We all know there is nothing better than wading into a stream as the crystal clear water ripples past us, carrying our flies down with it, in hopes that we may get a bite. Unfortunately, in today’s age, these crystal clear streams are becoming less common, with those in the upper Midwest struggling to hold on to their standard of health. The process of maintaining healthy, aquatic ecosystems is under-regulated, leading to devastating effects down stream from the input of excess nutrient pollution. Hopefully this article re-ignites your passion, not only for hauling in big fish, but for appreciating and protecting our natural aquatic ecosystems in the process.
An ecosystem is loosely defined as the interaction between all organisms in an area combined with the relationship between organisms and the habitat. This means that in streams we not only take into consideration the individuals interacting within the food webs (trophic system) but also how the habitat is contributing to those interactions. Productivity of a stream (production of algal biomass via photosynthesis and sequestration of carbon) is highly dependent on interactions between organisms and surrounding habitat because other nutrients that help to create algal biomass may be limiting factors in the organism’s growth, such as nitrogen and phosphorus. This means that although the organism may have sufficient amounts of one nutrient, another may be limiting (in high demand and low concentrations) which will halt the potential growth of the organism.
Eutrophication is the hyper-productivity of an aquatic ecosystem, which commonly leads to low levels of oxygen dissolved in water. This is the outcome when aquatic systems have high levels of nutrients dissolved in the water, such as nitrogen and phosphorus, which stimulate the growth of primary producers (algae).
This is generally thought to be a good thing for ecosystems; as larger populations of primary producers will photosynthesize releasing oxygen and also serve as a food source for zooplankton; and more zooplankton means more food for baby trout. Eventually this boom in growth at the bottom of the food web will run out of the high nutrient source, leaving these populations to starve out and die. This high volume of dead organisms descend down the water column and are broken down by microbes via respiration using oxygen and energy, thus depleting the amount of oxygen dissolved in the water. This cycle viciously strips valuable dissolved oxygen from the aquatic habitat as time goes on. As you know, many of the larger organisms that reside in the aquatic habitat, (trout for example) rely on high levels of oxygen to be able to persist, grow and ultimately reproduce. When oxygen levels become depleted, these tasks become much more difficult and force organisms to find new habitats that can sustain their needs, altering our previously defined image of an ecosystem with all of its parts living in sync.
Readers may be questioning why it is important for us in Wisconsin to worry about this phenomenon if we still have high quality streams persisting. As mentioned previously, eutrophication is commonly driven by excess nutrients being loaded into the aquatic system. Wisconsin is especially susceptible to this occurrence, as it is a large agricultural state that uses fertilizers with high nutrient concentrations. In the spring when we generally have high amounts of precipitation and snow-melt, we also tend to be prepping our fields for planting by adding fertilizer. Large amounts of fertilizer can end up being released into surrounding streams via this wet period, loading in nutrients like nitrogen and phosphorus in large pulse concentrations. We can see the effects of this in our own backyard, when streams empty into low flow areas like lakes and reservoirs, threatening the possibility of using these areas for recreation, fishing, or as a water source. We can see much larger effects of the same phenomena, when we travel downstream of the Mississippi River and see the accumulation of these nutrients in delta areas like the large hypoxic (“low oxygen level”) zone on the coast of the Gulf of Mexico.
Why is it important?
Though the future of our aquatic ecosystems may seem dismal, we are also starting to engage more concerned individuals, who are researching solutions to maintain healthy aquatic ecosystems by assessing management processes that help to prevent excess nutrients from entering our streams and contributing to explosive growth in algal biomass which threaten other aquatic organisms.
Unfortunately, in today’s age, these crystal clear streams are becoming less common.
My research focus has been to see how these nutrients, specifically phosphorus (P), cycle through the water column. This is important information that can be used to inform others about where P is getting taken up in streams and ultimately for methods to better control P input from agricultural runoff based on stream type.
During the summer of 2015, I conducted an experiment in the beloved Spring Coulee Creek in Coon Valley looking at relative P uptake rates between different types of substrate found in streams. For the purpose of this study substrate was defined as anything that could be found in the water column that took in soluble reactive P. When surveying the site area it was noted that macrophytes (aquatic vegetation), filamentous algae, sand, and periphyton (algae attached to rocks) were the major substrates in the stream. I was able to mimic stream conditions by creating microhabitats in 30L plastic totes in order to keep environmental factors equal across all treatment types.
The experimental design shows that each tub was submerged into the stream to control for temperature, open to air and sunlight to control for environmental conditions, and mixed with a power head to mimic flow. In order to assess uptakes rates of phosphorus of each substrate, each tote, 30L of stream water (control) or 30L stream water + substrate type, was allowed to mix and settle to its natural condition in the stream. A known amount of P was added to each tote, and in timed intervals, water samples were collected and stored on ice for later analysis of P concentrations. At the end of the trial all of the substrate in the tote were collected and stored to be later weighed for organic mass because, in order to be able to compare uptake across all substrate types, we needed to have a “unit” that applied all substrate types equally. For this we looked at the overall rate of uptake for each 30L tote and divided it by how many milligrams of organic matter were in the tote (show equation). This helped to normalize results so that we were comparing the amount of uptake per mg of carbon, or uptake by mg organic matter, in the sample. Results averaged from three different trials show that filamentous algae was found to take up P from the stream at significantly higher levels than all other substrates and that sand was actually shown to release phosphorus into the water.
This data can be considered extremely helpful from an application standpoint, allowing us to manipulate areas of streams that might be heavily exposed to nutrient runoff from landscapes around them. For example, if we have a stream adjacent to agricultural fields, it may be a good idea to incorporate native species filamentous algae downstream from possible nutrient input areas (field run off or cow crossings) to create a “buffer zone”. These “buffer zones” physically slow down the P pulses from flowing down stream and can decrease the size of the pulse by being excellent areas for other organisms, (that may uptake P as well) to inhabit. Along with this, filamentous algae can then be reused later as a fertilizer for more crops. Everybody wins!
Although consumers of products with high P concentrations have started to take responsibility and change the way they are applying them, it truly does take a community to protect a natural ecosystem. I sincerely hope the next time you have the opportunity to step into one of our world class streams, or any body of water, you not only thank the fish for the fun you are having, but also the amazing vegetation and smaller parts of the stream responsible for keeping the fish and ecosystem healthy.