Mortality associated with a phytoplankton bloom off Penzance in Mounts Bay

Since May 1977 the Penzance Branch of the British Sub-Aqua Club has carried out a routine survey, as a contribution towards the Underwater Conservation Year Programme, on the fauna and flora of a sublittoral reef situated about 300 m to the south west of the Gear Reef in Penzance Bay. The reef is situated in water of about 17 m depth and is generally surrounded by firm sand with patches of Zostera . The reef itself is about 30 m long and 17 m wide at its widest point and 8 m high with vertical faces and overhangs at its northern end. The sides slope by stages to the sand at the southern end and contain many deep fissures with a large one at the northern end widening to a cave that passes through from east to west. The reef is aligned with its longitudinal axis running north-south and provides a diverse environment for a rich fauna and flora, a considerable background knowledge of which has been gained by the survey: by late August 1978 a total of over 30 dives, each of approximately one hour's duration, had been made in this area.