As discussed in our last blog post, Phosphorus in the Environment: Conception, Cycling, and Fate in an Ecosystem, the greatest mechanisms for Phosphorus (P) loss from an ecosystem are erosion and run-off. Once lost from an ecosystem, where does it go? What is the fate of this lost P and what does it mean for Ohio and its waterways if the P makes its way to a stream or lake?
Once in water, the P is generally bound to sediment, which means that it is not readily available for use by plants and microbes. Some of the P, however, is dissolved in water and ready for biological uptake. The P ready for uptake is often called bioavailable P (BAP). BAP provides necessary nutrients for waterborne plants and microbes to grow. When there is too much BAP for plants (water willow, cattail, duckweed, etc.) to utilize, algae and other microorganisms will proliferate due to the excess nutrients that are readily available. This hyper-enriched condition is referred to as eutrophic, and the process of its formation is eutrophication.
Algae thrives in water with high nutrient content, warm conditions, and moderate sunlight. Under normal circumstances, algae is consumed by grazers (macroinvertebrates and fish). These grazers are in turn consumed by predatory fish, many of which are valued as “sport fish.” In this way, algae makes up an invaluable part of the food chain in many waters as a primary producer. There are, however, many kinds of algae with varied modes of survival. As you could probably guess, different species of algae thrive under specific environmental conditions. Under most circumstances, available nitrogen (N) is far more prevalent in ecosystems than BAP, and therefore it is common to have high N:P ratios in plants, soil, and water. Most microorganisms have specific N:P ratios similar in magnitude to the ecosystems in which they thrive. Green algae, the stringy algae with which most people are familiar, has a relatively high N:P ratio and is likely to dominate in most environments. However, with high P inputs from surrounding areas, the N:P ratio tends to decrease, favoring microorganisms with lower N:P ratios (typically lower than 10:1) or organisms that are more competitive for N.
One kind of algae, cyanobacteria (the blue-green algae), can fix nitrogen (N), meaning it can pull N from the atmosphere and turn it into a form usable for itself. This gives cyanobacteria an advantage in those environments with lower N:P ratios because they can counter-balance the excess of BAP. Once cyanobacteria-laden waters receive an excess of BAP due to inputs from surrounding lands, the cyanobacteria can dominate the system and the grazers that consume the algae simply cannot keep up. This is the beginning of a blue-green algae bloom. Once algae begins to bloom, it quickly outcompetes other waterborne microorganisms because of the thick mats they tend to form. Algae often covers entire ponds and streams from bank to bank. Once covered, algae has preferential access to nutrients, sunlight, and even physical space. Additionally, a byproduct of cyanobacteria is a set of compounds called cyanotoxins. These toxins are what turn algal blooms into harmful algal blooms (HABs).
There are many different compounds exuded, and some target a number of different human organs. Microcystins are the main group of compounds used to determine if cyanotoxins are present. These compounds were measured at levels found to be harmful to humans in 2014 in Lake Erie near Toledo, OH. Cyanotoxins can cause liver damage, contact dermatitis, and gastro-intestinal illness. Additional effects are likely to occur, but these have not been well studied. Cyanobacteria in a bloom eventually reach a point in the season when resources, length of daylight, or drops in temperature cause a dieback. The bloom recedes and much of the bacteria die or go into a suspended state forming a cyst which lies in the sediment until conditions are favorable again.
The large, now dead, algal mats left over from the bloom continue to have consequences on water quality long after the bloom has subsided. Like any digestive process, the decomposition of these algal mats by aerobic bacteria requires a large amount of oxygen, creating what is known as a biologic oxygen demand (BOD). Since aerobic bacteria require oxygen to decompose the algae, dissolved oxygen (DO) concentrations can be depleted to levels far below the 2 parts per million (ppm) minimum needed for many macroinvertebrates and fish to survive. This consequence of eutrophication, is a common cause of fish kills around the country. These effects can occur naturally, but are often exacerbated by anthropogenic inputs of nutrients (fertilizers, sediment disturbance, etc.) into these aquatic systems.
It takes time for balance to return to an ecosystem after a large bloom. Often, water flow and dropping temperatures eventually raise DO to a point where populations of macroinvertebrates, fish, and other heterotrophic organisms can recover. If HABs are patchy and more widely distributed, this can occur within 6 months. However, if the HABs are relatively ubiquitous throughout the lake or stream, population recovery could be slow, since fish and macroinvertebrates will need to migrate in from further away. Recovery of these systems can be slow, but nearby, healthy, and resilient systems can help to ensure the eventual recovery of even highly impaired water bodies.
Although aquatic systems can improve after becoming eutrophic, protecting our lakes and streams from eutrophication is critically important. How do we prevent HABs in our aquatic systems, and avoid the associated loss of ecologic function and human and animal health risks? What can be done to limit the frequency and severity of HABs? In our next blog, we will discuss what we, as stewards of the land, can do to keep phosphorus where it belongs: in the soil and not in our waterways! Watch for this in another few weeks!
For additional information on HABs see the Ohio EPA HAB webpage, the Army Corps of Engineers HAB webpage, or the National Oceanic and Atmospheric Administration Great Lakes Environmental Research Laboratory webpage.