The estimation of metabolism in the mesopelagic zone: Disentangling deep-sea zooplankton respiration

Respiration in the mesopelagic layer is rather difficult to estimate but is of paramount importance to assess active flux, the downward carbon transport carried out by diel vertical migrants in the ocean. Migrant biomass and respiration in the mesopelagic zone are required to estimate respiratory flux, an important component of active flux. Three main approaches were used in the past to assess respiration in the mesopelagic zone: (1) The use of Q10 values, (2) published equations relating respiration, body mass, and temperature, and (3) the electron transfer system (ETS) enzymatic activity, calibrated to obtain respiration rates. The latter method is also a commonly used approach for measuring mesopelagic community respiration. Here we report respiration rates and ETS activity of migrant copepods captured at night in the epipelagic layer. The estimated mesopelagic respiration obtained from conversion of epipelagic respiration at night using a Q10 value for mesopelagic zooplankton was compared to the estimated respiration obtained using the equations given by Ikeda (1985), Ikeda (2014), and ETS activity using a respiration to ETS (R/ETS) ratio of 1. They were not statistically different. However, differences were observed between those latter values and the R/ETS ratio of 0.5. These results show that the commonly used R/ETS ratio of 0.5 promotes quite conservative values of the respiratory flux, and the equation of Ikeda (2014) displayed values in the middle of both R/ETS ratios, and it may be used with confidence. Relatively high specific ETS activities were observed in the mesopelagic zone, also coinciding with a previous observation in Eastern Equatorial Pacific. We discuss this observation as an adaptation of migrant zooplankton to endure the adverse conditions of low temperature and low oxygen in deep waters