Projects & Programs
Evaluation of Methods to Increase Light Under Ferry Terminals
To address concerns of resource agencies about the potential impacts of ferry terminal expansion on valuable habitat functions and resource use of nearshore areas, the PNNL, in partnership with the Washington State Department of Transportation, conducted field trials with off-the-shelf products that promote light passage through dock structures. These products included a SunTunnel, deck prisms and a metal halide greenhouse light. Light measurements and photosynthetically active radiation, (PAR), were also recorded beneath glass blocks and a metal grating installed at Clinton Ferry Terminal on Whidbey Island, Washington. A review of other studies measuring the effects of dock shading and alternate dock materials was conducted.
PAR measurements from this study were related to minimum requirements for eelgrass, Zostera marina, photosynthesis and to the known maximum photosynthetic "saturation" rate for Z. marina. We also related PAR measurements to what we know about light requirements for macroalgae and kelp and about light effects on juvenile salmonid feeding and passage under overwater structures.
Light technologies were tested as to whether they could provide adequate light for eelgrass growth underneath a ferry terminal being expanded or rebuilt if certain requirements are met. For example, all of the technologies (some singly and some in multiples of two to four) would provide adequate light for eelgrass if the dock were no more than 3 m (10 ft) above the eelgrass. At increased distances between the deck and eelgrass, the water-to-air ratio as well as the total distance light must travel begin to limit the technologies that could be successfully applied.
In general, the order of the products predicted to provide the most to the least light to eelgrass are the grating, SunTunnel, metal halide greenhouse light and prisms. Light levels required to allow fish to feed and form schools are low (~ 1-2 µmol/m2/s) and much lower than those required for photosynthesis. Based on work by others, our research indicates that installing any of the tested light products would likely maintain light levels under the dock above those required for active feeding by juvenile salmonids.