An Evaluation of Glass Prisms in Boat Docks to Reduce Shading of Submerged Aquatic Vegetation in the Lower St. Johns River, Florida

The effectiveness of glass prisms in boat docks was assessed to determine if shading impacts to submerged aquatic vegetation (SAV), primarily Vallisneria americana, were reduced. Six experimental docks, three with prisms and three without prisms, were constructed in the lower St. Johns River, Florida. SAV percent cover and photosynthetically active radiation (PAR) were monitored under each dock and in an adjacent control area with no experimental docks. Subsurface PAR during the growing season of the first year of the study was not significantly greater beneath docks having prisms than beneath docks without prisms. Postconstruction SAV monitoring (February 2000 to May 2002) revealed no significant differences in SAV percent cover between dock treatments, although coverage declined in both dock treatments and the control area. Declining water quality conditions at the study site clearly impacted the health of the SAV habitat as indicated by the decline in SAV coverage in the control area initially in the study. Given the subsequent resurgence of SAV in the control area, the additional light transmitted through the prisms did not appear to be biologically significant or adequate to counteract effects from larger-scale environmental stressors.