Abstract
AbstractThe complex patterns observed in marine phytoplankton distributions arise from the interplay of biological and physical processes, but the nature of the balance remains uncertain centuries after the first observations. Previous observations have shown a consistent trend of decreasing variability with decreasing length scale. Influenced by similar scaling found for the properties of the water that the phytoplankton inhabit, “universal” theories have been proposed that simultaneously explain the variability seen from meters to hundreds of kilometers. However, data on the distribution of phytoplankton alone have proved insufficient to differentiate between the many causal mechanisms that have been suggested. Here we present novel observations from a cruise in the North Atlantic in which fluorescence (proxy for phytoplankton), nitrate, and temperature were measured simultaneously at scales from 10 m to 100 km for the first time in the open ocean. These show a change in spectra between the small scale (10–100 m) and the mesoscale (10–100 km) which is different for the three tracers. We discuss these observations in relation to the current theories for phytoplankton patchiness.
Highlights
Satellite images regularly show visually striking cases of phytoplankton patchiness, a phenomenon which may play a role in ecosystem stability [Bracco et al, 2000; Perruche et al, 2011] and phytoplankton diversity [Karolyi et al, 2000; Perruche et al, 2010]
The power spectrum—which quantifies the amount of variability as a function of length scale—has proved a popular tool to describe phytoplankton spatial variability
Both nitrate and chl-a display a similar spectral shape, often described as a ‘‘knee’’ [Steele, 1978] when plotted on a logarithmic scale, which is characteristic of a shift in power law behavior. This shift is less pronounced for the nitrate spectrum (a 5 21.29 for scales 8–115 km; a 5 21.75 for scales 10–100 m) than for phytoplankton (a 5 20.65 for 8–115 km; a 5 22.64 for 10–100 m)
Summary
Satellite images regularly show visually striking cases of phytoplankton patchiness, a phenomenon which may play a role in ecosystem stability [Bracco et al, 2000; Perruche et al, 2011] and phytoplankton diversity [Karolyi et al, 2000; Perruche et al, 2010]. Such heterogeneous spatial structures arise from the combined action of biological and physical processes, at the mesoscale and submesoscale (1–500 km) where both act on similar timescales. Observations suggest that it is invariably negative; the steeper the line, the less variability exists at smaller scales relative to larger ones
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