Coupling of carbon, nitrogen and oxygen cycles in sediments from a Mediterranean lagoon: a seasonal perspective

Abstract

International audienceExperimental data and simulations were used to investigate the seasonal coupling between carbon, nitrogen and oxygen cycles in marine sediments from a eutrophic shallow lagoon in the Mediterranean Sea area. A negative seasonal correlation was observed between oxygen consumption and coupled nitrification–denitrification rates in surface sediments. Elevated values of oxygen consumption rates were reached during warm periods (up to 87.7 mmol m–2 d–1) whereas nitrification and denitrification rates remained close to the lowest rates reported for coastal sediments (values around 0.021 to 0.35 mmol N m–2 d–1 for nitrification and 0.014 to 0.045 mmol N m–2 d–1 for denitrification). A steady-state diagenetic model closely represented the seasonal negative correlation of oxygen uptake, coupled nitrification–denitrification rates, the vertical distribution patterns of pore water oxygen and the solid phase distribution of organic carbon when nitrification inhibition by sulfide was included. Simulation adjusted to field data also highlighted the importance of oxygen penetration depth in the seasonal variation of nitrification. The modelling indicated that anaerobic metabolism was the most significant pathway (65 to 80%) during organic matter mineralization with a clear seasonal increase during warm periods. These warm periods were also characterized by the higher benthic demand of oxygen mostly used to re-oxidize the by-products from anaerobic reactions (from 57 to 82%), the other part being used for carbon mineralization

Modelling biogeochemical processes in sediments from the north-western Adriatic Sea: response to enhanced particulate organic carbon fluxes

“…Model results showed that organic matter accumulation in the farming area enhanced the anaerobic metabolism. Oxygen microprofiles recorded by Dedieu et al (2007) inside and outside the mussel farm presented differences which are comparable to those recorded in the present work, with a decrease of 50 % in oxygen penetration depth, and an increase up to 3 times of diffusive O 2 fluxes (30 mmol O 2 m −2 d −1 versus 90 mmol O 2 m −2 d −1 ). In Dedieu et al (2007) this was accompanied by a remarkable enhancement in NH + 4 concentrations, which was not visible in the field data reported in the present study.…”

“…Oxygen microprofiles recorded by Dedieu et al (2007) inside and outside the mussel farm presented differences which are comparable to those recorded in the present work, with a decrease of 50 % in oxygen penetration depth, and an increase up to 3 times of diffusive O 2 fluxes (30 mmol O 2 m −2 d −1 versus 90 mmol O 2 m −2 d −1 ). In Dedieu et al (2007) this was accompanied by a remarkable enhancement in NH + 4 concentrations, which was not visible in the field data reported in the present study. This can certainly be related to the difference in the rate of biodeposit accumulation, although other factors can also play a role (changes of local macrobenthic activity and of coupled nitrification-denitrification rates).…”

“…In our work, the background flux (OM1 + OM2 flux at EST1) was lower, 11.6 mmol C m −2 d −1 , with an increase of 8.2 mmol C m −2 underneath the farm, at IN1. The relative increase, with respect to the background flux, was 71 %, which is comparable to the 69 % increase found by Dedieu et al (2007) in winter. Nonetheless, in absolute terms, Dedieu et al (2007) reported a difference of approximately 50 mmol C m −2 d −1 in summer, in a system already characterised by large fluxes of organic matter, and hence dominated by SO 2− 4 reduction, whereas in this study, the in-crease is 5 times less −10 mmol C m −2 d −1 , over a system which has mostly oxic and suboxic diagenesis.…”

“…This can certainly be related to the difference in the rate of biodeposit accumulation, although other factors can also play a role (changes of local macrobenthic activity and of coupled nitrification-denitrification rates). Mean OC fluxes estimated by Dedieu et al (2007) by calibrating the diagenesis model at the mussel farm site were 38.4, 108.0, and 108.0 mmol C m −2 d −1 , respectively in winter, spring, and summer, with increases by 26.4, 53.4, and 52.4 mmol C m −2 d −1 with respect to a station of reference located outside of the farm influence, in the respective seasons. In our work, the background flux (OM1 + OM2 flux at EST1) was lower, 11.6 mmol C m −2 d −1 , with an increase of 8.2 mmol C m −2 underneath the farm, at IN1.…”

“…Finally, it is worth remarking that resuspension is a common mechanism in shelf trap measurements: in the northern Adriatic Giani et al (2001) estimated the contribution of resuspension to the gross flux sedimented in traps reaching 43.7 %. Dedieu et al (2007) compared seasonal cycles of C, N, and O inside and outside a shellfish farming area in a lagoon located in southern France, by combining a steady state diagenetic model (Soetaert et al, 1996) with a comprehensive set of experimental data. Model results showed that organic matter accumulation in the farming area enhanced the anaerobic metabolism.…”