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Zooplankton I; II
Soil & Water Conservation Society of Metro Halifax (SWCSMH)
|September 23, 2007
Animals of fresh waters are extremely diverse, and include representatives of nearly all phyla. The zooplankton include animals suspended in water with limited powers of locomotion. Like phytoplankton, they are usually denser than water, and constantly sink by gravity to lower depths. The distinction between suspended zooplankton having limited powers of locomotion, and animals capable of swimming independently of turbulence-the latter referred to as nekton- is often diffuse. Freshwater zooplankton are dominated by four major groups of animals: protozoa, rotifers, and two subclasses of the Crustacea, the cladocerans and copepods. The planktonic protozoa have limited locomotion, but the rotifers, cladoceran and copepod microcrustaceans, and certain immature insect larvae often move extensively in quiescent water. Many pelagial protozoa (5-300 µm) are meroplanktonic, in that only a portion, usually in
the summer, of their life cycle is planktonic. These forms spend the rest of their life cycle in the sediments, often encysted throughout the winter period. Many protozoans feed on bacteria-sized particles (most cells <2µm), and thereby utilize a size class of bacteria and detritus generally not utilized by large zooplankton. Although most rotifers (150µm-1mm) are sessile and are associated with the littoral zone, some are completely planktonic; these species can form major components of the zooplankton. Most rotifers are nonpredatory, and omnivorously feed on bacteria, small algae, and detrital particulate organic matter. Most food particles eaten are small (<12µm in diameter). Most cladoceran zooplankton are small (0.2 to 3.0 mm) and have a distinct head; the body is covered by a bivalve carapace. Locomotion is accomplished mainly by means of the large second antennae. Planktonic copepods (2-4 mm) consist of two major groups, the calanoids and the cyclopoids. These two groups are separated on the basis of body structure, length of antennae, and legs.
Filtration of particles is the dominant means of food collection by rotifers and cladocerans. Filtering rates tend to increase with both increasing body length and increasing temperatures. Size of particles ingested is generally proportional to body size. Among cladocerans, the feeding rates commonly stabilize or decrease as concentrations of food particles increase. The effectiveness of zooplankton grazing varies greatly seasonally and among lakes. Throughout much of the year, zooplankton grazing only filters a small proportion of the water volume (<15% per day). At certain times of the year, grazing can remove large portions of the phytoplankton and can cause marked reduction in phytoplankton productivity.
Algal species succession can also be altered by intensive, selective (usually size specific) grazing and concommitant regeneration of
nutrients. Certain algae can survive gut passage and their growth can be enhanced by contact with high nutrient levels within the gut of zooplankton.
Assimilation efficiency is variable, but is usually less than 50%. Efficiency of assimilation increases somewhat with higher temperatures and decreases markedly with increasing food concentrations. Food quality also influences assimilation efficiencies. Rates of assimilation are low when zooplankton are feeding on detritus particles, higher with bacteria, and generally highest when they are feeding on algae of acceptable size and type. Much of autotrophic production is not utilized by herbivorous
zooplankton, but instead enters detrital pathways as nonpredatory particulate and dissolved organic matter. Although particulate detritus has less energy content than living algae, detritus often augments the diet of suspension-feeding zooplankton.
Many zooplankton, particularly the Cladocera, exhibit marked diurnal vertical migrations. The adaptive significance of diurnal migrations is unclear but likely evolved as a mechanism to avoid predation by fish, much of which is a visual process requiring light. Most species migrate upward from deeper strata to more surficial regions as darkness approaches, and return to the deeper areas at dawn. The lower vertical boundary of zooplanktonic filter feeding was found to be closely defined by the 1 mg/l isopleth of DO concentration. Filtering and respiration rates decrease rapidly at oxygen concentrations below 3 mg/l. Grazing rates of
suspension feeders are usually several times greater during the dark period when they have migrated to surface strata.
The horizontal spatial distribution of zooplankton in lakes is often uneven and patchy. Pelagial cladocerans and copepods also migrate away from littoral areas (avoidance of shore movements) by behavioral swimming responses to angular light distributions. In many cases, nonrandom dispersion of zooplankton is caused by water movements, in particular Langmuir circulations and metalimnetic entrainment of epilimnetic water.
Seasonal polymorphism, or cyclomorphosis, is found among many zooplankton, but is most conspicuous among the Cladocera. Adaptive significance of cyclomorphic growth likely centers on reducing predation by allowing continued growth of peripheral transparent structures without enlarging the central portion of the body visible to fish. Small cladocerans that increase size by cyclomorphic growth reduce capture success by invertebrate predators like copepods. A combination of environmental parameters has been shown to induce internal growth factors (hormones) that influence differential growth: increased temperature, turbulence, photoperiod, and food enhance cyclomorphosis in daphnid cladocerans. Changes in rotifer growth form include elongation in relation to body width, enlargement, reduction in size, and production of lateral spines which reduce predation success. Cyclomorphosis is lacking in copepods, which, by means of rapid, evasive swimming movements, can defend themselves better from invertebrate predators than can most rotifers and cladocerans.
Planktivorous fish can be important in regulating the abundance and size structure of zooplankton populations. Prey are visually se- lected, in most cases, on an individual basis, although the gill rakers of certain fish collect some zooplankton as water passes through the mouth and across the gills. Planktivorous fish select large zooplankters and can eliminate large cladocerans from lakes. When size selection by fish is not in effect, and when large zooplankters are present, smaller-sized zooplankton are generally not found to co-occur with the larger forms. The cause is likely a result of size-selective predation of smaller zooplankton by invertebrates (copepods, phantom midge larvae, and predaceous Cladocera).
The production rate (=net productivity) of zooplankton is the sum of all biomass produced in growth, including gametes and exuviae of molting, less maintenance losses from respiration and excretion. Efficiency of assimilation is nearly always less than 50%. Assimilated energy expended in respiration is usually less than 50%; the remainder is used for growth and reproduction. Assimilation and respiration rates generally increase at higher trophic levels, and production decreases. Emigration (e.g., outflow losses) and immigration from streams and other lakes of zooplankton are usually negligible. A general, positive correlation exists between the rates of production of phytoplankton and of zooplankton. The productivity of suspension-feeding zooplankton is higher than that of predaceous zooplankton.
Separation of the niche hyperspace with relatively small regions of species overlap minimizes interspecific competition and contribute to the large diversity of population interactions that have evolved to permit coexistence in limnetic zooplankton communities.
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