Understanding the extent of universality in phytoplankton spatial properties

Abstract

Phytoplankton are one of the most visible signs of life in our oceans. They also are a key component of the global carbon cycle and of the marine food web. Their complex patterns at the sea surface are routinely seen in satellite images, though the first observations go back centuries.

The motivation of this thesis is to explore the spatial properties of phytoplankton. Inspired by ‘universal’ theories for the dynamics of turbulence, several ones have been proposed to explain phytoplankton patchiness as a balance between turbulent stirring by the water and biological processes involving the phytoplankton. The thesis examines the extant theories of plankton patchiness using a novel twin tracer approach, specifically using in situ simultaneous Chlorophyll-a and nitrate measurements from a cruise in the North Atlantic. A significant difference is observed between the variability spectra of the two biochemical variables, an outcome potentially explained only by one theory.

More generally, although numerous observations testify to the existence of scaling behaviour of phytoplankton spatial variability, the collation of these studies indicates considerable variability, and hence uncertainty, in the power law behaviour, specifically the value of the spectral ‘slope’. The many different techniques used to evaluate the spectrum, the different sources of data, and the geographical and temporal limitations associated with the data all contribute to adding noise and uncertainties in the estimates for the slope and make a comparison between studies difficult. In this thesis, the existence of the universal scaling properties of phytoplankton are tested over a wide range of spatial (sub-regional and regional) and temporal (few days to a year) scales using in situ, satellite data and model output. For this purpose a robust method is developed that reliably evaluates the spectrum of phytoplankton. A power-law behaviour in the phytoplankton spectrum is consistently found across the sources of data used and the range of scales studied (from 10 m to 130 km). However, stronger universality for the phytoplankton spectrum, defined as constant or uniform slope, is undermined by the significant variability in spectral slope that is consistently demonstrated across the spatial and temporal scales studied.

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