Combining pigment analysis and imaging microscopy to identify seasonal patterns in biomass and diversity of plankton

Multi-decade ocean monitoring of plankton is important to distinguish between short-term variability due to environmental drivers and longer-term changes due to climate stressors. Traditional methods for plankton enumeration are time consuming, have long lag times, and preservation may result in cell shrinkage. Fluorometric measurements of Chlorophyll-a concentrations exclude pigment separation, which is required to identify and quantify major taxonomic groups. To address these challenges, flow imaging microscopy (FIM) and high-performance liquid chromatography (HPLC) pigment analysis were combined to optimize the detection of plankton biomass seasonal patterns and their environmental drivers in the Bedford Basin, Nova Scotia, Canada. Eleven plankton sub-groups were detected, three of which overlapped between methods (pigmented dinoflagellates, diatoms and euglenoids). The HPLC was able to detect small cells (e.g. nano-diatoms), while FIM added higher taxonomic resolution and detection of non-pigmented cells, including two of the highest biomass groups, ciliates and non-pigmented dinoflagellates. Hierarchical clustering of the composite community dataset revealed four communities: C1-diatom/cryptophytes, C2-ciliates/chlorophytes, C3-dinoflagellates/haptophytes and C4-diverse community, which were influenced by seasonal cycles associated with nutrient replenishment and stratification. The complementary approaches increased monitoring efficiency of plankton population dynamics, which is essential for understanding the marine ecosystem and potential impacts of anthropogenic climate change.