Hennepin County

This county-based case study was developed in support of a report entitled “A review of transparency trends in lakes.” It is one of several county-based case studies, which provide a brief lake-by-lake description of transparency and TP trends and potential reasons for the observed trends. Corresponding transparency (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.

Hennepin County had 19 lakes with significant trends. Of these, 14 had increasing transparency and five were decreasing. Eurasian water milfoil was a common exotic among these lakes and was present in 13 lakes. In most cases, confirmation dated to the late 1980s and mid 1990s. Zebra mussels have been present in Lake Minnetonka since about 2010. In general, there was good correspondence between transparency and TP trends for 13 of 14 lakes with increasing transparency. Of the five lakes with decreasing transparency, there was an increase in TP over time in four. Summary comments were derived from TMDL reports, communication with MPCA project managers, and local resource managers including Three Rivers Park District (TRPD; Rich Brasch and Brian Vlach).

Increasing transparency

Lake Calhoun is a large, deep lake located in and is the focal point for the Minneapolis Chain of Lakes Regional Park. It exhibits a significant increase in transparency based on a continuous record from 1988-2014. These data are augmented by sporadic measures dating to 1971. Much of the increase in transparency occurred from 1990 through the early 2000’s. The TP record is continuous since 1988 and a decrease over time is evident. Measurements from 1993 and forward are characterized by lower within year variability (as indicated by standard error), in contrast to measurements prior to this time. TP has been low and rather consistent from 2001-2014. Eurasian water milfoil has been in the lake since 1989 and curly-leaf is present as well. There have been numerous watershed projects and in-lake measures implemented over time and it appears these measures have been successful in reducing in-lake TP, Chl-a, and helping to maintain high transparency.

Wirth Lake is a small lake in Golden Valley and is the focal point for the Theodore Wirth Park. The lake was placed on the impaired waters list in 2004.The lake exhibits a significant increase in transparency based on a continuous record from 1992-2014 and some additional data from the 1970s. An inflection point in the Transparency readings occurred near 1999; whereby there was a distinct increase thereafter. Transparency reached an asymptote in about 2008. TP was very high in early 1990s but decreased steadily over time and leveled off in about 2008. Eurasian water milfoil has been present since 1996. A formal watershed and management plan was developed for Bassett Creek Water Management Commission in 1996 that described several proposed projects to reduce nutrient loading to the lake. An approved TMDL report was completed in 2010. Implementation projects were completed and the lake was delisted from the Impaired Waters list in 2014.

Lower Twin Lake is a small but deep lake at Crystal and is part of a chain of lakes: Upper, Middle and Lower Long Lakes and Ryan Lake. Its transparency was rather stable at ~2 feet until about 2000 when a marked increase was noted. Since that time, mean transparency is ~4 feet. TP data suggest a decrease over time from hypereutrophic to moderately eutrophic by 2003-2012. Eurasian water milfoil was confirmed in 2006. All four of the lakes are on the impaired waters list and a TMDL implementation plan was developed as part of the Shingle Creek Watershed Commission Watershed TMDL. Watershed implementation was completed and the lake was delisted from the Impaired Waters list in 2014.

North Anderson is moderate-sized but very shallow lake at Eden Prairie and associated with the Three Rivers Park District. The recent record indicates a significant increase in transparency since 2007, accompanied by a distinct decrease in TP over the same time. Transparency data from 1981 suggests the lake had extremely low transparency in earlier years.

wq-s2-08j 1 Mitchell is moderate-sized, shallow lake at Eden Prairie (Riley-Purgatory WD). The lake winterkills periodically with the most recent in 2010. Transparency increased significantly over time with the largest increase since 2008. TP exhibited a significant linear decrease since about 1999, with recent measures in the mildly eutrophic range. The lake was confirmed for Eurasian milfoil in 2002.

Diamond is a large but very shallow lake at Rogers. Because of its shallowness, it does not appear to be managed actively as a fishery. The watershed is undergoing a shift from rural/agricultural to urban land uses. The lake exhibits a significant increase in transparency that is most marked from 2000-2010. TP exhibits a distinct decrease over time since 2000 and has shifted from hypereutrophic to moderately eutrophic conditions by 2013. Diamond is on the impaired waters list and a TMDL report was drafted in 2015 for it and other lakes in the Elm Creek Watershed. Diamond Lake is affected by heavy curly-leaf pondweed growths in some years. Some of the recent years exhibiting good water quality have corresponded with curly-leaf herbicide treatment efforts by the lakeshore residents. The treatments were intended to be localized but may have had a lake-wide treatment effect. Bullheads are abundant in the lake as well. Anecdotal observations indicate the lake has transitioned from algal-dominated to plant-dominated condition in recent year.

Lake Minnetonka is an assemblage of several basins/lakes and is the source of Creek. Several bays on Lake Minnetonka exhibit significant increases in transparency. This includes Lower Lake, Upper Lake, Carson’s Bay, Crystal Bay, and Maxwell Bay. Eurasian milfoil was first confirmed in 1987 and is likely present in most of the bays. Zebra mussels were first confirmed in 2010 and spread across most, if not all, of the bays since that time.

While there has been a significant increase in transparency in all five bays, the length of record and trajectory (slope) varies among the lakes. The Lower Lake, for example, increased from 5-7 feet in the 1970s to 15-17 feet in 2012-2014. A decrease in TP is evident for this period with values ranging from 50-60 ppb in the 1970s to 18-20 ppb in 2012-2014. The Upper Lake has an equally long record but transparency increase is more subtle and peaks at about 10 feet. TP decreased as well, and was in the 20-30 ppb range in 2012-2014. Carson’s Bay has a shorter record with a steep increase in transparency from 2006-2014. TP decreased from 35 ppb to 15 ppb over this same period. TP reduction in the 1970’s and 1980’s was attributed, in part, to the removal of WWTF discharges (e.g., Long Lake and Maple Plain) that previously drained to Lake Minnetonka.

South Whaletail is moderate-sized and shallow and is the southern portion of Whaletail Lake at Minnetrista. The watershed is predominately rural and includes Little Long Lake, which has very good water quality. Both North and South Whaletail are on the impaired waters list. South Whaletail had a subtle but significant increase in transparency since 1990. TP was quite variable based on a record that dates to 1999. It was confirmed for Eurasian milfoil in 1996. TMDL development for both lakes was underway in 2015 and the draft TMDL indicates internal recycling of P may be a major source.

Ryan is a small and shallow lake at Brooklyn Center. It exhibits a significant increase but highly variable transparency over time. TP has decreased over time and most recently is in the 40 ppb range. It was on the impaired waters list and included in the Shingle Creek Watershed Commission Watershed TMDL. Successful watershed implementation led to delisting of the lake in 2014.

Schmidt is a small and predominately shallow lake at Plymouth. It was confirmed for Eurasian milfoil in 1990 and the lake has curly-leaf as well. It exhibits a significant increase in transparency over time but is somewhat cyclic. Reports from TRPD indicated summer aeration had been implemented by the lake association in the years when they monitored the lake (2004-2008). This aeration may explain the cyclic nature of the transparency record, however, this requires knowledge of when it was put in place and during which summers it was used. With that knowledge, the trend data (transparency and TP) could help discern the efficacy of this in-lake management practice.

TP has decreased since 2000 but has risen lately from 2011-2014 and Transparency has decreased accordingly. The lake was placed on the Impaired Waters list in 2002 and a TMDL implementation plan was developed by the Shingle Creek Watershed Management Organization. Successful implementation led to delisting the lake in 2014.

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Decreasing transparency

Diamond (27-0022) is a moderate-sized but very shallow lake in Minneapolis. The lake is not actively managed as a fishery and may be better described as a wetland. The lake has exhibited a significant decrease in transparency over time, with peak measures in 1990-1995 and decreasing since then. There is an extensive TP record but no significant trend. TP has generally remained in the hypereutrophic range over the 26-year record.

Fish is a small but relatively deep lake at Maple Grove. The lakes’ watershed has been fully built-out, with medium density residential as the dominant land use. The lake was confirmed for Eurasian milfoil in 1993. It exhibits a significant decrease in transparency based on a record that extends to 1973. Peak transparency was in the 1980-1990 period, with a decrease thereafter. While mean transparency is typically 3-5 feet, the range between the annual minima and maxima is very large. Based on a continuous record from 1997-2014 there is no significant change in TP with most measurements from 40-50 ppb. Fish Lake is on the impaired waters list and is addressed in the Elm Creek Watershed Management Commission TMDL report. Internal P recycling was identified as a significant source of P to the lake in the TMDL report.

Half Moon is a small shallow lake in Baker Park Reserve at Loretto. The lake has a huge watershed: lake area ratio 110:1 and a variety of land uses in the watershed. The lake exhibited a significant decrease in transparency based on a record that dates to 1991. In the 1990s, transparency averaged 5-10 feet as compared to 3-5 feet in 2008-2014. TP data from 2004-2014 indicates distinct increase in TP, with all measures in the hypereutrophic range. Upstream Spurzem Lake has been identified a large external source of TP to Half Moon Lake. Spurzem Lake, in turn, receives extensive P loading from its watershed as well as stormwater from the City of Loretto and the upstream discharge of the Loretto WWTF. The lake will likely be included on the 2016 impaired waters list. In anticipation of this, it was included in Pioneer-Sarah Creek Watershed TMDL and WRAPS project.

North Little Long is a small but deep lake at Minnetrista and is part of Three Rivers Park District. It has a very small watershed. TRPD recently acquired land surrounding much of the lake and is restoring it to natural condition. The lake was confirmed for Eurasian milfoil in 1991. The lake exhibited a significant decrease in transparency with measures dating to 1980. The steepest decrease was from 2001-2010, with an increase since that time. TP has increased over this period as well, with a shift from borderline oligotrophy to mesotrophy. It is important to acknowledge that even with these changes transparency remains very high and this allows for extensive and diverse rooted plant growth across the lake. The lake was included in Pioneer-Sarah Creek Watershed WRAPS project and this, combined with recent TRPD initiatives, will lead to protection for this lake and a reversal of the observed transparency and TP trend.

Northwood is a small shallow lake at New Hope. It exhibited a significant decrease in transparency from 2000-2012. TP has remained consistently in the hypereutrophic range over this period and no distinct trend is evident. All values are in the hypereutrophic range. The lake is on the impaired waters list and Bassett Creek WMC and City of New Hope have been developing implementation plans to reduce nutrient loading to the lake. TRPD is contributing to this effort by monitoring runoff from an 835-acre subwatershed (45% of Northwood’s watershed) in Plymouth. These data will be valuable for TMDL development and eventual implementation.

Cedar Island is a moderate-sized but very shallow lake at Maple Grove. The lake exhibited a significant decrease in transparency based on a record that dates to 1973. Based on a continuous record from 1990-2014, the steepest decrease was from 2002-2010 (went from 2 feet to 1 foot), with a slight increase thereafter. There was minimal TP data, but the available data indicated an increase. All measures were in the hypereutrophic range.

3 Figure 1. Transparency and TP trend charts for Hennepin County case study.

Increasing transparency Calhoun

Wirth

Lower Twin

4 North Anderson

Mitchell

Diamond

Lake Minnetonka - Lower Lake

5 Minnetonka - Upper Lake

Minnetonka - Carson’s Bay

Minnetonka - Crystal Bay

6 Minnetonka - Maxwell Bay

Ryan

Schmidt

7 South Whaletail

Decreasing transparency Diamond

Fish

8 Half Moon

North Little Long

Northwood

Cedar Island

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