Near bottom dinoflagellate populations on the northwest Florida Shelf.
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2010-04-21
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GRABOWSKI, KATHRYN EVE. Near bottom dinoflagellate populations on the northwest Florida Shelf. (Under the direction of Dr. Daniel Kamykowski)
The toxic dinoflagellate Karenia brevis is most commonly sought near the sea surface 18-74 km from shore due to the historical occurrence of bloom events. Hydrographic conditions on the west Florida shelf alternate between two seasonal conditions. From November through April, the water column generally is well-mixed to the bottom with an offshore gradient of colder, less saline inshore temperatures to warmer, more saline offshore temperatures. From May through October, vertical stratification exists across the shelf. Phytoplankton succession models suggest that diatoms may be favored near-bottom at least early in the succession sequence where the euphotic zone reaches the sediments, while dinoflagellates, using vertical migration to transit between the nutrient-rich sediments to the base of the euphotic zone, may be favored near-bottom as the sediments descend below the euphotic zone. Field and laboratory observations as well as computer models focused on Karenia brevis suggest that seed populations for coastal blooms under upwelling favorable conditions may exist near the sediment interface after water column stratification sets in and the sediments contain higher nutrient levels than the water column. Cruises during July 2009 and October 2008 provide a seasonal sequence of the cross-shelf expression of the summer/fall condition on the northwest Florida shelf. Transects between the 20-70m depth contours were sampled for hydrographic character using ACROBAT and CTD
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surveys and for nutrients, pigments and phytoplankton composition using CTD/rosette water collections. Water samples collected from selected depths including near the sediment interface were analyzed on the FlowCAM for phytoplankton community composition. During July 2009 cross-shelf, a pycnocline existed between 10-20 m depth, the 1% light level reached to about 45 m depth, nitrate-nitrite concentrations started increasing about 10 m above the sediment interface out to 50 m depth and then increased below 40m depth across the rest of the outer shelf, and a chlorophyll a maximum occurred between 20-30 m depth. Near-bottom, dinoflagellates were 2-8 times more abundant than diatoms everywhere on the shelf, especially between 35-55m, except at one 20 m station. Time series samples, collected along the 50 m contour following a drogue with a holey sock extending between 28-40 m, suggest trends of a dinoflagellate distribution pattern suggestive of diel vertical migration. Dinoflagellates clearly dominated the near-bottom water over diatoms across the shelf probably due to late succession influences like selective grazing on diatoms and microphytobenthos activity inshore as well as increased hydrographic influences. Dinoflagellates found near bottom also have the migration capability to access vertically separated light and nutrient resources offshore. The October 2008 cruise results provide insight into changes in physical, chemical and biological changes and phytoplankton succession patterns associated with the transition from horizontal stratification to well-mixed water columns that form a cross-shelf hydrographic gradient. A wind event between successive transects during the October cruise showed how disturbance events can influence near bottom phytoplankton distributions. Before the wind event the distribution of near bottom phytoplankton was highly structured and after the distribution of near bottom
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phytoplankton was more evenly distributed along the bottom. This wind event served as the transition between seasonal succession cycles of near bottom phytoplankton populations. Such near-bottom populations may serve as seed possible near-shore dinoflagellate blooms, responding to upwelling favorable winds and behavioral accumulation at coastal fronts. The October 2008 cruise results provide insight into changes in physical, chemical and biological changes and phytoplankton succession patterns associated with the seasonal transition from horizontal stratification in the summertime to well-mixed water columns that form a cross-shelf hydrographic gradient in the wintertime, promoting succession changes.
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Keywords
phytoplankton, Gulf of Mexico, Biological Oceanography
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Degree
MS
Discipline
Marine, Earth and Atmospheric Sciences
