Long-Term Changes in the Unique and Largest Seagrass Meadows in the Bohai Sea (China) Using Satellite (1974–2019) and Sonar Data: Implication for Conservation and Restoration

Shaochun Xu,Mingjie Liu,Ruiting Gu,Shidong Yue,Yongliang Qiao,Yu Zhang,Yi Zhou,Xiaomei Zhang,Shuai Xu

doi:10.3390/rs13050856

Abstract

Seagrass meadows play critical roles in supporting a high level of biodiversity but are continuously threatened by human activities, such as sea reclamation. In this study, we reported on a large seagrass (Zostera marina L.) meadow in Caofeidian shoal harbor in the Bohai Sea of northern China. We evaluated the environmental impact of sea reclamation activities using Landsat imagery (1974–2019) by mapping seagrass meadow distribution changes. ISODATA was adopted for the unsupervised classification and mapping of seagrass beds. The error matrix developed using the in situ data obtained from acoustic surveys for Landsat 8OLI image classification was 87.20% accurate. The maps showed rapidly increasing changes in seagrass meadows as the amount of reclaimed land increased. Some seagrass meadows experienced large-scale changes, and sea reclamation has been suggested as the main factor responsible for habitat loss, which results from physical damage, excessive sedimentation, and increased turbidity caused by reclamation. In addition, habitat degradation may have resulted from three storm surges induced by typhoons in 1992–1998. Fortunately, land reclamation, forming an artificial “longshore bar”, buffers seagrass meadows from wave actions, providing relatively sheltered conditions, which has allowed a large habitat increase since 2012. These were the largest eelgrass meadows (3,217.32 ha), with a peripheral area of ~100 km2, in the Bohai Sea of northern China in 2019. However, the existing largest eelgrass beds in China are threatened by trawling, clam harvesting (especially clam sucking), channel dredging, and culture pond construction. Our work will help coastal managers monitor the environmental impacts of reclamation activities on seagrass meadows on a large spatio-temporal scale and will also provide information for seagrass restoration using artificial “longshore bars”.

Highlights

Introduction conditions of the Creative CommonsSeagrasses form productive plant communities, providing habitats, foods, and nurseries for a variety of marine organisms [1,2,3,4,5,6,7]
Five species within three genera are found in temperate northern China according to the national seagrass resource survey (2015–2020), but only the seagrass Zostera marina L. is found at our study site
Our work provides new seagrass distribution data from temperate northern China and fundamental information for implementing appropriate seagrass restoration and conservation strategies

Summary

Introduction

Seagrasses form productive plant communities, providing habitats, foods, and nurseries for a variety of marine organisms [1,2,3,4,5,6,7]. With multiple stressors threatening these aquatic plants, seagrass meadows have been declining worldwide since 1990 at a rate. Many studies have been conducted to effectively understand changes in seagrass habitats’ spatial distributions and temporal fluctuations [9,10,11]. There are 72 species of seagrass in six families and 14 genera worldwide [12]. Five species within three genera are found in temperate northern China according to the national seagrass resource survey (2015–2020), but only the seagrass Zostera marina L. (eelgrass) is found at our study site.

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