Phosphorus is one of the key elements necessary for the growth of plants and animals and in lake ecosystems, it tends to be the growth-limiting nutrient.
In plants, phosphorus is essential for photosynthesis, respiration, seed production, root growth and other critical functions. Phosphorus in animals is critical for proper bone and muscle growth, metabolism, reproduction, and overall animal performance.
Phosphorus, however, largely limits phytoplankton growth in freshwater systems. Excessive phosphorus loading into lakes and reservoirs can lead to cyanobacteria blooms and potential toxicity.
Effect of flow rates
Reservoirs are impoundments of large rivers that drain extensive watersheds, advective flow, flushing rate, and residence time can also regulate phytoplankton dynamics. For instance, during storm runoff and elevated flow, residence time can decrease to less than a week, while flushing rate increases.
Even though soluble phosphorus loading can be high and available for phytoplankton uptake during these periods, rapid flushing that exceeds the algal growth rate can lead to anomalously low chlorophyll in the reservoir due to washout of the algal community.
During lower flow, residence time increases, while flushing rate decreases. Anticedent phosphorus loads assimilated by phytoplankton during these periods can result in bloom development and chlorophyll increases because the phytoplankton doubling time now exceed flushing rate.
My research measured seasonal variations in phosphorus and chlorophyll concentrations and Secchi transparency in relation to hydrodynamics and nutrient (primarily phosphorus) availability in Tainter Lake. We hypothesized that although phosphorus concentrations would increase in Tainter Lake during periods of high flow, chlorophyll would be abnormally low due to the high flushing rate. Conversely, as flow and phosphorus loading subsided and residence time increased, high phosphorus availability would lead to phytoplankton blooms and chlorophyll increases if algal doubling time exceeded the flushing rate.
Our results showed that during high flow periods, total phosphorus and soluble reactive phosphorus (SRP) increases in the head waters. This is due to runoff flowing into the lakes from a rain storm. Longitudinal gradients decreases total phosphorus and SRP concentration toward the dam. SRP is very high and available for algal uptake. During this time, residence time is low, resulting in a washout of the phytoplankton community. Chlorophyll decreases vastly because the flushing rate increases causing a washout of the chlorophyll.
During low flow periods, total phosphorus and SRP decreases. In fact, SRP declines tremendously near the dam in conjunction with massive chlorophyll increases, signifying uptake for growth. At this time, residence time is much higher and phytoplankton doubling exceeds the flushing rate. Rapid algae blooms occur because the residence time increases and it causes the lakes to become toxic, green and smelly.
Long road to recovery
In conclusion, algal growth can be limited by advection [the transport of something from one region to another] and transport in rapidly flushed reservoirs like Tainter Lake even though phosphorus concentrations are high and available for phytoplankton uptake. In an ideal world, we could wish for more rain in order to continue the washout of phosphorus, but that’s not a definitive solution. My project was designed to give a scientific insight on what’s occurring in the lakes.
I believe that this gives community members a better understanding of not only the environmental issue, but also the steps necessary to take in order to maintain and improve the lakes water quality and beauty. This problem is not pointed at one individual, but a community as a whole has to take responsibility for their own actions so that change can be made.
This REU summer internship has changed the way I look at certain issues. For instance, initially I thought that it could all be solved if farm practices would be conservatory.
As I dug deeper into my research, I realized that the issue is much more complex. The community as a whole has to take responsibility for this issue and look for ways in which they could contribute to a solution. It can be as simple as informing your neighbor about the issue and spreading the word.
For those who live on the shorelines, one might consider investing in buffer so that runoff into the lakes can be limited. For farmers who have adopted conservative practices, it would be beneficial to inform other farms about the lake issue and how they may be contributing to its continuous problem. With everyone educated about the issue, we can someday come to a solution that has both a positive economic and environmental impact. This is only the start to a long road to recovery.