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4.2.1.4 Water Quality and DNR Data Although Silver Lake currently exhibits generally good water quality, somewhat poor water quality was noted prior to 1985. The DNR believes the change in water quality was associated with a change in the lake's fish population. Prior to 1978, the lake was subject to seasonally low oxygen levels resulting in winterkills. Frequent winterkills prevented the establishment of a gamefish population and enabled a large black bullhead population to flourish in the lake. Since 1978, however, winter aeration has enabled a gamefish population to become established in the lake. The gamefish population controls the benthivorous bullheads and planktivorous panfish, both of which contribute to degraded water quality. The establishment of a gamefish population has caused a dramatic decline in the bullhead population in the lake, due to the effects of competition between fish species. Initially, the decline was in the form of an elimination of smaller bullheads. In 1985, the death of large numbers of older bullheads resulted in a dramatic decline in the bullhead population. The. DNR reports that trapnet catches of bullheads declined from : ■ 130 in 1980 to ■ 19.5 in 1985 to ■ 1.5 in 1990. Prior to 1985, the large numbers of bullheads in the lake added nutrients to the water by stirring up bottom sediments and excreting nutrients into the water. The reduction in nutrients in Silver Lake during 1985, and in subsequent years, appears to be largely due to the decline in the bullhead population. Drought conditions during 1987 through 1989 reduced nutrient loading to the lake which improved its water quality. The water quality improvement occurring during the late 1980s appears to be a combination of a change in the fish population and climatic conditions. The water quality changes noted as a result of the change in fish population within the lake are expected to continue without interruption. Ramsey County operated a portable pump and aeration baffle system from 1977 through 1985 to prevent winterkill in the lake. A permanent pump and aeration baffle system was installed in 1986 to insure the survival of the gamefish population in the lake. Therefore, the bullhead population is expected to remain low, and water quality improvements are expected to continue. The DNR currently manages the lake's native bass and panfish population and stocks walleye and tiger muskie on a regular basis. Ramsey County has conducted water quality sampling on Silver Lake since 1974; annual sampling has been conducted since 1984. Water quality data collected from the lake are presented on Figure 4-2. The 1993 Silver Lake data were compared to criteria used to denote a lake's nutrient "status", and the effects of the nutrients on the general water quality and/or trophic conditions of a water body (NALMS, 1988). Based on a comparison of the lake's nutrient (i.e., total phosphorus) concentration to these criteria, Silver Lake would be assigned a trophic status of eutrophic. This simply means that the lake is rich in nutrients. However, an evaluation of the effects of the nutrients on the general water quality of the lake (i.e., chlorophyll a concentrations and Secchi disc transparency) results in a classification of mesotrophic. This means that the lake is not responding to its nutrient load as expected, and that few algae are found in the lake. Because of the low algal population, water clarity is quite good. 23\82\045\SEC4.RPT\KMH 3 DRAFT: June 23, 1994