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Abstract Submission No. | ABS-2022-06-0448 |
Title of Abstract | Unexpected temperature trends in plankton growth and grazing along a 110°E transect in the eastern Indian Ocean |
Authors | Michael R. Landry*, Raleigh R. Hood, Claire H. Davies, Karen E. Selph, David Antoine, Lynnath E. Beckley |
Organisation | Scripps Institution of Oceanography, University of California at |
Address | SIO/UCSD, 9500 Gilman Dr. La Jolla, CA, United States Pincode: 92093-0227 Mobile: 9999 E-mail: mlandry@ucsd.edu |
Country | United States |
Presentation | Oral |
Abstract | With mid- to low-latitude waters of the Indian Ocean (IO) warming faster than other major oceans, many questions can be raised about the potential impacts of increasing temperature on IO productivity and food webs. Most models would predict strong increases in phytoplankton growth rate with rising temperature and declining intermediate consumers like zooplankton due to the combined effects of reduced nutrient input, enhanced metabolism, and phytoplankton size shifts that reduce trophic transfer efficiency. On R/V Investigator cruise IN2019V03 (May-June 2019), we investigated phytoplankton production, growth and grazing processes along the historic IIOE 110°E transect from 39.5 to 11.5°S in the eastern IO. Stations spanned a 14°C gradient in mean euphotic zone temperature from temperate and subtropical to tropical water masses. Production and grazing were strongly coupled over the transect and balanced on average. Increasing growth conditions (light and temperature) were mainly manifest as more rapid biomass turnover and mesozooplankton biomass accumulation, which was elevated by 3 fold in tropical waters. Over uniformly oligotrophic subtropical-tropical waters spanning a 10°C temperature increase, growth of the phytoplankton biomass dominant, Prochlorococcus, showed no rate response, violating expected Q10 rules and predictions of the Metabolic Theory of Ecology. However, biomass-normalized grazing of mesozooplankton increased 6 fold over the transect, showing a strong temperature relationship (r2 = 0.87) that exceeded expected temperature effects on gut turnover and metabolic rates. Such results might be explained by an increasing role of mixotrophy in warmer tropical waters and selection for defenses that minimize phytoplankton mortality to protistan grazers, the latter as an alternative to growing faster in highly coupled oligotrophic systems where production, grazing and nutrient cycling processes must balance. Predictions of future states of the warming ocean will be difficult to make from relationships derived from lab cultures incorporated into models. Our results highlight the importance of using existing natural gradients of adapted complex communities like the eastern IO as laboratories for observation and hypothesis testing. |