1. a School of Biological and Environmental Sciences , University of Stirling , Stirling, FK9 4LA, UK
2. b Isotope Biochemistry Laboratory , Scottish Universities Environmental Research Centre , Rankine Avenue , Scottish Enterprise Technology Park , East Kilbride, G75 0QF, UK
3. c Balaton Limnological Research Institute of the Hungarian Academy of Sciences , Tihany, POB 35, H‐8237, Hungary