Assessing nature and dynamics of POM in transitional environment (the Curonian Lagoon, SE Baltic Sea) using a stable isotope approach

The nature of the particulate organic matter (POM) as well as its temporal and spatial distribution and dynamics in the Curonian Lagoon (south-eastern part of the Baltic Sea) were investigated. The organic matter was characterized by the organic carbon and nitrogen content, delta C-13 and delta N-15 signatures as well as POC/Chl-a and C/N ratios. Additionally, data on hydrological, chemical and biological parameters were used for better understanding the POM distribution and dynamics. The sampling was performed at 13 stations in the Curonian Lagoon and its outflow in the Baltic Sea during the 2012-2013 period. Samples were also collected at the Nemunas River mouth in order to test the riverine impact. Obtained results showed that isotopic values of carbon and nitrogen ranged from -36.1 parts per thousand to -25.2 parts per thousand and from -0.9 parts per thousand to 15.5 parts per thousand, respectively. The isotopic composition, together with the low C/N molar (similar to 7) and POC/Chl-a ratios ( < 100) of the POM, suggested the dominance of living phytoplankton in POM throughout the year with the higher input of detrital material (C/N > 10, POC/Chl-a ratios > 100) in late autumn - winter. The results of multivariate analysis evidenced a spatial distinction of POM distribution in the northern transitional and central confined areas and allowed us to distinguish the main driving factors. The seasonal variation of the delta C-13 and delta N-15 values in POM (towards higher delta C-13 and lower delta N-15 values in the summer - early autumn period and lower delta C-13 and higher delta N-15 values in the late autumn - spring period) was determined by combination of factors such as availability of inorganic carbon and nitrogen, the riverine discharge, seasonal phytoplankton succession and by the short-term saline water intrusion to the northern-transitional part of the lagoon.