Abstract
Sagami Bay, Japan is influenced by both the warm Kuroshio Current and the cold Oyashio Current and rich nutrients are supplied from multiple river sources and the deep-sea, forming a dynamic ecosystem. The aim of the present study was to investigate eukaryotic and bacterial communities in the coastal waters of Sagami Bay, using 16S rRNA and 18S rRNA sequencing and to assess the seasonal and vertical dynamics in relation to physicochemical and biological conditions. Eukaryotic and bacterial communities showed synchronous seasonal and vertical changes along with environmental variability. Diversity of plankton community suspended in the surface was lower than those at the subsurface layers in both the eukaryotes and bacteria communities; however, community diversity showed different characteristics in the subsurface where the eukaryotic community at the deeper layer (100 m) was as low as the surface and highest in intermediate depth layers (10–50 m), while that of bacterial community was highest in the deeper layer (100 m). The annual variability of the coastal microbial communities was driven, not only by the seasonal changes of abiotic and biotic factors and short-term rapid changes by river water inflow and phytoplankton blooms, but also largely influenced by deep-seawater upwellings due to the unique seafloor topography.
Highlights
Marine pelagic ecosystems are interconnected by two types of food chains: grazing food chains and microbial food chains
Classical grazing food chains begin with phytoplankton photosynthesis, which become food for zooplankton, such as herbivorous copepods and further preyed upon by larger organisms, such as fishes (Fenchel 1988; Azam 1998)
The microbial loop is a relatively new food chain concept beginning with bacteria introducing dissolved organic material (DOM) to the classical food chain via heterotrophic protist and microzooplankton (Pomeroy 1974; Azam et al 1983; Pomeroy et al 2007)
Summary
Marine pelagic ecosystems are interconnected by two types of food chains: grazing food chains and microbial food chains (microbial loop). Classical grazing food chains begin with phytoplankton photosynthesis (producers), which become food for zooplankton, such as herbivorous copepods and further preyed upon by larger organisms, such as fishes (Fenchel 1988; Azam 1998). The microbial loop is a relatively new food chain concept beginning with bacteria introducing dissolved organic material (DOM) to the classical food chain via heterotrophic protist and microzooplankton (Pomeroy 1974; Azam et al 1983; Pomeroy et al 2007). Physical factors, such as currents, temperature, density and light, chemical factors, such as nutrients and oxygen and biological factors related directly or indirectly with marine organisms, regulate the system. Several studies have shown that diversity, abundance and the community structure of marine plankton correspond with changes in physical and chemical factors and interaction between marine plankton (Sin et al 1999; Hays et al 2005; Sogawa et al 2013; Signori et al 2014; Vajravelu et al 2018; Ibarbalz et al 2019).
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