2.1. MACROPHYTES
2.2. MICROPHYTES
2.3. THE PHYSICAL AND CHEMICAL HABITAT CHARACTERISTICS
2.4. AQUATIC VEGETATION AND INVERTEBRATES
2.5. AQUATIC MACROPHYTES AND FISH
Aquatic plants do not belong to one distinct taxonomic group but rather form a collection of many plant taxa. The term "aquatic macrophytes" is commonly used for all macroscopic forms of aquatic vegetation; it includes macroscopic algae (stoneworts and the alga Cladophora), some ferns and mosses (pteridophytes) and many flowering plants (angiosperms). On the basis of their emergence or submergence and the manner of attachment or rooting in the bottom sediment, two main groups, with 3 subdivisions are commonly distinguished (Wetzel, 1975).
i. Emergent aquatic macrophytes
Reed is often found in monospecific stands, but also mixed with Typha spp., Scirpus lacustris , Acorus calamus, Dseudacorus, Butomus umbellatus and Sagittaria sagittifolia. Emergent macrophytes are rooted in the sediment and may grow to a water depth of ca. 1 m. During the growing season all members of this group produce aerial leaves and flowers.
ii. Floating-leaved aquatic macrophytes
The floating-leaved plant communities are often predominated by Nymphaea spp., Nuphar lutea and Nymphoides peltata. Potamogeton natans and Polygonum hydropiper also belong to this group. The floating-leaved plants may root in water depths up to 3 m. and have floating or aerial flowers (reproductive organs).
iii. Submersed macrophytes
This group includes the stoneworts (charophytes) Chara and Nitella, a few moss species like Fontinalis antipyretica and many flowering plants e.g. Myriophyllum spicatum, Elodea nuttallii, Potamogeton pectinatus and E. perfoliatus. The submersed macrophytes complete their life cycle under the water surface. Some species cause "nuisance growth". The degree of nuisance depends on the pursued management aims (water transport, recreation, fishery management or nature conservation) in the water body.
All aquatic macrophyte species mentioned in this review are listed in Table 1. A few species and some terms are visualized in Fig. 1.
The phytoplankton consists of a large assemblage of microscopic algae. These plants freely move in the water column. Some species become buoyant after dying most algae sink to the water bottom. There are many taxonomic groups of phytoplankton algae and their systematics are very complex. Many species of the freshwater planktonic algae belong to the extremely diverse chlorophytes (including also the macrophytes like Chara). Other important taxonomic groups
Table 1. The aquatic macrophytic species as referred in the text. Sm = submersed; fl = floating-leaved; em = emergent; ter = terrestial; B = not rooted in the sediment
Charophytes (stoneworts) |
-. Hydrocharitaceae |
||
Chara spec. |
sm |
Hydrocharis morsus-ranae L. |
B/fl |
Nitella spec. |
sm |
Stratiotes aloides L. |
B/fl |
Nitellopsis spec. |
sm |
Elodea canadensis Michx. |
B/sm |
Elodea nuttallii (Planchon) St. John |
B/sm | ||
Bryophytes (mosses) |
Hydrilla verticillata (L. f.) Royle |
B/fl | |
Fontinalis antipyretica L. |
sm |
Vallisneria americana Michx |
B/sm |
Pteridophyta (ferns and allies) |
-. Potamogetonaceae ("pondweeds") |
sm | |
Azolla spec. ("duckweed") |
B/fl |
Potamogeton compressus L. |
sm |
Isoetes spec. |
sm |
Potamogeton crispus L. |
sm |
Potamogeton densus L. |
sm | ||
Spermatophyta (flowring plants) |
Potamogeton filiformis pers. |
sm | |
-. Polygonaceae |
Potamogeton lucens L. |
sm | |
Polygonum amhibium L. |
Ter/fl |
Potamogeton natans L. |
fl |
Polygonum hydropiper L. |
fl |
Potamogeton obtusifolius L. |
sm |
Potamogeton pectinatus L. |
sm | ||
-. Nymphaeceae ("water-lilies") |
Potamogeton perfoliatus L. |
sm | |
Nuphar lutea (L.) |
fl |
Ruppia spec. |
sm |
Nymphea alba L. |
fl |
Zannichellia palustris L. |
sm |
Nymphea candida presl. |
fl |
Zostera marina L. |
sm |
-. Ceratophyllaceae ("Hornworts") |
-. Najadaceae |
||
Ceratophyllum demersum L. |
B/sm |
Najas marina L. |
sm |
-. Ranunculaceae |
-. Iridaceae |
||
Ranunculus circinatus Sibth. |
sm |
Iris pseudacorus L. |
em |
-. Onagraceae |
-. Gramineae |
||
Epilobium hirsutum L. |
Ter. |
Glyceria maxima (Hartmann) Holmberg |
em |
Phragmites australis (Cav.) Trin. Ex steudel |
em | ||
-. Haloragaceae |
-. Araceae |
||
Miriophyllum spicatum L. ("water milfoil") |
sm |
Acorus calamus L. |
em |
Hippuridaceae |
-. Lemnaceae (duckweeds") |
||
Hippuris vulgaris L. |
sm |
Lemna trisulca L. |
B/fl |
Lemna minor L. |
B/fl | ||
-. Primulaceae |
|||
Hottonia palustris L. |
em |
-. Typhaceae ("cattails") |
|
Typha latifolia L. |
em | ||
-. Menyanthaceae |
Typha angustifolia L. |
em | |
Nymphoides peltata (Gmel.) O.Kuntze |
fl |
||
-. Cyperaceae |
|||
-. Lentibulariaceae |
Scirpus lacustris L. |
em | |
Utricularia vulgaris L. |
B/sm |
Carex acuta L. |
em |
-. Alismataceae |
|||
Sagittaria sagittofolia L. |
em |
||
-. Butomaceae |
|||
Butomus umbellatus L. |
em |
Fig 1. Schematic representation of the zonation in the aquatic vegetation and major terms used in this report
that contain phytoplankton algae are (among others) the diatoms (Bacillariophyceae) and the blue-green algae (cyanophytes or Myxophyceae). Blue-green algae usually occur in colonies; many species can fix atmospheric nitrogen, like some bacterial species. They taxonomically differ much from the other groups of algae, like green algae and diatoms. Many detailed works deal with the identification of phytoplankton species; for an introduction to functional and ecological aspects see Golterman (1975) and Wetzel (1975).
The microphytes growing on the bottom sediments, stones, submersed macrophytes and other objects in the water column are called periphyton. If only growing on the submerged parts of the aquatic macrophytes, this periphyton is also referred as "epiphytes". Often the entire commun;ty of m;croscop;c algae (with many diatom species), bacteria, protozoa and detritus on the submerged surfaces of the leaves is meant. The macrophytes benefit the epiphytes by providing organic carbon and nutrients (Allen, 1971). The presence of epiphytes may protect the leaves from grazing by invertebrates like snails, crustaceans and insect larvae, because not the plant itself is consumed but the epiphytes are scraped off (Carpenter & Lodge, 1986).
The aquatic macrophytes strongly affect the physical environment in the water. Within stands of aquatic vegetation the light intensity quickly decreases with depth, although great differences exist in the degree of light attenuation between specs (Titus & Adams, 1979). Not only the light regime but also the temperature in plant stands differs from open water sites. The vertical temperature gradient within vegetation may be very steep. So there are great differences between the surface temperature (where locally tropical temperatures can be reached) and the water layer just above the sediment (Grosch, 1978; Carpenter & Lodge, 1986)
Aquatic macrophytes influence the water movement and the sedimentation of particulate mineral and organic matter because the water plants suppress water turbulence. Suspended small particles settle faster to the bottom sediment in quiet water.
Aquatic macrophytes change the chemical properties of the water. Because green plants produce oxygen they contribute to the oxygen concentration in the water. Pokorny & Rejmankova (1983) measured a net oxygen production up to 5.7 mg/l daily in dense Ceratophyllum demersum stands in small fish ponds. Oxygen is not only released in the water column. Many plant species have air spaces in their tissue, in which photosynthetically produced oxygen is transported by diffusion. In this way the plants transport oxygen to their roots. Subsequently the oxygen is often released in the sediment (Carpenter & Lodge, 1986).
On the other hand, aquatic macrophytes may also indirectly cause oxygen depletion. Pokorny & Rejmankova (1983) found decreased oxygen concentrations in duckweed-covered water, because the plants reduce the light penetration into the water. Respiration, not compensated by oxygen production during night time. and the decay of macrophytes will directly take oxygen from the water. Usually the main part of the decay will take place in autumn, when the water temperature is low and the oxygen demand of invertebrates and fish is slight. However. in abnormal circumstances. especially in eutrophic, warm, shallow, poorly circulated waters, high amounts of oxygen will be derived from the water column (Godshalk & Wetzel, 1978; Carpenter & Greenlee, 1981).
Rooted submersed and floating leaved macrophytes form a living link between sediment and water column. Nutrients can be transported from the sediments to the water: the f1oating-leaved plants like Nymphaea spp. and Nuphar lutea for instance potentially can function as an important nitrogen and phosphorus "pump" (Brock et al. 1983). However, all (aquatic) macrophytes may also act as a sink and can immobilize nutrients. During periods of active growth e.g. Potamogeton pectinatus and the attached epiphytes can act as a sink for phosphorus (PO4-P) (Howard-Williams, 1981)
This internal nutrient cycle is basically a continuous process during the growing season. On the one hand aquatic macrophytes actively excrete nutrients and organic substances; on the other hand nutrients and organic substances are passively released in the water (Pomogyi et al., 1984). Nutrients are released when living parts of the plants are damaged by animals, or by means of autolysis, leaching and microbial breakdown. The role of muskrats (Ondatra zibethicus), birds (e.g. coots, Fulica atra), insects and crustaceans in decomposition of Nymphoides peltata is extensively studied (Lammens & van der Ve1de. 1978; van der Velde et al., 1982; Wallace & O'Hop, 1985). In laboratory investigations the loss rates of nutrients from N. peltata showed the following order: K>Na>P>Mg>C>N>Ca>Fe (Brock et al., 1983; Brock, 1984). Very local environmental circumstances. for instance the grazing activity of small animals. may influence this decomposition process, but field studies on Nuphar lutea revealed that the decompostion of the leaves was also strongly dependent on the alkalinity of the water body (Brock et al., 1985).
The residence time of nutrients in the biomass of living organisms depends on the length of their life cycle. This is several months for aquatic macrophytes. which is nearly always intermediate between the residence time in plankton (several days or weeks) and in fish (several years) (Carpenter & Lodge, 1986).
The emergent aquatic macrophytes are an interface between the surrounding land and the water. They are, to a greater degree than the rooted submersed and floating leaved aquatic macrophytes. influenced by the quality of the groundwater and the composition of the soil of the surrounding land (Verhoeven, 1983). Emergent aquatic macrophytes can remove nutrients from the water column. For instance reed (Phragmites) beds may remove nitrogen and (PO4)-phosphorus from the water. The phosphorus is adsorbed to the sediments and the nitrogen is released from the sediment as ammonia or nitrogen gas due to denitrification by bacteria (Kickuth. 1978). During the decompostion of Phragmites leaves, the released nutrients occur initially in the water but are successively adsorbed to the sediment (Best et al., 1982).
Summary: the aquatic vegetation strongly influences the light conditions. temperature. oxygen concentration. sedimentation rate and turbulence in the water body. The submersed and floating leaved macrophytes have a central position in the process of internal recycling of materials in the aquatic ecosystem. Whereas the emergent aquatic macrophyte can reduce the external nutrient loading.
Invertebrates are the main food source for most fish species. The quantity of invertebrate organisms is related to the amount of aquatic vegetation (Murphy & Eaton. 1981; Dvorak & Best. 1982; Whitefield. 1984). The invertebrates use the macrophytes as substrate. shelter and habitat for feeding. The (American) crayfish. Orconectes limosus occurs mainly in the aquatic vegetation. Juvenile animals feed on filamentous algae, but also vascular plants. Larger individuals (>25 mm) are omnivorous and also consume molluscs. Grosch (1978) reported a dramatic decline in the catches of this crayfish. when the aquatic vegetation in the Heiligensee (West-Berlin)
Fig.2. Annual catch (in numbers) in the Berlin area of the American crayfisch (Orconectes limosus Raff.) (Unpublished data by courtesy of dr. Ulrich Grosch)
decreased (Fig. 2). The macrophyte itself is rarely consumed. but the epiphytes on the surface of the leaves and other submerged parts of the plants are consumed. Excessive growth of epiphytes is harmful to the macrophyte. because insufficient quantities of light and inorganic carbon (HCO3- and CO2) can reach the plant and consequently the macrophyte photosynthesis is hampered (Sand-Jensen. 1977; Sand-Jensen & Sondergaard. 1981). Therefore the amount of epiphytes is an ecologically important factor which. in its turn. is strongly dependent on invertebrate grazing (Orth & van Montfrans. 1984). Hootsmans & Vermaat (1985) found under experimental conditions a five fold increase in growth of Zostera marina plants in the presence of grazing snails in comparison with non-grazed plants.
Recently the relations between snails and aquatic macrophytes have been investigated in more detail. Bronmark (1985b) found high correlations between the species diversity of plants and gastropods (snails) within a number of eutrophic ponds. Lodge & Kelly (1985) showed a rapid recovery of the Lymnae peregra population concomitant with the come-back of submersed macrophytes after their sudden disappearance.
The aquatic macrophytes provide shelter to "large" cladoceran zooplankton species (Leah et al. 1980; Timms & Moss. 1984). The grazing activity of zooplankton benefits the smaller edible phytoplankton species that grow faster because of more efficient nutrient uptake. If the larger zooplankton species become scarce (for whatever reason) the phytoplankton community will change to slow-growing bigger, mainly unedible blue-green species. These bigger. often colonial blue-green phytoplankters compete more easily in a nutrient-rich environment with the small edible species. They form a high population biomass with chlorophyll-a concentrations of several hundreds ~g/l. Because high chlorophyll concentrations hamper the light penetration. the conditions for submersed aquatic macrophytes will become worse (Moss. 1976; Moss & Leah. 1982; Moss et al. 1985).
Many fish species of the temperate zone need sites with macrophyte stands in the spawning season to deposit their eggs. For successful hatching the oxygen concentrations have to be sufficient. while after emerging from the eggs. the larvae need substrate and oxygen. The oxygen supply for the larvae is maximal close to the surface of the living leaves. Young fish also need the plants to protect themselves from predation by piscivorous species (Grosch. 1978) or to avoid cannibalism (Grimm. 1981). Several indigenous adult fish species of , the Northwestern and Middle Europe like tench (Tinca tinca), rudd, (Scardinius eryhtrophthalmus) and pike (Esox lucius) live in stands of emergent and floating-leaved aquatic macrophytes. The submersed aquatic macrophytes form important feeding habitats for perch (Perca fluviatilis), roach (Rutilus rutilus), bleak (Alburnus alburnus) and eel (Anguilla anguilla), while the ruff (Gymnocephalus cernua) prefers the Chara beds (Table 2). Some species need relatively soft submerged plants to eat: especially rudd and roach consume considerable amounts of macrophytes. But also in the diet of tench, bream (Abramis brama), white bream (Blicca bjoerkna), carp (Cyprinus carpio), chub (Leuciscus cephalus), ide (l. idus), crucian carp (Carassius carassius), gudgeon (Gobio gobio) and stickleback (Gasterosteus aculeatus) small or minimal amounts of plant material was found (Prejs, 1984).
Table 2. Preferred zones in North-German lakes by adult fish species. 1: Eulittoral (zone with emergent aquatic macrophytes). 2: Littoral (zone with floating-leaved and submersed aquatic macrophytes). 3: Lower littoral (zone with scattered Chara-beds). 4: Littori-profundal (no or very few living aquatic macrophytes). 5: Pelagial (open water zone) (after Grosch, 1978)
fish species |
zone in the lake | ||||
1 |
2 |
3 |
4 |
5 | |
Anguilla anguilla (eel) |
+ |
+ |
+ |
||
Osmerus eperlanus (smelt) |
+ | ||||
| Esox lucius (pike) | + |
||||
Abramis brama (bream) |
+ |
||||
Alburnus-alburnus (bleak) |
+ | ||||
Blicca bjoerkna (white bream) |
+ |
||||
Carassius auratus gibelio (gibel carp) |
+ |
||||
Carassius carassius (crucian carp) |
+ |
||||
Cyprinus carpio (carp) |
+ |
||||
Gobio gobio ( gudgeon ) |
+ |
||||
Leucaspius delineatus (German: Moderlieschen) |
+ |
||||
Leuciscus cephalus (chub) |
+ |
||||
Rhodeus ser;ceus (b;tterling) |
+ |
||||
Rutilus rutilus (roach) |
+ |
||||
Scardinius erythrophthalmus (rudd) |
+ |
||||
Tinca tinca (tench) |
+ |
||||
Misgurnus fossilis (weatherfish) |
+ |
||||
Silurus glanis (wels or European catfish) |
+ |
+ |
|||
Lota lota (burbot) |
|||||
Gasterosteus aculeatus (st;ckleback) |
+ |
||||
Gymnocephalus cernua (ruffe) |
+ |
||||
Perca fluviatilis (perch) |
+ |
+ |
+ |
+ |
+ |
Stizostedion lucioperca (pikeperch) |
+ | ||||
Traditionally. the abundant occurrence of reed is not considered as a desirable situation for the exploitation of fish populations (Grosch, 1978). However, Deufel (1978) stressed the importance of the reed swamps and other emergent vegetation for fish populations in lake Constance (Bodensee) (Table 3). There are seven species in his list which are of commercial importance.
The fish can directly influence the dispersion of the plants. Carpenter & McCreary (1985) showed that the nesting behaviour of three sunfish species (the American Perciformes: Lepomis gibosus, Micropterus sa1moides and M. dolomieui) strongly influenced the zonation of aquatic macrophytes. The nest sites of these fishes are firstly cleared of vegetation by the fish. This vegetation spreads vegetatively via horizontal stems and mainly consists of a Myriophyllum-species. In August, after use, the nest sites are colonized by diaspore-propagated plant species (among others an Isoetes- species) which do not spread vegetatively. Every following spawning Season about 20% of the original nest sites are not occupied.
Table 3. List of fish species in Lake Constance, temporarily living in the zone of emergent vegetation. 1. Feeding habitat; 2. Spawning area; *. Commercially important species (Deufel. 1978)
| Fish species | function of emergent vegetation | |
1 |
2 | |
Anguilla anguilla (eel) * |
+ |
|
Esox lucius (pike) * |
+ | |
Abramis brama (bream) * |
+ | |
Blicca bjoerkna (white bream) |
+ | |
Carassius carassius (crucian carp) |
+ | |
Cyprinus carpio (carp) * |
+ | |
Gobio gobio ( gudgeon ) |
+ |
+ |
Leuciscus cephalus (chub) |
+ | |
Leuciscus leuciscus (dace) |
+ | |
Phoxinus phoxinus (minnow) |
+ |
|
Rhodeus sericeus (bitterling) |
+ | |
Rutilus rutilus (roach) * |
+ |
+ |
Scardinius erythrophtalmus (rudd) * |
+ |
+ |
Tinca tinca (tench) * |
+ |
+ |
Misgurnus-fossilis (weatherfish) |
+ |
+ |
Noemacheilus barbatulus (stone loach) |
+ |
+ |
Silurus glanis (wels or European catfish) |
+ | |
Gasterosteus aculeatus (stickleback) |
+ | |
Cottus gobio( bullhead ) |
+ |
|
Patches of a mixed vegetation arises on these abandoned nest sites of Myriophyllum (recolonizing the site from the edges) and the diaspore propagated plant species. These patches can persist several years. Thust the basses actively contributed to the creation of a mosaic pattern of different aquatic plant species.
Among others Savino & Stein (1982) have shown that moderately complex habitat structures are important for young fish to hide and feed and that they will produce more stable prey-predator relations between fry and piscivorous fish. Crowder & Cooper (1982) proved that for bluegill sunfishes (Lepomis macrochirus) in experimental ponds, a habitat structure with intermediate macrophyte density is favourable for the fish as well for the prey organisms. In this situation they found the highest growth rates for the fish and more variation in its diet.
Grimm (1981, 1983) studied the effect of stocking artificially propagated young pike (Esox lucius) on the natural pike population in a number of water bodies in the Netherlands. He concluded that the presence of aquatic plants and not stocking with young piket determined the abundance of adult pike. In vegetated areas the survival of young pikes was higher than in unvegetated waters. Intraspecific predation (cannibalism) was the most important factor that regulated the abundance of adult pike. Stocking had no effect on the composition of the adult pike population. In a Florida lake in the presence of a coverage by 75% of Hydrilla verticillata also a higher survival was also found for the young of the year of Esox niger and some other important American sportfishes (Shireman et al., 1983).
Stein & Magnuson (1976) found that the feeding behaviour of the crayfish Orconectes propinquus in the U.S.A. is influenced by the presence of predators like the smallmouth bass, Micropterus dolomieui. The crayfishes will hide more often in the substrate and spend less time on grazing. So the basses indirectly affect the feeding relations between plants and crayfishes.
Predation by young of the year fish of smelt (Osmerus eperlanus), perch (Perca fluviatilis), roach (Rutilus rutilus) and bream (Abramis brama) can affect the body size and biomass of the zooplankton community (Hrbàcek et al., 1961; van Densen, 1985 and many others). This predation on the "larger" zooplankton can affect the phytoplankton community because the most important phytoplankton grazers disappear. The unedible biggert colonial algal species start to dominate (see Section 2.4.2.). Therefore the removal of fish from an eutrophic lake may cause clearer water and the return of rapidly-growing small phytoplanktonic species (Andersson et al. 1978; Leah et al. 1980; Reinertsen & Olsen, 1984; Brabrand et al. 1986).
Moss et al. (1979) mentioned a decline in catches in the Norfolk Broads and Rivers after a dramatic decrease of the aquatic vegetation. The perch (Perca fluviatilis) and gudgeon (Gobio gobio) declined, probably due to decreased habitat diversity.
At the beginning of the 20 th century about 28 truly indigenous species occurred in the Havel lakes (West-Berlin). In 1965 these species were still present. However, in 1979 6 species became extinct among which barbel (Barbus barbus), dace (Leuciscus leuciscus) and weatherfish (Misgurnis fossilis). About 9 species were almost extinct or sporadically found; among them were: smelt (Osmerus eperlanus), chub (Leuciscus cephalus), rudd (Scardinius erythrophthalmus), gudgeon (Gobio gobio), wels (Silurus glanis) and burbot (Lota lota). Among the 5 species that became rare, pike (Esox lucius), crucian carp (Carassius carassius) and tench (Tinca tinca) were mentioned. These changes probably were connected with the decrease in aquatic plants (see also Section 2.5.1. and Table 1). On the other hand bream (Abramis brama), white bream (Blicca bjoerkna), perch (Perca fuviatilis), ruff (Gymnocephalus cernua) and pikeperch (Stizostedion lucioperca) increased. Only the eel (Anguilla anguilla) -due to intensive stocking- and the bleak (Alburnus alburnus) remained constant since 1965. The pikeperch catches increased considerably (Fig. 3). This species usually stays in the pelagial zone and requires bare sediment for spawning (Grosch, 1978, 1980).
Fig. 3. Annual yield (in tonnes) of pikeperch (Stizostedion lucioperca) in the Berlin area (Unpublished data. by courtesy of dr. Ulrich Grosch, 1978)
Large areas with submersed Potamogeton disappeared from the hypertrophic Tjeukemeer between 1971 and 1981. De Nie (1987) produced evidence for a strong decrease in the availabi1ity of gammarids and worsened feeding conditions for eel (Anguilla anguilla) since that time. Lammens (1986) observed an increased abundance of pikeperch and a decrease in biomass of roach (Rutilus rutilus). perch (Perca fluviatilis) and ruff (Gymnocephalus cernua) in the Tjeukemeer during 1977-85 in comparison with the previous six years (1971-77). Lammens et al., (1986) showed that the three decreased fish species are more adapted to foraging in a 1ittora1 habitat (see a1so Tab1e 2. Section 2.5.1.).
Eutrophication caused in Lake Ontario (Canada) an expklosive growth of the filamentous algae Cladophora sp. and a collapse of the walleye (Stizostedion vitreum) population. Adverse effects on egg hatching of salmonids and coregonids were reported too. The (introduced) carp (Cyprinus carpio) and (native) yellow perch (Perca flavescens) populations increased because of the effects of eutrophication (Oster, 1980).
Price et al. (1985) reported a negative effect of eutrophication on the striped bass (Morone saxatilis) population In the Chesapeake Bay area (U.S.A. ). Although presented as a "speculative hypothesis", they argue that the surface area with sufficient oxygen has dramaticly decreased and so the hatching area of this fish species. This consequently endangers the well-being of the entire population.
Summary: Fish species need oxygen and most species also need substrate for their eggs and larvae. Young and adult fish need oxygen, food and shelter. The aquatic vegetation must be present to satisfy these needs, which differ in space and time. Therefore not only the presence of a few water plant species, but a diverse aquatic vegetation is essential to maintain a diverse fish fauna. Fish are an important part of a complex network of relations between nutrients, phytoplankton, epiphytes, grazers and aquatic macrophytes: directly because of their predation on herbivorous zooplankton and larger invertebrate organisms in aquatic vegetation, and indirectly because of piscivorous predation on planktivorous (small) fish.
