Sampling sites

Sites were selected to correspond to long-term monitoring sites from the statewide Texas Seagrass monitoring data set (see Congdon et al. 2019 for a more detailed description of pre and post Hurricane sampling sites). We focused on 20 of the long-term sampling sites that varied in magnitude and type of impact from Hurricane Harvey.

Eight of the sites were located in southern Aransas Bay (Redfish Bay area) experienced high freshwater runoff and longer retention time of freshwater (> 2 months). These sites included 4 that experienced high degrees of physical seagrass damage (>50 seagrass cover loss) and those with minimal seagrass loss (< 20% change in percent cover).

Eight of the sites were located in northern Corpus Christi Bay (Redfish Bay area) and experienced freshwater runoff with shorter retention time (< 6 weeks). These sites included 4 that experienced high degrees of physical seagrass damage (>50 seagrass cover loss) and those with minimal seagrass loss (< 20% change in percent cover).

Finally, 4 sites were located in the East Flat region of Corpus Christi bay which was further outside of the major impact zone and experienced lower degrees of seagrass loss and freshwater runoff.

The 16 Redfish Bay area sites were sampled in November 2017, March 2018, July 2018, and November 2018. The East flats sites were sampled during July and November 2018.

Hydrographic measurements

At each sampling site, the data sonde was lowered into the water from the side of the boat so that the instrument probes are completely submerged. We measured hydrographic measurements including water depth (m), conductivity (μS/cm), specific conductivity (μS/cm), temperature (C), salinity, dissolved oxygen (% and mg/L), chlorophyll a fluorescence (μg/L), and pH were collected using a YSI 6920 data sonde. Parameter measurements were recorded once readings stabilized at the water surface. In the field, dissolved oxygen levels were checked for accuracy based on 100% saturation at the water-atmosphere interface and recalibrated as necessary.

Care was taken to avoid agitating the benthos since this can re-suspend microalgae and compromise the accuracy of the in situ chlorophyll probe. All sonde measurements and water samples were obtained prior to the deployment of benthic sampling equipment. Upon return to the laboratory, a post-calibration check was performed on salinity, pH, dissolved oxygen, and chlorophyll a. If data sonde probes could not be calibrated or did not maintain their calibration, they were replaced.

Sonde handling:

In the field, dissolved oxygen levels were checked for accuracy based on 100% saturation at the water-atmosphere interface and recalibrated as necessary. The YSI 6920-V2 data sonde was calibrated daily prior to each use in the field. Because the sonde uses temperature compensation, it was important that the temperature probe was properly verified with a traceable digital thermometer in a bucket of tap water. The temperature probe was checked on a daily basis by comparing the readings of multiple data sondes and was verified weekly using a digital thermometer. For conductivity and pH calibration, the calibration cup and sensors were pre-rinsed three times with a small amount of the standard prior to calibration. The conductivity sensors were then submerged completely in the solution, and gently shaken to dislodge any visible air bubbles that may be trapped by the sensor. Three-point calibration was used for pH and calibration steps were followed according to manufacturer’s instructions depending on the particular buffer solutions used. Dissolved oxygen calibration was performed in a 100% saturated Freshwater bath for 10 minutes following manufacturer instructions. The chlorophyll sensor was calibrated in DI water, running the wiper at least once to ensure a zero reading on the sensor. If necessary, a two-point calibration was conducted according to manufacturer’s specifications using dye standards. Any error messages during any of these calibrations were investigated and appropriately solved before utilizing any data output from the data sonde. Instrument calibration was recorded in a logbook. Upon return to the laboratory, a post-calibration check was performed on salinity, pH, dissolved oxygen, and chlorophyll a. If data sonde probes could not be calibrated or did not maintain their calibration, they were replaced.

Seagrass percent cover

Species composition and areal coverage were obtained from two replicate quadrat samples per station at two cardinal locations from the vessel boat (starboard-bow and starboard-stern). Percent cover of areal biomass was estimated by direct observation, looking down at the seagrass canopy through the water using a 0.25 m2 quadrat framer subdivided into 100 cells.