Swamp Stomp

Volume 14, Issue 49

After an algae bloom forced the drinking water plant in Toledo, Ohio, to close in August 2014, scientists began to study several components of the Great Lakes habitats in order to further understand the causes of toxic algae. They discovered that the established causes of algae blooms—commercial farm runoff, animal manure, sewage spills, faulty septic tanks, and other sources contributing to the rising phosphorus and nitrogen levels in water—are not the only factors responsible for algal growth.

While invasive species do not provide the initial cause of toxic algae, they do encourage algal growth and worsen the effects of the toxin. The University of Michigan and the National Oceanic and Atmospheric Administration co-authored a paper in the scientific journal Water Resources Research, whereby they assert that the effect of invasive mussels contribute to the development of algae blooms after the initial phosphorus inputs.

After a 20-year absence between 1975 and 1995, toxic blooms have not only reappeared on Lake Erie, but continue to grow larger year after year. While there have been rising levels of phosphorus in the water since the return of the blooms, the lead author of the paper, Daniel Obenour, formerly of the University of Michigan Water Center, claims that other factors are decisive to the growth of toxic algae. He says, “Phosphorus loading doesn’t explain everything.”

David Culver, a retired zoologist from Ohio State University, is one of several scientists who think invasive mussels both spit out microcystis, the most dominant form of toxic algae found in Lake Erie, and excrete the majority of the phosphorus they absorb due to an inability to digest it. Invasive mussels have tiny sensors that identify when microcystis approach, allowing them to eat what they want and spit out what they don’t. Furthermore, their bodies absorb the phosphorus from the water as if they had a nutrient deficiency, however, due to the large amount of phosphorus in the water these days, most of the phosphorus ingested is merely excreted back into the water.

The initial introduction of zebra mussels into the Great Lakes in 1986 seemed to cleanse the water, however, scientists at numerous conferences since then have commented on how the quality of both the lakes aesthetics and delicate ecological balance have backslid. Furthermore, the last decade has seen a mussel abundance in the lakes as quagga mussels, the larger and more adaptable cousin of the zebra mussel, began to appear.

Don Scavia, director of the Graham Sustainability Institute, a University of Michigan aquatic ecologist, and a co-author of the aforementioned paper, claims that this increase in mussels must contribute to Lake Erie’s susceptibility to algae growth. He says, “We’re thinking it may have been the increase in mussels.”

A mussel may seem insignificant to the lake as a whole, but the amount of mussels now inhabiting the lake proves serious. Obenour asserts that these mussels “don’t remove phosphorus from the system; they just change how it is cycled through the system. Now because of changes in the lake, it requires less of a phosphorus load to initiate these blooms.”

The mussels, then, do nothing to cleanse the lake, but merely contribute to the risk of algae blooms. While initial phosphorus levels instigate toxic algae, it is the growing amount of invasive mussels in the water that cultivates the toxins and allows blooms to occur.