Abstract
While primary productivity in the oligotrophic North Pacific Subtropical Gyre (NPSG) is changing, the micro-size plankton community has not been evaluated in the last 4 decades, prompting a re-evaluation. We collected samples over three years (2016-2018) from depths of 10 to 200 m (n = 127), and the micro-size plankton were identified and counted to understand the heterogeneity of micro-size plankton community structure. The assemblages were consistent to the those of 4 decades ago. Dinophyceae (dinoflagellates) were the most numerically abundant, followed by Cryptophyceae and Bacillariophyceae (diatoms). The other micro-size plankton classes (Cyanophyceae, Haptophyceae, Dictyochophyceae, Euglenophyceae, and Prasinophyceae) were not always detected, whereas only Trichodesmium spp. was counted in the Cyanophyceae. Other unidentified autotrophic and heterotrophic flagellates were also significantly present, and their numeric abundance was higher than or at the same level as was that of the Dinophyceae. In the Dinophyceae, Gymnodiniaceae and Peridiniales were abundant. The chlorophyll a concentration and these class-level assemblages suggested micro-size plankton is not a major primary producer in this area. We applied generalized additive models (GAMs) and principal coordination analyses (PCoAs) to evaluate the habitats of every plankton group and the heterogeneity of the assemblages. The GAMs suggested that every classified plankton abundance showed a similar response to salinity, and we observed differences in habitats in terms of temperature and nitrate concentrations. Based on the PCoAs, we observed unique communities at the 200 m depth layer compared with those at the other sampling layers. The site scores of PCoAs indicated that the micro-size plankton assemblages are most heterogeneous at the 10 m depth layer. At such depth, diazotrophic Cyanophyceae (Trichodesmium spp.) are abundant, particularly in less-saline water. Therefore, nitrogen fixation may contribute to the heterogeneity in the abundance and assemblages in the western NPSG.
Highlights
The size of primary producers is one of the keys to controlling the productivity of marine ecosystems [1]
We evaluated the habitats of plankton groups by using empirical statistical models and multivariate analyses
The subsurface chlorophyll maximum (SCM) layer was formed at 115 ± 9 m in the KY1604, 94 ± 10 m in the KY1704, and 138 ± 10 m in the KY1801 cruises
Summary
The size of primary producers is one of the keys to controlling the productivity of marine ecosystems [1]. In waters where pico-size phytoplankton are dominant, more trophic levels are required to convert primary production to useful forms; biomass transfer efficiency is low [1]. Energy fixed by primary production is more efficiently transferred to higher trophic organisms in micro-plankton dominated waters [1]. The abundance and composition of micro-size plankton comprise necessary information for evaluating the biological productivity of the ocean. The stratification of the surface water column develops throughout the year, and nutrient supplies from the deeper layer are extremely limited; phytoplankton abundance is low and limited in this layer. In the subtropical open ocean, pico- and nano-size phytoplankton is the dominant primary producers; micro-size phytoplankton is present in significant numbers [5,6,7]. Diatoms (Bacillariophyceae) are sometimes abundant (> 108 cells m-2) in the surface mixed layer [5, 8, 9], with most being nitrogen-fixing diatom symbioses [10]
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