Study of beds of Ruppia Maritima and Halodule wrigthii at Grand Bay National Estuarine Research Reserve, Mississippi

Submerged aquatic vegetation (SAV), a unique group of flowering plants that have adapted to live fully underwater, are a valuable resource and indicator of aquatic habitat quality. The cumulative effects of alteration of natural habitats and decline in coastal environmental quality are causing a decline of coastal SAV.

We hypothesized that there are significant spatial and short-term fluctuations in the coverage of Ruppia/Halodule beds. The hypothesis was tested by conducting bi-annual surveys at several seagrass beds at Grand Bay National Estuarine Research Reserve (GBNERR). The SAV depth distribution was surveyed at five sites at GBNERR using line-intercept method, twice a year, since 2005. At each site, three 200-m transects, parallel to each other and perpendicular to the shore, were established using reel tapes and metal/PVC posts. Each transect was surveyed by snorkeling, while recording the start and stop points of SAV patches that were intercepted by the transect line. Water depth was measured using a graduated wooden stick at every 3 meters along the transects.

We analyzed the SAV abundance, species composition, and distribution recorded along the gradients of water depth and shore type using three-way ANOVA with fixed variables of site, survey time, and water depth. Our results indicate that the coverage and distribution of the beds dominated by Rupia/Halodule beds of the tidal bay area vary substantially primarily due to changes in R. maritima abundance between summer and fall, and between years. Based on the ANOVA analysis, SAV coverage was significantly difference among sites, survey time, year, and water depth (P<0.001). Also, Halodule wrightii abundance at a given site was not changed significantly between summer and fall of the same year. Our results suggest that shore orientation and wind-driven energy within the estuarine system might be contributing factors to the spatial difference in the shallow estuary.