The impacts of rainfall runoff on tidal creek algal and bacterial production

Field and mesocosm experiments were performed to examine the affects of different nutrient concentrations on production of bacteria and phytoplankton in three New Hanover County, North Carolina, tidal creeks of differing watershed impervious cover. Radiotracer assays were performed to determine production rates using tritiated-thymidine for bacteria and 14-C bicarbonate for phytoplankton. The field study compared production rates and nutrient concentrations monthly during dry and rain conditions and the mesocosm experiment compared production rates given different combinations of nutrients approximating concentrations from rain events (control, NP, NPSi, NPC). Pages Creek had mean daily heterotrophic and autotrophic production rates of 36.4 and 395.4 mgC m-3day-1, respectively. Physical parameters control production in Pages Creek, while nutrients are also a limiting factor in Howe and Bradley Creeks. Nutrient concentrations are generally similar between creeks, all of which are also nitrogen limited. Mean heterotrophic and autotrophic production rates in Howe Creek were 38.3 and 636.7 mgC m-3day-1, respectively. Production rates in Bradley Creek were 41.1 mgC m-3day-1 heterotrophically, and 547.2 mgC m- 3day-1 autotrophically. Production ratios in Bradley Creek differ from the other creeks, indicating interdependence between bacteria and algae. Both types of production correlated with dissolved carbohydrates in this creek, suggesting that phytoplankton supply dissolved carbohydrates to support bacterial growth. This research shows that though nutrient concentrations and production appear to be similar among creeks, there are differences in response to nutrient concentrations and grazing pressure, which may be anthropogenically altered