![]() We developed an automated method to detect phytoplankton blooms using MODIS images (Extended Data Fig. The dataset was derived using global, 1-km resolution daily observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard NASA’s Aqua satellite, and all 0.76 million images acquired by this satellite mission between 20 were used. We generated a satellite-based dataset of phytoplankton bloom occurrence to characterize the spatial and temporal patterns of algal blooms in coastal oceans globally. To fill this knowledge gap, we developed a method to map global coastal algal blooms and used this tool to examine satellite images between 20, addressing three fundamental questions: (1) where and how frequently global coastal oceans have been affected by phytoplankton blooms (2) whether the blooms have expanded or intensified over the past two decades, both globally and regionally and (3) the identity of the potential drivers. However, the technical difficulties in dealing with complex optical features across different types of coastal waters have so far prohibited their application globally 20. Alternatively, satellite data have been used to monitor the ocean surface continuously since 1997 and have enabled bloom detection in many coastal regions 17, 18, 19. For example, although HAB events have been compiled into the UNESCO (United Nations Educational, Scientific, and Cultural Organization) Intergovernmental Oceanographic Commission Harmful Algae Event Database (HAEDAT) globally since 1985, bloom trends are difficult to resolve, owing to inconsistent sampling efforts and the diversity of the eco-environmental or socio-economic effects 6. Owing to substantial heterogeneity in space and time, algal blooms are challenging to characterize on a large scale 5, 14, and thus present knowledge does not allow us to answer one of the most fundamental questions: whether algal blooms have changed in recent decades on a global basis 6, 15, 16. Unfortunately, algal bloom frequency and distribution are projected to increase with future climate change 12, 13, with some changes causing adverse effects on aquatic ecosystems, fisheries and coastal resources. In other cases, the decay of a dense algal bloom can deplete oxygen in bottom waters, forming anoxic ‘dead zones’ that can cause fish and invertebrate die-offs and ecosystem restructuring, with serious consequences for the well-being of coastal communities 1, 11. For instance, the toxins produced by some algal species can accumulate in the food web, causing closures of fisheries as well as illness or mortality of marine species and humans 8, 9, 10. However, proliferations of algae that cause harm (termed harmful algal blooms (HABs)) have become a major environmental problem worldwide 5, 6, 7. Many algal blooms are beneficial, fixing carbon at the base of the food chain and supporting fisheries and ecosystems worldwide. ![]() Although many blooms can occur naturally, nutrients linked to anthropogenic eutrophication are expected to intensify their frequency globally 2, 3, 4. Phytoplankton blooms are accumulations of microscopic algae in the surface layer of fresh and marine water bodies. Our compilation of daily mapped coastal phytoplankton blooms provides the basis for global assessments of bloom risks and benefits, and for the formulation or evaluation of management or policy actions. We documented the relationship between the bloom trends and ocean circulation, and identified the stimulatory effects of recent increases in sea surface temperature. Globally, the spatial extent (+13.2%) and frequency (+59.2%) of blooms increased significantly ( P < 0.05) over the study period, whereas blooms weakened in tropical and subtropical areas of the Northern Hemisphere. We found that algal blooms occurred in 126 out of the 153 coastal countries examined. Here we map daily marine coastal algal blooms between 20 using global satellite observations at 1-km spatial resolution. ![]() ![]() Phytoplankton blooms in coastal oceans can be beneficial to coastal fisheries production and ecosystem function, but can also cause major environmental problems 1, 2-yet detailed characterizations of bloom incidence and distribution are not available worldwide.
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