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and Hoff, 1983

Striped bass (yearlings)

76 (N)

Amphipods

Gardinier and Hoff, 1983

Striped bass (2-yrold)

14 (N)

Fish

Gardinier and Hoff, 1983

Tessellated darter

>50 (N)

Chironomids, microcrustaceans

Duryea and Schmidt, 1986

importance is expressed as % of number (N) or volume (V) of items in the gut contents that were benthic invertebrates.

YOY = young-of-year fish

Modified from Strayer and Smith (2001).

importance is expressed as % of number (N) or volume (V) of items in the gut contents that were benthic invertebrates.

YOY = young-of-year fish

Modified from Strayer and Smith (2001).

Trichoptera (caddisflies), Coleoptera (beetles), Ceratopogonidae (no-see-ums), and Chaoboridae (phantom midges).

While annelids, mollusks, crustaceans, and insects dominate the Hudson's zoobenthos, many other animals are present. Porifera (sponges), Cnidaria (hydras, jellyfish), Turbellaria (flatworms), Nematoda (roundworms), and Acari (mites) may be locally abundant in the Hudson and add to its biological richness.

The Hudson's fauna resembles that of other tidal rivers in northeastern North America, from the James to the St. Lawrence. The macrozoobenthos of the freshwater parts of these rivers is usually strongly dominated by Limnodrilus hoffmeisteri and other tubificid oligochaetes, and often contains dense populations of predatory chironomids (for example, Coelotanypus scapularis, Procladius spp., Cryptochironomus spp.) and sphaeriid clams (Massengill, 1976; Crumb, 1977; Vincent, 1979; Ettinger, 1982; Diaz, 1989). Most of the freshwater species in these tidal rivers also occur widely in lakes and warm water rivers, but the fauna is distinctive in two ways. Several species common in the Hudson and other northeastern estuaries (for example, the cumacean crustacean Almyracumaproximoculi, the amphipodMonocu-lodes edwardsi, the isopods Chiridotea almyra and Cyathura polita, and the snail Littoridinops tenuipes) usually live in oligohaline estuaries and coastal waters, and introduce a distinctively estu-arine element to the "freshwater" fauna. Also, net-spinning caddisflies and burrowing mayflies, two groups of suspension-feeding insects that are important in many large rivers worldwide, are very rare in the freshwater tidal rivers of the Northeast, perhaps because rapidly changing tidal currents interfere with the construction and operation of the fixed burrows and nets used in feeding.

Spatial Variation in the Hudson Zoobenthos

Benthic communities vary enormously from place to place along the Hudson, in terms of both the number and kinds of animals that are present. Four factors are correlated with this variation: position along the course of the river, salinity, the presence or absence of rooted plants, and the nature of the bottom (hard vs. soft).

It appears that the density of benthic macroin-vertebrates in the Hudson follows a W-shaped pattern, with peaks near Manhattan, Kingston, and Albany, and deep, broad troughs between these peaks (Fig. 19.2). This pattern is very strong, with densities in the peaks about 100-fold higher than

River kilometer

Figure 19.2. Long-river variation in density of benthic macroinvertebrates in the Hudson River. Data from mid-channel samples from Ristich et al. (1977) (black circles) and Simpson et al. (1984) (white circles), and from cross-channel transects in 1990-92 by Strayer et al. (unpublished). Because the three studies were done at different times and used different methods, the data are not exactly comparable across studies.

River kilometer

Figure 19.2. Long-river variation in density of benthic macroinvertebrates in the Hudson River. Data from mid-channel samples from Ristich et al. (1977) (black circles) and Simpson et al. (1984) (white circles), and from cross-channel transects in 1990-92 by Strayer et al. (unpublished). Because the three studies were done at different times and used different methods, the data are not exactly comparable across studies.

in the troughs. The W-shaped pattern may arise through acombinationofstressandfoodsubsidies. Unstable salinities in RKM 20-100 and unstable, sandy sediments in RKM 170-210 may suppress benthic communities (cf. Simpson et al., 1986). Inputs of sewage from New York City, and of phyto-plankton from the Bight and near RKM 150 (Cole, Caraco, and Peierls, 1992) may further contribute to the development of the peaks.

The composition of benthic communities in the Hudson is a strong function of salinity (Fig. 19.3). Near Manhattan, the fauna is dominated by characteristically marine animals (polychaetes, bivalves such as Mya andMacoma), while above Newburgh, the benthos is dominated by freshwater species of oligochaetes, insects, and bivalves. In the intermediate zone of moderate and fluctuating salinity, the fauna contains a few species (for example, thepoly-chaete Marenzelleria viridis, the amphipod Lepto-cheirus plumulosus) that thrive in brackish water. Nevertheless, there is a good deal of blurring of the fauna along the salinity gradient, and it is common to find supposedly marine or brackish-water animals (e.g., the crab Callinectes sapidus, the cumacean Almyracuma proximoculi) well into the freshwater Hudson (Simpson et al., 1985).

The nature of the substratum also has a strong influence on the character of the zoobenthos (Table 19.2). Compared to nearby unvegetated habitats, beds of rooted vegetation support di verse communities that are rich in insects and snails. Dozens of species of benthic animals in the Hudson are essentially confined to plant beds (Strayer and Smith, 2001). Likewise, rocky bottoms support more diverse communities than un-vegetated soft sediments, including animals like mayflies and beetles that are rare elsewhere in the river. In contrast, the communities ofvarious kinds of soft sediments (that is, sand vs. mud) differ little from one another, at least in the freshwater part of the Hudson (Strayer and Smith, 2001).

Nevertheless, most of the site-to-site variation in benthic communities in the Hudson and other large rivers is unexplained by factors like salinity, rooted plants, the grain size and organic content of the sediments. For example, the amphipod

Unionidae (Bivalvia) Sphaeriidae (Bivalvia) Hydrolimax grisea (Turbellaria) Gammarus fasciatus (Amphipoda) Limnodrilus hoffmeisteri (Oligochaeta) Dreissena polymorpha (Bivalvia)

_Chironomidae (Diptera)_

Cyathura polita (Isopoda) ? Chaoborus punctipennis (Diptera)

?__Littoridinops tenuipes (Gastropoda)

Hydrobia spp. (Gastropoda) Marenzelleria viridis (Polychaeta) Leptocheirus plumulosus (Amphipoda) Macoma balthica (Bivalvia) Polydora websteri (Polychaeta)

Mya arenaria (Bivalvia) Eteone heteropoda (Polychaeta) Nereis succinea (Polychaeta) Streblospio benedicti (Polychaeta)

POLY MESO IOLIGOI FRESH ,

0 50 100 150 200 250 River kilometer Figure 19.3. Approximate longitudinal distribution of dominant benthic animals in the Hudson River estuary, showing succession along the salinity gradient. The typical late-summer salinity zonation is shown just above the X-axis. FRESH = freshwater(< 0.5ppt), MESO = mesohaline (5-18 ppt), OLIGO = oligoha-line (0.5-5 ppt), POLY = polyhaline (18-30 ppt). Based on Ristich etal. (1977),Weinstein (1977), Simpsonetal. (1986), and Strayer and Smith (2001). Uncertainties indicated by dashes and question marks.

Table 19.2. Composition of macrobenthic communities in three habitats of the freshwater tidal Hudson near Kingston, based on % numerical abundance

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