In the Province of Manitoba, Canada lies Lake Winnipeg. It is the 10th largest lake in the world by surface area and is relatively shallow with an average depth of twelve meters. Its watershed is vast, spreading an astounding 950,000 square kilometers. Many rivers feed into the lake, but 60% of water introduced to the lake comes from the Red, Winnipeg, and Saskatchewan Rivers. In recent years, urbanization and agricultural developments have been booming in the watershed of Lake Winnipeg; it is now home to over seven million people and countless agricultural sights.
These facts all contribute to the eutrophication of Lake Winnipeg (Mosquin 1995).
Eutrophication is the overloading of nutrients to a body of water, usually resulting in a dense growth of algae. Algae require light, nitrogen, and phosphorus to grow. However, since nitrogen is abundant and readily absorbed from the atmosphere, phosphorus is usually the limiting factor for algal growth. Phosphorus exists in sediment and animal waste, and primarily enters Lake Winnipeg through its tributaries.
As stated above, Lake Winnipeg has a vast watershed, approximately 950,000 square kilometers, so changes in phosphorus content in the watershed impact the concentration of phosphorus in the lake greatly. In recent decades, nutrient input has skyrocketed. In fact, between 1990 and 2000, the total concentration of phosphorus in the lake doubled. This can have potentially detrimental effects on the ecology of the lake. I will now discuss the changes that have resulted in the eutrophication of Lake Winnipeg, and some strategies to help handle this problem (Bunting 2016).
Microfossils found in the lake sediment show a more complete picture of the history of eutrophication of Lake Winnipeg.
To view the change in the biotic community of the lake over the past two centuries, a team of scientists took a core of sediment from the lake bottom and examined the microfossils within. Their results showed that the population of biota common in algal blooms was steady between 1800 and 1900 but thereafter began to increase dramatically. Between 1900 and the present day, the concentration of algal cells increased by 500%. It is clear that Lake Winnipeg has not always been eutrophic to the extent we see today. Changes in the watershed have led to the current state of the Lake Winnipeg ecosystem (Bunting 2016).
Lake Winnipeg is one of the most eutrophic lakes in the world. In 1969, cyanobacteria contributed to 56% of the biomass in the lake. Since 1994, cyanobacteria account for 90% of total biomass. This decrease in the diversity of the ecosystem is positively correlated with the rise in total phosphorus input. From 1971-to 1980 the phosphorus concentration increased by 56% (McCullough 2012), and in the 90 the concentration doubled (Schindler 2012). Over 90% of inputs to the lake come from four major rivers: Winnipeg, Saskatchewan, Dauphin, and Red River. Historically, the Red River has seen the most dramatic change in phosphorus concentration. While the other rivers average about 40 mg/m of phosphorus concentration, the Red River contributes a whopping 404 mg/m. Over 60% of the total phosphorus comes from this river, and so I will focus on changes to the Red River watershed that have led to increased nutrient loading to Lake Winnipeg (Hall 1999).
A lake is a dynamic and complex ecosystem. Nutrients enter lakes through the rivers in their watershed, which can span many thousands of square kilometers. Two major changes to Lake Winnipeg’s watershed have exacerbated eutrophication in the recent decades: climate change and anthropogenic loading. I will discuss each in turn.
Anthropogenic loading refers to ecosystem changes due to human activities or inputs. As stated above, approximately 7 million people live in the Lake Winnipeg watershed. However, human waste contributes to only a small portion of the increased Phosphorus loading in the lake. It is estimated that animal agriculture in the region contributes as much as a human population of 43 million people (Schindler). Today, more than 650,000 square kilometers of the watershed are devoted to animal agriculture (Bunting). Animal waste is rich in phosphorus, so the runoff from these agricultural areas is one of the main contributors to the eutrophication of lake Winnipeg (Schindler 2012).
Climate change has worked in tandem with anthropogenic loading to exacerbate the eutrophication problem. Spring flooding is a major contributor to the number of nutrients that reach Lake Winnipeg. Since 1995, runoff from the Red River valley has been steadily increasing. Huge floods occurred in 1997, 2005, 2009, and 2011, and it has been shown that years with excessive flooding are directly related to the concentration of phosphorus in the lake. The flooding increases the reach of the Red River, allowing it to flow over terrain that it would not otherwise be able to access (Schindler 2012). In the past, “closed” basins in the watershed were left untouched and were, therefore, able to sequester nutrients such as phosphorus. Precipitation has increased in the region by about 10% in the last century, and total runoff to the Lake has more than doubled. This has amplified the nutrient availability in Lake Winnipeg. The Red River Basin is very broad, making it nearly impossible to control the nutrient input via damming of the river (Donald 2015).
This is an important issue for many reasons. First of all, eutrophication can be very harmful to human health. The overloading of phosphorus often leads to large algal blooms occurring in lakes. Some algae can produce toxins that are deadly to mammals and marine life. This can render a body of water useless for human consumption and can be deadly for people who live nearby (Hall 1999).
Even if the algae is not toxic, large blooms eat up all of the oxygen in the water as they decay. This can cause anoxic conditions, which are very harmful to marine life. Also, when they grow out of control they make the water cloudy and can form piles that have a pungent odor. These blooms are bad for biodiversity, human health, and the ecology of lakes, so it would be in everyone’s best interest if we could get this issue under control (Hall 1999).
Most phosphorus in the lake is contained within the biota. When the algal cells die, they rain down into the bottom of the lake and become part of the sediment. Due to the shallow depth and large surface area, Lake Winnipeg is usually very well mixed. However, the lake does stratify weakly in both the summer and winter. During this time, Oxygen levels in the hypolimnion decrease to around 2.6mg/L. The anoxia in the hypolimnion promotes phosphorus release from the sediment. Small disturbances such as the wind can then easily break the stratification and sweep the phosphorus back into the epilimnion, where it can be used by cyanobacteria once again. This form of phosphorus recycling will delay recovery even if we succeed in reducing total phosphorus input from anthropogenic sources. (Schindler 2012).
Managing the anthropogenic inputs is the first step towards recovery for a eutrophic lake. However, a meta-analysis of 89 different eutrophic lakes shows that recovery is not always possible. In this study, they found that even when nutrient management systems were put in place, 30% of the ecosystems saw no improvement whatsoever. In the lakes that did recover, they found that after complete cessation from anthropogenic inputs, the ecosystem was able to return to baseline after 15 years on average. Partial reduction in nutrient input led to a recovery in approximately 31 years (McCrackin 2017).
Eutrophication is one of the greatest threats to inland water ecosystems. As our population grows, so too does our impact on the environment around us. Freshwater is a vital natural resource, and if we continue on this path of carelessly polluting our waterways then we will be in a serious water crisis. Lake Winnipeg is only one of many marine ecosystems that are suffering from this affliction. Eutrophication affects bodies of water all around the globe, especially in areas with a lot of human development and agriculture. More research needs to be done to find new ways to combat this issue or to find ways to stop the pollution at the source. It is our responsibility to find a solution to this problem so that we can leave a better world for the next generation.