Vanishing glaciers

The sky was cloudless, a spotless expanse of endless blue, and not a single breath of wind rippled the water of Bowman Lake, a long and slender finger of water nestled between two ridges in the northwestern corner of Glacier National Park.

After growing up on the east coast, nine years ago this summer I visited the American West for the first time, and I understood why Montana is nicknamed “Big Sky Country.” I spent eight weeks at the University of Montana’s Flathead Lake Biological Station, where my fellow students and I dove into our field ecology classes, exploring the mountains and lakes of northwestern Montana along the way.

We visited Glacier National Park on several of our field trips, and one day in particular sticks in my mind – the lake was a perfect mirror for the mountains and the sky, and it was too beautiful for us to leave without taking out our cameras.

The sky and surrounding ridges reflected in Bowman Lake, along with several other photographers, in mid-summer 2006.

(Image by Emily Benson)

The majestic scenery of Glacier National Park, however, is changing – scientists estimate that by 2030, all of the park’s glaciers will be gone, melted under the force of global climate change. As the National Park Service notes, “the park’s glacially fed streams provide a constant flow of cold water throughout the summer season, maintaining necessary water levels and regulating stream temperature for fish and other aquatic species. Plant and animal species throughout the park rely on this flow.”

One of those animal species is the western glacier stonefly, an aquatic insect that, in the past, has only been found in alpine streams in Glacier National Park.

Recently, a group of scientists (including one of the professors that I studied with during my summer in Montana) surveyed all of the locations where the western glacier stonefly has been found in the past, as well as some similar habitats, in order to determine whether their distribution is shrinking along with the glaciers in the park.

The researchers detected western glacier stoneflies in only one of the six streams where they’ve been found before; they also found the stoneflies in two new alpine sites within the park, as well as one site about 335 miles away in Grand Teton National Park.

The scientists note that further study on the “status, distribution, and vulnerability” of the western glacier stonefly is warranted, but the results they’ve already gathered “suggest that an extremely restricted historical distribution of [the western glacier stonefly] in [Glacier National Park] has been further reduced over the past several decades by an upstream retreat to higher, cooler sites as water temperatures increased and glacial masses decreased.”

At some point, the western glacier stonefly will run out of mountain as the population searches for higher and higher sanctuaries, and, left with no place to go, it may face extinction. Glacier National Park is still beautiful today, but it isn’t the same park that I visited nine years ago. In nine more years, will there still be a place within the park’s borders where the western glacier stonefly can feel at home?

An adult western glacier stonefly.

(Image by Joe Giersch/USGS)

Introducing mosquitofish

Western mosquitofish, small freshwater fish typically about two and a half inches long, have been distributed far and wide from their native habitat in the southern U.S. and northern Mexico. Because they feed on insect larvae, including, depending on the circumstances, substantial numbers of immature mosquitoes (hence their name), humans have introduced them to waterways throughout the world.

In fact, these introductions are still occurring today – if you live in California’s Alameda County, you can request a delivery of mosquitofish right to your own pond.

Of course, biological controls rarely function exactly as intended. Mosquitofish are aggressive toward other fish, and have even been known to replace native fish species – which, in some cases, are more “efficient mosquito control agents” than the introduced mosquitofish that displaced them.

Mosquitofish also eat lots of other types of insects, as well as zooplankton and aquatic vegetation – meaning the effects of their eating habits can ripple outward from multiple parts of their food webs. Far from solely reducing mosquito populations, mosquitofish can cause big changes in their new homes (which is why some states regulate the expansion of their range).

Water bodies are not isolated from the environments that surround them – scientists call the flow of nutrients and resources between aquatic and terrestrial areas a ‘boomerang flux’ or ‘reciprocal subsidy.’ For example, many aquatic insects (including mosquitoes) spend their larval stages underwater, and later emerge as winged adults – the energy they incorporate into their bodies from the aquatic environment is transferred to terrestrial environments when those adults are eaten by predators like spiders or birds. New research recently published in the journal Freshwater Science suggests that introduced mosquitofish can alter that transfer of energy.

A team of scientists working at Utah’s Fish Springs National Wildlife Refuge, a series of spring-fed wetlands in the middle of the desert 125 miles southwest of Salt Lake City, measured how well aquatic insects survive to adulthood in the presence of western mosquitofish, as well as two native fish species.

The researchers set up experimental tubs of spring-water, complete with sediment and aquatic vegetation, to which they added either one species of fish or a combination (they also kept fish out of some of the tubs, so they could compare the results from the fish-filled tubs to the fish-less ones). Then they captured the adult aquatic insects that emerged from the tubs’ water surface – presumably the insects that did not survive their underwater larval stages became fish food. (None of the adult insects they collected were mosquitoes.)

The scientists found that the biomass of aquatic insects that emerged unscathed from the tubs that didn’t contain any mosquitofish was much larger than the biomass captured above the tubs where they were present: about 70 percent of the insect biomass made it out of the tubs containing native fish species, while only about 40 percent survived the mosquitofish tubs.

As the researchers note in their paper, “[w]estern mosquitofish have the potential to negatively affect the flow of energy from springs to the terrestrial environment,” in the form of reduced adult insect biomass. Though we humans may cheer when those adult insects are mosquitoes, we should remember that mosquitoes are not the only insects western mosquitofish consume, and the effects of introducing the fish to new locations are rarely as straightforward as we might like.

Western mosquitofish are often intentionally introduced to ponds and lakes in an effort to reduce mosquito populations; such introductions can have unanticipated negative consequences. 

(Image by NOZO via Wikimedia Commons/Creative Commons license)

Winter fertilizer

By December, most stream ecologists in the northern U.S. have hung up their waders and retreated to their labs and offices, ready to spend the winter analyzing samples and writing reports after the end of another successful field season.

Most, but not all.

“I actually enjoyed going out to the streams in the winter, it was awesome seeing them change as the seasons changed,” Robert Mooney, a graduate student at University of Wisconsin La Crosse and lead author of a paper published this month in Freshwater Science, told me via email.

Mooney doesn’t mind the cold – he grew up in Wisconsin, where his interest in streams began with fishing and tying flies. Some of those flies would have been patterned after the adult forms of the aquatic macroinvertebrates that Mooney would go on to study in graduate school.

He and his co-authors investigated whether or not the excretions of Glossosoma intermedium – a caddisfly that builds a mobile, shell-like case for itself from tiny rocks and grains of sand that it finds on the streambed during its larval stage, when it lives underwater – could be supplying additional nutrients to the periphyton, or algae, that grows on the insects’ cases. In other words, they wondered if caddisfly poop could be fertilizing the periphyton.

Caddisfly larva in its case, balanced between two piles of sand grains. The red arrow is pointing to the head of the insect.

(Image by Robert Mooney)

They sampled streams in southwestern Wisconsin between November 2010 and February 2011, when densities of the caddisfly larvae were high, a period when average monthly air temperatures ranged from 14 to 37 degrees Fahrenheit in nearby La Crosse, Wis.

Mooney and his co-authors found that the larvae did seem to be fertilizing the periphyton on their cases in streams where the ratio of nitrogen to phosphorus was particularly high (a condition that suggests algae growth may be limited by a lack of phosphorus). In those streams, periphyton that grew on caddisfly cases was enriched relative to algae sampled from the streambed.

The nutrient-rich algae on their cases appeared to be an important food source for the caddisfly larvae – an analysis of their bodily nutrients matched the periphyton from the cases, but not the streambed, in the streams with the highest nitrogen to phosphorus ratios. (In the other stream, the periphyton from the cases and the streambed was too similar to distinguish which was the likely food source for the caddisflies.)

Other aquatic insects likely take part in the caddisfly case periphyton buffet, too. “I actually have observed other invertebrates living on the caddisfly cases,” Mooney said in an email, “and I would hypothesize that other invertebrates that feed on periphyton would utilize the case periphyton as a beneficial resource.”

Mooney stressed that this caddisfly, though only a single species, has an outsized effect on the ecosystem in which it lives. “Glossosoma intermedium is a keystone species in the streams [it] inhabit[s] and [is] sensitive to environmental changes. Possible declines in water quality could potentially reduce G. intermedium populations, removing the nutrient-rich periphyton resource.”

Sampling streams in the winter isn’t easy, but for Mooney and his colleagues, it was worth it to keep their waders out for a little bit longer as they explored the nutrient dynamics of Glossosoma intermedium, their poop, and the algae on their cases.

Periphyton growing on caddisfly cases may be an important food source for other macro invertebrates, too - here a different type of insect appears to graze on the algae on a caddisfly case. 

(Image by Robert Mooney)

Evaluating efficiency

Aquatic ecosystems need nutrients to survive, but excess nutrients can be a big problem – they can lead to blooms of algae in lakes, ponds, and bays (I’ve written about algal blooms before, here and here). Algal blooms are a natural phenomenon often exacerbated and made more frequent by human activities, primarily through the addition of nutrients to a watershed – a glut of nutrients adds fuel to the fire of a bloom.

Wastewater is one of the many sources of human-added nutrients in aquatic systems. Treatment plants collect wastewater from households (and sometimes industrial customers), process the water in some way, and release the treated water, or effluent, back into the local watershed. Depending on the type of facility, there may still be large amounts of nitrogen and phosphorous – the main nutrients in aquatic ecosystems – present in the effluent. Those nutrients will be washed downstream, where they can harm humans and natural systems by, among other things, contributing to algal blooms.

As older wastewater treatment plants are replaced by more efficient facilities, they need to be evaluated. The first step is deciding which metrics to use – what do you measure to see if an ecosystem is responding to a reduction in nutrients?

A research team working in France recently had the opportunity to answer that question when the city of Nîmes, in southwestern France, updated their wastewater treatment plant. The scientists measured a suite of metrics to assess water quality, before and after the new wastewater treatment plant opened; the results of their study were reported recently in the journal Freshwater Science.

Macroinvertebrate activity – the way aquatic insects are behaving – as well as their numbers and diversity can be a kind of barometer of stream health. In this study, the researchers found that the way the macroinvertebrates were functioning in the stream (the way they behaved) told a different story than the way they were structured in the stream (the number of different species, and the number of individuals of each species).

By the numbers, the sites below the wastewater treatment plant had begun to recover – they began to resemble a reference site above the treatment plant outfall, and sites in other, more pristine streams – within three months of the improved system coming online. The functional metrics, however, suggested that the health of the stream still had room to improve – by the end of the study, three years after the new treatment plant was built, the sites below the treatment plant still had not recovered according to many of those measures.

“Taxonomy-based metrics detected the first signs of river reach recovery rapidly,” the scientists write, “but combinations of trait-based metrics and taxonomic abundance-based metrics are more likely to identify functional recovery” of macroinvertebrate communities following nutrient reductions. In other words, in order to figure out if we’re cleaning up our act as much as we think we are when we make improvements to our wastewater treatment plants, we probably need to measure several different metrics of ecosystem response.

Even small towns often have wastewater treatment plants; this one serves a rural community of less than 1,000 people.

(Image by Emily Benson)