The Diel Cycle of Surface Ocean Elemental Stoichiometry has Implications for Ocean Productivity. Issue 3 (12th March 2022)

• Record Type: Journal Article
• Authors: Garcia, Nathan S.; Talmy, David; Fu, Wei‐Wei; Larkin, Alyse A.; Lee, Jenna; Martiny, Adam C.
• Is Part Of: Global biogeochemical cycles. Volume 36:Issue 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-12
• Subjects: phytoplankton -- growth -- carbon -- nitrogen -- productivity -- diel; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2021GB007092
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Environmentally driven variability in the elemental stoichiometry of ocean plankton plays a key role in ocean biogeochemical processes. Recent studies have identified clear regional variability in C:N:P, but less is known about the environmental regulation of diel variability in plankton elemental stoichiometry. Here, we quantified the amplitude of the diel variability in C:N of surface ocean particles (<30 μm, C:N amp ) across large latitudinal gradients in the Indian and Atlantic Oceans. We commonly observed diel oscillations in C:N and biome‐specific variability in C:N amp . Temperature emerged as the strongest predictor of C:N amp, relative to the supply of nitrate. We propose that C:N amp is positively related to photosynthesis and respiration and thus phytoplankton growth rates. We find that independent growth rate proxies and an ecosystem model support this hypothesis. In addition, the temperature sensitivity of C:N amp has a Q 10 of 1.78 corroborating studies of phytoplankton growth rates. Surface communities across the Indian Ocean transect had a very small dependency on nitrate, whereas recycled nitrogen sources were by far the most preferred and the ratio of recycled‐N:nitrate utilization increased with increasing C:N amp . To predict future changes in C:N amp, we combined our statistical model with data from the fifth Coupled Model Intercomparison Project for the years 1990 and 2090. The results suggest that future rising temperatures will yield increased C:N amp . Collectively, our results imply that rising surface ocean temperatures lead to elevated phytoplankton growth rates supported by recycled nutrients. Plain Language Summary: Fueled by light energy, phytoplankton are the base of ocean food webs and rely on carbon dioxide to build new cells. One future prediction suggests ocean warming will restrict the flow of nutrients such as nitrate from deep water into the sun‐lit surface where phytoplankton grow. Without a comprehensive perspective of growth rates, this negative view assumes nitrate is a limiting nutrient that phytoplankton also use to build new cells. We developed a unique measurement that involves the daily change in the ratio of carbon to nitrogen (C:N) in phytoplankton. During the day, C:N rises and at night it declines. The amplitude of this daily oscillation reflects physiological processes and is correlated with phytoplankton growth rates. We measured C:N amplitude across the Atlantic and Indian Oceans to understand how temperature and nitrate might influence phytoplankton growth. We then used global‐scale models to understand how C:N amplitude might change in the future oceans. Contrary to expectations, our models suggest that the dwindling supply of nitrate due to future warming will not affect phytoplankton growth rates to a large degree when compared to the strong, direct influence of rising temperature. Overall, our study casts new light on the effects of ocean warming on phytoplankton growth. Key Points: We observed significant daily oscillations in the C:N ratio of surface ocean ecosystems The daily C:N amplitude was positively related to temperature and phytoplankton growth rate Phytoplankton growth rate could increase in low latitude regions due to surface warming