Three years of experience in biomanipulating a small eutrophic lake: Lago di Candia (Northern Italy)

Results obtained from a step by step approach to the biomanipulation of a natural lacustrine environment (Lago di Candia, Northern Italy) are presented. Since the diversion of the municipal sewage of the small town of Candia, runoff and precipitation have been the sole contributors of nutrient to the lake. Fish population is mainly characterized by rudd (Scardinius erythrophthalmus) overstocking and by a low density of large-mouth-bass (Micropterus salmoides) and pike (Esox lucius). During 1986 about 12t of rudd (1–2 year old) were removed from the lake. Considering 1986 as ‘control year’, average Secchi disc transparency improved from 2.3 m in 1986 to 3.3 m in 1988; phytoplankton biovolume decreased from 114 to 58 mm³ l⁻¹ but zooplankton biovolume increased from 8 to 11.5 mm³ l⁻¹. The results achieved show that a grodual biomanipulation treatment can have a satisfactory outcome, and has the advantage of not producing catastrophic situations either in the biotic or in the abiotic compartments of the lake.     

Contribution of omnivorous tilapia to eutrophication of a shallow tropical reservoir: evidence from a fish kill

SUMMARY 1. We examined whether a large stock of tilapia (>750 kg ha^?1, in littoral areas >1300 kg ha^?1), mostly Oreochromis niloticus (L.) and Tilapia rendalli (Boulenger), could contribute to the eutrophication of a tropical reservoir (Lago Paranoá, Brasília, Brazil) by enhancing P‐loading. 2. We took advantage of an extensive fish kill (>150 tons removed) during May–August 1997 in a hypereutrophic branch of the reservoir to compare water quality characteristics 1 year before and after this event by means of BACI statistics. We also measured P‐excretion rates in laboratory trials to assess the P‐loading of the reservoir by the tilapia relative to tributary inputs and loading from a sewage treatment plant. 3. Concentrations of chlorophyll a (decline from 84 to 56 μg L^?1, P=0.018) and total P (decline from 100 to 66 μg L^?1, P < 0.001) decreased significantly in the branch of the reservoir affected by the fish kill, compared with a similar but unaffected branch that served as a control. Because P‐loading by both a sewage treatment plant and tributaries remained high after the incidence, the fish kill was likely to contribute to the observed water quality improvement. 4. Removing 150 tons of dead tilapia corresponded to 20 days of external total phosphorus load (TP‐load) to the branch, and resulted in a reduction of 5.1 kg P day^?1 in internal recycling via tilapia excretion, which is equivalent to 12% of the external TP‐load. 5. Implementing professional tilapia cast‐net fisheries could be an efficient biomanipulation approach to improve water quality and limit the occurrence of cyanobacteria blooms and fish kills in hypereutrophic branches of Lago Paranoá and similar tropical lakes.     

Some Evolutionary Arguments about what maintains the Pelagic Interval in Reef Fishes

In this paper, we address the question of the nature of evolutionary forces that account for the oceanic planktonic larva period. Putting the emphasis on coral reef fishes, we first present the most common theories and hypotheses, looking at them critically from the standpoint of individual selection and taking into account new data arising from genetic surveys. We concluded that each individual hypothesis based on short term advantages of a larva period cannot convincingly explain by itself the long term maintenance of such a complex life cycle. We then study the impact of the pelagic interval on species dispersal by compiling data sets from the literature. Following simple analysis of this data, we found that duration of the planktonic larva period drives gene flow in the Great Barrier Reef and colonisation throughout the Pacific. In speculating on the real nature of the short term selective forces responsible for the maintenance of the pelagic interval, we acknowledge the fact that long term constraints alone will not withstand erosion if they are not reinforced by some sort of short term mechanism. We tentatively arrive at the conclusion that these short term forces may be different from what could be expected from functional studies of planktonic life.     

Experimental study of the impacts of planktivorous fishes on plankton community and eutrophication of a tropical Brazilian reservoir

We examined the impacts of three facultative planktivorous fishes, Congo tilapia (Tilapia rendalli), bluegill (Lepomis macrochirus) and tambaqui (Colossoma macropomum), and an obligate planktivorous fish, silver carp (Hypophthalmichthys molitrix) on plankton community and water quality of a tropical eutrophic reservoir, Paranoá Reservoir, Brasília, Brazil, conducting both laboratory selective grazing experiments and an enclosure experiment. The first two species inhabit this reservoir and the remaining two are recommended for introduction. The field experiment was performed in ten limnocorrals (2 m³ each) and lasted five weeks. During the enclosure experiment, silver carp suppressed copepod nauplii, cladocerans and rotifers while the presence of tilapia and bluegill were associated with increased rotifers density. The dominant blue-green algae,Cylindrospermopsis raciborskii (98% of phytoplankton biomass) was enhanced in the presence of bluegill, tilapia and tambaqui, but reduced in the presence of silver carp. This impact on plankton is in agreement with the results of the laboratory feeding trials. The observed alterations in water quality parameters in fish limnocorrals are discussed in relation to plankton community and eutrophication of this ecosystem. It is suggested that the control of the undesirable algaeC. raciborskii directly by silver carp grazing is a promising management tool.     

Nitrogen Fuelling of the Pelagic Food Web of the Tropical Atlantic

We estimated the relative contribution of atmosphere (ic Nitrogen (N) input (wet and dry deposition and N fixation) to the epipelagic food web by measuring N isotopes of different functional groups of epipelagic zooplankton along 23°W (17°N-4°S) and 18°N (20-24°W) in the Eastern Tropical Atlantic. Results were related to water column observations of nutrient distribution and vertical diffusive flux as well as colony abundance of Trichodesmium obtained with an Underwater Vision Profiler (UVP5). The thickness and depth of the nitracline and phosphocline proved to be significant predictors of zooplankton stable N isotope values. Atmospheric N input was highest (61% of total N) in the strongly stratified and oligotrophic region between 3 and 7°N, which featured very high depth-integrated Trichodesmium abundance (up to 9.4×10⁴ colonies m^-2), strong thermohaline stratification and low zooplankton δ¹⁵N (~2‰). Relative atmospheric N input was lowest south of the equatorial upwelling between 3 and 5°S (27%). Values in the Guinea Dome region and north of Cape Verde ranged between 45 and 50%, respectively. The microstructure-derived estimate of the vertical diffusive N flux in the equatorial region was about one order of magnitude higher than in any other area (approximately 8 mmol m^-2 d ¹). At the same time, this region received considerable atmospheric N input (35% of total). In general, zooplankton δ¹⁵N and Trichodesmium abundance were closely correlated, indicating that N fixation is the major source of atmospheric N input. Although Trichodesmium is not the only N fixing organism, its abundance can be used with high confidence to estimate the relative atmospheric N input in the tropical Atlantic (r² = 0.95). Estimates of absolute N fixation rates are two- to tenfold higher than incubation-derived rates reported for the same regions. Our approach integrates over large spatial and temporal scales and also quantifies fixed N released as dissolved inorganic and organic N. In a global analysis, it may thus help to close the gap in oceanic N budgets.     

Implications of Food Web Interactions for Restoration of Missouri Ozark Glade Habitats

Ozark glades are gaps in forested areas that are dominated by grasses and forbs growing in rocky, nutrient‐poor soil. Historically, these open, patchy habitats were maintained by natural and anthropogenic fire cycles that prohibited tree encroachment. However, because of decades of fire suppression, glades have become overgrown by fire‐intolerant species such as Eastern red cedar (Juniperus virginiana). Current restoration practices include cutting down invasive cedars and burning brush piles, which represent habitat for Northern fence lizards (Sceloporus undulatus). Because Sceloporus actively consumes herbivores, we hypothesized that the presence of these lizards in and around brush piles might result in a trophic cascade, whereby damage on native plants is reduced. Field surveys across six Missouri glades indicated that lizard activity was minimal beyond 1 m from habitat structures. This activity pattern reduced grasshopper abundance by 75% and plant damage by over 66% on Echinacea paradoxa and Rudbeckia missouriensis near structures with lizards. A field transplant experiment demonstrated similar reductions in grasshopper abundance and damage on two other glade endemic species, Aster oblongifolius and Schizachyrium scoparium. These results demonstrate that future glade restoration efforts might benefit from considering top‐down effects of predators in facilitating native plant establishment.     

Food web manipulation in a small,eutrophic Lake Wirbel,Poland: effect of stocking with juvenile pike on planktivorous fish

In a four year experiment (1988–1991) carried out in a shallow, eutrophic lake, an increase in young-of-the-year pike (Esox lucius) density was used to control planktivorous fish assemblages consisting mainly of small-sized cyprinids: roach (Rutilus rutilus), Leucaspius delineatus, and white bream (Blicca bjoerkna). Stocking with pike fry of ca 30 mm in three successive springs resulted in large-scale mortality among prey of vulnerable sizes. Rotenone treatment at the end of the fourth year showed that roach and white bream populations contained few 0+ to 2+ fish, but very high numbers of 3+ to 6+ fish. By this time, short-lived, slow-growing L. delineatus, with no size refuge from predators, had been driven almost to extinction.     

Selective feeding by three native North American freshwater mussels implies food competition with zebra mussels

The response of a benthic macroinvertebrate assemblage to whole-lake biomanipulation was studied in a small Finnish mesotrophic lake. From 1993 to 1997, over 200 kg ha^–1 of fish, mainly roach (Rutilus rutilus (L.)) and bream (Abramis brama (L.)) were caught and the fish biomass was reduced by nearly 80%. The biomass and density of benthic invertebrates were investigated during the years of fish removal and for the following three years. The decrease in benthivorous fish stock led to a higher biomass and density of all major groups of benthic invertebrates during the early years of fish removal. Non-biting midges (Chironomidae), water mites (Hydrachnellae), mayfly nymphs (Ephemeroptera), sphaeriid clams (Bivalvia: Sphaeridae) and biting midges (Ceratopogonidae) seemed to respond most profoundly to changes in fish biomass. The biomass of most invertebrate groups correlated negatively with the catch-per-unit-effort (CPUE). The total biomass and density of invertebrates had strong negative correlations with the CPUE (r= -0.85, p = 0.016, r = -0.84, p = 0.019, respectively), but they did not correlate significantly with total phosphorus, chlorophyll a, or temperature. However, the variation in total biomass that was not explained by the CPUE, was significantly associated with total phosphorus.The fish stock recovered to almost its initial level within three years after fish removal had been discontinued. As an apparent response to increased predation pressure, the biomasses of many invertebrate groups decreased again in the years 1999–2000. The strong relationship between macrozoobenthos and fish populations in the studied lake is likely to be a consequence of the open and sparsely vegetated bottom, which offers minimal shelter to invertebrate prey. An additional factor behind the recent low biomass levels may be changes in primary production. Phosphorus and chlorophyll a concentrations started to decrease markedly after three years of fishing and they have remained at a low level.     

Food web manipulation in Lake Zwemlust: Positive and negative effects during the first two years

The hypertrophic Lake Zwemlust, a small water body used as a swimming pool, was characterized by algal blooms in summer, reducing the Secchi disk transparency to less than 0.3 m. Since in The Netherlands a Secchi disk transparency of 1 m is obligatory for swimming waters, corrective measures were called for to improve the light climate of the lake. In March, 1987, as an experiment, the lake was drained by pumping out the water to facilitate fish elimination. Planktivorous and benthivorous fish species, which were predominant, were removed by seine- and electro-fishing. After the lake had refilled by seepage it was restocked by a new simple fish community comprising pike (Esox lucius) and rudd (Scardinius erythrophthalmus) only. Stacks of willow twigs (Salix) and macrophytes (roots ofNuphar lutea and seedlings ofChara globularis) were introduced into the lake as spawning grounds and refuges for the pike against cannibalism and as shelter for the zooplankton. The effects of this food web manipulation on the light climate, phytoplankton, zooplankton, fish, macrophytes, macrofauna and on the nutrient concentrations were monitored during 1987 and 1988. In summer 1987, despite of high nutrient concentrations, the phytoplankton density was low, due to control by zooplankton, causing a Secchi disk transparency of 2.5 m, the maximum depth. Chlorophyll-a concentrations were low (<5 g Chl.l^–1), blooms of cyanobacteria did not occur and a shift from rotifers to cladocerans took place. In 1988, however, also some negative effects were noticed. Macrophytes and filamentous green algae reached a much higher biomass (50–60% cover of the lake bottom) than in 1987; some species, growing through the entire water column, interfered with the lake's recreational use. Associated with the macro-vegetation and possibly with the absence of larger cyprinids, the diet of which also comprises snails, a large scale development of the snail population, among themLymnaea peregra var.ovata took place. This species is known to act as an intermediate host of the bird-parasitizing trematodeTrichobilharzia ocellata, the cercariae of which cause an itching sensation at the spot of penetration of the human skin, accompanied by rash (schistosome dermatitis or swimmers' itch); in July, 1988, about 40% of the bathers complained about this itching. A positive effect of the macrophytes and filamentous green algae was the high uptake of nitrogen, resulting in a low nitrogen concentration in the lake and growth limitation of the phytoplankton population by nitrogen in the summer of 1988. In 1988 the cladocerans were abundant in April only; and unlike in 1987, in the summer of 1988 there was a shift from cladocerans to rotifers. Therefore, only in early spring (April) zooplankton grazing controlled phytoplankton growth and in summer nitrogen limitation was the major controlling factor, keeping chlorophyll-a concentrations low.     

Energy Content of Antarctic Mesopelagic Fishes: Implications for the Marine Food Web

For a better understanding of the role of mesopelagic fish in the Southern Ocean food web, the energy and water content of Bathylagus antarcticus, Electrona antarctica and Gymnoscopelus braueri from the Lazarev Sea were investigated. Mean dry weight energy content of B. antarcticus (20.4 kJ g⁻¹) was significantly lower than in E. antarctica and G. braueri (both 29.4 kJ g⁻¹). In E. antarctica, an increase of dry weight energy density with age was evident from 26.9 kJ g⁻¹ in juveniles of less than 1 year of age to 32.0 kJ g⁻¹ in 3-year-old fish. Water content decreased with size in all three species. Abundant high-energy species such as E. antarctica are at a key position in the food web. Due to a marked influence of age on energy content, population structure can be an important variable in estimates of energy fluxes in the Southern Ocean ecosystem.     

Biomanipulation additional to nutrient control for restoration of shallow lakes in The Netherlands

Eutrophication control is one of the major issues in the environmental policy in The Netherlands. As a result of international action programmes the average phosphorus loading of freshwater systems should decrease by 50% between 1985 and 1995. However, in many cases the restoration of water quality requires additional measures. Recovery is hampered by the structure and functioning of the present food-chain.

The feeding behaviour of the dominant fish species in Dutch lakes, bream and roach, tend to impose a homeostasis on the system, resisting restoration of water quality. In shallow lakes, biomanipulation, including drastic reduction of fish-stocks, may induce a shift from a stable ‘turbid-water state’ to a stable ‘clear-water state’.

To assess the possibilities of biomanipulation for the restoration of a particular lake, three questions are relevant: (1) is a drastic reduction of fish-stocks feasible?, (2) will a shift occur from ‘turbid to clear’ after the fish reduction? and (3) will the new situation of clear water be stable? This paper focuses attention on the last two questions. The increase in water clarity, following fish reduction, largely depends on the increase in the density of the Daphnia-population and the contribution of benthivorous fish to the resuspension of sediments. A ‘turbid to clear’ shift may be expected if the total biomass of planktivorous and benthivorous fish is reduced to levels<50 kg ha^?1. The stability of the achieved clear-water state largely depends on the development of submerged macrophytes in the lake and on the level of nutrient loading. It is tentatively concluded that a stable clear-water state may be expected at initial total-P concentrations<0.10 mg l^?1.

Because the water managers in The Netherlands have no fishing rights, they have to.co-operate with anglers and commercial fishermen to apply biomanipulation as a tool for water management.

     

Food competition and niche separation between fish and the Red-necked Grebe Podiceps grisegena (Boddaert, 1783)

During the breeding season, the aquatic bird Red-necked Grebe Podiceps grisegena feeds on small aquatic animals, which are also an important food source for many fish species. Because grebes have to provide substantial amounts of invertebrates to their brood, competitive interactions with fish may be expected during the breeding season of grebes. Based on this hypothesis, the distribution of Red-necked Grebes, the abundances of macroinvertebrates, fish, and vegetation, as well as the water depth were determined in a shallow lake during two consecutive breeding seasons. The Red-necked Grebes only utilised the reed bed as breeding site, whereas the adjacent Lagoon, although potentially suitable as breeding site, was not used. Macroinvertebrate biomass was 5–10 times higher in the reed beds than in the Lagoon. The abundance of fish was low in the reed bed but considerably higher in the Lagoon. The results indicate habitat separation between breeding Red-necked Grebe and fish in Lake Tåkern, and that the separation is related to distribution of food resources birds and fish have in common.     

Some positive and negative effects of stocking whitefish on the ecosystem redevelopment of Hjarbæk Fjord,Denmark

In 1984 the County of Viborg introduced a large-scale programme to improve the water quality in Hjarbæk Fjord, a freshwater fjord cut off from the sea in 1964. Measures were taken to reduce the discharges of nutrients from various sources. To reduce the nuisance of chironomids a large release of whitefish (Coregonus lavaretus L.) was incorporated in the programme. The effects of these efforts on the fish, benthos and plankton communities were studied during a period of five years. The whitefish established a self-reproducing population with a very good growth rate, and may have created quantitative and qualitative changes in the plankton community. Observed changes in the benthos community may also be related to predation by whitefish. The study did not confirm that whitefish could actually control the population of chironomids and diminish, thus, the nuisance caused by them. The stocking of whitefish created an exploitable fish stock of a valuable fish species in the hypertrophic Hjarbæk Fjord.

     

Engineering and biological approaches to the restoration from eutrophication of shallow lakes in which aquatic plant communities are important components

Engineering approaches (nutrient removal, sediment pumping, hypolimnion oxygenation, alum treatments) may be most appropriate to deep lakes where the aim of restoration from eutrophication is simply to reduce the production and crop of one component, the phytoplankton. They do not always give the desired results because the nutrient loading may only be reduced to a limited extent. There are additional problems in shallow lakes where change of state between community dominance (aquatic plants versus plankton) is wanted. Each community has powerful buffering mechanisms and biomanipulation may be essential to switch one state to another even with considerable nutrient reduction. For the phytoplankton-dominated community the buffers include the advantages of early growth, lower diffusion pathways for CO₂, overhead shading, and an absence of large cladoceran grazers. This later is because open-water shallow environments provide no refuges against predation for the large Cladocera which are both the most efficient grazers and the most favoured prey for fish. Restoration of aquatic plants may then require provision of refuges for the grazers. Different sorts of refuge are discussed using case studies of Hoveton Great Broad and Cockshoot Broad in the Norfolk Broadland.     

The Microbial Food Web in Eutrophic Shallow Estuaries of the Baltic Sea

Microbial food webs are responsible for the main carbon flow in shallow eutrophic estuaries of the Baltic Sea. Bacteria account for respirative use most of the autotrophic production. Bottom-up influences are mainly directed to the phytoplankton. Massive increase of phytoplankton biomass has only little effect on the biomasses of the heterotrophic plankton. The investigated ecosystem obviously differs by its high bacteria/non-bacteria heterotrophs-relation from other aquatic ecosystems.     

The fish community of the Loosdrecht Lakes and the perspective for biomanipulation

In a two-year-study, the fish community of the Loosdrecht Lakes area was characterized with regard to the professional fishery and the forage base. The lakes are shallow (mean depth 2 m) and eutrophic to hypertrophic. The bream,Abramis brama, dominates the fish biomass. The pikeperch,Stizostedion lucioperca, is the main predator.Bream up to 30 cm have a slow growth rate and are in a bad condition. A faster growth rate and a better condition are shown by bream of 30 cm and more. The small bream feeds on chironomid larvae, benthic cladocerans and zooplankton. The better condition of bream over 30 cm is explained by the more efficient feeding of larger bream onChironomus plumosus larvae. Pikeperch show a fast growth rate and a good condition. Recruitment is limited by the low densities of smelt,Osmerus eperlanus, leaving cannibalism as the most important way for the 0+ pikeperch to become piscivorous and to manifest a fast growth.The impact of the professional gillnet fishery on bream and pikeperch is small because the mesh sizes in use are as large as 75–100 mm bar mesh.The planktivorous 0+ pikeperch consumes mainly the carnivorous zooplanktersLeptodora kindtii and cyclopoids. The zooplankton community lacks large herbivorous species likeDaphnia hyalina, capable of consuming bluegreens. A possible experiment in biomanipulation with a view to find out whether the development ofD.hyalina is depressed by the small planktivorous cyprinids, is predator enhancement with the aid of a stocking programme for indoor-raised 0+ pikeperch in early summer.     

Impact of the removal of gizzard shad (Dorosoma cepedianum) on nutrient cycles in Lake Apopka,Florida

1. The St. Johns River Water Management District removed over 5.4 million kg of gizzard shad (Dorosoma cepedianum) from Lake Apopka, FL during 1993–2005, as a means of reducing lake phosphorus and phytoplankton concentrations and improving water clarity. Other steps included reduction of external nutrient inputs and operation of a treatment wetland. We measured nutrient excretion by Lake Apopka gizzard shad to quantify the nutrient effect of this biomanipulation. 2. Both N and P excretion were significantly affected by fish body mass and temperature. Larger fish had lower mass‐specific rates of excretion than smaller fish. 3. High water temperature increased P excretion to a much greater extent than N, resulting in a low N : P of nutrient excretion in midsummer. The N : P of excretion was lower than has been observed in other systems, probably because of higher water temperature. 4. Removal of gizzard shad >200 g prevented the annual release of 45 800 kg N year^?1 (3.46 kg N ha^?1 year^?1) and 7700 kg P year^?1 (0.62 kg P ha^?1 year^?1) on average. The actual impact on the P cycle varied substantially from year to year (range 7900–78 800 kg N year^?1; 1200–14 800 kg P year^?1), primarily because of fluctuations in the catch. 5. On an annual basis, the P directly removed in fish tissues was similar to that removed by the treatment wetland. The P excretion prevented by the removal of fish was approximately 20% of the reduction in external P loading achieved during 1993–2005. 6. In the short term, most of the P demand of planktonic primary producers is met through recycling of P, which greatly exceeds external P loading. Depending on population biomass, phosphorus excretion by the resident gizzard shad population was similar in magnitude to the P release by diffusive flux from the sediments.     

Food habits, feeding behaviour and impact of triploid bighead carp, Hypophthalmichthys nobilis, in experimental ponds

Food of sterile triploid bighead carp, Hypophthalmichthys nobilis , was examined in ponds receiving water from a hypereutrophic take in Florida. No distinctive seasonal changes in food composition were found. The fish fed selectively on Botryococcus braunii Kuetzing, a large nuisance algal species, which constituted 61% of the volume and 50% of the dry weight of the food. Zooplankton was highly selected, but constituted only 3% of the food volume because of the low zooplankton density in the ponds. The remaining proportion of food consisted mainly of blue-green algae. Final fish biomass was low, ranging from 60 to 97 kg ha⁻¹ in individual ponds. Low biomass was due to slow fish growth and high fish mortality. Despite low biomass, the fish lowered the ratio of blue-green/green algae in the ponds and tended to lower phyto- and zooplankton abundance.     

Comparison of losses of planktivorous fish by predation and seine-fishing in a lake undergoing long-term biomanipulation

SUMMARY 1. Piscivore stocking at artificially high densities and fishing are the two common approaches to reduce the amount of planktivorous and benthivorous fish in lake biomanipulation programmes. Both measures have advantages and disadvantages, but their relative efficacy has not previously been directly compared.
2. We calculated the average annual catch of roach and bream in a lake undergoing long-term biomanipulation (Feldberger Haussee, Germany) by seining each year between 1992 and 1998. We compared this value with a bioenergetics estimate of annual consumption rates of the dominant cohorts of piscivores, pikeperch and pike, in 1997 and 1998. We also determined species composition and length distribution of prey fish in stomachs of the piscivores.
3. Roach was the dominant prey species of both pikeperch and pike, whereas bream was rarely taken by either piscivorous species. Seining removed on average larger specimens of roach than were found in the stomachs of the piscivores.
4. Based on stocking densities of the piscivores, published mortality rates, and individual consumption rates, feeding of pikeperch and pike on roach exceeded the manual removal of roach by seining by a factor of 4–15 (biomass) in 1997 and 1998.
5. Based on these results, a combination of fishing and piscivore enhancement is recommended. Whereas the stocks of adult roach and bream have to be reduced mainly by fishing, the predation of piscivores should be directed predominantly towards the juvenile zooplanktivorous fish. Therefore, small size-classes of piscivorous fish should be promoted by fisheries management, including stocking and harvest regulations.