Collapse and reorganization of a food web of Mwanza Gulf, Lake Victoria

Lake Victoria in East Africa is the world's second largest freshwater system. Over the past century the ecosystem has undergone drastic changes. Some 30 years after the introduction of Nile perch (Lates niloticus) and Nile tilapia (Oreochromis niloticus) in the 1950s, the highly diverse community of native haplochromines collapsed, leaving a system dominated by only four species: the native cyprinid dagaa (Rastrineobola argentea) and shrimp (Caridina nilotica), as well as the introduced Nile perch and Nile tilapia. More recently, an unexpected resurgence of haplochromines has been reported. To understand these changes in terms of ecosystem functioning and of changes in growth of trophic groups, we created mass balances of the food web near Mwanza, Tanzania, before, during, and after the Nile perch boom (1977, 1987, and 2005), using the application ECOPATH. We connected these mass balances with a dynamic model assuming linear trends in net growth rates of the trophic groups. Our analysis suggests that the Nile perch boom initially altered the biomass distribution over trophic levels. Also, results indicate that not only fishing but also changes at the detritivores' trophic level might have played an important role in driving changes in the system. Both the mass balances and the dynamic model connecting them reveal that, after a major distortion during the Nile perch boom, the biomass distribution over the main trophic levels had largely recovered its original (1977) state by 2005. However, no such return appeared in terms of community structure. Biodiversity in the new state is dramatically lower, consisting of introduced species and a few native surviving species. We conclude that at an aggregate level Lake Victoria's ecosystem has proved to be resilient in the sense that its overall trophic structure has apparently recovered after a major perturbation. By contrast, its intricate functional structure and associated biodiversity have proved to be fragile and seem unlikely to recover.     

Cascading Effects of the Introduced Nile Perch on the Detritivorous/Phytoplanktivorous Species in the Sublittoral Areas of Lake Victoria

In the 1980s an explosive increase of the introduced Nile perch ( Lates sp.; Harrison 1991) in Lake Victoria caused the destruction of approximately 65% of the endemic haplochromine cichlids. The eradication of approximately 200 vertebrate species in less than a decade may well represent the largest extinction event among vertebrates during this century. The introduction of the Nile perch can be considered as a large-scale, albeit unintended, experiment. Farreaching changes in the food web are taking place. We present data on the importance of haplochromines in the ecosystem prior to the Nile perch boom. An analysis of the pelagic community in the sublittoral area of the Mwanza Gulf revealed that the phytoplanktivores formed 18% of the biomass of the total haplochromine community. In the benthic community, the detritivores that frequently included phytoplankton in their diet comprised 31% of the biomass. We examine the hypothesis that algal grazing was reduced by the disappearance of haplochromine phytoplanktivores and detritivores. The disappearance of these groups may have contributed to the recent algal blooms. To investigate this hypothesis we consider a second major change in the system. Coinciding with the decrease of the haplochromines, the number of atyid prawns, Caridina nilotica , has strongly increased. We present preliminary evidence that the stock of the detritivorous haplochromines that formerly comprised most of the demersal ichthyomass has (partially?) been replaced by the prawn. We discuss the possible mechanisms underlying this major change in the food web. With respect to the conservation of the Great African Lakes, tbe collapse of Lake Victoria's ecosystem should serve as an example of how easily a complex ecosystem can be irreversibly destroyed.     

Patterns of the Remnant Cichlid Fauna in Southern Lake Victoria

During the years 1984–1987 Lake Victoria in East Africa experienced what is probably the largest mass extinction of contemporary vertebrates. Within a decade about 200 endemic species of haplochromine cichlids disappeared. The extinctions that occurred in the 1980s have been documented predominantly on species of offshore and sub-littoral waters in the Mwanza Gulf of southern Lake Victoria. Although the littoral fauna of southern Lake Victoria had not been examined in detail, their diversity seemed less affected by the changes in the ecosystem. We give results of the first comprehensive inventory of the littoral cichlid fauna in southern Lake Victoria and discuss its conservation status. We also report on new developments in the sub-littoral fauna after 1990. More than 50 littoral and 15 sub-littoral stations were sampled between the years 1991 to 1995. Of the littoral stations, 34 were sampled for the first time. One hundred sixty three species of haplochromines were collected. Of these, 102 species were previously unknown. About two thirds of them live in rocky areas that were sampled for the first time. Littoral rocky habitats harbored the highest diversity. Since 1990, however, 13 more species disappeared from established sampling stations in littoral habitats. Fishing practices, spreading of exotic fishes, water hyacinth, and eutrophication are considered important threats to the littoral fauna. Also in the upper sub-littoral the number of species declined further. On deeper sub-littoral mud bottoms individual and species numbers increased again, although they are nowhere close to those found before the Nile perch (Lates niloticus) upsurge. This fauna differs from the well studied pre-Nile perch fauna. At well-established sampling stations, the sub-littoral zone is dominated by previously unknown species, and some known species have performed dramatic habitat shifts.     

Fish Faunas of the African Great Lakes: Origins, Diversity, and Vulnerability

The largest African Great Lakes, Tanganyika, Malawi, and Victoria, which have the richest lacustrine fish faunas of any of the world's lakes, provide a unique comparative series for studies of evolutionary mechanisms, community ecology, and fish behavior. Their colorful littoral fishes are also known to aquarists worldwide. This paper examines the origins of their fish diversity, looking at the history of the lakes, colonization from river systems, and evolution of endemic faunas within each lake. All three lakes support fisheries of great socioeconomic importance for the rapidly rising human populations. The paper also examines the vulnerability of the faunas to fishing pressures and introductions of exotic species. In Malawi and Victoria, bottom-trawling has altered the cichlid species composition. The loss of an estimated 200 taxa of endemic cichlid species from Lake Victoria's fauna, following introductions of exotic fishes (tilapias and predatory centropomid Lates ) 40 years ago, stresses the need to protect the unique fish faunas in Lakes Tanganyika and Malawi.     

Evolutionary and Conservation Biology of Cichlid Fishes as Revealed by Faunal Remnants in Northern Lake Victoria

Lake Victoria until recently harbored the world's second richest lacustrine fish assemblage, but it is now experiencing a mass extinction. Here we report on the current status of the endemic cichlid fishes from a preliminary biotic inventory conducted as part of limnological reconnaissance in Kenyan and Ugandan waters during 1989–1992. Sixty-two haplochromine taxa were recovered, half of them new. Some are local endemics, while others are more widely distributed, based on review of earlier collections. Comparison with data from 1982–1984 and 1988 suggests that some shoreline assemblages have been locally stable over the past decade. In addition to littoral rock and vegetation, four kinds of refugia for indigenous fishes are recognized: (1) schools of Rastrineobola , (2) benthic microbial mats in deep water, (3) satellite lakes, (4) the oxycline. Reconnaissance by remote operated vehicle (ROV) documented fish kills due to deoxygenation of the water column, indicating the risk of using the oxycline as a refugium. Several species formerly abundant in Lake Victoria were found only in satellite lakes, revealing the importance of these lakes in the conservation of indigenous species. Recent events in Lake Victoria shed light on mechanisms of speciation and mass extinction, insights relevant to conservation planning for the valuable remnants of the lake's indigenous fauna.     

Conservation of the African Great Lakes: A Limnological Perspective

The limnology of Lakes Malawi Tanganyika, and Victoria is discussed with the objective of examining how the physical, chemical, and biological properties of the lakes will determine the response of these ancient great lake ecosystems to human activities. Of the physical properties discussed, large dilution capacities and long flushing times can make the detection and removal of chemical pollutants in these lakes difficult. The outflows of all three lakes are small because of high evaporation losses, and as a result lake levels are responsive to climate changes that would alter evaporation:precipitation ratios. Increased nutrient input to these lakes will likely result in a decrease in the volume of oxygenated water and available fish habitat. Plankton community composition will also change, probably toward dominance by cyanobacteria. While the effects of eutrophication on fish production are difficult to predict, changes in plankton composition would almost certainly be accompanied by changes in fish community structure. Recent studies of water chemistry and plankton productivity in Lake Victoria provide evidence of possible eutrophication. Because land use has a diffuse but potentially large impact on these lakes, conservation strategies must take into account human activities within entire drainage basins. This requires cooperation between riparian countries and technological and financial input from the international community. An immediate need is the establishment of monitoring programs to determine the impact, real or potential, of human activities around the lakes.     

Recruitment of Hexagenia mayfly nymphs in western Lake Erie linked to environmental variability.

After a 40-year absence caused by pollution and eutrophication, burrowing mayflies (Hexagenia spp.) recolonized western Lake Erie in the mid 1990s as water quality improved. Mayflies are an important food resource for the economically valuable yellow perch fishery and are considered to be major indicator species of the ecological condition of the lake. Since their reappearance, however, mayfly populations have suffered occasional unexplained recruitment failures. In 2002, a failure of fall recruitment followed an unusually warm summer in which western Lake Erie became temporarily stratified, resulting in low dissolved oxygen levels near the lake floor. In the present study, we examined a possible link between Hexagenia recruitment and periods of intermittent stratification for the years 1997 2002. A simple model was developed using surface temperature, wind speed, and water column data from 2003 to predict stratification. The model was then used to detect episodes of stratification in past years for which water column data are unavailable. Low or undetectable mayfly recruitment occurred in 1997 and 2002, years in which there was frequent or extended stratification between June and September. Highest mayfly reproduction in 2000 corresponded to the fewest stratified periods. These results suggest that even relatively brief periods of stratification can result in loss of larval mayfly recruitment, probably through the effects of hypoxia. A trend toward increasing frequency of hot summers in the Great Lakes region could result in recurrent loss of mayfly larvae in western Lake Erie and other shallow areas in the Great Lakes.     

Forecasting the Spread of Invasive Rainbow Smelt in the Laurentian Great Lakes Region of North America

Abstract:  Rainbow smelt ( Osmerus mordax ) have invaded many North American lakes, often resulting in the extirpation of native fish populations. Yet, their invasion is incipient and provides the rationale for identifying ecosystems likely to be invaded and where management and prevention efforts should be focused. To predict smelt presence and absence, we constructed a classification-tree model based on habitat data from 354 lakes in the native range for smelt in southern Maine. Maximum lake depth, lake area, and Secchi depth (surrogate measure of lake productivity) were the most important predictors. We then used our model to identify lakes vulnerable to invasion in three regions outside the smelt's native range: northern Maine (52 of 244 lakes in the non-native range), Ontario (4447 of 8110), and Wisconsin (553 of 5164). We further identified a subset of lakes with a strong potential for impact (potential–impact lakes) based on the presence of fish species that are affected by rainbow smelt. Ninety-four percent of vulnerable lakes in the non-native range in Maine are also potential–impact lakes, as are 94% and 58% of Ontario and Wisconsin's vulnerable lakes, respectively. Our modeling approach can be applied to other invaders and regions to identify invasion-prone ecosystems, thus aiding in the management of invasive species and the efficient allocation of invasive species mitigation and prevention resources.     

Trace elements and organochlorine pollutants in selected fish species from Lake Phewa,Nepal

Lake water and fish livers and gills of sahar (Tor putitora), spiny eel (Mastacembelus armatus), African magur (Clarias gariepinus), and tilapia (Oreochromis niloticus) from Lake Phewa, Nepal, were assessed for the concentrations of trace metals/elements and persistent organic pollutants. The lake water was neutral with low ionic and metallic concentrations as compared to high-altitude lakes of Nepal. The four elements Cu, Zn, Se, and Cd had highest concentrations in livers, indicating uptake from diet, whereas four other metallic elements Cr, Mn, Ni, and Pb had highest concentrations in gills, indicating uptake from lake water. O. niloticus tended to have most of trace metals in the liver at higher concentrations than the other species but significant differences among the different species were found only for Mn, Ni, and Zn in the gills. A pilot study on the concentrations of persistent organic pollutants in muscle revealed that dichlorodiphenyltrichloroethane and related compounds were the dominant organochlorine pesticides, having highest concentrations in C. gariepinus and lowest in O. niloticus.     

Introduced trout sever trophic connections in watersheds: consequences for a declining amphibian

Trophic linkages between terrestrial and aquatic ecosystems are increasingly recognized as important yet poorly known features of food webs. Here we describe research to understand the dynamics of lake food webs in relation to a native riparian amphibian and its interaction with introduced trout. The mountain yellow-legged frog Rana muscosa is endemic to alpine watersheds of the Sierra Nevada Mountains and the Transverse Ranges of California, but it has declined to a small fraction of its historical distribution and abundance. Although remaining frogs and introduced trout feed in different habitats of alpine lakes, our stable-isotope analyses clearly show that the same resource base of benthic invertebrates sustains their growth. During one period, insect emergence from naturally fishless lakes was nearly 20-fold higher compared to adjacent lakes with trout, showing that fish reduce availability of aquatic prey to amphibious and terrestrial consumers. Although trout cannot prey on adult frogs due to gape limitation, foraging post-metamorphic frogs are 10 times more abundant in the absence of trout, suggesting an important role for competition for prey by trout in highly unproductive alpine watersheds. Most Sierran lakes contain fish, and those that do not are usually small isolated ponds; in our study, these two lake types supported the lowest densities of post-metamorphic frogs, and these frogs were less reliant on local, benthic sources of productivity. Since Rana muscosa was formerly the most abundant vertebrate in the Sierra Nevada, the reduction in energy flow from lake benthos to this consumer due to fish introductions may have had negative consequences for its numerous terrestrial predators, many of which have also declined. We suggest that disruptions of trophic connections between aquatic and terrestrial food webs are an important but poorly understood consequence of fish introduction to many thousands of montane lakes and streams worldwide and may contribute to declines of native consumers in riparian habitats.     

Predicting Future Threats to the Native Vegetation of Robinson Crusoe Island, Juan Fernandez Archipelago, Chile

Abstract:   The Juan Fernandez Archipelago, a Chilean national park and biosphere reserve, is 700 km west of continental Chile. Invasive plant species pose major threats to the native, highly endemic vegetation, especially on Robinson Crusoe Island (Mas a Tierra), where there is a permanent settlement. We used historical and recent vegetation map data as well as recent vegetational point data to reconstruct vegetational changes on the island since the early twentieth century. In addition, we used logistic regression models with environmental variables or their surrogates (elevation, solar radiation, topographic similarity index, slope position) to estimate the potential distributions of the worst invaders. Native vegetation has been affected most severely by Acaena argentea , Aristotelia chilensis , Rubus ulmifolius , and Ugni molinae , leading to a significant decrease of endemic plants in the montane forests and native shrublands. The native forest has decreased by approximately one-third. The area affected by Aristotelia chilensis increased from 6.5% to 14% of the total island area. Ugni molinae , once rare, is now abundant (4.6%), as is Acaena (11.9%). Rubus ulmifolius , not present 80 years ago, now covers about 7% of the island's surface. The distributions of all studied species except Rubus ulmifolius are significantly ( p < 0.05) controlled by the environmental factors we examined. Inferring from potential distributions of Aristotelia chilensis and Ugni molinae , 50% of the native montane forest could be invaded or replaced (with a probability of >60%) by these plants. Based on the invasion speed of the past 80 years, this would take another 80 years if conservation measures do not succeed.     

中国东部浅水湖泊沉积物总氮总磷基准阈值研究

受人类活动的影响,东部浅水湖泊沉积物中总氮、总磷负荷很高,当外来污染源得到控制时,底泥中的营养盐会逐渐释放出来,对湖泊水质与生态系统影响很大。为合理削减湖泊内源污染,控制沉积物中营养盐向上覆水体释放,研究制定东部浅水湖泊沉积物总氮、总磷基准阈值,分别测定了100个湖泊的896个表层沉积物样品和8个典型湖泊11个柱芯的沉积物总氮（TN）、总磷（TP）含量,分析了沉积物TN、TP浓度剖面分布特征。通过频度分布法对100个湖泊沉积物总氮总磷的污染状况进行评价,通过背景值比较法确定了8个典型湖泊沉积物的TN、TP背景值。结果表明,100个湖泊的表层沉积物TN浓度范围在479.70~5 573.65 mg·kg-1,TP浓度范围在248.44~1000.33 mg·kg-1,不同湖泊表层沉积物中TN、TP值差异较大。8个典型湖泊沉积物总氮、总磷含量整体上表现出随着深度增加而下降变化趋势,在深层沉积物中含量保持稳定。所调查8个湖泊TN均值为1443.83 mg·kg-1,变化范围为247.45~3719.46 mg·kg-1,各湖泊中TN均值表现为：沱湖〉焦岗湖〉花园湖〉七里湖〉北民湖〉大通湖〉城东湖〉瓦埠湖;TP均值为519.62 mg·kg-1,变化范围为225.41~1944.89 mg·kg-1,各湖泊中TP均值表现为：北民湖〉大通湖〉七里湖〉焦岗湖〉沱湖〉瓦埠湖〉城东湖〉花园湖。不同湖泊沉积物总氮、总磷背景值差异很大。通过对100个湖泊表层沉积物TN、TP的频度分析发现,沉积物营养盐含量上25%点位对应的TP质量浓度398.51mg·kg-1,TN质量浓度为1106.24 mg·kg-1,沉积物营养盐含量下25%点位对应的TP质量浓度664.58 mg·kg-1,TN质量浓度为2916.66 mg·kg-1。通过互相之间的比较分析,推荐采用背景值比较法确定的各湖泊沉积物总氮、总磷背景值均值与频度分步法25%点位对应的总氮值和40%点位对应的总磷值作为东部浅水湖泊沉积物总氮、总磷基准阈值。因此,?     

In situ evaluation of the genotoxic potential of the river Nile: I. Micronucleus and nuclear lesion tests of erythrocytes of Oreochromis niloticus niloticus (Linnaeus, 1758) and Clarias gariepinus (Burchell, 1822)

This study aimed to investigate the genotoxic potential of chemicals present along the course of the river Nile using frequencies of micronuclei (MN) and nuclear lesions (NL) in erythrocytes of Nile tilapia Oreochromis niloticus niloticus and African catfish Clarias gariepinus, as biomarkers. Results showed that most of the physicochemical parameters detected and heavy metal concentrations were significantly higher in the water collected from the estuaries of the river Nile compared to other sites of the upper Nile. The frequencies of MN and NL in peripheral blood erythrocytes of Nile tilapia and African catfish were significantly higher in estuary sites in Damietta and Rosetta compared to upper sites. The lowest level of genotoxicity was observed at two sites (Aswan and Kena), considered to be less contaminated. Our results suggested that higher frequencies of MN and NL determined at Damietta and Rosetta sites may be indicative of damage produced by pollutants in these areas. The most remarkable result was that MN and NL frequencies appear to be strongly related to water quality at different sites examined, indicating that MN frequencies may serve as a reliable biomarker for testing genotoxicity in situ. The positive correlation between MN and NL induction suggested that NL may be a useful complementary assay for genotoxicity analyses when fish are used as experimental animals. It was also found that seasonal variations in MN and NL frequencies might contribute to a better understanding of genotoxic responses in the field. The use of fish as indicator organisms for monitoring the presence of genotoxic-inducing contaminants in the environment seemed justified because the effects of exposure to a “complex mixture” such as river water were obtained. Nile tilapia appears to be a more suitable bioindicator species than African catfish in studying genotoxic chemical pollution in the river Nile attributed to a higher sensitivity.     

Impact of urbanization on Bellandur Lake, Bangalore--a case study

Lake and river water is the prime source for drinking, irrigation and other domestic purposes. Bellandur Lake is one of the major Lakes of Bangalore city. The addition of effluents from urbanized Bangalore city has changed the characteristics of the Lake from being a natural ecologically healthy Lake to an artificial reservoir of domestic sewage and industrial effluents. The DO of the Bellandur Lake water ranged from 3.8-6.3 mg/l. The Bellandur lake water BOD ranged from 89-99 mg/l due to absorption of pollutants by aquatic flora in lake system. If the present state of affairs continues for long, the Bellandur Lake may soon become an ecologically inactive Lake.     

Invasive salmonids and lake order interact in the decline of puye grande Galaxias platei in western Patagonia lakes

Salmonid fishes, native to the northern hemisphere, have become naturalized in many austral countries and appear linked to the decline of native fishes, particularly galaxiids. However, a lack of baseline information and the potential for confounding anthropogenic stressors have led to uncertainty regarding the association between salmonid invasions and galaxiid declines, especially in lakes, as these have been much less studied than streams. We surveyed 25 lakes in the Aysén region of Chilean Patagonia, including both uninvaded and salmonid-invaded lakes. Abundance indices (AI) of Galaxias platei and salmonids (Salmo trutta and Oncorhynchus mykiss) were calculated using capture-per-unit-effort data from gillnets, minnow traps, and electrofishing. We also measured additional environmental variables, including deforestation, lake morphometrics, altitude, and hydrological position (i.e., lake order). An information-theoretic approach to explaining the AI of G. platei revealed that by far the strongest effect was a negative association with the AI of salmonids. Lake order was also important, and using structural equation modeling, we show that this is an indirect effect naturally constraining the salmonid invasion success in Patagonia. Supporting this conclusion, an analysis of an independent data set from 106 mountain lakes in western Canada showed that introduced salmonids are indeed less successful in low-order lakes. Reproductive failure due to insufficient spawning habitat and harsh environmental conditions could be the cause of these limits to salmonid success. The existence of this effect in Chilean Patagonia suggests that low-order lakes are likely to provide natural ecological refugia for G. platei. Finally, pristine, high-order lakes should be actively protected as these have become rare and irreplaceable unspoiled references of the most diverse, natural lake ecosystems in Patagonia.     

The effect of male coloration on female mate choice in closely related Lake Victoria cichlids (Haplochromis nyererei complex)

We studied the effect of male coloration on interspecific female mate choice in two closely related species of haplochromine cichlids from Lake Victoria. The species differ primarily in male coloration. Males of one species are red, those of the other are blue. We recorded the behavioral responses of females to males of both species in paired male trials under white light and under monochromatic light, under which the interspecific differences in coloration were masked. Females of both species exhibited species-assortative mate choice when colour differences were visible, but chose non-assortatively when colour differences were masked by light conditions. Neither male behaviour nor overall female response frequencies differed between light treatments. That female preferences could be altered by manipulating the perceived colour pattern implies that the colour itself is used in interspecific mate choice, rather than other characters. Hence, male coloration in haplochromine cichlids does underlie sexual selection by direct mate choice, involving the capacity for individual assessment of potential mates by the female. Females of both species responded more frequently to blue males under monochromatic light. Blue males were larger and displayed more than red males. This implies a hierarchy of choice criteria. Females may use male display rates, size, or both when colour is unavailable. Where available, colour has gained dominance over other criteria. This may explain rapid speciation by sexual selection on male coloration, as proposed in a recent mathematical model. Received: 11 April 1997 / Accepted after revision: 27 July 1997     

基于RS与GIS的武汉城市湖泊演化研究

以武汉市主城区为例,利用1995和2005年Landsat5两期TM影像与2000和2010年Landset7两期ETM＋影像的解译结果,基于景观分形理论与GIS的空间分析功能相结合的研究方法,分别构建湖泊变化强度指数和湖泊分形维数变化指数。从湖泊面积变化和湖泊形态变化,以及湖泊水域和其他土地利用类型的转移变化特点与影响进行综合分析。来丰富湖泊演化的分析方法,并总结高速城市化背景下湖泊的变化规律,从而更深入地了解和认识人类活动因素与湖泊水域动态变化之间的响应关系,同时提出若干城市湖泊治理与管理的方式。研究结果表明,（1）15年间的湖泊水域面积总量的变化呈现萎缩的趋势,年变化量在逐渐减小,湖泊萎缩的速度得到了一定的控制。（2）1995-2010年湖泊分形维数也呈逐期较小趋势,说明湖泊几何形状趋于简单化,人为活动对湖泊的影响加大。（3）15年来主城区28个主要湖泊的变化强度指数均为负值,呈萎缩趋势。但从2000年开始,少数湖泊变化强度指数为正值,萎缩趋势得到初步的控制。（4）1995-2010年间湖泊的水域面积主要转移成为建设用地和耕地。其转移面积占转移总面积的87．02％。围湖造田和城市化率的迅速提高对城市湖泊的演化影响深刻。（5）城市化背景下湖泊面积与形态变化的规律并非完全与城市化发展速率呈正比的关系,当城市化率上升到一定的阶段后,城市内部对生态环境的保护与要求也不断提高。为改善城市环境和城市内部的生态用地,湖泊应得到立法等强制性保护。     

富营养化对浅水湖泊轮虫种群结构影响研究

石臼湖和固城湖是长江中下游两个浅水湖泊,富营养化问题日益突出。为探索湖泊富营养化对轮虫的影响,于2012年平水期和枯水期对这两个湖泊轮虫种群结构进行了调查,并检测水体理化因子,分析轮虫分布与水环境因子及水体富营养状况的关系。水质检测结果显示,平水期石臼湖除个别点水体综合营养状态指数（TLI）〉50为轻度富营养状态外,大多数站点TLI50,处于轻度-中度富营养化状态。水生生物调查检出轮虫35种,其中石臼湖33种,固城湖27种,平水期种类多于枯水期。出现的轮虫以臂尾轮虫（Brachionus）和异尾轮虫（Trichocerca）居多,优势种主要有螺形龟甲轮虫（Keratella cochlearis）、广生多肢轮虫（Polyarthra vulgaris）、疣毛轮虫（Trichocerca sp.）、沟痕泡轮虫（Pompholyx sulcata）、异尾轮虫属和臂尾轮虫属等。轮虫平均密度3023.19 ind·L-1,平均生物量745.72μg·L-1。现存量季节变动明显,大部分站点平水期大于枯水期。轮虫多样性指数也是平水期大于枯水期,石臼湖轮虫多样性大于固城湖。水体综合营养状态指数（TLI）与轮虫种类数和多样性成显著负相关,随着富营养化加剧,轮虫种类减少,多样性降低。典范对应分析（Canonical Correspondence Analysis,CCA）表明,影响轮虫种群分布的主要环境因子为NH4＋-N、TN、TP和CODMn。按轮虫丰度分布与水环境因子的关系排序,2次调查的24个站点可大致分成3个组。轮虫种群结构受水体富营养状态影响,每组站点的代表性种类对水体营养状态有较好的指示作用。组1的代表性种类为曲腿龟甲轮虫,指示营养级别O-β;组2代表性种类萼花臂尾轮虫,指示的营养级别为β-α;组3以O营养级种类为主。     

不同底质和透明度下马来眼子菜的表型可塑性研究

马来眼子菜(Potamogeton malaianusMiq.)是目前太湖沉水植物优势种之一。文章比较分析了3种不同底质和透明度情况下马来眼子菜的生长特征,探索其在太湖不断扩展的原因及其适应性。分别选取粘土质粉砂、粉砂和下蜀黄土底质上生长的马来眼子菜进行观察试验。结果表明,这3种底质在粒度组成、营养盐和分布特征上具有显著差异。在太湖粘土质粉砂和粉砂底质上马来眼子菜的生物量比下蜀黄土底质上的马来眼子菜高,其生物量随着底质营养盐的增加而增加。马来眼子菜与觅光相关的形态指标,如高度、节间距、节数、叶数、叶长及叶面积均随着底质营养盐的增加而显著增加。马来眼子菜的冠层高度与水体的透明度呈显著负相关,表明其对剧烈变化的湖泊光环境具有很强的适应能力。马来眼子菜在不同底质上的形态可塑性是其优先占据湖泊资源成为优势种的重要原因。     

广州市区人工湖泊PFU原生动物群落群集过程及其对水质差异的指示作用

应用PFU法(GB／T12990-91)对广州市区主要湖泊——流花湖、荔湾湖和麓湖的原生动物群落与水质进行研究．结果表明：用PFU法所测定和计算的原生动物群集的种类数、植鞭虫指数、群集参数(SeqG、T90%)、群落多样性指数d和群落污染值(CPV)等都能较好地指示3个人工湖泊水质的差异，水质由优到劣的排列顺序为麓湖＞荔湾湖＞流花湖．流花湖、荔湾湖和麓湖目前虽仍处于富营养状态，但清污截流等对城市湖泊污染的综合治理措施可以减少湖泊外部营养物负荷及沉积物内负荷，降低水体的营养盐含量，有助于原生动物群落的恢复．图3表3参16