Pelagic primary production in the coastal Mediterranean Sea: variability, trends, and contribution to basin-scale budgets

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 similar to 25 % each of total coastal production. Strong regional variability is revealed in coastal PP, from high-production areas (> 300 g Cm-2) associated with major river discharges to less productive provinces (< 50 g Cm-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 CM-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.

Automated summary

We estimated the pelagic primary production in the coastal Mediterranean Sea and found that it contributes approximately 12% of the total carbon fixation in the region. The eastern basin has the highest production, followed by the western and Adriatic shelves. There is strong regional variability in coastal primary production, with some areas having high productivity associated with major river discharges and others having lower productivity in the southeastern Mediterranean. While interannual variations dominate the variability, a significant decline in primary production has been observed since 2012, coinciding with increasing sea surface temperatures and positive climate indices. Long-term trends show slight declines in most coastal areas except for the Adriatic, where primary production is increasing. The study emphasizes the importance of temporal and spatial scales of variability and provides insight into the contribution of coastal waters to the carbon balances in the Mediterranean Sea.