Marine lake as in situ laboratory for studies of organic matter influence on speciation and distribution of trace metals Marina Mlakar n , Željka Fiket, Sunčana Geček, Neven Cukrov, Vlado Cuculić Ruđer Bošković Institute, Division for Marine and Environmental Research, Bijenička 54, 10001 Zagreb, Croatia article info Article history: Received 5 December 2014 Received in revised form 3 April 2015 Accepted 28 April 2015 Available online 29 April 2015 Keywords: Marine lake Geochemical processes Trace metals Sediments Organic matter abstract Karst marine lakes are unique marine systems, also recognized as in situ “laboratories” in which geo- chemical processes on a different scale compared to the open sea, can be observed. In this study, organic matter cycle and its impact on distribution of trace metals in the marine lake Mir, located on Dugi Otok Island, in the central part of the eastern Adriatic Sea, was investigated for the first time. Studied marine lake is small, isolated, shallow basin, with limited communication with the open sea. Intense spatial and seasonal variations of organic matter, dissolved and particulate (DOC, POC), and dissolved trace metals concentrations in the water column of the Lake are governed predominantly by natural processes. En- hanced oxygen consumption in the Lake during summer season, high DOC and POC concentrations and low redox potential result in occasional occurrence of anoxic conditions in the bottom layers with ap- pearance of sulfur species. Speciation modeling showed that dissolved trace metals Cu, Pb and Zn, are mostly bound to organic matter, while Cd, Co and Ni are present predominantly as free ions and in- organic complexes. Trace metals removal from the water column and their retention in the sediment was found to depend on the nature of the relationship between specific metal and organic or inorganic phases, sulfides, Fe-oxyhydroxydes or biogenic calcite. The above is reflected in the composition of the sediments, which are, in addition to influence of karstic background and bathymetry of the basin, sig- nificantly affected by accumulation of detritus at the bottom of the Lake. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Marine lakes represent a peculiarity of karstic coast. Formed by flooding of karst depression and hidden inland, they are char- acterized by poor water exchange with the sea, low tidal ampli- tudes, and temperature extremes, stratification of the water col- umn and occurrences of hypoxia/anoxia in the bottom layer. All these features combined with their isolated character differentiate marine lakes in geochemical as well as biochemical terms in comparison to adjacent sea. Approximately 200 marine lakes are documented worldwide so far (Hamner et al., 1982; Burnett et al., 1989; Landing et al., 1991; Orem et al., 1991; Bates et al., 1993; Lyons et al., 1996; Hamner and Hamner, 1998; Cerrano et al., 2006; Colin, 2009 and references therein). Listed literature mainly deals with biological aspects of marine lakes, while comprehensive geochemical studies of such systems, especially on distribution of trace metals and organic matter, are scarce. Among all the various pollutants, dangerous to fragile and isolated marine environments, heavy metals are the most toxic (Radix et al., 2000), persistent and abundant substances, prone to accumulation as well as resistant to biodegradation. In lacustrine sediments heavy metals can be of both natural as well as an- thropogenic origin. Once introduced into the aquatic environment, metals are redistributed throughout the water column, deposited or accumulated in sediments and consumed by biota (Fichet et al., 1998). Compared to metals, organic matter has a more dynamic cycle due to seasonal changes, and is influenced by contribution via wet deposition and surrounding soil. Nevertheless, in small isolated waterbody, eg. marine lake, biogeochemical reactions af- fecting the specified metal and organic matter distribution take place on an entirely different scale compared to the sea. Accord- ingly, these and similar systems can serve as natural “laboratories”, i.e. environments in which interactions between metal and organic matter can be studied under conditions almost like in a closed box model. Specific conditions are required to form a marine lake and therefore they represent unique and fragile marine environments. Knowledge on the distribution of trace metals and organic matter in water column and sediment of such systems is prerequisite for better understanding of the ongoing biogeochemical processes ensuring their sustainability and protection. This paper describes geochemical processes that affect the distribution of trace Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research http://dx.doi.org/10.1016/j.csr.2015.04.024 0278-4343/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Fax: þ385 1 4680 231. E-mail address: mlakar@irb.hr (M. Mlakar). Continental Shelf Research 103 (2015) 1–11