Microphytobenthos and chironomid larvae attenuate nutrient recycling in shallow‐water sediments

and SiO₂) in shallow‐water sediments of a hypertrophic freshwater lagoon. Fluxes were measured in the light and in the dark in reconstructed sediments with low (L = 600 ind/m²), high (H = 1,800 ind/m²) and no (C) addition of chironomid larvae, after 3 weeks of pre‐incubation under light/dark regime to allow for microalgal growth. Besides flux measurements, pore water nutrient (NH₄⁺, and SiO₂) and dissolved metal concentrations (Fe²⁺ and Mn²⁺) were analysed and diffusive fluxes were calculated.
1. Chironomid larvae increased sediment heterotrophy, by augmenting benthic O₂ demand and TCO₂ and N₂ dark production. However, on a daily basis, treatments C and L were net O₂ producing and N₂ sinks while treatment H was net O₂ consuming and N₂ producing. All treatments were net C sink regardless of chironomid density.
2. Microphytobenthos always affected benthic nutrient exchange, as significantly higher uptake or lower efflux was measured in the light compared with dark incubations. Theoretical inorganic N, P and Si demand by benthic microalgae largely exceeded both dark effluxes of NH₄⁺, and SiO₂ and their net uptake in the light, suggesting the relevance of N‐fixation, water column NO₃^? and solid‐phase associated P and Si as nutrient sources to benthic algae.
3. Chironomid larvae had a minor effect on inorganic N and P fluxes while they significantly stimulated inorganic Si regeneration. Their bioturbation activity significantly altered pore water chemistry, with a major reduction in nutrient (highest for NH₄⁺ and lowest for SiO₂) and metal concentration. Underlying mechanisms are combinations of burrow ventilation and bioirrigation with stimulation of element‐specific processes as coupled nitrification–denitrification, co‐precipitation and inhibition of anaerobic paths such as Fe³⁺ or Mn⁴⁺ reduction or re‐oxidation of their end products.
4. The combined activity of benthic algae and chironomid larvae may significantly attenuate internal nutrient recycling in shallow eutrophic ecosystems, and contribute to the control of pelagic primary production.