Coastal restoration policy needs to consider seaweed diversity

Abstract

Seaweed diversity and biomass is in decline in many regions worldwide. In China, for example, 44% of seaweed species have become locally extinct around Yushan Island since 1989 (ref. ¹); 286 species have been lost from Hainan since the 1970s²; and natural seaweed beds of Sargassum horneri in Nanji Islands have almost disappeared since the 1980s. These losses are alarming. Seaweeds (macroalgae) underpin rocky coastal ecosystems, where they provide habitat, food and spawning grounds, and drive nutrient cycling³. Seaweed species also benefit people as a direct source of food, medicines and chemicals, in addition to improving water quality and buffering storm surges^4,5. Yet acknowledgement of the ecological and economic importance of diverse seaweeds is conspicuously lacking from high-level policy discussions. Specific attention to the sustainable use and safeguarding of seaweed resources and biodiversity is sorely needed.

There are several drivers of seaweed declines. Urbanization and coastal development (including construction of artificial structures such as seawalls, ports, piers, pontoons and mariculture rafts) have transformed natural habitats and environments (Fig. 1a). This sprawl of artificial shorelines is happening worldwide; in China, it has increased from 24% to 71% of the coast over the past four decades⁹. Many seaweed species do not survive in these artificial environments; this leads to diversity loss and macroalgal blooms — often of single species that can grow well in these altered habitats¹⁰. In addition, extreme weather events driven by climate change (such as marine heatwaves and storm surges) can decimate seaweeds by killing and removing them from hard substrata^11,12. Seaweed species that trap gas for flotation (for example, Ulva and Sargassum spp.) can survive and drift on the sea surface; sinking and decomposition of species without this flotation can cause harmful reductions in oxygen levels. The overharvesting of edible species has also contributed to the sharp reduction in seaweed diversity, particularly on the extensive rocky shores of China.