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Biogeochemistry

Turnover Pattern and Mixing

Due to the fact that Mille Lacs Lake is fairly shallow, it mixes fairly regularly throughout the ice-free period, making it a cold polymictic lake that is not stratified ^{[1] [2]}. Constant mixing keeps oxygen and other nutrient levels fairly homogenous, but thermoclines have been known to form in the late summer, which can be associated with decreases in deeper dissolved oxygen levels ^[3].

Trophic State

Mille Lacs Lake has an overall trophic state index (TSI) of 43, making it moderately mesotrophic ^[4]. This trophic state index is within the expected overall TSI range of lakes within its ecoregion ^[4].

Nutrient Levels

Compared to other lakes within the ecoregion, the total phosphorus, chlorophyll a, and Secchi depth are all within expected ranges, while chloride, total suspended solids, and conductivity levels are all above their expected ranges ^[3]. Higher levels of planktivores affect food and nutrient availability needed by other species within the lake. ^[5]

Low total nitrogen to total phosphorous ratios indicate that the lake is phosphorus limited, which combines with relatively low total Kjeldahl Nitrogen to largely limit algal bloom growths ^[3].

Mediators of Eutrophication

Zebra mussels, an aquatic invasive species, became established in Mille Lacs Lake around 2005. ^[6] The presence of zebra mussels has reduced the trophic state index of the lake, with Secchi depth readings increasing noticeably since their arrival, likely due to their methods of filter feeding on suspended particles. ^[6] A sharp rise in water transparency of the lake after the year 2010, measured via Secchi depth, coincides with the zebra mussel invasion timeline. ^[5] As the water clarity is increased within the lake, levels of aquatic vegetation may also increase. ^[5] This has the potential to alter fish habitats, proving desirable for some species' productivity and undesirable for others'. ^[5] It is common for increased water clarity to contribute to decreased productivity and abundance of walleye, which is most commonly fished in the lake. ^[7] Zebra mussels are also known for taking in nutrients from the water column and sequestering it in sediments, thereby reducing the nutrients available to primary producers within the water column. ^[8] Levels of phosphorus and chlorophyll a, however, are reported to have remained consistent since the invasion, which has been seen before in unstratified lakes. ^{[2] [9]}

Historical Effects of Human Activity

Runoff caused by the growth of agriculture in the surrounding area, as well as other anthropogenic factors, caused increasing levels of sediment and phosphorus buildup from the 1960's until at least the early 2000's. ^[10] This was evident from the analysis of a sediment core drawn from the lake in 2002, which also determined that while few new microbial species had established themselves in the lake since European settlement, microbial communities have still been affected by nutrient and sediment loading. ^[10] In particular, the sediment core demonstrated the decline of bottom dwelling microbes in favor of microbial taxa that float freely in the water column, likely as a result of increased phosphorus availability within the pelagic zone of the lake. ^[10] Note that this sediment core was taken before the introduction of zebra mussels, which are known to increase the flux of nutrients from pelagic environments to sediments. ^[8]

Ground-water can also be an indication of anthropogenic effects. Ground-water in the surrounding areas of Mille Lacs Lake occurs in glacial and bedrock aquifers. ^[11] Water from these aquifers is released through well withdrawals, escape into nearby aquifers, and flow into bodies of water like Lake Mille Lacs and the Rum River. ^[11] Upon inspection of this groundwater in the late 1990’s, sodium, manganese, and iron levels repeatedly exceeded acceptable EPA standards for health and drinking water. ^[11]Multiple common pesticides, trace metals, and volatile organic compounds were also found in samples, but were below detection limits, except for zinc levels. ^[11] Possible sources of groundwater contamination within lakes due to anthropogenic effects can include but are not limited to land usage and commercial, industrial, and agricultural activities. ^[11]

References