Abstract: Coastal ecosystem services are of great significance for maintaining regional ecological security. Clarifying the tradeoff and synergy relationships, as well as the driving mechanisms of ecosystem services in coastal areas, is crucial for balancing the sustainable utilization of regional ecological resources with human socio-economic development. Utilizing the InVEST and RUSLE ecological models and synergy-tradeoff indices, we quantitatively assessed the spatial and temporal variations of ecosystem services and their synergistic/tradeoff relationships at both 30 m grid and municipal scales in the Bohai Rim coastal area from 1990 to 2020. Employing the geographic detector method, we revealed the spatial differentiation and influencing mechanisms. The results showed that: (1) Over the past 30 years, the four types of ecosystem services showed significant spatial and temporal variations, with spatial distribution becoming more clustered as the scale increases. Overall, soil retention services showed an increasing trend, rising by 77.4%, while carbon sequestration, biodiversity maintenance, and water purification services showed declining trends, decreasing by 7.3%, 5.0%, and 18.8% respectively. (2) The tradeoff and synergy relationships among the six pairs of ecosystem services exhibited scale effects. The quantitative relationships between services were more stable at the municipal scale but displayed more pronounced spatial heterogeneity. At the grid scale, carbon sequestration-soil retention and water purification-biodiversity maintenance primarily exhibited synergistic relationships, with low to moderate negative synergy. In contrast, water purification, soil retention, and biodiversity maintenance were predominantly characterized by tradeoffs. At the municipal scale, significant tradeoff relationships existed among four pairs of services, including carbon sequestration, water purification, soil retention and biodiversity maintenance, with an average synergy intensity index of over 0.6 for water purification and soil retention. (3) NDVI, temperature, precipitation, and landscape diversity index were the dominant factors influencing the spatial differentiation of the tradeoffs and synergies among ecosystem services, mainly through nonlinear enhancement or dual-factor enhancement interactions, with temperature and precipitation, and NDVI and precipitation being the primary interactive influences. By clarifying the tradeoffs, synergies, and driving factors among ecosystem services in the Bohai Rim coastal area, this study provides a scientific basis for regional ecological protection and management.

Key words: ecosystem services, tradeoff and synergy relationship, driving factor, Bohai Rim coastal area