Enhancement of Inorganic Nitrogen and Phosphate Release from Lake Sediment by Tubificid Worms and Chironomid Larvae

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Abstract
The effects of chironomid larvae and tubificid worms on the release of inorganic nitrogen and phosphorus from a eutrophic freshwater lake sediment were evaluated experimentally using mud-water microcosms. The larvae of Chironomus plumosus L. and Limnodrilus spp. caused an enhancement of inorganic nitrogen release (mainly ammonium nitrogen) from lake sediment. Increasing densities of tubificids markedly influenced release rates. Apparent diffusion coefficients of ammonium at the mud-water interface in microcosms with Limnodrilus spp. increased from 3.5 to 24× 10-5 cm2 s-1 over a range of 5 densities from 0 to 72500 worms m-2, while that for microcosms with C. plumosus increased from 2.6 to 48× 10-5 cm2 s-1 over a range of 5 densities from 0 to 6800 larvae m-2. Chironomids and tubificids also had an accelerating effect on phosphate release. Phosphate release rates became greater with an increasing density of organisms. Chironomids induced some enhancement of the release rate at low densities but caused a suppression at high densities. The excretion: release ratios indicated the importance of bioturbation for ammonium release and the importance of excretion for phosphate release.