Diatom responses to Holocene environmental changes in a karstic Lake Vrana in Dalmatia (Croatia)

Abstract

The 11 m sediment core of karstic Lake Vrana located at the Dalmatian coast of Eastern Adriatic (Croatia) was investigated through the analyses of diatoms. The diatoms exhibited six marked changes in Diatom Zones (DZ) with included water/sediment interface, as the record of environmental changes detected by marine and other terrestrial palaeoclimate records. The diatoms recorded at the deepest sediments were interpreted to represent Pleistocene/Holocene transition. The dated Holocene history is top 810 cm core section, ca. 8.1 cal kyr BP to the present. The first zone record (1101-1050 cm) was interpreted as alkaline, oligo-mesotrophic, oxic lentic ecosystem of the karstic flooding plain based on the dominance of Epithemia adnata. Enhanced bioproductivity of the shallow and unstable environment was noticed in the second diatom zone (1050-815 cm), which was interpreted as the beginning of the Holocene and it was divided into 4 subzones because of rapid environmental fluctuations. A warmer period of temperate climate with a pluvial phase initiated development of the oligo-β mesosaprobic shallow lake in the early Holocene. The expansion of the macrophytes and higher cyanobacteria production that caused acidification and oxygen depletion at the lake bottom was determined in the mid-Holocene (810-713 cm; 8.1–7.1 cal kyr BP). Mid-Holocene (8.1–4.7 cal kyr BP) is marked by oligohalobic freshwater Epithemia argus characteristic for alkaline water. High cyanobacteria production was determined as a consequence of algal production that induced anoxic conditions in the lake bottom sediments from the warmer period till today. The initial marine influx through the permeable karst underground was observed in the sediment record from ca. 5 cal kyr BP. The discovered marine influence on the lake indicating brackish water conditions related to the Holocene sea-level high stand. The sea intrusion had different intensities through time and is still observed today in the seasonal fluctuation of lake water salinity. The Late Holocene (2.7 cal kyr BP to the present) sediment record is characterized with the strongest marine influxes and highest biodiversity of the β mesosaprobic lake. Major changes in the water/sediment interface of very shallow karstic lake were caused by anthropogenic agricultural activity in Vrana. Diatom compositions of Lake Vrana indicate that they are sensitive to both paleoclimatic changes, freshwater influx and sea level rise (marine intrusion through permeable karst).