Water quality and avian inputs as sources of isotopic variability in aquatic macrophytes and macroinvertebrates

Different factors can affect the isotopic values of aquatic organisms. Nevertheless, water quality may be very important for aquatic organisms because they directly depend on it. In this article, we aimed to investigate if variations in the chemical and biological water characteristics affect the stable isotope values of aquatic organisms. We also wished to discuss alternative sources of isotopic variability. We analysed the water chemical characteristics, the input of extra nitrates from bird guano, and the δ15N and δ13C values for the macroinvertebrates and macrophytes present in freshwater irrigation ponds. Variability in the values of the analysed stable isotopes was high, even for the same species in different ponds. Water conductivity, nitrates, ammonium, organic nitrogen concentrations and COD (Chemical Oxygen Demand) directly correlated with the isotopic values. Besides, the input of extra nitrates from Larids’ (gulls and terns) guano might also increase the δ15N values at the ponds which these birds most intensively use. Nevertheless, the high δ15N values are difficult to explain in terms of water characteristics and excrements inputs and only general processes of denitrification could explain these values. Longer water residence times could cause extremely enriched isotopic values in both DIC (Dissolved Inorganic Carbon) and dissolved nitrates. This study shows different sources of isotopic variability which can prove useful to interpret stable isotopes studies.