Cell densities

Based on fifteen years of spatially extensive sampling, the grand mean bacterial abundance over the river reach examined is 7.6 x 109 cells/L and the distribution of observations shows quartiles of 4.6 and 9.5 x 109 cells/L. These abundances are in the upper portion of the range reported for estuaries in Ducklow and Shiah (1993). Not surprisingly, there is a strong seasonality to bacterial abundance (Fig. 8.1), the mean abundance in summer months (June through September) is about 50% higher than in spring (before June) or autumn (after September). There also is a significant positive correlation between bacterial abundance and

Figure 8.1. Seasonal mean abundance of planktonic bacteria averaged over twelve years (1988 through 2000) and six stations ranging from Haverstraw Bay to Castle-ton. Seasonal means differ significantly p < 0.001).

< June June-July Aug-Sept >Sept SEASON

< June June-July Aug-Sept >Sept SEASON

Figure 8.1. Seasonal mean abundance of planktonic bacteria averaged over twelve years (1988 through 2000) and six stations ranging from Haverstraw Bay to Castle-ton. Seasonal means differ significantly p < 0.001).

temperature (p < 0.05; r = 0.15) although it must be recognized that many other factors including river discharge and primary production co-vary with temperature. Superimposed on the expected seasonal changes in bacterial abundance have been interannual increases in abundance following the zebra mussel invasion (Findlay, Pace, and Fischer, 1998a) and these are discussed below.

Spatial variability shows a very consistent pattern with abundances about 25 percent higher in stations upriver of RKM 150 (Fig. 8.2). The range in spatial variability is only somewhat smaller than the range in seasonal variability, suggesting that whatever process, factor, or combinationisrespon-sible is equal with temperature in affecting bacterial numbers. The spatial pattern, although present in all seasons, varies over time with much smaller upriver-downriver contrasts (range = 23 percent of downriver mean) in spring (before June) than in mid-summer (range = 33 percent of downriver mean for Aug-Sept), implying the pattern may be related to residence time rather than simply loading and removal.

The reasonably high bacterial abundance multiplied by our estimate of cell carbon content yields values for bacterial biomass of 70 |g C/L, which is substantially larger than planktonic metazoan biomass (Findlay, Pace, and Fischer, 1996). The relative biomass of bacteria and phytoplankton has changed dramatically since the zebra mussel invasion (Strayer et al., 1999) with pre-zebra mussel ratios (bacterial/algal biomass) of less than 0.5 while post-zebra mussel values are typically 2.0 or higher (Findlay et al., 1998a).

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