By Kevin Bunch, IJC
Invasive zebra and quagga mussels are taking nutrients that would otherwise be in deeper waters and shunting them closer to the shore, which could make it more difficult to halt harmful algal blooms.
Known as the “nearshore nutrient shunt,” this migration of phosphorus and other nutrients used as food by plankton has led to some severe negative impacts in the existing Great Lakes food web. Algae, particularly Cladophora which grows on the hard surfaces near the mussels and feeds on the nutrients the mussels excrete, are thriving in those nearshore regions where nutrients are stockpiling.
The shift in nutrient locations also has benefitted other species that prefer nearshore and benthic – or lake floor – environments, according to Dr. Harvey Bootsma, associate professor in the School of Freshwater Sciences at the University of Wisconsin-Milwaukee.
Historically, a greater chunk of phosphorus entering the lakes has found its way offshore, where it serves as food to phytoplankton. Those in turn are eaten by zooplankton, and fish can feed on both types of plankton, as well as other aquatic species that eat them. Some phosphorus ends up finding its way into the benthic level, periodically getting kicked back up dissolved into the water, where it can continue to serve as fertilizer for phytoplankton.
With invasive mussels, more phosphorus is staying in the nearshore environment, cycling through and never making it into deeper waters. Nearshore currents also tend to keep dissolved phosphorus in the water column, where Cladophora gets the first crack at this food supply. Coupled with the mussels’ voracious appetites clarifying the water column, this can lead to greater harmful algal growth, resulting in the blooms seen on Lake Erie and in bays throughout the Great Lakes. The mussels also are capable of scavenging offshore plankton as it drifts into the nearshore zone, ultimately retaining the nutrients from the plankton in the mussels’ nearshore home.
The impact this has had on the food web is significant. Some species of fish that historically have lived offshore or in the water column are willing to enter nearshore or benthic regions for food. Round goby, an invasive fish that feeds on the mussels and other invertebrates, has a ready food source in the nearshore region. This has led to some native predatory fish, like the brown trout, steelhead trout and Atlantic salmon, venturing into the nearshore areas to feed on the gobies. Other species, such as Chinook salmon and coho salmon, don’t feed on round gobies and aren’t making that move into the nearshore. Instead, their food supply is declining as the offshore plankton production is limited by the mussels, and their populations are suffering.
The expanded Cladophora mats could be causing other problems too. Bootsma said studies have shown Cladophora can harbor higher concentrations of bacteria as it decomposes on beaches. In northern Lake Michigan there have been an increasing number of birds killed by avian botulism. Bootsma said there is evidence suggesting the Cladophora could be promoting growth of the bacteria that cause botulism. When round gobies end up eating the toxic bacteria and in turn get eaten by birds, the birds get sick and die.
This nutrient shunt has led researchers to conclude that more stringent controls on the amount of phosphorus and other nutrients making it into the Great Lakes are needed to improve water quality. While mussels are the primary culprit behind the resurgence of Cladophora, on Lake Erie it’s believed this is why harmful algal blooms and other water quality issues associated with excessive nutrients rebounded in the 1990s, despite existing regulations of phosphorus and other nutrients, and have continued to plague the lake in the decades since.
The United States and Canada have agreed to reduce phosphorus entering Lake Erie by 40 percent of 2008 runoff amounts, though neither government has unveiled its plan yet. Bootsma said based on historical data and numerical models, that reduction amount should be enough to reduce the problems of toxic algae and deep-water hypoxia – the formation of oxygen-deprived zones in the water – to acceptable levels. The IJC recommended similar reduction amounts in a 2014 report released as part of the Lake Erie Ecosystem Priority.
While reduced phosphorus loading may help to address phytoplankton blooms in Lake Erie, Bootsma said there’s still uncertainty as to how nearshore Cladophora growth will respond to a reduction in phosphorus entering the water. Lake Michigan, with its lower phosphorus concentration compared to Lake Erie, still has problems with the nearshore algae. This is leading scientists to question whether localized phosphorus reductions will impede Cladophora growth or if phosphorus concentrations in the entire lake need to come down first. Lower phosphorus concentrations in the offshore areas could further reduce the amount of plankton in those areas, hurting the food web in those areas even more than the mussels already have.
“What we need now is models, based on solid research, that tell us how both the offshore and the nearshore zones will respond to changes in phosphorus loading,” Bootsma said.