Temperature sensitivity of marine macroalgae for aquaculture in China

Highlights

• Species-specific and region-specific thermal sensitivity of macroalgae aquaculture in China were evaluated.

• The impacts of heat stress will increase while cold stress will decrease in the future in the face of climate change.

• Macroalgae farming potentially is running out of traditional geographies.

• Effective adaptation and mitigation efforts are urgent for the sustainable development of macroalgae aquaculture.

Abstract

Macroalgae aquaculture faces grand challenges from extreme temperature events in the context of global warming. For mapping aquaculture areas and assessing potential risks, it is urgent to evaluate the thermal sensitivity of commercially and ecologically important macroalgae. In the present study, thermal tolerance of commercial macroalgae species in China and sea surface temperature (SST) under current conditions and three Shared Socioeconomic Pathways (SSP1–2.6, SSP3–7.0, SSP5–8.5) scenarios in 2050 were collected for calculating thermal safety margin (TSM), an index of thermal sensitivity, for each species in the aquaculture area. Our results indicated that a few species were particularly sensitive to current conditions, and the kelp Saccharina japonica and Ulva pertusa were the most sensitive species to heat and cold stress, respectively. As indicated by the regional thermal sensitivity (RTS) (i.e., the mean of TSMs for all species in each grid cell), the commercial macroalgae located in southern China (20–25°N) were more sensitive to heat stress, while the commercial macroalgae located in the regions of 27–40°N was more sensitive to cold stress. Commercial macroalgae nearby the Yangtze River Estuary (~30°N) were sensitive to both heat and cold stress. Under the three SSP scenarios, macroalgae aquaculture would be more sensitive to heat stress, and the impacts from cold stress would relatively weaken in 2050. Our findings provide crucial information for the management of macroalgae aquaculture and for climate change mitigation and adaptation in China.

Introduction

With increasing temperature and extreme weather events (IPCC, 2019), climate change dramatically increases the risk, complexity, and uncertainty of aquaculture systems, and mariculture has emerged as the proverbial canary in the coal mine (Galappaththi et al., 2020). Macroalgae farming, as a feasible solution to capture carbon and sequester CO₂, is considered as a promising tool to mitigate and adapt to climate change (Duarte et al., 2017; Froehlich et al., 2019; Yong et al., 2022). Given the utility value and ecological implication of seaweeds, macroalgae industry is also regarded as a promising sector for the blue bioeconomy (Araújo et al., 2021; Chirapart and Ruangchuay, 2022). In addition, algal production is the fastest-growing sector in global mariculture, generating an excess of US$ 16.5 billion annually (FAO, 2022), and still holds remarkable potential for innovation on valuable products (Sudhakar et al., 2018). In general, macroalgae aquaculture has huge development potential, market prospects, and ecological implications in the world (FAO, 2018; Moreira et al., 2022).

At present, about 61 producing countries and territories report algae cultivation (FAO, 2022). The main seaweed producers are mostly in Asia, and China, Indonesia, Korea and Philippines contributed 56.75%, 27.86%, 5.09% and 4.2% of world seaweed production, respectively (FAO, 2021). As the largest producer of seaweed, China mainly produced eight categories of seaweeds (China Fishery Statistical Yearbook, 2011-2020; Li et al., 2021) (Fig. 1), including globally distributed species (Gelidium amansii and Gracilariopsis lemaneiformis), introduced species (Laminaria digitata and Laminaria hyperborea). Kelp was the main composition in the Chinese macroalgae market, followed by gracilaria, nori, and wakame. In the aspect of aquaculture area, nori occupied the largest macroalgae cultured areas in China, followed by kelp, gracilaria, and wakame. The area of macroalgae aquaculture kept increasing from 2011 (119,233 ha) to 2017 (145,263 ha) and remained stable at ~141,737 ha from 2019. Macroalgae aquaculture production in China maintains momentum of rapid growth at an annual rate of 6.3% and increased from 2011 (1,601,764 tons) to 2020 (2,615,136 tons) (Fig. 1).

However, sustainable development of macroalgae aquaculture is encountering high uncertainty in China in the context of climate change (Ji et al., 2016). The marginal seas of China show significant responses to climate change (Cai et al., 2021), and sea surface temperature (SST) anomalies exceeded 3 °C, 2.2 °C, and 1.4 °C over the baseline in the past 30 years in the East China Sea in 2016 (Oliver et al., 2021), the Yellow Sea in 2016 (Li et al., 2022) and the South China Sea in 2018 (Yao and Wang, 2021), respectively. Facing threats from increasing temperature and extreme temperature events, many macroalgae farms, such as Neopyropia yezoensis, Saccharina japonica and Sargassum fusiforme, have suffered from extreme temperature events (Liu et al., 2019; Liu and Lin, 2021; Wang et al., 2018) and will face more extreme temperature events in the future (Guo et al., 2022; Tanaka and Van Houtan, 2022; Woolway et al., 2021). For mitigating and adapting to climate change, it's urgent to assess the sensitivity of macroalgae for aquaculture to extreme temperature events.

Thermal safety margin (TSM) has been considered as a suitable index for assessing the temperature sensitivity of mariculture species and mapping aquaculture areas in the face of climate change (Cereja, 2020; Froehlich et al., 2016; Ma et al., 2021). TSM is calculated based on the species' physiological performance and the climatic variables (mostly temperature) for depicting how closely organisms are living to their lethal limits (Clusella-Trullas et al., 2021; Deutsch et al., 2008; García-Rueda et al., 2021; Liao et al., 2021; Sunday et al., 2014). In the present study, we calculated TSMs of main commercial macroalgae species in China for evaluating and predicting species-specific and region-specific sensitivity of macroalgae aquaculture to climate warming. These results would be crucial for risk assessment and then sustainable development of the macroalgae aquaculture.