Ecological interactions between Nile tilapia (Oreochromis niloticus, L.) and the phytoplanktonic community of the Furnas Reservoir (Brazil)

1. Exotic invasive species modify natural food webs in a way frequently hard to predict. In several aquatic environments in Brazil the introduction of Oreochromis niloticus (tilapia) was followed by changes in water quality. Yet, because of its rapid and easy growth, this fish has been used in many aquaculture programmes around the country. 2. To measure the effects of tilapia on the phytoplankton community and on water conditions of a large tropical reservoir in south‐eastern Brazil (Furnas Reservoir), we performed two in situ experiments using three controls (no fish) and three tilapia enclosures (high fish density). Abiotic and biotic parameters were measured at 4 day intervals for 28 days. 3. Fish presence increased nitrogen (N) and phosphorus (P) availability (ammonium 260 and 70% mean increase – first and second experiment; and total phosphorus 540 and 270% mean increase) via excretion. Nutrient recycling by fish can thus be significant in the nutrient dynamics of the reservoir. The higher chlorophyll a concentration in the experimental fish tanks (86 and 34 μg L^?1, first and second experiment, respectively) was the result of a positive bottom‐up effect on the phytoplankton community (approximately 2 μg L^?1 in the reservoir and control tank). 4. Because tilapia feed selectively on large algae (mainly cyanobacteria and diatoms), several small‐sized or mucilaginous colonial chlorophyceans proliferated at the end of the experiments. Thus, the trophic cascade revealed strong influences on algal composition as well as on biomass. 5. Tilapia can contribute to the eutrophication of a waterbody by both top‐down and bottom‐up forces. In particular, by supplying considerable amount of nutrients it promotes the increase of fast growing algae. Tilapia must be used cautiously in aquaculture to avoid unexpected environmental degradation.     

Predicting the potential long‐term influence of climate change on vendace (Coregonus albula) habitat in Bassenthwaite Lake,U.K.

1. The long‐term suitability of Bassenthwaite Lake as a habitat for vendace (Coregonus albula) was assessed using two models. The first was the phytoplankton model (PROTECH) that provided temperature and phytoplankton biomass outputs that were used to drive a second model of lake oxygen (LOX). 2. Both temperature and oxygen concentrations were used to define the available habitat for the adult vendace, using 18 °C as an upper and 2 mg L^?1 as a lower threshold, respectively. The outputs of both models were compared with 4 years of observed data for the purposes of validation and produced good simulations of water temperature, total chlorophyll a and oxygen concentrations in the epilimnion, hypolimnion and at the lake bottom. 3. Using the outputs of a regional climate model (RCM) simulating 20 years of both present and future climate conditions for this part of the United Kingdom, both models were re‐run. These data suggest the future climate will cause a mean increase of >2 °C in water temperature, little change in overall phytoplankton biomass and a 10% decline in oxygen concentration. 4. Using the thresholds defined above, the habitat volume will decline greatly under the future climate scenarios, with all of the 20 years simulated having periods of zero habitat volume for >7 consecutive days, primarily caused by high temperature. These results suggest that the long‐term viability of the lake as a habitat for this rare fish is extremely low.     

The phytoplankton of S'Ena Arrubia Lagoon (Centre-Western Sardinia) between 1990 and 1995

Abstract

The phytoplankton study on the S'Ena Arrubia Lagoon started in 1990, after dystrophic events, leading to high fish mortality and consequently great economic losses. This study presents a general picture of the conditions of the lagoon between 1990 and 1995, and reports on the quali-quantitative changes in the phytoplanktonic populations, as well as the trends of temperature, salinity, pH, dissolved oxygen, algal nutrients and chlorophyll a. The lagoon was characterized by great changes in salinity between years and within the same year, by high nutrient contents and by a phytoplankton trend characterized by sudden intense blooms mostly due to the Chlorophyceae and the Bacillariophyceae. The number of species found, decreased from 1990 to 1994, followed by a slight increase in 1995. The same trend was also found for total density and chlorophyll a.     

Bloom formation of Gloeotrichia echinulata and Aphanizomenon flos-aquae in a shallow,eutrophic, Danish lake

Over a period of four years, the seasonal periodicity of dominant phytoplankton species in a shallow, eutrophic Danish lake changed markedly. Cyanophytes prevailed during the summer period of all four years. In the first three years, species of Microcystis, Anabaena and Aphanothece dominated, whereas in the fourth year of investigation, these algae were replaced by Gloeotrichia echinulata (J. E. Smith) Richter and Aphanizomenon flos-aquae (L.) Ralfs. The most striking environmental differences in the fourth year as compared with the previous three years, were an increase in tranparency, from about 0.5 meter in 1989–1991 to more than 2 metres preceding the summer maximum in 1992, and a simultaneous occurrence of low oxygen concentrations. A collapse of the fish population was followed by an increased proportion of large Cladocerans in the zooplankton. Improved light conditions at the bottom and grazing pressure from large Cladocerans favoured growth of the large colony forming blue-green algae, Gloeotrichia echinulata and Aphanizomenon flos-aquae. These species germinate from resting spores in the sediment and are able to sustain some growth there before migration to the lake water. The transfer of algal biomass from the bottom sediment to the water phase was accompanied by a marked increase in concentrations of particulate phosphorus and nitrogen in the entire lake.     

Ecological and socio‐economic impacts of invasive water hyacinth (Eichhornia crassipes): a review

1. Water hyacinth (Eichhornia crassipes) is one of the world’s most invasive aquatic plants and is known to cause significant ecological and socio‐economic effects. 2. Water hyacinth can alter water clarity and decrease phytoplankton production, dissolved oxygen, nitrogen, phosphorous, heavy metals and concentrations of other contaminants. 3. The effects of water hyacinth on ecological communities appear to be largely nonlinear. Abundance and diversity of aquatic invertebrates generally increase in response to increased habitat heterogeneity and structural complexity provided by water hyacinth but decrease due to decreased phytoplankton (food) availability. 4. Effects of water hyacinth on fish are largely dependent on original community composition and food‐web structure. A more diverse and abundant epiphytic invertebrate community may increase fish abundance and diversity, but a decrease in phytoplankton may decrease dissolved oxygen concentrations and planktivorous fish abundance, subsequently affecting higher trophic levels. 5. Little is known about the effects of water hyacinth on waterbird communities; however, increases in macroinvertebrate and fish abundance and diversity suggest a potentially positive interaction with waterbirds when water hyacinth is at moderate density. 6. The socio‐economic effects of water hyacinth are dependent on the extent of the invasion, the uses of the impacted waterbody, control methods and the response to control efforts. Ecosystem‐level research programmes that simultaneously monitor the effects of water hyacinth on multiple trophic‐levels are needed to further our understanding of invasive species.     

Interactions between phytoplankton,zooplankton and fish in a shallow,hypertrophic lake: a study of phytoplankton collapses in Lake Søbygård,Denmark

Since 1983 severe phytoplankton collapses have occurred 1–4 times every summer in the shallow and hypertrophic Lake Søbygård, which is recovering after a ten-fold decrease of the external phosphorus loading in 1982. In July 1985, for example, chlorophyll a changed from 650 µg l^–1 to about 12 µg 1^–1 within 3–5 days. Simultaneously, oxygen concentration dropped from 20–25 mg O₂l^–1 to less than 1 mg O₂l^–1, and pH decreased from 10.7 to 8.9. Less than 10 days later the phytoplankton biomass had fully recovered. During all phytoplankton collapses the density of filter-feeding zooplankton increased markedly, and a clear-water period followed. Due to marked changes in age structure of the fish stock, different zooplankton species were responsible for the density increase in different years, and consequently different collapse patterns and frequencies were observed.The sudden increase in density of filter-feeding zooplankton from a generally low summer level to extremely high levels during algae collapses, which occurred three times from July 1984 to June 1986, could neither be explained by changes in regulation from below (food) nor from above (predation). The density increase was found after a period with high N/P ratios in phytoplankton or nitrate depletion in the lake. During that period phytoplankton biomass, primary production and thus pH decreased, the latter from 10.8–11.0 to 10.5. We hypothesize that direct or indirect effects of high pH are important in controlling the filter-feeding zooplankton in this hypertrophic lake. Secondarily, this situation affects the trophic interactions in the lake water and the net internal loading of nutrients. Consequently, not only a high content of planktivorous fish but also a high pH may promote uncoupling of the grazing food-web in highly eutrophic shallow lakes, and thereby enhance eutrophication.A tentative model is presented for the occurrence of collapses, and their pattern in hypertrophic lakes with various fish densities.     

Two mesocosm experiments investigating the control of summer phytoplankton growth in a small shallow lake

1. Mesocosm experiments were carried out to examine the relative importance of top down (fish predation) and bottom up (nutrient addition) controls on phytoplankton abundance in a small shallow lake, Little Mere, U.K., in 1998 and 1999. These experiments were part of a series at six sites across Europe. 2. In the 1998 experiment, top‐down processes (through grazing of large Cladocera) were important in determining phytoplankton biomass. The lack of plant refugia for zooplankton was probably important in causing an increasing chlorophyll a concentration even at intermediate fish density. Little Mere normally has abundant macrophytes but they failed to develop substantially during both years. Bottom‐up control was not important in 1998, most probably because of high background nutrient concentrations, as a result of nutrient release from the sediments. 3. In 1999 neither top‐down nor bottom‐up processes were significant in determining phytoplankton biomass. Large cladoceran grazers were absent even in the fish‐free enclosures, probably because dominance of cyanobacteria and high phytoplankton biomass made feeding conditions unsuitable. As in 1998, bottom‐up control of phytoplankton was not important, owing to background nutrient concentrations that were even higher in 1999 than in 1998, perhaps because of the warmer, sunnier weather. 4. The differing outcomes of the two experiments in the same lake with similar experimental designs highlight the importance of starting conditions. These conditions in turn depended on overall weather conditions prior to the experiments.     

Massive fish kills within the Nyanza Gulf of Lake Victoria,Kenya

The morphometry and hydrology of the Nyanza Gulf of Lake Victoria can be greatly affected by violent storms. This can result in a condition in which nutrient-rich bottom mud is mixed with the sediment-laden runoff water from neighbouring marshes and rivers. This situation can lead to massive fish kills. It is such an event in 1984 which will be described that caused the sudden death of over 400 000 fish weighing over 2400 tonnes.The unusually low precipitation in the catchment area during that year resulted in a drop in the lake level from 12.4 to 11.8 m. This low water level, combined with a severe storm created the conditions which caused this large fish kill. The species affected were Lates niloticus and Oreochromis niloticus. Mortality was attributed to several factors which included high levels of suspended material in the water column (detritus and algae) which clogged the gills of the fish, low dissolved oxygen, low pH, and high concentrations of algae.     

Overwinter body condition,mortality and parasite infection in two size classes of 0+ year juvenile European bitterling Rhodeus amarus

Body condition and parasite abundance were examined in two size classes of European bitterling Rhodeus amarus during the first overwintering period in two seasons (2007–2008 and 2009–2010). Body condition of large fish did not change during winter, and increased significantly in March. From November to February, small fish showed a decreasing trend in condition. Despite a significant increase in March condition of small fish only reached the same level as before winter. Total parasite abundance increased significantly in winter in both fish size classes, reflecting a seasonal increase in monogenean infection. Large fish were parasitized significantly more than small fish during winter, but only in small fish was a negative correlation between parasite infection and condition found and a significant decrease in parasite abundance recorded after wintering, indicating mortality of heavily infected individuals with low condition during the winter. A trend for higher overwinter mortality in small fish was found under semi‐experimental conditions. The decrease in condition during the winter period in small fish may reflect faster energy depletion generally expected in smaller individuals. The results indicate that parasite infection may contribute to the overwinter mortality of 0+ year R. amarus, with a stronger effect in smaller individuals.     

Plankton dynamics in a tropical floodplain lake: fish, nutrients, and the relative importance of bottom-up and top-down control

1. Two enclosure experiments were carried out in Laguna Bufeos, a neotropical várzea lake located in the floodplain of River Ichilo (Bolivia). The experiments aimed (i) to assess the relative importance of bottom‐up and top‐down control on the plankton community, (ii) to assess the relative impact of direct and indirect effects of planktivorous fish on the zooplankton, and (iii) to attempt to identify the mechanisms responsible for these effects. 2. During the first experiment, bottom‐up control seemed to dominate the planktonic food web. Compared with fishless enclosures, oxygen concentrations, chlorophyll a levels and the population densities of all cladoceran zooplankton taxa increased in enclosures with fish. Birth rates of Moina minuta, the dominant taxon, were substantially higher in the presence than in the absence of fish, whereas death rates did not differ between treatments. These results are the first to suggest that the positive effects of fish on crustacean zooplankton via effects on nutrient cycling and the enhancement of primary production can compensate for losses because of fish‐related mortality. 3. During the second experiment, the direction of control appeared to vary between trophic levels: the phytoplankton appeared to be bottom‐up controlled whereas the zooplankton was mainly top‐down controlled. Chlorophyll a concentrations were enhanced by both fish and nutrient additions. The majority of the zooplankton taxa were reduced by the presence of fish. Birth rates of most cladoceran taxa did not differ between treatments, whereas death rates were higher in the enclosures with fish than in the fishless enclosures. Bosminopsis deitersi reached higher densities in the presence of fish, probably because of a release from predation by Chaoborus. 4. We convincingly showed strong deviations from trophic cascade‐based expectations, supporting the idea that trophic cascades may be weak in tropical lakes.     

Why biomanipulation can be effective in peaty lakes

The effects of fish stock reduction (biomanipulation) was studied in an 85 ha shallow peaty turbid lake. The lake cleared in a 4-week period in April–May 2004, which demonstrated that biomanipulation can be effective in peaty lakes. We demonstrated that it is possible to reduce the fish stock to <25 kg ha⁻¹ benthivorous fish and <15 kg ha⁻¹ planktivorous fish, sufficiently low to switch the lake from a turbid to a clear state. Knowledge of lake morphology, fish stock, fish behaviour, and a variety of fishing methods was necessary to achieve this goal. It is expected that continuation of fisheries to remove young of the year planktivorous species is needed for several years, until macrophytes provide sufficient cover for zooplankton and can compete with phytoplankton. Cladocerans developed strongly after fish removal. The clearing of the lake coincided with a sudden decrease of filamentous cyanobacteria and suspended detritus, and a strong increase of Bosmina. We assume that Bosmina was able to reduce filamentous prokaryotes and detritus. After the disappearance of the cyanobacteria, Bosmina disappeared too. After the clearing of the lake Daphnia dominated in zooplankton and apparently was able to keep phytoplankton levels low. In our case, wind resuspension did not prevent biomanipulation from being successful. No correlation between windspeed and turbidity was found, neither in an 85 ha nor in a 230 ha shallow peaty lake. Regression analysis showed that on average 50% of the amount of suspended detritus can be explained by resuspension by fish and 50% by phytoplankton decomposition. The main goal of this biomanipulation experiment, clear water and increased submerged plant cover in a shallow peaty lake, was reached.     

Biomanipulation development in Norway

Since 1974 several studies have been carried out in Norway to investigate the interactions between planktivorous fish, zooplankton, phytoplankton and water chemistry. Since 1978 a long-term national research program has been conducted by the Norwegian Council for Scientific and Industrial Research (NTNF). In this program several whole lake manipulations of the fish stocks have been performed to test hypotheses about trophic interactions. It was predicted that manipulations of planktivorous fish populations, might also improve water quality in lakes undergoing eutrophication. Two examples are given to illustrate the achieved results. I: Whole lake fertilization experiment (1974–1978) carried out by Langeland and Reinertsen. The results revealed the importance of top-down effects in the lake ecosystem. When cladocerans dominated, the zooplankton community was able to maintain a more or less constant phytoplankton biomass and a rather low phytoplankton production even when nutrient levels were increased. During years with rotifer dominance, algal biomass and productivity increased, despite the low amounts of added nutrients. II: Experiment performed by Reinertsen, Jensen, Koksvik, Langeland and Olsen in the eutrophic Lake Haugatjern, total elimination of the fish populations by rotenone in late 1980, resulted in a 4-fold decrease in the algal biomass. The species composition changed from the dominance of large-sizedAnabaena flos-aquae andStaurastrum luetkemuelleri to smaller, fastgrowing species and gelatinous green algae. The results are discussed in relation to management of inland waters by combined techniques of biomanipulation and reduced external nutrient supply which increase food-chain efficiency.     

The Effects of Periphyton, Fish and Fertilizer Dose on Biological Processes Affecting Water Quality in Earthen Fish Ponds

The potential of periphyton-based aquaculture in South Asia is under investigation in an extensive research program. This paper is a further analysis of data from four experiments carried out in that framework, to explore periphyton, fish and fertilizer dose effects on water quality. Factor analysis and ANOVA models applied to a data matrix of water quality parameters in ponds with and without artificial substrates (bamboo poles and kanchi sticks), with and without fish (filter feeders catla and rohu, with and without bottom feeder kalbaush), and with a standard or 50% increased fertilizer dose, allowed us to identify the underlying ecological processes governing this novel periphyton-based pond system, and construct conceptual graphic models of the periphyton–environment relationships observed. We clearly established that the phosphorus flow is mainly linked to phytoplankton activity in the water column and decomposition on the pond bottom, while nitrogen flow is mainly linked to autotrophic (photosynthesis) and heterotrophic (decomposition and nitrification) processes that take place in the periphyton in addition to the water column and pond bottom. Consequently, disruption of the pond bottom by bottom feeding fish primarily promoted phosphate cycling and phytoplankton, while periphyton development on the supplied substrates and fertilization mainly improved oxygen balance and nitrogen related processes developing in the water column. The use of bamboo poles led to better results than kanchi sticks, related to the greater autotrophic periphyton development on bamboo and to the larger surface of bamboo poles that facilitate fish grazing and periphyton dislodgment that in turn have a renewal effect on periphyton. Stocking bottom feeding fish produces a fertilizing effect through the food web that benefits the filter-feeding fish and that makes it unnecessary to increase the dose of inorganic and organic fertilizers applied to the ponds. Thus, the output of this analysis will help the fish farmers in resource constrained countries to improve their production in periphyton-based ponds just by choosing bamboo substrates, stocking a bottom feeder fish together with the filter feeders, and saving money on fertilizers.     

Ecosystem size structure response to 21st century climate projection: large fish abundance decreases in the central North Pacific and increases in the California Current

Output from an earth system model is paired with a size‐based food web model to investigate the effects of climate change on the abundance of large fish over the 21st century. The earth system model, forced by the Intergovernmental Panel on Climate Change (IPCC) Special report on emission scenario A2, combines a coupled climate model with a biogeochemical model including major nutrients, three phytoplankton functional groups, and zooplankton grazing. The size‐based food web model includes linkages between two size‐structured pelagic communities: primary producers and consumers. Our investigation focuses on seven sites in the North Pacific, each highlighting a specific aspect of projected climate change, and includes top‐down ecosystem depletion through fishing. We project declines in large fish abundance ranging from 0 to 75.8% in the central North Pacific and increases of up to 43.0% in the California Current (CC) region over the 21st century in response to change in phytoplankton size structure and direct physiological effects. We find that fish abundance is especially sensitive to projected changes in large phytoplankton density and our model projects changes in the abundance of large fish being of the same order of magnitude as changes in the abundance of large phytoplankton. Thus, studies that address only climate‐induced impacts to primary production without including changes to phytoplankton size structure may not adequately project ecosystem responses.     

The role of solid waste materials as habitats for macroinvertebrates in a lowland dam reservoir

Eight hypereutrophic phytoplankton dominated ponds from the Brussels Capital Region (Belgium) were biomanipulated (emptied with fish removal) to restore their ecological quality and reduce the risk of cyanobacterial bloom formation. Continuous monitoring of the ponds before and after the biomanipulation allowed the effects of the management intervention on different compartments of pond ecosystems (phytoplankton, zooplankton, submerged vegetation and nutrients) to be assessed. Fish removal resulted in a drastic reduction in phytoplankton biomass and a shift to the clear-water state in seven out of eight biomanipulated ponds. The reduction in phytoplankton biomass was associated with a marked increase in density and size of large cladocerans in six ponds and a restoration of submerged macrophytes in five ponds. The phytoplankton biomass in the ponds with extensive stands of submerged macrophytes was less affected by planktivorous fish recolonisation of some of the ponds later in the summer. The two non-vegetated ponds as well as one pond with sparse submerged vegetation showed a marked increase in phytoplankton biomass associated with the appearance of fish. Phytoplankton biomass increase coincided with the decrease in large Cladocera density and size. One pond lacking submerged macrophytes could maintain very low phytoplankton biomass owing to large Cladocera grazing alone. The results of this study confirmed the importance of large zooplankton grazing and revegetation with submerged macrophytes for the maintenance of the clear-water state and restoration success in hypereutrophic ponds. They also showed that large Cladocera size is more important than their number for efficient phytoplankton control and when cladocerans are large enough, they can considerably restrain phytoplankton growth, including bloom-forming cyanobacteria, even when submerged vegetation is not restored. The positive result of fish removal in seven out of eight biomanipulated ponds clearly indicated that such management intervention can be used, at least, for the short-term restoration of ecological water quality and prevention of noxious cyanobacterial bloom formation. The negative result of biomanipulation in one pond seems to be related to the pollution by sewage water. Guest editors: B. Oertli, R. Cereghino, A. Hull & R. Miracle Pond Conservation: From Science to Practice. 3rd Conference of the European Pond Conservation Network, Valencia, Spain, 14–16 May 2008     

Fishery-aspects of eutrophication

Summary Eutrophication influences among others food supply, prey catchability, reproduction success, growth and mortality of fish. The first stages of eutrophication are favourable for many fish species. Proceeding eutrophication interferes so strongly with the environment that fish is brought in a vulnerable position: vegetation, which is a prerequisite for a number of species, disappears; oxygen depletion near the bottom affects fish food organisms; turbidity hampers catchability of preyfish and decomposition of dead phytoplankton or vegegation may especially at the end of the summer result in lethal oxygen contents. Hence at higher eutrophic levels the fish population surpasses the optimum and decreases again. These optima vary with the species; for the most important species the sequence from oligotrophic to hypertrophic is: 1. Coregonids and Salmondis, 2. pike, 3. roach and perch, 4. pikeperch, 5. bream. Hence the fish populations of very turbid hypertrophic waters are dominated by bream and pikeperch. This population will furthermore usually consist of smaller quantities of eel, smelt, ruffe and white bream. The total fish population in such a hypertrophic water is considerably larger than in a oligotrophic or mesotrophic lake. The vulnerability of the fish community, however, has increased too. In the hypertrophic situation the oxygen supply is the weak spot and every interfering negative influence, as for example sewage discharge, may have disastrous consequences. The many severe fish mortalities prove this to be a real danger.     

Bottom–up carbon subsidies and top–down predation pressure interact to affect aquatic food web structure

Human impacts such as eutrophication, overexploitation and climate change currently threaten future global food and drinking water supplies. Consequently, it is important that we understand how anthropogenic resource (bottom–up) and consumer (top–down) manipulations affect aquatic food web structure and production. Future climate changes are predicted to increase the inputs of terrestrial dissolved organic carbon to lakes. These carbon subsidies can either increase or decrease total basal production in aquatic food webs, depending on bacterial competition with phytoplankton for nutrients. This study examines the effects of carbon subsidies (bottom–up) on a pelagic community exposed to different levels of top–down predation. We conducted a large scale mesocosm experiment in an oligotrophic clear water lake in northern Sweden, using a natural plankton community exposed to three levels of glucose addition (0, 420 and 2100 μg C l^?1 total added glucose) and three levels of young‐of‐the‐year perch Perca fluviatilis density (0, 0.56 and 2 individuals m^?3). Bacterioplankton production doubled with glucose addition, but phytoplankton production was unaffected, in contrast to previous studies that have manipulated carbon, nutrients or light simultaneously. This suggests that carbon addition alone is not sufficient to reduce autotrophic production, at least in an oligotrophic lake dominated by mixotrophic phytoplankton. Larval perch grazing did not produce a classical trophic cascade, but substantially altered the species composition of crustacean zooplankton and ciliate trophic levels. Glucose addition increased the biomass of rotifers, thus potentially increasing energy transfer through the heterotrophic pathway, but only when fish were absent. This study illustrates that changes in community structure due to selective feeding by top‐predators can determine the influence of bottom–up carbon subsidies.     

Behavioural responses of different blackfly species to short-term oxygen depletion

Behavioural changes of four larval blackflies (Simulium noelleri, S. ornatum (complex), S. trifasciatum [syn. S. spinosum] and S. variegatum) to experimental reduction of dissolved oxygen were investigated with videotechniques. Since these species are characterised by different habitat preferences, they were expected to respond differently to oxygen depletion. During experiments lasting 120–140 min., dissolved oxygen (DO) was reduced to 0.3–0.6 mg/1 within the first hour and kept constant for another hour. Water velocity in the laboratory channel was 135 cm/s. Transparent plastic walls allowed video-recording of larval behavioural responses to oxygen depletion. Blackflies did not engage in undulative or other ventilation movements, and they did not increase locomotory activity, nor did they actively drift. Oxygen depletion mainly altered filter-feeding activity and non-locomotory movements (bending and sweeping). With decreasing DO concentrations larvae of all four species reduced and finally ceased filter-feeding. Non-locomotory movements first temporarily increased and then also ceased, except in S. variegatum, where these movements were immediately suspended. DO-threshold values for cessation of filter-feeding and movements, as well as mortality indicated S. variegatum to be the most sensitive species in respect to oxygen depletion, while S. noelleri seems to be the most tolerant among the four blackfly species tested here.     

主养草鱼高密度池塘溶氧收支平衡的研究

采用原位生态学的方法测定广东省中山市9口主养草鱼高密度池塘中浮游植物光合作用产氧量、水柱呼吸耗氧量、底泥呼吸耗氧量和鱼呼吸耗氧量, 并用数学模型计算增氧机增氧量及用差减法计算大气扩散作用引起的得氧或失氧, 对高密度养殖池塘中溶氧收支平衡状况进行了研究。结果显示: 在水深为1.5-2.0 m的主养草鱼高密度池塘中, 光合作用产氧量随着水深的增加而显著降低(P0.05), 底层出现负值呈现氧债现象。水呼吸耗氧量在表层、中层和底层之间没有显著差异(P0.05)。表层水光合作用产氧量显著大于水呼吸耗氧量(P0.05), 而中层和底层水光合作用产氧量却显著小于水呼吸耗氧量(P0.05)。在主养草鱼高密度池塘溶氧的收入中, 浮游植物光合作用产氧量、增氧机增氧量和大气扩散溶入氧量分别占总溶氧来源的44.7%、42.3%和13.0%, 机械增氧作用已接近光合作用, 成为溶氧来源的主要贡献者; 在池塘溶氧的支出中, 水呼吸、鱼呼吸和底泥呼吸耗氧量分别占总耗氧量的45.9%、45.0%和9.1%, 鱼呼吸耗氧与水呼吸耗氧相当, 成为水体中氧气的主要消耗者。结果表明在草鱼高密度养殖过程中, 合理使用机械增氧是池塘溶氧管理的有效措施。       

Impacts of mosquitofish (Gambusia affinis) predation on plankton communities

An investigation of the effects of mosquitofish (Gambusia affinis) predation was conducted in 12 experimental ponds in southern California over a period of 10 months.Gambusia essentially eliminatedDaphnia pulex andCeriodaphnia sp. populations, reducedDiaptomus pallidus andKeratella quadrata populations, had little impact onCyclops vernalis, and caused large increases inK. cochlearis, Polyarthra sp.,Synchaeta sp., andTrichocerca spp. populations and in total phytoplankton.Gambusia caused a decrease in the PIE (probability of interspecific encounter) of the planktonic crustaceans and an increase in the PIE of the planktonic rotifers. Hemiptera, such as neustonicMicrovelia sp. and nektonicBuenoa sp. andNotonecta sp., andHyla regilla tadpoles were absent from fish ponds but sometimes abundant in control ponds.Gambusia caused higher pH and oxygen levels, presumably via its effect on the phytoplankton. The impact ofGambusia on the pond ecosystems was less in winter, when fish numbers and feeding rates were low, than in summer. Results of other fish-plankton studies are summarized in tabular form. A model is proposed to account for variation in the calanoid/cyclopoid ratio; evidence is summarized suggesting that in general calanoids are more susceptible to predation by predaceous zooplankters while cyclopoids are more susceptible to fish predation. Some parallels are drawn between the effects ofGambusia predation and those of insecticide treatments.