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)