Abstract
The current phytoplankton community structure is expected to change, with small phytoplankton becoming dominant under ongoing warming conditions. To understand and evaluate the ecological roles of small phytoplankton in terms of food quantity and quality, the carbon uptake rates and intracellular biochemical compositions (i.e., carbohydrates, CHO; proteins, PRT; and lipids, LIP) of phytoplankton of different sizes were analyzed and compared in two different regions of the western East/Japan Sea (EJS): the Ulleung Basin (UB) and northwestern East/Japan Sea (NES). The average carbon uptake rate by the whole phytoplankton community in the UB (79.0 ± 12.2 mg C m–2 h–1) was approximately two times higher than that in the NES (40.7 ± 2.2 mg C m–2 h–1), although the average chlorophyll a (chl a) concentration was similar between the UB (31.0 ± 8.4 mg chl a m–2) and NES (28.4 ± 7.9 mg chl a m–2). The main reasons for the large difference in the carbon uptake rates are believed to be water temperature, which affects metabolic activity and growth rate, and the difference in euphotic depths. The contributions of small phytoplankton to the total carbon uptake rate were not significantly different between the regions studied. However, the rate of decrease in the total carbon uptake with increasing contributions from small phytoplankton was substantially higher in the UB than in the NES. This result suggests that compared to other regions in the EJS, the primary production in the UB could decrease rapidly under ongoing climate change. The calorific contents calculated based on biochemical compositions were similar between the small (1.01 ± 0.33 Kcal m–3) and large (1.14 ± 0.36 Kcal m–3) phytoplankton in the UB, whereas the biochemical contents were higher in the large phytoplankton (1.88 ± 0.54 Kcal m–3) than in the small phytoplankton (1.06 ± 0.18 Kcal m–3) in the NES. The calorific values per unit of chl a were higher for the large phytoplankton than for the small phytoplankton in both regions, which suggests that large phytoplankton could provide a more energy efficient food source to organisms in higher trophic levels in the western EJS.
Highlights
Phytoplankton, as primary producers, play an important role in the food web as well as the biogeochemical cycling of aquatic ecosystems
An increase in the contribution of small phytoplankton to the total phytoplankton community has been observed in various oceans (Agawin et al., 2000; Li et al., 2009; Morán et al., 2010; Hilligsøe et al., 2011; Mousing et al., 2014; Joo et al., 2017), which indicates a growing importance of small size phytoplankton as a basic food source for higher trophic level organisms in the marine ecosystems
This study in the two different regions (i.e., UB and NES) of the western EJS reported the influence of small phytoplankton contribution to the primary production and different energy efficiencies between small and large phytoplankton based on their biochemical components
Summary
Phytoplankton, as primary producers, play an important role in the food web as well as the biogeochemical cycling of aquatic ecosystems. Primary production by the phytoplankton community is an important factor in controlling the quantity of food sources for higher trophic level organisms and subsequently could affect the recruitment, biomass, and production of fishery resources (Whyte, 1987; Kleppel and Burkart, 1995; Kang et al, 2017). Several studies have reported that recent climate change could lead to an increase in the contribution of small phytoplankton to the total phytoplankton biomass; determining the ecological role of small phytoplankton as primary producers providing basic food sources in marine ecosystems is important under ongoing warming conditions (Agawin et al, 2000; Morán et al, 2010; Hilligsøe et al, 2011; Mousing et al, 2014)
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