Abstract. Phytoplankton spring bloom phenology was derived from a 15-year time series (2000–2014) of ship-of-opportunity chlorophyll a fluorescence observations collected in the Baltic Sea through the Alg@line network. Decadal trends were analysed against inter-annual variability in bloom timing and intensity, and environmental drivers (nutrient concentration, temperature, radiation level, wind speed).

Spring blooms developed from the south to the north, with the first blooms peaking mid-March in the Bay of Mecklenburg and the latest bloom peaks occurring mid-April in the Gulf of Finland. Bloom duration was similar between sea areas (43 ± 2 day), except for shorter bloom duration in the Bay of Mecklenburg (36 ± 11 day). Variability in bloom timing increased towards the south. Bloom peak chlorophyll a concentrations were highest (and most variable) in the Gulf of Finland (20.2 ± 5.7 mg m⁻³) and the Bay of Mecklenburg (12.3 ± 5.2 mg m⁻³).

Bloom peak chlorophyll a concentration showed a negative trend of −0.31 ± 0.10 mg m⁻³ yr⁻¹. Trend-agnostic distribution-based (Weibull-type) bloom metrics showed a positive trend in bloom duration of 1.04 ± 0.20 day yr⁻¹, which was not found with any of the threshold-based metrics. The Weibull bloom metric results were considered representative in the presence of bloom intensity trends.

Bloom intensity was mainly determined by winter nutrient concentration, while bloom timing and duration co-varied with meteorological conditions. Longer blooms corresponded to higher water temperature, more intense solar radiation, and lower wind speed. It is concluded that nutrient reduction efforts led to decreasing bloom intensity, while changes in Baltic Sea environmental conditions associated with global change corresponded to a lengthening spring bloom period.