Impact of simulated drought on ecosystem biomass production: an experimental test in stream mesocosms

Climate models predict widespread shifts in precipitation patterns and increases in the frequency of extreme events
such as droughts, but consequences for key processes in affected ecosystems remains poorly understood. A 2-year
manipulative experiment used a series of stream mesocosms to test the effect of recurrent drought disturbance on the
composition and secondary production of macroinvertebrate consumer assemblages and functional groups. On
average, secondary production in drought-disturbed communities (mean 4.5 gm2 yr1) was less than half of that that
in controls (mean 10.4 gm2 yr1). The effects of the drought differed among functional feeding groups, with
substantial declines for detritivore shredders (by 69%) and engulfing predators (by 94%). Contrasting responses
were evident among taxa within most functional feeding groups, ranging from extirpation to irruptions in the case of
several small midge larvae, but production of most species was suppressed. Taxon-specific responses were related to
body mass and voltinism. The ratio of production to biomass (community P/B) increased under drought, reflecting a
shift in production from large long-lived taxa to smaller taxa with faster life cycles. This research provides some of the
first experimental evidence of the profound effects that droughts can have on both the structure and functioning of
aquatic ecosystems.