Bioturbation as a potential mechanism influencing spatial heterogeneity of North Carolina seagrass beds

The frequency and duration of bioturbation pits and their potential role in altering or maintaining the spatial heterogeneity of seagrass beds were evaluated within beds dominated by a mix of Zostera marina and Halodule wrightii near Beaufort, North Carolina, USA. Our evaluations were performed systematically over large (1/4 ha) sites in order to make generalizations as to the effect of bioturbation at the scale at which seagrass landscape patterns are discerned. Eighteen 50 x 50 m sites, representing a wide range of seagrass bottom cover, were surveyed seasonally for 2 yr at 1 m resolution to describe the spatial heterogeneity of seagrass cover on the sites. We measured a number of environmental factors, including exposure to waves, tidal current speed, percent seagrass cover, sediment organic content and silt-clay content, seagrass shoot density, above- and belowground seagrass biomass, and numbers of bioturbation pits in the bottom. Seagrass bed cover ranged from 13 to 100% of the site, with 0 to 1.3% of the seafloor within the site showing discernable bioturbation pits. Three sites were selected for detailed study where we marked both existing bioturbation pits and new ones as they formed over time. Pits were measured every 1 to 3 d for width, length and depth until the pit was obscured; pit duration averaged similar to 5 d with an observed maximum of 31 d. We found a bimodal frequency distribution of pits, with small pits (0.05 to 0.4 m(2)) occurring more frequently than large pits (>0.4 m(2)), although the fewer large pits could account for over a third of bed margin disruption on a site. Even with a small amount of the open sand areas showing disturbance at any one time (less than or equal to 1.3%), based on the rate of new pit formation, the return interval for disturbance of a given square meter of unvegetated bottom among seagrass patches was similar to 1.2 yr. Pits often occurred within 1 m of seagrass bed margins and were of sufficient depth to fully disturb the seagrass rhizosphere. Because of the frequency of pit formation, their depth and proximity ro patch edges, it appears that bioturbation may play an important role in the maintenance of seagrass landscape pattern in the Beaufort area through disruption of the bed margins and, potentially seedling recruitment.