Seasonal Refuge in a Tropical Transition Zone: Hotter Temperatures Reduce Seasonal Productivity of Seaweeds But Do Not Cause Direct Mortality

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. The results uncovered 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. The results suggest that the discrepancy amongst peak photosynthetic rates and growth may not lead to the predicted mortality or range shifts in warmer future oceans and increased solar budgets. Instead, a shorter growth season and longer dormancy is predicted when considering bi-phasic seaweeds in climatic transition zones, potentially reducing system-wide productivity for these algal forests.

Keywords: temperate reef, algal phenology, primary productivity, ocean warming, marginal habitat, coastal productivity