Seasonal effects of the zebra mussel (Dreissena polymorpha) on sediment denitrification rates

Abstract: Zebra mussels (Dreissena polymorphn) have altered the structure of invaded ecosystems and exhibit charac-teristics that suggest they may influence ecosystem processes such as nitrogen (N) cycling. We measured denitrification rates seasonally on sediments underlying zebra mussel beds collected from the impounded zone of Navigation Pool 8 of the Upper Mississippi River. Denitritication assays were amended with nutrients to characterize variation in nutrient limitation of denitritication in the presence or absence of zebra mussels. Denitrification rates at zebra mussel sites were high relative to sites without zebra mussels in February 2004 (repeated measures analysis of variance (RM ANOVA), p = 0.005), potentially because of high NO,--N variability from nitrification of high NH, ' zebra mussel waste. Denitri-fication rates were highest in June 2003 (RM ANOVA, p c 0.001), corresponding with the highest NO,--N concentra-tions during the study (linear regression, R ' = 0.72, p < 0.001). Denitrification was always N-limited, but sites without zebra mussels showed the strongest response to N amendments relative to sites with zebra mussels (two-way ANOVA, p 2 0.01). Examining how zebra mussels influence denitrification rates will aid in developing a more complete under-standing of the impact of zebra mussels and more effective management strategies of eutrophic waters. RCsumC: Les moules zCbr6es (Dreisser~n polymorpha) ont rnodifii la structure des Ccosystemes qu'elles ont envahis et elles possedent des caracteristiques qui laissent croire qu'elles peuvent influencer les processus Ccosystemiques, tels que le cycle de l'azote (N). Nous avons mesure les taux saisonniers de dinitrification dans les sediments sous-jacent