How do microbial communities keep living in the Mediterranean outflow within northeast Atlantic intermediate waters?

Abstract

Abstract As part of the France-JGOFS programme, the Mediterranean outflow within the Atlantic Ocean was investigated during MEDATLANTE cruises (January and August 1989). The present work is focused on the export and fate into the Atlantic of the microbial communities carried away from the Mediterranean Sea by this outflow that generates Meddies (Mediterranean eddies). The respiratory activity of micro-organisms (≤200 μm) was monitored through their electron transport system (ETS) activity in Meddy “Nicole” (36°25′N, 10°35′W), Meddy “Yseult” (36°20′N, 16°W), and in waters of Mediterranean origin (MOW) at 36°57′N, 17°02′W. On each cruise, profiles of ETS activity through the water column (0–2000 m) were established in the Mediterranean Sea to serve as a reference for the ETS activity profiles in the Atlantic. ETS activity within the Meddies or MOW was similar to that in the Mediterranean Sea at the same period and significantly higher (up to three times) than that calculated for intermediate Atlantic waters. Within investigated Meddies and MOW, rates of metabolic CO2 production varied from 64.8 mg C m−2 day−1 (Meddy Nicole) to 200.5 mg C m−2 (Meddy Yseult). This represents up to 10 times the rate of metabolic CO2 production within the corresponding 500–1500 m layer in the Sargasso Sea. Considering the life-time of micro-organisms (a few days) and the age of Meddies and MOW (up to 1 year), the microbial communities evidenced by their ETS activity at stations far from the Strait of Gibraltar must have either reproduced themselves or been replaced by opportunistic communities. The initial loading of the outflow in fresh organic material, close to Gibraltar Strait, cannot account for the relatively higher rate of mineralization by microorganisms. We suggest a hypothesis based on the dynamics of Meddies.