Centimetre-scale spatial structure of estuarine in vivo fluorescence profiles

The spatial complexity of phytoplankton distributions at large (metre to kilometre) scales has long been recognised. Here we present evidence of structure identified in the spatial distributions of phytoplankton measured at decimetre and centimetre scales. To characterise the temporal and spatial variation in the structure of phytoplankton distributions, fractal analysis was applied to vertical in vivo fluorescence profiles sampled at 25 cm and 5 cm sampling intervals. At the decimetre scale, mean fractal dimensions of in vivo fluorescence profiles were highly variable and not statistically distinct from those of complementary temperature and salinity profiles, suggestive of similar structuring processes for both biological and physical distributions. Sampling at the centimetre scale resolved increased structural complexity in in vivo fluorescence distributions, including the presence of multiple scaling regions, indicative of multiple structuring mechanism, together with the presence of distinct changes in the vertical structure of distributions for profiles separated by a horizontal distance of 7 cm. Shifts in centimetre-scale structural complexity were attributed to the behavioural reorganisation of cells due to a phototactic response to light and a gyrotactic or rheotactic response to localised shear. These results verify the importance of biological-physical structuring mechanisms down to the centimetre scale, where cell distributions and interactions hold significant ecological relevance.