The negative impacts of invasive species and extreme climate events on seagrass meadows and their soil organic carbon (SOC) pools

Seagrass meadows play a crucial ecological role, with Nanozostera japonica being the most widely distributed seagrass species in China. However, it faces severe degradation. In the Yellow River Delta, Shandong, China, the largest N. japonica meadow once existed. Invasive Spartina alterniflora has encroached upon N. japonica habitats. UAV (unmanned aerial vehicle) tracking from 2015 to 2019 revealed a 516-fold increase in the distribution area of S. alterniflora within the study region. Between April and July 2019, it advanced 14 meters into seagrass areas, spreading long distances via seed dispersal and short distances through clonal growth. With increasing levels of S. alterniflora invasion, the growth of N. japonica was significantly suppressed. In early August 2019, Typhoon Lekima severely impacted the seagrass meadows in the Yellow River Delta, reducing their distribution area from 1,031.8 hectares to less than 10 hectares and decreasing the surface soil organic carbon (35 cm depth) by 35%. During the typhoon, N. japonica was in its flowering stage, and its sexual reproduction was severely affected, resulting in a significant reduction in seed production. Soil erosion caused by the typhoon led to massive seagrass mortality, drastically decreasing the density of overwintering shoots. As a result, natural recovery of the seagrass in this region is highly challenging, necessitating artificial restoration efforts to expedite meadow recovery.