Global spread of hypoxia in freshwater ecosystems during the last three centuries is caused by rising local human pressure

The spread of hypoxia is a threat to aquatic ecosystem functions and services as well as to biodiversity. However, sparse long‐term monitoring of lake ecosystems has prevented reconstruction of global hypoxia dynamics while inhibiting investigations into its causes and assessing the resilience capacity of these systems. This study compiles the onset and duration of hypoxia recorded in sediments of 365 lakes worldwide since AD 1700, showing that lacustrine hypoxia started spreading before AD 1900, 70 years prior to hypoxia in coastal zones. This study also shows that the increase of human activities and nutrient release is leading to hypoxia onset. No correlations were found with changes in precipitation or temperature. There is no evidence for a post‐1980s return to well‐oxygenated lacustrine conditions in industrialized countries despite the implementation of restoration programs. The apparent establishment of stable hypoxic conditions prior to AD 1900 highlights the challenges of a growing nutrient demand, accompanied by increasing global nutrient emissions of our industrialized societies, and climate change.     

Local human pressures influence gene flow in a hybridizing Daphnia species complex

Anthropogenic environmental changes are considered critical drivers of the genetic structure of populations and communities through, for example, the facilitation of introgressive hybridization between syntopic species. However, the mechanisms by which environmental perturbations trigger changes in the genetic structure of populations and communities, such as the processes that determine the directionality of hybridization and patterns of mitochondrial introgression over many generations, remain largely unexplored. In this study, the changes in genetic structure of hybridizing members of the Daphnia longispina species complex were reconstructed over the last 100 years for three large temperate lakes under strong anthropogenic pressures via palaeogenetic analyses of resting egg banks. Drastic changes in the genetic structure of the Daphnia community, associated with hybridization events between D. longispina and D. galeata and subsequent introgression, were detected in Lakes Geneva and Bourget. In Lake Bourget, these changes were induced by the successful establishment of D. galeata with rising phosphorus levels and reinforced by the sensitivity of D. longispina to fish predation pressure. In Lake Geneva, the pattern of hybridization during eutrophication is more likely a function of the original taxonomic composition of the species complex in this lake. Lakes seem to require at least a meso‐oligotrophic status to allow D. galeata populations to establish and accordingly no D. galeata genotypes were found in the egg bank of oligotrophic Lake Annecy. In contrast to the generally assumed pattern of unidirectional hybridization in this species complex, bidirectional hybridization was recorded in Lakes Geneva and Bourget. Our results also demonstrate complex genetic trajectories within this species complex and highlight the irreversibility of changes in the genotypic architecture of populations driven by local human pressures. Finally, we show that extensive hybridization and introgression do not necessarily result in a large and homogenous hybrid swarm.     

Food Preference of Perch (Perca fluviatilis) Stocked in a Previously Fishless,Acid Bog Lake and Development of the Fish Population in Absence of Piscivory

The almost fishless, acid Lake Fuchskuhle was physically partitioned into 4 sections and stocked with young, 10‐ to 13‐cm perch. Their body growth was normal only up to age 4 years. Although the perch spawned successfully, reproduction was negligible because only few offspring survived the first year of life. Food for young perch was plentiful, benthic invertebrates composing the main source. Because the population included only 2 to 3 year classes, the perch could not shift from benthic feeding to piscivory as they grew. As a result, growth rate decreased in older fishes (over 4–5 years old). The benthic food supply alone could not satisfy the energy requirements of the older perch.     

Microbial Food Webs in an Artificially Divided Acidic Bog Lake

The microbial loop of a naturally acidic bog lake, Große Fuchskuhle (Northeastern Germany), that had been artificially divided into 4 basins, was investigated. In the northeast (NE) and southwest (SW) basins, which differ strongly in chemistry and primary production, we conducted intensive studies of the main carbon fluxes through microbial food webs. In the less acidic, NE basin, much higher phytoplankton as well as bacterial biomass and production were found in parallel with negligible numbers of larger zooplankters. Weakly top-down controlled populations of protists were characterized by an exceptionally low numerical proportion of heterotrophic nanoflagellates (HNF) to ciliates (-1.5-3.5). The ciliate community was dominated by a scuticociliate, Cyclidium sp. (>95% of total ciliates), with an estimated grazing rate equal to 46–80% of heterotrophic bacterial production. In contrast, in the more humic, SW basin, both phyto- and bacterioplankton dynamics seemed to be top-down controlled by abundant populations of small fine-filter feeding cladocerans, Ceriodaphnia quadrangula and Diaphanosoma brachyurum. Consequently, ciliates disappeared from the food web structure of the SW basin, HNF dropped to negligible numbers and bacteria showed very uniform morphology, dominated by small cocci or short rods. Our investigations have shown that the division of the lake into separate compartments can lead to very different microbial food web structures with extreme species compositions.     

Seasonal patterns in feeding ecology of three small fishes in the Biandantang Lake, China

Feeding ecology of three small fish species, Hypseleotris swinhonis, Ctenogobius giurinus and Pseudorasbora parva was studied seasonally in the Biandantang Lake, a small, shallow lake in central China. Gut length, adjusted for total body length, was significantly higher in spring than in other seasons for all the three species. Seasonal changes in gut length were not associated with changes in food quality. Weight of fore-gut contents, adjusted for body weight, was significantly higher in winter and spring than in summer and autumn in H. swinhonis and C. giurinus , and significantly higher in autumn than in spring and summer for P. parva. Percentage of empty fore-guts was highest in summer and lowest in spring for H. swinhonis and C. giurinus , and highest in winter and lowest in autumn for P. parva. Diet of the three small fishes showed apparent seasonal changes, and these changes reflected partly the seasonal fluctuations of food resources in environment. Diet breadth was high in winter and low in autumn for H. swinhonis , high in winter and low in spring and summer for C. giurinus , and high in autumn and low in spring for P. parva. Diet overlaps between pairs of species were biologically significant in most cases, except between H. swinhonis and P. parva in summer and autumn and between C. giurinus and P. parva in autumn.     

Modeled Effects of Dissolved Organic Carbon and Solar Spectra on Photobleaching in Lake Ecosystems

Dissolved organic matter (DOM) contains molecules that absorb light at various wavelengths. This chromophoric DOM (CDOM) influences the transmission of both visible and ultraviolet energy through water. The absorption of light by CDOM often causes structural changes that reduce its capacity to further absorb light, a process termed ‘photobleaching‘. A model was designed to assess photobleaching through the entire water column of lake ecosystems. The model uses lake morphometry and dissolved organic carbon (DOC) concentration in conjunction with a defined solar spectrum and experimentally measured photobleaching rates to compute the total water columm photobleaching. The model was initially applied to a theoretical ‘average‘ lake using solar spectra for both the north (N) and south (S) temperate western hemispheres and variable DOC from 0.3 to 30 mg L⁻¹. The consequences of varying waveband-specific photobleaching coefficients and lake morphometry were explored in a second set of simulations. Finally, the model was also applied to four temperate northern lakes for which we had prior measurements of CDOM photobleaching rates. The model demonstrates that all three wavebands of solar radiation (UVB, UVA, and PAR) contribute significantly to total water column photobleaching, with UVA being most important. The relative contributions of the three wavebands were invariant for DOC more than 3 mg L⁻¹. Total water column photobleaching at 440 nm was three to five times faster under the UV-enriched solar spectrum of the southern hemisphere. Increasing the lake’s mean depth (from 0.37 to 9.39 m) resulted in five- or 15-fold slower rates of total water column photobleaching for DOC concentrations of 1 or 10 mg L⁻¹, respectively. Varying the waveband-specific photobleaching coefficients by 10-fold resulted in a similar change in total water column photobleaching rates. Applying the model to four specific lakes revealed that photobleaching for the entire water column would reduce CDOM light absorption by 50% in 18–44 days under summer conditions. Received 17 November 1998; accepted 27 June 2000.     

Socioeconomic Mechanisms Preventing Optimum Use of Ecosystem Services: An Interdisciplinary Theoretical Analysis

Ecosystems provide a wide range of services to society. Some forms of use affect the quality of the ecosystem, reducing its value for other users. This leads to a conflict of interest that is often settled through political processes, resulting in some form of regulation. We link theory on ecosystem response to theories from the socioeconomic branches of science to analyze the mechanisms behind two widespread problems associated with such political solutions. First, they often represent a compromise rather than an integrative solution. We demonstrate that, particularly in sensitive ecosystems, integrative solutions yield a higher average social utility and imply a higher ecosystem quality. Integrative solutions require insight into ecosystems responses to different forms of use and a complete overview of ecosystem services to society. Second, there is a systematic bias away from optimal shared use toward activities that are detrimental to ecosystem quality. This bias arises from the fact that utilities depending on ecosystem quality are often shared by large diffuse groups, whereas pollution and harvesting activities can usually be traced to relatively small and well-organized groups. Theory and data indicate that this type of concentrated group is systematically better at mustering political power than large groups, which find it difficult to realize collective action due to what is known in game theory as “free-rider problems.” Our analysis suggests that the following three key ingredients are needed to correct the problems of bias and compromise: (a) clear insight into ecosystem dynamic responses to human use, (b) a broad inventory of credible measurements of ecosystem utilities, (c) avoidance of bias due to differences in the organizational power of groups of stakeholders. We argue that good ecosystem models, institutionalized ecosystem valuation, and innovative tax-setting schedules are essential to achieving a socially fair and sustainable use of ecosystems by societies. In addition, we highlight the fact that many environmental problems remain unresolved for a long time and briefly identify the social mechanisms responsible for this delay. Received 10 November 1999; accepted 28 April 2000.     

洞庭湖区退田还湖后不同恢复模式下土壤酶活性的变化

以洞庭湖区退田还湖后不同恢复模式下(恢复自然水域、种植芦苇、种植杨树)的典型堤垸（青山垸、官垸和小集成）为研究对象,通过与相邻堤垸农田生态系统（水田、旱田）土壤中蔗糖酶、脲酶、磷酸酶和过氧化氢酶活性的比较,揭示不同退田还湖模式下土壤酶活性的变化及其与土壤化学特性的相关关系.结果表明：与水田、旱田相比,在表层土壤中,以种植杨树为恢复方式的小集成样地的过氧化氢酶活性显著增加,其他酶活性无显著变化;以种植芦苇为恢复方式的官垸样地的蔗糖酶和磷酸酶活性有所下降,其他酶活性无显著变化;恢复自然水域的青山垸样地中,除磷酸酶活性下降外,其他酶活性没有表现出明显的差异性.相关分析表明,土壤有机质含量与4种酶活性均表现出显著的正相关关系（P<0.01）,说明退田还湖后系统恢复模式导致的土壤有机质含量的变化是土壤酶活性变化的主要原因.     

The Stratification of Pools in the Alluvium of the River Lužnice

The stratification of two shallow and small pools, situated in a floodplain ecosystem, was investigated. A new methodological approach was applied. The parameter called summarized chemical and biological stratification (SCB), was derived from basic limnological parameters in order to evaluate the global intensity of chemical and biological stratification and to compare it with a thermal one. Three situations were described when the seasonal courses of thermal and SCB stratification were compared: 1. Both SCB and thermal stratification are present. 2. The water column is homogeneous — there is no stratification in the pool. 3. SCB stratification is present, but the water is homoiothermic. The thermal stratification is very unstable because of the small volume of the pools and frequent floods. The SCB stratification is of higher stability and it may persist in a homoiothermic water column. In comparison with temperate deep lakes, the thermal stratification does not play such an important role in initiating the chemical and biological stratification. The surface/volume ratio and the depth in the pools are lower, which cause the “bottom-initiated” quick renovation of stratification even in homoiothermic water. The differences in oxygen regime, volume and localization of both investigated pools cause the differences in stratification patterns. Frequent oxygen depletions in one of the pools increase the intensity of stratification and influence its character. The anaerobic conditions lead to steeper gradients of basic nutrients, pH, alkalinity and water color. The occurrence of a unique phytoplankton community (Cryptophyceae) is probably closely related to the described stratification patterns.     

洞庭湖湿地与农田土壤动物多样性研究

为探讨洞庭湖退田还湖工程的生态恢复进程, 2005年10月对洞庭湖退田还湖区3类典型生境7个样地土壤动物群落结构进行了调查, 共获土壤动物标本8,484头, 隶属于5门11纲32个动物类群。其中, 还湖湿地捕获土壤动物类群26类, 优势类群为线虫类、蜱螨目; 未还湖农田生境捕获土壤动物类群28类, 优势类群为线虫类、蜘蛛目; 原始湿地(对照)仅捕获土壤动物13类, 优势类群为腹足类。对Shannon-Wiener多样性指数(H')、Pielou均匀性指数(E)、Simpson优势度指数(C)、Margalef丰富度指数(D)和复杂性指数(C_j) 5个多样性指标进行分析, 结果表明: (1)与Shannon-Wiener多样性指数(H')相比, 复杂性指数(C_j)表征的土壤动物多样性更能客观地反映土壤动物分布的真实情况; (2)群落多样性与均匀度显著相关(P<0.05), 而与其他指数关系不密切; (3)在类群数上, 还湖湿地和未还湖农田都多于对照的原始湿地, 有极显著差异(P<0.01); 个体数量上看, 未还湖农田多于原始湿地, 存在极显著差异(P<0.01), 还湖湿地与原始湿地相比却没有显著差异(P>0.05)。这说明: 洞庭湖退田还湖后生态恢复较慢, 恢复效率较低。