Abstract
The spatial structure of phytoplankton distributions in the mesoscale eddy field of the North Atlantic (vicinity of 20°W, 59°N) was examined with a series of high resolution, towed transects. Data were obtained after the fall transition in August 1991 as part of the Marine Light-Mixed Layers (MLML) program. Measurements of chlorophyll fluorescence (CF) and the beam attenuation coefficient anomaly (
Δc
490) show that chlorophyll and particles are confined to the surface mixed layer and seasonal pycnocline. Two water masses account for the potential temperature–salinity (
θ–
S) structure in the region: North Atlantic Central Water from the sub-tropical gyre and Sub-Arctic Intermediate Water from the sub-polar gyre. These water masses carry distinct particle distributions based on measurements of CF and
Δc
490. Spatial distributions of CF and
S in the mixed layer are spatially coherent for horizontal wavenumbers
k less than a transition wavenumber
k
t≈0.15 cycles
km
-1 (transition wavelength
λ
t=
k
-1
t≈7
km). The horizontal distribution of
θ in the mixed layer is uncorrelated with
S. Autospectra of
θ,
S, CF, and
Δc
490 vary approximately as
k
-2 for
k less than
k
t. For
k⩾
k
t, CF and
Δc
490 decrease approximately as
k
-3, while S spectra continue as
k
-2. These observations are consistent with physical control of phytoplankton distributions for
k<
k
t by eddy advection. They are consistent with control by non-conservative biological processes for
k⩾
k
t. This contradicts some theoretical models of chlorophyll spectra, which predict physical control of phytoplankton distributions above
k
t and biological control below. The contradiction may arise because the scales of variance sources assumed in the models are not consistent with observed scales in the MLML region.