Sensitivity of transects across a depth gradient to measuring changes in aerial coverage and abundance of Ruppia megacarpa Mason

Effective management of estuarine systems that contain submerged aquatic macrophytes, requires knowledge of whether macrophyte populations are stable, increasing or decreasing in terms of aerial coverage and abundance. This study established three transects within Wilson Inlet and monitored them five times during 1996 for percent cover of Ruppia megacarpa and maximum depth limits of these meadows. Secchi depth data collected weekly in the Inlet for water quality monitoring were used for comparison. Percentage cover of R. megacarpa varied along the length of the transects and also between sampling times. This variation was partially explainable by variation in light availability, but was affected by other factors within the system. Overall sites and times, the maximum depth limit of R. megacarpa was at approximately 24% of incident irradience. This value is high in comparison to other submerged macrophytes and suggests that R. megacarpa is not limited by light. Its distribution may be limited by sediment type, with the seagrass being confined to coarser sediments in the Inlet. It was concluded that R. megacarpa depth distribution in Wilson Inlet is not currently limited by light, and so a minimum acceptable Secchi depth cannot be recommended. Repeated measurement of permanent transects for percent cover and canopy height, replicated within each meadow and along each transect, is recommended.

Keywords: Estuaries, Transects, Seagrass, Light, Depth, Management