Physical controls and mesoscale variability in the Labrador Sea spring phytoplankton bloom observed by Seaglider

Abstract

We investigated the 2005 spring phytoplankton bloom in the Labrador Sea using Seaglider, an autonomous underwater vehicle equipped with hydrographic, bio-optical and oxygen sensors. The Labrador Sea blooms in distinct phases, two of which were observed by Seaglider: the north bloom and the central Labrador Sea bloom. The dominant north bloom and subsequent zooplankton growth are enabled by the advection of low-salinity water from West Greenland in the strong and eddy-rich separation of the boundary current. The glider observed high fluorescence and oxygen supersaturation within haline-stratified eddy-like features; higher fluorescence was observed at the edges than centers of the eddies. In the central Labrador Sea, the bloom occurred in thermally stratified water. Two regions with elevated subsurface chlorophyll were also observed: a 5 m thin-layer in the southwest Labrador Current, and in the Labrador shelf-break front. The thin layer observations were consistent with vertical shearing of an initially thicker chlorophyll patch. Observations at the front showed high fluorescence down to 100 m depth and aligned with the isopycnals defining the front. The high-resolution Seaglider sampling across the entire Labrador Sea provides first estimates of the scale dependence of coincident biological and physical variables.

Figure 4. Fluorescence quenching (a) Near-surface (0–10 m, black line) and deeper (20–30 m, gray line) values of fluorescence-to-backscatter ratio F : bbp , and SeaWiFS iPAR (instantaneous incident light, hatched regions, described in Section 2) for July 4–9, 2005. Lower surface F : bbp ratios are observed during the day (higher iPAR), indicative of fluorescence quenching. (b) Day and night vertical profiles of F and bbp show the depth range over which fluorescence is quenched (0–30 m), while bbp is relatively constant in the surface layer.

Figure 12. Chlorophyll within and between fresh lenses. Mean vertical profiles of (a) salinity and (b) fluorescence bloom intensity inside (black) and at the edges of (blue-gray) lenses 1–3 in the north region. Lower salinities are in the lenses by definition, while higher average F values are at the edges of the lenses. Shaded regions around the profiles indicate one standard deviation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Eleanor Frajka-Williams

Professor of Ocean Dynamics in a Changing Climate

I am a physical oceanographer who uses ocean observations to investigate ocean dynamics and circulation in a changing climate. I have a particular interest in problems spanning scales (from micro- to large-scale) or spheres (biogeosphere, cryosphere, atmosphere), and in methods that leverage traditional observations with new platforms and satellite data.