Description: 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 ( F v /F m ), the functional absorption cross-section of PSII ( PSII ) and the rate of reoxidation of the primary PSII electron acceptor (1/ Qa ), which were dependent on the hydrographic structure of the water column. FRR-derived parameters were used to estimate the chlorophyll a -normalized, photosynthetic rate ( P B ); the photosynthesis vs. irradiance ( P – E ) curves were derived from in situ P B 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 ( E k ) 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 P B 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.

Description: To examine the relationship between carbon and nitrogen stable isotope (SI) ratios ( 13 C and 15 N) of zooplankton, we analyzed samples collected bimonthly from March to October 2009, from the euphotic layers of the Oyashio current along the A-line in the western North Pacific. Isotopic ratios of higher trophic levels such as predatory zooplankton and/or long-lived zooplankton varied little with season, while those of short-lived zooplankton were variable on the 15 N– 13 C map. We also analyzed preserved samples taken from the warm-core ring 86-B derived from the Kuroshio extension region. Although the zooplankton groups in the two regions exhibited different values in 15 N, the 15 N versus 13 C slopes for each ecosystem do not show significant differences. Statistical analysis conducted together with previously published data from the Antarctic Ocean and the Gulf of Alaska suggested a similar 15 N versus 13 C slope throughout the four regions. We attributed this common slope to physiological aspects of feeding processes (e.g. the kinetic isotope effects inherent in the processes of amino acid synthesis). The common pattern for all four oceanic regions suggests that SIs may be used to elucidate general patterns in ecosystems and biogeochemical cycles.