Bacterial growth

Bacterial production has been estimated from the rate of thymidine (TdR) incorporation into DNA (Findlay et al., 1991). For purposes of examining temporal and spatial patterns we use the actual rate of TdR incorporation while for carbon budgeting purposes we apply an empirically-derived conversion factor to arrive at a rate of carbon production (Findlay et al., 1991). As for bacterial abundance, thymidine incorporation is strongly seasonal (Fig. 8.3) with greater than two-fold variation from spring to summer and a strong positive

Figure 8.2. Spatial variability in planktonic bacterial abundance for six stations ranging from Castleton in the north (228 km north of the Battery) to the upper end of HaverstrawBay in the south (66 km north of the Battery). Station means are significantly different (p < 0.0001).

228 193 152 122 79 66 River KM

228 193 152 122 79 66 River KM

Figure 8.2. Spatial variability in planktonic bacterial abundance for six stations ranging from Castleton in the north (228 km north of the Battery) to the upper end of HaverstrawBay in the south (66 km north of the Battery). Station means are significantly different (p < 0.0001).

Figure 8.3. Seasonal mean growth rates (DPM of thymidine incorporation /L/h) for planktonic bacteria averaged over twelve years (1988 through 2000) and six stations ranging from Haverstraw Bay to Castleton. Seasonal means are significantly different (p < 0.001).

< June June-July Aug-Sept SEASON

< June June-July Aug-Sept SEASON

(p < 0.05; r = 0.51) correlation with water temperature. The spatial pattern in growth mirrors abundance with upriver stations showing rates roughly double the growth rates, relative to downriver stations (Fig. 8.4). Given these patterns, it is not surprising that there is a significant positive correlation (p < 0.05; r = 0.3) between abundance and growth.

In a number of estuarine systems, bacterial abundance, growth and metabolic activity have been shown to vary dramatically with particle abundances, particularly in and near the turbidity maximum (e.g., Hollibaugh and Wong, 1999; Crump and Baross, 1996). For the lower Hudson River, bacterial abundance and growth were positively associated with salinity with no obvious increase in the zone of the turbidity maximum (Sanudo-Wilhelmy and Taylor, 1999). In the tidal freshwater Hudson River there was no correlation between bacterial abundance or thymidine incorporation and total suspended matter (p > 0.05; r = -0.1 and -0.04, respectively). Potential limitation of bacterial growth by inorganic nutrients has been shown for a variety of aquatic ecosystems (e.g., Brett et al., 1999) although such limitation seems unlikely given the relatively high concentrations of dissolved inorganic nitrogen and phosphorus in the tidal freshwater Hudson (Lampman, Caraco, and Cole, 1999). Despite the apparent surplus of inorganic nutrients, Roland and Cole (1999) observed a significant stimulation of bacterial growth and respiration following the addition of nitrogen and/or phosphorus in bioassays. Moreover, assays of phosphatase in the mainstem Hudson show reasonably high values in spite of the apparently high availability of soluble reactive phosphorus (SRP) in the water column. These observations taken in concert suggest a more complex interaction of inorganic nutrients and bacterial dynamics than is usually suggested simply from ambient nutrient availability.

In absolute terms, bacterial secondary production is large relative to other components of

Figure 8.4. Spatial variability in bacterial growth rates for six stations ranging from Castleton to the upper end of Haverstraw Bayin the south. Stations differ significantly at p = 0.002.

3 4.5e6

3 4.5e6

228 193 152 122 79 66

River km

228 193 152 122 79 66

River km

secondary production within the tidal freshwater Hudson. The grand mean thymidine incorporation translates to a carbon production of 216 |igC/L/h using conversion factors detailed in Findlay et al. (1991). The rates of growth and abundance estimates yield bacterial turnover times ranging from as long as 3.4 days in autumn to about once per day during summer. Although marginally significant (p = 0.06), there is a pattern of shorter turnover times in the upriver stations with values above RKM 150 ranging from 1.3 to 1.8 days, while downriver sites were 2.3 to 2.6 days (data not shown). This pattern of shorter turnover is due to the more rapid decline of growth with distance downriver relative to the decline in abundance.

0 0

Post a comment