Distribution of Redox-sensitive Elements in Bottom Waters, Porewaters and Sediments of Rogoznica Lake (Croatia) in Both Oxic and Anoxic Conditions

Abstract

Abstract. Geochemical, mineralogical and sedimentological analyses were carried out to contrast two different sites (respectively characterized by permanently oxic and anoxic conditions) in a small, meromictic, seawater lake. In fact, due to relatively high organic matter content, and reduced water exchange, the Rogoznica Lake has almost permanent anoxic conditions below the depth of 12 m, where sediment can be considered an anoxic-sulphidic sedimentary environment. Different water column and sediments redox conditions affect the distribution and speciation of major redoxsensitive metals (Fe, Mn, Mo), reduced sulphur species (RSS) and dissolved organic C (DOC). Trace metals, especially those that accumulate in anoxic-sulphidic environments (Fe, Mo) showed a marked enrichment in the solid phase, whereas the low solubility of sulphides leads to low porewater concentrations. The relatively high sedimentary enrichment of Mo (up to 81 mg/kg) also confirms highly anoxic conditions within the Rogoznica Lake sediments. Results clearly show that chemical species within the sediments will tend towards equilibrium between porewater and solid phase according the prevailing environment conditions such as redox, pH, salinity, DOC.

Community Shift from Phototrophic to Chemotrophic Sulfide Oxidation following Anoxic Holomixis in a Stratified Seawater Lake

“…Interestingly, the abundance of Deltaproteobacteria in the hypolimnion is comparable throughout seasons (see Table S6 in the supplemental material), while RSS concentrations are increased during the summer months (see Table S5 in the supplemental material). Higher water temperatures, higher productivity in the epilimnion, and greater export of cells and organic matter from the surface (15,20) could facilitate increased sulfate reduction rates in the anoxic bottom layers and sediments during the summer months. However, epilimnion production, organic matter export to the hypolimnion, and sulfate reduction rates in anoxic sediments and waters remain to be quantified to test this hypothesis.…”

“…The epilimnion is similar to waters of the adjacent Adriatic Sea in temperature and salinity, while the hypolimnion is highly enriched in nutrients (NH 4 ϩ , up to 150 M; PO 4 3Ϫ , up to 22 M; SiO 4 4Ϫ , up to 400 M) and dissolved organic carbon (DOC) (up to 6 mg liter Ϫ1 ) (15)(16)(17)(18). This is assumed to be a consequence of extensive organic matter remineralization (19,20) coupled to microbial sulfate reduction in anoxic lake sediments and the hypolimnion (21), where sulfide concentrations reach up to 5 mmol liter Ϫ1 (15,17,19,21,22). The depth of the chemocline increases from winter to summer months (15).…”

Enhanced viral activity and dark CO2 fixation rates under oxygen depletion: the case study of the marine Lake Rogoznica

“…In this regard, the marine Rogoznica Lake in the Adriatic Sea is an ideal model for investigating the impacts of global change, and especially oxygen depletion, on marine systems . This easily accessible natural macrocosms is characterized by the presence of strong gradients in temperature (T), salinity (S), dissolved oxygen (O 2 ) and pH (Ciglenečki et al, 2006;, and presents both oxygenated areas (displaying natural environmental conditions of the Adriatic Sea; Pjevac et al, 2015 and ref. therein) and oxygen-depleted zones.…”

“…therein) and oxygen-depleted zones. The physicalchemical characteristics of the marine Rogoznica Lake have been investigated in detail and over the long term (Ciglenečki et al, 2006;2015 and ref. therein).…”