LONG-TERM RESPONSES TO FISH-STOCK REDUCTION IN SMALL SHALLOW LAKES - INTERPRETATION OF 5-YEAR RESULTS OF 4 BIOMANIPULATION CASES IN THE NETHERLANDS AND DENMARK

The effects of fish stock reduction have been studied in 3 Dutch lakes (Lake Zwemlust, Lake Bleiswijkse Zoom and Lake Noorddiep) and 1 Danis h lake (Lake Vaeng) during 4-5 years. A general response is described. The fish stock reduction led in general to a low fish stock, low chlo rophyll-a, high Secchidisc transparency and high abundance of macrophy tes. Large Daphnia became abundant, but their density decreased, due t o food limitation and predation by fish. The total nitrogen concentrat ion became low due to N-uptake by macrophytes and enhanced denitrifica tion. In Lake Bleiswijkse Zoom the water transparency deteriorated and the clear water state was not stable. The fish stock increased and th e production of young fish in summer was high. Clear water occurred on ly in spring. Large daphnids were absent in summer and the macrophytes decreased. In Lake Zwemlust, Lake Vaeng and Lake Noorddiep the water remained clear during the first five years. In summer of the sixth yea r (1992) transparency decreased in Lake Zwemlust (with high P-concentr ation of 1.0 mg P l-1). Also in Lake Vaeng (with a low nutrient concen tration of 0.15 mg P l-1) a short term turbid stage (1.5 month) occurr ed in summer 1992 after a sudden collapse of the macrophytes. Deterior ation of the water quality seems to start in summer and seems related to a collapse in macrophytes. At a low planktivorous fishstock (e.g. L ake Vaeng) the duration of the turbid state is shorter than in presenc e of a high planktivorous fish biomass (e.g. Lake Zwemlust, and later years of Lake Bleiswijkse Zoom).