Seasonal dynamics and net production of dissolved organic carbon in an oligotrophic coastal environment

To understand dissolved organic carbon (DOC) seasonal dynamics in a coastal oligo trophic site in the north-western Mediterranean Sea, we monitored DOC concentrations monthly over 3 yr, together with the meteorological data and the food-web-related biological processes involved in DOC dynamics. Additional DOC samples were taken in several inshore-offshore transects along the Catalan coast. We found DOC concentrations of similar to 60 mu mol C l(-1) in winter, with increasing values through the summer and autumn and reaching 100 to 120 mu mol C l(-1) in November. There was high inter-annual variability in this summer DOC accumulation, with values of 36, 69 and 13 mu mol C l(-1) for 2006, 2007 and 2008, respectively. The analysis of the microbial food-web processes involved in the DOC balance did not reveal the causes of this accumulation, since the only occasion on which we observed net DOC production (0.3 +/- 1 mu mol C l(-1) d(-1) on average) was in 2007, and the negative DOC balance of 2006 and 2008 did not prevent DOC accumulating. The DOC accumulation episodes coincided with low rates of water renewal (average 0.037 +/- 0.021 d(-1) from May to October) compared with those of winter to early spring (average 0.11 +/- 0.048 d(-1) from November to April). Indeed, the amount of DOC accumulated each year was inversely correlated with the average summer rainfall. We hypothesize that decreased DOC turn-over due to photochemical or biological processes-mostly active during the summer-and low water renewal rate combine to determine seasonal DOC accumulation and influence its inter-annual variability.