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Manipulative laboratory studies provide strong evidence that phytoplankton primary production (PP), stoichiometry, and taxonomic composition affect marine copepod production (CP), which is the biomass-dominant zooplankton group. However, field observations investigating the simultaneous effects of prey stoichiometric quality, PP, and phytoplankton and copepod taxonomic composition on CP remain relatively rare. Here, we examined how in situ CP is affected by carbon:nitrogen:phosphorus (C:N:P) molar ratios of prey, PP, and phytoplankton and copepod composition in the East China Sea (ECS) and Dongsha Atoll in the South China Sea. Field estimates of CP were measured directly as the product of in situ instantaneous growth rate estimates by artificial cohort method and copepod biomass. We found that CP was low when prey C:N and C:P ratios were high, but the variation of CP was large when prey C:N and C:P ratios were low. CP did not, however, show a strong positive relationship with PP. Multivariate regression indicates that prey C:N ratio explains most of the variation of CP, followed by phytoplankton and copepod compositions, while PP exerts a weak influence on CP. Our findings suggest that copepod community production is affected by prey stoichiometry, with further modification by copepod and phytoplankton compositions in the ECS. However, the total variance explained by those key factors is less than 50%, indicating that marine copepod growth and biomass production are influenced by complex interactions in nature.

Trophic strategy determines stoichiometry of plankton. In general, heterotrophic zooplankton have lower and more stable C:N and C:P ratios than photoautotrophic phytoplankton whereas mixotrophic protists, which consume prey and photosynthesize, have stoichiometry between zooplankton and phytoplankton. As trophic strategies change with cell size, body size may be a key trait influencing eukaryotic plankton stoichiometry. However, the relationship between body size and stoichiometry remains unclear. Here, we measured plankton size-fractionated C:N ratios under different intensities of light and nutrient supply in subtropical freshwater and marine systems. We found a unimodal body size-C:N ratio pattern with a maximum C:N ratio at ~50 µm diameter in marine and freshwater systems. Moreover, the variation in C:N ratios is mainly explained by body size, followed by light intensity and nutrient concentration. To investigate the mechanisms behind this unimodal pattern, we constructed a size-based plankton food web model in which the trophic strategy and C:N ratio is an emergent result. Our model simulations reproduce the unimodal pattern with C:N ratio of photoautotrophs ≤ 50 µm increasing with body size due to increase of photosynthetic carbon, whereas C:N ratios of organisms > 50 µm decreases with size due to decreasing photoautotrophic but increasing heterotrophic uptake. Based on our field observations and simulation, we extend the classic “light-nutrient” theory that determines plankton C:N ratio to include body size and trophic strategy dependency. We conclude that body size and size-dependent uptake of resources (light, nutrients and prey) determine plankton stoichiometry at various light and nutrient supplies.

Manipulative laboratory studies provide strong evidence that phytoplankton primary production (PP), stoichiometry, and taxonomic composition affect marine copepod production (CP), which is the biomass-dominant zooplankton group. However, field observations investigating the simultaneous effects of prey stoichiometric quality, PP, and phytoplankton and copepod taxonomic composition on CP remain relatively rare. Here, we examined how in situ CP is affected by carbon:nitrogen:phosphorus (C:N:P) molar ratios of prey, PP, and phytoplankton and copepod composition in the East China Sea (ECS) and Dongsha Atoll in the South China Sea. Field estimates of CP were measured directly as the product of in situ instantaneous growth rate estimates by artificial cohort method and copepod biomass. We found that CP was low when prey C:N and C:P ratios were high, but the variation of CP was large when prey C:N and C:P ratios were low. CP did not, however, show a strong positive relationship with PP. Multivariate regression indicates that prey C:N ratio explains most of the variation of CP, followed by phytoplankton and copepod compositions, while PP exerts a weak influence on CP. Our findings suggest that copepod community production is affected by prey stoichiometry, with further modification by copepod and phytoplankton compositions in the ECS. However, the total variance explained by those key factors is less than 50%, indicating that marine copepod growth and biomass production are influenced by complex interactions in nature

It has been estimated that more than 48% of global methane emissions from lakes and reservoirs occur at low latitudes (<24°). To improve this estimate, knowledge regarding underexplored ecosystems, particularly deep lakes and reservoirs in Asian monsoon regions, is needed because the magnitude of methane emissions is influenced by lake bathymetry and climatic conditions. We conducted long-term studies beginning in 2004 at Feitsui Reservoir (FTR) in Taiwan, a subtropical monomictic system with a maximum depth of 120 m to monitor seasonal and interannual variations of three key characteristics and to understand the mechanisms underlying these variations. Key characteristics investigated were as follows: (1) the balance of primary production and heterotrophic respiration as a determinant of vertical oxygen distribution, (2) methane production at the bottom of the reservoir, oxidation in the water column, and emissions from the lake surface, and (3) the contribution of methane-originated carbon to the pelagic food web through methane-oxidizing bacteria (MOB). This review highlights major achievements from FTR studies integrating isotopic, microbial, and modeling approaches. Based on our findings, we proposed two conceptual models: (1) a model of methane dynamics, which addresses the differences in methane emission mechanisms between deep and shallow lakes, and (2) a spatially explicit model linking benthic methane production to the pelagic food web, which addresses the diversity of MOB metabolisms and their dependence on oxygen availability. Finally, we address why long-term studies of subtropical lakes and reservoirs are important for better understanding the effects of climate on low- to mid-latitude ecosystems.