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| Titel |
Multidecadal time series of satellite-detected accumulations of cyanobacteria in the Baltic Sea |
| VerfasserIn |
M. Kahru, R. Elmgren |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 11, no. 13 ; Nr. 11, no. 13 (2014-07-04), S.3619-3633 |
| Datensatznummer |
250117506
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| Publikation (Nr.) |
 copernicus.org/bg-11-3619-2014.pdf |
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| Zusammenfassung |
| 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. |
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