Simulating the potential effects of a carp virus fish kill on water quality and phytoplankton in lentic environments

Decaying fish play an important role in delivering nutrients into rivers and lakes but can create water quality issues. The release of cyprinid herpesvirus-3 (CyHV-3) in Australia with an associated mass common carp mortality may have serious effects on water quality in lakes. To evaluate the effect of a virus-induced fish kill, different biomasses of dead common carp (250–6000 kg ha–1) were placed into 2000-L mesocosms within Prospect Reservoir, Australia, for up to 40 days. Decaying carp created anoxic conditions within all treatments except the 250 kg ha–1 treatment, in which oxygen saturation dropped to 30%. A higher biomass of carp led to longer periods of anoxia. Total nitrogen (TN) increased from a baseline of 0.25 to 1.5–30 mg L–1 in the different treatments, whereas total phosphorus (TP) increased from 0.01 to 0.05–5.0 mg L–1. Chlorophyll-a levels increased from 5 µg L–1 to levels between 100 and 1000 µg L–1 in the different treatments. Mean nutrient levels (TN and TP), chlorophyll-a and phytoplankton biovolume all showed a significant (P 0.05) linear relationship with carp biomass. This relationship can be used to support the quantification of water quality risk from a known biomass of carp. Our experiment suggests that carp mortality may significantly reduce water quality in shallow lakes where the biomass of dead carp is above moderate levels (250 kg ha–1).