BSEP116B Biodiversity in the Baltic Sea - Helcom

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Spring bloom index
1200
1000
800
600
400
200
0
The shift from diatoms to dinoflagellates may
have implications for the nutrient dynamics in
the summer and the input of organic matter to
the sediment, as diatoms usually sediment to the
seabed at the end of the bloom, whereas dinoflagellates
are mostly remineralized in the upper water
layers (Tamelander & Heiskanen 2004). However,
a general decrease in diatoms has not yet been
found in the Belt Sea, as confirmed by Wasmund et
al. (2008) for the Kiel Bight for the past 100 years.
Based on high-frequency monitoring data on
chlorophyll-a collected on merchant ships, the
spring bloom intensity has been monitored since
1992 in the Arkona Basin, the northern Baltic Proper
and the western Gulf of Finland (Fleming & Kaitala
2006). The index values of 0–1 060 from the period
2000–2006 are comparable to those in previous
years and do not indicate any clear trends, although
the average values have been slightly higher in the
2000s, particularly in the Gulf of Finland (Figure
3.1.2; Fleming & Kaitala 2006).
1992-1999
2000-2006
Arkona Basin
Northern Baltic
Proper
Western Gulf of
Finland
Figure 3.1.2. Phytoplankton spring bloom index in the open
western Gulf of Finland, northern Baltic Proper and Arkona Basin.
The bars represent average values with standard deviations for the
periods 1992–1999 and 2000–2006. Alg@Line data modified from
Fleming & Kaitala (2006).
Area covered (km 2 )
80000
70000
60000
50000
40000
30000
20000
10000
0
1997
1998
1999
2000
Bloom intensity
Area covered
2001
2002
2003
Figure 3.1.3. Area coverage and intensity of cyanobacterial blooms,
as integrated for the Baltic Sea for 1997–2007. Annual summary
values are based on the analysis of satellite image data. Modified
from Hansson (2007).
2004
2005
2006
2007
14000
12000
10000
8000
6000
4000
2000
0
Bloom intensity (km 2 days)
Cyanobacterial blooms
Cyanobacteria are a natural component of the
phytoplankton community in most parts of the
Baltic Sea area (HELCOM 1996b, Hajdu et al.
2008). They usually dominate in summer in the
coastal and open areas of most sub-basins of the
Baltic Sea, with the exception of the Belt Sea and
the Kattegat (e.g. Jaanus et al. 2007, Wasmund &
Siegel 2008).
The cyanobacterial biomass has been lower in
the 2000s than in the 1980s–1990s in the Gulf
of Riga, Eastern Gotland Basin and Arkona Basin
(Jaanus et al. 2007). In contrast, late-summer
biomass of cyanobacteria has been reported to
have increased in the open northern Baltic Sea
since the late 1970s (Suikkanen et al. 2007; see
also Kahru et al. 2007).
Cyanobacterial blooms in the Baltic Proper are
typically formed by the diazotrophic species Aphanizomenon
flos-aquae, Anabaena spp. and Nodularia
spumigena that can fix molecular nitrogen
(Laamanen & Kuosa 2005, Mazur-Marzec et al.
2006, Hajdu et al. 2007). N. spumigena blooms
are potentially toxic, whereas no toxic blooms of
A. flos-aquae have been recorded in the Baltic
Sea. The blooms of N 2
-fixing cyanobacteria as
such do not necessarily indicate strengthened
eutrophication (Gasiūnaitė et al. 2005, Toming &
Jaanus 2007).
Satellite images covering the Baltic Sea area show
that the frequency and magnitude of the accumulation
of cyanobacteria on the surface water have
varied during 1997–2007, but without a clear
trend (Figure 3.1.3, Hansson 2007). However, the
average frequency of cyanobacterial accumulations
was 39% higher in 1998–2006 than in
1979–1984, although the difference is not significant
(Kahru et al. 2007). It should be noted that
satellite images describe the surface accumulation
of N. spumigena relatively well, but mostly ignore
A. flos-aquae which generally locates deeper in
the water column (Kahru et al. 2007).
The surface blooms are typically short in duration,
i.e., from days to a few weeks (Hansson 2007), but
their influence may last longer through the effects
on near-bottom oxygen conditions and potential
food-web effects (Vahtera et al. 2007). A low
nitrogen-to-phosphorus ratio and calm and warm