Persistence in a tropical transition zone? Sargassum forests alternate seasonal growth forms to maintain productivity in warming waters at the expense of annual biomass production

Rhyn W Y Cheung-Wong; Jonne Kotta; Deevesh A Hemraj; Bayden D Russell

doi:10.1016/j.scitotenv.2022.158154

Persistence in a tropical transition zone? Sargassum forests alternate seasonal growth forms to maintain productivity in warming waters at the expense of annual biomass production

Sci Total Environ. 2022 Dec 10;851(Pt 1):158154. doi: 10.1016/j.scitotenv.2022.158154. Epub 2022 Aug 19.

Authors

Rhyn W Y Cheung-Wong¹, Jonne Kotta², Deevesh A Hemraj¹, Bayden D Russell³

Affiliations

¹ Swire Institute of Marine Science and Area of Ecology and Biodiversity, School of Biological Sciences, The University of Hong Kong, Hong Kong SAR, China; Institute for Climate and Carbon Neutrality, The University of Hong Kong, Hong Kong SAR, China.
² Estonian Marine Institute, University of Tartu, Tallinn, Estonia.
³ Swire Institute of Marine Science and Area of Ecology and Biodiversity, School of Biological Sciences, The University of Hong Kong, Hong Kong SAR, China; Institute for Climate and Carbon Neutrality, The University of Hong Kong, Hong Kong SAR, China; The Dove Marine Laboratory, School of Natural and Environmental Sciences, Newcastle University, Newcastle-upon-Tyne, UK. Electronic address: brussell@hku.hk.

PMID: 35995150
DOI: 10.1016/j.scitotenv.2022.158154

Abstract

Macroalgal forests provide productivity and biomass that underpins the function of many coastal ecosystems globally. The phenology of forests is seasonally driven by environmental conditions, with the environment-productivity relationship understood for most coastlines of the world. Climatic transition zones, however, have characteristics of temperate and tropical regions, creating large fluctuations in environmental conditions, and potentially limiting productivity and the persistence of macroalgal forests. The response of a forest-forming, dimorphic seaweed (Sargassum hemiphyllum) to seasonal temperature and light conditions in a rapidly warming tropical-temperate transitional zone (Hong Kong) was quantified by measuring in situ growth, net primary productivity (NPP), respiration, and photosynthetic potential. These physiological responses of S. hemiphyllum were then experimentally tested in response to changing temperatures (16.5-27 °C) and irradiances (20, 110, and 300 μmol m^-2 s^-1) in laboratory mesocosms. In contrast to predictions, S. hemiphyllum demonstrated asynchronous NPP and growth patterns, with growth maximized in cooler conditions but, counter-intuitively, highest photosynthetic rates in summer after annual senescence and dormancy were established. This discrepancy between peak photosynthetic rates and growth may provide regional populations of S. hemiphyllum the ability to survive higher temperatures in the near future, resisting the predicted range shifts under ocean warming. In contrast, warming is likely to drive a shorter growth season, longer dormancy, and reduced annual biomass production in bi-phasic seaweeds inhabiting climatic transition zones, potentially reducing system-wide productivity of these algal forests.

Keywords: Algal phenology; Coastal productivity; Marginal habitat; Ocean warming; Primary productivity; Temperate reef.

MeSH terms

Biomass
Ecosystem
Forests
Sargassum*
Seasons
Seaweed*