2007 Results
In 2007 Linwood Lake had average or slightly below average water quality for this region of the state (NCHF Ecoregion), receiving an overall C grade. The lake is slightly eutrophic. In 2007 water quality in late summer was disappointingly worse than the other most recent years. At that time of year phosphorus levels were high (maximum 77 ug/L) and a substantial algae bloom developed (maximum chlorophyll-a 51 mg/L). ACD staff’s subjective observations of the lake’s physical characteristics were that there was a “definite” algae presence until July when algae levels became “high.” The lake went from “slightly swimming impaired” to a staff subjective assessment of “no swimming, boating ok.” Still, when compared to all data over the last 10 years, it seems that the severity of the 2007 late summer algae bloom is not particularly unusual. Seven of the last 10 years have been monitored, and the maximum total phosphorus and chlorophyll-a has been higher than 2007 in two of those years.
Trend Analysis
Thirteen years of water quality data have been collected by the Metropolitan Council (1980, ‘81, ’83, ’89, ’94, and’97) and the ACD (1998-2001, 2003, 2005, and 2007). Water quality has not significantly changed from 1980 to 2007 (repeated measures MANOVA with response variables TP, Cl-a, and Secchi depth; F2,10=0.78, p=0.49).
Discussion
While several of the most recent years of monitoring suggested that Linwood Lake may not belong on the Minnesota Pollution Control Agency’s (MPCA) list of impaired waters, 2007 reaffirmed the listing is appropriate. The threshold for listing is summertime average total phosphorus exceeding 40 ug/L, but in 2007 Linwood’s was 47.4 ug/L and a substantial algae bloom persisted throughout late summer.
The primary inlet to Linwood Lake comes from Boot Lake, a scientific and natural area, and it is likely that this water is of high quality. It is likely that factors degrading water quality originate from the lake itself, activities within the roughly 1/3 of the shoreline that is developed, or other portions of the watershed. Threats to this lake may include rough fish , failing shoreland septic systems, poor lakeshore lawn care practices, and natural sources such as nutrient-rich lake sediments. High powered boats may be impacting water quality by disturbing sediments because the lake is large enough for these boats to get up to full speed, but is mostly shallow.