Graduation Year

2006

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Marine Science

Major Professor

Gabriel A. Vargo, Ph.D.

Co-Major Professor

Karen A. Steidinger, Ph.D.

Committee Member

John J. Walsh, Ph.D.

Keywords

Algal Biomass, Chlorophyll a, Pigments, Phytoplankton, Variability

Abstract

The West Florida Shelf (WFS), typically characterized as being oligotrophic, is

one of the most productive continental shelves in the United States. In addition to

supporting a large fishing industry, the WFS also supports high biomass blooms of the

toxic dinoflagellate

Karenia brevis. Because of the large ecological and economic

impacts these blooms have on the area, the ECOHAB: Florida program was developed to

gain a better understanding of red tides and their initiation, maintenance, and dispersal.

This interdisciplinary program consisted of monthly cruises from June 1998 through

December 2001, with a hiatus from January through March of 2001. Hydrography,

nutrients, chlorophyll

a, phaeopigments, and a wide variety of other factors were

measured during the cruises. In this paper chlorophyll

a and phaeopigment

concentration, nutrients, and hydrographic data were examined to explain the temporal

and spatial distribution of chlorophyll on the shelf.

Average surface chlorophyll values were 0.55 mg/m

3 with near bottom values

averaging 0.85 mg/m

3. Chlorophyll was found to be highest near the estuaries of Tampa

Bay and Charlotte Harbor with a decreasing gradient seaward. Near bottom chlorophyll

values were generally two to fourfold greater than surface values. Midshelf stations (35-

50 m) were characterized by high near bottom chlorophyll, whereas the offshore stations

(86-200 m) were characterized by a subsurface chlorophyll maximum ranging between

40 to 80 m deep. Nutrients were generally low across the shelf except for 1998 when a

subsurface intrusion of nutrient rich slope water reached to the 20 m isobath.

Temperatures ranged from 14.00

° C to 31.47° C. Salinity ranged from 30.5 to 37.50 in

the study area.

Four blooms of

Karenia brevis, lasting several months, contributed to the high

chlorophyll concentrations along the inner shelf. Maximum chlorophyll concentrations

of 27.10 mg/m

3 were a result of the October 1999 to March 2000 red tide. Blooms of

Trichodesmium

and diatoms also were contributors to patterns seen on the shelf.

Maximum chlorophyll values were generally highest in the late summer and fall except

for offshore values which showed little to no seasonality. Inshore of the 50 m isobath,

average phaeopigments comprised from 43 to 68 percent of the measured Chl

a, while

offshore values were from 68 to over 100 percent.

Inshore chlorophyll distributions were attributed to riverine and estuarine flux of

nutrients, localized upwelling, and recycling of nutrients aided by salinity and

temperature fronts. Midshelf distributions were attributed to the movement of

biologically important material through the bottom Ekman layer from offshore to the

inshore regions of the shelf. Offshore distributions were attributed to Loop Current

upwelling and synoptic scale processes associated with seasonal meteorological forcing.

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