Benthic microalgae are more 13 C-depleted than phytoplankton in temperate flooded rice fields
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Benthic microalgae are more 13C‑depleted than phytoplankton in temperate flooded rice fields Natsuru Yasuno1 · Gen Kanaya2 · Yuya Takagi3 · Shuichi Shikano4 · Eisuke Kikuchi5 Received: 23 June 2020 / Revised: 30 September 2020 / Accepted: 7 October 2020 © The International Society of Paddy and Water Environment Engineering 2020
Abstract We investigated seasonal changes in the δ13C levels of particulate organic matter (POM), which mainly consist of phytoplankton and microphytobenthos from three flooded rice fields treated with different types of fertilizer (chemical NPK, chemical PK, or organic fertilizers) in northeast Japan. Generally, microphytobenthos in lentic freshwater systems show higher levels of δ13C (about− 26‰) than phytoplankton (about− 32‰) due to 13C enrichment under limited resources of dissolved inorganic carbon (DIC) in the benthic boundary layer (BBL). However, our results showed that POM was more 13C-enriched than microphytobenthos regardless of the type of fertilizer applied. Overall mean POM and microphytobenthos δ13C levels in the fields ranged from − 27.5 to − 25.2‰ and − 30.3 to − 27.0‰, respectively. In contrast, δ13C levels in soil organic matter were temporally stable, with means ranging from − 27.6 to − 26.3‰. High δ13C levels in POM were due to limited DIC under conditions of high phytoplanktonic productivity and water stagnation in paddy systems, resulting in D I13C intake 13 during photosynthesis. Conversely, microphytobenthos used more C-depleted DIC in the BBL, which was microbially regenerated from the SOM pool. Microphytobenthos in the rice fields showed similar seasonal variations in δ13C, decreasing sharply in late June and then increasing again. This was possibly caused by changes in DIC availability or the growth rates of microphytobenthos during that period. The present results provide basal information about producer-specific δ13C variations in two major microalgal producer groups, phytoplankton and microphytobenthos, in temperate rice paddy wetlands. Keywords Benthic boundary layer (BBL) · Food web · Microphytobenthos · Paddy field · Particulate organic matter (POM) · Temporary water
Introduction
* Natsuru Yasuno [email protected]; [email protected] 1
Center for Environmental Science in Saitama, Kamitanadare 914, Kazo, Saitama 347‑0115, Japan
2
National Institute for Environmental Studies, 16‑2 Onogawa, Tsukuba, Ibaraki 305‑8506, Japan
3
Tochigi Prefectural Shioyaminaminasu Agricultural Promotion Office, Kashima 20‑22, Yaita, Tochigi 329‑2163, Japan
4
Center for Northeast Asian Studies, Tohoku University, 41 Kawauchi, Aoba‑ku, Sendai, Miyagi 980‑8576, Japan
5
Research Institute for Teacher Training and Development, Miyagi University of Education, 149 Aramaki Aza Aoba, Aoba‑ku, Sendai, Miyagi 980‑0845, Japan
Phytoplankton and benthic microalgae are major primary producers in freshwater lentic systems (e.g., Zohary et al. 1994; Vadeboncoeur et al. 2001). Stable isotope ratios are widely used to investigat
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