The indirect effects of current velocity on algal abundance through interactions with Ceratopsyche larvae

Background/Question/Methods

Streambed current can have direct and indirect effects on benthic organisms. The distribution and abundance of streambed algae, for example, may be directly influenced by current velocity via subsidy/stress effects.  Current may indirectly affect algae by influencing the distribution of benthic organisms, such as retreat-building hydropsychid caddisfly larve. As passive filter-feeders, hydropsychid larvae respond to current and may negatively influence algal abundance by competing for space, altering microhabitats and by actively removing algae.  We hypothesized that algal abundance and hydropsychid density would each show a positive relationship to current, but an inverse relationship to one another.  To test this, we examined the streambed community in a reach of the Chippewa River in Eau Claire, WI, USA, underlain with exposed bedrock and colonized by hydropsychids of the genus Ceratopsyche.  Forty-five benthic samples were collected across a range of current spanning 5-100 cm s^-1.  For each sample algae and benthic macroinvertebrates were removed from a 225 cm² area of the bed, preserved, and separated in the lab. We quantified Ceratopsycheabundance, benthic invertebrate richness, and the algae biomass.  The data were analyzed using linear regression and a structural equation model (SEM) to estimate causal relationships.

Results/Conclusions

There was a positive relationship between current velocity and Ceratopsyche density, and between Ceratopsyche density and algal abundance, but algae abundance showed a hump-shaped response to current.  Algal biomass increased with current up to 70 cm s^-1, then decreased as velocities became faster. The SEM model showed that current velocity affected Ceratopsyche density and algal abundance (up to 70 cm s^-1) in the manner predicted, but contrary to our predictions, Ceratopsyche density and algal abundance were positively correlated. These results suggest there is a subsidy/stress threshold around 70 cm s^-1 at which algal mats reach optimal biomass, and that Ceratopsyche larvae respond to algal abundance rather than vice versa.  Although our data support the notion that current velocity mediates the relationship between Ceratopsyche and algal density, it did not occur in the manner anticipated (i.e., Ceratopsyche displacing algae).  On-going research will investigate the causal linkages proposed by our SEM and experimentally test the manner in which current velocity indirectly influences Ceratopsyche density and algal biomass on the Chippewa River streambed.