Influence of ocean circulation on the temporal variability of marine phytoplankton populations

Marine phytoplankton community dynamics are complex and noisy. At intra-seasonal time scales, ecological time series can be difficult to interpret and relate to environmental variability. This is in part due to the dispersal of phytoplankton by ocean currents and mixing. In order to quantify the contribution of ocean circulation to shaping ecological variability, we contrast changes observed in phytoplankton abundance and size structure through time at stationary locations and following individual water parcels (i.e., Eulerian and Lagrangian perspectives). In essence we ask: Does ecological variability in these two perspectives differ, by how much, and does the difference vary regionally and seasonally? We performed a global analysis of matching observations from the Global Drifter Program buoy trajectories (~13000 trajectories) and satellite-estimated chlorophyll and phytoplankton size fraction (total biomass of small and large cells). Chlorophyll variability following buoy trajectories is generally lower than observed at stationary locations along their path. We hypothesize that deviations from the latter general pattern are linked to ecological gradients in different regions of the ocean, and to the strength of dispersal. In regions with weak advection and mixing, ecological variability in Eulerian and Lagrangian perspectives are similar. In contrast, phytoplankton dynamics are more variable in the Lagrangian perspective when the buoy trajectory travels across large ecological gradients. Many ecological and biogeochemical observing programs in the ocean are Eulerian in nature (e.g., HOT, BATS), and this study assists in interpreting data from stations such as these by explicitly partitioning whether ecological variability is due to the passage of water or local dynamics.