1. Extension of the vegetation period of phytoplankton
Introduction
Extension of the vegetation period of phytoplankton
in the western Baltic Sea – response to climate change
by Norbert Wasmund, Günther Nausch and Monika Gerth
Leibniz Institute for Baltic Sea Research Warnemünde (IOW), Germany
What is available?
Data basis: Weekly sampling
1 June 1988 – end of 1998: pier in Warnemünde
1999 to present: pier in Heiligendamm
No phytoplankton samples Sept-Dec 1994 and in 1997
Variables:
Temperature+salinity
Nutrients (N, P, Si)
Phytoplankton
Chlorophyll a (Chla)
Additional weather data from
central Bay of Mecklenburg ( ):
Sea surface temperature (SST)
Sun shine duration
from Deutscher Wetterdienst (DWD)
Challenge:
Extract scientific news from
the routine coastal monitoring

2. Extension of the vegetation period of phytoplankton
Original data
Can something more be extracted from the data?... Some scientific highlights?
→ compare data from the beginning and from the end of the data series
Extension of the
vegetation period
Vegetation period
Same results with
two independent
methods *
Standard methods
not described here
For calculations:
Looking for the
best fitting
thresholds:
● Biomass:
750 mg m-3
● Chla:
2.3 mg m-3
Vegetation period

3. Extension of the vegetation period of phytoplankton
Results
Day of year when a threshold value is first reached
(DF=”day first”)
Day of year when a threshold value is last reached
(DL=”day last”)
Duration between the first and last reaching
of a threshold value (DD=”day duration”)
All linear trends are highly significant (α=0.1 %).
Representative, conservative estimates based on the trend lines:
the spring bloom starts earlier with a rate of 1.4 days/year,
the end of the autum bloom delays with 3.1 days/year
the vegetation period extends by 4.5 days/year
in the period from 1989 to 2017.
That means: Increase in vegetation period from 160 days (in 1989)
to 288 days (in 2017), i.e. 1.8-fold.

4. Extension of the vegetation period of phytoplankton
Results
Summer
Gap
Vegetation period
Spring
bloom
Autumn
bloom
Vegetation period
Gap

5. Extension of the vegetation period of phytoplankton
Results
Reasons ? → Check correlations between duration of vegetation period (DD-C) and abiotic variables
Season
Salinity
Temp.
Sunshine duration
Nitrate
Silicate
Phosphate
Biomass
Chla
DD-SST10
Spring
0.076
0.016
0.298**
0.217*
0.097
0.432**
0.068
0.272**
0.268**
Autumn
0.220*
0.349**
0.002
0.160*
0.142
0.411**
0.208*
0.431**
Coefficient of determination (r²) for correlations of seasonal means of different variables in spring and autumn
with the duration of the chla-based vegetation period (DD-C) at significance levels (α) of ** α = 0.01 and * α = 0.05.
DD-SST10 = Number of days per year exceeding a threshold of 10 °C.

6. Extension of the vegetation period of phytoplankton
Outlook
Global view
Terrestrial organisms (trees, birds, butterflies etc.):
earlier spring timing of 2.3 days/decade
(Parmesan & Yohe 2003)
Phytoplankton (Baltic Sea)
vegetation period extends by 45 days/decade
Kahru et al. (2016): 73 days/decade
Open questions:
What causes the extension of the vegetation period: Temperature ? Directly or via a mediator? Which? Stratification, grazing?
Is this a Baltic-wide phenomenon? Acc. to Kahru et al. (2016): yes. Global ?
Is the extension of the growing season occurring also in zooplankton, fish etc.?
Further research: Problem: process finished in our area, can only be retraced by historical data
Probably the process is going on in the central/northern Baltic → research possible.
This contents will be published as: Wasmund, N., Nausch, G., Gerth, M., Busch, S., Burmeister, C.,
Hansen, R., Sadkowiak, B.: Extension of the vegetation period of phytoplankton in the western
Baltic Sea – response to climate change. Marine Ecology Progress Series (in press)
Please refer to DOI:

7. Extension of the vegetation period of phytoplankton
Appendix
Additional slides

8. Extension of the vegetation period of phytoplankton
Appendix
Trends in biomass and chla:
Seasons separated → consistent increase in autumn, partly in spring
Summer: biomass decrease
Additional slides

9. Extension of the vegetation period of phytoplankton
Appendix
Reasons ? → Look for trends in abiotic factors
Climate change = Temperature increase
Eutrophication?
Increase in solar radiation
Increase in CO2 etc. No
data
Unlikely
Trends in the seasonal averages of
nitrate (red) and phosphate (green)
Cumulative sum of sunshine duration in spring and autumn in the Bay of Mecklenburg
Trends in the seasonal averages of
water temperature and salinity
Season
Salinity
Temp.
Sunshine duration
Nitrate
Silicate
Phosphate
Biomass
Chla
DL-SST10
Winter
+**
Spring +*
-***
-**
Summer -*
Autumn
+****
+***
Trend in the start (DF-SST10) and
end (DL-SST10) of the period with
SST>10°C
Significance of trends for different variables at different significance levels (α):
**** α=0.001, ***α=0.01, ** α=0.05, * α= means increase, - means decrease.

10. Extension of the vegetation period of phytoplankton
Phytoplankton data
Biomass trends in the main phytoplankton groups and species:
Skeletonema marinoi
+ Rhizosolenia setigera
Ceratium spp.
Dactyliosolen
fragilissimus
Cerataulina pelagica Actinocyclus spp.
Pseudosolenia
calcar-avis

11. Extension of the vegetation period of phytoplankton
Phytoplankton data
Species occurring both in
spring and autumn
New species occurring in
autumn and winter
Shift from spring to winter
Shift from summer to autumn

12. Extension of the vegetation period of phytoplankton
Discussion
Explanations ? Which mechanisms ?
Light: In spring: More light enables earlier blooms
In autumn: Light limitation? Mixotrophy (e.g. Ceratium)
Nutrients: Not expected as cause, probably even affected by phytoplankton
We could not separate pre-bloom and post-bloom nutrient data
Temperature: In spring: Earlier stratification (but not at coast, but coupling with open sea)
In autumn: Extended stratification, zooplankton ?
Zooplankton: grazing effect unknown (no zooplankton data, zooplankton not considered)
In spring: mismatch phyto – zooplankton? → reduced grazing pressure?
In summer: higher temperature stimulates zooplankton → phytoplankton gap
In autumn: extended grazing pressure? Autumn bloom shifts to Nov./Dec.
Large species (e.g. Ceratium) not fed?

13. Extension of the vegetation period of phytoplankton
Phytoplankton data
Phytoplankton biomass (wet weight) data from the coastal station for every investigation year. The threshold value is the black line.

14. Extension of the vegetation period of phytoplankton
Phytoplankton data
Chla data from the coastal station for every investigation year. The threshold value is the black line.

15. Extension of the vegetation period of phytoplankton
Complimentary data
Trends in the seasonal averages of water temperature and salinity at the investigation station.

16. Extension of the vegetation period of phytoplankton
Complimentary data
Trends in the seasonal averages of inorganic nutrients at the investigation station.