Epiphytic microbial biomass (as chlorophyll a ) was measured monthly in North Inlet Estuary, South Carolina, for 16 months on spatially distinct stem sections (bottom and middle) of dead and living Spartina alterniflora growth forms (tall, medium, and short) exposed at low tide. The highest biomass was located on the bottom section of tall plants, presumably due to their relatively longer contact with creek water and associated phytoplankton, and their closer proximity to marsh sediments with associated benthic microalgae, both recruitment sources for epiphytes. Dead plants left standing from the previous year’s growth cycle had higher epiphytic biomass than living plants, which occurred mostly in late spring through fall. Epiphytic biomass was highest in the winter (mean of 1.77 mg chl a (m 2 marsh) −1 ) and lowest in the summer (mean of 0.34 mg chl a (m 2 marsh) −1 ). Because phytoplankton and Spartina production are lowest in the winter, the results emphasize the relative importance of epiphytes to growth of herbivores in this season.

Salt marsh estuaries emit high levels of dimethylsulfide (DMS), yet little is known about the contribution of tidal creeks, which are rich in phytoplankton that can potentially produce dimethylsulfoniopropionate (DMSP). Quantitative data is presented on the relationship between phytoplankton assemblage structure during tidal cycles in North Inlet, a high-salinity salt marsh estuary near Georgetown, South Carolina. While there was little or no correlation between chlorophyll a (ch1 a) and phytoplankton DMSP (DMSPp) in tidal-creek waters, the DMSPp:ch1 a ratio showed a strong correlation with tidal stage, being highest at high slack tide and lowest at low slack tide, Phytoplankton assemblage structure determined from HPLC pigment profiles and CHEMTAX analysis (a matrix factorization program to derive taxonomic composition from photopigment ratios) showed that DMSP-rich taxa were highly correlated with the high DMSP:ch1 a values which occurred at high tide, These were haptophytes, dinoflagellates, cyanobacteria and cryptophytes. Diatoms had a lower correlation coefficient but, because they represented almost 40% of the algal biomass during the tidal cycle, this group could be a significant contributor of DMSPp at high tide. Chlorophytes, prasinophytes, and some chrysophytes showed a strong negative correlation coefficient (r) with the DMSPp:ch1 a peak. We conclude that the increase in the DMSPp:ch1 a ratio at high tide is due to an increased contribution of DMSP-rich phytoplankton taxa that enter the creeks from coastal waters during flood tide, and low values resulted from low DMSP-containing resuspended benthic microalgae, advected from the adjacent salt marsh into the tidal creeks during ebb tide. The data indicate a strong tidal effect on DMSP concentration that is a function of change in phytoplankton assemblage structure.

CHEMTAX is a matrix factorization program used to derive taxonomic structure of phytoplankton from photosynthetic pigment vitios. The program was originally developed from and applied to the analysis of oceanic phytoplankton assemblages. We found that application of the original CHEMTAX reference matrix to southeastern United States estuarine systems produced inaccurate results, as verified by microscopy. Modification of the matrix, based primarily on the pigment ratios of 33 estuarine isolates, improved the predictive capabilities of CHEMTAX for our samples. Limitations of the method included an overstimation of diatom biomass (due to the inability to differentiate diatoms from taxa with chloroplasts derived from diatom endosymbionts, notably some dinoflagellates) and a tendency to exclude some raphidophyte species. In complement with microscopic verification, the method was shown to improve assessment of phytoplankton taxonomic composition.

ABSTRACT Phytoplankton community pigment composition and water quality were measured seasonally along salinity gradients in two minimally urbanized salt marsh estuaries in South Carolina in order to examine their spatial and temporal distributions. The North Inlet estuary has a relatively small watershed with minimal fresh water input, while the Ashepoo, Combahee, and Edisto (ACE) Basin is characterized by a relatively greater influence of riverine drainage. Sampling stations were located in regions of the estuaries experiencing frequent diurnal tidal mixing and had similar salinity and temperature regimens. Phytoplankton community pigment composition was assessed by using high-performance liquid chromatography (HPLC) and multivariate statistical analyses. Shannon diversity index, principal-component, and cluster analyses revealed that phytoplankton community pigments in both estuaries were seasonally variable, with similar diversities but different compositions. The temporal pigment patterns indicated that there was a relatively weak correlation between the pigments in ACE Basin and the relative persistence of photopigment groups in North Inlet. The differences were presumably a consequence of the unpredictability and relatively greater influence of river discharge in the ACE Basin, in contrast to the greater environmental predictability of the more tidally influenced North Inlet. Furthermore, the timing, magnitude, and pigment composition of the annual phytoplankton bloom were different in the two estuaries. The bloom properties in North Inlet reflected the predominance of autochthonous ecological control (e.g., regenerated nutrients, grazing), and those in ACE Basin suggested that there was greater influence of allochthonous environmental factors (e.g., nutrient loading, changes in turbidity). These interestuarine differences in phytoplankton community structure and control provide insight into the organization of phytoplankton in estuaries.