1997 Green Lake Survey: The Warmwater Fish Community of an Urban Lake Plagued by Algal Blooms and Eurasian Watermilfoil

Green Lake is a highly eutrophic, relatively shallow (mean depth = 3.8 m; max depth = 9.1 m) body of water located within the city limits of Seattle, King County. The lake is fed by rainfall, stormwater runoff, city drinking water, and subsurface seepage. There are no natural surface water inflows to Green Lake. Surface water exits the lake (surface area = 105 ha) through three man-made outflows located along the northeast shoreline. The dominant emergent aquatic plant is white water lily (Nymphaea odorata), which is located only along the western shoreline. The dominant submerged aquatic plant is, unfortunately, invasive Eurasian watermilfoil (Myriophyllum spicatum). Others include waterweed (Elodea canadensis), coontail (Ceratophyllum demersum) and brittlewort (Nitella sp.). Details of these characteristics can be found in previous studies prepared for the Seattle Department of Parks and Recreation (SDPR) by private contractors (URS 1987; KCM 1995) and through the Washington Department of Ecology (Kirk Smith; personal communication).

Because of its central location, just north of downtown Seattle, Green Lake is valued by residents for its unique, aesthetic qualities as well as its recreational opportunities. Shoreline activities include walking, jogging, bicycling, roller skating, and fishing [the lake is stocked with rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta), and channel catfish (Ictalurus punctatus)]. Water sports include swimming, wind surfing, and crew (rowing). The SDPR maintains all public access, including a small, primitive boat launch at the south end of the lake.

Throughout most of the 20 century, intense blue-green algae blooms have hindered recreational activities around the lake. Both internal and external sources of nutrients, primarily phosphorous, are known to induce the blooms (URS 1987; KCM 1995). These include aquatic plant senescence and decay, translocation by vertically migrating blue-green algae, bioturbation by bottom-feeding common carp (Cyprinus carpio), waterfowl excretion, and stormwater runoff (KCM 1995). Furthermore, during the 1980's, Green Lake was invaded by Eurasian watermilfoil which covered up to 90% of the lake bottom. In recent years, restoration efforts included treating the lake with alum (aluminum sulfate) and sodium aluminate to inactivate sediment phosphorous (1991), relocating or sterilizing resident Canada geese (ongoing since 1991), mechanical harvesting of lake vegetation (ongoing since 1992), and diverting/filtering stormwater runoff (drain improvements completed in 1993).

In addition to the activities described above, the Green Lake management program, which was largely funded by the Washington Department of Ecology and the United States Environmental Protection Agency (EPA), recommended enhancing the warmwater fish community [e.g., largemouth bass (Micropterus salmoides) or crappie (Pomoxis sp.)] to control the common carp population through predation, and to provide better fishing opportunities (KCM 1995). Given its physical characteristics, Green Lake is well suited for warmwater fish species. Therefore, in an ongoing effort to monitor the success of the restoration program, as well as improve the warmwater fishery at the lake, personnel from the Washington Department of Fish and Wildlife’s (WDFW) Warmwater Enhancement Program conducted a fisheries survey of Green Lake during the early fall of 1997.