Open watersheds and their rivers play a pivotal role in biogeochemical cycles, forming crucial links between land and ocean ecosystems. This study utilized remotely sensed and reanalysis datasets to investigate the land-ocean Gross Primary Productivity (GPP) dynamics of 15 major watersheds: Mississippi, Nelson, St. Lawrence, Amazon, Parana, Congo, Zambezi, Niger, Orange, Ganges Brahmaputra, Yangtze, Huang He, Amur, Danube, and Murray Darling. It looked at the dynamics and vulnerabilities of major watersheds to identify the influences of land use and land management, moisture balance, and temperature on the patterns of how primary producers absorb CO2 from the atmosphere. Intensive human interventions, particularly in agriculture-dominant watersheds like Mississippi, Ganges Brahmaputra, Yangtze, Huang He, and Murray Darling, were observed to substantially influence land productivity, with consequent cascading effects on adjoining ocean ecosystems. In contrast, the vast rainforests of Amazon and Congo illuminated their intrinsic vulnerability to moisture dynamics, accentuating potential threats under shifting climatic regimes. Watersheds like the Parana and Zambezi offered insights into ecosystem susceptibilities arising from temperature variations. As climatic anomalies continue to redefine land-ocean interactions, our study not only underscores the imperative for adaptive conservation strategies but also to study adjacent land and marine systems as intricately linked entities.