Metabolism and elemental composition of the euphausiids Euphausia pacifica and Thysanoessa inspinata during the phytoplankton bloom season in the Oyashio region, western subarctic Pacific Ocean

Rates of oxygen consumption (R: μl O2 [individual]−1 h−1), and ammonia excretion (E: μg NH4-N [individual]−1 h−1), O:N ratios (by atoms) and body water contents (% of wet mass [WM], as an index of lipid accumulation) of Euphausia pacifica and Thysanoessa inspinata were monitored during 9–14 March and 6–30 April 2007, including the onset of the spring phytoplankton bloom in the Oyashio region. Regression analyses revealed that variations in R and E were generally correlated with dry body mass (DM) for both euphausiids. When R and E were standardized to a body size of 10 mg DM, Rstd, Estd, O:N ratios and water contents fluctuated from 6.3 to 10.1, 0.07 to 0.34, 31 to 232 and 73.8 to 78.1, respectively, for E. pacifica, and from 8.7 to 11.3, 0.06 to 0.71, 24 to 295 and 75.1 to 82.2, respectively, for T. inspinata. None of these variables were significantly correlated with temporal variations in SST (1.7 to 5.7 °C) or chlorophyll a standing stock (29.4 to 252.6 mg m−2, 0–100 m) at the study site. However, April E and O:N ratio data pooled for both euphausiids were significantly less and greater, respectively, than the pooled values in March, suggesting preferential utilization of dietary protein for body growth or reproduction under conditions of abundant food supply as found in April. No substantial differences were observed in water content, ash, C, N or C:N ratios (by mass) of the two euphausiids collected in either March or April. Water content, C, N, C:N ratio and ash data averaged for E. pacifica and T. inspinata over the March and April cruises were similar: 76.3 and 78.1% of WM, 36.1 and 37.5% of DW, 9.4 and 10.1% of DW, 3.82 and 3.71 by mass, and 10.6 and 10.8% of DM, respectively. Combining the R-DM and E-DM relationships established with the population structure data of Kim et al. (2010), we estimate that daily ingestion and ammonia-N regeneration by the combined populations of the two euphausiids were 1.5–24.1% (integrated mean: 4.9%) and 0.18–1.32% (0.41%), respectively, of primary production and associated N demand during the bloom.