AS HEARD ON THE GCN RADIO NETWORK! RADIO LISTENERS RECEIVE 5% OFF CERAMIC FILTER SYSTEMSAs heard on GCN! 5% off CERAMIC FILTER SYSTEMS
menuclose
Fast and Free Shipping on orders over $50!
questions? call 877-992-3753 or visit helpful resources >>

Clear Water in Lakes May Actually Mean Poor Water Quality

Don't judge a book by its cover, and by the same token, don't judge the quality of a lake by its clarity.

Lakes dotted around agricultural hotspots typically tend to be bright green in color. This is largely due to phytoplankton and algal growth fueled by nutrients (nitrogen and phosphorus) present in agricultural fertilizers, which get washed into rivers and lakes with runoff.

However, after analyzing water quality data collected from 139 lakes located in agricultural hotspots of Iowa over a 13 year period, scientists found that even though lakes had high concentrations of nutrients, they were remarkably clear.

Aerial view of a small lake near the city of Clear Lake, Iowa, which represents typical landscapes surrounding the lakes in this study. About 92 percent of land within Iowa is in production agriculture and crops on these lands receive large amendments of nitrogen as anhydrous ammonia and phosphorus. Excessive algae growth caused by these nutrient inputs have turned many of the lakes in this region bright green. Surprisingly, a number of lakes in this study were clearer and appeared bluer than expected, yet are far from healthy. The study authors hypothesize that very high nitrogen levels, often >10 mg/L, suppress high chlorophyll (algae) concentrations. Aerial view of a small lake near the city of Clear Lake, Iowa, which represents typical landscapes surrounding the lakes in this study. About 92 percent of land within Iowa is in production agriculture and crops on these lands receive large amendments of nitrogen as anhydrous ammonia and phosphorus. Excessive algae growth caused by these nutrient inputs have turned many of the lakes in this region bright green. Surprisingly, a number of lakes in this study were clearer and appeared bluer than expected, yet are far from healthy. The study authors hypothesize that very high nitrogen levels, often >10 mg/L, suppress high chlorophyll (algae) concentrations.

The study, which was recently published in the scientific journal Inland Waters, shows that the excessive fertilizer added to the lakes from agricultural runoff was so high that it killed chlorophyll containing phytoplankton and algae, which typically give polluted lakes the bright green color.

According to lead author, Chris Filstrup, a research associate at the UMD Large Lakes Observatory and Minnesota Sea Grant, it is dangerous to mistake an increase in water clarity for an improvement in water quality, as in actual fact, the opposite is very often true. Water quality in clear lakes with high levels of nutrients is worse than that of lakes where more algae is present, yet nutrient levels are lower.

Water clarity is often used to measure water quality, yet this study suggests this approach may not necessarily be appropriate for all regions.

Nutrient levels rise to excessive levels after nitrogen and phosphorus from surrounding agricultural fields, animal feed lots, suburban gardens and urban landscaping gets washed into rivers and lakes with rain and melted snow. Yet, while these nutrients generally spur algal growth, they can in fact kill algae when concentrations become excessive.

"In some of the Iowa lakes in our study we noted phosphorus levels 10 times what we'd expect to see in a northern Minnesota lake," said Filstrup. "We were astonished to see that the nitrogen levels were more than 30 times higher."

When nutrient levels become so extreme they kill phytoplankton and algae present in the waterbody, making the lake appear clearer. In the same way that applying excessive amounts of fertilizer to soils can harm plants, rendering the soil barren, excessive amounts of nutrients in freshwater systems can kill water plants, effectively ridding lakes of algae, thus improving water clarity.

"We thought that the low appearance of algae at high nitrogen concentrations might be due to imbalances of other nutrients, or too much shade for algae to grow, or that some algae are less green or that zooplankton eat more algae when there's a lot of nitrogen," said co-author John A. Downing, director of Minnesota Sea Grant, a scientist at the UMD Large Lakes Observatory and a professor in the UMD Department of Biology. "But none of those hypotheses panned out. The only explanation that makes sense, so far, is that high nitrogen is bad for algae."

According to the scientists, the decrease in algae in these lakes is most likely caused by an interplay of nitrogen, phosphorus, sunlight and the landscape, which in combination can cause the excess nitrate particles to form reactive oxygen species that burst the cell walls and cell membranes of algae, damaging or killing them.

According to Filstrup, its a bit like pouring hydrogen peroxide onto a wound. The hydrogen peroxide bursts the bacteria, making the wound fizz. A similar reaction occurs within lakes, but while there is no fizz, the reactive oxygen species that forms from nitrate can kill organic matter, including algae and phytoplankton.

As the increased demand for agricultural crops continues to rise, along with the application of fertilizers to stimulate rapid growth of commercially produced crops, the scientists hope their study will provide some insight to other agricultural regions where extreme nutrient loading may be a cause for concern.

Downing suggests that excessive application of nitrogen based fertilizers is not only a waste of money, it also leads to unhealthy freshwater systems, and ultimately also causes ocean dead zones such as that in the Gulf of Mexico. It is therefore important that we grow crops and manage animals wastes appropriately to avoid polluting and degrading the environment.

Journal Reference

Christopher T. Filstrup, John A. Downing. Relationship of chlorophyll to phosphorus and nitrogen in nutrient-rich lakes. Inland Waters, 2017; 1 DOI: 10.1080/20442041.2017.1375176

Leave a Reply