Zooplankton and the Zebra Mussel Invasion

The invasion of the zebra mussel had important impacts on the Hudson ecosystem. Zebra mussel grazing resulted in an 80-90 percent decline inphytoplanktonbiomass and primary production (Caraco et al., 1997). Zebra mussels also caused a reduction in populations of native benthic bivalves and other benthic animals that feed on plankton (Strayer and Smith, 2001). Some zooplankton populations as well as zooplankton community biomass also declined (Pace et al., 1998).

The general effect of zebra mussels is evident from examining mean annual abundance from long term data. Assessments of zebra mussel effects are based on a comparison of means before and after 1992. Rotifers have clearly declined from mean densities greater than 100 l-1 to means less than 50 l-1 since the invasion of the zebra mussel (Fig. 16.4). Other microzooplankton groups (data not shown) such as tintinnid ciliates and copepod nauplii exhibited a similar decline (Pace et al., 1998). Post-naupliar copepods have varied within a narrow range of 2-6 l-1 with no obvious trends, although four years of relatively low mean abundance (<3l-1) have all occurred following the invasion of the zebra mussel (Fig. 16.4). While copepod densities may have declined slightly, no substantial change has occurred, a conclusion supported by time series analysis (Pace et al., 1998).

Any change in cladocerans is difficult to assess based simply on an inspection of the annual means (Fig. 16.4). Mean abundance was low in some years following 1992 (e.g., 1996 and 2000) while other years appear quite similar to those observed prior to 1992 (e.g., 1998 and 1999). The input of fresh water to the Hudson as measured by flow over the dam at Troy, New York, has an important influence on plankton dynamics (Pace et al., 1991; Caraco et al., 1997). When mean cladoceran abundance is plotted against freshwater flow, the annual means fall cleanly into two groups representing the pre- and post-zebra mussel period (Fig. 16.5). Abundance is low in wet years and high in dry years. This pattern persists with zebra mussels but abundance has shifted to a lower level (Fig. 16.5). Thus, the mean abundance of cladocerans declined in association with the zebra mussel invasion.

Figure 16.5. Mean annual abundance of cladocerans and mean flow for years prior to (pre) and following (post) the zebra mussel invasion. Mean flow from daily measurements of discharge made by the United States Geological Survey at the head of the estuary, Green Island, New York.

Flow (m3 sec"1)

Figure 16.5. Mean annual abundance of cladocerans and mean flow for years prior to (pre) and following (post) the zebra mussel invasion. Mean flow from daily measurements of discharge made by the United States Geological Survey at the head of the estuary, Green Island, New York.

Loss of phytoplankton as food is an obvious possible mechanism that couldlead to a decline inzooplankton. The most direct test of this idea, however, using available field data, is contradictory. Long-term records of egg production for Bosmina indicate little change in eggs per individual or clutch size following the invasion (Pace et al., 1998). Reproductive parameters are sensitive indicators of food limitation in zooplankton populations. Thus, the mechanisms whereby zebra mussels influence cladocerans are uncertain. Nevertheless, years of high freshwater flow and abundant zebra mussels are associated with reduced abundances.

Declines in various microzooplankton including tintinnid ciliates, rotifers, andcopepodnaupliimay be related to a loss of food, but it is equally plausible that zebra mussel predationlimits these groups. Rates of zebra mussel feeding based on laboratory studies (MacIsaac et al., 1992; Wong et al., 2003) in combination with estimates of abundance in the Hudson suggest predation on microzooplankton is comparable to maximum estimated population growth rates (Pace et al., 1998).

Overall, declines in zooplankton observed in the Hudson in association with the zebra mussel invasion were size-dependent. The largest declines were observed for organisms with individual body mass less than 0.1 |g dry weight while substantial declines occurred in organisms with body mass in the range of 0.1-1 |g dry weight. For organisms with an average size greater than 1 |g (e.g., copepods) no significant change was observed. Many resident and anadromous fish exploit zooplankton especially during their earliest life history stages. Limited declines observed in larger, preferred prey items like copepods (Limburg et al., 1997) may have reduced the effects of zebra mussels on fish populations.

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