Small-scale nutrient patches in tidally mixed coastal waters

In the eastern English Channel, characterized by its megatidal regime and the resulting very high turbulence, phytoplankton biomass and purely passive scalars such as temperature and salinity are basically regarded as homogenised by turbulent fluid motions. However, recent studies have demonstrated-on the basis of innovative statistical analyses in marine ecology-that these parameters were heterogeneously distributed at a small scale, We extend these concepts to high-resolution time series of dissolved nitrogen (NO2-). We first demonstrate the validity of our sampling procedure of continuously measuring nitrite concentration (3 s temporal resolution), Second, we describe how these time series recorded at different times of 4 tidal cycles can be characterized as heterogeneously distributed using fractal and multifractal parameters, and then can be described in terms of small-scale patches. In addition, we show how multifractal parameters can be regarded as being both qualitatively and quantitatively more informative than a single fractal dimension. These parameters showed very specific temporal patterns revealing the absence of density-dependence of nitrite distribution. In contrast, nitrite distributions clearly appeared to be less heterogeneously distributed under higher turbulence, suggesting a physical control of small-scale nutrient patchiness, Nevertheless, taking into account the different structures of nitrite distributions under high and low turbulence, we suggest that the observed small-scale nutrient patches could be the result of complex interactions between hydrodynamic conditions, biological processes related to phytoplankton populations, and the productive efficiency of bacterial populations, This hypothesis is supported by observations that the structure of temperature and salinity-regarded as passive scalars under purely physical control of turbulent motions-recorded simultaneously to the nitrite data remained similar under all hydrodynamic conditions.