Historical variability in past phytoplankton abundance and composition in Doubtful Sound, New Zealand

•We present the first data of historical phytoplankton abundance and composition for Doubtful Sound.•Diatoms have been prevalent in the phytoplankton community of Doubtful Sound for at least 350 years.•Sedimentary pigment and diatom records demonstrate natural variability across the near-pristine fjord.•Complementary biomarkers and multiple sites needed for palaeoecological studies in fjords.

Doubtful Sound, New Zealand, provides an exceptional opportunity to study a ‘baseline’ coastal ecosystem with an intact watershed. We present the first data on historical changes in phytoplankton abundance and community composition for three sites in Doubtful Sound using sediment records. Profiles of sedimentary concentrations of β-carotene (a proxy of total algal abundance, 0.021–1.345 mmol g organic carbon−1) and carotenoids were generally depleted, indicating low autochthonous production. Phytoplankton pigments and diatom frustules in Doubtful Sound indicate that diatoms have been prevalent for at least the last ca. 350 years; however, the relative importance of marine and freshwater diatoms has varied through time. Further, the timing of change in phytoplankton biomass and community composition differed among the sites within Doubtful Sound. This finding highlights the need to use multiple sites and complementary biomarkers when studying historical changes in phytoplankton communities in complex ecosystems with strong physicochemical gradients such as fjords.