Using fossilised pigments to understand cyanobacterial blooms in New Zealand lakes

Phytoplankton (including cyanobacteria) are a natural component of lake ecosystems and are the base of many food webs. However, changes in the lake catchment, the lake itself and the wider environment (e.g., climate change) can alter the composition of phytoplankton communities. Of recent concern is the increase in the abundance of cyanobacteria and the formation of blooms in many of New Zealand’s low-land lakes (>30% of those that are monitored). Because regular monitoring data does not stretch back more than 10-30 years and many lakes are not regularly monitored, it is difficult to ascertain whether the intensity of present-day blooms are a new phenomenon or are part of a pre-existing cycle. The pigments produced by cyanobacteria (and other phytoplankton) are deposited in lake sediment and can be extracted from sediment cores and surface sediment samples. Analysis of these pigments by high-performance liquid chromatography has allowed us to reconstruct the historical phytoplankton community from nine New Zealand lakes and to track the emergence of cyanobacteria in impacted lakes. We will present data on the performance of different cyanobacteria pigment indicators and several case studies to demonstrate how fossilised pigment data can be used to understand shifts in lake phytoplankton communities.