Variability and sustenance of the deep-chlorophyll maximum over a narrow shelf, Augusta Gulf (Sicily)

This work reports on the temporal variability of the deep-chlorophyll a maximum (DCM) in a Mediterranean coastal environment. In contrast to a supply through vertical diffusion normally found offshelf, in the study area the nutrients were supplied laterally through intermittent, pycnoclinal intrusions over the shelf by the enriched Messina Mixed Water mass. A 48 h time series revealed a strong variability in the nutrient concentrations and size-fractioned chlorophyll a distributions. This variability appeared to be less correlated with the diurnal biological cycle and more correlated with the availability of newly advected nutrients. The observations also demonstrated that the planktonic community of DCM remains vertically non-homogenous when exposed to strong vertical shears caused by deformations in the depth of the pycnocline. For this shelf situation, assumptions regarding the distribution of the DCM, e.g., steady state, vertical or horizontal coherence, were all counterindicated.