Background.

Harmful algal blooms have increased in frequency, intensity and distribution in the last decades around the world. This increase has also been observed for the waters of the Mediterranean Sea. In November 2023, several reddish patches were observed in İzmir Bay (Aegean Sea); the dinoflagellate Polykrikos hartmannii was the causative agent.

Goal.

The purpose of this study was to report for the first time a bloom of this species in İzmir Bay.

Methods.

During the bloom, surface samples of seawater were collected using 1 liter plastic bottles at eleven sampling stations and fixed with Lugol for cell counting, which were performed under an OLYMPUS BX-50 microscope, using 1 ml Sedgewick-Rafter chambers. Samples were collected for live analysis and correct identification of the responsible species.

Results.

The moderate bloom occurred in November 2023 was caused by Polykrikos hartmannii. During the bloom two-celled chains were the predominant species, with single cells occurring less frequently. The highest cell abundance (4.8 x 104 cells L-1) was observed at station 29 in the inner bay. Cell sizes were 29.7 - 34.6 μm long, and 39.6 - 44.5 μm wide.

Conclusions.

Living samples allowed correct identification of P. hartmannii. Cells tend to become round or disrupted when preserved with Lugol’s solution. This may be why this species has not been previously reported. The maximum abundance of P. hartmannii was recorded in İzmir Bay at a water temperature of 14.84 °C and a salinity of 38.02 ‰, coinciding with highest values of nutrients. The bloom occurred in the shallowest area with limited water circulation and significant riverine inflow.