The influence of macronutrient form on the spatial and seasonal variability of phytoplankton assemblages and bacterial abundances in four coastal South Carolina systems
The South Carolina (SC) coastal zone is undergoing rapid development and urbanization such that land development rates often exceed population growth rates, including areas in and around the city of Charleston. However, regions of relatively unaltered coastal systems remain. This range of land uses is likely to influence nutrient delivery (nitrogen - N, phosphorus - P, and dissolved organic carbon - DOC) to the coast, as well as associated phytoplankton and bacterial communities. This study utilized <i>in situ</i> bioassays deployed seasonally (2011-13) at four coastal SC systems with distinct land usages: a forested/agricultural creek in Winyah Bay, an urbanized creek in the Charleston Harbor (CH) system, a stormwater detention pond on Kiawah Island (KI), and a forested/undeveloped creek in the ACE (Ashepoo, Combahee, and Edisto River) Basin. Bioassays were used to examine the spatial and seasonal variability in phytoplankton assemblages and bacterial abundances in response to N (ammonium, nitrate, urea) and/or P (orthophosphate) additions. Field sampling included assessments of seasonal DOC, bacterial abundance, and phytoplankton levels. Results showed that these sites are primarily N-limited and diatoms were typically the major contributor to total community biomass (chlorophyll <i>a</i>). Treatments containing urea were generally associated with the greatest phytoplankton growth responses at the relatively more developed sites (CH and KI), where harmful algal bloom species were observed. DOC concentrations exhibited seasonal variability, and increased DOC in treatments containing urea suggest that increased nitrogen inputs may lead to greater phytoplankton contributions to the total DOC pool. Environmental variables (temperature, salinity, and precipitation) were important drivers of nutrient, phytoplankton, and bacterial dynamics within these coastal systems. Findings herein can be used by water quality managers for regulating coastal N and P inputs.