Variable Oxygen Levels Lead to Variable Stoichiometry of Benthic Nutrient Fluxes in a Hypertrophic Estuary

Harmful blooms of cyanobacteria may extend over long time spans due to self-sustaining mechanisms. We hypothesized that settled blooms may increase redox-dependent P release and unbalance the stoichiometry of benthic nutrient regeneration (NH4+:SiO2:PO43− ratios). We tested this hypothesis in the hypertrophic Curonian Lagoon, the largest in Europe. During summer, at peak chlorophyll and water temperatures, sediment cores were collected over 19 stations representing all the lagoon sedimentary environments. Sediment organic content, granulometry, aerobic respiration, and oxic and anoxic fluxes of dissolved inorganic nutrients and metals—Fe2+ and Mn2+—were measured. Loads and stoichiometry of regenerated nutrients were compared with those from the watershed. Analyzed sediments had elevated oxygen demand (−1.90 to −5.66 mmol O2 m−2 h−1), generally uncoupled to their variable organic matter content (1–23%) and median particle size (30–300 μm). Under oxic conditions, summer internal recycling equaled (SiO2) or exceeded, by a factor of ~66 and ~ 2, external loads of NH4+ and PO43−, respectively. Transient anoxia produced a general decrease of NH4+ and SiO2 regeneration, likely due to decreased macrofauna activity or inefficient mineralization, whereas it doubled average PO43− fluxes. In sandy, well-flushed areas, anoxia had a minor effect on PO43−, but stimulated a large production of Mn2+. Muddy sediments in lagoon areas with slow water renewal displayed large redox-dependent PO43− mobility, coupled to Fe2+ release. Settled algal blooms and hypoxic conditions might unbalance benthic regeneration stoichiometry and sustain blooms. The sedimentary pool of Mn4+ may represent a natural buffer preventing iron reduction and PO43− mobility.