We investigated the photoacclimation state and photosynthetic rate of cells in the highly eutrophic upper Gulf of Thailand (UGOT), using fast repetition rate (FRR) fluorometry. Observations revealed differences in photosystem II (PSII) parameters: the maximum photochemical quantum efficiency of PSII (Fv/Fm), the functional absorption cross-section of PSII (σPSII) and the rate of reoxidation of the primary PSII electron acceptor (1/τQa), which were dependent on the hydrographic structure of the water column. FRR-derived parameters were used to estimate the chlorophyll a-normalized, photosynthetic rate (PB); the photosynthesis vs. irradiance (P–E) curves were derived from in situ PB profiles. The P–E parameters differed markedly between the upper stratified waters and the mixed water column; that is, the maximum photosynthetic rate |$P_{{\rm max}}^{\rm B} $| and the light saturation parameter (Ek) were higher in the upper stratified waters, while the initial slope of the P–E curve (α) was higher in the mixed waters. This indicates that cells acclimated to relatively bright light dominated the upper stratified waters, while cells acclimated to lower light dominated the mixed waters. A significant, positive relationship was found between the average PB in the euphotic zone and surface photosynthetically available radiation, from all profiles at both sampling sites, which suggests that phytoplankton photosynthesis in the UGOT was controlled primarily by irradiance. Furthermore, as stress from nutrient-limitation is unlikely in the UGOT, cells might realize their photosynthetic potential by means of photoacclimation, even under the different light regimes present in the UGOT.
During 2010–2012 in the northwestern region of the North Pacific subtropical gyre, we examined seasonal variability of the phytoplankton community with respect to structure and photo-physiological status using the chemotaxonomy program CHEMTAX and fast-repetition-rate fluorometry. Total chlorophyll a standing stock in the upper 150 m (ʃTChl a) varied from 25.5 to 89.0 mg m−2 during periods of relatively deep vertical mixing (40–207 m), but was fairly constant (18.6–27.2 mg m−2) during stratification periods. Prymnesiophytes, chlorophytes, chrysophytes, prasinophytes, and/or diatoms comprised major portions of the ʃTChl a during mixing periods. Diatoms became the most abundant group (29–43 %) in February 2011, when large phytoplankton blooms were observed, whereas Prochlorococcus was the dominant component (15–46 %) during stratification periods. During mixing periods, nitrate and nitrite (NO3 + NO2) concentrations occasionally exceeded 0.5 μmol kg−1 in surface waters, and F
v/F
m (photochemical efficiency of photosystem II) ranged from approximately 0.40–0.50 within the euphotic zone. During stratification periods, however, NO3 + NO2 concentrations were very low (<0.1 μmol kg−1) from the surface to 50–95 m, and F
v/F
m was in the range of 0.25–0.40 in the upper mixed layer. The seasonal variability of phytoplankton community structure in this region was related to the nutrient supply from deep waters. Additionally, we compared our data to published values at the time-series station ALOHA, located in the eastern region of the NPSG. At Sta. ALOHA in 2011, ʃTChl a was almost constant (19.5–26.4 mg m−2) throughout the year and was consistently dominated by Prochlorococcus (61–81 %). These results suggest that the phytoplankton community in the northwestern region of the NPSG is seasonally variable and composed of diverse groups compared with the eastern region of the NPSG.
The genetic diversity of one of the most abundant species in the Arctic and subarctic oceans, the pelagic snail Limacina helicina, has not yet been characterized in the north Pacific. This species has different 'forma' (L. helicina forma helicina, acuta, pacifica and ochotensis), but whether or not the morphological differences between these forma are caused by phenotypic plasticity or genetic differentiation remains unclear. Here, we analysed partial nucleotide sequences of the mitochondrial cytochrome c oxidase subunit I gene in L. helicina from the subarctic western North Pacific Ocean (SWNP; L. helicina f. acuta) and compared them with those from Svalbard (L. helicina f. helicina) and other localities (Beaufort Sea, eastern Pacific, northern Sea of Japan and western Atlantic). The results show clear genetic differentiation between populations in the SWNP and Svalbard (ΦCT = 0.59282, P < 0.001). These genetic differences are consistent with the previous description of the two forma L. h. f. acuta (SWNP) and L. h. f. helicina (Svalbard) based on shell morphology.
Abstract We conducted a current velocity and hydrographic time series observation in the upper ocean by deploying a mooring buoy at station K2 ( N, E) from July 2015 to June 2019 to understand the physical conditions that affect the acidification of the winter mixed layer (ML) water in the western subarctic North Pacific. Disturbances with substantial current velocity shear were excited by the wind changes remotely related to the 2015/2016 and 2018/2019 El Niño events. The density stratification beneath the ML was also weakened in winter, particularly the El Niño years. Based on the bulk Richardson number mixing scheme, we found that the El Niño‐related strengthening of the current velocity shear enhanced the vertical diffusivity beneath the winter ML driven by the near‐inertial current velocity variation. The weakening of the density stratification is ineffective in enhancing the vertical diffusivity. It is suggested that the El Niño‐related large vertical mixing accelerates the acidification of the winter ML water through the enhancement of the entrainment of the deep water that is rich in dissolved inorganic carbon.
