Biomass and chemical composition of net-plankton down to greater depths (0–5800 m) in the western North Pacific Ocean

As part of the research program Western Pacific Environment Study on CO2 Ocean Sequestration for Mitigation of Climate Change (WEST-COSMIC), plankton sampling was carried out down to great depths (maximum: 5800 m) at four stations (44°N, 155°E; 39°N, 147°E; 30°N, 147°E and 25°N, 137°E) located in the western North Pacific Ocean. Ranges of plankton standing stock integrated over the water columns were wet mass (WM) 22–256 g m−2, dry mass (DM) 1.3–27.4 g m−2, carbon (C) 0.4–12.3 g C m−2, nitrogen (N) 0.10–2.44 g N m−2, ash (ash) 0.43–4.35 g m−2, ash-free dry mass (AFDM) 0.9–21.8 g m−2 and energy 20–582 kJ m−2. Log-linear equations were computed to allow interconversions among measurements in different mass units. Plankton mass was greater at higher latitudes. At each station, plankton mass decreased exponentially with increasing depth. This decrease was greater at subarctic (44°N) and subtropical (30°N and 25°N) stations than that at the transitional station (39°N) due to high amounts of diapausing copepods in meso- and bathypelagic depths at the latter station. Water content (% of WM), C:N ratios and ash (% of DM) increased with increasing depth, while C, N, AFDM (% of DM) and energy contents (J mg−1 DM) showed an opposite pattern. Extremely low C (24–25% of DM), N (3.2–4.4% of DM), AFDM (48–56% of DM) and energy contents (10–12 J mg−1 DM), accompanied by high C:N ratios (5.7–8.0 by weight), of plankton samples from depths below 3000 m suggest possible dominance of detritus in samples from that layer. The overall effect of detritus in measuring chemical composition of plankton samples is discussed.