Comparison of calculated and measured nutrient fluxes reveals buffer saturation in dystrophic lagoon sediments

In shallow lagoons dissolved oxygen and nutrients fluxes regulate oxygen levels, water chemistry and primary production and are generally measured via dark incubations. Nutrient fluxes can also be calculated from sediment respiration and C:N:P stoichiometry or from diffusive gradients between bottom and pore waters. Rarely are these different approaches used together, even though they can provide valuable insights into benthic functioning. In this study O₂, NH₄⁺ and PO₄^3- fluxes were seasonally measured (i.e., dark core incubation) and calculated (i.e., from sediment C:N:P and pore water profiles) at 4 areas of a shallow eutrophic lagoon. Variable differences between measured and theoretical fluxes, higher in winter than summer due to more effective biogeochemical buffer mechanisms, were hypothesized. The results obtained suggest large O₂ respiration and NH₄⁺ and PO₄^3- regeneration at all stations and seasons, supported by labile organic matter inputs. Measured and theoretical fluxes from respiration and stoichiometry were always comparable, suggesting ineffective benthic buffer mechanisms retaining or removing nutrients. Limited nitrification and denitrification of the ammonified organic N, and limited capacity to retain PO₄^3- were likely due to poorly bioturbated and chemically reduced sediments with negligible O₂ penetration. Diffusive fluxes calculated from pore water profiles were lower than those measured or calculated from stoichiometry, with a few exceptions during colder periods. The use of multiple approaches to quantity benthic fluxes and to infer about biogeochemical buffers and the potential limits of diffusive fluxes calculation in eutrophic lagoons are discussed.