Fluid dynamical niches of phytoplankton types

Abstract

The biogeochemical role of phytoplanktonic organisms strongly varies from one plankton type to another, and their relative abundance and distribution have fundamental consequences at the global and climatological scales. In situ observations find dominant types often associated to specific physical and chemical water properties. However, the mechanisms and spatiotemporal scales by which marine ecosystems are organized are largely not known. Here we investigate the spatiotemporal organization of phytoplankton communities by combining multisatellite data, notably high-resolution ocean-color maps of dominant types and altimetry-derived Lagrangian diagnostics of the surface transport. We find that the phytoplanktonic landscape is organized in (sub-)mesoscale patches (10-100 km) of dominant types separated by physical fronts induced by horizontal stirring. These physical fronts delimit niches supported by water masses of similar history and whose lifetimes are comparable with the timescale of the bloom onset (few weeks). The resonance between biological activity and physical processes suggest that the spatiotemporal (sub-)mesoscales associated to stirring are determinant in the observation and modeling of marine ecosystems.

Fig. 1.

Ecological and physical (sub-)mesoscale structure from satellite data. (A) Total chlorophyll distribution (μg/L). (B) Dominant types identified from 8-d SeaWiFS composite by the PHYSAT algorithm during the spring bloom (November 24–December 1, 2001). Colors in B indicate diatoms (green), Prochlorococcus (red), Synechococcus (dark blue), nanoeukaryotes (yellow), Phaeocystis (magenta), and coccolithophorids (cyan). Boundaries among dominant types do not separate necessarily chlorophyll patches of different concentration, but may appear in regions of relatively homogeneous total Chl values (see Fig. S2). (C) Mean circulation. (D) Sea surface height (contours indicate millimeters of height above the reference geoid).

Fig. 2.

Identification and history of the fluid dynamical niches. (A) PHYSAT-detected dominant types at the beginning of the bloom season (August, 20, 2001). (B) Initial distribution of numerical tracers (August 20, 2001). (C) Distribution of the numerical tracers advected by the altimetry-derived large-scale circulation (altimetry filtered in space and time by 200 km and 3 mo) after 3 mo (November 28, 2001). (D) Distribution of the numerical tracers advected by altimetry-derived geostrophic currents after 3 mo (November 28, 2001). (E) Stirring rates (altimetry-derived finite-size Lyapunov exponents) in the region of interest. (F) Observed dominant phytoplankton types; yellow here is an aggregation of picoplankton (Prochlorococcus and Synecococcus) and nanoeukariotes, other colors as in Fig. 1. Superimposed in gray, physical fronts of panel E.