Eddy stirring and phytoplankton patchiness in the subarctic North Atlantic in late summer

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 (Δc₄₉₀) 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 Δc₄₉₀. 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(t)/^-1 ≃ 7 km). The horizontal distribution of θ in the mixed layer is uncorrelated with S. Autospectra of θ, S, CF, and Δc₄₉₀ vary approximately as k^-2 for k less than k(t). For k ≥ k(t), CF and Δc₄₉₀ 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.