Differences in the ammonium excretion rate of functional copepod groups

Ammonium/ammonia (NH 3 ) excretion by zooplankton is a key nitrogen source sustaining oceanic primary production. However, the controlling factors of NH 3 excretion have been poorly understood. We measured NH 3 excretion rates of copepods using a highly sensitive colorimetric method in single-individual incubation experiments. A total of 152 individuals, representing ten genera from seven families, were collected at five stations in the Kuroshio region of the western North Pacific during spring. Copepods were incubated at two biologically relevant temperatures (14°C and 18°C) for 0.8–7.4 h. Body-nitrogen-specific excretion rates (NH 3 excretion rates normalized to copepod’s body nitrogen content) ranged from 5.0 × 10 –5 to 2.63 × 10 –2 h −1 . Four findings were revealed with a generalized additive model: 1) smaller copepods excreted more NH 3 per unit body nitrogen; 2) temperature broadly did not influence NH 3 excretion; 3) their prolonged incubation period reduced the specific NH 3 excretion rate; and 4) significant differences among taxonomic groups. Middle-sized carnivores ( Candacia and the Euchaetidae) and small-sized, cruise-feeding non-carnivores ( Oncaea and Clausocalanus ) exhibited higher specific NH 3 excretion rates than middle- to large-sized filter-feeding non-carnivore taxa (Aetideidae, Calanus , Mesocalanus , Nannocalanus , and Eucalanus ), highlighting the influence of feeding habits on NH 3 excretion. The higher specific NH 3 excretion rates observed in smaller specimens suggested greater metabolic activity in small copepods than in large ones. In addition, copepod metabolic activity declined with time during the incubation, implying that previous studies may have underestimated NH 3 excretion rates. Consequently, copepod NH 3 excretion likely supports a larger fraction of oceanic primary production than previously recognized.