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

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 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 transportResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 456: 7-19, 2012 (Zweifel et al. 1995), the north-western Mediterranean (Copin-Montégut & Avril 1993) and the Sargasso Sea (Carlson et al. 1994). Accumulated DOC can be subsequently transported by convective or mixing processes and surface currents, therefore contributing to the carbon balance of adjacent regions. The potential for DOC transfer is thus dependent on its residence time in the water and on the biological availability of the DOC pool to bacteria.In terms of its availability, DOC consists of a broad spectrum of molecules: (1) labile DOC that is readily usable by bacteria turning over in time scales of hours to days, (2) semi-labile DOC that resists rapid microbial degradation turning over on seasonal to annual time scales and (3) refractory DOC with turnover times of millennia (Carlson 2002). Some processes, such as protozoan grazing and viral lysis, are known to be particularly important in producing refractory DOC, as reported by Strom et al. (1997) and Suttle (2007). Both, the refractory nature of DOC and the limited availability of inorganic nutrients can inhibit DOC utilization by bacteria and can thus contribute to DOC accumulation (Legendre & Le Fèvre 1995, Thingstad et al. 1997. As a consequence, the composition and re fractory nature of DOC can vary depending on the trophic structure and the environmental conditions of the ecosystem . Despite the widespread occurrence of DOC accumulation in a wide variety of oceanic and coastal eco...

Horizontal and Vertical Distributions of Transparent Exopolymer Particles (TEP) in the NW Mediterranean Sea Are Linked to Chlorophyll a and O2 Variability

“…Bacteria did not seem to be a significant TEP source in June as no significant correlations between TEP and BP or TEP and the BP/Chla ratios were observed. Hence, TEP may accumulate in the sea surface at the beginning of the stratification period, similar to other phytoplankton-derived organic matter (Avril, 2002 ; Vila-Reixach et al, 2012 ; Romera-Castillo et al, 2013 ). These substances may be not taken up immediately by bacterioplankton due to nutrient defficiency, which is common in summer (Sala et al, 2002 ; Pinhassi et al, 2006 ).…”

Coastal Bacterioplankton Metabolism Is Stimulated Stronger by Anthropogenic Aerosols than Saharan Dust

“…TOC concentrations in surface waters of the Catalan coast range from 50 to ca. 150 μmol l -1 ( Vila-Reixach et al, 2012 ; Romera-Castillo et al, 2013 ; Table 1 ). Thus, the amount of TOC supplied by aerosols is usually much lower than the actual concentration in the seawater.…”

Marked seasonality of aerobic anoxygenic phototrophic bacteria in the coastal NW Mediterranean Sea as revealed by cell abundance, pigment concentration and pyrosequencing of pufM gene

“…Contrarily, at a temporal scale, we found a negative trend between AAP abundance and Chl a (http://onlinelibrary.wiley.com/doi/10.1111/1462-2920.12278/suppinfo) and inorganic nutrients. Furthermore, higher numbers of AAP bacteria and pigment content were measured in summer when dissolved organic carbon (DOC) accumulates annually and becomes chemically and structurally more complex in this coastal station (Vila‐Reixach et al ., ; Romera‐Castillo et al ., ). This observation may indicate that AAP bacteria rely on phototrophy as auxiliary energy source both when labile and/or refractory DOC are available, and that in spite of lower inorganic nutrient availability, summer conditions are clearly favourable for the growth of AAP bacteria.…”