Dolichospermum lemmermannii (Nostocales) bloom in world’s deepest Lake Baikal (East Siberia): abundance, toxicity and factors influencing growth

Mass development of the cyanobacteria Dolichospermum (D. lemmermannii as the dominant species) was reported in the coastal zone of Bol’shye Koty Bay (western coast of the southern basin), towards the end of July to the beginning of August in 2019. Blooms were observed as 1–1.5 m wide bands adjoining the shoreline and stretched uninterrupted over 2 km. Abundance of cyanobacteria in blooms varied within 7.2–71.9 thousand cell mL-1, with 0.73–7.20 g m-3 biomass attained. Maximal concentration was observed opposite the biological station of Irkutsk State University, a frequently visited place that was hosting participants of several conferences at that time. Hydrochemical analysis of samples collected four days after continuous heavy rains showed much higher concentrations of nutrients in contrast with the data obtained a week before (24 July), which is normal for that season. Nutrient concentrations were elevated relative to long term averages by 3 to 30 fold: with phosphate concentrations up to 0.200 mg L-1, ammonium ions 0.29 mg L-1, and nitrates 0.31 mg L-1. Possible reasons for the harmful freshwater cyanobacterial bloom that is unusual for this part of the lake are discussed. Excessive proliferation of common cyanobacteria D. lemmermannii in the open lake areas was affected by several factors: long-lasting heavy rains, zero wind, high air (from 13–15°С at night to 29°С in daytime) and water temperatures (from 15°С to 19.2°С); and absence of isolated septic tanks in the nearshore zone.