Phosphorus mobility under short-term anoxic conditions in two shallow eutrophic coastal systems (Curonian and Sacca di Goro lagoons)

The effects of short-term anoxia on phosphorus mobility were investigated in estuarine sediments collected from two eutrophic European lagoons. Study areas differ with regard to climate and salinity: the boreal Curonian Lagoon (Lithuania) is oligohaline while the Mediterranean Sacca di Goro (Italy) is mesohaline. In each lagoon two sites were chosen representing areas impacted by river plume discharge and areas impacted by high organic matter deposition. Benthic fluxes of dissolved oxygen, inorganic phosphorus (DIP), iron (Fe²⁺) and manganese (Mn²⁺) were measured by dark intact core incubations in oxic and anoxic conditions. The latter were induced by incubation prolonged by up to 40h depending on oxygen consumption rates. Solid phase P pools and pore water profiles of DIP, Fe²⁺, Mn²⁺ and sulfides were also measured before and after the induction of anoxia. Although incubation temperature and organic matter content were similar in plume and organic impacted areas of the two estuaries, higher benthic oxygen consumption rates were observed in Sacca di Goro, suggesting higher reactivity of the organic pool. Short-term anoxia had a significant effect on benthic fluxes of DIP (sulfide and Fe²⁺) only at the organic-rich and saline site of Sacca di Goro while internal buffers prevented inorganic P regeneration at the remaining sites. However, the analysis of pore water and solid phase pools revealed large variations during the transition and provided insights on the mechanisms controlling P dynamics. Both stations influenced by river plume deposition had high total inorganic P content (but most of it was refractory) and high reactive Mn pools, continuously regenerated by ventilation and irrigation of sediments by macrofauna. In these sites, oxygen depletion resulted in large accumulation of reduced Mn in the pore water but high fluxes of Mn²⁺ were also measured in the oxic part of the incubation suggesting the relevance of this respiration process. Thus Mn reduction preserved oxidized Fe pool and ultimately prevent P mobilization. Our results suggest that fluxes of DIP across the sediment-water interface are controlled by an array of different and site-specific mechanisms. These biogeochemical buffers are effective and contrast massive P release to the water column in highly productive systems as coastal lagoons, where episodes of transient oxygen shortage are frequent.