Georgetown Lake’s odd chemistry studied

“There’s been so much information over the past year, and it turns out there’s a lot of good things going on,” said Stokke, who is president of the Georgetown Lake Homeowners Association. “We’re feeling really good about it.”

For example, the lake’s water appears to be growing clearer rather than cloudier, in part because it churns its contents in a fashion researchers didn’t expect. That’s just one of the odd findings from an examination of Georgetown’s fish and plant life, chemistry and geology underwritten by Granite County and Montana’s Natural Resource Damage Program.

“It’s only been there 100 years, so it runs in a different trajectory from lakes that have been around since the last ice age,” said University of Montana freshwater ecologist Craig Stafford, who did much of the water quality work. “It’s behaving a little differently.”

Georgetown Lake is actually a reservoir with an average depth of about 5 meters (16 feet). Nevertheless, it hosts one of the few trophy brook trout fisheries in the western United States, and regularly ranks among Montana’s Top 10 fishing destinations (based on state Fish, Wildlife and Parks surveys of angler hours on the water). It’s up there with larger places like Fort Peck Reservoir and Canyon Ferry. Its rainbow trout population remains equally popular.

“The typical pattern, as places get built up, you see more nutrients getting into the water,” Stafford said. “You start to see the lake change, with more plant growth, especially phytoplankton turning the water green. And as plant materials decay, they use up oxygen. This happens under the ice in winter, and toward end of summer.

“That was the concern of what we might be seeing up in Georgetown,” he went on. “But that’s not what we’re seeing. It’s more the opposite pattern.”

Georgetown Lake’s odd chemistry offers good and bad news for fish health, Stafford said. The improving clarity comes from relatively low levels of algae and plankton. Those levels appear due to a combination of low phosphorus and low oxygen in the water.

Low phosphorus is good because that means people’s septic tanks and wastewater aren’t leaking and causing algae blooms that cloud the water and reduce its oxygen. But that makes accounting for the low oxygen somewhat confusing.

Stafford said in a typical lake, decaying plant life would build up on the bottom, releasing phosphorus that would support plankton and zooplankton. Those tiny organisms would in turn consume much of the lake’s oxygen.

But the dam that formed Georgetown Lake draws water off the bottom, and flushes much of the phosphorus with it. That results in lower nutrient levels and clearer water. It also helps the more complex plants in the lake’s weed mats outcompete the plankton for nutrients.

The downside shows up in the lake’s kokanee salmon populations, which feed on zooplankton. FWP fisheries biologist Brad Liermann said while kokanee numbers remain strong, their sizes appear to have shrunk. Smaller kokanee are harder to catch.

One way to improve that situation seems counterintuitive: See if the brook trout can eat more kokanee. Liermann said more predatory pressure on the kokanee will clear out the smaller fish, allowing the remainder to grow larger.

To do that, FWP is considering tinkering with the brook trout populations, either by adding more stock fish or changing the regulations on what anglers can keep. After their numbers dropped to troublesome lows around the turn of the century in 2000, brooks were put on catch-and-release regulations for much of the decade.

“Since then we’ve seen really nice improvement,” Liermann said. “In 2010 and 2011, we were seeing 10 times higher numbers of fish gill-netted than we had in 2004 and 2005.”

Anglers may now keep two brook trout as part of their five-fish daily limit. Liermann said one tweak might be to restrict one or both brook trout to a certain size, as a way of keeping more big (kokanee-eating) fish in the lake. Those changes await further study.

Georgetown’s oxygen levels indicate some odd water behavior too. In summer, the basin’s high winds churn the shallow lake so much, it keeps a relatively stable temperature at all depths. But when it ices over in winter, the water separates into distinct temperature and oxygen-content layers.

The bottom runs out of oxygen first, concentrating much of the fish within a meter of the surface. Occasionally, this led to fish kills in March and April. Visitors around the Flint Creek dam at the lake’s northern corner may recall a rotten-egg smell in early spring, as hydrogen sulfide bubbles up from the oxygen-starved bottoms.

But oddly, there haven’t been any documented fish kills since the late 1970s, despite the downward trend in dissolved oxygen and increased shoreline development. Stafford said he doesn’t have a firm explanation for why, but the research has shown some interesting possibilities.

Montana Tech hydrogeologist Glenn Shaw used traces of radon to pinpoint where groundwater was seeping into Georgetown’s basin. He found just under half the lake’s supply is coming from underground, much of it around Stuart Mill Bay. Large limestone deposits there appear to be the source.

“Usually groundwater is low in oxygen, but these flows are the opposite,” Stafford said. “In winter, that groundwater has a lot more oxygen than the bottom of the lake does. So maybe it’s creating a refuge for fish where they can hide out in that corner of lake.”

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Information from: Missoulian, http://www.missoulian.com