Multidecadal time series of satellite-detected accumulations of cyanobacteria in the Baltic Sea

[摘要] Cyanobacteria, primarily of the species \textit{Nodularia spumigena}, form extensive surface accumulations in the Baltic Sea inJuly and August, ranging from diffuse flakes to dense surface scums. The areaof these accumulations can reach ~ 200 000 km². We describe thecompilation of a 35-year-long time series (1979–2013) of cyanobacteriasurface accumulations in the Baltic Sea using multiple satellite sensors.This appears to be one of the longest satellite-based time series inbiological oceanography. The satellite algorithm is based on remote sensingreflectance of the water in the red band, a measure of turbidity. Validationof the satellite algorithm using horizontal transects from a ship ofopportunity showed the strongest relationship with phycocyanin fluorescence(an indicator of cyanobacteria), followed by turbidity and then bychlorophyll a fluorescence. The areal fraction with cyanobacteriaaccumulations (FCA) and the total accumulated area affected (TA) were used tocharacterize the intensity and extent of the accumulations. The fraction with cyanobacteriaaccumulations wascalculated as the ratio of the number of detected accumulations to the numberof 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 weredetected at least once during the season. The fraction with cyanobacteriaaccumulations and TA were correlated (R² = 0.55) and both showed large interannual and decadal-scalevariations. The average FCA was significantly higher for the second half of thetime 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 butdecadal-scale oscillations. Cyanobacteria accumulations were common in the1970s and early 1980s (FCA between 11–17%), but rare (FCA below4%) during 1985–1990; they increased again starting in 1991 and particularlyin 1999, reaching maxima in FCA (~ 25%) and TA(~ 210 000 km²) in 2005 and 2008. After 2008, FCA declined tomore moderate levels (6–17%). The timing of the accumulations hasbecome earlier in the season, at a mean rate of 0.6 days per year, resultingin approximately 20 days advancement during the study period. The interannualvariations in FCA are positively correlated with the concentration ofchlorophyll a during July–August sampled at the depth of ~ 5 m by a ship of opportunity, but interannual variations in FCA are more pronouncedas the coefficient of variation is over 5 times higher.

[学科分类] 地球化学与岩石