The role of stromatolites in explaining patterns of carbon, nitrogen, phosphorus, and silicon in the Sečovlje saltern evaporation ponds (northern Adriatic Sea)
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In summer 2007, biweekly benthic fluxes of the biogenic elements carbon (C), nitrogen (N), silicon (Si), and phosphorus (P) were studied in the Sečovlje saltern (salt-making facility) in the northern Adriatic Sea, Slovenia in order to determine the impact of stromatolite (“petola”) on the geochemical properties of saltern sediments.
Materials and methods
The brine and pore waters were analyzed for salinity, NH 4 + , NO 3 − , PO 4 3− , SiO 4 4− , total dissolved nitrogen, total dissolved phosphorus, and fluorescent dissolved organic matter. The sediment was analyzed for organic carbon (OC), total nitrogen (TN), total and organic phosphorus (OP), and biogenic Si concentrations, as well as values of δ 13COC and δ 15NTN.
Results and discussion
Nutrient concentrations in brine water increased along the salinity gradient due to different processes, such as the evaporative concentrations of seawater, bacterial activity, more pronounced transformation and degradation of organic matter, and regeneration of nutrients. The petola from the Sečovlje saltern, which is predominately composed of cyanobacterial and diatom communities, develops during the early evaporation stage and survives during high salinity and halite crystallization. Nitrogen fixation and P removal were the principal biogeochemical processes controlling dissolved inorganic N and P concentrations. At higher salinities, N limitation was more important. Microbes decomposed at higher salinities, and the remineralized N and P nutrients were released from surface pore waters to the brine. OP remineralization was also an important process influencing the distribution of PO 4 3− concentrations in pore waters deeper in the sediments. The increasing SiO 4 4− concentrations with increasing salinity in the brine waters were due to dissolution of diatom frustules, while the decrease in pore water SiO 4 4− was probably the consequence of microbial uptake.
This study provides a better understanding of nutrient cycling and the geochemical processes in the Sečovlje saltern.
KeywordsCyanobacteria Hypersaline environment Northern Adriatic Nutrients Stromatolites
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