Abstract
Abstract. We estimated pelagic primary production (PP) in the coastal (<200 m depth) Mediterranean Sea from satellite-borne data, its contribution to basin-scale carbon fixation, its variability, and long-term trends during the period 2002–2016. Annual coastal PP was estimated at 0.041 Gt C, which approximately represents 12 % of total carbon fixation in the Mediterranean Sea. About 51 % of this production occurs in the eastern basin, whereas the western and Adriatic shelves contribute with ∼25 % each of total coastal production. Strong regional variability is revealed in coastal PP, from high-production areas (>300 g C m−2) associated with major river discharges to less productive provinces (<50 g C m−2) located in the southeastern Mediterranean. PP variability in the Mediterranean Sea is dominated by interannual variations, but a notable basin-scale decline (17 %) has been observed since 2012 concurring with a period of increasing sea surface temperatures in the Mediterranean Sea and positive North Atlantic Oscillation and Mediterranean Oscillation climate indices. Long-term trends in PP reveal slight declines in most coastal areas (−0.05 to −0.1 g C m−2 per decade) except in the Adriatic where PP increases at +0.1 g C m−2 per decade. Regionalization of coastal waters based on PP seasonal patterns reveals the importance of river effluents in determining PP in coastal waters that can regionally increase up to 5-fold. Our study provides insight into the contribution of coastal waters to basin-scale carbon balances in the Mediterranean Sea while highlighting the importance of the different temporal and spatial scales of variability.
Highlights
Coastal ocean waters (i.e., < 200 m depth) are an important link between the land and the open ocean
In this study we focused on the contribution of coastal waters to the overall pelagic primary production (PP) in the Mediterranean Sea
While the mean coastal values for the Mediterranean (100±91 g C m−2) are somewhat lower than the mean values over the continental shelves of the world ocean (160 ± 40 g C m−2; Smith and Hollibaugh, 1993), the impact of coastal pelagic PP on total basin production (12 %) is in the high range of the estimations for other seas (Muller-Karger et al, 2005). This estimation is subject to the uncertainties inherent to using satellite ocean color, which is limited to the upper ocean and has poor performance in some areas (i.e., Case-2 waters)
Summary
Coastal ocean waters (i.e., < 200 m depth) are an important link between the land and the open ocean. They act as a buffer between terrestrial and human influences and the open ocean (Liu et al, 2000) Despite their relatively reduced extension (∼ 7 % of ocean surface area; Gattuso et al, 1998), they behold some of the most productive habitats on the planet. They have a disproportionate importance in many basin-scale biogeochemical and ecological processes, including carbon and nitrogen cycling, and in the maintenance of marine diversity (Cebrian, 2002; Coll et al, 2010; Dunne et al, 2007). Coastal seawaters support high primary production (PP), contributing to some 10 % of global ocean PP and up to 30 % if estuarine and benthic production is considered (Ducklow et al, 2001; Muller-Karger et al, 2005).
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