Organisms that live in aquatic environments face certain challenges that their terrestrial counterparts do not. One of the obvious differences is the motion of the fluid medium, which presents opportunities and drawbacks that are unique to animals and plants that live suspended in the water column. Among the benefits this lifestyle offers are enhanced dispersal of the population, which may be achieved at a relatively low energy cost, the resultant high gene flow among dispersed populations, and the ability to readily expand into new habitats.
Aquatic organisms with limited swimming ability relative to the strength of ambient currents are said to be planktonic. The term plankton is derived from the Greek word planktos, which means wandering or drifting. Organisms such as these, whose distributions are closely tied to the movement of the water mass in which they reside, are at risk of being transported away from conditions that are necessary for their survival.
Biologists typically classify plankton into three general categories based on their phylogeny: phytoplankton are microscopic algae and other photosynthetic organisms; zooplankton are animals, mainly invertebrates; and ichthyoplankton comprise the larval fish component of the plankton.
Zooplankters are classified based not only on their taxonomy, but frequently they are grouped according to their size Zooplankton size classification ). Larger planktonic organisms (mesoplankton and above) are usually collected by towing finely woven conical plankton nets behind a vessel or streaming nets out from a fixed object in a swift current. The size of the openings in the netting material (mesh size) depends on the size-class of plankton being targeted. The smaller classes of plankton (microplankton and below) are generally collected by trapping water in bottles because nets fine enough to retain them clog rapidly when they are towed.
A third way of classifying zooplankters is based on the relative length of their planktonic life. Organisms that remain planktonic throughout the entire duration of their life cycle are referred to as holoplankters, and these are the permanent zooplanktonic residents of the water column. Other organisms, which spend only a portion of their lives as plankters, usually during the larval stages, are called meroplankters. Most of the common benthic invertebrates of coastal and estuarine waters have meroplanktonic larvae.
The estuarine zooplankton are of considerable trophic importance. Many copepods and other zooplankters, especially estuarine species, are omnivores that derive the majority of their nutrition by feeding on heterotrophic protists such as ciliates and dinoflagellates, although under some circumstances they may rely more heavily on microphytoplankton (Kleppel et al. 1998). In localities where macrophytes are abundant, such as salt marshes or seagrass beds, zooplankton standing stocks may obtain much of their nutrition by feeding on detritus (Roman et al. 1983). In estuaries, heterotrophic protists are an important component of the microzooplankton, since they provide a link between bacterial production and higher trophic levels (Heip et al. 1995). Their importance in the diets of many marine and freshwater zooplankton species was emphasized by Sanders and Wickham (1993), who noted that protists serve as a necessary link in the transfer of bacterial biomass to larger organisms.
Zooplankton density and volume specific biomass are usually greater in estuaries than in other aquatic habitats, reflecting the generally higher productivity of an estuarine environment. The species of fish and shellfish responsible for over 85 percent (by weight) of the commercial fisheries landings of the southeastern Atlantic states are estuarine or estuarinedependent at some life stage (Burrell 1975a). For many of these species that depend on estuaries as spawning or nursery grounds (e.g., Atlantic croaker, Atlantic menhaden, seatrout, drum, blue crab, and white shrimp), an abundant zooplanktonic population is necessary. Recently, Allen et al. (1995) described how competition for zooplankton as food in a high salinity South Carolina estuary may be minimized by vertical and lateral partitioning and temporal shifts in dietary selectivity. Similar partitioning of zooplankton food sources, based upon prey size, has been documented for freshwater fish species such as the threadfin shad and blueback herring introduced to the Jocassee Reservoir in the 1970s (Davis and Foltz 1991).
Certain mesoplankters, particularly copepods and cladocerans, are essential as food for early fish larvae and for larger predacious zooplankters, which in turn are fed upon by late larval and postlarval fish and other organisms. In estuaries, macroplankters such as mysid shrimp and gammarid amphipods may be the most important food chain link in habitats bounded by extensive salt and brackish marshes, which themselves often are important fish nursery grounds (Ragotzkie 1959; Van Engel and Joseph 1968; University of Georgia Marine Institute 1971). In fresh water, most larval fish are zooplanktivores, frequently selecting small-bodied organisms like rotifers and copepods. Cladocerans, which are generally larger, are preferentially selected by older fish.
Although the diverse assemblages of zooplankton in marine, estuarine and freshwater habitats are all subjected to the vagaries of the water in which they reside, they do not all respond similarly to the forces that cause the water to move. By using selective behavior in response to various physical cues, even planktonic organisms can exert some influence on the ultimate outcome of their transport (Epifanio 1988). Thus, by responding to salinity cues, some planktonic species may be distributed only within restricted zones in coastal waters, such as the low-salinity regions of estuaries, while others with may reside only in coastal waters and the high-salinity reaches near the estuary mouth.
Another important aspect of zooplankton behavior is the periodic vertical migration exhibited by many copepods (Steele and Henderson 1998). The diel (or daily) vertical migration (DVM) of many planktonic organisms may be influenced by the abundance of both food items and predators, as well as other environmental cues such as light, salinity, and temperature. In addition to locating food and avoiding predators, zooplankton may benefit from the changes in their bioenergetics that result from metabolic rates that differ on either side of the thermocline (McLaren 1963) in stratified waters. Avent et al. (1998) recently provided evidence that a common species of the estuarine copepod genus Acartia exhibits an endogenous vertical migration with a period that coincides with the semi-diurnal tide in San Francisco Bay Vertical migration .
Studies of the zooplankton of freshwater habitats in coastal regions of the southeastern states are limited, and virtually nothing has been published on the freshwater plankton of the ACE Basin. Sandifer et al. (1980) reviewed the literature describing the general characteristics of freshwater zooplankton in riverine, palustrine and lacustrine habitats of the coastal sea islands of the southeast United States. Among the early studies were those of Turner (1910), who described the copepod and cladoceran fauna of wetland habitats near Augusta, Georgia. The zooplankton of the temporary and permanent ponds and ditches sampled by Turner (1910) included 4 species of calanoid copepods, 10 cyclopoids, 1 harpacticoid, and 24 species of cladocerans. The copepod Cyclops serulatus and the cladoceran Simocephalus serrulatus were the most widely distributed taxa.
More recent research has described the species richness and population dynamics of zooplankton in another type of palustrine habitat, the Carolina bays of the Savannah River site of the U.S. Department of Energy. These geological features are shallow, poorly drained elliptical or oval depressions that number in the hundreds of thousands throughout the Atlantic coastal plain from New Jersey to Florida. Their distribution and ecological status in South Carolina was addressed by Bennett and Nelson (1991), who noted that 20 of these features are located in Colleton County; however their precise locations, and consequently their inclusion within the ACE Basin characterization area, was not described. Sharitz and Gibbons (1982) discussed the ecology of southeastern Carolina bays, but made no mention of their zooplankton.
Mahoney et al. (1990) reported that Carolina bays on the upper South Carolina coastal plain support exceptionally rich zooplankton communities, compared with temporary ponds elsewhere. These communities are generally dominated early in the wet season by crustacean taxa with long generation times, such as anostracans, conchostracans and calanoid copepods. In the 23 bays studied, seven species of the calanoid genus Diaptomus were common; none of which are typically found in nearby permanent waters. Another group of crustaceans, the cladocerans, were represented by 26 genera and at least 44 species, many of which showed considerable overlap between the fauna of the temporary bay ponds and permanent reservoir waters. Other major invertebrate taxa collected in the Carolina bays were cyclopoid and harpacticoid copepods; the crustacean orders Amphipoda, Isopoda, and Ostracoda; the insect orders Ephemeroptera, Odonata, Coleoptera, Trichoptera; the Dipteran families Ceratopogonidae, Chaoboridae, Chironomidae, and Culicidae; and oligochaetes, nematodes and aquatic mites.
The population dynamics of zooplankton in Rainbow Bay, one of the 23 Carolina bays mentioned above, were studied by Taylor and Mahoney (1990). They observed a temporal pattern in that Carolina bay pond that was typical of many others. The community was initially dominated by the copepods Diaptomus stagnalis and either Acanthocyclops vernalis or Diacyclops haueri, but later in the hydroperiod by cyclopoid copepods and cladocerans, including Daphnia laevis and Simocephalus spp. Experiments conducted on sediments from the dry pond bed suggested that the time of emergence from resting stages was a determinant of the initial succession of species in this temporary aquatic habitat. Predation by amphibian larvae (primarily salamanders) was not sufficient in this pond to limit the abundance of the predominant zooplankters; thus, population growth was limited for extended periods by insufficient food. (See related section: Geomorphology.)
The zooplankton of lakes and rivers is generally dominated by the free-living non-photosynthetic protists, rotifers and microcrustaceans; however, the species composition of these groups may be quite different in lacustrine habitats than in riverine ones (Sandifer et al. 1980). Hudson (1975) described the zooplankton of Keowee Reservoir, a man-made lake in the South Carolina piedmont region. Of the 53 species of copepods and cladocerans identified from the reservoir, only about 15 were common in the plankton, while the remainder were littoral or benthic species. Diaptomus mississippiensis, Mesocyclops edax, and Tropocyclops prasinus were the most abundant copepods, while Diaphanosoma branchyurum, Holopedium amazonicum, Daphnia ambigua, and two species of Bosmina were the most abundant planktonic cladocerans. More recent research on the zooplankton of reservoirs in South Carolina focused on the spatial heterogeneity of the plankton communities (Betsill and Van den Avyle 1994) and the effects of thermal stresses caused by nuclear reactor cooling effluents (Taylor et al. 1993).
Riverine zooplankton of coastal South Carolina has not been intensively studied. Herlong and Mallin (1985) noted that the zooplankton below an impoundment on Black Creek, South Carolina, was augmented by the impoundment outfall, making it much denser than that upstream from the impoundment. Dames and Moore Associates (1975) sampled freshwater creeks and portions of the Cooper River, collecting 12 taxa of rotifers, 4 taxa of copepods, and 2 taxa of cladocerans. Rotifers and copepods together comprised nearly 90 percent or more of the total number of zooplankters at all six sample sites. The most abundant rotifers were Polyarthra sp. and Keratella cochlearis, while the only genus of copepod identified was Diaptomus. The cladocerans Bosmia longirostris and Alonella sp. were dominant within that taxon.
The calanoid copepod Acartia tonsa was by far the most abundant mesozooplankter in the Edisto River at Bluff Point. Its numerical dominance was even more pronounced in the nearby saltwater impoundments, where it was 1 to 2 orders of magnitude greater in abundance than any other species Taxonomic composition and abundance . In contrast to A. tonsa, the second most abundant species in the river, the harpacticoid copepod Euterpina acutifrons, did not successfully colonize the ponds. Rotifers and barnacle larvae (cirripedes) were among the remaining dominant species in samples from the river, along with the calanoids Parvocalanus crassirostris, Pseudodiaptomus coronatus, and copepod nauplii.
The total zooplankton abundance reported by Knott (1980) in the North Edisto River was similar to that described at North Inlet, South Carolina, by Lonsdale and Coull (1977), who used comparable methods and equipment to investigate composition and seasonality of mesozooplankton in that high salinity estuary. The overall mean abundance in the North Edisto (10,148 indiv./m3) was only slightly greater than at North Inlet (9,257 indiv./m3). Similarity between the species composition of these two locations was also high, with a 67 percent coincidence among the 12 dominant mesoplanktonic taxa collected at each site. Furthermore, similarity between the mesozooplankton of these two sites with that described qualitatively by Burrell (1975b) from the Wando River, South Carolina, suggests that estuarine waters of the ACE Basin are likely to support a comparable community. Although the literature contains scant reference to studies of estuarine zooplankton of the ACE Basin itself, one might expect it to resemble that which typically inhabits many southeastern and Gulf coast estuaries, based on the similarities between estuarine zooplankton in the North Edisto River and elsewhere in South Carolina and that described from North Carolina (Mallin 1991), Georgia (Stickney and Knowles 1975), the Florida Gulf coast and Keys (Grice 1960), and the Gulf of Mexico (Buskey 1993).
The relative contribution of meroplanktonic organisms to total zooplankton abundance was uniformly low (3 to 21 percent) in the North Edisto River. The predominant meroplanktonic taxa were gastropod veligers (which peaked in spring), barnacle larvae (which peaked in winter/early spring), and decapod crustacean larvae (which peaked in spring and summer). Although a variety of decapod crustacean larvae were collected (at least 20 species ), they contributed relatively few numbers to the total mesoplankton community (Taxonomic listing of species ). Many of the planktonic larvae of decapods are macroplanktonic, and they may not have been efficiently captured by the 30 centimeter (12 inch) diameter net with 147 mm mesh deployed by Knott (1980). Samples collected in North Inlet, South Carolina, by Lonsdale and Coull (1977), using a nearly identical plankton net, also contained relatively few meroplanktonic larvae (25 percent by number), suggesting a sampling bias against some of the larger crustacean larvae that might be able to avoid capture by these small fine-meshed nets.
Copepods were predominant among the mesozooplankton of the North Edisto River, both in terms of species richness (63 different species) and abundance (78 percent in the river; 95 to 98 percent in the ponds)(Knott 1980). The copepods comprised 24 truly planktonic species in the orders Calanoida and Cyclopoida and a rich representation of the Harpacticoida (39 species), all but three of which were typically benthic organisms that were suspended at the shallow river station by tidal turbulence.
White shrimp and blue crab are two decapod crustaceans with significant commercial value in South Carolina. (See related section: Commercial Fisheries.) Both of these species have a life history that includes offshore larval development and an estuarine nursery habitat. Much is known about the use of South Carolina saltmarsh nursery habitats by these two species (Boylan and Wenner 1993; Mense and Wenner 1989; Wenner and Beatty 1993), but less is understood of the links between their offshore and estuarine life history stages. Consequently, recent studies (1993-94) in the ACE Basin Characterization Area by SCDNR focused on the way in which coastal oceanographic and meteorological processes influence the movement of planktonic postlarvae of these two species from the inner continental shelf through the North Edisto Inlet, to their estuarine nursery grounds. Postlarvae of both species are macrozooplankton that are potentially influenced by strong tidal forces, wind stress, bottom friction, and buoyancy fluxes. The SCDNR Ingress studies in the North Edisto Inlet were designed to explain some of the ways in which postlarval decapod distributions (both spatial and temporal) are related to the physical processes of transport. These specifically address ways in which periodic phenomena such as tides and daily or lunar cycles, and less predictable ones such as wind-generated currents, interact to influence the transport of planktonic larvae of these two species through the inlet and into the estuary.
Studies by other researchers working in the North Inlet estuary of South Carolina provide additional insight into the composition and dynamics of a macroplanktonic community that is likely to closely resemble that in the ACE Basin. Tidal, day-night, and day-to-day patterns of macrozooplanktonic abundance were described by Houser and Allen (1996), who observed large pulses of crab and shrimp larvae originating from nocturnal hatching events in the upper reaches of a tidal creek. The most abundant organisms in their 6-month series of daily samples were fish larvae (primarily the goby Gobiosoma), larval and postlarval decapod crustaceans (including the snapping shrimps Alpheus spp., the fiddler crabs Uca spp., the grass shrimps Palaemonetes spp., and the commercially valuable shrimps Penaeus spp.), juvenile bivalves, the holoplanktonic chaetognaths (arrow worms), gammarid amphipods and hydromedusae. Further seaward in Town Creek, near the inlet of the same estuary, Moore and Reis (1983) observed a similar macrozooplanktonic community dominated by the mysid crustacean Neomysis americanus. At that locality they also noted greater numbers of the holoplanktonic decapod crustaceans Acetes americanus and Lucifer faxoni, which are more typical residents of shallow coastal oceanic environments and high salinity inlets. Further documentation of such tidal incursions of coastal macrozooplankton into this estuary was provided by Costello and Stancyk (1983), who described the mechanism by which the macroplanktonic appendicularian Oikopleura dioica enters the North Inlet from the ocean.
D. Knott, SCDNR Marine Resources Research Institute
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