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Big Pelican Lake Chlorophyll-a

Chlorophyll-a is the pigment in plants and algae that makes them green. Chlorophyll-a is a measurement of algal concentration in the water. Chlorophyll a concentrations greater than 10 ug/L are perceived as a mild algae bloom, while concentrations greater than 20 ug/L are perceived as a nuisance algae bloom.

more information on Chlorophyll-a

Pelican chlorophyll-a at site 206 has been monitored from 1996 to 2011, and has an average of 5.2 ppb. A trend analysis showed that there has been no trend in the data over this 15 year period, meaning that the algal concentration hasn't gotten better or worse, it has just stayed the same.

2011 Results

In 2011, all three lake sites were monitored for algal concentration (chlorophyll-a). All chlorophyll-a results at sites 201, 205, and 206 were similar. From these results, we can conclude that the algal concentration is fairly uniform across the lake. The 15-year average chlorophyll-a for Site 206 is 5.2; however, the average for 2011 among all sites were slightly lower to the historical average.

Site 205 had the highest chlorophyll-a concentration, which makes sense because the Pelican River is the largest contributer of phosphorus to Pelican Lake. Phosphorus feeds algae. The Pelican River enters Pelican Lake on the east end. For results from each individual site, please see below.

Table 2. 2011 Chlorophyll-a results.

In 2011, chlorophyll a concentrations at all sites 201, 205, 206 never reached 10 ug/L, indicating no minor algae blooms throughout the summer.

Site 206

Chlorophyll a was evaluated at site 206 in Pelican Lake from 2002-2011. Chlorophyll a concentrations start out low in June and then increase to a peak in mid to late August (see figure above). This pattern follows the phosphorus concentration because phosphorus is food for the algae.

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.

Site 201

Site 201 was added in 2007 to see if the algal concentration 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.

This pattern seen at site 201 follows typical algae cycles. In May, the water is cold and algae abundance is low. In June the water warms up and nutrients are available so the algae multiply. In late June zooplankton (tiny crustaceans that feed on algae) populations are high, temporarily decreasing the chlorophyll a. In late summer the zooplankton populations are reduced by small fish predation, the weather is warm and the sun is strong, causing algae populations to increase again.

Site 205

Site 205 was added in 2007 to see if the algal concentration was different on the east end of the lake due to the Pelican River inlet flowing through Little Pelican Lake. The average chlorophyll-a at site 205 in 2011 was 4.7 ppb. The chlorophyll a is slightly higher at site 205 than sites 206 and 201. It could be higher due to the fact that the Pelican River is the largest source of phosphorus loading to the lake, and phosphorus is food for algae.

This pattern seen at site 205 follows typical algae cycles. In May, the water is cold and algae abundance is low. In June the water warms up and nutrients are available so the algae multiply. In late June zooplankton (tiny crustaceans that feed on algae) populations are high, temporarily decreasing the chlorophyll a. In late summer the zooplankton populations are reduced by small fish predation, the weather is warm and the sun is strong, causing algae populations to increase again.

to next page: Big Pelican Total Phosphorus

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