Bioturbating invertebrates enhance decomposition and nitrogen cycling in urban stormwater ponds

Abstract

Stormwater retention ponds manage runoff in developed areas, reducing flooding and erosion, and improving downstream water quality. Urban ponds also serve as habitat for aquatic organisms in an otherwise terrestrial habitat. However, the relationship between common pond species and biogeochemical function of ponds is poorly understood. Using a microcosm experiment, we evaluated the influence of two bioturbating invertebrates, Chironomus sp. and Lumbriculus variegatus, on sediment–water column nutrient exchange and benthic metabolism in stormwater ponds with divergent management histories and sediment organic matter (OM) content. Both organisms facilitated net release of reactive inorganic nitrogen to the water column, flipping the net reactive nitrogen balance of sediments relative to controls. Release of ammonium to the water column and sediment oxygen consumption were enhanced by both species, especially during colonization, but sediment uptake of nitrate was strongly enhanced by Chironomus and inhibited by L. variegatus. Comparison with direct animal respiration and excretion rates allows attribution of direct relative to indirect effects of bioturbation and bioirrigation. The influence of L. variegatus is predominantly direct, whereas Chironomus has strong direct and indirect biogeochemical effects. After 30 days, microalgal photosynthetic efficiency was reduced in all L. variegatus treatments, and context specific interactions appeared, with a reduction in OM in sediment with initially higher values, and reduced nitrate uptake in sediments with lower OM. Animal impacts on key processes ultimately influence downstream water quality and this study illustrates the important role of tolerant benthic invertebrates in mediating ecosystem functions in even the most disturbed aquatic ecosystems.