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Abstract
Abstract. Cyanobacteria, primarily of the species \\textit{Nodularia spumigena}, form extensive surface accumulations in the Baltic Sea in July and August, ranging from diffuse flakes to dense surface scums. The area of these accumulations can reach ~ 200 000 km2. We describe the compilation of a 35-year-long time series (1979–2013) of cyanobacteria surface accumulations in the Baltic Sea using multiple satellite sensors. This appears to be one of the longest satellite-based time series in biological oceanography. The satellite algorithm is based on remote sensing reflectance of the water in the red band, a measure of turbidity. Validation of the satellite algorithm using horizontal transects from a ship of opportunity showed the strongest relationship with phycocyanin fluorescence (an indicator of cyanobacteria), followed by turbidity and then by chlorophyll a fluorescence. The areal fraction with cyanobacteria accumulations (FCA) and the total accumulated area affected (TA) were used to characterize the intensity and extent of the accumulations. The fraction with cyanobacteria accumulations was calculated as the ratio of the number of detected accumulations to the number of cloud-free sea-surface views per pixel during the season (July–August). The total accumulated area affected was calculated by adding the area of pixels where accumulations were detected at least once during the season. The fraction with cyanobacteria accumulations and TA were correlated (R2 = 0.55) and both showed large interannual and decadal-scale variations. The average FCA was significantly higher for the second half of the time series (13.8%, 1997–2013) than for the first half (8.6%, 1979–1996). However, that does not seem to represent a long-term trend but decadal-scale oscillations. Cyanobacteria accumulations were common in the 1970s and early 1980s (FCA between 11–17%), but rare (FCA below 4%) during 1985–1990; they increased again starting in 1991 and particularly in 1999, reaching maxima in FCA (~ 25%) and TA (~ 210 000 km2) in 2005 and 2008. After 2008, FCA declined to more moderate levels (6–17%). The timing of the accumulations has become earlier in the season, at a mean rate of 0.6 days per year, resulting in approximately 20 days advancement during the study period. The interannual variations in FCA are positively correlated with the concentration of chlorophyll a during July–August sampled at the depth of ~ 5 m by a ship of opportunity, but interannual variations in FCA are more pronounced as the coefficient of variation is over 5 times higher.
Highlights
The lifetime of a typical satellite sensor is too short to collect quantitative interannual time series of sufficient length
3.1 Validation of satellite detection of cyanobacteria accumulations with horizontal transects measured on ships of opportunity
The ship transects on 12 July and on 20 July were both along the edge of the major area of accumulations in the Southern Baltic Proper, and that probably caused a large part of the variability in the average cyanobacteria score there
Summary
The lifetime of a typical satellite sensor is too short to collect quantitative interannual time series of sufficient length. In the era of anthropogenic climate change it is common and justified to look for trends in environmental variables, but the observed changes in relatively short time series are most often due to decadal or interannual variability rather than long-term trends. Surface or near-surface accumulations of cyanobacteria are common in the Baltic Sea during the summer months of July and August. They are caused by massive blooms of diazotrophic cyanobacteria, primarily Nodularia spumigena and Aphanizomenon sp. In order to build a time series suitable for investigating long-term trends in cyanobacterial surface accumulations in the Baltic Sea and the mechanisms causing them, we have used multiple satellite sensors to create the longest possible time series of comparable observations
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