Bacterioplankton growth rate

	Author: Johan Wikner, Umeå Marine Sciences Centre, Umeå University, SE-910 20, Hörnefors, Sweden

Key Message

Bacterioplankton growth rate is a powerful indicator of the decomposition of organic matter and thereby trophic status of the Sea. The decomposition is closely connected to the oxygen consumption in the water column.

The bacterioplankton growth rate represented at least good condition 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. The bacterial growth has been stable for the past 13 years, suggesting a balanced nutrient supply to and organic production in the two basins. A tendency of declining bacterioplankton growth, and thereby trophic status, has occurred for the past 7 years.

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 (cf. BOD₇). Bacterioplankton growth rate thereby indicate 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 ecosystem¹. 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 other limiting factors (i.e. inorganic nutrients, temperature) do not therefore directly appear to control the bacterial community biomass production at typical environmental conditions. This agrees with empirical observations that bacterial growth rate over larger scales correlate with trophic status of a system ^{1, 2} , and at smaller scales between water layers and seasons ³.

Results

The bacterioplankton growth in the deep water (40-100 m) was stable during 1994-2006 in both the off-shore areas of the Bothnian Bay and Bothnian Sea. No significant linear trends could be demonstrated for the whole period (Table 1).

A statistically significant decline in bacterioplankton growth could, however, be demonstrated in the northern Bothnian Bay (2000-2006), with a similar tendency at all stations albeit not statistically secure. Significance levels for negative trends at stns. A13 and C1/3 during the same time period were 0,13 and 0,18 , respectively.

Periods of higher and lower growth in the deep water, however, occurred at all stations (c.f. Statistics). The most marked peak occurred at station BMP-A13 during 1997 and BMP-A5 2002.

The bacterioplankton growth was on average 28,5 nmol C dm^-3 day^-1 in the Bothnian Bay, while being 26% higher in the Bothnian Sea (p<0.01 , Table 1). The higher value in the Bothnian Sea was in good accordance with the expected higher trophic status of this basin.

The bacterioplankton growth in the Bothnian Sea corresponded to a bacterial oxygen consumption of 1.14 µmol O₂ dm^-3 day^-1, assuming a bacterial growth to growth efficiency function according to del Giorgio and Cole ⁴. 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 respiration⁵, average daily oxygen consumption would correspond to 0,51 and 0,85 % day ^-1, respectively.

Figure 1: The bacterioplankton growth rate below the stratified layer has been stable in the Bothnian Bay since 1994. From year 2000 a decrease has occurred at stn. A5. 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 has been stable in the Bothnian Sea. Lines defined as in figure 1.

Assessment

The bacterioplankton growth rate in the deep water of the Bothnian Bay and Bothnian Sea was at a level assessed as representing at least good conditions. This quality factor does therefore not support that these basins are disturbed by eutrophication.

The conclusion was based on that the corresponding oxygen consumption was low enough on a daily basis to allow sufficient replenishment of oxygen by advection of surface water. The lack of significant decrease of oxygen levels in both basins 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.

Furthermore, the absolute rates of total oxygen consumption calculated from the bacterial growth (assuming 50% due to bacterial growth) was only 50 % higher than the mean oxygen consumption, measured at the same depth interval in the ocean⁵. As elevated nutrient levels are unlikely to occur in the open ocean this further support that the current level of oxygen consumption in the Bothnian Bay and Sea are at the status “good”  or better.

The lack of significant long term and monotonic trends further suggest that the trophic conditions in these basins have been stable for the past decade. Any monotonic trends occurring must be less than 3,5 % year^-1 based on the power of the time series (Table 1).

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 (n=4). Typically 8-20 samplings distributed over the year has been performed.

Figure 3. 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 analysed time series were interpolated. Time series were seasonally decomposed in the software SPSS^®.

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 8.0^®).

The power of the time serie was estimated from the standard deviation of the autoregression, 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 tested by an ANOVA where area and year were fix factors, and month a co-variate.

Tables

Table 1. A statistically significant decreasing linear trend below the stratified layer could be shown at stn. A5 in the Bothnian Bay. This analysis was based on the whole time serie period. The confidence interval (±95% C.I.), also shows how large the trend at least would have to be to achieve statistical significance. The significance (α) shows the risk to be wrong if stating that there is a trend, which should be below 5% for a statistical significant statement. The series mean of bacterial growth rate is shown.

References

1.         Billen, G., P. Servais, and S. Becquevort, Dynamics of bacterioplankton in oligotrophic and eutrophic aquatic environments: bottom-up or top-down control ? Hydrobiologia, 1990. 207: p. 37-42.

2.         Cole, J.J., S. Findlay, and M.L. Pace, Bacterial production in fresh and saltwater ecosystems: a cross systems overview. Mar. Ecol. Prog. Ser., 1988. 43: p. 1-10.

3.         Wikner, J. and Å. Hagström, Bacterioplankton intra-annual variability at various allochthonous loading: Importance of hydrography and competition. Aquat. Microb. Ecol., 1999. 20: p. 245-260.

4.         del Giorgio, P.A. and J.J. Cole, Bacterial growth efficiency in natural aquatic systems. Annu. Rev. Ecol. Syst., 1998. 29: p. 503-541.

5.         Robinson, C. and P.L. Williams, Respiration and it´s measurement in surface marine waters, in Respiration in aquatic ecosystems, P.A. del Giorgio and P.L. Williams, Editors. 2005, Oxford University Press: Oxford. p. 147-180.

For reference purposes, please cite this indicator fact sheet as follows:

[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2007. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.

Last updated: 7.12.2007