Temperature effects on decomposition of a Posidonia oceanica mat

Posidonia oceanica mats act as an important carbon (C) sink in the Mediterranean by accumulating roots, rhizomes and leaf sheaths, and allochthonous organic matter over millennia due to low rates of decomposition of organic matter. Thus far, rates of decomposition have only been modeled, whereas few attempts have been made to measure decomposition rates under in situ conditions in P. oceanica mats. We studied decomposition of P. oceanica mats by simulating in situ conditions at different temperatures. Samples from 5 different depths in a 3 m thick vertical profile of a P. oceanica mat were each incubated under anoxic conditions for 7 mo at 5 different temperatures (15, 20, 25, 30, and 35C). Our measured low mat mineralization rates confirm slow decomposition and support that P. oceanica mats act as important long-term C sinks in the Mediterranean. However, between 15 and 25C, both C and nitrogen (N) mineralization increased with increasing temperature, and generally declined above 25C, probably due to unbalanced bacterial growth in the mat. Relatively, N mineralization was stimulated more than CO 2 release (i.e. up to 14.3 and 4.5 times, respectively), suggesting that regeneration of N in P. oceanica mats is enhanced with increasing temperatures up to 25C. This may affect the nutrient dynamics in this oligotrophic ecosystem. Our data suggest that CO 2 release due to decomposition of the mat is potentially enhanced under a scenario of climate change, thus reducing the C sequestration capacity of the mats during this century.