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4.1 Water quality

Water bodies with medium and dense macrophyte cover are characterized by a low concentration of suspended sediments, hence high water transparency. Where aquatic macrophytes disappear, such as under the impact of eutrophication, water transparency is reduced. In lakes where macrophytes reappear after a period of absence, water transparency gradually improves with the increasing vegetation cover (Fig. 14). The success of prey capture by fish which locate their prey visually, such as pike, depends on good water transparency. Pikeperch tolerates a lower water transparency than pike.

Fig. 14

Fig. 14. Turbidity (mean June-September) and percentage lake area covered by submersed macrophytes Chara tomentosa and Potamogeton pectinatus in Lake Krankesjon 1984–1990. (From Hargeby et al., 1994).

Several of the chemical parameters influenced by plants that have been observed to reach lethal limits for common freshwater fish are dissolved oxygen, acidity and ammonia. Frodge et al. (1987, 1995) measured dissolved oxygen and pH values in patches of Brasina scherberi in the northwest of the USA and found their values ranging from above (in open water) to below (in the patches of the macrophyte) lethal limits for rainbow trout (Oncorhynchus mykiss) within 10 m of each other in the same lake. Rainbow trout in cages positioned at 1 m depth below Nymphaea odorata, where dissolved oxygen concentration was consistently less than 4 mg L-1, were found dead within 12 hours. Significant mortalities of largemouth bass occurred at both the surface and at 1 m depth in dense beds of the floating leaved macrophyte Brasenia schreberi, and in the bottom water of 2 m or more depth of both floating leafed and submersed species, where dissolved oxygen concentrations were less than 2 mg L-1. The authors suggested that high densities of aquatic plants can have significant detrimental localized effects on fish and reduce the amount of habitat available to fish within the system. Perna and Demilio (1986) found that dissolved oxygen concentrations in the Ticino River, Italy, with submersed aquatic plants, changed by up to 4.0 mg L-1 between day and night. A trout farm, receiving water fom this river, experienced difficulties as a result of the temporary low oxygen concentrations.

Dissolved oxygen content in water may substantially change between day and night when large stands of aquatic macrophytes are present. This may result in substantial diel movement of fish within the plant bed to avoid low dissolved oxygen concentrations in the morning and high pH in the afternoon (Killgore et al., 1991).

In many Amazonian floodplain lakes dissolved oxygen concentrations are often very low and anoxia is frequent at least in deeper waters. Lago Camaleao in Brazil is situated about 15 km above the confluence of the Amazon River with Rio Negro. During low water, the lake is dry, apart from a small, muddy pool of 1–2 ha of 50–80 cm depth (Junk et al., 1983) and the dry lake bottom is covered by terrestrial and semi-aquatic herbaceous plants, which produce considerable amount of organic material. When the water level rises, the lake becomes flooded, the terrestrial plants die and decompose, while aquatic plants develop in great quantities, with grasses being the most abundant among them. When some of the aquatic plants start dying, about 60% of the lake is still covered by the grass Echinochloa polystachia, and floating plants Eichhornia crassipes and Salvinia auriculata become abundant. The highest levels of dissolved oxygen have been recorded in the lake when there are still large open water spaces, but a heavy oxygen deficit is present when there is a large-scale plant decomposition in progress. This may reduce the values at the water surface to 0.5 mg L-1. An increase in dissolved oxygen concentration is noticed with the reduction in the macrophyte cover. Junk et al. (1983) describe the consequences of the low dissolved oxygen concentrations in this lake: the fish is forced to expend much energy and time in the search for oxygen, for instance coming to the surface to swallow air, or staying near the surface to use the oxygen-rich surface layer. This may increase the danger of being caught by predators. Food diversity is considerably reduced because phytoplankton and zooplankton become scarce. The bottom fauna is lacking because of the lack of oxygen. The authors also list the advantages of the low dissolved oxygen concentrations among aquatic macrophytes of Lake Camaleao: interspecific competition for food is reduced, as only a small number of fish species utilizes the existing food sources; and there is a reduction in predators. The presence of many juvenile fish under hypoxic conditions may be related to this factor. In Lake Camaleao, about 40 species of fish were able to live under extremely low oxygen conditions, 12 of which were air breathers, and 8 possessed other adaptations to the oxygen deficiency. (See also Section 2). For example, Colossoma macropomum, is known to be able to increase its haemoglobin content and erythrocyte counts associated with seasonal hypoxia. This species initiates aquatic surface respiration when dissolved oxygen level falls below 0.5 mg L-1 (Saint-Paul, 1984). Jedicke et al. (1989), who recorded diurnal changes in dissolved oxygen concentration beneath floating macrophytes in a varzea lake, showed that while the maximum concentrations exceed 12 mg L-1 in the afternoon hours, in the early morning hours the water is virtually deoxygenated.

Junk (1973) measured vertical oxygen profiles in stands of Leersia hexandra in Lago dos Passarinhos (25 km from Manaus, Brasil). The profile was similar to that measured in dense stands of Paspalum repens of varzea: high dissolved oxygen concentrations at the surface, rapid decrease into the roots, and a slight increase at the lower margin caused by an inflow of oxygenated water from open areas. Within 20 cm depth the dissolved oxygen concentration dropped to less than 0.5 mg L-1, and H2S sometimes formed at a depth of 30 to 140 cm. In a dense, floating island in Lago Paru, 10 m inside the mat of Paspalum repens, the dissolved oxygen concentration dropped from 6–8 mg L-1 at the surface to almost nil in a depth of 20 cm, to increase to about 3 mg L-1 at 30 cm and deeper. Junk (1973) also noted that low conductivity of water is not a limiting factor for faunal abundance in the floating meadows of the varzea, until it falls below 30 μS20.

In the reservoir Bung Borapet in Thailand, Junk (1977) recorded dissolved oxygen profiles underneath stands of several aquatic macrophytes (Fig. 15). The lowest concentrations were measured 10 m inside the stand of Coix aquatica, where there was no oxygen present already at 10 cm depth. Very low concentrations were also measured underneath Leersia hexandra and Ischne globosa, while underneath water hyacinth the lowest dissolved oxygen concentration, measured above the bottom, was 2.5 mg L-1. All four plants form floating islands which drift around the reservoir.

Fig. 15

Fig. 15. Vertical profiles of dissolved oxygen concentrations within the stands of four aquatic macrophytes in reservoir Bung Borapet, Thailand. (From Junk, 1977).

Hypoxia is widespread in tropical fresh waters, particularly in floodplain pools and permanent swamps. It is prevalent in extensive papyrus swamplands (Petr, 1973), underneath sudd (Junk, 1977; Junk and Robertson, 1997), in deeper waters of the newly established reservoirs (Viner, 1970), and it may pose a formidable barrier to the dispersion of many fish groups. While some non-air breathing fish may occasionally penetrate in the almost deoxygenated papyrus swamps of East Africa, the fish inhabitants there are primarily air breathers such as Protopterus aethiopicus, Polypterus bichir, Clarias lazera, Ctenopoma muriei (Beadle, 1981). In a small cyprinid Barbus neumayeri, not an air-breathing fish, the respiratory mode affected its dispersal through hypoxic papyrus swamps (Chapman and Liem, 1995). This fish, which feeds on insect larvae and aquatic plants, was found to inhabit swamp waters with dissolved oxygen concentrations between 0.34 and 1.24 mg L-1, but always in places with at least 0.6 m2 of open water surface where it has access to the water surface and higher oxygen. In response to severe hypoxia it uses aquatic surface respiration at the air-water interface, where diffusion produces a very thin layer of well oxygenated water, despite oxygen scarcity in the rest of the water column. Chapman and Liem (op.cit.) also found significant differences in the gill morphology between B. neumayeri from the papyrus swamp and those from the main river, thus affirming the need for a morphological adaptation, as well as having access to the open water within the swamp.

But high water quality is not a necessary prerequisite for optimal fish production, unless fishery managers are to satisfy a demand for fish species that can exist only in oligotrophic waters, such as salmonids and coregonids. With most cyprinids, the higher the trophic level, the higher fish production - to a certain level. This approach has been used in common carp ponds in Europe and Asia. On the other hand, where sport fishery targets fish species in clean rivers, any deterioration of water quality results in the reduction or loss of such species.

4.2 Eutrophication and aquatic macrophytes

Eutrophication has become a problem in many countries. De Nie (1987) has briefly reviewed long-term changes in aquatic macrophytes of selected water bodies under the impact of eutrophication in Finland, Germany, the Netherlands, Poland, Switzerland, UK, and the USA. For example in some small water bodies in the Netherlands, the area covered by submersed vegetation declined by a factor 6. In the polish lake Mikolajskie (460 ha) over the period 1963–1980 the decline in the biomass of aquatic macrophytes was 16.6% per year, which equalled to 94% over the entire period. In five eutrophicated lakes of Finland the submersed vegetation had greatly decreased or became restricted to small areas. Also in Germany the eutrophication influenced especially the submersed macrophytes, and in Lake Constance the depth of occurrence of the Characeae changed from a maximum of 25 m in the 1950s to 5–7 m in 1978. In shallow lakes of the Norfolk Broadlands, palaeolimnological study has shown that starting with the 19th century Strumpshaw Broad was rich in Characeae and epiphytes, with productive fisheries and rich in wildlife (Moss, 1990). In the beginning of the 20th century the Characeae were replaced by taller macrophytes, such as Potamogeton pectinatus, Myriophyllum spicatum and Ceratophyllum demersum. During that period the eutrophication started by sewage effluents from the nearby city and cultivation and fertilization of the surrounding land. The fisheries was still productive. This was followed by changes towards the middle of the 20th century, and especially afterwards: by the early 1970s aquatic macrophytes disappeared and phytoplankton started to predominate. The fisheries became less productive and there was an increase in the erosion of the banks and in sedimentation.

Inputs of nitrogen and phosphorus especially in shallow freshwater bodies have led to considerable changes in the aquatic community structure, enhancing phytoplankton production, which in turn has caused suppression or complete disappearance of aquatic submersed macrophytes. Such changes have also meant changes in aquatic invertebrate and fish species composition and biomass. Based on their experiments inducing artificially eutrophication using high nitrate treatment, Daldorphy and Thomas (1991) identified three possible mechanisms which may be responsible for the rapid transition to phytoplankton dominance: (i) competition by phytoplankton for resources such as light and nutrients, where high phytoplankton density attenuates the photosynthetically active radiation impinging on macrophytes; (ii) inhibition of macrophyte growth by toxic factors of algal and bacterial origin. During algal blooms, considerable quantities of organic matter may accumulate in the sediment. The addition of such material greatly reduces the growth of submersed macrophytes, possibly due to the phytotoxic organic acids released during decomposition; (iii) inhibition of macrophytes by epiphytic algae.

4.3 Biomanipulation

The term “biomanipulation” was used by Shapiro et al. (1975) to include lake improvement procedures that alter the food web to favour grazing on algae by zooplankton, or that eliminate fish species that recycle nutrients. Cooke et al. (1988) expanded this term to the elimination of certain species or restructuring of the fish community to favour the dominance of piscivorous instead of planktivorous fish. In broad terms, biomanipulation is the management of aquatic communities by controlling natural populations of organisms to produce desired conditions. Lazzaro (1997) refers to biomanipulation as a set of deliberate modifications of certain key communities, and/or their habitats, to improve water quality/clarity and/or (less often) fisheries characteristics. Van Donk (1998) uses this term in a sense of man-made modification of fish populations. The topic of biomanipulation has been widely covered in a symposium on biomanipulation, held in 1990 (Gulati et al., 1990). A Guide to the Restoration of Nutrient-enriched Shallow Lakes (Moss et al., 1996) also includes a chapter on Biomanipulation, and the topic is a subject of a special FAO fisheries publication, to be published shortly. Numerous papers exist on this topic, especially discussing the long-term observations on Dutch shallow lakes, especially Lake Zwemlust. For the purpose of the present publication only the biomanipulation component of using fish to achieve improvement of water quality, and hence the return or enhancement of aquatic macrophytes, is discussed.

The principal objective of biomanipulation in shallow lakes is to generate a period of clear water of sufficient length to allow macrophytes to establish (Perrow et al., 1997). Both a relative increase in the piscivorous fish stock and a reduction in nutrient levels are thought to be important in stabilising the system in the long term. With increase in lake trophy, the area occupied by and biomass of submersed macrophytes decreases, and in hypertrophic lakes aquatic plants are scarce or absent (Ozimek et al., 1990). Disappearance of macrophytes leads to other changes in the aquatic communities. Highly eutrophic to hypertrophic lakes have a characteristically high phytoplankton production and in Europe large stocks of plankton-feeding fish, such as bream (Abramis brama) and roach (Rutilus rutilus). To improve the water quality, as well as to establish a higher fish species diversity, including fish of fishery significance, a technique of biomanipulation has been developed. Biomanipulation aims at decreasing the phytoplankton biomass, therefore increasing water transparency, which in turn leads to re-establishment of aquatic macrophytes. The macrophytes provide then a substrate for periphytic fauna and lead also to an increase in bottom-dwelling invertebrates, both of which diversify the source of food for fish.

In a shallow and small Lake Zwemlust in the Netherlands, biomanipulation involved complete removal of all planktivorous and benthivorous fish species (ca. 1000 kg ha-1), introduction of a new simple fish community (pike and rudd) as well as of “seedlings” of Chara globularis and roots of Nuphar lutea. While before the manipulation no submersed vegetation was present in the lake, the second year after the manipulation about 50% of the lake bottom was covered by macrophytes dominated by Elodea nuttallii, increasing to 80% in the third year (Van Donk et al., 1990). The expansion of macrophytes was possible due to the drastic reduction in phytoplankton, especially cyanobacteria. Other changes resulting from the biomanipulation was development of strong populations of zooplankton and benthic/epiphytic macrofauna, especially snails Lymnaea peregra, and a pike/rudd fish community. In summer and autumn the abundance of macrophytes and filamentous algae led to nitrogen limitation of the phytoplankton. The persistence of clear water in the summer after the biomanipulation was believed to be caused by macrophytes, rather then by zooplankton grazing on algae. Van Donk (1998) followed the changes in Lake Zwemlust for 10 years and noted that after the initial biomanipulation the lake shifted several times between the turbid and the clearwater state. In years 4–5 Ceratophyllum demersum became the dominant macrophyte. Both Elodea and Ceratophyllum compete strongly with phytoplankton for nutrients, especially nitrogen. In the next three years, these plants became nearly absent, and Potamogeton berchtoldii became the dominant species in spring. However, periphyton growth on this plant led to its early collapse, which was followed by an increase in phytoplankton and the lake became turbid during late summer and autumn. In 1995 E. nuttallii reappeared and became again the dominant macrophyte. Next year, it became scarce and Potamogeton abounded, and the lake shifted again to the turbid state (Van Donk, 1998). She concluded that the presence of submersed macrophytes seems to be essential in keeping this lake clear. However, she added that herbivory, especially by coots (Fulica atra), and to a lesser extent by rudd, was probably an important factor in triggering the shift from perennials (i.e.Elodea and Ceratophyllum) to non-perennials (Potamogeton). The number of larger rudd (only larger rudd are herbivorous) was quite low until 1990 but increased in 1991 to 297 kg ha-1, to stabilize at 200–300 kg ha-1 during 1992–1996. As rudd feeds only when water temperature exceed 16°C, and avoids Ceratophyllum, Van Donk (1998) suggested that it had much lower impact on total macrophyte biomass than coot.

The biomanipulation of Lake Zwemlust by re-establishment of macrophytes and filamentous algae has created a habitat for periphytic fauna and changed the environmental and trophic conditions for fauna inhabiting the bottom sediments. This was achieved mainly by changing the physical properties of bottom sediments and the water, especially water transparency and the dissolved oxygen concentration. Zoobenthos achieved a higher biomass than phytofauna, except in submersed plants, especially Elodea nuttallii, which occupied the majority of the bottom surface (Kornijow et al. (1990). Filamentous algae (e.g. Mougeotia sp. and Oedogonium sp.) were readily eaten live by chironomid larvae and during the algal decay in autumn, when sedimenting, also by the benthic Lymnaea peregra, Asellus meridianus and Gammarus pulex.

In order to improve lake water quality in Lake Vaeng in Denmark, a total of 2.5 tons of bream and roach were removed, reducing the biomass of planktivorous/benthivorous fish by approximately 50% (Sondergaard et al., 1990). Again, there was a change from cyanobacteria and small diatoms to a dominance of large diatoms, larger greens and cryptophytes, change in zooplankton from the dominance of rotifers to large cladocerans, and increase in water transparency and dissolved oxygen content. Submersed macrophytes, mainly Potamogeton crispus and Elodea canadensis, increased in abundance. In the following years, Elodea coverage fluctuated. This was paralleled by the fluctuation of aquatic birds coot and mute swans, with their high numbers recorded only in years with high macrophyte density (Sondergaard et al., 1998). A similar parallel between the density of aquatic macrophytes and coot were also observed on another Danish lake Stigsholm. The grazing intensity in Lake Vaeng is suspected to inhibit colonization of the lake by aquatic plants. In both Danish lakes the results of the Sondergaard et al. (1998) studies suggest that waterfowl may suppress macrophyte biomass and development in shallow lakes, and thus have a negative impact on some lake fish.

In a large eutrophic lake in Minnesota, USA (Lake Christina) the water was highly turbid prior to biomanipulation (summer transparency less than 30 cm), virtually devoid of macrophytes, with large stocks of planktivorous fish bluegill (Lepomis macrochirus), small yellow perch (Perca flavescens), and big-mouth buffalo (Ictiobus cyprinellus). Benthivorous black bullheads (Ictalurus melas) were also abundant (Hanson and Butler, 1990). The zooplankton community was typical of those subjected to intense planktivory by fish, being dominated by small-bodied species of cladocerans, by Cyclops bicuspidatus thomasi, and several rotifers. The fish removal by rotenoning, which led to virtually all fish being killed, followed by restocking, resulted in a much improved water transparency and dramatic changes in zooplankton, phytoplankton, and in submersed vegetation (Chara, Najas, Ruppia and Myriophyllum) revival. Unlike other lakes, in Lake Christina the post-manipulation increases in water clarity and submersed macrophytes were caused by the increases in zooplankton grazing, with the peak daphnid abundance in late May-June. These months are the critical growth period for submersed macrophytes.

In Europe, pike and pikeperch have been used to suppress the small zooplanktivorous fish such as bream, roach and perch. In fully overgrown lakes, a high cyprinid (<15 cm) production as high as ca. 300 kg ha-1 can be consumed by a high pike biomass (100 kg ha-1) (Grimm and Backx, 1990). Such a high production of planktivorous cyprinids is associated with a total phosphorus concentration of ca 400 μg P L-1. At a higher biomass of cyprinids the large-bodied zooplankton disappears, phytoplankton increases, causing a decrease in the macrophytes area, and leading to the dominance of Potamogeton pectinatus and plants with floating leaves. This in turn leads to changes in fish stocks, with the pike-tench associations being replaced by bream-pikeperch populations. There is an increase in benthic-feeding fish, which in turn increases the internal loading and turbidity. Grimm and Backx (1990) concluded that in shallow eutrophic lakes of northern Europe aquatic vegetation and northern pike are effective tools to maintain water quality, but these are limited by the maximum nutrient concentration aquatic vegetation can sustain.

Pikeperch is more efficient then pike in turbid water, therefore, in a more eutrophicated environment (Moss et al., 1996). Addition of piscivores, however, will not have a lasting effect unless they are stocked every year. Therefore, removal of the fish stocks and maintaining them at very low levels for a number of years gives better effect. The available data suggest that in phytoplankton-dominated lakes, the total fish biomass will be between 200 and 500 kg ha-1. Where only half, or somewhat more of thek biomass has been removed, there has been no real improvement (Moss et al., 1996).

Bronmark and Weisner (1992) highlighted the indirect effects of the fish community on submersed vegetation in shallow eutrophicated lakes: eutrophication and the accompanying phytoplankton and periphyton dominance shade off the macrophytes leading to their demise. This leads to decline of piscivorous fish species, and an increase in phytoplankton feeding fish. Return from the phytoplankton-dominated state depends on an increase in piscivore stocks, either through restocking or growth of the small surviving piscivores. The resulting decrease in phytoplankton biomass and increase in light availability will promote the reestablishment of submersed macrophytes. In reality, this should be accompanied by substantial reduction of the planktivorous fish stocks, to achieve good results (Moss et al., 1996; Van Donk and Otte, 1996).

A study of lakes in Florida has shown that leaving a small fringe of vegetation around a lake for the purpose of water quality improvement will have little or no effect on the lake's trophic status (Canfield and Hoyer, 1992). Significantly reducing macrophyte coverage of a lake, for example from 60% to 20% or from 40% to 0% will cause major and observable water quality (water clarity) changes. Conversely, the cover must be raised to 30% to 50% before significant improvement in water clarity will be observable.

The general contention of biomanipulation is that removal of fish from shallow lakes may invoke “cascading impacts” that enhance water clarity, stimulate macrophytes, and establish a new steady state. Recent, large-scale biomanipulations have made it possible to update earlier recommendations regarding when, where, and how biomanipulation should be performed. Hansson et al. (1998) formulated more applicable recommendations as follows: (1) the reduction in the biomass of planktivorous fish should be 75% or more; (2) the fish reduction should be performed efficiently and rapidly (within 1–3 years); (3) efforts should be made to reduce the number of benthic feeding fish; (4) the recruitment of young-of-the-year fish should be reduced; (5) the conditions for establishment of submersed macrophytes should be improved; and (6) the external input of nutrients (phosphorus and nitrogen) should be reduced as much as possible before the biomanipulation. Recent biomanipulations have shown that, correctly performed, the method also achieves results in large, relatively deep and eutrophic lakes, at least within 5 years. Moss et al. (1996) provide decision trees for biomanipulation and for aquatic macrophytes establishment.

Lazzaro (1997) addressed the question whether trophic cascades and ‘classical’ biomanipulation apply to the tropics. The ‘trophic cascade hypothesis’, developed for temperate systems, does not seem to be a good candidate for tropical lakes, for several reasons, among which is its typical linear structure from piscivores → zooplanktivores → zooplankton → phytoplankton, which disregards the widespread omnivory over size-selective zooplanktivory per se. In direct reference to fish Lazarro mentions the seasonality of spawning in temperate waters compared to the spawning of tropical fish, which may spawn at different times, therefore juvenile fish maintaining a permanent pressure on zooplankton. Filtering omnivores (not visual zooplanktivores) often dominate the planktivore community in tropical waters, without being limited by the presence/absence of zooplankton, nor controlled by fish predators as because of their fast growth they soon reach size larger than the predator's mouth gape. Tropical waters have smaller, ‘sedentary’ carnivorous fish, unlike temperate waters where large-sized piscivores dominate. Macrophytes in tropical waters grow faster and all the year. They can provide serious management problems, as their proliferation is not controlled in eutrophic systems, and they represent a highly variable and unstable sink or source of nutrients. Concluding, Lazzaro says that the above characteristics of tropical lakes and reservoirs weaken the top-down links between piscivores and planktivores, and between zooplankton and phytoplankton. Because of their weakness, there is little chance for a successful biomanipulation.

4.4 Aquatic macrophytes as wastewater treatment system, and fish

Lakes and ponds bordered by swamps and marginal vegetation are widely exploited for municipal, industrial and agricultural wastewater treatment. Aquatic macrophytes may also be used for the removal of aquaculture effluents. This assists in maintaining the water quality in the water body concerned at an acceptable standard for fish in natural water bodies and reservoirs.

In the 1970s the treated effluent from the often overloaded Kampala (Uganda) main sewage works entered the papyrus along the north shore of Lake Victoria, and its distinct chemical signature was completely erased after it passed through a kilometer of swamp into the lake (UNDP/WHO, 1970). The use of the large emergent species Schoenoplectus lacustris and Phragmites australis in Europe, and Eichhornia crassipes in USA, for treating industrial and urban effluents has been mentioned by Finlayson and Mitchell (1982). Water hyacinth prefers eutrophic conditions, and an increase of nitrogen level from one to 25 mg L-1 in water increases their dry weight and plant number linearly (Chadwick and Obeid, 1966). Dense submersed macrophyte beds with their associated epiphytic algae are considered in some areas to be useful nutrient filters for absorption of nitrogen and phosphorus (Howard-Williams, 1981).

Use of constructed wetlands for surface-flow effluent treatment in the USA and Canada was initiated by a series of pilot-scale trials at Arcata in northern California, Gustine in southern California, Orlando and Lakeland in Florida, Listowel and Port Ferry in Ontario, Canada (Gaerheart, 1992). In the Netherlands constructed wetlands are applied to intermittent flows such as campsites, or sewage disposal from small communities.

Aquatic macrophytes have been also successfully used for removal of nutrients originating from fish production. In the experimental use of macrophytes for removal of nutrients originating from fish production (Oreochromis aureus) tanks, water hyacinth was the fastest growing plant in the wastewater treatment tank and removed the greatest amount of nitrogen per square metre of growing area (Rakocy and Allison, 1981). The submersed Egeria densa was also an efficient plant in nitrogen removal, while Vallisneria sp. grew slowest and removed the lowest amount of nitrogen. The combined plant population, consisting of two floating and two submersed plants, removed 12 to 15.5% of the waste nitrogen in the wastewater treatment tank. More information on the removal of nutrients from fish culture using aquatic plants is the subject of another FAO publication.

Wastewater discharge to wetlands is often expressed in litres per square meter per day, which is termed the hydraulic loading rate (HLR). In a study carried out by Schwartz and Boyd (1995) a free water surface wetland was constructed adjacent to a commercial fish pond used to raise channel catfish (Ictalurus punctatus), and its efficiency in removing potential pollutants from pond water was evaluated. Two experimental wetlands were constructed in series, with the first receiving the effluent directly from the pond, being planted with Scirpus californicus and Zizaniopsis miliacea, and the other - receiving the effluent from the first one - planted with Panicum hemitomon. The HLR were 77–91 L m-2 day-1. The concentrations of potential pollutants were much lower in the effluent from the wetlands than in the inflows from ponds, with the following reductions in concentrations recorded: total ammonia nitrogen, 1–81%; nitrite-nitrogen, 43–98%; nitrate-nitrogen, 51–75%; total Kjeldahl nitrogen, 45–61%; total phosphorus, 59–84%; biochemical oxygend demand, 37–67%; suspended solids, 75–87%; volatile suspended solids, 68–91%; and settleable solids, 57–100%. Overall performance of the wetland was best when operated with a 4-day hydraulic residence time (HRT) in the vegetative season, but good removal of potential pollutants was achieved for shorter HRTs and when vegetation was dormant. The disadvantage of wetlands for treating aquaculture pond wastes is the large amount of space necessary to provide an adequate HRT. Wetlands also could be used for treating overflow from ponds after rains. The experiments have also shown that S. californicus and Z. miliacea provided much thicker stands than P. hemitomon, with the S. californicus being the fastest growing plant. Hence the use of S. californicus alone or together with Z. miliacea would provide for quicker plant establishment and higher standing crop in wetlands than the combination used in the study.

Wetlands have also been used on a limited scale for the removal of heavy metals. However, total elimination of toxic materials, such as heavy metals, may not be achieved. Copper mining in the foothills of the Ruwenzori Mountains, Uganda, has been a source of heavy metals from the leaching dumps and tailings, with the toxic metals reaching a swamp along the northern shore of Lake George (Fig. 16). The succession of plants ends at the lakeward edge with floating rafts of papyrus. In spite of some removal of the heavy metals, fish caught at the swamp edge have distinctly higher concentrations of copper in their liver compared with fish from the outflow from Lake George (Kazinga Channel) and those from the nearby Lake Edward. As no elevated concentrations of heavy metals in the fringing aquatic macrophytes have been found, Denny et al. (1995) suggested that a proportion of metal-enriched water and sediments must pass through the swamp. The highest concentrations are in the phytoplankton and the floating Pistia stratiotes near the swamp fringe. Elevated copper concentrations were also found elevated in zooplankton and in the livers of juvenile tilapias (O. leucostictus), which fed on it.

To learn more on the use of natural and constructed wetlands for wastewater treatment in Europe, the reader is advised to consult other publications, such as Hamer (1990), Moshiri (1994), Vymazal et al. (1998) and Vymazal (1999). For constructed wetlands for the treatment of landfill leachates see the publication by Mulamoottil et al. (1998).

4.5 Intensification of fish production in fish ponds, and aquatic macrophytes

Fish ponds in southern Bohemia (Czech Republic) are a classic example of the impact of human activities on aquatic environment, as manifested by changes in aquatic plants and fish production in these water bodies. The fish ponds, constructed in the 14th to 16th centuries on poor soils, had initially oligotrophic to dystrophic character, as their water originated from large peat bogs. Such ponds had a high aquatic macrophyte species diversity, high water transparency, and well developed periphyton and benthos, grazed and consumed by molluscs, insect larvae and fish. The dissolved oxygen concentration was high and almost constant, about 100 % saturation (Pokorny et al., 1990). Fish production in such fish ponds was very low. Intensification of agriculture in pond catchments, which started in the 1930s, the input of manure and fertilizers directly into the ponds, and in some cases also provision of fish feed, have led to the establishment of eutrophic and even hypertrophic conditions. This was accompanied by increasing fish production, but also by increasing fluctuations in pH and dissolved oxygen concentrations (Pokorny et al., 1992). In nutrient-rich ponds well-developed periphyton started shading off the macrophytes leading to their decline. Within twenty years the vegetation typical of oligo-mesotrophic waters disappeared, being replaced in the inshore areas by Potamogeton perfoliatus, tolerating the eutrophicated environment (Fig. 17).

Fig. 16

Fig. 16. Lake George, Uganda. A major wetland in the north receives a river with a heavy loading of heavy metals. (From Greenwood, 1976).

Fig. 17

Fig. 17. Disappearance of the aquatic vegetation typical for the Czech oligo-mesotrophic fishponds during the process of intensification of fish production. (From Husak and Krahulec, 1994).

The submersed macrophytes completely disappeared from the deeper water of fish ponds (Photos 2 and 3).


PHOTO 2: Fish pond in southern Bohemia, Czech Republic.


PHOTO 3: Harvesting fish pond in southern Bohemia, Czech Republic. The harvested fish is dominated by common carp Cyprinus carpio.

In summer there was a strong development of phytoplankton, which was dominated by chlorococcal algae, and this added to the shading effect, further reducing water transparency. Such ponds are suitable for carp culture, with fish production reaching up to 1 000 kg ha-1, but they have low fish species diversity, especially being short of predators. With virtually no permanent aquatic macrophytes that would help to stabilise the aquatic community, pond managers have to keep the fish pond ecosystem in a steady state ensuring high fish production on the one hand and reasonable water quality on the other, as fish ponds in southern Bohemia serve also other functions, such as recreation.

The Czech fish ponds are an example of a situation where macrophytes are considered redundant, as these ponds are directed toward fish production along the shortest possible food chain, i.e. phytoplankton-zooplankton-fish (Pokorny et al., 1990). In eutrophic/hypertrophic fish ponds the major threat to the fish stocks comes from the highly variable and sometimes hazardous environment. Such conditions, common in densely stocked and fertilised or manured fish ponds, can be tolerated by a few fish species such as the large-sized common carp, but less so by fry or fingerlings. Pokorny et al. (1990) suggest that in order to improve the ecological conditions for fish, and especially for the fry, it would be useful to reconsider the role of aquatic macrophytes as a component stabilising the fish ecosystem. The main stabilising properties of aquatic macrophytes in fish ponds are their relatively slow biomass turnover and high net production, their relatively high uptake of nutrients which remain fixed in their biomass or detritus for some time and may be harvested when required, and the provision of shelter for young fish. Phytophilous aquatic macroinvertebrate fauna is an important component of the fish diet, and some waterfowl feed intensively on some macrophytes or their parts (see also Section 3.3 - birds). The detritus accumulating in macrophyte stands binds mineral nutrients and organic matter at times of their abundance in the fish ponds and releases them gradually according to the dissolved oxygen pattern and diffusion in the sediments. The detritus serves as a source of food for benthos and also for filtrating zooplankton when algal production is low. Detritus is also the most important substrate for microbial decomposers.

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