The geographical and seasonal occurrences of dinoflagellate cysts at a depth of 10 m in the North Atlantic and North Sea are described from samples taken by the Continuous Plankton Recorder between June 1973 and Dec 1975. A special study was made of the large cyst of Polykrikos schwartzii Butschli in the autumn of 1973 when CPR samples were supplemented by vertical sampling in the upper 500 m with a Longhurst-Hardy Plankton Recorder at Ocean Weather Station, INDIA. Only 10 isolated records of cysts were made in the western Atlantic. All others occurred to the east of 30 degree W in both oceanic and neritic waters and especially in the shallow waters of the North Sea. Cysts were found in the spring plankton but were most abundant in the autumn. They have been recorded down to 500 m but occur most frequently in the top 30 m of the water column. Large aggregations of cysts in bottom sediments probably reflect concentration over many years since they cannot be explained by the low numbers found in the plankton. Generally, the distribution of cysts in the plankton is reflected in bottom sediments but concentrations in sediments may be outside the centres of production, implying movement by currents or sediment transport.
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