Utility of an automatic water sampler to observe seasonal variability in nutrients and DIC in the Northwestern North Pacific

Concentrations of nutrients (NO₃, NO₂, Si(OH)₄, PO₄ and dissolved inorganic carbon (DIC) were measured in a series of seawater samples collected over approximately 15 months in 2005 and 2006 by an automatic water sampler (Remote Access Sampler, RAS) in the Northwestern North Pacific. Seasonal variability and concentrations of NO₃ + NO₂ (NO_x and Si(OH)₄ were comparable to previous shipboard observations, although there were small errors associated with measurements of PO₄ and DIC. Concentrations of these nutrients began to decrease in late April. After the end of June, NO_x and Si(OH)₄ decreased rapidly, with large fluctuations. After October, these nutrients increased again until late spring 2006. The ratio of the decrease of Si(OH)₄ to that of NO_x suggests that numbers of biogenic opal-producing creatures, such as diatoms, increased after the end of June. This conclusion was supported by a rapid increase in biogenic opal flux recorded in a sediment trap at 150 m. The relationship between NO_x concentrations at the RAS depth of 35 m and NO_x integrated over the upper 100 m was determined using previous shipboard hydrocast data. This relationship was used to estimate integrated mixed layer NO_x concentration from RAS data. Estimated new production based on seasonal drawdown of integrated NO_x averaged approximately 156 mg-C m⁻²day⁻¹ annually, which agrees with previous estimates. Thus, an automatic seawater sampler that documents annual maximum and minimum nutrient concentrations and episodic events such as storms and spring blooms, which might be missed by an ordinary research vessel, will contribute to time-series observations of nutrients and, by extension, biological pump activity.