The Effect of Plankton Aactivita on the Stratification of Dissolved Oxygen and Orthophosphates in a Karstic Lake in the Mediterranean Coastal Area

The Vrana Lake is a karstic lake on the island of Cres located near the Adriatic coast. It is a natural water supply reservoir (74.5 m deep, 5.5 sq. km surface and 220 million cu. m volume). The Vrana Lake is a cryptodepression situated in carbonate rocks, and is classified as an oligotrophic lake. Because different types of karstic lakes are wide spread in this area, the aim of this study was to determine how phytoplankton activity influences the vertical distribution of dissolved oxygen and orthophosphates. The Vrana Lake is a monomictic lake characterized by beginning of the thermal stratification in spring, after winter isothermal conditions. The stratification is stabilized during summer. In epilimnion (from surface to the depth of 10 m) the mean temperature was 23 o C. Thermocline was established at the depth of 10 to 20 m. The mean hypolimnetic temperature was 10.7 o C. In late autumn and winter the cold wind caused total mixing and an isothermal water column temperature below 7 o C. Vertical stratification of dissolved oxygen indicated an orthograde distribution with the trend of increasing in epilimnion during spring and autumn. The established oxygen curve correlated with two chlorophyll a content peaks in spring and autumn. Concentration of orthophosphates decreased in spring and autumn. Its deficit was found in epilimnion. These facts suggested that: (1) orthophosphates deficit has been caused by phytoplankton activity and (2) during summer in the hypolimnion the orthophosphate content increase has been a consequence of phosphorus recycling. Principal Component Analysis confirmed our hypothesis that the plankton activity was an important biotic factor significantly affecting the stratification of oxygen and orthophosphate content in karstic lakes located in carbonate rocks of the Mediterranean coastal area.