More Information about Phytoplankton:    

 Our measure of phytoplankton biomass in the water column is based on chlorophyll-a concentrations.  Other phyto-plankton pigments were also examined using HPLC analyses, but they will not be summarized in this report since further analyses are required before the pigments can be accurately classified as to the type of phytoplankton.

 The average chlorophyll-a concentration in creek habitats was 12.8 mg/L compared to an average of 9.7 mg/L at the open water sites.  This difference was statistically significant (p < 0.02).  Additionally, a cumulative distribution function  (CDF) analysis indicated that approximately 13% of the state’s tidal creek habitat had > 20 mg/L of chlorophyll-a, which is considered to be elevated by Bricker et al. (1999).  In comparison, only 3% of the open water habitat had elevated chlorophyll-a concentrations.  The higher chlorophyll concentrations in tidal creeks may be reflective of the higher nutrient concentrations observed in the creeks.  It may also reflect possible re-suspension of benthic algae from the creek bottoms and nearby marsh surfaces. 

In order to evaluate whether nutrient concentrations are influencing the chlorophyll-a concentrations observed, several regression and correlation analyses were conducted.  Based on the two-year data set, there were no clear relationships between either TN or TP concentrations and the chlorophyll-a concentrations observed in either the creek or open water habitats, or when both habitats were considered together (r² < 0.1).  Comparison of TDN (measured in 2000 only) did show a stronger positive correlation in creek habitats (r² = 0.6), but not in open water areas (r² < 0.01). Neither TP or TDP showed any relationship with chlorophyll-a concentrations in creek or open water habitats, or when both habitats were considered together (r² < 0.01).  The relationship between nutrient concentrations and measures of phytoplankton biomass may become clearer with a larger data set that can be partitioned by tidal stage and time of day.  While all of the nutrient and chlorophyll-a samples collected in 2000 were taken simultaneously, this was not always the case in 1999 and chlorophyll-a concentrations are known to vary by both tide stage and time during daylight hours (Day et al., 1989).   

 Additional chlorophyll-a data collected through this study and through the Harmful Algal Bloom program currently being conducted in South Carolina will provide a much better understanding of what chlorophyll-a concentrations represent “eutrophic” conditions in South Carolina.  The NOAA study conducted by Bricker et al. (1999) included very little data from either South Carolina or Georgia, which represent two states with very different tidal amplitudes compared to other southeastern states.  Until further data are available, we have not incorporated the phytoplankton data in our overall measure of biotic condition for this report, but plan to do so in the future.