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Zooplankton I; II
Soil & Water Conservation Society of Metro Halifax (SWCSMH)
April 10, 2015 |
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Contents:
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.
- Wetzel, 1983:
- Ciliated protozoans and rotifers become more important in the
zooplankton among eutrophic, subtropical lakes . Although nearly all
Protozoa are aerobic, a majority can grow very well even when oxygen
concentrations are very low. This microaerophilic ability is
conspicuous among the planktonic and benthic ciliates, and is attested
to by their major development in organic-rich and polluted waters.
Populations of ciliates often develop in strata greatly reduced in or
devoid of oxygen in which bacterial populations tend to be dense.
- As lakes become more eutrophic, a greater proportion of
the phytoplankton biomass and productivity often results from large
algae (mostly colonial or filamentous). The larger algae interfere with
food collection to a greater extent in larger cladocerans, causing
reduced growth and fecundity, than in smaller cladoceran species that
feed on small particles. Such interspecific competition, in addition to
size-selective predation, could contribute to reduction of larger
zooplankton.
- Carney, 1990:
- In mesotrophic systems edible and nutritious algae are in higher
concentrations than in more nutrient-poor waters, and the proportion of
these algae is greater than in more eutrophic systems. In these
intermediate systems there are also sufficient concentrations of
cladoceran herbivores. A number of species in the genus Daphnia
have particularly high per capita filtering rates. Cladocerans also
regenerate nitrogen and phosphorus in the soluble available forms. This
enhances phytoplankton productivity, speeds nutrient cycling, and
tightens coupling between these trophic levels. In oligotrophic systems
concentrations of edible algae are lower, so zooplankton concentrations
are also lower. Perhaps as important, there is a shift in dominance to
copepods which have lower per capita filtering rates and excrete faecal
pellets rather than dissolved nitrogen and phosphorus. All these
factors contribute to reduced coupling at this interface. At the other
end of the spectrum, in very eutrophic lakes and ponds inedible algae
(for example relatively large filamentous blue-greens and greens)
become important. This may represent a state in the coevolutionary
process in which, following substantial grazing impact and natural
selection, these algal species have "won". In any case, both grazing
pressure and nutrient regeneration are lower in these systems.
- Percentage of production of Rotifer, Cladoceran, and Copepod Zooplankton in various lakes, from oligotrophic to eutrophic (Wetzel, 1983):
Lake | Rotifers | Cladocera | Copepods |
Mirror (oligotrophic), New Hampshire | 39.8 | 40.9 | 19.3 |
Krivoe, USSR | 15.2 | 36.1 | 48.7 |
Krugloe, USSR | 19.2 | 71.8 | 8.9 |
Naroch, USSR | 43.5 | 31.2 | 25.2 |
Myastro, USSR | 23.9 | 47.6 | 28.6 |
Batorin, USSR | 29.3 | 45.5 | 25.1 |
Lake 239, Ontario, Canada | 67.2 | 5.4 | 27.4 |
- Some characteristic zooplankton groups,
Ryding and Rast, 1989, and Standard Methods, 18th Ed. 1992:
Oligotrophic waterbody | Eutrophic waterbody |
Bosmina obtusirostris (0.4 mm) (Phylum Arthropoda, Class Crustacea, Order Cladocera) | Bosmina longirostris (0.4 mm) (Phylum Arthropoda, Class Crustacea, Order Cladocera) |
B. coregoni (0.4 mm) (Phylum Arthropoda, Class Crustacea, Order Cladocera) | Daphnia culcullata (2 mm) (Phylum Arthropoda, Class Crustacea, Order Cladocera) |
Diaptomus gracilis (2 mm) (Phylum Arthropoda, Class Crustacea, Order Copepoda) | |
MAPC, 1972; Wetzel, 1983:
- The 1971 summer zooplankton samples (mostly single
samples) from 38 lakes revealed that 30 lakes were dominated by either
Diaptomus minutus
(22 lakes) or Mesocyclops edax (4 lakes) and/or Tropocyclops prasinus
(4
lakes). Four other species were dominant in the other 8 lakes, and they
were Diaphanosoma brachyurum (3 lakes), Bosmina sp. (3 lakes),
Holopedium
gibberum (1 lake), and Epischura nordenskiöldi (1 lake).
- Lakes sampled were Albro, Bell, Bissett, Bluff,
Charles, Chocolate, Colbart, Cranberry, First, Governor, Henry,
Kearney, Kidston, Lemont, Long, Long Pond, Loon, Lovett, Maynard,
MicMac, Morris, Oat Hill, Otter, Paper Mill, Penhorn, Powder Mill,
Power Pond, Rocky, Russell, Sandy,
Second, Spruce Hill, Third, Three Mile, Topsail, Webber, William
(Halifax)
and Williams (Waverley).
- A list of the more common pelagic zooplankters found:
Phylum (Division) Arthropoda, Class Crustacea, Order Cladocera | Phylum (Division) Arthropoda, Class Crustacea, Order Copepoda |
Diaphanosoma brachyurum | Diaptomus minutus (Suborder Calanoida) |
Bosmina sp. | D. spatulocrenatus (Suborder Calanoida) |
Daphnia pulex | Epischura nordenskiöldi (predacious) |
Leptodora kindtii (predacious) | E. lacustris (predacious) |
Polyphemus pediculus (predacious) | Mesocyclops edax (carnivorous) (Suborder Cyclopoida) |
Holopedium gibberum | Tropocyclops prasinus (Suborder Cyclopoida) |
Strong, 1986; Wetzel, 1983; and
Standard Methods, 18th Ed. 1992:
- Plankton collections were obtained at each of 36 lakes during summers
and early autumns of 1983 or 1984, and the collections were made about
midday, each lake being sampled on one date only. A total of 27 taxa was
identified. Most lakes contained 3-7 species (excluding rotifers and
copepod nauplii) and were dominated by 1-3 species. The calanoid copepod
Diaptomus minutus was the most numerous, followed closely by cyclopoid copepod, Mesocyclops edax, the cladoceran Bosmina longirostris and the rotifer Keratella taurocephala.
Each of these species occurred in at least 75% of all lakes sampled.
These species also occurred very commonly in the study by Carter et al., (1986) in their study of Nova Scotia and New Brunswick lakes; and the copepods D. minutus (22 lakes), M. edax (4 lakes) as well as T. prasinus (4 lakes) dominated 30 Metro Halifax lakes in the study by MAPC (1972).
- Simple statistics such as the number of species, diversity index, and
evenness index were poorly correlated with abiotic variables. The best
correlations indicated that diversity and evenness were negatively
correlated with water temperature, water transparency and lake area, and
positively correlated with conductivity. The calanoid copepod Diaptomus
minutus was associated with warm, turbid waters of decreased acidity,
whereas the cladoceran Bosmina longirostris dominated in the opposite conditions. The cyclopoid copepod, Mesocyclops edax was usually dominant in clear, deep lakes, and the cladoceran Daphnia catawba was often dominant in lakes with highly coloured water.
- The metro Halifax lakes sampled were Dollar, Echo, Paces, Second and
Fourth. Following were the zooplankton taxa from plankton net collections
from the five Metro lakes:
Phylum (Division) Arthropoda, Class Crustacea, Order Cladocera | Phylum (Division) Arthropoda, Class Crustacea, Order Copepoda | Phylum (Division) Aschelminthes, Class Rotifera [Rotatoria] |
Bosmina longirostris (~0.4 mm) | Diaptomus minutus (Suborder Calanoida) (~2 mm) | Keratella taurocephala (~200 µm) |
Daphnia catawba (~2 mm) | Mesocyclops edax (carnivorous) (Suborder Cyclopoida) | K. cochlearis (~200 µm) |
Diaphanosoma birgei (~1.5 mm) | Epischura nordenskioldi (predacious) | Kellicotia longispinus (~1 mm) |
D. brachyurum (~1.5 mm) | Tropocyclops prasinus (Suborder Cyclopoida) | Polyarthra sp. (~175 µm) |
Holopedium gibberum | unidentified nauplii | |
Planktonic crustaceans in Canada: several species south of the 60th parallel
Patalas, 1990; Standard Methods, 18th Ed. 1992; Wetzel, 1983
- Phylum (Division) Arthropoda, Class Crustacea, Order Cladocera (mostly
0.2-3 mm):
- Daphnia schoedleri (2 mm)
- D. catawba
- D. parvula
- D. retrocurva
- D. magna
- D. galeata mendotae
- D. hyalina ceresiana
- D. thorata
- D. ambigua
- D. dubia
- Ceriodaphnia affinis
- C. lacustris
- C. quadrangula
- C. reticulata
- Eubosmina coregoni (0.4 mm?)
- E. tubicen
- Diaphanosoma brachyurum (1.5 mm)
- D. leuchtenbergianum
- Leptodora kindtii (~9 mm) (predaceous)
- Polyphemus pediculus (1.5 mm) (predaceous)
- Phylum (Division) Arthropoda, Class Crustacea, Order Copepoda (mostly
2-4 mm), Suborder Cyclopoida (primarily littoral benthic, but planktonic
forms comprise major components in the copepod population, especially in
small shallow lakes):
- Diacyclops bicuspidatus thomasi
- Eucyclops agilis (herbivorous)
- Macrocyclops albidus (carnivorous)
- Mesocyclops edax (carnivorous)
- Orthocyclops modestus
- Tropocyclops prasinus
- Phylum (Division) Arthropoda, Class Crustacea, Order Copepoda (mostly
2-4 mm), Suborder Calanoida (mostly planktonic):
- Diaptomus kenai
- D. leptopus
- D. nudus
- D. oregonensis
- D. pygmaeus
- D. reighardi
- D. siciloides
- D. spatulocrenatus
- D. tyrreli (filter feeder)
- Phylum (Division) Arthropoda, Class Crustacea, Order Copepoda (mostly
2-4 mm), Suborder Harpacticoida (mostly littoral):
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