MEASUREMENTS OF THERMAL TRENDS AND ANOMALIES IN THE BED OF AN INTERTIDAL SALT MARSH CHANNEL USING FIBER-OPTIC DISTRIBUTED TEMPERATURE SENSING

Direct connections between surface water and groundwater in the intertidal zone are difficult to identify amidst the dynamics of tidal and diurnal forcing. Temperature can be a useful proxy for identifying fluid exchanges when the groundwater and surface water are at different, well-known temperatures. A fiber-optic Distributed Temperature Sensing (DTS) system was deployed along 330-m of two intertidal salt marsh channel beds in northern California to better understand the relative influences of warmer tidal water (17.4 ± 0.5 oC) and cooler groundwater (11.5 ± 0.2 oC) on the benthic tidal-channel environment. DTS data, averaged over 10-minute intervals, were collected at a spatial resolution of 1-m during one-and-a-half tidal cycles. The precision of the temporally aggregated DTS temperature measurements was calculated to be 0.7 oC. Continuous measurements of tidal stage, meteorological variables, groundwater temperature, and bay temperature were recorded separately. A strong temperature gradient (loss of ~2 oC/100 m inland) was discovered along the channel beds. Bed temperatures decreased from high values at the channel mouths, reflecting relatively warm and temperature-invariant bay water, to low values inland, corresponding to nearly constant-temperature, cool groundwater. A strong positive correlation between channel bed temperature and the number of hours of tidal submergence suggests that intertidal channel bed temperatures are a function of the competition between cool groundwater seepage and a warming tidal-water thermal blanket. Locations of anomalous bed temperatures up to 4 oC above or below the overall trends were also identified. These local thermal anomalies coincided with microtopographic tributaries that are hypothesized to release mixed tidal- and ground-waters held as bank storage. DTS, coupled with more traditional measurement of potential thermal end-members, is shown to be a promising approach for investigating spatio-temporal thermal patterns that may be important in hydroecological studies of intertidal systems.