Yesterday we completed our second cross-sectional transect of the upwelling filament we have been studying. This transect complemented the four-day ‘cycle’ we just finished. During cycles, we pick a parcel of ocean water, deploy drifting instruments, and follow those instruments (and the water) for several days, sampling as we go. For transects, we use satellite images of sea surface temperature and phytoplankton chlorophyll to determine the overall extent of the filament, and then draw a line through it and sample along that. Ideally, the midpoint of the line captures the core salty, upwelled, and (presumably) high-productivity waters of the filament, while the endpoints capture lower-productivity waters on either side.
Transects are fast-paced, all-night-through-afternoon affairs. This one encompassed a broad section of the filament, which meant 11 sampling stations spaced 5 nautical miles, or about 45 minutes, apart. So by the time we finished collecting water and processing plankton nets from one station, we turned around and sampled at the next station.
Our sampling produced successful and very interesting results! The central part of the transect (filament core) showed cooler temperatures and elevated salinity and fluorescence, consistent with the waters that we measured in Cycle 1, closer to the coast at the newly-upwelled origins of the filament. Plankton samples captured green, phytoplankton-rich waters across the filament, with a notable drop-off to very blue water and reduced biomass at our last station (outside of the filament).
We have moved farther west for Cycle 3, to sample the narrow leading tip of the filament – presumably the initial upwelled waters that have since evolved biologically as they have been transported away from shore. We are still seeing lots of big diatoms (a type of phytoplankton that indicates highly-productive waters), but they are mostly dying and sinking, suggesting that they have already bloomed and run their course. The fact that we are finding them offshore provides evidence for exactly the questions we are out here to explore: how much are nearshore upwelling blooms transported offshore, and what does that mean for the ecosystem?
Posted by: Laura Lilly, SIO