Lake boundaries

“Historically, lake ecosystems were viewed as isolated ‘habitat islands in a sea of lands’ (Forbes 1887).”

So begins a paper recently published in the journal Oikos by a team of scientists working in northeast Germany. The paper they reference in their opening was written by Stephen Alfred Forbes, a prominent biologist from the 1870s until his death in 1930, and “the founder of the science of ecology in the United States,” according to L. O. Howard, writing for the National Academy of Sciences.

Though Forbes was one of the first scientists to investigate how species interact and influence one another, and emphasized the importance of studying a community rather than a single species in isolation, he did not recognize the ways in which the seemingly distinct habitats of lakes and land are interconnected. In his 1887 paper, he wrote:

“The animals of [a lake] are, as a whole, remarkably isolated – closely related among themselves in all their interests, but so far independent of the land about them that if every terrestrial animal were suddenly annihilated it would doubtless be long before the general multitude of the inhabitants of the lake would feel the effects of this event in any important way. … It forms a little world within itself.”

Recent research suggests that the boundaries of the ‘little world’ of a lake are not impermeable – energy and resources are exchanged between lakes and the terrestrial environments in which they are nestled. Sometimes, as in the study recently described in Oikos, the vector of that exchange is an insect with both aquatic and terrestrial life stages.

The researchers added corn leaves to part of two lakes (each lake was divided in half by a plastic curtain). Corn leaves have a different chemical signature than the other vegetation present in or near the lakes, allowing the researchers to trace the fate of the energy in the leaves as it made its way around the food web, from prey to predator.

In the sides of the lakes to which corn leaves had been added, the scientists found values consistent with the leaves in aquatic insect larvae that ate them, the adult forms of those insects, and the terrestrial spiders that ate the insects. In what they describe as a “boomerang flux,” they demonstrated the transfer of terrestrial carbon (in the form of corn leaves) first to aquatic organisms, and then back to a terrestrial setting.

Referring back to Forbes’ idea of lakes as distinct entities, separate from their surroundings, the scientists write: “a viewpoint of such isolation does not reflect the continued back-and-forth ‘boomerang’ cycling of organic matter (and probably nutrients) across the borders of terrestrial and aquatic systems.”

Forbes’ prescient insights into the importance of the relationships among species introduced the world to the concept of ecology and changed the way future scientists would think about and conduct their studies; with time, those future scientists would realize that the interconnectedness among species extends beyond the bounds of Forbes’ original idea of a lake, and beyond the shores of lakes themselves.

Many insects that spend their immature life stages underwater emerge to live in the terrestrial environment as adults. 

(Original image by Bj.schoenmakers via Wikimedia Commons)

Microplastic mud

Marine debris – the collection of discarded, man-made objects that accumulates in the world’s oceans – is a well-known environmental problem (NOAA even has a program dedicated to studying and ameliorating the issue). Ocean currents and wind patterns can gather marine debris into giant, floating masses of refuse, like the Great Pacific Garbage Patch, and encounters between wildlife and the litter can have devastating consequences.

Since it was published earlier this week, this paper from the journal PLoS ONE concerning the amount of plastic that humans have dumped into the world’s oceans has garnered a lot of media attention. One of the interesting findings in that paper was that, consistent with earlier studies, the researchers found a lot less of one type of plastic than they expected to find during their surveys – microplastic, or pieces smaller than five millimeters (that’s a little bit less than a quarter of an inch).

Another study published earlier this month offers another perspective on microplastic pollution – it appears to be ubiquitous in the sediment of at least one large river, the St. Lawrence River in eastern Canada. According to the scientists who conducted the study, other researchers have found microplastic floating in freshwater lakes and on their shorelines when they’ve looked for it (plastic debris have been much more extensively studied in marine than in freshwater environments), but, they write, “[n]o studies to date have addressed the presence of microplastics in North American freshwater sediments.” 

To address that deficiency, the researchers collected sediment from 10 sites on the St. Lawrence River, most between Quebec City and Montreal (two were below Montreal); they found microplastics at eight of the sites. The densities ranged from low (an average of seven pieces of plastic per square meter at the site with the lowest densities) to extraordinarily high – the single sample containing the most microplastic had 3,980 pieces per liter. Imagine a Nalgene water bottle full of mud from a riverbed, suffused with that many tiny bits of plastic.

Plastic pollution is a problem in the ocean, but it’s also a problem in freshwater environments, where it’s just beginning to be explored. If fish confuse morsels of microplastic for food, the effects on freshwater food webs could be devastating – and, as the authors of the paper note, this is an area ripe for future research: “[t]he extent to which microplastics have become incorporated into the St. Lawrence River food web – and the consequences for biotic communities – remain to be determined.”

Microplastic pollution is an environmental issue in both marine and freshwater environments. 

(Image by Wright via Wikimedia Commons)

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)

Artificial light

Humans interact with streams in lots of ways – we like to swim in them, catch the fish that live there, and fill our water glasses and irrigate our fields with the water flowing through them. We also change rivers and streams to suit our needs – we straighten stream channels in cities, we cut down the riparian vegetation that grows on stream banks when it gets in our way, and, in some places, we put up streetlights with little regard for how that artificial light might affect the animals and plants that live in streams.

These changes influence aquatic organisms and the ecosystem overall, but sometimes it’s difficult to know what effect each change, individually, is having – something conservation officers or resource managers might want to know if they’re trying to decide which restoration project to dedicate limited funds to. In order to know how streams or rivers respond to different anthropogenic changes, it’s necessary to study systems where only one of them is occurring. And in order to experimentally test what the effects of a change are, it’s necessary to conduct the study in a place where researchers can manipulate streams.

“I was trying to find a system to work in that was basically light-naïve and would be simple to manipulate,” says Liz Perkin, currently a post-doctoral researcher at the University of British Columbia and the lead author on a paper recently published in the journal Freshwater Biology.

Perkin found a system that would work for her research – the Malcolm Knapp Research Forest, located in British Columbia. She and her team set up high-pressure sodium streetlights at four streams within the forest, then compared the insect community, leaf decomposition, and fish growth in the experimental reaches with four similar areas that weren’t lit by streetlights at night, to see what effect the streetlights had on the streams.

The scientists found that the number of aquatic macroinvertebrates drifting in the streams – a primary food source for fish – was much lower in the reaches lit by streetlights. This wasn’t surprising; as Perkin and her co-authors write, “[p]revious studies have shown that the activity of aquatic insects is at least partially controlled by light levels.” Insects are more likely to stay on the stream bottom, rather than drift through the water, when there’s enough light for fish to see and eat them.

Other macroinvertebrate activities, as well as leaf decomposition and fish growth rates, were the same at the lit and unlit streams. Perkin has some ideas about why that might be – they were only able to keep their streetlights in the research forest for one month during the summer, and it’s possible that the duration of the experiment simply wasn’t long enough to induce changes in the ecosystem, or that the effects of the streetlights would have been more pronounced during the spring, when days are shorter and nights are longer.

Another possibility is that the type of streetlight the researchers used in the experiment may explain why they didn’t see larger differences between the lit and unlit streams.

“They weren’t as bright as they could be,” Perkin says, of the streetlights they used. “We used high-pressure sodium because they’re very efficient and they’re currently the most commonly used streetlights across the globe.” But LED lights, which are gaining popularity as they become cheaper and more efficient, are brighter and have unique spectral qualities that may make them more influential in aquatic ecosystems. “If you look at the spectrum of a high-pressure sodium light, they tend to be more in the orange and red end of the spectrum, and those wavelengths are very readily absorbed by water, whereas with LEDs you have things more on the blue end of the spectrum, and those are more likely to penetrate water and be brighter in the water.”

Moving forward, Perkin plans to look at fish behavior in more detail. She suspects that even though macroinvertebrates drift less in streams lit by streetlights – meaning less food is available for fish in those streams – the extra light may make it easier for fish to spot the insects that are drifting.

 “What we know from this [study] is that artificial light does have the ability to affect stream communities,” Perkin told me. “The changes [we] saw were pretty small, but because they’re added at an important level in the food web, there’s definitely the potential for there to be bigger changes on a longer timescale.”

Experimental streams with high-pressure sodium streetlights installed above them had fewer drifting insects than streams without streetlights, but didn't seem to differ in other ways.

(Image of experimental streetlight set-up by Nora Schlenker)