A Hydrodictyon reticulatum bloom at Loe Pool, Cornwall

An extensive bloom of Hydrodictyon reticulatum was observed on Loe Pool, a eutrophic coastal freshwater lake in Cornwall. In previous years the Pool has experienced cyanobacterial blooms, but in 1993 Hydrodictyon became dominant for the first time. It was estimated that approximately 24% of the Pool was covered by Hydrodictyon and dissolved oxygen saturations were observed to range between 220% and 34% within a mat of Hydrodictyon. The reason for the shift in algal dominance remains unclear, though large numbers of zooplankton were observed sheltering around the stands of Hydrodictyon. These may have restricted the growth of phytoplankton once the Hydrodictyon had become established. It is concluded that further study is required to determine the causal mechanism which may be responsible for the explosion in Hydrodictyon. The possibility of the growth of a more vigorous strain cannot be discounted.