A disturbing mix

Posted on June 19, 2017

The common carp (Cyprinus carpio) has been introduced to many locations around the world and is highly invasive.  Its habit of foraging by digging in the bottom substrate significantly alters freshwater systems by uprooting plants, resuspending sediment, and increasing turbidity and the release of nutrients.  However, the extent to which aquatic nutrient flows are affected by carp digging hasn’t been well documented.  To address this gap in knowledge, a study was carried out in a shallow urban lake in Minnesota, U.S.A..  The lake had resident carp and  had been treated with alum (aluminum sulfate and buffered sodium aluminate) as part of a restoration project to reduce the sediment phosphorus load. The aluminium introduced in this way was used as a tracer to estimate sediment mixing by carp.  Immediately after alum treatment, the researchers installed circular (5 m diameter) plastic mesh structures to exclude fish from certain areas. The carp in the lake had a mean weight of 3.4 kg and a biomass density of 180 kg per hectare. Comparisons of sediment cores taken before and after the addition of alum showed, firstly, that the treatment successfully reduced concentrations of mobile phosphorus by converting it to Al-P, and secondly, that the sediment mixing depth (the depth to which the added aluminium was detected) was significantly greater in areas with carp (around 13 cm) than in areas from which carp had been excluded (5 cm).  This showed that carp can dig deeply in lake bottoms, and the sediment profiles indicated that, by extending the mixing depth, carp increased the amount of mobile phosphorus available for release by 55-92%.  Carp-induced increases in sediment mixing depth are also likely to reduce the effectiveness of aluminium-based treatments designed to stabilise bottom phosphorus.

Reference:   Huser, B.J. et al.  2016.  Effects of common carp (Cyprinus carpio) on sediment mixing depth and mobile phosphorus mass in the active sediment layer of a shallow lake. Hydrobiologia 763,23–33. https://www.maisrc.umn.edu/sites/maisrc.umn.edu/files/effects_of_common_carp_on_sediment_mixing_depth.pdf