However, two remote seagrass meadows around the Isles of Scilly and Mannin Bay off Connemara, Ireland - were relatively healthy.
Writing in Royal Society Open Science, Mr Jones and Dr Unsworth said: "Our study provides the first strong quantitative evidence that seagrass meadows of the British Isles are mostly in poor condition in comparison with global averages, with tissue nitrogen levels 75% higher than global values."