Responses of lake phytoplankton to micronutrient enrichment: a study in two New Zealand lakes and an analysis of published data

Natural and anthropogenic changes in nutrient concentrations can affect phytoplankton in marine and freshwater environments. However, potential micronutrient limitation of phytoplankton productivity in fresh waters is often overlooked. To investigate the responses of lake phytoplankton to micronutrient enrichment, we conducted a study in two contrasting New Zealand lakes, and analysed data from the published international literature.We undertook nutrient enrichment bioassays of phytoplankton communities sampled from a mesotrophic reservoir and an eutrophic coastal lake to determine the relative occurrence of micronutrient (iron, boron, cobalt, copper, molybdenum) and macronutrient (nitrogen, phosphorus) limitation. In the mesotrophic reservoir, phytoplankton productivity was phosphorus limited. No evidence of micronutrient limitation was found in six bioassays in summer. In the eutrophic lake, tenfold enrichment of ambient micronutrient concentrations increased the primary productivity in four of 11 bioassays. During a cyanobacterial bloom in the eutrophic lake, experimental enrichment with boron, cobalt, copper or molybdenum increased primary productivity by 40%. These four micronutrients are commonly applied as agricultural fertiliser in the lake's catchment. Nitrogen or phosphorus enrichment had no effect on phytoplankton productivity at this time. Micronutrient limitation has been reported in more than 40 lakes internationally, and our analysis of published data suggests that the prevalence of micronutrient limitation is unrelated to lake size or trophic state. As micronutrient enrichment can significantly increase phytoplankton productivity in a range of lakes types, the potential contribution of micronutrient enrichment to eutrophication should not be overlooked.