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Phosphorus bioavailability is a major issue in aquatic environments, where it generally limits primary production. In this work, the analysis of the pore water and the solid phase of the sediment was carried out over a 9-month monitoring period between February 2020 and April 2021 in two drained marshes (Marans and Genouillé, France) distinct by their uses and management tools. Soluble reactive phosphorus (SRP) enrichment in the sediment was intimately controlled by iron oxide dissolution. The latter seemed highly controlled by seasonal nitrate inputs (winter and early spring) that favoured denitrification as a major benthic mineralization process promoting iron curtain development and stability. Following benthic mitigation of nitrate other anaerobic metabolisms developed such as iron dissolutive reduction promoting P recycling and planktic bioavailability. Surprisingly, sulphur cycle seemed to affect P dynamics, especially in the absence of nitrate. The absence of NO 3 − triggered high sulphate reduction rates in the two first centimeters depth, reaching −8.9 E−03 ± 0.5 E−03 and -5.0 E−03 ± 0.2 E−03 nmol cm−3 s−1 in August and July at Marans and Genouillé respectively. These values placed this process at higher rates than the denitrification (maximum in May at Marans with −5.0 E−03 ± 1.1 E−03 nmol cm−3 s−1) and reduced iron production (maximum in July at Genouillé with 0.5 E−03 ± 0.1 E−03 nmol cm−3 s−1). The rapidity with which process changes occur (monthly scale) testified to the dynamism of these systems. The similarity in geochemical patterns regarding NO 3 − pressure at both sites underlines the importance of diffuse pollution in coastal systems for nitrogen mitigation and phosphorus trapping. The results obtained in this study could lead to the development of a generalized diagenetic operating model for temperate systems with high agricultural pressure. This would enable to target management efforts to both optimize the purification function and limit eutrophication risks in these systems. [Display omitted] • Drivers for P mobilization are partially unknown in P-limited drained marshes. • Porewater and sediment analysis were used to conceptualize P dynamics. • NO 3 − is a main actor for P retention in the sediment by promoting iron curtain. • Significant geographical impact of NO 3 − dependent on water management. • Strong evidence of iron, sulphur and methane interaction for mineral P mobilization. [ABSTRACT FROM AUTHOR]

Coastal marshes are submitted to huge management due to anthropogenic pressure and thus, it is essential to preserve their biodiversity, their ecological functions and the ecosystem services they can provide. This study investigates the diversity and abundance of planktonic communities (heterotrophic prokaryotes, heterotrophic protists, microphytoplankton and metazooplankton). The aim of this study is to provide a first quantitative inventory on the plankton communities present in two marshes and to construct the different typologies of planktonic food webs in these marshes. Seasonal samplings (4) for environmental variables, nutrient concentrations and planktonic communities were conducted at 2–3 stations on each marsh during a year. A total of five different types of planktonic food web were determined, three of them were found in both marshes. The saltmarsh phytoplanktonic communities were dominated by Cryptophyta, nanoflagellates and Cyanobacteria. The freshwater marsh was dominated by Cryptophyta in autumn and winter and by a diverse phytoplankton community in spring. Marine copepods (Calanoida and Harpacticoida) characterized the saltwater metazooplanktonic communities, Cladocerans and Rotifera the freshwater ones. Overall, the planktonic diversity was higher in the freshwater marsh (102 taxa) than in the saltwater marsh (54 taxa). Phytoplankton blooms represented nutrient and CO 2 uptake in both marshes and this purification function seemed most efficient in autumn in the saltmarsh. Considerable zooplankton communities represented a potential for nursery. Of the three management actions performed, only periodic water renewals might affect the seasonal dynamics of planktonic communities. • Planktonic diversity differs highly between the salt- and freshwater marsh. • Similar planktonic ecological functions can be found in both marshes. • Seasonal variations might be affected by management operation. • Monitoring planktonic communities is essential to adapt coastal marsh management. [ABSTRACT FROM AUTHOR]

• Benthic P remobilization coincides with phytoplankton bloom periods. • Strong Fe and P coupling in sediment. • Watershed NO 3 − inputs inhibits benthic Fe respiration and P regeneration. • Primary production and sediment denitrification drive water purification capacity. • Strong contribution of phytobenthos to sediment OM input and water pigment exchange. Wetlands, especially marshes, support many services such as carbon catchment control or water purification led by primary producers such as phytoplankton and microphytobenthos (PB). The impact of the sedimentary compartment, as source and sink of essential nutrients for the water column, is often neglected in the study of their dynamics and water purification capacity of the systems. This work compared monthly (between February 2020 and April 2021) the benthic and pelagic primary producers' dynamics in two anthropised freshwater marshes (Marans and Genouillé), with the simultaneous follow-up of physico-chemical parameters of the water column and nutrient fluxes at the sediment-water (SWI) interface. It was suggested a strong contribution of phytoplankton (pumping) and the benthic compartment (denitrification) to the water purification of these two nitrates (NO 3 −)-rich marshes. Total phytoplankton production fluctuated between ∼5 (winter) and 1500 mg C m−3 d−1 (fall) at Marans and between 40 (winter) and ∼750 mg C m−3 d−1 (spring) at Genouillé. At Marans, soluble reactive phosphorus (SRP) benthic effluxes (-2.101 to -6.102 µmol m−2 d−1 in fall and summer, respectively) coincided with phytoplankton bloom periods. These effluxes were inhibited by NO 3 − penetration in the sediment (0 to 5.104 µmol m−2 d−1), by inhibiting iron respiration. At Genouillé, inhibition of SRP effluxes depended on denitrification rate and on P stocks in the sediment, where slight SRP effluxes (-101 µmol m−2 d−1) could have co-occurred with slight NO 3 − influxes (5.102 µmol m−2 d−1) in spring. The presence of PB (between 10–60 and 40–120 mg g sed −1 at Marans and Genouillé, respectively), suggested a strong contribution of the benthic compartment to the total primary production (benthic and pelagic through resuspension processes) in these environments. This work encourages to consider the benthos and the pelagos as a unicum to provide better sustainable management of such systems and limit eutrophication risks in coastal areas. [Display omitted] [ABSTRACT FROM AUTHOR]