Effects of vanadium on freshwater phytoplankton photosynthesis

Abstract

In six of the seven lakes studied, addition of vanadium at concentrations in the 2 to 165×10−7 M range decreased photosynthetic rates of phytoplankton in six lakes. The depression was not a result of photosynthate loss. The response of phytoplankton photosynthesis to vanadium addition was assessed as the slope of photosynthetic rate (as percent of control) plotted against added vanadium concentration. We then used univariate and multivariate statistics to determine whether the response to vanadium (“slope”) was influenced by phosphorus availability (estimated by 32P-turnover-time), phytoplankton biomass, and proportions of six taxonomic groups (as % of total phytoplankton biomass): Chlorophyta, Bacillariophyta, Chrysophyta, Pyrrophyta, Cryptophyta, and Cyanobacteria. Simple correlation analysis revealed that only biomass and cyanobacteria were significantly correlated (p<0.05) with the response to vanadium. To reduce redundancy among the eight ecological variables, we conducted a principal component analysis using data from the 22 experiments. The first two principal components accounted for 59% of the variance in the original variables. PC1 loaded highly and positively on biomass and cyanobacteria, and negatively on Bacillariophyta and Chrysophyta. PC2 loaded positively on Cryptophyta and Pyrrophyta. Only PC1 was significantly correlated (p <0.05) with the response to vanadium (“slope”). We conclude that lakes characterised by high phytoplankton biomass, high proportion of cyanobacteria, and low proportion of Bacillariophyta and Chrysophyta, are most vulnerable to inhibition of photosynthesis by vanadium. In the surface waters studied, dissolved vanadium at concentrations above the detection limit of 5.0×10−8 M was found only in the inshore areas of Lake Erie and in Hamilton Harbour, Lake Ontario.