The net effect: caddis flies and streambed flow

Posted on March 26, 2022

The beds of streams are dynamic interfaces where water, nutrients and living organisms move between surface and sub-surface environments, and streambed permeability has a significant influence on processes such as nitrogen cycling and organic matter decomposition.  Although the physics of streambed exchange are well understood, interactions between physical and biological processes have received relatively little attention, and there have been only a few studies on the ways in which stream invertebrates affect vertical flow in gravel-bed rivers.  In this context, net-spinning caddisfly larvae (hydropsychid trichopterans) play an important role as widely distributed ecosystem engineers in streambed sediments.  Commonly occurring at densities of up to 10,000 per square metre, they construct silk nets to catch food in the form of algae, organic particles and small invertebrates, and they also build silk structures for refuge.  American ecologists measured the impact of caddisflies on vertical streambed flow under controlled laboratory conditions by circulating water through plastic columns filled with coarse sand and medium gravel (particle size 1.4 – 22 mm) from local streams.  In some trials, which lasted 8-11 days, the columns were inoculated with species of Arctopsyche  and Hydropsyche at a density of 2000 caddis larvae per square metre.  These “caddis only” trials were compared with those in other treatments in which algal cultures were circulated through the columns (with or without caddis), or where sterilised or unsterilised water was circulated without caddis.  In the control treatment (sterilised water) the hydraulic conductivity of the sediment columns didn’t change with time, but in the non-caddis treatments that permitted biological growth (algae and unsterilised water) there was a drop in conductivity due to the build-up of organic biofilms on sediment particles.    However, there was a much more marked change with the caddis only treatment, where there was a near-linear drop in conductivity from 15 to 7 cm/sec over nine days.  Also, the initial conductivity was lower than with non-caddis treatments, so it seemed that the caddis had an immediate effect.  At the end of the experiment, about twice as much organic matter had accumulated in the sediments occupied by caddis relative to sediments without caddis.  Unexpectedly, adding algae as well as caddis didn’t further reduce permeability.  In discussing their results, the researchers suggest that caddis increase their net building activities when their algal food is scarce, thereby slowing water flow through sediment pores and allowing biofilms to better process dissolved and suspended nutrients, some of which derive from caddis faecal pellets.  In this way, when streambeds are disturbed early-colonising caddis probably help to improve hydraulic conditions for developing biofilm communities.

Reference:  MacDonald, M.J. et al. 2021.  Ecosystem engineering in the streambed: net-spinning caddisflies influence hydraulic properties.  Ecohydrology 14:e2266.