Seasonal Phytoplankton Blooms in Coastal Waters and Their Impact on Sustainable Water Quality Management and Bioenergy Recovery: A Case Study from the Bay of Gdansk, Poland

Seasonal phytoplankton blooms in coastal waters are an increasing environmental challenge affecting water quality, ecosystem stability, and the sustainable management of marine resources. At the same time, excessive algal biomass may constitute a valuable substrate for renewable energy production within circular economy frameworks. The aim of this study was to evaluate the seasonal variability of phytoplankton bloom biomass in the Bay of Gdansk (southern Baltic Sea, Poland) and assess its suitability for bioenergy recovery through anaerobic digestion. Biomass samples were collected between May and October during 2023–2024 and subjected to quantitative, taxonomic, and chemical analyses as well as methane fermentation under mesophilic batch conditions. Cumulative biogas and methane production curves were fitted using the modified Gompertz model. The results demonstrated pronounced seasonal changes in phytoplankton structure, with green algae and dinoflagellates prevailing in spring, cyanobacteria dominating during summer blooms, and diatoms occurring mainly in autumn. These variations strongly affected biomass composition and anaerobic digestion performance. The highest methane yield, reaching 270 ± 13 mL CH4/g VS, and the highest methane production rate of 32.5 ± 1.6 mL CH4/g VS·d were recorded in August during intensive cyanobacterial blooms. This period was characterised by elevated contents of organic carbon, sugars, and lipids in the biomass. In contrast, the lowest biomethane potential was observed in autumn biomass dominated by diatoms. The findings indicate that seasonal phytoplankton blooms should be considered not only as a symptom of eutrophication but also as a recoverable bioresource. Controlled harvesting and anaerobic utilisation of bloom biomass may support sustainable coastal water management, eutrophication mitigation, and renewable bioenergy recovery in coastal engineering systems.