Food Web Impacts

Light Fuels Our Lake's Food Web
Some fish, especially during their first weeks of life, feed on tiny organisms called zooplankton which are suspended or swimming weakly in the water column. Unable to swim against currents, zooplankton include a diverse set of species which vary in size. Most zooplankton feed on even smaller drifting plants called phytoplankton, but some eat other zooplankton or even small fish.

 

The combined efforts of diverse phytoplankton species serve as the power plant that fuels life in the lake. Phytoplankton capture the energy in the light particles from sunlight through photosynthesis, and store the energy in organic compounds such as sugars, complex sugars, and oils. Because they are light-dependent, most phytoplankton live near the surface of the water. Phytoplankton play an important role in the aquatic food web as well as Earth’s atmosphere by sequestering carbon dioxide and releasing oxygen. Plankton abundance is heavily dependent on available nutrients. Phosphorus and nitrogen are common limiting nutrients in freshwater.

Mussel Invasion
Then, starting in 1988, zebra mussels arrived in the Great Lakes via ballast water from the international shipping industry. Within a few years, the invasive zebra mussel, native to the Dnieper River in Ukraine, had spread throughout the Great Lakes. Efficient at filtering nutrients from the water column, zebra mussels began to change the nutrient composition of the nearshore coastal zones which resulted in a decrease of food available to Phytoplankton.

 

By 1993, quagga mussels, another aquatic hitchhiker, also native to the Dnieper River and similarly brought to the Great Lakes by ship, had begun to make its mark on the Great Lakes. In 2002, quagga mussels were observed on the Mid Lake Reef Complex in Lake Michigan demonstrating their adaptability to both cold and warm water far exceeded that of zebra mussels, which are only active in warm water. Between 2002 and 2006, quagga mussel colonization was complete in the Great Lakes entirely replacing zebra mussels and far exceeding them abundance.

With an increasing population of highly efficient filterers, quagga mussels began to change the nutrient makeup of the Great Lakes, reducing phytoplakton abundance.

Phytoplankton Loss & Cladophora Growth
Invasive mussels caused a dramatic nutrient shift in the Great Lakes with the diversion of energy from the open water to the lake bottom resulting in decreasing phytoplankton abundance.
These mussel induced shifts have had numerous implications on Lake Michigan’s food web. Many native aquatic species are negatively affected.


An implication of quagga mussel proliferation is that their collective filtration clears the water allowing sunlight to penetrate deeper in the water. Another implication of quagga filtration that you probably know is the smelly seaweed that washes up on shore. This algae, scientifically known as Cladophora, attaches to rocks and hard surfaces in the lakes. Once washed up on shore, the animals in the algae decompose causing a distasteful smell and attracting birds eager to enjoy the feast. The bird excrement on the beach causes E.coli contamination resulting in beach closings.

Benthification
Using their siphons, the invasive zebra and quagga mussels have dramatically changed the Great Lakes by shifting primary production out of the water column and into the bottom (or benthic) zone. As a result, there is an increase in invertebrate food for bottom feeders. This process of shifting the importance and energy flow of the lakes to the bottom is known as benthification. Bottom-dwelling species such as round gobies, also invaders, are well suited to take advantage of this aquatic ecosystem shift. Native to the same region as the invasive mussels, round gobies feed on quagga mussels. The research expedition highlighted in the film led by University of Wisconsin–Milwaukee School of Freshwater Sciences graduate student Abby DeBofsky focused on round gobies and what kind of toxicity they may be accumulating.

-This text written in 2013 by Catherine Simons