MEPS 456:7-19 (2012)  -  DOI: https://doi.org/10.3354/meps09677

ABSTRACT: To understand dissolved organic carbon (DOC) seasonal dynamics in a coastal oligotrophic 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 ~60 µmol C l⁻¹ in winter, with increasing values through the summer and autumn and reaching 100 to 120 µmol C l⁻¹ in November. There was high inter-annual variability in this summer DOC accumulation, with values of 36, 69 and 13 µmol 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 µmol 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.

KEY WORDS: Coastal DOC · Seasonality · Primary production · Bacterial metabolism · Physical transport