Long-term changes in macrophyte vegetation after reduction of fish stock in a shallow lake

Starting in the middle of the 1970s, submerged macrophytes began to disappear from shallow Lake Warniak due to feeding pressure by grass carp (Ctenopharyngodon idella). In the middle of the 1980s, the lake was stocked with seston-feeding silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis). By 1993, the biomass of silverhead carp and bighead carp had declined. This allowed charophytes to recolonize the bottom of the lake. The main charophyte species at this time were Chara globularis and Chara rudis. Since then, five other stonewort species have been found: Chara contraria, C. filiformis, C. tomentosa, C. aspera and Nitellopsis obtusa. Seventeen species of aquatic angiosperms have also been found. There were distinct changes in the relative abundance and spatial distribution of particular species. C. rudis developed most intensely in the shallow parts of the lake near the southern and western shores. C. globularis gradually took over the deeper central part of the lake. In 2001, C. rudis began to retreat again. The relative abundance and spatial distribution of charophytes was correlated to water clarity (r = 0.87, p < 0.05), total phosphorus level (r = −0.78; p < 0.05), and chlorophyll a content (r = −0.79; p < 0.05).