Salinity as a determinant of salt lake fauna: a question of scale

High and often variable salinity is an obvious feature of salt lakes. Correspondingly, salinity is usually assumed to be an important ecological determinant in such lakes. An investigation of the macroinvertebrate fauna of 79 lakes (salinities from 0.3 to 343 g 1^–1) in the Western District of Victoria, Australia, examined this assumption. Over the total range of salinity, species richness and composition are highly correlated with salinity. However, these relationships become nonsignificant over intermediate ranges of salinity. Furthermore, many taxa have very broad tolerances to salinity at these intermediate ranges, implying that factors other than salinity may determine their distribution. An appreciation of scale (that is, the range of salinity over which observations are considered) resolves the paradox that, despite these broad tolerances by most taxa, species richness and composition strongly reflect salinity over the entire salinity range.     

Climate–water quality relationships in three Western Victorian (Australia) lakes 1984–2000

To achieve robust simulations of past and future climate and their effect on aquatic biota, it is desirable to integrate results from palaeolimnology, contemporary monitoring and process modelling. Here we analyse over 15 years of water quality monitoring data from three lakes, Purrumbete, Colac and Bullen Merri from Western Victoria, Australia and their relationship to climate. In the context of a large number of limnological and palaeoenvironmental studies from the region, we seek to refine understanding of the primary forcing mechanisms that explain present, past and potentially future water quality variability in the lakes. Our analysis shows that there are strong relationships between climate and water quality in these lakes of varied size and salinity (average conductivity range 740–14,000 μS cm⁻¹). The strongest climate–water quality relationship exists between air and water temperature, particular during the colder months. Strong relationships also exist with air temperature and other parameters, most notably nutrient concentrations. Effective precipitation also appears to exert a strong influence on water quality in these lakes. This influence is, by contrast with the influence of air temperature, less direct. Nevertheless, these lakes of varying salt concentration exhibit a coherent pattern of conductivity response to variation in effective precipitation, particularly during times of high moisture stress.     

Saline systems of the Great Plains of western Canada: an overview of the limnogeology and paleolimnology

In much of the northern Great Plains, saline and hypersaline lacustrine brines are the only surface waters present. As a group, the lakes of this region are unique: there is no other area in the world that can match the concentration and diversity of saline lake environments exhibited in the prairie region of Canada and northern United States. The immense number of individual salt lakes and saline wetlands in this region of North America is staggering. Estimates vary from about one million to greater than 10 million, with densities in some areas being as high as 120 lakes/km². Despite over a century of scientific investigation of these salt lakes, we have only in the last twenty years advanced far enough to appreciate the wide spectrum of lake types, water chemistries, and limnological processes that are operating in the modern settings. Hydrochemical data are available for about 800 of the lake brines in the region. Composition, textural, and geochemical information on the modern bottom sediments has been collected for just over 150 of these lakes. Characterization of the biological and ecological features of these lakes is based on even fewer investigations, and the stratigraphic records of only twenty basins have been examined. The lake waters show a considerable range in ionic composition and concentration. Early investigators, concentrating on the most saline brines, emphasized a strong predominance of Na⁺ and SO₄ ^-2 in the lakes. It is now realized, however, that not only is there a complete spectrum of salinities from less than 1 ppt TDS to nearly 400 ppt, but also virtually every water chemistry type is represented in lakes of the region. With such a vast array of compositions, it is difficult to generalize. Nonetheless, the paucity of Cl-rich lakes makes the northern Great Plains basins somewhat unusual compared with salt lakes in many other areas of the world (e.g., Australia, western United States). Compilations of the lake water chemistries show distinct spatial trends and regional variations controlled by groundwater input, climate, and geomorphology. Short-term temporal variations in the brine composition, which can have significant effects on the composition of the modern sediments, have also been well documented in several individual basins. From a sedimentological and mineralogical perspective, the wide range of water chemistries exhibited by the lakes leads to an unusually large diversity of modern sediment composition. Over 40 species of endogenic precipitates and authigenic minerals have been identified in the lacustrine sediments. The most common non-detrital components of the modern sediments include: calcium and calcium-magnesium carbonates (magnesian calcite, aragonite, dolomite), and sodium, magnesium, and sodium-magnesium sulfates (mirabilite, thenardite, bloedite, epsomite). Many of the basins whose brines have very high Mg/Ca ratios also have hydromagnesite, magnesite, and nesquehonite. Unlike salt lakes in many other areas of the world, halite, gypsum, and calcite are relatively rare endogenic precipitates in the Great Plains lakes. The detrital fraction of the lacustrine sediments is normally dominated by clay minerals, carbonate minerals, quartz, and feldspars. Sediment accumulation in these salt lakes is controlled and modified by a wide variety of physical, chemical, and biological processes. Although the details of these modern sedimentary processes can be exceedingly complex and difficult to discuss in isolation, in broad terms, the processes operating in the salt lakes of the Great Plains are ultimately controlled by three basic factors or conditions of the basin: (a) basin morphology; (b) basin hydrology; and (c) water salinity and composition. Combinations of these parameters interact to control nearly all aspects of modern sedimentation in these salt lakes and give rise to four 'end member' types of modern saline lacustrine settings in the Great Plains: (a) clastics-dominated playas; (b) salt-dominated playas; (c) deep water, non-stratified lakes; and (d) deep water, "permanently" stratified lakes.     

Australian salt lakes: their history,chemistry, and biota — a review

A vast number of large lakes ( 100 km²) are typically very old features of the Australian landscape; they occupy areas which have changed little tectonically (e.g., they occupy ancient drainage systems in Western Australia or lie in deep depressions such as the Great Artesian Basin: Lake Eyre) and have not been transgressed by the sea since at least the Palaeogene. Other salt lakes, most of which are small ( 50 km²), have been affected morphologically during recurring glacial-interglacial cycles (e.g., lakes associated with gypsum or clay lunettes, sabkhas, pans, lakes near the coast behind barrier dunes as a result of sea-level changes) and their sedimentary records represent comparatively much shorter periods of time. There are also a number of unusually young (< 30 000 years) crater lakes, some of which are the best studied lakes in Australia.The major ions encountered today in Australian salt lakes consist of sodium and chloride although some lakes are also calcium sulphate rich. The origin of these ions is briefly discussed. Sodium carbonate lakes are rare in Australia today. Under past climatic/hydrological conditions the chemistry of a number of lakes was apparently different.The biota of Australian salt lakes is mostly endemic; it is highly diversified as witnessed by the crustacean fauna and is well adapted to the harsh conditions prevailing in saline water. This is the result of a long history of aridity in Australia. The characteristics of this biota are presented together with data on its distribution which is primarily related to climatic conditions.     

Eutrophication control strategies for three shallow Vecht lakes in the province of North Holland

Like many shallow surface waters in the Netherlands the North Holland Vecht lakes, formerly known for their rich variety of flora and fauna, now face a serious eutrophication problem. Nutrient enrichment has been mainly in the form of (treated) wastewater discharges, and the continuing ingress of nutrient-loaded water from the river Vecht. Yet, this water has to be supplied in order to compensate for water shortages resulting from (i) changes in the groundwater flow pattern due to reclamation of the deep polder Horstermeer, (ii) extensive groundwater extraction in the Gooi hills, and (iii) extensive drainage for agricultural purposes.The present policy of eutrophication abatement and restoration of the Ankeveen and Kortenhoef lakes ecosystems is focused on eliminating wastewater discharges and Vecht water supply. It also allows for additional dredging measures. Because of the un-suitable major ion composition of the Vecht, the aim is to compensate for this water supply by (i) partial restoration of the original groundwater flow from the Gooi hills and (ii) periphere additional supply with fresh seepage water from the skirts of the Horstermeer polder. However, uncertainty exists about the amounts of water needed.Water balances and phosphorus budgets have been established to ascertain the water demands of the lakes and to gain a detailed insight into the nutrient fluxes through the lakes. A groundwater flow model is used to assess the beneficial effects of the proposed measures.The results obtained, question the current unilateral restoration objectives. Calculations reveal that, both in the present situation and after (total) reduction of groundwater extractions in the future, the available quantity of fresh seepage water from the skirts of the polder Horstermeer is not sufficient to replace the inlet from the river Vecht into the Kortenhoef lakes. Additional supply options are available but the ones favoured from an ecological viewpoint are either the most expensive or less favoured from a social point of view. Although the sediments of the lakes appear to be a major source of eutrophication, the possibility of dredging the lakes will be considered only after reviewing results of a pilot-dredging project in the Hollands Ankeveen lakes in 1991.     

Is speciation of demersal fishes in Lake Tanganyika restrained by physical limnological conditions?

Both Lake Malawi and Lake Victoria have many species of offshore demersal cichlids, most of which are stenotopic. In Lake Tanganyika there are fewer, more eurytopic, demersal species. These differences may be the effect of the physical regimes of the lakes. In Lake Victoria there is no permanent anoxic layer. In both the other lakes, water below 250 m depth is permanently anoxic and the seasonal cycle is dominated by upwelling. In Lake Malawi this is mainly derived from intermediate water containing free oxygen. In Lake Tanganyika upwelling involves hypolimnetic water, which may rise to within 80 m of the surface, causing great short-term changes in oxygen concentration over the depth range 50–250 m. This has inhibited the development of deep water species restricted to narrow depth ranges.     

Spatiotemporal water dynamic modelling of Ramsar-listed lakes on the Victorian Volcanic Plains using Landsat,ICESat-2 and airborne LiDAR data

Spatiotemporal dynamic information on surface water area and level is a prerequisite for effective wetland conservation and management. However, such information is either unavailable or difficult to obtain. In this study, for the first time, we leverage Landsat imagery, ICESat-2 and airborne LiDAR data to develop time series of water body dynamics over the last 35 years (1987–2021) using machine learning method on a cloud computing platform for lakes identified as international importance in the Western District Lakes Ramsar site in Victoria, Australia. Our results reveal distinct seasonal (dry and wet) variation patterns and long-term changes in trends of lake water areas and levels in response to seasonal rainfall variations and regional climate changes for the periods of before, during and after the Millennium Drought when southeast Australia experienced unprecedented dry conditions. Lake water bodies have not recovered to the status of pre-Millennium Drought, and many permanent Ramsar-listed lakes in the region have become to ephemeral lakes due to climate change. The outcome of this study provides a baseline to help understand the historical and ongoing status of the Ramsar-listed lakes in a warming and drying climate in support of the development of strategic plan to implement international obligations for wetlands protection under the Ramsar Convention.     

Effects of invertebrate predation on the seasonal succession of a zooplankton community: a two year study in Lake Aydat,France

Saline playa lakes represent major geomorphic and hydrologic components of internal drainage basins in the arid to semiarid interior of Australia. These lakes mark the outcrop areas of regional shallow groundwater; thus, they are effective hydro-chemical sinks for elemental concentration and authigenic formation of carbonate, evaporite, and silica/silicate minerals.Field observations and petrochemical characterization of playa sediments from drainage basins in Western and Central Australia indicate that localized discharge of groundwater, from shallow aquifers in calcrete deposits, plays a fundamental role in geochemical evolution of playa-lake marginal facies. The available data indicates also that although evaporative concentration and salt recycling are major controls on geochemistry of the playas, yet a simple evaporative concentration model does not provide a complete explanation for brine evolution and particularly the geochemical process-product relationships observed in the individual playa lakes. The distribution of the chemical facies in the playas, in relation to geomorphic setting of the internal drainage basins, reflects a significant impact of variation in groundwater discharge pattern on the geochemical evolution of the playa lakes. Accordingly, the development of chemical facies in individual playas have progressed through repeated episodes of evaporative concentration, groundwater-level fluctuations and ion-exchange processes.     

Groundwater rise,soil salinization and the decline of Casuarina in southeastern Australia during the late Quaternary

Abstract The decline in interglacial importance of Casuarina over the late Quaternary across southeastern Australia is documented. Three previously proposed causes for the decline (change in fire regime, change to a wetter climate and competitive exclusion by eucalypts) are shown to be inadequate for explaining the majority of cases. Re-examination of the evidence shows the decline at most sites occurred synchronously with a rise in groundwater or soil salinization, or both. From a review of biological literature, it is established that Casuarina stricta, the main species considered to have been affected by the decline, is likely to be disadvantaged by high water tables and saline soils. A link is demonstrated between groundwater salinity and the nodulation status of Casuarina in Victoria. It is concluded that the late Quaternary Casuarina decline was caused by a combination of rising groundwater levels and soil salinization. Soil salinization and groundwater level must therefore be considered as major factors determining vegetation patterns in southeastern Australia through the Quaternary up to the present day.     

Effects of Changing Rainfall on the Limnology of a Mediterranean,Flowthrough‐Seepage Chain of Lakes

Relationships between groundwater and lake ecology are often overlooked, but they may be strong, particularly in seepage lakes. As a result, the nature and degree of groundwater effects on lakes are usually neglected. In this study interactions among rainfall, groundwater and surface water and their limnological effects were traced seasonally for two years of changing rainfall in a Spanish flowthrough, seepage lake complex. Cumulative rainfall dictated recharge of groundwater with delays of nine months. Groundwater discharge, in turn, increased surface discharge downstream. Mediated by the geographical setting of lakes, both fluxes impinged on lake water renewal time, but effects of the latter on limnological variables were much stronger at the district scale than at the single lake scale. These water‐renewal effects included the following: decreasing salinity, total phosphorus concentration and phytoplankton biomass and increasing water transparency and total nitrogen concentration as water renewal shortened, the nitrogen effect arising because of nitrate‐rich water entering the lakes as groundwater levels rose. This complex response of a Mediterranean lake district to water availability may also be expected in cold temperate lakes as climate change effects become stronger. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Chinese and Mongolian saline lakes: a limnological overview

More than half of China's lakes are saline (viz. have salinities > 3 g L⁻¹). Most salt lakes are in northwestern China (Tibet, Qinghai, Sinkiang, Inner Mongolia). Most Mongolian salt lakes are in the west of that country. Tectonic movements have been of the greatest importance in lake origins, but aeolian activity and deflation have also played a role. Many salt lakes in Qinghai-Tibet lie at altitudes > 4 000 m.a.s.l.; Aiding Hu (Sinkiang) lies at −154 m.a.s.l. Again, many lakes are large in area and deep. Small, shallow lakes are also common. Dimictic thermal patterns prevail in deep lakes, polymictic patterns in shallow ones. The highest salinity recorded is 555 g L⁻¹. The salinity of Qinghai Lake, the largest Chinese salt lake, is 14 g L⁻¹, but mean lake salinity on the northern Tibetan plateau is about an order of magnitude greater. Lop Nor has a salinity of ∼ 5 g L⁻¹. Dominant ions are Na and Cl; Mg, Ca, SO₄ and HCO₃ + CO₃ are important in certain lakes. Most major ions originate by weathering and leaching from rocks. pH values are generally high (often > 9.0). There are no bird or fish species confined to salt lakes, though many are associated with lakes of low or moderate salinity. Artemia occurs widely inland and in coastal salt pans, but is the only major macroinvertebrate of highly saline lakes. In lakes of only low to moderate salinity, invertebrate communities comprise widespread halotolerant freshwater forms and halophiles, some regionally endemic. Submerged and emergent macrophytes occur in lakes of low salinity, but phytoplankton species are more halotolerant. Ctenocladus circinatus, a green alga, is known from a Tibetan salt lake with a salinity of 200 g L⁻¹. There is a dearth of basic limnological information on Chinese and Mongolian salt lakes. More work in particular is needed on a variety of geographically widespread lakes to (a) document seasonal physico-chemical events, and (b) compile comprehensive biological inventories of taxa present. Chinese salt lakes are significant sites for palaeoclimatic research, for conservation purposes, and for the resolution of several important biological questions (especially of an ecological and biogeographical sort). They also have important economic values. Unfortunately, the natural existence of many appears to be threatened by decreased inflows, largely the result of human impact on catchments.     

Characteristics of catchments disturbed by lignite mining—case study of Schlabendorf/Seese (Germany)

In the Lower Lusatian lignite mining district, large areas have been mined and recultivated. An even larger area has been affected by groundwater lowering. The specific characteristics of catchments disturbed by lignite mining are presented for the Schlabendorf/Seese region, where a close relation between water issues and open-cast lignite mining exists. In general, this relationship starts prior to mining and persists even after the land is reclaimed.The most obvious feature of this post-mining landscape is the large number of forming post-mining lakes. A further feature is the recovery of the groundwater table. Dry streams and springs now are starting to flow again, and areas with a formerly deep groundwater table are now directly influenced by rising groundwater.The disturbance causes various acidification phenomena. The aeration of pyrite-bearing material leads to a drastic deterioration of the groundwater quality, which affects receiving streams and lakes. Thus there is the tendency for some parts of post-mining lakes to become acid. Additionally, in the marginal areas of the groundwater recovery, acidification of streams occurs, causing water quality problems in ponds and lakes located downstream.The development of each individual water body with respect to water quantity and quality depends on its specific hydrogeological and hydrological setting. The region is experiencing dynamic changes, and shows that the future development of the post-mining catchments is difficult to predict.     

A legacy of climate and catchment change: the real challenge for wetland management

Wetland managers are faced with an array of challenges when restoring ecosystems at risk from changing climate and human impacts, especially as many of these processes have been operating over decadal–millennial timescales. Variations in the level and salinity of the large crater lakes of western Victoria, as revealed over millennia by the physical, chemical and biological evidence archived in sediments, attest to extended periods of positive rainfall balance and others of rainfall deficit. The recent declines in the depth of these lakes have been attributed to a 15% decline in effective rainfall since AD 1859. Whilst some sites reveal state shifts following past droughts, the response of most wetlands to millennial-scale climatic variations is muted. Regional wetland condition has changed comprehensively, however, since European settlement, on account of extensive catchment modifications. These modifications appear to have reduced the resilience of wetlands limiting their capacity to recover from the recent ‘big dry’. These sedimentary archives reveal most modern wetlands to be outside their historical range of variability. This approach provides a longer-term context when assessing wetland condition and better establishes the restoration challenge posed by the impact of climate change and variability and human impacts.     

Impact of flowering on bird community dynamics in some central Victorian eucalypt forests

Many species of Eucalyptus in Australia provide copious amounts of nectar during their reproductive seasons. The nectar is used by many animal species but especially by birds, insects and some bats, which act as pollinators. One of the major features of eucalypt flowering in southern Australia is the patchy, asynchronous flowering of different species, which appears to drive mass nomadism of nectarivorous birds among regions and among habitats. Here we explore whether flowering asynchrony or climate is primarily responsible for the influxes and effluxes of vast numbers of nectarivorous birds in central Victoria, Australia. By using a structured sampling program, we show that winter flowering by red ironbark Eucalyptus tricarpa is the most likely agent controlling avian-nectarivore dynamics rather than climatic differences among regions. Densities and species richness of nectarivores, and numbers of nectarivory events, are all closely related to measures of flowering intensity. However, nonnectarivores, such as insectivores and granivores, show no relationships with either habitat or region. We discuss how dependence on a patchily distributed but highly rewarding resource such as nectar influences population densities and community structure in birds.     

Biological diversity of the Yala Swamp lakes, with special emphasis on fish species composition, in relation to changes in the Lake Victoria Basin (Kenya): threats and conservation measures

During the second half of the last century, the Lake Victoria ecosystem has undergone drastic ecological changes. Most notable has been the decline in the populations of many endemic cichlid fishes. The lake has lost nearly 200 haplochromines and one tilapiine, Oreochromis esculentus. The above changes have been attributed to effects of species stocking and, in particular, from predation pressure by the introduced Nile perch, Lates niloticus. Other factors that have led to the decline of the endemic species include intensive non-selective fishing, extreme changes in the drainage basin, increased eutrophication, and the invasion of the lake by the water hyacinth, Eichhornia crassipes. However, the remnants of some species that had disappeared from Lake Victoria occur abundantly in the Yala Swamp lakes (Kanyaboli, Sare and Namboyo). This paper discusses the biodiversity of the swamp and the three lakes and gives suggestions for their conservation.     

Human-induced changes in the composition of fish communities in the African Great Lakes

Summary Large natural African lakes contain unique and diverse fish faunas which have evolved within each lake in a comparatively short period of time. members of the family Cichlidae are particularly diverse, although there is strong evidence to show that the haplochromines in Lake Victoria, and possibly Lake Malawi, are monophyletic. The unique faunas in Lakes Victoria and Kyoga have been subject to perturbations from the introduction of exotic fish, and the faunas in these and other lakes have been disturbed by fishing activities and other human endeavours.Factors governing the establishment of exotic species are not clearly understood. The exotic fish must be physiologically adapted to their new environment, able to compete successfully both for habitat and for food at each stage of their life history, able to avoid predation and must have a suitable reproductive potential. Although about 50 species of fish have been introduced into African inland waters, including reservoirs, only comparatively few, in particular Nile perch (Lates niloticus), various cichlids (especially tilapias) and clupeids (Limnothrissa miodon), have been successful in establishing themselves. Those that have become established have had obvious but unquantifiable impacts on the indigenous faunas.It is difficult to differentiate between the effects of fishing and of the presence of alien fish on the fish species composition of the lakes (Witte et al., 1992). Many of the lakes were overfished before introductions were made, with a resultant decline in some species, especially the larger ones, and the virtual disappearance of others. Some lake fish faunas, such as those of Lakes Kyoga and Victoria, which have been subjected to the perturbations described above, continue to change rapidly (Ogutu-Owayo, 1990b).There is a fundamental need to collect biological information on the fish communities of African lakes for effective management, resulting not only in the conservation of unique fish faunas but also the production of sustainable fish yields for the people relying on this source of protein. This information is required before any more introductions of exotic fish are made.     

Limnological imbalances: an antipodean viewpoint

SUMMARY. 1. It is argued that modern limnology is excessively concerned with the study of fresh waters in the northern temperate region. Limnology has been and is unbalanced in its interests and emphases and, as a result, many widely held limnological concepts need revision. 2. The genesis of the argument is outlined. It involved the initial assumption that Australia was limnologically distinctive. Later, it was realized that since most of the world is arid, semi-arid, tropical or semi-tropical, Australia was much less distinctive than originally thought. 3. Other limnological assumptions considered in the paper concern saline lakes and salinization, the importance and nature of temporary bodies of standing fresh water, biological diversity in tropical fresh waters, the evolutionary importance of permanent freshwater lakes, and the global applicability of the River Continuum Concept. Consideration suggests that some widely-held views on these matters are erroneous. 4. Thus, saline lakes are more important (scientifically, commercially), widespread and numerous environments than is generally thought. Salinization is a significant environmental hazard for many rivers in semiarid regions. Temporary bodies of standing fresh water are geographically widespread and abundant aquatic environments with high species diversity, and in which many species have restricted distributions and low dispersal abilities. Biological diversity appears to be low in tropical fresh waters. Permanent freshwater lakes are perhaps less important loci for the evolution of the freshwater biota than generally thought, temporary fresh waters more so. Finally, the River Continuum does not seem to be applicable to many running waters outside the north temperate region. 5. A number of suggestions are made of how a better balance in limnology may be arrived at: how a more globally comprehensive intellectual framework may be built. Suggestions include: an increased awareness of the frequently different natures of waters within and beyond the northern temperate region; recognition of the potential benefits to be gained from the study of waters outside the northern temperate region;     

Some factors controlling the major ion chemistry of small lakes: Examples from the prairie parkland of Canada

The management of recreational lakes in the prairie region of Canada requires that their physical, chemical and biological setting be clearly defined. This study relates the major ion chemistry of two lakes to significant ion contributions from groundwater, surface runoff and snow. The major ion concentrations in addition are related to several important limnological processes.Lake Wabamun is a 84.6 km² lake with a basin set in Cretaceous bedrock. Groundwater studies in the watershed indicate that rather large quantities of groundwater enter the lake from fractured coal and bedrock units. Hastings Lake has a considerably smaller surface area and is formed in a shallow depression in glacial drift. Generally, smaller quantities of groundwater enter this lake because the glacial drift has a low permeability.Evaluation of the data indicates that relative quantity and quality of groundwater inflow is probably the most important factor controlling the major ion chemistry of the lakes. Other factors are mineral precipitation, freezing out and the timing of major water inflows.     

The seasonality of phytoplankton in African lakes

Although some study of the subject began in 1899, wide-ranging information from African water-bodies has only become available since 1950. Important developments included the establishment of long-term centres of research, the adoption of improved methods for quantitative algal sampling, the more intensive study of environmental conditions, the beginnings of experimental testing, and the improvement of taxonomic knowledge.At higher latitudes (> 20 °) examples of pronounced algal seasonality are long-established; they are accompanied and influenced by marked changes in radiant energy income and so water temperature, and often by effects of seasonal water input. Illustrations are given from lakes in Morocco and South Africa.More generally in Africa, including the tropical belt, annual patterns of phytoplankton seasonality are usually either dominated by hydrological features (water input-output) or by hydrographic ones (water-column structure and circulation). Examples of both types are discussed, together with instances (e.g. L. Volta) of combined hydrological and hydrographic regulation. In both the seasonal abundance of diatoms is often distinct and complementary to that of blue-green algae, with differing relationships to vertical mixing and water retention.Horizontal variability in the seasonal cycle is especially pronounced in the larger or morphometrically subdivided lakes. Some inshore-offshore differentiation is also known to affect phytoplankton quantity (e.g. L. George) and species composition (e.g. L. Victoria). Longitudinal differentiation is common in elongate basins especially when with a massive or seasonal inflow at one end (e.g. L. Turkana, L. Nubia, L. Volta); occasional terminal upwelling can also be influential (e.g. southern L. Tanganyika). Such examples grade into the longitudinally differentiated seasonality of flowing river-reservoir systems, as studied on the Blue and White Niles.The annual amplitude of population density, expressed in orders of magnitude (=log₁₀ units), is one measure of seasonal variability. It can exceed 3 orders both in systems subject to hydrological wash-out (e.g. Nile reservoirs) and in the more variable species components of lakes of long retention (e.g. L. Victoria). Low amplitudes can be characteristic of some components (e.g. green algae in L. Victoria) or of total algal biomass (e.g. L. George, L. Sibaya).Seasonal changes may be subordinated to inter-annual ones, especially in shallow and hydrologically unstable lakes (e.g. L. Nakuru).     

山东潍坊地下水硝酸盐污染现状及δ15N溯源

采用均匀布点选取了山东潍坊居民区、粮田和蔬菜种植区等区域地下水为研究对象,分析了地下水硝态氮含量及污染来源,结果表明:潍坊地区地下水硝态氮平均含量为28.1 mg/L ,按照国家地下水质量标准(GB/T 14848-93)属于Ⅲ类水;饮用水井硝态氮平均含量为23.3 mg/L, 最高值达到了150 mg/L,对国家饮用水标准(10 mg/L)超标率高达73.5%,严重超标率达50%;不同土地利用方式下地下水硝态氮含量不同,设施蔬菜种植区最高,其次是露地蔬菜种植区, 小麦-玉米种植区地下水硝态氮含量较低,但都超过了WHO饮用水中硝酸盐的最大允许含量50 mg/L的规定(折合为硝态氮11.3 mg/L);硝酸盐与水质离子之间的相关性以及水质分析相关的派珀图分析显示地下水硝酸盐污染与氮肥施用有关;根据硝酸盐δ¹⁵N的稳定同位素溯源分析,潍坊地区地下水硝酸盐41.5%来自于化肥,14.6%来自于生活污水,其他是来自化肥、生活污水和家畜粪尿的混合污染。综上,潍坊市地下水硝酸盐污染非常严重,已经对当地居民的身体健康造成了潜在的威胁;因此,亟需从源头控制做起,减少肥料的过量投入和生活污水的随意排放,以控制硝酸盐的继续污染及改善当地水环境。