Abstract
Abstract. The stratification of the upper oligotrophic ocean has a direct impact on biogeochemistry by regulating the components of the upper-ocean environment that are critical to biological productivity, such as light availability for photosynthesis and nutrient supply from the deep ocean. We investigated the spatial distribution pattern and diversity of phytoplankton communities in the western Pacific Ocean (WPO) in the autumn of 2016, 2017, and 2018. Our results showed the phytoplankton community structure mainly consisted of cyanobacteria, diatoms, and dinoflagellates, while the abundance of Chrysophyceae was negligible. Phytoplankton abundance was high from the equatorial region to 10∘ N and decreased with increasing latitude in spatial distribution. Phytoplankton also showed a strong variation in the vertical distribution. The potential influences of physicochemical parameters on phytoplankton abundance were analyzed by a structural equation model (SEM) to determine nutrient ratios driven by vertical stratification to regulate phytoplankton community structure in the typical oligotrophic ocean. Regions with strong vertical stratification were more favorable for cyanobacteria, whereas weak vertical stratification was more conducive to diatoms and dinoflagellates. Our study shows that stratification is a major determinant of phytoplankton community structure and highlights that physical processes in the ocean control phytoplankton community structure by driving the balance of chemical elements, providing a database to better predict models of changes in phytoplankton community structure under future ocean scenarios.
Highlights
Phytoplankton contribute nearly half of global primary production (Field et al, 1998) and represent an important part of biogeochemical cycling and transformation (Falkowski et al, 1998)
Samples were collected by net, and net-collected samples reduced phytoplankton abundance in small volumes, thereby underestimating the phytoplankton abundance in the ocean under investigation
Phytoplankton samples were collected from water samples, which better reflected the phytoplankton community structure and abundance
Summary
Phytoplankton contribute nearly half of global primary production (Field et al, 1998) and represent an important part of biogeochemical cycling and transformation (Falkowski et al, 1998). Marine phytoplankton link the cycling of different elements through their demand for multiple nutrients such as nitrogen (N), phosphorus (P), or iron and their relative availability (Hillebrand et al, 2013). The nutrient requirements of phytoplankton are limited by the environmental conditions in which they grow, and nutrient limitation increases the N : P ratio of primary production (Carlson, 2002; Fogg, 1983; Karl et al, 1998). Nitrogen fixation by phytoplankton may deplete phosphorus from the upper ocean, causing an increase to the N : P ratios (Karl et al, 2001). The photosynthesis does not cease, even when there are not enough nutrients to grow (Bertilsson et al, 2003; Geider et al, 1998; Goldman et al, 1979)
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