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significant not onlyfor understanding zooplankton dynamics but also because the bloom of Bosmina overlaps with critical periods in the development of larval fishes in the Hudson (e.g., Limburg, Pace, and Arend, 1999). Assuming an average of 100 animals l-1 between river km 80 and 160 and a volume of 900 billion liters for this section of the river, the abundance of Bosmina was 90 trillion animals. An individual Bosmina weighs about 0.7 |g dry wt or 0.3 |g C. This translates to a total biomass of 27,000 kg C or 27 metric tons (mt). In 1992, as in most years, this biomass developed and disappeared over a two-week period. Specifically, densities were greater than 10 l-1 between May 26 and June 10, 1992, at the Kingston station (Fig. 16.3). Assuming a production-to-biomass ratio of 1 over this two-week time interval, secondary production was about 2 mt per day. While these calculations are approximate, they indicate that there is large productivity available for exploitation by fish and invertebrate predators. The calculations also indicate how spatial and temporal data are needed to assess riverwide abundance. Sampling conducted only at a station above, below, or within the bloom

Figure 16.2. Seasonal dynamics of rotifers, copepods, and cladocerans for the years 1991 and 1999 at a freshwater station near Kingston, New York (river km 152).

the result of a single species, Bosmina freyi, and is remarkably consistent from year to year (Fig. 16.2 and Limburg et al., 1997).

Zooplankton also vary across the large spatial area of the Hudson ecosystem. For example, the early June increase in Bosmina freyi (Fig. 16.2) is not observed uniformly throughout the river. In 1992 for example, large increases in the population occurred primarily between river km 80 and 160 (Fig. 16.3). Below river km 50, Bosmina are rarely abundant in association with the transition from freshwater to oligohaline conditions. In contrast to the pattern for Bosmina, calanoid copepods are often most abundant toward the oligohaline end of the estuary (Fig. 16.3).

Spatial and temporal data on the period of high abundance ("bloom") of Bosmina freyi in 1992 can be used to project an estimate of the river-wide biomass and production. This projection is

Bosmina June 1993

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