Effects on Productivity

Light Intensity and Primary Production

Studies from Lake Washington: Overwater structures reduce the amount of light available to phytoplankton and aquatic macrophytes, which can ultimately reduce primary production. White (1975) compared light intensity and primary production/biomass ratios of phytoplankton at sampling stations under overwater structures, and at control stations outside of overwater structures in Lake Washington. As expected, light intensity was higher at all control stations compared with intensities measured under overwater structures. Surface light intensities at open water stations on sunny days inhibited phytoplankton production in the upper 60 cm of the water column. Production/biomass ratios measured at stations outside of piers reached maximum levels at depths from 1 to 2 meters. Surface phytoplankton production/biomass ratios under narrow residential piers or at the edge of large overwater structures exceeded those measured at open water stations due to the reduction in surface light intensity. However, phytoplankton production/biomass ratios from outside stations exceeded those from under-pier stations from a depth of about 60 cm to the maximum depth measured. White (1975) suggested that, while narrow residential piers do not significantly reduce surface phytoplankton production, the reduction is inversely proportional to shading, as illustrated by the low production/biomass ratios observed under a residential pier with skirting, a boathouse, and an overwater apartment complex.

White (1975) did not comment on the reduced production/biomass ratios of shaded sites at depth, compared to open-water control sites, nor did he measure periphyton or macrophyte abundance and production under and outside of overwater structures. The reduced light intensity observed under all overwater structures when compared with open-water control sites, resulted in reduced total water-column phytoplankton production/biomass ratios, and likely reduced periphyton and macrophyte production as well. White (1975) did not attempt to measure the cumulative loss in primary productivity that would ultimately result from the collective overwater coverage of numerous residential and commercial structures. While the loss in productivity from a single, narrow, residential structure may be insignificant, the cumulative impact of thousands of narrow piers is likely to be a significant reduction in primary productivity.

A comparison of the benthic invertebrates under, and outside of, overwater structures in Lake Washington revealed complex patterns of abundance and/or distribution in the benthic community, with some organisms more abundant under, than outside of, piers in spring and less abundant in fall (White 1975). Possible explanations for the observed patterns include variations in phototaxis with life-history stage and differences in forage availability during fall and spring (White 1975). Macrophytes were absent or sparse under piers; grazing invertebrates would be found outside piers where macrophytes were abundant in the fall, but under piers, where they could graze on periphyton, in the spring when macrophytes were sparse (White 1975).

Studies from other systems: Loflin (1995) reported that docks in two Florida marine locations produced distinct areas in their shadow that were nearly devoid of seagrass, and that were significantly correlated with total dock surface area. Shading from docks also produced changes in seagrass species composition and reduced epiphitic loading on grass blades (Loflin 1995). The percent cover of epifauna on primary kelp blades was less under piers than on perimeter piles at a marine site in Portsea, Australia (Fletcher and Day 1983). Shaded piles in Sydney, Australia had different epibiotic assemblages than unshaded piles or adjacent rocky reefs (Glasby 1999a; 1999b). Epibiotic assemblages on unshaded piles were composed of filamentous and foliose algae (primary producers), while communities on shaded piles were composed of filter feeders (Glasby 1999b).

At a Long Island Sound location, Iannuzzi et al. (1996) predicted that construction of an 800-slip marina would reduce macroalgal production by 17%, but that reduction would be compensated for by microalgal production on the hard attachment surfaces of the marina. The Long Island Sound site was a high-energy marine system with radical changes in energy, sediment composition, and turbidity expected to result from construction of the marina (Iannuzzi et al. 1996). The references from this and the preceding paragraph were studies of marine systems. Extrapolating their results to freshwater systems may not be entirely appropriate. However, the responses of macro- and microalgae to reductions in light intensity resulting from overwater structures would be expected to be similar among systems.

Another expected effect of shoreline structures includes the loss of allocthonous nutrient input resulting from the removal of shoreline vegetation. France and Peters (1995) estimated the annual litter input to a northern Ontario lake from 1 m of forested shoreline to be 32-g dry weight. Allocthonous litter input per unit offshore distance was related to the size of riparian trees, their proximity to the shoreline, and the elevation of their canopy (France and Peters 1995). Riparian deforestation resulted in annual reductions of up to 17.8 g of dissolved organic carbon and 2.9 g of total phosphorous per meter of shoreline in oligotrophic Canadian shield lakes, reducing primary production by up to 9 percent (France et al. 1996). The implications are less serious for mesotrophic urban lakes where increased phosphorous loading is problematic. However, Eggers et al. (1978) concluded that the littoral benthic community in Lake Washington was resource limited. Increasing allocthonous litter input from shoreline vegetation on Lakes Washington and Sammamish could increase forage for juvenile salmonids and the forage fish of bull trout in the littoral zones. The permanent removal of shoreline vegetation for bulkhead construction, and for unobstructed views may affect the forage base of ESA-listed salmonids by reducing allocthonous input to the littoral zone. An incidental effect of shoreline vegetation removal is likely to be an increase in diel temperature fluctuation in the littoral zone due to loss of shade (Steedman et al. 1998), especially in littoral areas that have been isolated from the main water mass by artificial structures.

In summary: Evidence from Lake Washington indicates that single narrow residential piers do not significantly reduce surface planktonic primary productivity, but the productivity losses below 60 cm, and from reduced macrophyte productivity were not investigated. Cumulative reductions in primary productivity resulting from numerous overwater structures were not measured, but could be substantial. Comparisons of benthic primary production (epiphytes) in Lake Washington were not made. The effects of overwater structures on invertebrate production in Lake Washington have not been conclusively established. Evidence from marine systems indicated that epibiotic assemblages are affected by shade, with primary producers being replaced by consumers. Removal of shoreline vegetation could reduce allocthonous input of nutrients. While the deck prisms mentioned above that are being mandated by some municipalities as mitigation for overwater coverage have not been tested for efficacy at reducing bass attraction to piers, they do transmit ambient light. Thus, they may be useful for retrofitting existing piers to restore the primary productivity loss from those structures.

Aquatic Vegetation Control

Control of "nuisance" aquatic macrophytes by lakeshore property owners is another potential source of lost productivity. Aside from the importance of macrophytes in primary production, numerous studies have indicated the importance of littoral vegetation for increased fish production (e.g., Randall et al. 1996). The most significant effect on fish of development of the shoreline of an Iowa lake was the removal of aquatic macrophytes by lakeshore residents (Bryan and Scarnecchia 1992). Methods for vegetation control include active harvesting, chemical controls, and covering of the substrate with materials that block vegetation growth. Engel (1984) compared removable and non-removable materials for aquatic vegetation control, and concluded that even removable screens with large pore sizes nearly eliminated benthic invertebrates. Despite the generally undesirable effects of macrophyte removal, there are situations where the reduction of aquatic macrophytes may benefit fish. Dissolved oxygen (DO) levels under dense patches of Eurasian milfoil and fragrant white water lily (Nymphaea odorata) were lethal to caged steelhead trout in Lake Washington in a study by Frodge et al. (1995). Native species of aquatic macrophytes found in Lake Washington typically do not form large monotypic stands with dense surface mats such as those found to reduce DO concentrations (Frodge, pers. comm., 10 July 2000).

Physical Effects Lake Washington: The physical effects of bulkheads on benthic organisms are expected to depend upon both the designs of bulkheads and the material from which bulkheads are constructed. White (1975) compared benthic invertebrate abundance at various depths in front of bulkheaded shorelines, developed shorelines without bulkheads, and along natural shorelines in Lake Washington. White's (1975) results were inconclusive, indicating no clear trends in invertebrate abundance. However, White (1975) did not report the position relative to OHW of the bulkheads in his study, nor did he measure invertebrate abundance immediately waterward (at the toe) of bulkheads.Results from White's (1979) study provide inconclusive evidence for an adverse affect of bulkheads on the benthic community within local lakes. Smooth vertical structures would be expected to reflect wave energy in a non-random manner. Complex non-vertical bulkheads, such as those constructed of boulders, would be expected to reflect wave energy in a random manner. Reflected wave energy would produce a chaotic, high-energy environment for epibenthic and infaunal invertebrates within a zone adjacent to the bulkhead. The benthic community within this zone would be expected to have lower invertebrate abundance, richness, and diversity than lower energy zones.Estuarine Systems: Only one study was obtained that specifically examined the effects of shoreline armoring on infaunal organisms, and the study system was an estuary with a sand beach. Spalding (1998) concluded that sediments were finer and better sorted immediately adjacent to bulkheads than at similar elevations at control sites, and that meiofauna densities increased with distance from bulkheads in a New Jersey estuary. Bulkheads had the greatest influence on sediment characteristics and meiofaunal densities when located waterward of wavebreak, or when subjected to high wave energy (Spalding 1998). Spalding's (1998) study was of sandy estuarine beaches with tidal influences - drastically different from the typical beaches in local lakes; she was also sampling organisms within the swash zone of the beach. Thus, Spalding's (1998) results may have limited applicability to our local lakeshores. However, the findings of Spalding's (1998) study should not be ignored. Chironomid larvae are the primary prey items for juvenile chinook and coho salmon, and could be adversely affected by bulkhead-induced changes in sediment composition. A lack of affect of bulkheads on infaunal organisms should not be concluded without further investigation. Byrne (1995) described a year-long study designed to test whether there was a difference in the species composition and abundance of macroinvertebrates and fish inhabiting bulkheaded and non-bulkheaded shore zones in a manmade estuarine lagoon. Results from Byrne's (1995) study could not be obtained, only a summary of the study design.A comparison of the macroinvertebrate community structure on rip-rap bulkheads and smooth retaining walls was conducted using simulated rip-rap and patio blocks of similar surface area placed along three different shorelines (rip-rap, vertical retaining wall, and natural shoreline) in three Wisconsin lakes (Schmude et al. 1998). As expected, the complex artificial substrate had significantly greater abundance and taxa richness of macroinvertebrates than the two-dimensional patio blocks, but no significant differences in abundance or richness were observed among shoreline types.

In summary, only one study investigating the effects of shoreline armoring on benthic organisms in freshwater was located, and it did not provide conclusive evidence for an adverse affect. The findings of Spalding (1998) indicating a change in sediment composition and meiofauna density in front of bulkheads along sandy estuarine shorelines imply that bulkheads could adversely affect benthic organisms in freshwater lakes, and that further investigation is necessary.