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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 2–940153–87–6 244 NEAR curriculum in natural environmental science 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. Plankton and benthic flora 245 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, 246 NEAR curriculum in natural environmental science 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, Plankton and benthic flora 247 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) 248 NEAR curriculum in natural environmental science 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 Plankton and benthic flora 249 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 250 NEAR curriculum in natural environmental science 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). Plankton and benthic flora 251 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). Nybakken, J., W. and Bertness, M., D. 2005 Marine biology; an ecological approach, Pearson, 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. Aquat. Plant Manage, 39: 117–125. 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, 2(2). 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.