Beltrami County. This county-based case study was developed in support of a report entitled âA review of transparency trends in. Minnesota lakes.â It is one of ...
Beltrami County This county-based case study was developed in support of a report entitled “A review of transparency trends in Minnesota lakes.” It is one of several county-based case studies, which provide a brief lake-by-lake description of Secchi and TP trends and potential reasons for the observed trends. Corresponding Secchi (R-generated seasonal Kendall with median, max, and min for each year) and summer-mean TP graphs for each lake are included following the lake-by-lake descriptions. Further details on these charts and overall trend assessments, are found in the report cited above. Beltrami County had eight lakes with significant trends in transparency. Of these, six exhibited significant increases and two significant decreases. Of the eight lakes, there was good correspondence between transparency and TP trends on six of the lakes. We assembled individual comments from MPCA records, MDNR LakeFinder, and observations from Beltrami County Environmental Services (Brent Rud and Bill Patnaude). Increasing transparency Lake Andrusia is a 1,600-acre, deep lake and is part of the Cass Lake Chain. Water levels in the overall system are affected by dam operations on the Mississippi River (MDNR LakeFinder). It exhibited a significant increase in Secchi over time. There is a large break in the data, with the first records in 1976-1979 and the recent data record that begins in 2004. There is a distinct decline in TP from the 1970s to the 2000s data, with the more current record being quite consistent from 2004-2014. The 1970s data reflect the impact of the Bemidji wastewater treatment facility (WWTF), which discharged upstream of the lake. P limits were placed in the NPDES permit for Bemidji in the 1980s and this reduced P loading to downstream lakes. Andrusia was listed for zebra mussels in 2014. Continued increase in transparency is likely as zebra mussels become more established in the lake. Blackduck Lake exhibited a significant increase in transparency based on a fairly continuous record from 1975-2014. TP data from the 1970s (USEPA National Eutrophication Survey) was much higher than the more recent data from the 2004-2014 and reflected the influence of the Blackduck WWTF. Reductions in the WWTF since the 1980s, was the primary reason for the TP reduction and improvement in transparency. Long Lake (4-0076) is a deep, moderate-sized, and spring-fed lake near Turtle River. It exhibited a subtle but significant increase in transparency over time. TP has been collected only since 2004 and shows a slight increase over time; however, this record is rather short and does not provide enough information to discern if there is a trend. Wolf Lake is located upstream of Andrusia and both are linked by the Mississippi River. It exhibits a pattern similar to Andrusia and would also be affected by Bemidji’s WWTF. It also was listed for zebra mussel in 2014. Medicine is a 460-acre, spring-fed lake of moderate depth. The lake exhibits a significant increase in transparency but has a rather short record with six summers of data from 2007-2014 and one complete summer in 1975. TP data is very limited and does not provide an indication of a trend. Movil is an 853-acre lake of moderate depth located near Bemidji. It is part of a chain of lakes and has a large watershed. The lake exhibits a significant increase in transparency over time. While most of the record is from 2000-2014, there are summer data from 1974 and 1990 as well. TP data is from the period 2000-2014 and a decline over time is noted. Decreasing transparency Marquette is a 527-acre lake of moderate depth. The lake exhibited a decreasing trend in transparency based on a continuous record from 1998-2014 and summer 1975. TP data, available for 2007-2014, are not sufficient for trend assessment; however, there is good correspondence among TP, Chl-a, and Secchi for the summers with data. The lake has a large watershed and during periods of high precipitation, lake levels can be quite elevated (Brent Rud, Beltrami County Environmental Services, personal communication). These large events carry high phosphorus loads that could wq-s2-08c
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lead to increased algal growth and reduced transparency. Excessive shoreline erosion could be a contributor as well. The PHDI and precipitation records (Climate Division 1) from 1993-2011 indicates high soil moisture and several summers with above normal precipitation. Long Lake (4-0227) is a large, but shallow lake with (over 90% littoral) near Puposky. It exhibited a significant decline in transparency over time based on a record from 2000-2014. TP exhibited an increasing trend over the same period.
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Figure 1. Transparency and TP trend charts for Beltrami County case study. Increasing transparency Andrusia
Blackduck
Long
3
Wolf
Medicine
Movil
4
Decreasing transparency
Marquette
Long (4-0027)
5
