriented
water samples generally showed that low concentrations of dinoflagellate populations
were found to dominate between 30-50m depth contours, while diatoms dominated
etween 20-30m depth contours. In water columns deeper than 50m both dinoflagellates
and diatoms populations decline as the euphotic zone separates from the sediment
interface by more than a 12hr. swimming speed ascent.