Patterns and Rates of Nitrogen Release by Benthic Chironomidae and Oligochaeta

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Abstract
Metabolic mineralization of ammonium by tubificid worms and chironomid larvae appears to be an important mechanism contributing to nitrogen regeneration from aerobic lake sediments. Mean weight [ash free dry weight (AFDW)] – specific ammonium release rates ranged from 3 to 15 nmol NH4 (mg AFDW)]−1∙h−1 for chironomid and tubificid species collected at different times and temperatures from nearshore Lake Michigan sediments. Although mean rates of nitrogen release were similar for the two groups of benthic invertebrates, the patterns of release were different. Tubificids released nitrogen (ammonium plus primary amines) continuously, whereas chironomids released it in spurts several times per hour. Mean ammonium-release rates were generally constant with time after the animals were removed from food for both species. This implies that ammonium regeneration for these benthic animals is primarily an endogenous process. Conversely, primary amine nitrogen release often decreased with time after food removal and may reflect either egestion of partially digested materials from the animals' guts or a decrease in the metabolic pool size of free amino acids. Based on previous measurements of phosphorus release, mean molar NH4:PO4 ratios in excreted materials were calculated to be 15:1 for chironomids and 35:1 for tubificids.Key words: nitrogen release, benthic invertebrates, chironomids, tubificids, ammonium excretion, amino acid release