Abstract
Large populations of copepod-dominated meiofauna are found in the sand filters of the St. Lawrence marine mesocosm at the Montreal Biodome. Experiments were conducted in heterotrophic microcosms to quantify how populations of micro- and meiofaunal organisms affect ammonia oxidation (nitrition) and nitrite oxidation (nitration) using apparent nitrition rate (ANiR) and apparent nitration rate (ANaR) as proxies. ANiR and ANaR were not related to ciliate density. Meiofauna had no effect on ANiR, but a significant relationship between ANaR and meiofaunal biomass was observed, which varied with the particulate organic nitrogen (PON) content of the sediment. The relationship was negative at low PON and positive at high PON. These results suggest a direct negative action by predation on nitrifying bacteria and an indirect positive action by reducing competitors through grazing of heterotroph bacteria. The negative effect of 1 g meiofauna·m-2 at low PON (-20% of ANaR0, i.e., ANaR without meiofauna) is much smaller than its positive effect at high PON (+172 to +571% of ANaR0). Copepod-dominated meiofaunal biomasses less than 0.16 g·m-2 increase two to five times the nitrification rate in heterotrophic habitats rich in PON.
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