Organic carbon and bacterial heterotrophic activity in the maximum turbidity zone of the Seine estuary (France)

Abstract

The aim of this study is to understand the sources, biodegradability and fate of organic carbon entering the maximum turbidity zone (MTZ) of the Seine estuary. Dissolved (DOC) and particulate organic matter (POC), and their biodegradable fractions (BDOC and BPOC) were investigated immediately upstream and within the MTZ in various hydrological conditions together with bacterial production of free-living and particle-associated cells. At the entrance of the MTZ, average DOC was 3.6 mg C l -1 with a biodegradable fraction of 23 %. The DOC and BDOC/DOC ratio decreased linearly with increasing salinity, indicating a dilution of river freshwater with a higher DOC and higher BDOC/DOC ratio than marine waters. The organic content of suspended matter (SM) at the entrance of the MTZ was high (POC/SM ratios up to 10.6 %), especially during phytoplankton blooms when high BPOC/POC ratios were also found. Within the MTZ, the carbon content of SM was lower and less variable; the biodegradable fraction of POC in the MTZ was lower than upstream, illustrating the progressive bacterial degradation of BPOC within the MTZ. Total bacterial production in the MTZ fl uctuated between 0.2 and 5.1 µg C l -1 h -1 ; average production of free-living bacteria was 0.77 µg C l -1 h -1 while that of attached bacteria was 1.47 µg C g SM -1 h -1 . Growth rates of attached bacteria were, on average, three times higher than those of free-living ones. Bacterial activity can explain the decrease in the POC/SM ratio observed between summer and fall in the MTZ.

Relationship between nutrients and plankton biomass in the turbidity maximum zone of the Pearl River Estuary

“…For example, the periodic resuspension of particles at the TMZ may facilitate the oxidation of recalcitrant particulate organic matter by recharging the metal-oxide phases due to repeated exposure to oxygenated water (Aller, 1994(Aller, , 1998. Several studies (Crump and Baross, 1996;Ploug et al, 2002;Servais and Garnier, 2006) have shown that bacteria attached to particles dominated the total bacterial activity in TMZ, and bacterial mineralization of the organic carbon was enhanced by the high suspended substances concentration in the TMZ. In addition, the high concentrations of the suspended particles in the TMZ decrease the light penetration which restricts phytoplankton growth (Garnier et al, 2001).…”

High alkaline phosphatase activity in phosphate replete waters: The case of two macrotidal estuaries

“…However, it is well known that bacterial production is particularly high in estuaries, with the highest values usually found in the maximum turbidity zone (Rochelle-Newall et al 2004;Servais and Garnier 2006). This high carbon production is supplied by the biodegradability of organic carbon, particulate organic carbon being the main source (Garnier et al 2008).…”

Significant decomposition of riverine humic-rich DOC by marine but not estuarine bacteria assessed in sequential chemostat experiments

“…These findings of enhanced bacterial decomposition of humic-rich DOC by a polyhaline bacterial community at high salinities relative to that by oligohaline bacterial communities of the upper sections of the Weser estuary are consistent with reports of a generally conservative mixing of DOC in these estuarine regions. Such enhanced bacterial decomposition provides an explanation for the rapid disappearance of DOC in the outer estuarine regions at high salinities (Cauwet 2002, McKenna 2004, Servais & Garnier 2006). Other differences were the 0.1 d -1 dilution rate and incubation at 20°C of the Wad30 experiment compared to a dilution rate of 0.15 d -1 and incubation at 15°C of the Wes15 and Wes5 experiments.…”

“…This implies that the DOC is largely recalcitrant and explains its conservative behavior of mixing with seawater, i.e. dilution without significant bacterial decomposition (Cauwet 2002, Servais & Garnier 2006. If non-conservative mixing occurs, it is mainly the result of production of labile DOC in the estuarine region and a function of the residence time of the DOC (McKenna 2004).…”