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”.

Eelgrass (Zostera marina L.) is a dominant seagrass species in the temperate waters of the northern hemisphere and is experiencing global declines. The use of eelgrass seeds is increasingly being recognized as a viable option for large-scale restoration projects. Eelgrass reproductive shoots are often collected to obtain seeds or to broadcast seeds in restoration projects. Here, a new method to accurately evaluate the seed provision efficacy of detached eelgrass reproductive is proposed. Viable seeds of detached eelgrass reproductive shoots were collected using in situ net cages at Swan Lake, China. The seed yield and quality of detached eelgrass reproductive shoots under five different treatments (with or without rhizomes and roots, vegetative shoots, and sediment) were compared to select the optimal treatment for this method. The results showed that each detached eelgrass reproductive shoot produced approximately 50 viable seeds on average and the potential seed yield overestimated the actual seed yields by around four times. Seed quality evaluated by size and weight was consistent with that of the natural eelgrass meadow in Swan Lake. Comparing the five different treatments, the simplest treatment (a single reproductive shoot) was convenient and robust for this method. The results indicate that this method is worth further extending to other populations to improve the efficiency of seed use and for effectiveness evaluation in restoration projects.