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

Abstract

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.

Keywords:

• Ionian Sea
• Deep-chlorophyll maximum
• Shelf processes
• Chla time dependency

Acknowledgements

The Italian National Research Council provided support for this study and writing of the manuscript. The Italian Ministry of University and Scientific and Technological Research (MURST), Programme ‘SAM’ (Automatic Monitoring System), funded the cruise. The authors are grateful for the excellent support provided by the captain and crew of the R/V Thetys during the cruise. They would also like to thank Dr. Ermanno Crisafi and Prof. Bruno D'Argenio of the CNR-IAMC, for their full support of the program and this study. We are grateful to Dr. Mariagrazia Galletta for her collaboration on the ship and in the laboratory, Mr. Michele Furnari for his technical support, and our colleagues at the institute for their various support.