Short-term variability of water quality parameters in two shallow estuaries of North Carolina

Abstract

We quantified the effects of nutrient loading following precipitation events (≥ 1.25 cm) in 2 tidal creeks varying in size and anthropogenic input during the winter and summer seasons of 1996. Several water quality parameters were repeatedly measured in the water column every 3 h for several days after each event (4–5 per season). Total nitrogen (TN) and total phosphorus (TP) behaved nonconservatively with salinity and appeared as pulsed additions, occasionally doubling within 1 to 2 tidal cycles following significant rain events. Average values for TN, TP, and chlorophylla were 10–15 μM, <4 μM, and <7 μg l⁻¹, respectively for winter events and 30–35 μM, >4 μM, and ≥ 7 μg l⁻¹, respectively for summer events. However, response times were variable, depending on the magnitude and duration of the event as well as temperature. Chlorophylla biomass often increased after nutrient additions, especially in the summer when increased nutrient loading took place. Dissolved silica (DSi) behaved conservatively with salinity; low values were observed at high tide and vice versa. Average DSi ranges for winter and summer events were 5–45 μM and 10–85 μM, respectively. DSi range values increased proportionally with the amount of freshwater loaded into the system. Recovery times for salinity were usually greater than the recovery times for nutrients. Dissolved oxygen displayed a diel pattern, increasing after daytime productivity and decreasing during nighttime. In conclusion, each rainfall event was unique and responses were variable depending upon rainfall history, seasonality, and the duration and intensity of the rainfall event. Several other variables, such as water viscosity, percolation rates, and evapotranspiration rates which were not quantified in this study, could have also explained parameter responses.