ABSTRACT

We conducted shipboard observations in Funka Bay, Hokkaido, Japan, in 2015, 2016 and 2019 to observe temporal variations of isoprene (C₅H₈) concentration in water and sediment. We found increases in C₅H₈ concentrations below and within the euphotic zone, which were associated with chl-a peaks and changes in nutrient and dissolved oxygen (DO). We found a C₅H₈ peak in the subsurface layer within the euphotic zone at 20–30 m depth in May–June 2019 that coincided with a decrease in DO, suggesting that there was net oxygen consumption. We hypothesize that the rapid cycle of regenerated production in the subsurface layer in early summer resulted in C₅H₈ production (1.00–1.75 pmol (μg chl-a) d^–1 at 30 m), decreased DO, relatively high chl-a levels, and nutrient depletion. We propose that C₅H₈ can be used as a new chemical parameter reflecting primary production, in addition to the common parameters of chlorophyll-a, nutrients, and oxygen. We attributed a C₅H₈ peak at the surface in May–June 2019 to photosynthetic production of C₅H₈ associated with new production, likely stimulated by the sporadic input of nutrients to the sea surface. In addition, this is the first study to report the presence of C₅H₈ in seafloor sediments. We found very high C₅H₈ concentrations in the sediment from the surface to 5 cm depth. We propose that C₅H₈ production and degradation by the bacterial community in coastal marine sediments are approximately in balance.