The effects of biomass depth distribution on phytoplankton spring bloom dynamics and composition in an Arctic fjord

Abstract

Fjord systems are among the most productive and best described of Arctic marine habitats. Contributing substantially to that overall productivity, spring phytoplankton blooms are one of the most important features of fjord systems and of mid- and high-latitude oceans in general. Understanding specific mechanisms that control the timing, magnitude, and composition of these blooms is among the most central, and yet unresolved, questions within biological oceanography. To elucidate how the distribution of phytoplankton with depth affects bloom dynamics, we analyzed a comprehensive dataset on spring blooms in an Arctic fjord from three consecutive years, covering environmental drivers as well as ecological and biogeochemical dynamics. Our data show that the build-up of biomass correlated positively with the chlorophyll layer depth (CLD, defined as the depth at the bottom of the layer containing a threshold concentration of chlorophyll a), with highest rates of biomass accumulation occurring in more depth-extended distributions despite lower light availability at greater depths. Based on our results, we hypothesize that this relationship is caused by reduced grazing pressure under conditions of deepening mixing layers. Further, we postulate that changes in the depth to which phytoplankton biomass is distributed have the potential to control the species composition of the Arctic phytoplankton spring bloom, with diatoms dominating in situations with shallow CLDs and Phaeocystis pouchetii with deepening CLDs, which may impact the biogeochemistry of the studied fjord system differently.