Hypoxia increase and the correlation to Cyanobacteria Blooms in the Western Gotland Basin from 1998 to 2024

Sammanfattning

Cyanobacterial blooms and bottom-water hypoxia have long been studied and connected to the eutrophication in the Baltic Sea. This thesis investigates the dynamic of the spatiotemporal correlation between cyanobacterial summer bloom and dissolved oxygen at the sea floor in the Western Gotland Basin, as per defined by HELCOM 2022. The thesis is conducted using weekly and monthly data from 1998 to 2024, provided by Copernicus Marine Service, on known geophysical and geochemical drivers of Harmful Algal Blooms, HAB’s. Basin wide Spearman correlations, Mann-Kendall trend analysis, cross-correlation and partial correlation methods were applied to aggregated monthly data of June, July and August. The basin was divided by the approximate halocline depth of 80 m, comparing correlations for areas with different vertical mixing prerequisites. No statistically significant correlation for pixel-wise and basin mean correlations between sea floor oxygen minimum and the cyanobacteria bloom frequency was found. However, a lagged cross-correlation between the same variables of -12 and + 12 months, not statistically significant, still indicated a negative relationship, with different Spearman ρ peaks for waters deeper than 80 m and above it. Dissolved oxygen at the sea floor showed a statistically significant decrease over time, with a p value of p = 8.15e-06 and Sen slope of −21.7 mmol m⁻³/decade while SST, salinity and chlorophyll a exhibited consistently increasing values over the 27 years.