This dataset describes two experiments done in seagrass beds in Back Sound, North Carolina. Experiment 1 was located in a large contiguous shallow seagrass bed near Cape Lookout, NC (34.668121, -76.509455) and Experiment 2 was located in the Rachel Carson Estuarine Reserve, Beaufort, NC (34.698799, -76.595439). Experiment 1 was a clam-addition/control experiment, and 2018/2019 summer growth rates, 2018/2019 summer biomass cores, and 2018/2019 epiphytic load on Zostera marina and Halodule wrightii were sampled. Experiment 2 was a two-factor experiment looking at clam-addition and excavation and 2019 summer growth rates, 2019/2020 summer biomass cores, and 2019/2020 recolonization and percent cover were sampled. To document the spatial characteristics of the experimental areas, we mapped the extent of the contiguous seagrass bed and the coordinates of the experimental plots and subplots with a Trimble R10 Integrated GNSS system in May 2018, June 2018, and July 2019 for Experiment 1 and April 2019 and July 2019 for Experiment 2 in the NAD83 coordinate system.

Biomass cores were taken in May 2019 and July 2019 for Experiment 1 and June 2019, July 2019, and June 2020 for Experiment 2 and analyzed for Z. marina and H. wrightii shoot density, aboveground biomass, and belowground biomass. For Experiment 1, cores were taken near the center of each plot. For Experiment 2, cores from the first year of the experiment were taken from the outside edge of subplots to determine the effects of disturbance and clam addition on neighboring shoots. Cores taken in June 2019 were collected from early-season disturbance subplots that had been disturbed 1.5 months earlier; July 2019 cores were collected from late-season disturbance subplots that had been disturbed 1 month earlier. For all core samples, we used a metal corer with a diameter of 10 centimeters that was pushed into the sediment to a depth of 15 centimeters. The core was extracted and sieved in the field to remove sediment, shells, and faunal biomass. The cores were stored in mesh or Ziploc bags in a freezer until processing; all cores were processed within sixty days. To process, cores were thawed under warm running water and then carefully separated into the following categories: aboveground Z. marina biomass, aboveground H. wrightii biomass, and belowground biomass from both species as it was not possible to distinguish belowground biomass between the two species. Shoot count for each species was recorded for each core. Biomass samples were dried in a Fisher Scientific 180L Gravity Convection Oven at 60°C until fully dry and then weighed.

Summer growth rates were sampled in 2018 and 2019 for Experiment 1 and 2019 for Experiment 2. To quantify Z. marina growth rate, we used the leaf marking technique first put forth by Zieman (1974) and modified by Dennison (1990). A random location was selected within the treatment plot for Experiment 1 and along the outside edge of the subplots for Experiment 2 and all Z. marina shoots within a three-inch diameter of that random location were pricked completely through the sheath below the meristem. Marked shoots were collected fourteen days later and brought back to the University of North Carolina’s Institute of Marine Science (UNC IMS) where they were processed within twenty-four hours. Up to five shoots were used for each treatment plot or subplot per sampling point. New growth was defined as any tissue below the scar created from the push pin puncture and separated from the old biomass. Belowground biomass was discarded. Samples were dried in a Fisher Scientific 180L Gravity Convection Oven at 60°C until fully dry and then weighed. The growth rate (GR) was calculated as:

GR=New Biomass (g)Number of Shoots*Days Between Pricking and Collection           (Eq 1)

Due to the small size of H. wrightii, it was not possible to mark leaves and thus we used the clipping method from Virnstein (1982). A location within the plot for Experiment 1 or along the outside edge of the subplot for Experiment 2 was selected randomly during each sampling point and the shoots were trimmed with scissors flush to the sediment in a triangular area roughly ~40 cm². The trimmed area was marked and we returned after fourteen days to collect trimmed shoots for processing in the lab. Up to five shoots per plot or subplot were processed. Growth was determined from the average height of all processed shoots.

For Experiment 1, we estimated the epiphytic load on Z. marina and H. wrightii shoots using epiphytic Chlorophyll A as a proxy (see Parsons et al. 1984) in May 2018, July 2018, May 2019, June 2019, and July 2019. We haphazardly selected four individual seagrass blades of each species from each plot (except for Z. marina during the July 2018 and July 2019 sampling points, where only one shoot was collected due to very few Z. marina shoots remaining in the meadow and the high load of epiphytic biomass on the Z. marina shoots by this point in the summer). Shoots were carefully floated into a Ziploc bag with a small amount of seawater. Samples were stored in a cool, dark container for transport to UNC IMS and processed within twenty-four hours. In the laboratory, each sample was transferred to a sorting pan with a small amount of filtered seawater. Blades were carefully scraped to remove all epiphytes using a glass microscope slide and the total surface area of each blade was recorded. The epiphytes and seawater were vacuum-filtered through a Whatman GF/F 0.7μ filter and frozen for no longer than eight weeks until they could be extracted. The filters were sonicated in 90% Acetone for sixty seconds and extracted for 12-24 hours in a freezer. Chlorophyll A concentrations were measured on a Turner Designs Trilogy Laboratory Fluorometer and chlorophyll concentrations were normalized to seagrass surface area.

In Experiment 2, we estimated seagrass percent cover to assess seagrass regrowth into experimentally disturbed areas and compare it to percent cover of non-disturbed areas with and without clam additions. Percent cover of subplots was recorded three times in the fall of 2019 (September 3^rd, September 16^th, and October 2^nd) and once in the spring of 2020 (June 6^th). Between the two September sampling points, Hurricane Dorian made landfall along the North Carolina coast as a Category 1 hurricane on September 6^th, 2019.

Occasionally replicate samples were not able to be collected or analyzed for each response variable due to uncontrollable circumstances (i.e., unable to locate subplots in unideal weather, oven malfunction, etc.). In these cases, data is represented by NA and NOT the value zero. 

Each Excel file has a metadata worksheet to aid in understanding the data presented.