Stable C and N Isotope Composition of Suspended Particulate Organic Matter in the Neva Estuary: The Role of Abiotic Factors, Productivity, and Phytoplankton Taxonomic Composition

Mikhail S Golubkov,Sergey M Golubkov,Vera N Nikulina,Alexei V Tiunov

doi:10.3390/jmse8120959

Abstract

Knowledge of carbon and nitrogen isotopic ratios in organic matter and their changes is important when studying nutrient cycles in aquatic ecosystems. Relationships between δ13C and δ15N values of suspended particulate organic matter (POM), water temperature, salinity, pH, redox potential, chlorophyll a 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, δ13CPOM (−16.8–−27.6‰) and δ15NPOM (2.3–7.3‰) changed synchronously. Statistical analysis showed that for both isotopes, the photosynthetic activity and taxonomic composition of phytoplankton are important. For 13CPOM, the second most important factor was water salinity, which was apparently associated with the transition of algae from CO2 to HCO3 consumption during photosynthesis in estuarine waters. For 15NPOM 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.

Highlights

Stable isotope analysis is a standard technique for tracking organic matter transport and trophic interactions in both terrestrial and aquatic food webs [1]
Pairwise correlations and multi-regression showed that for both isotopes, and especially for δ13CPOM values, the photosynthetic activity of algae is important, the indicator of which is the concentration of chlorophyll a and the level of plankton primary production
Statistical analysis showed that for δ13CPOM values, the second most important factor is water salinity, which is apparently associated with the transition of algae during active photosynthesis from CO2 consumption to HCO3

Summary

Introduction

Stable isotope analysis is a standard technique for tracking organic matter transport and trophic interactions in both terrestrial and aquatic food webs [1]. It is known that the carbon isotope ratios 13C/12C or “isotopic signature” expressed using the delta notation (δ13C) of organic matter of continental and marine origin is different. The largest active pool in the global carbon cycle is dissolved inorganic carbon ( DIC) in the oceans. The 13C/12C isotope ratio in the DIC ocean pool depends on the atmospheric CO2 input and the concentration of marine carbonates. The concentration of 13C in DIC depends on the ratio of dissolved atmospheric CO2, emission of carbon dioxide during the dissolution of rock carbonates, and CO2 release during the mineralization of soil organic matter coming from the catchment [12]. The isotopic ratio of rock carbonates, depending on the composition, varies from +2% in carbonate-rich to −12% in carbonate-poor soils

Results

Discussion

Conclusion