Benthic microalgae are more 13C-depleted than phytoplankton in temperate flooded rice fields

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 DI13C intake during photosynthesis. Conversely, microphytobenthos used more 13C-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.