Origin and composition of sediments in a highly stratified karstic estuary: An example of the Zrmanja River estuary (eastern Adriatic)

Abstract

Estuaries are among the most important environments of the coastal zone and are altered to a large percentage by human activities. Thus, their sustainable management, in which sediment and sedimentation processes have a leading role, is becoming a necessity. The present study investigates sediment sources and both natural and anthropogenic factors contributing to the sediment composition in a highly stratified karstic estuary of the Zrmanja River (eastern Adriatic coast, Croatia). This is a small semi-enclosed bay, comprising the riverine part and two other small interconnected bays, the Novigrad Sea and the Karin Sea.The results show that sedimentation in the Zrmanja River estuary is governed by its geological setting (karstic catchment with flysch deposits in the lower part of the estuary), prevailing hydrogeological conditions (influence of the karstic Zrmanja River, the small Karišnica River, and ephemeral streams draining flysch deposits, combined with limited circulation of the basin with open sea), biological production and geomorphology of the basin (general isolation of the basin, especially the Karin Sea). Although the sedimentation processes in the studied area are still primarily determined by natural factors, sediment records also indicate an influence of the hydropower plant in the upper part of the Zrmanja River.A simplified end-member mixing model (EMMA) was proposed to explain possible contributors to the sediment composition in the estuary and their relative mixing proportions. The presented model allows prediction of contribution of different material sources, even at contributions of less than 10%, on the basis of basic and easily measurable parameters (Mz, percentages of sand, silt and clay, and percentages of CaCO3 and TOC).