Community Composition

Our discussion of zooplankton in this chapter includes both animalzooplanktonandheterotrophic protists (also called protozoans). The principal planktonic microbial protists that feed on other organisms are ciliates and flagellates. Ciliates and flagellates are often not well characterized taxo-nomically, but they are important consumers that may be quite significant inrivers andestuariesboth as grazers on phytoplankton as well as consumers of bacteria (Gasol and Vaque, 1993).

Fresh water dominates much of the Hudson. Principal freshwater zooplankton include flagellates, ciliates, rotifers, and crustaceans. The latter group includes cladocerans and copepods - both are common in the Hudson. There are numerous species of flagellates that do not contain chloro-plasts and are presumed to feed primarily on bacteria. Little is known about the species in this group. The ciliates are typical of planktonic forms, primarily composed of choreotrichs (inclusive of olig-otrichs and tintinnids), a group distinguished by the lack of somatic ciliature but with complex ciliated structures associated with the feeding apparatus (Fig. 16.1). Rotifers are very small animals, with many of the species in the Hudson being less than 100 |im in maximum length. Two genera, Keratella and Polyarthra, are found in almost any freshwater sample taken from the Hudson. A variety of other genera are common, e.g., Asplanchna, Asco-morpha, Brachionus, Collotheca, Filinia, Kellicotia, Notholca, Pleosoma, and Synchaeta. Copepods include representatives of the three main groups: cyclopoids, calanoids, and harpacticoids. The latter are primarily benthic, although harpacticoid cope-pods are found in Hudson zooplankton samples. Two species of cyclopoid copepods, Diacyclops bicuspidatus thomasi and Halicyclopssp., are common. The calanoid copepod, Eurytemora affinis, while more typically associated with low salinity conditions, also occurs in the freshwater Hudson. The most common cladoceran is Bosmina freyi (Fig. 16.1). Other cladoceran species of the genera Daphnia, Diaphanosoma, and Ceriodaphnia occur sporadically at low abundance. Additionally, the large, predatory cladoceran Leptodora kindtii is found in the Hudson where it feeds on a variety of prey including Bosmina freyi.

Nauplii

f Oithona Lepadella f to! A

Figure 16.1. Representative Hudson River zooplankton including a tintinnid, copepod nauplius, a rotifer, Lepadella,acladoceranBosminafreyi, andacyclopoid copepod Oithona sp. Diagram is not to scale. Tintin-nids, rotifers, and nauplii are small zooplankton in the size range of 50-200 |m. Bosmina are typically 200400 |im, and Oithona >500 |im.

In saline sections of the Hudson, a true estuarine fauna occurs. Freshwater forms decline in the oligohaline sections and do not occur in the southern portion of the estuary. As in the freshwater Hudson, flagellates and ciliates can be abundant. Functionally the species resemble freshwater forms, but there is a greater diversity of ciliates, especially tintinnid species (Fig. 16.1). Estuarine rotifers and cladocera (i.e., Evadne) can occasionally be important (Lonsdale, Cosper, and Doall, 1996) as are avariety of larvae of benthic organisms (referred to as meroplankton) including those of polychaetes, barnacles, and crabs. Copepods, however, are the most abundant animals (Fig. 16.1). There are a variety of calanoid copepods that vary in salinity preferences and seasonal occurrence. The most common species include Eurytemora affinis, Acartia tonsa, and A. hudsonica. In addi tion, species of Centropages, Pseudocalanus, and Temora are found. Oithona is an important cy-clopoid copepod. There are also larger, predatory zooplankton such as ctenophores (e.g., Mnemiop-sisleidyi) andmysids (Neomysis americana). These zooplankton taxa are typical of temperate estuaries along the east coast of the United States (e.g., Narragansett Bay, Rhode Island, Long Island Sound, New York, and Chesapeake Bay, Maryland).

As water moves from the Hudson River to New York Harbor and offshore, the estuarine zooplankton community is replaced by a coastal zooplankton community (Stepien, Malone, and Chervin, 1981). Copepods of the genera Eurytemora and Acartia are emblematic ofthe estuarine-nearshore community, while further offshore, the New York Bight is characterized by Calanus, Pseudocalanus, Centropages, and Temora.

Another useful way of viewing the zooplankton community is based on size. Size is well related to metabolic activity, feeding rates, and predator-prey interactions. One convention is to organize the communityinto size classes. These are nano- (2-20 |im in maximum dimension), micro-(20-200 |im), meso- (200-2,000 |im), and macro-(>2,000 |m) plankton (Omori and Ikeda, 1984). Based on this organization, flagellates and some of the smaller ciliates constitute the nano- size class. Microzooplankton are principally ciliates, rotifers, copepodnauplii, some meroplankton, and some of the smaller cladocerans and copepods. Mesozoo-plankton are mostly crustaceans, especially post-naupliar life stages of copepods and cladocerans. Finally, macrozooplankton are some of the largest copepods and various, largely predatory zooplankton such as Leptodora, ctenophores, mysids, and even fish larvae. This largest size class of organisms is much more capable of directed swimming, and so at some point these animals are better considered nekton, but there is no strict separation of the groups.

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