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Pelican Lake Water Transparency

more information water transparency

Pelican water transparency at site 206 has been monitored from 1996 to 2011, and has an average of 13.0 feet. A trend analysis showed that there are no trends in the data over this 15 year period, meaning that the Secchi readings are not getting better or worse, but staying the same.

2011 Results

In 2011, all three lake sites were monitored for water transparency with a Secchi disk. The 15-year average Secchi depth is 13.0 feet; therefore, the averages in 2011 were similar to the historical average. For results from each individual site, please see below.

The transparency varies year-to-year due to changes in weather, precipitation, lake use, flooding, temperature, lake levels, etc. Sites 206 and 205 in Pelican Lake have been monitored from 1996-2011, and Site 201 has been monitored from 2007-2011. The annual means for Pelican Lake range from 9.6-15.4 ft. The figure below shows that Site 206 in the middle of the lake consistently has the best transparency in the lake. Site 205 on the east end of the lake consistency has the lowest transparency, which makes sense because the Pelican River is the largest contributor of phosphorus to Pelican Lake . The Pelican River enters Pelican Lake on the east end. Transparency was highest in 2002, 2006, 2008, and 2011. Transparency was lowest 1998, 2001, 2003 and 2007.

Pelican Lake transparency ranges from 6.5 to 26 feet throughout the summer. The figure above shows the seasonal transparency dynamics. Pelican Lake transparency is highest in May and early June and then declines throughout the summer. After the lake turns over in September the transparency improves in late September - October. This pattern is typical for a lake of this depth in Minnesota. The transparency dynamics have to do with algae population dynamics and lake turnover.

It is helpful for lake association members to understand the seasonal transparency dynamics so they don't worry when the transparency declines in late August. Figure 4 can show them that it is typical for the lake to decline in late August when the algae are most abundant, and that the transparency then rebounds in October after fall turnover.

Site 206

Site 206 has been monitored from 1996 to 2011.

Tracking trends in data over several years can indicate improving or declining water quality. Generally, it is best to have 8-10 years of data with 4 or more measurements per year to be confident in an emerging trend. Statistically, the probability that a trend is truly describing the water quality and not just a random trend is important. A probability over 90% is required by the MPCA to really be confident in the observed trend. This means that there is a 90% probability that the trend is a true pattern in the data and a 10% probability that it is a random pattern.

The median transparency at this lake from 1972 to 2010 increased by 0.00 feet per decade. Given years, there is no evidence yet of a long-term trend in either direction. A plausible range for the long-term trend is between no trend and an increase of 0.36 feet per decade.

Site 201

Site 201 was added in 2007 to see if the water transparency was different on the west end of the lake due to two inlets. Bob Creek and Spring Creek both flow into Pelican Lake on that end. The average Secchi depth at site 201 in 2011 was 13.3 feet, nevertheless site 201 had similar secchi averages as site 205 and 206.

Site 205

Site 205 was added in 2007 to see if the water transparency was different on the east end of the lake due to the Pelican River inlet flowing through Little Pelican Lake. The average Secchi depth at site 205 in 2011 was 13.6 feet, which was slightly less than site 206. This is most likely because the Pelican River is the largest contributor of nutrients to the lake.

to next page: Big Pelican Lake Chlorophyll-a results

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