ABSTRACT. Five short cores (top 40-45 cm of sediment) from 4 lakes of the Plitvice Lakes system (Croatia) were measured for 210 Pb, 137 Cs, a 14 C, δ 13 C, and δ 18 O in order to study the influence of environmental changes on the sediment system in small and large lakes. Sediment chronology based on the constant flux (CF) 210 Pb model was the most reliable. Lake sediments consisted mainly of autochthonous carbonates with higher sedimentation rates in small lakes. Sediments from 2 large lakes, ProšÊe and Kozjak, showed constant stable isotope profiles for the carbonate fraction and full agreement between the 137 Cs and 210 Pb chronologies. Sediments from 2 small lakes, Gradinsko and Kaluerovac, showed synchronous increases in 14 C and δ 13 C and disturbed 137 Cs records. All lakes showed an increase in a 14 C in the carbonate sediments above the first occurrence of 137 Cs, which was interpreted as a damped (~10 pMC increase in a 14 C) and decades-delayed consequence of the bombinduced increase in a 14 C in atmospheric CO 2 . For the small lakes, increased δ 13 C in the last 2 decades and part of the a 14 C increase is probably due to an increase in primary productivity, which enhanced biologically induced calcite precipitation with concomitant changes in the carbon isotopic composition of carbonate sediments. δ 13 C values of a near-shore sediment core close to the confluence of one of the tributaries of Lake Kozjak showed that the carbonates in this core are a mixture of autochthonous and eroded allochthonous mineral carbonate. This core had a higher fraction of organic material. The sedimentation rate at this core site was high, but rates could not be quantified by 210 Pb, 137 Cs, or 14 C.