Download Plankton and benthic flora

Plankton and benthic flora
Teodora Maria Onciu
Ovidius University of Constanta, Romania
1. Introduction
Organisms can be classified at the functional level by their sources of energy and of carbon.
Autotrophic organisms use inorganic carbon dioxide to produce organic matter and
heterotrophic organisms depend on organic carbon (glucose). Phototrophs derive energy directly from sunlight in photosynthesis and the chemotrophs use a chemical energy source. In
aquatic ecosystems, the base of food chains is formed by photoautotrophs (unicellular planktonic algae) and benthic flora (macroalgae and higher aquatic plants). They represent also the
most important material for decomposers, which represent a great amount of the organic carbon
source, i.e. particulate and dissolved organic matter. Higher in the food chains are the
heterotrophic animals, which use different material for their growth: plants are sued by the herbivores, particulate organic matter is used by the detritivores and animals are used by carnivores
(Goldman and Horne, 1983; Kalff, 2001).
The plankton community, generally referred to as “the plankton” is a mixed group of
photosynthetic, single-cell organisms (phytoplankton), minute herbivorous, omnivorous and
carnivorous invertebrates (zooplankton), and decomposers floating, drifting or feebly swimming in the water mass. The term “plankton” was first used by Hensen in 1887, referring to
living organisms and the non-living particulate matter (Cole, 1983; Kalff, 2001).
At the bottom of the aquatic ecosystem lives a large community, usually termed benthos,
consisting of macroalgae and pondweeds (macrophytobenthos) and herbivorous, omnivorous
and carnivorous organisms (zoobenthos) and decomposers (Cole, 1983; Kalff, 2001).
Freshwater ecosystems include lakes, streams, rivers, estuaries and reservoirs. Wetlands
are transition zones between terrestrial and aquatic systems where soils are waterlogged for at
least part of the year or covered by shallow water and which are typically occupied by rooted
aquatic vegetation. The littoral zone of lakes and rivers forms a continuum with wetlands which,
at the same time, are ecotones between terrestrial and aquatic systems (Kalff, 2001; Mitsch and
Gosselink, 2007).
NEAR Curriculum in Natural Environmental Science, 2010, Terre et Environnement, Vol. 88, 243–251,
ISBN 2–940153–87–6
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Box 1 Criteria for plankton classification
Dimensions
Plankton not captured by a net
Net plankton
nannoplankton (5–50 µm)
microplankton (50–1,000 µm)
mesoplankton (1–5 mm)
Life cycle
Holoplankton
Meroplankton
Nutrition
Photoautotrophic
Heterotrophic
phytoplankton
zooplankton
Taxonomy
Phytoplankton
Zooplankton
Cyanobacteria
Anabaena, Oscillatoria, Aphanisomenon
Chlorophyta
Volvox, Chlorella, Scenedesmus
Euglenophyta
Euglena
Desmidiaceae
Closterium, Staurastrum, Micrasterias
Pyrrophyta
Ceratium
Bacillariophyta
Cyclotella, Asterionella, Fragillaria
Ciliata
Paramecium, Frontonia, Colpidium
Rotatoria
filter feeders
Brachionus, Filinia, Keratella
carnivorous
Asplanchna
omnivorous
Hexarthra, Synchaeta
Cladocera
filter feeders
Daphnia, Bosmina, Chydorus
carnivorous
Leptodora, Polyphemus
Copepoda
Calanida filter feeding,
herbivorous
Diaptomus
Cyclopoida filter
feeding or carnivorous Cyclops
2. Plankton
Holoplanktonic organisms are always present in the plankton, as distinct from meroplanktonic
organisms, which spend some of their life cycle on the bottom of the waterbody in a dormant
form (for single-cell organisms) or adult form (mollusca, crustaceans, etc.). Plankton can be
divided into two main groups on the basis of size: the net plankton, which is retained by a
plankton net (mesoplankton having dimensions between 1 and 5 mm and microplankton between 50 and 1,000 µm) and those which are not captured by the net (nannoplankton between 5
and 50 µm) (Pérès and Devèse, 1963; Dussart, 1966). The criteria for classification of plankton
are summarised in Box 1.
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Even if planktonic organisms are very small they are slightly heavier than water and, even
those that have an active form of movement, eventually sink. Sinking represents a major
problem for planktonic organisms, especially for the photosynthetic single-cell organisms,
because leaving the high irradiance zone near the water surface reduces or prevents photosynthesis. The buoyancy of planktonic organisms is influenced by water density and viscosity. Both
physical characteristics depend on two factors: temperature and the concentration of salts.
Organisms can do nothing about the density or the viscosity of the water and therefore any adaptations by the organism to reduce their sinking rate must either reduce their weight or increase
their surface resistance. The first strategy involves physiological mechanisms to replace heavy
chemical ions in the body fluids with lighter ones or to produce gas vacuoles or oil droplets. The
surface of resistance may be increased in a number of ways (Kalff, 2001; Nybakken and
Bertness, 2005):
+
spherical, acicular or lamellar body shape and small size, or
+
increasing the surface of resistance by developing various forms of appendages or
spines.
2.1 Phytoplankton
The taxonomic diversity of freshwater phytoplankton is less important that those of marine
single-cell algae. Cyanobacteria – the so-called blue-green algae (Microcystis, Oscillatoria,
Anabaena, Aphanizomenon), Chlorophyta – the green algae (Volvox, Chlorella, Scenedesmus
and Spirogyra species), and species assigned to another large taxonomic unit, the Euglenophyta,
are present in freshwater phytoplankton. Desmidiaceae (Closterium, Staurastrum,
Micrasterias) are found in acid bogs or oligotrophic lakes. Pyrrophyta (peridinians or
dinoflagellates) are also represented in freshwater (especially Ceratium species). The most important group (approximately 5,000 species) is Bacillariophyta, i.e. the diatoms such as
Cyclotella, Asterionella, Fragillaria, Melosira (Cole, 1983; Goldman and Horne, 1983;
Kalff, 2001).
2.2 Zooplankton
Freshwater
zooplankton
presents
a
remarkable
species
diversity.
Non-pigmented
(heterotrophic) protozoa, Zoomastigophora, which are representative of two orders of amoeba
(Diphlugia) and a great number of ciliates (Paramecium, Frontonia, Colpidium) are present in
all freshwater ecosystems and in wetlands. Rotifers (about 1,800 species) inhabit a wide range of
aquatic habitats, occurring in wetlands, in lotic system (rivers) and in lakes (in the water column
or associated with the sediments or vegetation from the littoral zone). Rotifers have a ciliated
corona at the anterior end of their body. The cilia serve for locomotion and for spinning food into
the mouth. The digestive tract contains a set of jaws to grasp food particles. Most species are
filter feeders (consuming algae or particulate organic matter) (Brachionus, Keratella, Notholca,
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Filinia), others are omnivores (Synchaeta, Hexarthra), or even predators (Asplanchna)
(Rudescu, 1960; Cole, 1983; Goldman and Horne, 1983; Kalff, 2001).
Larger zooplankton belong to the class Crustacea, organisms with jointed appendages and
the body enclosed in a protective exoskeleton made of chitin. One major group consists of filter
feeding organisms that use, with high efficiency, the particulate organic matter suspended in the
water column. These are the cladocerans (suborder Cladocera), known also “water fleas”
(Daphnia, Bosmina, Chydorus). There are also large carnivorous genera (Leptodora,
Polyphemus) that grasp their prey. The cladocerans, like rotifers, are parthenogenic with a short
life cycle under favourable conditions of temperature, food and photoperiod. Females (2n) produce unfertilized eggs (2n) that develop into females (2n). Consequently, rotifers and
cladocerans produce many generations each year and can rapidly increase in abundance under
favourable conditions. When conditions change, some eggs develop in males (which produce
haploid gametes) and females produce haploid eggs that require fertilization. A hard and resistant case covers the diploid eggs, making it possible for them to persist awaiting the return of
more favourable environmental conditions. The resistant eggs of cladocerans are sometimes
contained in a sculptured case known as an ephippium (Rudescu, 1960; Cole, 1983; Goldman
and Horne, 1983; Negrea, 1983; Kalff, 2001).
Copepods (class Copepoda) are the second group of freshwater zooplankton. Copepods
are dependent on sexual reproduction and therefore females and males occur in the population
(presenting a remarkable dimorphism). After hatching, a larva known as a nauplius emerges and,
after five or six moults, it becomes an immature form known as a copepodit. After another five
moults, the juvenile transforms into an adult. Copepods produce fewer generations per year than
cladocerans, and usually only one generation in temperate freshwaters. Copepods are divided
into the filter-feeding, herbivorous calanoids (Diaptomus, Calanipeda, Eurytemora), and the
cyclopoids which feed raptorially or occasionally by filtration (Cyclops, Acanthocyclops,
Eucyclops) (Damian-Georgescu, 1963, 1966; Cole, 1983; Goldman and Horne, 1983;
Kalff, 2001).
3. Macrophytes
Aquatic macrophytes may dominate the primary production in shallow lakes and in wetlands.
Most of them are flowering plants (angiosperms), but also aquatic ferns (Salvinia, Marsilea),
mosses (Sphagnum), liverworts (Marchantia) and even large pluricellular algae (Cladophora,
Chara) occur. Macrophytes are classified according to their habitat because they comprise such
a diverse taxonomic group (Box 2). The major division is based on their attachment by roots to a
solid substrate. Rooted macrophytes may have all or part of their vegetative and sexually reproductive parts above the water, i.e. emergent plants (Phragmites, Typha, Scirpus, Carex,
Sagittaria, Butomus, Nuphar, Nymphaea, Trapa, Urticularia, Vallisneria, Potamogeton,
Myriophyllum), or they may be completely submerged, i.e. submergent plants (Cladophora,
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Box 2 Criteria for benthic plant classification
Taxonomy
Macroalgae
Mosses
Liverworts
Ferns
Angiosperms
Cladophora, Chara
Sphagnum
Marchantia
Salvinia, Marsilea
Ecology
Rooted macrophytes
Non-rooted macrophytes
Attached to the substrata
emergent plants
Phragmites, Typha, Carex, Scirpus
vegetative and
reproductive parts
partially under water
Nuphar, Nymphaea, Trapa,
Butomus, Sagittaria, Utricullaria,
Vallisneria, Potamogeton, Myriophyllum
submergent plants
Elodea, Cladophora, Chara
submergent
Ceratophyllum
free floating plants
Salvinia, Lemna, Azola, Hydrocharis
periphyton
Chara, Elodea). There are also non-rooted submergent plants (Ceratophyllum). Free floating
plants are not fixed to the substrata (Salvinia, Hydrocharis, Lemna, Azolla). Also important are
the single-cell algae on the surface of macrophytes or rocky substrata (i.e. epiphytic algae which
form the periphyton). The most striking feature of an aquatic macrophyte assemblage is the distinct zonation observed from land to progressively deeper water (Figure 1) (Cole, 1983;
Goldman and Horne, 1983; Kalff, 2001).
Figure 1 Zonation of aquatic macrophytes (After: Cole, 1983; Goldman and Horne, 1983; Kalff,
2001)
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4. Major objectives of plankton and benthic flora studies
The study of phyto- and zooplankton is carried out in order to determine species diversity, the
adaptations for flotation in algae and in small invertebrates, spatial and temporal distribution of
organisms, feeding behaviours and interactions between predators and their prey (Box 3). To
investigate microalgae living at the bottom of an aquatic ecosystem is quite difficult and it requires special devices and sampling conditions. The macrophytobenthos includes the bottom
dwelling pluricellular algae and the aquatic macrophytes (pondweeds). Research on these
requires knowledge of biodiversity, adaptation to abiotic conditions and their contribution to the
whole food web of the ecosystem.
The study of plankton and benthic flora is also important for determining the health of the
aquatic ecosystem or wetland. Plants, both dead and alive, form a structural habitat for many
species to live and thrive in. Vegetation plays a major role in determining the boundary of the
wetland, and it contributes many different features to the wetland community. There is great
diversity of planktonic and aquatic plants species, with differing responses to human disturbance. Ecological tolerances are known for many species, and thus changes in the composition
of both planktonic and benthic communities might be used to diagnose the cause of the stress.
Because macrophytes are primarily immobile (apart from some floating aquatic species), they
can indicate long-term, chronic stress in a wetland (Kalff, 2001; Mitsch and Gosselink, 2007).
Aquatic plants, and especially the reed, mace reed and bulrush, serve as a filter system for
the aquatic ecosystem by taking up nutrients, and also contaminants, which may be present
(Friedrich et al., 1999).
Box 3 Objectives of plankton and benthic plant studies
To gain knowledge of biological diversity
To ascertain the adaptations for flotation by plankton and of pondweeds to the environmental conditions
To explain the food web in an ecosystem, identifying feeding behaviour and interactions
between predators and their prey
To determin wetland features and boundaries
To diagnose the health of the aquatic ecosystem or wetland
5. Methods of study
In order to determine the health of an aquatic ecosystem or of wetlands, a protocol must be established to determine the baseline conditions (Box 4). The baseline will then aid in monitoring
future wetland conditions. The design of a scientific study depends on the purpose of study. This
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involves identification and delimitation of scientific problems and objectives to be resolved and
consequently the spatial and temporal scale of the investigation. An important next step is to
draw a thorough sketch of the research zone: distinct features of the freshwater ecosystem or
wetland and surrounding area, different vegetative communities (i.e. grasses, rushes, reed plots,
sedges and willows). The sketch must also contain details of the sampling network (different
sample locations and photographs of sampling points) to ensure that samples and pictures are
taken from the same location when revisited. The site location must be determined by a Global
Positioning System (GPS) device. Sampling methods must be selected according to the scientific purpose, the features of the habitat, the investigator’s expertise and the associated cost. This
involves determining which abiotic parameters should be measured and the specific methods to
be used. It is useful to take both live and preserved samples. Formalin and alcohol are the most
common agents to fix and preserve living organisms. In the laboratory, primary analyses determine the qualitative structure of different populations of organisms (taxonomical diversity,
highlighting eventual invasive species) which makes it possible to establish the structure and dynamics of organism associations. Statistical consideration should be a central element in the
sampling design and in drawing conclusions. An important step is to standardise sampling and
evaluation methods (Skjoldal et al., 2000; Kalff, 2002).
Box 4 Protocol for scientific investigations of plankton in wetlands
Identification of scientific problems
Spatial and temporal scale of investigations
Drawing up of a thorough sketch of the research area
Deciding on sample locations and photographic points (points marked by GPS)
Selection of physical and chemical water parameters to be analyzed
Selection of sampling methods
Sampling in the field and taking of photographs
Determination of area covered by certain plants or plant associations
Taxonomical analysis of organisms (using identification keys)
Determination of quantitative structure of each species association
Creation of database
Statistical analysis of results from database
Drawing conclusions from information obtained above
Comparison and evaluation of global results by consulting references and metadata
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Phytoplankton samples can be collected using sampling bottles (with known water
volume) in order to perform quantitative studies, or with a sampling net (inefficient for very
small species) or a jug. Microscopic identification and counting is difficult and it is very important that living cells are compared with the preserved cells. Preservation can be done with iodine
(usually known as Lugol’s solution), formalin or alcohol. Observation and counting are performed using a microscope, in special settling chambers or on glass slide under a cover slip.
Using identification keys, the species of algae can be determined and, at the same time, their
morphological characteristics can be noted. Quantitative data are given as cells per litre (or cell
per cubic decimetre).
Zooplankton samples can be collected using traps (most utilised is Schindler-Patalas trap),
or nets for both horizontal and vertical tows. It is difficult to obtain representative animal samples from a known volume of water. Silk or nylon fabric with a determined mesh size (between
90 and 125–200 µm) is used to make zooplankton nets. Observations on live organisms are very
important, especially the protozoan, but are difficult to perform because of the swimming velocity of the larger organisms (crustaceans). Preservation can be done with formalin or alcohol.
In the laboratory, the collected material is concentrated into a determined volume and 1–5 cm3
(1–5 ml) samples are observed using a stereomicroscope (magnifying glass). In order to identify
organisms, individuals must be studied with the microscope, either intact or after the dissection
of their appendages. The zooplankton can be identified using identification keys, by taking notice of their morphological characteristics. Quantitative data are given as numbers of individuals
per cubic metre.
In order to study the benthic flora, boat-based and in-water sampling methods can be used.
From a boat, the plants can be collected by hand (wearing appropriate gloves), or using a dredge
or pitchfork. The in-water method involves the use of 0.1–0.5 m2 quadrate PVC frame deployed
by a diver in order to determine the spatial distribution of pondweeds. Alternatively, the diver
pulls a PVC bag over the submergent plant and removes it together with the rhizome and roots
(Parson et al., 2001). The plants must be observed as fresh material, transported to the laboratory
in a small quantity of water. At the same time, it is possible to observe invertebrates living on the
leaves. Therefore, it is recommended that transparent laboratory containers are used. It is important to notice the morphological characteristics of the plants and their adaptations to the environmental conditions (especially water velocity) and, using this information, plants can be
identified using identification keys. Quantitative data are given as grams per square metre.
Plants are gently dried in paper bags or in a fan oven at low temperature prior to weight determination. Any epiphytes (a plant that grows upon another plant) present must be observed with a
microscope in order to identify them and quantitative data are given as epiphyte plant weight per
support plant weight. Quantitative data for animals that live on vegetation are given as number
of individuals per gram wet weight of plants (Cole, 1983; Goldman and Horne, 1983; Sameoto et
al., 2000; Kalff, 2002; Mitsch and Gosselink, 2007).
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Usually, during field trips, students collect qualitative samples of phytoplankton, zooplankton (using a net for horizontal tows) and pondweeds. In some situations, quantitative
zooplankton samples can be collected using a net for vertical tows or by filtering water through a
sieve. In the laboratory, samples can be observed for morphological adaptations to the environmental conditions and they can be identified to their major taxonomical groups.
6. References
Cole, G. A. 1983 Textbook of Limnology, C.V. Mosby Company, Toronto.
Damian-Georgescu, A. 1963 Copepoda Fam Cyclopidae (forme de apã dulce) Fauna RPR,
Editura Academiei RPR, Bucureºti: 4(6).
Damian-Georgescu, A. 1966 Crustacea Copepoda Calanoida (forme de apã dulce) Fauna RPR,
Editura Academiei RPR, Bucureºti: 4(11).
Dussart, B. 1966 Limnologie : l’étude des eaux continentales, Gauthiers-Villars, Paris.
Friedrich, J., Dinkel, Ch., Grieder, E., Radan, S., Steingruber, S. and Wehrli, B. 1999 Delta lakes
as nutrient sinks – a process study in the Danube Delta. Geo-Eco-Marina, 4: 5–19.
Goldman, Ch.R. and Horne, A.J. 1983 Limnology, McGraw-Hill Book Company, Toronto.
Kalff, J. 2001 Limnology, Prentice Hall, New Jersey.
Mitsch, W., J. and Gosselink, J.,G. 2007 Wetlands, John Wiley & Sons, Inc., Hoboken, New
Jersey.
Negrea, St. 1983 Cladocera, Fauna R.S.R., Ed. Academiei RSR, Bucureºti, 4(12).
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San Francisco.
Parsons, J. K., Hamel, K. S., Madsen, J. D. and Getsinger, K.D. 2001 The use of 2,4-D for
selective control of an early infestation of Eurasian watermilfoil in Loon Lake, Washington. J.
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Pérès, J.-M. and Devèse, L. 1963 Océanographie biologique et biologie marine, Presses
universitaires de France, Paris.
Rudescu, L. 1960 Trochelminthes. Rotatoria, Fauna R.P.R. Editura Academiei RPR, Bucureºti,
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Sameoto, D., Wiebe, P., Runge, J., Postel, L., Dunn, J., Miller, C. and Combs, S. 2000 Collecting
zooplankton. In: Harris, R., Wiebe, P., Lenz, J., H.-R., Skjoldal and Huntley, M. [Eds] ICES
Zooplankton methodology manual, Elsevier.
Skjoldal, H.-R., Wiebe, P. H. and Foote, K.G. 2000 Sampling and experimental design. In: Harris,
R., Wiebe, P., Lenz, J., H.-R., Skjoldal and Huntley, M. [Eds] ICES Zooplankton methodology
manual, Elsevier.