Zostera Epifauna Beaufort, NC, USA

Plant biomass and animal species abundance were measured after disturbance of eelgrass (Zostera marina) at two sites at Drum Shoals (34 deg 42’40”N, 76 deg 46’20”W) in Bogue Sound and two sites at Middle Marsh (34 deg 41’32”N,76 deg 37'20"W) in Back Sound. Drum Shoals is located at least 250 m from the nearest Spartina marsh whereas Middle Marsh sites are located from 10 to 80 m from the nearest Spartina marsh and are almost surrounded by it. The disturbance was the denudation of eelgrass.
To denude plots, I removed sediment to a depth of 5 cm in 120 squares (0.25 m squared each) with a shovel in September 1975.
I sampled plants and animals using a square box quadrat macrobenthic sampler (947 cm squared bottom area – approximately 36.5 liter volume including the top 3 cm of sediment) having 0.8 mm mesh netting.
Most of the sediment was removed through the mesh sides of the sampler in the field; after transfer to buckets, samples were brought back to the lab, washed again on a 0.8 mm mesh sieve to remove residual sediment, and frozen in plastic bags. All samples were taken in the day time within +/- 3 hours of low tide; water depth was 5 to 60 cm while sampling. Samples from denuded plots were taken in the center of each plot.
Fauna were separated from shell material and plant material in a white tray. Animals were identified to species except for some polychaete families.
Plant material in each sample was separated into live Z. marina leaves, Z. marina root-rhizomes, macroalgae (by species) macrodetritus and Halodule wrightii (leaves plus root-rhizomes). Macrodetritus was defined as brown or dark leaf material not connected to rhizomes, rhizomes without attached leaves with short (< 1 cm) or no roots and other dead plant material retained on a 0.8 mm mesh screen. All plant material was dried at 90 deg C to constant weight, weighed and ashed at 550 deg C for at least 2 hours. Ash free dry weight (AFDW) was determined by difference.
Water temperature was measured in the field by a mercury thermometer. Water samples were taken on site each month and measured for salinity using a hydrometer.
Depths were measured on a grid for 156 points at each of the 4 sites and adjusted for changes during the sampling period and corrected to mean low tide by comparing contemporaneous readings on a tidal guide at Pivers Island, Beaufort, NC.
Results:
Water temperature ranged from 6 deg C in winter to summer highs of 31 detg C. Standing pools at low tide occasionally reached 37 deg C. Salinity ranged from 30.7 ppt to 36.8 ppt at Drum Shoals and from 31.5 ppt to 36.8 ppt at Middle Marsh.
Two hundred natural samples of epifauna and shallow benthos yielded over 93,00 individuals (approx. 5,000/square meter). Most were gastropods (43%), or amphipod crustaceans (41.5%) followed by polychaete annelids (5.2%), decapod crustaceans (3.5%), bivalve molluscs (3.4%) and isopod crustaceans (1.7%). Zostera leaf biomass in natural samples varied greatly with location, season, and year and was usually higher at Middle Marsh (41 g AFDW / square m versus 25 g AFDW / square m at Drum Shoals).
Initial regrowth of seagrasses (99% Z. marina) after denudation occurred within six months at location M1, followed closely by D1 and D2. Within the first year after disturbance Z marina leaf biomass was similar to that of undisturbed subplots at D1, D2, and M1. Regrowth of M2, however, was almost absent the first year and leaf biomass was not equal to undisturbed subplots by June of the second year, normally the month with the highest plant biomass. Drift algae apparently settled in the slight depressions left after the denudation at M1 and M2. I occasionally observed algae in these denuded subplots while there were no algae on the bare sediment surrounding the depression. In the Middle Marsh (M1 and M2) denuded plots Sargassum contributed most (58%) of the seaweed biomass, followed by Agardhiella (23%), Codium (18%) and a small amount of Ulva (less than 1%).
Number of species and individuals was correlated with Zostera leaf biomass and macroalgae biomass but did not seem to have a simple linear relationship.