Phytoplankton and nutrient variability along a cross-shelf transect off Savannah, Georgia, U.S.A.

Abstract

Monthly cruises were conducted for one year to examine temporal and spatial variability in nutrients, chlorophyll, and the relative rate of photosynthesis of the <10 μm and total size fraction along a cross-shelf transect off Savannah, Georgia. Additionally, an upwelling event on the outer shelf was examined in some detail to determine its effect on primary production, phytoplankton species composition, nutrient and chlorophyll concentrations. Our results show that chlorophyll concentrations are always high (1.9–8.0 μ l −1 ) close to shore with little cruise-to-cruise variability. At offshore stations (>20–30 km) nutrients and chlorophyll concentrations were generally low (nitrate <0.1 μ m , chlorophyll <1.0 μg l −1 ). The outer shelf ( c . 100 km offshore), however, exhibited considerable variability. When we found upwelling at the shelf break, chorophyll a concentrations at the surface were as high as 6.0 μg l −1 . At other times surface chlorophyll at the same locations averaged 0.1–0.2 μg l −1 with nitrate concentrations <0.1 μ m . Phytoplankton >10 μm dominated the near shore stations. The <10 μm fraction dominated the phytoplankton of the mid and outer shelf, except during upwelling when diatoms such as Skeletonema costatum and Asterionella japonica were found in high abundance. Our study shows that phytoplankton abundance exhibits little seasonal variability in the Georgia Bight, but that upwelling events induce high frequency (days to weeks) variability on the outer shelf.