Effects of container size, density, and incubation period on ammonium excretion rate measurements of planktonic copepods: Validation with a highly sensitive analytical method

Abstract

Ammonium excretion by planktonic copepods is an important process supplying nitrogen to primary producers and other microbial activity in pelagic ecosystems. Because of methodological constraints, conventional analyses using sealed chamber methods have required the adoption of unnatural experimental settings, such as a small container, high density, or a long incubation for ammonium excretion measurements. Therefore, most of the estimated ammonium excretion rates are potentially linked to stresses on the experimental animals during incubation. In this study, the effects of container size, density, and incubation period on the ammonium excretion rates of five copepod species (Calanus sinicus, Eucalanus californicus, Metridia pacifica, Pleuromamma abdominalis, P. gracilis) were examined using a highly sensitive analytical method that has recently been developed to measure ammonium concentration at the nanomolar level. The results indicate that responses to experimental stresses are species-specific, while incubation in a small container was generally the most significant factor causing overestimation (up to 2.6 times) of the excretion rates. This study shows that this highly sensitive analysis method, which allows short incubations with a single individual in a relatively large-sized container is an appropriate method to estimate ammonium excretion of pelagic copepods in the field using the sealed chamber method.