Fine-Scale Detection of Estuarine Water Quality with Managed Freshwater Releases

Freshwater pulses to subtropical estuaries often occur on time scales less than 1 week. In particular, introduction of low-level pulses are potentially important during the dry season (November–April) when freshwater is scarce. Determining potential ecological benefits of pulses requires an innovative method of data acquisition at the appropriate spatial and temporal scales. The South Florida Water Management District conducted a pilot study to assess changes in water column attributes with pulse releases to the Caloosahatchee River Estuary (CRE) from January to April 2012. An average inflow of 450 cfs was targeted for a series of freshwater pulses. This study utilized an onboard, flow-through system to record surface water temperature, salinity (S), pH, dissolved oxygen, turbidity, and in situ chlorophyll a (in situ CHL) at 5 s intervals along the 42-km length of the estuary. On each of seven research cruises, the vessel stopped at multiple stations to conduct vertical water column profiles. Salinity increased throughout the CRE as inflow decreased during the study period. Simple correlation and partial least squares regression were used to determine that the downstream locations of the S?=?10 isohaline and the maximum CHL concentration (in situ CHL_max) were positively related to inflow. While the in situ CHL_max was located 12–20 km downstream on five of the cruises, it was only a few kilometer from the estuary head on the first (1/12) and last (4/11) dates. It is possible that two circumstances related to freshwater inflow accounted for this pattern. First, water column stratification before January could have stimulated remineralization and primary production. Second, inflow ceased as water temperature increased to 26.0 °C by April to promote algal growth. Further study of the relationships among inflow, water level, flushing time, and CHL is warranted. Future efforts will examine the range of wet season discharge by incorporating a sensor for colored dissolved organic matter to fully connect inflow, salinity, submarine light, and phytoplankton attributes in the CRE.