Graduation Year


Document Type




Degree Granting Department

Marine Science

Major Professor

Gabriel A. Vargo, Ph.D.


West Florida shelf, Redfield ratio, Nutrient limitation, Phytoplankton, Karenia brevis


The southwestern Florida shelf marine environment has often been characterized as oligotrophic, yet these waters can support large, high biomass, persistent phytoplankton blooms, including blooms of the toxin producing dinoflagellate Karenia brevis. Little is known regarding which major nutrient potentially limits primary production in these waters as both inorganic nitrogen and phosphorus concentrations are often near the limits of analytical detection and it is difficult to estimate what percentage of the dissolved organic pool is available for phytoplankton uptake. To assess the nutrient status of phytoplankton populations on the southwest Florida shelf, this project examines the particulate nutrient stoichiometry of ambient phytoplankton assemblages from 1998-2000 as part of the ECOHAB: Florida Program. Particulate C, N, P concentrations and particulate ratios display a large range of values across the West Florida Shelf (WFS).

The average particulate stoichiometry is well above the classic Redfield ratio with a geometric mean of 410C:56N:1P. Frequency percentages of particulate ratio values to total sample number binned according to potential nutrient limitation indicate that 39% (C:N) of the data have values suggesting N limitation and that from 88% (N:P) to 95% (C:P) of the data have values which suggest P-limitation. It is difficult to discern whether phytoplankton biomass is truly P-limited as related to the nutrient regime on the WFS or whether detrital contributions, which can potentially be large on this shallow shelf, are skewing the N:P and C:P ratios towards higher values. Errors which could potentially be related to the different methodologies of determining C, N and P concentrations must also be considered when interpreting the particulate nutrient ratios. The data were also analyzed as subsets to determine near-shore to offshore, latitudinal, seasonal, inter-annual and K. brevis bloom versus non-bloom trends.

The near-shore to offshore transect indicates decreasing concentrations of particulate C, N, P concentrations and increasing C:N, N:P, C:P ratios with increasing distance offshore. Particulate nutrient concentrations and particulate ratio values are very similar between the Tampa Bay, Sarasota and Fort Meyers transects indicating that these latitudes are not spatially distinct with regards to these variables. There does not appear to be any relationship between the particulate C, N, P concentrations or C:N, N:P, C:P ratios and rainfall as indicated by Spearman Ranking Correlation coefficients. However, there does appear to be monthly trends across the shelf where peak particulate nutrient concentrations and particulate ratio values occur during the spring, summer and fall. The average particulate nutrient concentrations and ratios differ for each year as well as each K. brevis bloom which occurred during the study period.

In summary, the particulate C, N, P concentrations and particulate nutrient ratios vary both spatially and temporally on the WFS and are potentially related to the flexibility of phytoplankton uptake kinetics in response to the varying nutrient regimes of the WFS.