HELCOM Baltic Sea Environment Fact Sheet 2015, Published: 14 December 2015
Author: Siv Huseby1 and Johan Wikner1,2
1Umeå Marine Science Center, Umeå University, SE-90571 Hörnefors, Sweden2The Swedish Institute for the Marine Environment, Unit at Umeå University, SE-905 71, Hörnefors, Sweden
Bacterioplankton growth rate is an indicator of the decomposition of
organic matter and thereby trophic status of the Sea. The pelagic bacterioplankton
metabolism accounts for about half of the mineralization of organic matter and
thereby pelagic oxygen consumption.
The bacterioplankton growth rate indicates good trophic status in the
off-shore Bothnian Bay and Bothnian Sea. Deep water growth rates were only 50 %
higher than at corresponding depths in the Atlantic Ocean, lacking excess
enrichment. The decline that has earlier been reported has now ceased. During
the last few years the growth rates has increased some and there is now no
significant trends either increasing nor decreasing in the Bothnian Bay or
Bacterioplankton growth rate is an indicator of the nutrient status in aquatic environments. It is an estimate of the consumption of organic carbon in the ecosystem and therefore closely related to the biochemical oxygen demand in situ. Bacterioplankton growth rate thereby indicates the rate of oxygen consumption that may lead to oxygen deficiency in the water column, when exceeding oxygen supply. The bacterial growth rate indicator may be used in all aquatic environments.
Bacterioplankton growth rate is a relatively unambiguous indicator of the flux of organic matter in the pelagic ecosystem (Billen, et al. 1990). Even if the relationship between the factors specific growth rate and abundance may vary, their product representing biomass production reflects the substrate requirement of the bacterial community. Density limitation (i.e. competition) or temperature does therefore not directly appear to control the bacterial community biomass production. This agrees with empirical observations that bacterial growth rate over larger scales correlate with trophic status of a system (Cole, et al. 1988, Billen, et al. 1990) , and at smaller scales between water layers and seasons (Wikner and Hagström 1999).
The bacteria growth in the Botnian Bay and Bothnian Sea deep water
(40-100m) show no significant trends for the years 1994-2014 (Table 1).
The bacterioplankton growth was on average 38 nmol C dm-3 day-1
in the Bothnian Bay, while being 12 % higher in the Bothnian Sea (ANOVA, total df=654,
p=0.040). The higher value in the Bothnian Sea was in accordance with the
expected higher trophic status of this basin. Compared to the difference in
phytoplankton production between the basins this difference was however low.
The ability of bacterial community growth to measure changes in nutrient
supply was shown by the seasonally recurrent variation in the deep water by a
factor of 3 (data not shown). Highest values occurred during the summer months
when also the highest sedimentation is expected.
The bacterioplankton growth in the Bothnian Sea corresponded to a bacterial
oxygen consumption of 1.3 µmol O2 dm-3 day-1,
assuming a bacterial growth to growth efficiency function according to del
Giorgio and Cole (del Giorgio and Cole 1998). The
oxygen consumption could be compared to the average oxygen concentrations of 370
and 235 µmol dm-3, respectively, for the Bothnian Bay and Bothnian
Sea (mean values in deepwater 2011-2014). Assuming bacterioplankton to account
for half of the total respiration (Robinson and le B Williams 2005), average
daily oxygen consumption would correspond to 0,67 and 1,14% day -1,
The bacterioplankton growth rate in the deep water of the Bothnian Bay
and Bothnian Sea was at a level assessed as representing good conditions. This
indicates that the export of organic matter by sedimentation to the deep water occur
within allowable limits. This quality factor does therefore not support that
these basins are disturbed by eutrophication.
The status classification is based on that the level of bacterioplankton
growth found in the Bothnian Bay and Sea are comparable to rates at similar
depth of oceanic water (Dufour and Torreton 1996). Another
way to compare this is by absolute rates of total oxygen consumption calculated
from the bacterial growth (assuming 50% due to bacterial growth). The level in
the Gulf of Bothnia was 50 % higher than
the mean oxygen consumption measured at the same depth interval in the ocean (Robinson and le B Williams 2005). Higher productivity, and thereby oxygen
consumption, is expected in coastal waters compared to oceanic, so it´s not
obvious that the higher rates in the Gulf of Bothnia are above allowable limits.
The lack of significant decrease of oxygen levels in the Bothnian Bay
since the beginning of the 1970´s, corroborate that oxygen supply to the deep
water has been and remained sufficient to account for total respiration. This
supports that the deep water bacterial growth is within allowable limits. Decline
in oxygen concentration that has been observed in the Bothnian Sea during the
past 40 years, however, makes the classification more difficult for this basin.
It seems unlikely that the only 12% higher growth rate in the Bothnian Sea than
Bay alone could explain the observed oxygen decline. Also a negative rather
than positive correlation between bacterial growth and oxygen decline is
expected (i.e. both currently decline). Other factors like import of oxygen
deficient water from the Baltic Proper or increased stratification may rather
explain the development of oxygen concentrations in the Bothnian Sea.
A management consequence of the low rates observed is that it will be extraordinarily
difficult to reduce them further by nutrient reductions. This would also require
a questionable aim to achieve an oxygen consumption level similar to those
found in the Atlantic ocean at corresponding depth.
The bacterioplankton growth estimates are based on uptake of the
nucleotide thymidine into the DNA (i.e. chromosome) of the bacteria. The
original method is published in international scientific journals and has been
used in many marine research studies since the beginning of the 1980´s. The
method has been part of the Helsinki commission guidelines for a longer period
of time (Baltic Sea Environment Proceedings No. 27D) and is recently re-introduced
into the COMBINE manual part C (Annex C-12).
Values were averaged for the depths sampled within the water layer
chosen (median n=4). Typically 8-20 samplings distributed over the year has
been performed. Data is taken from the Swedish national marine monitoring
supported by the Swedish Agency for Marine and Water Management and the Swedish
Environmental Protection Agency.
Figure 3. Sampling stations in the Gulf of
Missing values were replaced by linear interpolation and averaged to
yield monthly time series, as required for the seasonal decomposition. About
20% of the values in the analyzed time series were interpolated. Time series
were seasonally decomposed by a multiplicative model and endpoints weighted by
0.5 in the software SPSS® v.20.
Time series were typically serially autocorrelated by one lag and
therefore analyzed for trends by first order autoregression (SPSS®).
It uses a least-square technique where errors follow a first order
autoregression. A non-parametric test for analysis for trends were also performed
using Multitest, an Excel-based program for Mann Kendall tests for monotone
trends. Results are not shown, but did not give any significant time trends.
To show the general trends in the time series a negative exponential
smoother was applied. This is a local smoothing technique using polynomial
regression and weights computed from the Gaussian density function (SigmaPlot v.
The power of the time series was estimated from the standard deviation
of the autoregression (Root Mean Square Error), the non-centrality parameter
and the non-central t-distribution. As a guidance to assess significant
changes, a difference of 25 % in 2 years is expected to be detected with 80 %
The difference between the Bothnian Bay and Bothnian Sea was determined
from marginal means in an ANOVA on natural logarithm transformed bacterial
growth, where area and year were fixed factors, and month a co-variate.
Figure 1. The
bacterioplankton growth rate below the stratified layer in the Bothnian Bay show
no significant trend since 1994. A negative exponential smoother is shown
together with seasonally adjusted data. The 3 x SD lines represent the upper
and lower 99% confidence limits of the smoothed time series, suggested as
action limits for the quality factor. Year ticks are located at the 1 st of
January each year.
Figure 2. The
bacterioplankton growth rate below the stratified layer in the Bothnian Sea show
no significant trend since 1994. Lines defined as in figure 1.
Billen, G., P. Servais and S. Becquevort. 1990.
Dynamics of bacterioplankton in oligotrophic and eutrophic aquatic
environments: bottom-up or top-down control ? 207:37-42
Cole, J. J., S. Findlay and M. L. Pace. 1988. Bacterial
production in fresh and saltwater ecosystems: a cross systems overview. 43:1-10
del Giorgio, P. A. and J. J. Cole. 1998. Bacterial growth
efficiency in natural aquatic systems. 29:503-541
Dufour, P. H. and J. P. Torreton. 1996. Bottom-up and
top-down control of bacterioplankton from eutrophic to oligotrophic sites in
the tropical northeastern Atlantic Ocean. 43:1305-1320
Robinson, C. and P. J. le B Williams. 2005. Respiration
and it´s measurement in surface marine waters. 1:147-180
Wikner, J. and Å. Hagström. 1999. Bacterioplankton
intra-annual variability at various allochthonous loading: Importance of
hydrography and competition. 20:245-260
For reference purposes, please cite this Baltic Sea environment fact sheet as follows:
[Author's name(s)], [Year]. [Baltic Sea environment fact sheet title]. HELCOM Baltic Sea Environment Fact Sheets. Online. [Date Viewed], http://www.helcom.fi/baltic-sea-trends/environment-fact-sheets/.