Abstract
Freshwater phytoplankton blooms are increasing in prevalence and there are conflicting views on whether trace metals limit growth of key species and thus bloom formation. The Taupō Volcanic Zone (TVZ), New Zealand, was formed by multiple eruptions of a super-volcano which emitted rhyolitic tephra leaving lakes depleted in trace metals. This provides an opportunity to test the potential of trace metal limitation on freshwater phytoplankton growth under nanomolar concentrations. Growth responses of two algal species isolated from Lake Taupō, Dolichospermum lemmermannii (cyanobacteria) and Fragilaria crotonensis (diatom), to six biologically important trace metals (manganese, iron, zinc, cobalt, copper and molybdenum) were examined in culture experiments. These were conducted at three trace metal concentrations: (1) ambient, (2) two-times ambient, and (3) ten-times ambient concentrations in Lake Taupō. Elevated concentrations of iron significantly increased growth rates and maximum cell densities in D. lemmermannii, whereas no significant concentration dependence was observed for other trace metals. Fragilaria crotonensis showed no significant growth response to elevated concentrations of trace metals. These results highlight the importance of iron as a growth limiting nutrient for cyanobacteria and indicate that even small (twofold) increases in Fe concentrations could enhance cyanobacteria growth rates in Lake Taupō, potentially causing cyanobacterial blooms.
Highlights
Increasing anthropogenic pressures, such as the use of fertilizers to support agricultural practices, alterations in water flow regimes via the implementation of dam systems, and land use intensification through accelerated urbanisation[1] are decreasing the health of freshwater systems globally[2]
The Taupō Volcanic Zone (TVZ) lakes and the phytoplankton residing in their waters are uniquely suited to study the effect of trace metal limitation on the formation of cyanobacterial harmful algae blooms’ (CHABs) due to their naturally low trace metal concentrations and growth requirements
The respective concentrations of Mn and Co were a factor of 1.7 and 1.3 higher in D. lemmermannii with respect to target concentrations, whereas no significant increase was observed for F. crotonensis following the addition of culture cells to the medium (Table 2, Supplementary Data S6, online)
Summary
Increasing anthropogenic pressures, such as the use of fertilizers to support agricultural practices, alterations in water flow regimes via the implementation of dam systems, and land use intensification through accelerated urbanisation[1] are decreasing the health of freshwater systems globally[2]. The TVZ lakes and the phytoplankton residing in their waters are uniquely suited to study the effect of trace metal limitation on the formation of CHABs due to their naturally low trace metal concentrations and growth requirements. Further research is required to clarify the role of trace metals in CHAB formation and to distinguish concentration thresholds that result in enhanced growth of cyanobacteria and other algal taxa, such as diatoms (siliceous phytoplankton). While a previous investigation of two cyanobacteria species isolated from Lake Taupō waters showed that increased concentrations of Fe, Co and Mo positively affected growth rates and maximum cell c oncentrations[36], the effect of elevated trace metal concentrations on diatom growth was not tested. Confirmation of our hypotheses through the results of this study, would indicate that increasing trace metal concentrations would likely lead to enhanced phytoplankton growth and CHAB formation in lakes containing low trace metal concentrations
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