Marine biodiversity and ecosystem functioning including seaweed communities - in the territorial waters of the UK and its Overseas Territories are facing unprecedented pressures. Key stressors are changes in ecosystem functioning due to biodiversity loss caused by ocean warming (species replacement and migration, e.g. affecting kelp forests), sea level rise (e.g. loss of habitats including salt marshes), plastic pollution (e.g. entanglement and ingestion), alien species with increasing numbers of alien seaweeds (e.g. outcompeting native species and parasite transmission), overexploitation (e.g. loss of energy supply further up the food web), habitat destruction (e.g. loss of nursery areas for commercially important species) and ocean acidification (e.g. skeletal weakening of ecosystem engineers including coralline algal beds). These stressors are currently affecting biodiversity, and their impact can be projected for the future. All stressors may act alone or in synergy. Marine biodiversity provides crucial goods and services. Climate change and biodiversity loss pose new challenges for legislation. In particular, there are implications of climate change for the designation and management of Marine Protected Areas and natural carbon storage by marine systems to help control the global climate system. The UK currently has legal obligations to protect biodiversity under international and European law.