Satellite detection of multi-decadal time series of cyanobacteria accumulations in the Baltic Sea

Abstract

Cyanobacteria, primarily of the species Nodularia spumigena, form extensive surface accumulations in the Baltic Sea in July and August, ranging from diffuse flakes to dense surface scum. We describe the compilation of a 35 year (1979-2013) long time series of cyanobacteria surface accumulations in the Baltic Sea using multiple satellite sensors. 5 This appears to be one of the longest satellite-based time series in biological oceanog-raphy. The satellite algorithm is based on increased 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 10 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. FCA 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). TA was calculated by adding the area of pix-15 els where accumulations were detected at least once during the season. FCA and TA were correlated (R 2 = 0.55) and both showed large interannual and decadal-scale variations. The average FCA was significantly higher for the 2nd 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 ac-20 cumulations were common in the 1970s and early 1980s (FCA between 11-17 %), but rare (FCA below 4 %) from 1985 to 1990; they increased again from 1991 and particularly from 1999, reaching maxima in FCA (∼ 25 %) and TA (∼ 210 000 km 2) 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 25 year, resulting in approximately 20 days advancement during the study period. The interannual variations in FCA are positively correlated with the concentration of chloro-3320