Copepod's ammonia excretion rate in the Kuroshio

The copepods were collected in the Kuroshio area, south of Japan, at five stations (30°N, 31°N, 33°35′N, 34°N, and 34°30′N) along 138°E in March 2013. The copepods were collected by vertical hauls from 50 m depth to the surface using a 0.8-m-diameter plankton net (100-μm mesh, 3 L cod-end), which was towed slowly (We set the experimental temperature to 14°C in the coastal area (34°N and 34°30′N) and to 18°C in the offshore area (30°N, 31°N, and 33°35′N). These temperatures were slightly colder than those in the surface mixing layer. Incubations for NH3 excretion measurements were conducted for 1–7 h. The NH3 concentration in the seawater was measured using indophenol blue reaction colorimetry with a 1-m-long capillary cell. After NH3 measurement, the copepod was fixed in 5% neutral-buffered formalin. In the on-land laboratory, the detailed species identification was conducted, and their prosome lengths (PLs) were measured under a microscope. Nitrogen contents of individual copepods were estimated based on PLs, according to Uye (1982), and the body-nitrogen-weight specific NH3 excretion rate (h-1) was calculated.