Declining ecosystem performance of a restored lake and marsh associated with a rapidly expanding population of invasive common carp (Cyprinus carpio)

Background/Question/Methods The invasive common carp (Cyprinus carpio) from Eurasia has long been associated with altered ecosystem functioning in North American shallow lakes, but the relationship between carp and system performance has seldom been quantified. This study documented changes in the ecology of Hennepin & Hopper Lake, a recently restored shallow lake and marsh system in north-central Illinois, throughout a period in which it experienced a rapidly increasing carp population followed by a piscicide application in 2010. Using mark and recapture techniques, carp abundance and biomass were estimated in 2007, the sixth year following initiation of restoration activities in 2001. Carp population structure was then back-calculated through time and correlated with changes in various lake and marsh metrics indicating ecosystem performance, including water quality, macrophyte cover and species richness, and fall migratory waterfowl use.

Results/Conclusions Carp biomass remained relatively low for three years following the onset of restoration, but then increased to over 60 kg/ha in the fourth year after a strong recruitment event. Although this increase in 2005 had little immediate effect on vegetation (macrophyte cover >85% from 24 species) or waterfowl use (>150,000 use days), carp biomass increased to over 100 kg/ha in 2006 and over 300 kg/ha by 2007, coinciding with dramatic declines in vegetation (<25% macrophyte cover from 12 species) and waterfowl use days (<10% of peak counts). Mean water clarity also declined throughout this period, although no change in NO₃-N concentration was detected. Data on carp population size was not collected after 2007, but declines in vegetation, waterfowl use, and water clarity continued through 2009 (<3% macrophyte cover from six species, waterfowl use days <5% of the 2004 peak, and mean Secchi readings ~30% of pre-impact levels). All metrics began recovering following application of a piscicide in March 2010. These data suggest that carp can rapidly devastate the ecological integrity of shallow lake and marsh systems with densities exceeding 100 kg/ha (a value less than half that found in many shallow Midwestern lakes), and that these systems are capable of dramatic recovery if carp are removed.