Leaf processing in an impaired southeastern Minnesota stream: The impact of flow velocity on detritivores

Background/Question/Methods

Leaf decomposition is a fundamental process in most stream ecosystems, and understanding factors that control rates of decomposition are of critical importance to understanding many ecological processes in streams.  This study investigated the effects of flow velocity, suspended nutrients and past intermittent hypoxia events on the rate of leaf decomposition and associated benthic macroinvertebrate communities in a small agricultural stream in central Minnesota. Replicate mesh leaf bags with holes for detritivore entry were placed at three sites along the length of the stream that varied in flow velocity, substratum type, and summertime hypoxia exposure. Replicate samples were removed weekly from each site and analyzed for leaf pack biomass and macroinvertebrates. Colonization of individual taxa was used to assess flow velocity preferences and to help explain site differences in leaf processing rates. Water samples were also collected to assess the levels of available N and P at each site.

Results/Conclusions

The slowest rate of leaf decomposition and the fewest macroinvertebrate shredders were observed at the site with the slowest flow velocity. The stream site with a history of summertime hypoxia still had significantly lower dissolved oxygen saturation than the other sites, but as temperatures declined in September, hypoxic conditions were alleviated. The high flow velocity site had significantly higher concentrations of nitrates than the medium and low flow sites. While the concentration of phosphates did not vary significantly between sites, it did experience a significant decline over the study period at all sites, suggesting that detritivores may be limited by phosphorus in this stream. We hypothesize that areas experiencing low flow velocity have slower leaf processing rates because of lower shredder densities. Low flow rates can impede the activity of shredders by either reducing the delivery of phosphorus to the detritivores or by creating small pockets of hypoxia around the site of leaf breakdown. Future analyses of leaf stoichiometry may help us determine if nutrient or oxygen delivery is limiting the detritivore activity in this small agricultural stream.