Dataset: Seagrass nutrients post-Hurricane

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.

Field Procedures
At each station, two replicate cores were taken. A 15 cm inner diameter (ID) core was used to sample the seagrass species present within each quadrat. A PVC (polyvinyl chloride) core was used for the collection of below-ground and above-ground material. Care was taken to keep only the shoots that reside within the diameter of the core. Following placement of the 15 cm core on the seabed, the rubber stopper was removed from the top of the core. Before pressing the core into the sediment, the diver ran their fingers carefully around the bottom of the core. If grass was pulled under the core, it is removed. The diver pressed and twisted the core down into the sediment (10-15 cm depth). The stopper was re-installed in the 15 cm core, and the core was rocked back and forth. The diver worked their hand under the core and removed it from the grass bed, making sure to keep their hand under the bottom of the core in order to prevent loss of sample. Samples were then placed in pre-labeled Ziploc bags and immediately placed on ice.

Laboratory Procedures
Cores samples were kept in a refrigerator (4°C) until processing within one week of collection. Cores samples were sieved with filtered seawater through a 500 µm sieve to remove excess sediment. As Thalassia testudinum was the dominant species across sites, we focused our tissue analysis on this species. Seagrass tissue was carefully separated from infauna and shell harsh before further processing. From each core sample, six seagrass shoots were selected at haphazardly. Newly formed leaves (the youngest leaf in a shoot bundle) were gently scraped and rinsed in Milli-Q water to remove algal and faunal epiphytes. The rinsed tissue samples were then dried to a constant weight at 60°C and homogenized by grinding to a fine powder using a mortar and pestle. Tissue samples were re-dried in an oven overnight at 60ºC. Once dry, the samples were manually homogenized with a mortar and pestle and/or a Wig-L-Bug device. Approximately 1 mg of sample was weighed into tin capsules at the University of Texas at Austin – Marine Science Institute, Core Isotope Facility in Port Aransas, Texas. Samples were analyzed using an automated system for coupled nitrogen- and carbon-isotope measurements using a Thermo Fisher Scientific Flash EA-Isolink CNSOH elemental analyzer connected to a Thermo Fisher Scientific Delta V Plus isotope-ratio-mass-spectrometer. The isotope results are presented using the conventional δ-notation:

δ 13C (or δ 15N) = [(R sample /R standard) - 1] (in ‰)

where R sample and R standard = 13C/ 12C (or 15N/ 14N). All δ-values are reported relative to VPDB for carbon and AIR for nitrogen, unless otherwise stated (Coplen 1996). A two-point calibration of δ 13 C to VPDB and to δ 15 N to AIR is achieved using USGS-40 (−26.39‰,−4.52‰) and USGS-41a (+36.55‰, +47.55‰), respectively. Internal laboratory standards Peach Leaf and Casein were used to evaluate the accuracy and precision of the carbon and nitrogen isotope results, which should have a standard deviation of less than 0.2‰.