AbstractThe European flat oyster,Ostrea edulisL., has been successfully cultured in Cork Harbour on the south coast of Ireland, using non‐tidal, man‐made ponds for over 20 years. Broodstock placed in ponds spawn and the released larvae grow and settle on mussel shell cultch. This study investigates the nutrient dynamics and plankton cycles that occur from the time the ponds are filled. The incoming water to the ponds, when filled in May‐June, is usually nutrient rich, with high numbers of diatoms. The nutrients become depleted quickly and the phytoplankton becomes dominated by smaller algal species, particularly flagellates and by dinoflagellates. Following mussel shell addition, orthophosphate levels increase and this pulse in nutrients is generally followed by an increase in microplankton, particularlyChlorellaspecies. This increase often reaches bloom proportions and, if so, ponds are flushed. This addition of water increases the nutrient level and changes the phytoplankton composition, usually by introducing new dinoflagellate and diatom sp