| Summary: | Knowledge of carbon and nitrogen isotopic ratios in organic matter and their changes is important when studying nutrient cycles in aquatic ecosystems. Relationships between δ<sup>13</sup>C and δ<sup>15</sup>N values of suspended particulate organic matter (POM), water temperature, salinity, pH, redox potential, chlorophyll <i>a</i> concentration, primary production, and biomasses of different taxonomic groups of phytoplankton in the Neva Estuary were statistically analyzed. We tested the hypothesis that the studied physicochemical and biogeochemical characteristics, as well as the species composition of phytoplankton and its productivity, can be significant predictors of changes in the isotopic ratios of suspended particulate organic matter in estuaries. In the Neva Estuary, δ<sup>13</sup>C<sub>POM</sub> (−16.8–−27.6‰) and δ<sup>15</sup>N<sub>POM</sub> (2.3–7.3‰) changed synchronously. Statistical analysis showed that for both isotopes, the photosynthetic activity and taxonomic composition of phytoplankton are important. For <sup>13</sup>C<sub>POM</sub>, the second most important factor was water salinity, which was apparently associated with the transition of algae from CO<sub>2</sub> to HCO<sub>3</sub> consumption during photosynthesis in estuarine waters. For <sup>15</sup>N<sub>POM</sub> changes, the most important abiotic factor was pH. The study showed that the dependences of POM isotopic ratios on environmental variables obtained for continental and oceanic waters are also valid in transitional zones such as the Neva Estuary.
|
|---|