2005 Results:
In 2005 East Twin Lake had excellent water quality for this region of the state (NCHF Ecoregion), receiving an overall A grade; the same as in nine of the previous 10 years monitored. The lake is mesotrophic. Of particular notability is the 20 ft Secchi reading in spring 2002 and 17 ft in 2005; the 2002 reading is the deepest at any Anoka County lake since at least 1996. Later in summer TP held relatively steady, chlorophyll-a was variable but always low, and Secchi transparency was reduced to a minimum of 9.6 feet. These are still excellent. Subjective observation by ACD staff ranked physical and recreational conditions optimal.
Trend Analysis:
Eleven years of water quality data have been collected by the Metropolitan Council (1980, ’81,’83, ’95, and ’98), the Minnesota Pollution Control Agency (1989), and the Anoka Conservation District (1997, ‘99, 2000, 2002, and 2005). Water quality significantly improved from 1980 to 2005 (repeated measures MANOVA with response variables TP, Cl-a, and Secchi depth, F 2,8= 5.77, p=0.03). One-way ANOVAs revealed that reductions in Cl-a is the most important factor in this trend, followed by TP reductions. Secchi depth changes are not contributing to the trend. The improvements have been small and slow. The most obvious differences are from the 1980’s data and the post-1980’s data. Since 1990 only Cl-a has trended downward. Reasons for the improvement are unknown, but improved land use practices throughout the watershed and by lakeshore homeowners are suspected.
Discussion:
The ecology of this lake is different from that of many other Anoka County Lakes because it is deep. Sediment and dead algae sink to the bottom, mostly lost from the system. Resuspension by wind or rough fish is minimal. In shallower lakes, these factors can make the lake itself a major source of nutrients and turbidity. Additionally, the lake’s direct watershed is small, so there is a small area from which polluted runoff might enter the lake. Aquatic vegetation is also healthy, but not so prolific as to be a nuisance, further contributing to high water quality. One exotic invasive plant is present in the lake, curly leaf pondweed, though its growth is moderate and restricted in extent due to lake depth. A notable recent change at this lake is the improvement of the boat landing in 2005. Now many watercraft that are large relative to the lake size can use the lake. The larger, faster boats have a greater likelihood of disturbing sediments and nutrients in shallow areas, and causing shoreline erosion.