| Summary: | Following a review of published reports and small-scale experiments performed on coastal waters and sediments of the English Channel, the currently known impacts of ocean acidification on the microbial nitrogen cycle are presented to enable discussion of direct and indirect effects which are relevant to the biogeochemistry of the Mediterranean Sea. There is great potential for changes in the balance of the nitrogen nutrients NO3- and NH4+ and between nitrogen and other nutrients including phosphorus, which ultimately infer changes in community composition and ecological status. Data from the English Channel support theoretical predictions that the balance of theNH4+:NH3 equilibrium in seawater favours NH4+ under acidic conditions, which when coupled with stable PO43- concentrations may act to increase the ratio of dissolved N:P. In surface waters, nitrifying bacteria were found to be sensitive to conditions of elevated CO2, so that nitrification rates were reduced by approximately 30% as a result of a reduction in pH of 0.18. In coastal sediments, the removal of NO3- from overlying water, increased under high CO2, as did the activity of denitrifying and/or annamox bacteria which coupled with changes in nitrification may lead to depletion of pelagic NO3- in absolute terms and relative to NH4+.Yet robust information on many of these processes under the influence of enhanced pCO2 is scarce. Recent research related to the fixation of carbon into organic and inorganic material, which is relevant to this study, has intensified, but has produced information that is at times contradictory. A condition which fuels our call for focussed experimental and model studies of decreasing pH in the Mediterranean basin. We tentatively propose that ocean acidification will act to increase the oligotrophic nature of the Mediterranean Sea and increase the degree of phosphorus limitation currently found, which will almost by definition, contribute to reduced productivity and carbon export.
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