Abstract
As a part of Korean-Russian joint expeditions in the East/Japan Sea during 2012 and 2015, a set of total and small (<2 μm) phytoplankton NO3− and NH4+ uptake rate estimations were carried out. The study aimed to assess the spatio-temporal variations in dissolved inorganic nitrogen (DIN) assimilation by the total and small phytoplankton. The results show that the total NO3− uptake rates during 2012 varied between 0.001 and 0.150 μmol NL−1h−1 (mean ± SD = 0.034 ± 0.033) and that the total NH4+ uptake rates ranged between 0.002 and 0.707 μmol NL−1h−1 (mean ± SD = 0.200 ± 0.158). The total uptake rates during 2015 were ranged from 0.003 to 0.530 (mean ± S.D. = 0.117 ± 0.120 μmol NL−1h−1) for NO3− and from 0.008 to 1.17 (mean ± S.D. = 0.199 ± 0.266 NL−1h−1) for NH4+. The small phytoplankton NO3− and NH4+ uptake rates during 2015 ranged between 0.001 and 0.164 (mean ± S.D. = 0.033 ± 0.036) μmol NL−1h−1 and 0.010–0.304 (mean ± S.D. = 0.101 ± 0.073) μmol NL−1h−1, respectively. Small phytoplankton’s contribution to the total depth-integrated NO3− and NH4+ uptake rates ranged from 10.24 to 59.36% and from 30.21 to 68.55%, respectively. The significant negative relationship observed between the depth-integrated total NO3− and NH4+ uptake rates and small phytoplankton contributions indicates a possible decline in the DIN assimilation rates under small phytoplankton dominance. The results from the present study highlight the possibility of a reduction in the total DIN assimilation process in the East/Japan Sea when small phytoplankton dominate under strong thermal stratification due to sea surface warming. The present study’s findings agree with the model projections, which suggested a decline in primary production in the global warming scenario.
Highlights
The East/Japan Sea is a small, semi-enclosed marginal sea of the Pacific Ocean with boundaries shared by Korea, Japan, and Russia; there is no direct water mass exchange with Pacific Ocean waters [1,2,3]
Lee et al [24] found that the small phytoplankton contributions to primary productivity based on the chlorophyll a (Chl-a) concentrations were similar to the previous results observed in the East/Japan Sea during summer [34]
There were no significant correlations between environmental parameters and the dissolved inorganic nitrogen (DIN) uptake rates observed during the present study; the higher DIN uptake rates in the surface layers than in the bottom waters suggest the potential influence of light conditions
Summary
The East/Japan Sea is a small, semi-enclosed marginal sea of the Pacific Ocean with boundaries shared by Korea, Japan, and Russia; there is no direct water mass exchange with Pacific Ocean waters [1,2,3]. Previous studies suggest that the East/Japan Sea possesses higher primary production (246.8 g C m−2 y−1) rate than the open waters in various global oceans (55 g C m−2 y−1 to 102 g C m−2 y−1) [10]. The higher primary production is likely because the East/Japan Sea receives or possesses sufficient nutrients in the euphotic water column through various physico-chemical processes. Inorganic nutrient uptake is highly dependent on various factors, such as water temperature, salinity, nutrient availability, and chlorophyll a (Chl-a) concentration [11]. Apart from these parameters, DIN uptake is highly dependent on the types of phytoplankton species and their abundances [12]
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