The impact of natural water column mixing on iodine and nutrient speciation in a eutrophic anchialine pond (Rogoznica Lake, Croatia)

Abstract

The effect of natural water column mixing on the inorganic iodine and nutrient speciation in an intensely eutrophicated anchialine pond has been studied in late seasons of 2003 and 2004. As a result of an extremely warm and very dry European summer of 2003 this system attained isohaline and isothermal conditions in late-summer. Vertical mixing between highly reducing deep water with that in the upper layers produced intermediate redox conditions and homogeneous nutrient distribution within the water column. Nitrogen speciation additionally suggests a pronounced activity of nitrifying bacteria and/or archaea arising from an influx of the ammonium nitrogen. Meanwhile, in 2004 the water column attained nearly isohaline conditions late in November. Partial mixing of the water column resulted in downward transport of dissolved oxygen and moderate upward fluxes of nutrients into the surface layers, whereby the flux of ammonium was sufficiently high to promote nitrification. A response of the inorganic iodine to mixing of the water column was in accord with those of major constituents and nutrients. The results demonstrate that, in spite of marked changes in speciation, total inorganic iodine remained preserved within the system over each survey. Oxic to hypoxic redox conditions formed immediately after the overturn in September 2003 favoured iodate reduction, while slow mixing in November 2004 promoted iodide oxidation. In this system dominated by the sedimentary influence a degree of oxygenation of deep water seems to control the inorganic iodine inventory for the pond.