[A-HW24] Material transportation and cycling at the land-sea interface: from headwaters to the ocean

[AHW24-04] Nitrogen subsidy from profundal methane-oxidizing bacteria cascades across pelagic food web in a deep meso-oligotrophic reservoir

*Erica Silk Perez dela Paz¹, Pei-Chi Ho⁴, Fan-Sian Lin², Chih-hao Hsieh², Chaochen Lai³, Fuh-Kwo Shiah³, Chia-Ying Ko¹, Noburo Okuda^5,6 (1.Institute of Fisheries Science, National Taiwan University, Taipei, Taiwan, 2.Institute of Oceanography, National Taiwan University, Taipei, Taiwan, 3.Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan, 4.Graduate School of Life Science, Tohoku University, Japan, 5.Research Center for Humanity and Nature, Kyoto, Japan, 6.Research Center for Inland Seas, Graduate School of Science, Kobe University, Kobe, Japan)

Keywords:Freshwater ecosystem, Methanotrophy , Nitrogen subsidy , Pelagic food web, Zooplankton

Nutrient subsidy within the plankton community can influence the biological activities of zooplankton in the pelagic food web. While understanding of the potential direct effects of C-subsidies on zooplankton is growing, little is known about profundal MOB N-subsidies to pelagic food web and their effects on zooplankton and phytoplankton productivity during less productive seasons. We hypothesized that profundal MOB can subsidize N to pelagic food web during mixing period. We investigated the variation of MOB reliance, C/N ratio of zooplankton, zooplankton biomass, and phytoplankton mass-specific productivity as well as their relationships in response to profundal MOB N-subsidy during mixing periods in a subtropical deep meso-oligotrophic reservoir, Fei-Tsui Reservoir, Taiwan, from 2010 to 2014. As zooplankton relied on the MOB, their C/N ratios become lower, possibly due to lower C/N ratio for bacteria than for phytoplankton. The zooplankton C/N variability in turn influenced their biomass, implying an ecological stoichiometry response of zooplankton growth to nitrogen subsidy. This N-subsidy effect did not cascade to phytoplankton biomass, contrary to our prediction. The N-subsidy effect might have been masked by the stronger effect of zooplankton grazing during mixing period. The biological responses of zooplankton to MOB N-subsidy in the benthic-pelagic link clearly influenced the pelagic food web, demonstrating that N was physically linked within pelagic habitat in this deep meso-oligotrophic reservoir. Profundal MOB enhanced pelagic-benthic coupling and served as a N support for pelagic animals during less productive seasons, adding to our understanding of methanotrophic bacteria's role in the aquatic food web in this deep meso-oligotrophic reservoir.