Zooplankton biomass in rivers and estuaries is typically lower than in lakes even when comparing systems with comparable levels of phytoplank-ton (Pace et al., 1992). This comparison implies other features such as advective losses are important in limiting populations. During the cold temperature, high-flow periods of the year (November to May), zooplankton are rapidly advected downstream. After May, however, water residence time in the Hudson is several months. This allows ample time for zooplankton populations to increase, raising the question of what limits the abundance and biomass of zooplankton in the Hudson during the warmer, low-flow periods of the year (June to October). We hypothesize zooplankton are limited by a combination of food and predators with the latter being most important during summer.
Estuaries are areas with abundant and diverse food for consumers and in the Hudson some combination of bacteria, algae, detritus, and microzooplankton provide food for zooplankton. Does food limit the abundance of populations and overallbiomass of the community? As noted above since the early 1990s, zebra mussels have caused a very large reduction of phytoplankton in the Hudson. The lack of a decline in freshwater copepods (Fig. 16.4) and the relatively constant egg ratios (eggs per female) in cladocerans indicate food limitation was not necessarily the reason for the changes in zooplankton that accompanied the zebra mussel invasion. There is evidence, however, of food limitation of egg production by copepods in the lower, saline portion of the estuary. Supplementing natural food levels with an edible algal species resulted in increased egg production by calanoid copepods relative to ambient conditions at most times (Lonsdale et al., 1996). Moreover, egg production rates were positively related to total depth-integrated primary production (also see Chervin et al., 1981). Food limitation also affects copepod production in other estuaries (Durbin etal., 1983).
Limitation by food quality is more difficult to evaluate. Ample dissolved nutrients in the Hudson argue against skewed stoichiometric ratios ofmajor elements in the foods of zooplankton. Thus, the relative amounts of carbon, nitrogen, and phosphorus inresources are likely within ranges that do not cause food quality limitation, as has been observed in freshwater systems (DeMott and Gulati, 1999).
Nevertheless, if zooplankton are sustained by substantial quantities of detritus and/or bacteria, there is the possibility of food quality problems associated with essential fatty acids (Jonasdottir, 1994) andperhaps other nutritional requirements (for example, sterols; Harvey, Ederington, and McManus, 1997). Interestingly, the most extreme demonstration of food limitation of copepod egg production (Pseudocalanus sp.) in the Hudson estuary was found during a spring bloom of the diatom Skeletonema costatum (~3-7 x 106 cells ml-1) when the concentration of chlorophyll a was relatively high (>10 |gL-1) (Lonsdale etal., 1996). Itis possible that this reduction in egg production rate was due to food quality, as diatoms as a dominant food source may provide a nutritionally inadequate diet (Kleppel, 1992).
Based on measurements of the abundance of planktivorous fishes and invertebrates, zooplankton may be heavily preyed on in the Hudson. For example, the combined abundance of young-of-year alosids, white perch, striped bass, and bay anchovy (size <100 mm) varies in the range 0.1 to 100 m-3 during summer (Limburg, 1994). A rough calculation of the impact of planktivores can be made by assuming a planktivore consumes 100 copepods d-1, a rate well within the broad range observed in feeding studies. At planktivore densities of 10 m-3 and a copepod density of 1-10 L-1, the estimated daily predation on copepods is 10 to 100 percent of the standing stock. Such mortality rates are significant given likely specific cope-pod growth rates in the range of 0.1-0.75 d-1 for summer water temperatures (Huntley and Lopez, 1992). Obviously, lower densities of planktivores or lower consumption rates would indicate a more modest impact, but it is very likely that the fish surveys underestimate actual abundance and under many conditions the abundance of invertebrate planktivores (Leptodora, Neomysis, ctenophores, amphipods) is at least 1 animal m-3 and often far greater.
The inference of significant predatory regulation of crustacean zooplankton during the warmer, low-flow period of the year is consistent with findings from other systems. Ki0rboe (1998) using data on growth rates for copepods in coastal systems concluded that food was generally not the primary limiting factor. Case studies from other temperate es tuaries provide evidence that planktivory may be limiting for copepodids and adult copepods, particularly in summer and fall (Mehner and Thiel, 1999; Adrian et al., 1999).
Two additional features of the Hudson zooplankton support the notion of strong predatory effects. Community structure is indicative of a system where vertebrate planktivory is important. In the freshwater section, small-bodied species less visible to predators and those with good escape abilities (cyclopoids) are dominant. The Hudson assemblage resembles that found in lakes that experience intense planktivory. In the estuarine section larger-bodied calanoids are dominant. Some of these species (for example, Acartia), however, have numerous morphological and life history traits thought to be adaptations to predation including body shape, rapid escape responses, and broadcast spawning of eggs (Ki0rboe, 1998).
Not all planktivores are subject to regulation by predators. Ctenophores are, at times, very abundant in the lower Hudson and probably exert strong predatory effects on zooplankton, including copepods, rotifers, and ciliates (Deason and Smayda, 1982; Stoecker et al., 1987). There are a variety of predators of ctenophores (e.g., scypho-zoan medusae; Feigenbaum and Kelly, 1984), but ctenophores are not apparently subject to intense predation by fish commonly found in the Hudson (Kremer, 1994). Thus, planktivory may be little affected in this case by alterations in the fish predator community.
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