VI. B. 3. Impacts - Physical Processes and Biological Considerations |
Nightingale and Simenstad (2001b) provide a comprehensive summary of the primary literature related to the physical and biological impacts of overwater structures, which we attempt to summarize here. Overwater structures can alter a variety of the physical processes controlling the development and distribution of nearshore habitats. These include the ambient light regime, hydrology, substrate conditions, physical disturbance, and water quality (Conceptual Model, Figure II-3). However, reduction of ambient light conditions (e.g., light attenuation and shading) is one of the primary mechanisms by which ecological impacts are often ascribed to docks, floats, pilings, and moored vessels.
Light reduction, or shading, by overwater structures has implications for both vegetation and animals. For submerged aquatic plants such as eelgrass (Zostera marina), shading reduces levels of photosynthetically active radiation (PAR) necessary for survival. As previously discussed (Chapter IV), eelgrass is considered a critically important habitat in Puget Sound, serving primary production, feeding, refuge, and reproductive functions to a variety of marine species. Light regimes show considerable variation, depending upon the characteristics of the structure itself, including height above the bottom, orientation, piling density, and construction material. Increased dock height diminishes the intensity of shading by providing a greater distance for light to diffuse and refract around the dock surface before reaching the eelgrass canopy (Nightingale and Simenstad 2001b). Comparatively, floating docks allow no light to penetrate beneath them and the water’s surface. Marinas may further enlarge the shade footprint through the increased water surface area covered by floating moorages and vessels. A north-south dock orientation has been shown to increase underwater light availability by allowing varying shadow periods as the sun moves across the sky, thereby reducing stress imposed on eelgrass. The PAR variations may also affect epiphyte and macroalgae production. High densities of support pilings, which serve as attachment substrate for macroalgae themselves, may increase shading to benthic substrates and eelgrass beds.
Light is a determining factor in fish migration, prey capture, and predator avoidance (Nightingale and Simenstad 2001b). Overwater structures, such as piers, floating docks, and marinas, may substantially reduce light levels necessary to these functions. A variety of studies have shown that salmon fry migrate along the edges of shadows rather than penetrate them (Simenstad et al. 1999). Prey abundance and capture rate may also be reduced under piers as compared with open-water areas for some fish species (Duffy-Anderson and Able 1999). Light behavior criteria identified by Nightingale and Simenstad (2001b) suggest that feeding and schooling behavior of some fishes may not be sustained at the low light levels observed under some industrial docks. Overwater structures may also increase the exposure of juvenile salmon to potential predators by providing predator habitat, reducing refugia such as eelgrass, and diverting juveniles into deeper waters, although little empirical evidence exists to support these hypotheses. Fish distribution studies have also documented the affinity of small juvenile fish for protected embayments that include marinas, although this preference likely reflects their reliance on shallow nearshore habitats and avoidance of under-dock areas.
Overwater structures may also influence local hydrology. Pilings change the flow of water over adjacent substrates, causing scouring, changes in bathymetry, and alteration of sediment transport, especially at high piling densities (Nightingale and Simenstad 2001b). Floating piers are also known to affect sediment movement and are not recommended in areas of significant littoral transport.
Bottom substrates associated with some overwater structure features can be impacted by encrusting communities. For example, support pilings provide surface area for mussels, barnacles, and other sessile organisms. Predation by sea stars and crab results in a large deposition of shell hash on the adjacent substrates and changes in biological communities associated with these substrates (Nightingale and Simenstad 2001b). Changes in benthic substrate composition impacts eelgrass production, and may increase disturbance of eelgrass meadows by seastars and burrowing crab.
Overwater structures also may cause physical disturbances to local habitats. Construction activities associated with the driving and insertion of pilings directly affects benthic communities, whereas noise associated with piling driving operations may affect the distribution and behavior of salmon and other fish and wildlife species (Feist et al. 1996). Indirect habitat impacts associated with improperly sited structures include grounding, scouring, and prop-wash effects. Low tides present the greatest risk of contact between floating structures (floating docks, mooring buoys) and marine vegetation and substrates. Grounding of floating docks, mooring buoys, and vessels often leads to the total loss of eelgrass beds and alteration of the benthic invertebrate community (Nightingale and Simenstad 2001b). Heavy fastening chains or anchor lines that drag across the bottom during tide or wind events can cause scouring and disturbance of vegetation. Vessels commonly associated with many overwater structures can cause prop scouring of sediment, disturbing submerged vegetation and benthic communities.
Water-quality impacts are another potential issue associated with overwater structures. Marinas and covered moorages are typically associated with heavy boat traffic and human use, and may subject adjacent waters to potentially more frequent exposure to petroleum, household cleaning, pesticide, and herbicide products (Nightingale and Simenstad 2001b) and sewage. Similarly, boat paint and maintenance products can pose an increased risk of contamination to the marine food web through accidental spills. Poor water circulation in marinas can create a buildup of organic sediment, low dissolved oxygen concentrations, and dinoflagellate blooms.
Wood pilings treated with creosote, ACZA (Ammoniacal Copper Zinc Arsenate), and CCA Type C (Chromated Copper Arsenate) pose an additional risk of leaching contaminants into the water column (Poston 2001). These wood preservatives may release contaminants into aquatic habitats via three mechanisms: rain or snow melt runoff, dislodging of treated wood fibers by activities, or leaching into sediment. Exposure of aquatic organisms can occur in the water column, in adjacent sediment, or via direct attachment of tissue or eggs. All of these compounds have various levels of toxicity to marine organisms (Poston 2001). Port Madison Bay is one of three locations in Puget Sound where mass mortality of herring spawn has been documented (Jim West, WDFW, personal communication, 2002). Preliminary studies have suggested a link between a waterborne toxic substance, such as polynuclear aromatic hydrocarbon (PAH) compounds, and these mortalities, though definitive studies have yet to be conducted.
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