Abstract

We conducted time-series observations in Funka Bay, Hokkaido, Japan, from 15 February to 14 April 2019. The diatom spring bloom peaked on 4 March and started declining on 15 March. Funka Bay winter water remained below 30-m depth, which was below the surface mixed-layer and dark-zone depths on both dates. At depths of 30–50 m, concentrations of NO3–, PO43–, and Si(OH)4 decreased by half between these dates even in darkness. Incubation experiments using the diatom Thalassiosira nordenskioeldii showed that this diatom could consume nutrients in darkness at substantial rates. We conclude that the nutrient reduction in the subsurface layer (30–50 m) could be explained by dark consumption by diatoms that had been growing in the surface waters and then sank to the subsurface layer. We believe that this is the first study to present observational evidence for the consumption of the main nutrients by diatoms in the dark subsurface layer during the spring bloom. Nutrient consumption in this layer might have a substantial influence on the primary production during and after the spring bloom.

Highlights

  • 25 The supply of nutrients to the surface euphotic zone has a potent influence on regulating marine primary production

  • A massive spring bloom dominated by diatom species occurs in March every year before the Oyashio water flows into the surface of the bay, and it lasts until late March or early April, when Oyashio water occupies the surface of the bay (Odate 1987; Maita and Odate 1988)

  • Most previous studies in Funka Bay have focused on nutrient consumption in the surface euphotic zone and nutrient regeneration in the bottom water, Kudo and Matsunaga (1999) have pointed out that NO3- concentrations in the dark subsurface layer decreased during the spring blooms in 1991 during their observations from 1988 to 1992, and they mentioned that the 50 decrease was due to dilution of water by vertical mixing

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Summary

Introduction

25 The supply of nutrients to the surface euphotic zone has a potent influence on regulating marine primary production. Most previous studies in Funka Bay have focused on nutrient consumption in the surface euphotic zone and nutrient regeneration in the bottom water, Kudo and Matsunaga (1999) have pointed out that NO3- concentrations in the dark subsurface layer decreased during the spring blooms in 1991 during their observations from 1988 to 1992, and they mentioned that the 50 decrease was due to dilution of water by vertical mixing. We examine the temporal variation of nutrient concentrations in Funka Bay from the early phase of the diatom 55 bloom (February) to post-bloom (April) through time-series observations in 2019, focusing on the processes affecting nutrient reduction in the dark subsurface layer during the bloom. An overview 60 of chlorophyll-a (chl-a) concentrations in 2018–2019 in the bay in association with the VOI changes has already been reported (Ooki et al, submitted)

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