By Kevin Bunch, IJC
Scientists have known since the 1972 Great Lakes Water Quality Agreement that nutrient runoff from fertilizer and wastewater is responsible for harmful algal blooms. They’re also aware that invasive quagga mussels are to blame for changes in how nutrients move around the lakes and its food web. Now a recent modeling study has better defined the role quagga mussels play in the use and movement of nutrients within Lake Michigan, which could assist efforts to reduce blooms throughout the Great Lakes.
Quagga mussels are a dreissenid species native to the Ponto-Caspian region of eastern Europe that most likely entered the lakes through ballast water in the 1980s. The tiny creatures form hard shells and latch onto practically any hard surface before filter feeding out plankton in the water column. The mussels thrive in nearshore regions, with quagga mussels outcompeting their fellow invasive zebra mussels in many areas. Quagga mussels also are capable of surviving in deeper water than zebra mussels, pushing the invasion further offshore.
Lake Michigan has seen a decline in offshore primary productivity – essentially the rate of how organisms at the base of the food web convert sunlight and nutrients into biomass energy – for around the past decade, according to Darren Pilcher, research scientist with the Joint Institute for the Study of the Atmosphere and Ocean at the US National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory. Since these phytoplankton species are eaten by a wide variety of other creatures, including invertebrates and eventually fish, understanding what’s driving that decline is important, he said. There are two major drivers that have been suggested: the decline of nutrient pollution into the lake from runoff since the GLWQA went into force and the growth of the quagga mussel population.
“These mussels are able to just graze and eat the phytoplankton as they grow, and that’s one mechanism showing how productivity has been reduced in the lake,” Pilcher said.
The model Pilcher and his partners built shows representations of the lake’s productivity under different conditions. Researchers can see what the lake would look like with a reduced number of mussels, or with a reduced amount of nutrient runoff (based on the observed decline going back to before the mussels invaded). This allows them to tease out the effect of each process. The model accounts for how water moves throughout the lake and for its ecosystem, particularly phytoplankton and zooplankton – organisms that eat phytoplankton.
Pilcher said they found that the growth of the quagga mussel population and drops in total nutrient runoff have impacted the lake, but at different times and places. Quagga mussels have the biggest impact in the spring and late autumn due to how the water moves within the lake, Pilcher said. The mussels live at the bottom of the lake, while phytoplankton tend to live in the upper layers to soak in the sun’s rays. During spring and late autumn those water layers tend to mix, bringing those plankton down to where the mussels can eat them. In the summer, the water layers stratify and rarely mix, denying quagga mussels that food source outside of the nearshore zone where waters are shallower. The researchers had hypothesized that would be the case, and with the modeling they were able to see how the quagga mussel’s seasonal dependence worked.
“The phytoplankton sit in the top layer (of water) and no mussels are grazing on them, so they’re able to continue as business as usual at that point,” Pilcher said.
However, since mussels in shallower nearshore zones can continue to feed on phytoplankton, they’re clarifying the water and leaving a niche for other algae and bacterial species to move in and use the phosphorus there to grow and expand into blooms. This also has the effect of shuffling nutrients predominantly to the nearshore zone and processing them into a more bioavailable form, in what’s called the nearshore nutrient shunt. Runoff can exacerbate that, and as a result the impact of the runoff is mostly seen in the summer, when harmful algal blooms can grow and species like Cladophora, a kind of algae that lives on the lake bottom, can spread into large mats.
Since those nutrients aren’t making it into the offshore zones in the first place, species in the deep water zones are still out of luck. Pilcher said that some fish in nearshore zones still can conceivably eat algae or the mussels, but those in the offshore regions have a harder time finding food unless they’re willing and able to venture closer to shore. This has been thought to be contributing to the population declines of several fish species in the Great Lakes, such as lake whitefish, as they struggle to find food where they live.
In the future, Pilcher said they’d like to see benthic algae such as Cladophora added to the models to better understand how the mussels are affecting their growth. With the mussels clearing out more desirable phytoplankton in nearshore regions, phosphorus is going unused by those species, giving undesirable algae and bacteria an opening to grow. This information could be helpful as the Great Lakes states and Ontario work on reducing phosphorus loading into the Great Lakes, particularly Lake Erie, by helping refine phosphorus loading targets and expectations. Pilcher added that models using similar parameters could be developed for the other four Great Lakes; they already exist to some degree for Lake Erie.
Kevin Bunch is a writer-communications specialist at the IJC’s US Section office in Washington, D.C.