Bacterioplankton growth

Author: Johan Wikner, The Swedish Institute for the Marine Environment, Unit at Umeå University, SE-910 20, Hörnefors, Sweden

Key Message

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 bacterial growth has declined for at least the past 10 years, suggesting a reduced sedimentation of organic matter in the two basins and thus poorer nutrient status. A recovery in growth rates was observed at all stations during 2011.

Relevance of the indicator for describing developments of the environment

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).

Results

A statistically significant decline in bacterial growth was observed in the deep water (40-100 m) at both stations in the Bothnian Sea and Bay during the past 10 years (median -5,9 % yr^-1 , Table 1). The bacterioplankton growth showed a slower decline during 1994-2010 in both basins. Thus the decline in deep water bacterial growth seems to have accelerated after the millennium shift, although this difference was not statistically significant.

The bacterioplankton growth was on average 36 nmol C dm^-3 day^-1 in the Bothnian Bay, while being 12 % higher in the Bothnian Sea (ANOVA, total df=545, p=0.033 , Table 1). 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 383 and 267 µmol dm^-3, respectively, for the Bothnian Bay and Bothnian Sea. Assuming bacterioplankton to account for half of the total respiration (Robinson and Williams 2005), average daily oxygen consumption would correspond to 0,61 and 0,99 % day ^-1, respectively.

Assessment

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 therefore does  not support that these basins are disturbed by eutrophication. One may instead express concerns about the remarkably low rates currently reached, however, showing some recovery in 2011.

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 using the 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. A decline in oxygen concentration has been observed in the Bothnian Sea during the past 40 years, making 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.

Data description

Values

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.

Figure 1. Sampling stations in the Gulf of Bothnia.

Statistics

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.

 

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. 11.0^®).

 

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-distibution. As a guidance to assess significant changes, a difference of 25 % in 2 years is expected to be detected with 80 % probability.

 

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.

Figures

 

Figure 2. The bacterioplankton growth rate below the stratified layer show a significant decline in the Bothnian Bay 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 3. The bacterioplankton growth rate below the stratified layer declined in the Bothnian Sea since 1994. Lines defined as in figure 2.

Tables

Table 1. Bacterial community growth below the pycnocline level has declined in the whole Gulf of Bothnia since 1994. The 95 % confidence interval (C. I.) of the trend is shown. The significance (α) shows the risk to be wrong if stating that there is a trend, which should be below 0,05 for a statistical significant statement. The series mean of bacterial growth rate is shown.

Sea area	Station	Period	Trend	±95 C.I.	α	Years	Bacterial growth rate
			(% year-1)	(% year-1)	(-)		(nmol C dm-3 day-1)
Bothnian Bay	A5	2000-2011	-3,5	4,7	0,117	11	34
Bothnian Bay	A13	2000-2011	-2,2	3,3	0,201	11	36
Bothnian Sea	C1 and C3	2000-2011	-9,6	6,1	0,001	11	42
Bothnian Sea	C14	2000-2011	-5,2	3,0	0,000	11	40
Bothnian Bay	A13	1994-2011	-2,6	2,1	0,068	17	38
Bothnian Sea	C1 and C3	1994-2011	-2,1	1,8	0,035	17	44

References

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. L. 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 Sheets as follows:

[Author’s name(s)], [Year]. [Baltic Sea Environment Fact Sheets title]. HELCOM Baltic Sea Environment Fact Sheet(s) 2012. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.

Last updated: 26.10.2012