The impact of salinity pulses on the emergence of plant and zooplankton from wetland seed and egg banks

1. In this study we compared the emergence of aquatic biota from sediments under 14‐day pulses of high (5000 mg L⁻¹) and low (1000 mg L⁻¹) salinity with emergence under freshwater and equivalent constant salinity levels. We tested the hypothesis that pulses of high salinity and short duration have no impact on the emergence of aquatic plants and zooplankton from wetland sediment. 2. The way salt is moved through the landscape may alter the response of biota to increases in salinity. Under natural hydrological regimes in rivers and floodplains salinity pulses occur often at concentrations that exceed predicted tolerance levels for aquatic biota. The impacts of natural pulses of high salinity followed by rapid return to fresh conditions may be used to inform management guidelines for the potential release of non‐natural saline water into river systems with minimal impact. 3. For both aquatic plants and zooplankton the abundance and richness of the emerging taxa decreased at higher salinities kept at constant levels. In contrast, pulses of salinity followed by return to freshwater conditions did not have a negative impact on the emergence of aquatic plants or zooplankton. For many taxa of zooplankton a positive impact was demonstrated with higher emergence following the salinity pulse. 4. The responses of aquatic plant and zooplankton taxa are grouped into five response types. Type 1: negatively impacted by all salt regimes. Type 2: preference for constant salinities. Type 3: no difference between fresh and either pulse regime. Type 4: preference for high concentration pulses. Type 5: emergence higher under a low concentration pulse. 5. Although previous studies indicate that constant high‐level salinity in rivers and wetlands can decrease the species richness of aquatic communities, this current study shows pulses may not have the same impact. Our results support the hypothesis that pulses of high salinity and short duration do not impact on the emergence of aquatic plants and zooplankton from wetland sediments. For zooplankton, pulses of salt may trigger emergence. 6. These trends may be used to explore the potential to use managed water releases to move salt through the landscape with minimal impact of salinity on aquatic biota. However, before such preliminary results are applied in management of saline water releases we need to determine the implications for interacting processes in natural ecosystems.     

Changes in biotic communities developing from freshwater wetland sediments under experimental salinity and water regimes

1. Reduction in diversity of both freshwater aquatic and terrestrial ecosystems has been attributed to salinity increase and such increases are a symptom of changes to land use. Hydrological alteration to ground and surface water are likely to be associated with salinity increase and its influence on biodiversity. However the combined effects of salinity and hydrology on aquatic biodiversity have not been elucidated fully in either field or experimental situations. 2. The effect of salinity and water regime on the biota in sediments from seven wetlands from inland south‐eastern Australia was tested experimentally using germination of aquatic plant seeds (five salinity and two water levels) and emergence of zooplankton eggs (five salinity levels). Salinity levels were <300, 1000, 2000, 3000, 5000 mg L^?1 and water regimes were damp (waterlogged) and submerged. 3. Aquatic plant germination and zooplankton hatching was not consistent for all seven wetland sediments. Four of the wetland sediments, Narran Lakes, Gwydir Wetlands, Macquarie Marshes and Billybung Lagoon showed similar responses to salinity and water regime but the other three wetland sediments from Lake Cowal, Great Cumbung Swamp and Darling Anabranch did not. 4. As salinity increased above 1000 mg L^?1 there was a decrease in the species richness and the abundance of biota germinating or hatching from sediment from four of the wetlands. 5. Salinity had a particularly strong effect in reducing germination from sediments in damp conditions when compared to the flooded conditions. In parallel, salts accumulated in the sediment in damp conditions but did not in flooded conditions. 6. There is potential for increasing salinity in freshwater rivers and wetlands to decrease the species richness of aquatic communities and thus of the wetland community as a whole, resulting in loss of wetland biodiversity. This reduction in diversity varies between wetlands and is at least partly related to hydrology. For aquatic plants the reduction in diversity will be more marked for plants germinating from seed banks at the edges of wetlands where plants are not completely submerged than for the same seed bank germinating in submerged conditions.     

Drought and aquatic community resilience: the role of eggs and seeds in sediments of temporary wetlands

1. A long‐lived bank of propagules consisting of eggs, seeds and spores is one mechanism that allows aquatic communities to survive drought. A drying (drought) event is, for aquatic organisms in a temporary wetland, a phase from which communities must recover. Such a dry phase is often considered a disturbance but should not be considered adverse or catastrophic for the organisms that have evolved to live in temporarily wet habitats. 2. This paper explores the parallels between the egg bank of zooplankton and the seed bank of aquatic plants as means of survival in temporary wetlands. The resilience of communities in temporary wetland ecosystems is assessed by examining dormancy, hatching, germination, establishment and reproduction of animals and plants from the egg and seed banks of wetlands with a range of wetting and drying regimes. 3. Both the zooplankton and aquatic plants of the temporary wetlands studied rely on their egg and seed banks as a means for surviving drying. These communities recover after the disturbance of drying by means of specific patterns of dormancy, dormancy breakage, hatching, germination, establishment and reproduction. Spatial and temporal patterns of species richness allow resilience through dormancy, as not all species are present at all sites and not all species hatch and germinate at the same time. Multiple generations in the egg and seed bank and complexity of environmental cues for dormancy breakage also contribute to the ecosystem's ability to recover after a drying event. A persistent egg and seed bank allows species‐rich communities to hatch, germinate and develop rapidly once dormancy is broken. Rapid establishment of species‐rich communities that reproduce rapidly and leave many propagules in the egg and seed bank also facilitates community recovery on flooding of a temporary wetland after a drying event. 4. To maintain the diversity of temporary wetland communities through droughts and floods we need to manage the dry and wet phases of wetlands. To conserve a wide range of wetland types, we need to maintain a variety of hydrological patterns across the landscape.     

Assessing the potential for biotic communities to recolonise freshwater wetlands affected by sulfidic sediments

1. The formation of sulfidic sediments in response to factors such as secondary salinisation and fertiliser usage is an emerging concern for the management of many freshwater wetlands. However, fundamental knowledge regarding the influence of sulfidic sediments on the aquatic biota is still lacking. 2. This study investigated the potential for biota to recolonise wetlands affected by sulfidic sediments, by assessing zooplankton hatching and aquatic plant germination following inundation with freshwater. Sediment samples were collected from 16 wetlands in the southern Murray‐Darling Basin, Australia, that ranged in condition from non‐impacted to possessing a known history of sulfidic sediments and/or acidification. 3. Principal Components Analysis indicated that the wetlands separated out into five different groups based on their sediment chemistry: non‐impacted, sulfidic, sulfidic and highly saline (sediment EC 46 800–209 000 μS cm^?1), sulfidic and potentially acidic (sediment pH 5.81–6.45 and ANC 0.07–0.31% CaCO₃), and sulfidic and acidic (sediment pH 4.37 and ANC 0.00% CaCO₃). 4. A viable dormant propagule bank was present in all wetlands, but the taxon richness of zooplankton and aquatic plants was significantly lower in wetlands affected by sulfidic sediments compared with those that were non‐affected. 5. This suggests that zooplankton and aquatic plants will be capable of recolonising wetlands that have accumulated sulfidic sediments via their propagule banks if the appropriate remediation measures are undertaken, although the communities developing are likely to be less diverse compared with those in non‐affected wetlands.     

Oxygenation of anoxic sediments triggers hatching of zooplankton eggs

Many coastal marine systems have extensive areas with anoxic sediments and it is not well known how these conditions affect the benthic–pelagic coupling. Zooplankton lay their eggs in the pelagic zone, and some sink and lie dormant in the sediment, before hatched zooplankton return to the water column. In this study, we investigated how oxygenation of long-term anoxic sediments affects the hatching frequency of dormant zooplankton eggs. Anoxic sediments from the brackish Baltic Sea were sampled and incubated for 26 days with constant aeration whereby, the sediment surface and the overlying water were turned oxic. Newly hatched rotifers and copepod nauplii (juveniles) were observed after 5 and 8 days, respectively. Approximately 1.5 × 10⁵ nauplii m⁻² emerged from sediment turned oxic compared with 0.02 × 10⁵ m⁻² from controls maintained anoxic. This study demonstrated that re-oxygenation of anoxic sediments activated a large pool of buried zooplankton eggs, strengthening the benthic–pelagic coupling of the system. Modelling of the studied anoxic zone suggested that a substantial part of the pelagic copepod population can derive from hatching of dormant eggs. We suggest that this process should be included in future studies to understand population dynamics and carbon flows in marine pelagic systems.     

Anatomical patterns of aerenchyma in aquatic and wetland plants

A well-developed aerenchyma is a major characteristic of aquatic plants. However, because such tissues are also found in wetland and terrestrial plants, it is not always possible to use their presence or absence to distinguish aquatic species. Whereas patterns of aerenchyma in roots have been studied in detail, those of the shoots have not. We collected and tested 110 species of various aquatic and wetland plants, including ferns (5), basal angiosperms (5), monocots (65), and eudicots (35). Three common and two rare types of aerenchyma were observed in their roots (three schizogeny and two lysigeny), plus five types of schizogeny in their shoots. We re-confirmed that, although a well-developed aerenchyma is more common in most organs of aquatic plants than in wetland plants, this presence cannot be used as strict evidence for the aquatic quality of vascular plants. Here, aerenchyma patterns were stable at the genus level, and the consistency of pattern was stronger in the roots than in the shoots. Furthermore, significant trends were verified in several higher taxa, and those consistencies of patterns partially coincided with their phylogeny.     

Effects of aquatic insect predators on zooplankton in fishless ponds

We removed the surface-orienting aquatic insects from a fishless pond to determine their predation effects on zooplankton behavior and size structure. A second fishless pond served as the unmanipulated reference system in this two year study. In the reference pond and the treatment pond prior to manipulation, daphnids exhibited pronounced diel vertical migrations. Following the removal of surface-orienting aquatic insects from the treatment pond, daphnid migration changed to a reverse migration strategy that was significantly different from that observed in the reference system. Average daphnid body size increased significantly following predator removal in the treatment system. Our data indicate that predation by aquatic insect predators, such as notonectids and dytiscids, may affect daphnid migration behavior in fishless systems. Vertical migration by daphnids may allow coexistence with surface-orienting insects in ponds that are deep enough to provide a spatial refuge from these predators.     

高原湿地草海水生植物多样性变化研究

采用文献对比,结合野外补点调查和观测,研究高原湿地草海水生植物多样性变化.结果表明：近三十年来,到草海越冬的黑颈鹤种群有逐渐增大趋势,1985年223只、2005年506只,到2011年约为1000只.2005年草海共有维管束植物49种,隶属25科37属,较1983年新增5科10属11种,2012年调查发现草海水生植被朝沼泽植被方向演替发展,荆三棱群落与水葱、李氏禾、水莎草和灯芯草群落一起发展为草海湖滨带主要优势挺水植物群落.空心莲子草在水体东部、东南及东北部入侵危害严重.外界干扰是草海生物多样性变化的主要原因,减少人为负面干扰、维护草海及周边环境稳定是保护和增加水生植物多样性的重要举措.     

Empirical relationships between the element composition of aquatic macrophytes and their underlying sediments

A simple view of the role of rooted macrophytes in element cycling sees them as pumps retrieving buried elements from the sediment profile. To investigate the relationship between the elemental composition of plants and sediments, we analysed published data for 39 elements. The best general model explained 84% of the variance of the log of plant element concentration: LPE = - 0.81 + 0.90 Log Sediment Element (ug/g dry wt.) – 0.12 Sediment Organic Content (ug/g drt wt.) + 0.67 Atomic radius (nm) (r ² = 0.84; n = 39)This close relationship between the concentrations of an element in plant tissues and in the underlying sediment indicates that acquatic plants do not differ markedly in element composition from the sediments in which they grow. T-tests between mean residuals indicated that these aquatic plants do not discriminate between essential and nonessential elements. Model II regression analyses showed no difference between the slopes of the functional relationships for individual elements and that of the general model. When the elements were separated into three groups (alkali, transition and related metals, and halogens), Log Sediment Element accounted for 75–96% of the variation in LPE. Element physicochemical parameters were also significant independent variables explaining an additional 3–12% variation in LPE. The relative importance of the independent variables differed for the three groups of elements.Abbreviations AN Atomic number - AR Atomic radius (nm) - ELEC Electronegativity - IPD Ionization potential differential (eV: the difference in ionization potential between the used and next lower oxidation number) - IR Ionic radius (nm) - LPE Log plant element (ug/g dry wt.) - LSE Log sediment element (ug/g drt wt.) - OP Oxidation potential (eV) - ORG Sediment organic content (ug/g dry wt.)     

The influence of water level and salinity on plant assemblages of a seasonally flooded Mediterranean wetland

We studied the key environmental variables shaping plant assemblages in Mediterranean abandoned ricefields with contrasting freshwater inputs over saline sediments. Plant species cover, water levels and soil variables were studied following a stratified random sampling design. Multivariate analysis identified water regime, particularly summer and autumn irrigation, as the most important environmental variable associated with vegetation composition. Distribution of annual and emergent macrophytes was not associated to salinity as found at the study site (0.57–4.1 mS/cm). Increased soil salinity, caused by summer irrigation near the soil surface did affect shallow-marsh assemblage distribution. These key environmental characteristics allowed us to identify six main assemblages. Annual macrophytes (such as Zannichellia palustris) were defined by high (over 10 cm) annual mean water level (MWL) and early successional conditions; emergent macrophytes (such as Typha spp., Scirpus lacustris) by annual MWL of 10 to − 25 cm and continuous shallow flooding in summer and autumn (MWL of 0–10 cm). The shallow-marsh group, correlated with annual MWL − 25 to − 100 cm, separated into two subgroups by salinity: grassland (including Paspalum distichum) with summer and autumn MWL below − 25 cm and brackish (with Juncus subulatus or Agrostis stolonifera) with summer and autumn MWL just below the soil surface (0 to − 25 cm). Water levels for the grassland subgroup may equate with a salinity ‘refuge’ for P. distichum. Time was a further determinant of variation in the full data set. Abundance of a large group of agricultural annuals (such as Sonchus tenerrimus) and damp ground annuals (including ricefield weeds such as Ammania robusta) decreased with time as bare ground disappeared. Maintenance of spatial vegetation heterogeneity in abandoned ricefields is contingent on continued water regime management.     

三江平原典型沼泽湿地螺类组成生态指示

螺类作为湿地的重要生物类群,对环境变化响应敏感,这使得螺类成为潜在的环境指示物种。为了研究中国东北沼泽湿地不同类型湿地螺类群落结构的差异以及螺类作为不同类型湿地指示物种的可能,在2014年9月和2015年5月对小叶章沼泽化草甸湿地、臌囊苔草湿地、毛苔草湿地、漂筏苔草湿地共17个采样点进行螺类样品采集。共采集到了螺类8科13属17种4452个。研究表明,螺类以扁卷螺科Planorbidae、椎实螺科Lymnaeidae、膀胱螺科Physidae为主;4种不同类型湿地螺的种类组成不同,这些螺类的种类组成与不同类型湿地的水深、植物类型组成等湿地特征是相对应。螺类的生物多样性指数(ShannonWiener指数和Marglef指数)在不同类型湿地之间也存在一定差异,筛选了指示螺类6种,无褶螺是小叶章沼泽化草甸的指示物种,小土蜗、半球多脉扁螺和虹蛹螺是臌囊苔草湿地的指示物种,琥珀螺是毛苔草湿地的指示物种,平盘螺是漂筏苔草湿地的指示物种,这表明了螺类是沼泽湿地类型的重要指示生物。也为螺类生物多样性资源的保护、恢复和生态评价提供科学依据和资料积累。     

Microcosm analysis of salinity effects on coastal lagoon plankton assemblages

A microcosm experiment was conducted to assess the effects of salinity on coastal lagoon plankton assemblages. Five salinity levels were replicated four-fold in 3801 fiberglass tanks. Salinity levels used were 0, 8.5, 17, 34 and 51 ppt, or 0, 25, 50, 100 and 150 percent seawater. These were achieved by mixing concentrated lagoon water and tapwater in different proportions. Tanks were inoculated with plankton collected from San Dieguito Lagoon (Del Mar, San Diego County, California) and other fresh and saline waterbodies in the area. Selected physical-chemical variables, phytoplankton, zooplankton, and other invertebrate populations were monitored on five sampling dates over a 114 day period (13 August–5 December 1986).Total phytoplankton abundance increased with salinity, for salinities >17 ppt. Most taxa showed marked effects of salinity, though the pattern of the effects often varied greatly from date to date. Chlorophytes tended to be most abundant at 51 ppt. Pyrrhophytes were most abundant at 0 or 51 ppt, and least abundant at 8.5 or 17 ppt. Cryptophytes increased with increasing salinity. Euglenophytes exhibited no salinity effect on any date. Bacillariophytes were most abundant at 8.5–34 ppt and least abundant at 51 ppt, with individual taxa showing maxima at 0–17 ppt (Navicula, Synedra), 8.5–34 ppt (Surirella, Amphora), and 34 ppt (Cylindrotheca).Total zooplankton abundance decreased with salinity, for salinities > 17 ppt. The dominant taxa were protozoans, rotifers, cladocerans, and copepods, and all but the first group showed strong salinity effects. Protozoan abundance was unaffected by salinity. Rotifers were most abundant at 0 ppt (Keratella, Filinia) or 8.5 ppt (Brachionus). With few exceptions, cladocerans (Alona, Ceriodaphnia, Scapholeberis) were found only at 0 ppt. Abundance of calanoid copepods decreased with increasing salinity, with individual taxa showing maxima at 0 ppt (Diaptomus), 8.5–17 ppt (Pseudodiaptomus, Eurytemora), and 34 ppt (Acartia). Cyclopoid copepods were most abundant at 17 ppt, with individual taxa showing maxima at 0 ppt (Eucyclops), 8.5 ppt (Halicyclops), and 17 ppt (Oithona). Harpacticoid copepods (Cletocamptus, Tachidius) were most abundant at 17–34 ppt. Ostracods and mosquito (Culex) larvae were most abundant at 8.5 ppt and absent at 34 and 51 ppt. Polychaetes generally were most abundant at 17–34 ppt, and water boatmen (Trichocorixa) at 8.5–34 ppt. Various physical and chemical variables also showed significant variations with salinity. Tending to increase with salinity were temperature, ammonia and orthophosphate concentrations. Decreasing with salinity were pH, dissolved oxygen and silica concentrations. The causes and interrelationships of these salinity effects are discussed.     

Occurrence of aquatic invertebrates of the wheatbelt region of Western Australia in relation to salinity

The wheatbelt region of Western Australia has been extensively cleared of indigenous vegetation for agriculture and is now severely affected by dryland salinity. Wetlands that were once freshwater are now saline and others are under threat, as are the animals and plants that inhabit them. Rising groundwater is also affecting the many naturally saline playas. To provide a framework for setting conservation priorities in this region a biological survey was undertaken, including sampling of aquatic invertebrates at 230 wetlands. In this paper, we have used data from the survey to summarise occurrence of species in relation to salinity. Total species richness at a wetland showed no response to salinity below 4.1 g l⁻¹ and then declined dramatically as salinity increased. When halophilic species were excluded from consideration, species richness was found to decline from 2.6 g l⁻¹. These patterns are compared to previous studies of richness-salinity relationships. There is some evidence that the freshwater invertebrate fauna of the wheatbelt may be comparatively salt tolerant, with 46% of freshwater species collected at salinities above 3 g l⁻¹ and 17% above 10 g l⁻¹, though these proportions differed between various invertebrate groups. While this tolerance will provide a buffer against the effects of mild salinisation, many species are at risk of regional extinction as salinisation becomes more widespread.     

Oxygen depletion and carbon dioxide and methane production in waters of the Pantanal wetland of Brazil

This study examines dissolved O₂, CO₂ and CH₄ in waters of the Pantanal, a vast savanna floodplain in Brazil. Measurements are presented for 540 samples from throughout the region, ranging from areas of sheet flooding to sluggish marsh streams to the major rivers of the region. Dissolved O₂ is often strongly depleted, particularly in waters filled with emergent vascular plants, which are the most extensive aquatic environment of the region. Median O₂ concentrations were 35 M for vegetated waters, 116 M for the Paraguay River, 95 M for tributary rivers, and 165 M for open lakes (atmospheric equilibrium, 230–290 M). Airwater diffusive fluxes were calculated from dissolved gas concentrations for representative vegetated floodplain waters, based on data collected over the course of an annual cycle. These fluxes reveal about twice as much CO₂ evasion as can be accounted for by invasion of O₂ (overall means in nmol cm^-2 s^-1: O₂ 0.18, CO₂ 0.34, and CH₄ 0.017). Methanogenesis is estimated to account for ca. 20% of the total heterotrophic metabolism in the water column and sediments, with the remainder likely due mostly to aerobic respiration. Anaerobic respiration is limited by the low concentrations of alternate electron acceptors. We hypothesize that O₂ transported through the stems of emergent plants is consumed in aerobic respiration by plant tissues or microorganisms, producing CO₂ that preferentially dissolves into the water, and thus explaining most of the excess CO₂ evasion. This hypothesis is supported by measurements of gases in submersed stems of emergent plants.     

水文情势与盐分变化对湿地植被的影响研究综述

湿地植被是湿地生态系统的重要组成部分。水文情势与盐分变化直接影响到湿地植被的分布与演替。目前,全球气候变化和人类活动导致的水文情势改变与盐分聚集已造成大面积的湿地退化和盐渍化,已严重威胁全球淡水湿地生态系统的稳定和健康。系统总结了水文情势与盐分变化单一环境变量及其交互作用对湿地植物生理生态、物种多样性、群落结构与演替和植被动态等诸多方面的影响研究进展,并探讨了湿地水文动态-盐分变化-植被响应的综合模型研究现状,认为发展湿地综合模型预测未来水文情势与盐分变化情景下湿地演变,是应对气候变化湿地水盐管理和生态保护的重要工具,最后指出今后亟需加强的研究方向。     

Ecological effects of low-level phosphorus additions on two plant communities in a neotropical freshwater wetland ecosystem

We conducted a low-level phosphorus (P) enrichment study in two oligotrophic freshwater wetland communities (wet prairies [WP] and sawgrass marsh [SAW]) of the neotropical Florida Everglades. The experiment included three P addition levels (0, 3.33, and 33.3 mg P m^–2 month^–1), added over 2 years, and used in situ mesocosms located in northeastern Everglades National Park, Fla., USA. The calcareous periphyton mat in both communities degraded quickly and was replaced by green algae. In the WP community, we observed significant increases in net aboveground primary production (NAPP) and belowground biomass. Aboveground live standing crop (ALSC) did not show a treatment effect, though, because stem turnover rates of Eleocharis spp., the dominant emergent macrophyte in this community, increased significantly. Eleocharis spp. leaf tissue P content decreased with P additions, causing higher C:P and N:P ratios in enriched versus unenriched plots. In the SAW community, NAPP, ALSC, and belowground biomass all increased significantly in response to P additions. Cladium jamaicense leaf turnover rates and tissue nutrient content did not show treatment effects. The two oligotrophic communities responded differentially to P enrichment. Periphyton which was more abundant in the WP community, appeared to act as a P buffer that delayed the response of other ecosystem components until after the periphyton mat had disappeared. Periphyton played a smaller role in controlling ecosystem dynamics and community structure in the SAW community. Our data suggested a reduced reliance on internal stores of P by emergent macrophytes in the WP that were exposed to P enrichment. Eleocharis spp. rapidly recycled P through more rapid aboveground turnover. In contrast, C. jamaicense stored added P by initially investing in belowground biomass, then shifting growth allocation to aboveground tissue without increasing leaf turnover rates. Our results suggest that calcareous wetland systems throughout the Caribbean, and oligotrophic ecosystems in general, respond rapidly to low-level additions of their limiting nutrient.     

Effect of incubation temperature on the hatching of rotifer resting eggs collected from sediments

Sediment samples from Loch Leven, Scotland, were incubated at 5°C, 10°C and 15°C to induce hatching of rotifer resting eggs. The emergent animals were identified and counted. The temperature which induced hatching varied among the nine species studied. These results are discussed in relation to seasonality and temperature preferences previously recorded for the most abundant species. Resting egg densities of 2.2–13.9 eggs cm^–3 were recorded in the upper 5 cm of sediment.     

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

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

Geographical patterns of species turnover in aquatic plant communities

1. A classic theory in biogeography predicts that high latitude communities are unstable. This may be because of decreased species richness or decreased environmental predictability and productivity towards the poles.
2. We studied latitudinal patterns in long-term community persistence of aquatic vascular plants in 112 Finnish lakes, situated within a 1000-km range from the northernmost to the southernmost lake.
3. Contrary to theoretical predictions, we found that the turnover rate of plant species in 45 years was inversely related to latitude. That is, plant communities in northern lakes were more persistent than communities in southern lakes. When we used multiple regression to find the best predictors of species turnover rate (TR), latitude was the only variable that was highly significantly related to species turnover rate. Area, species number, water transparency, pH and change in transparency did not notably explain the gradient observed.
4. The latitudinal trend was mainly because of lower species immigration rates at higher latitudes, whereas extinction rate did not so strongly decrease with increasing latitude. Immigrations and extinctions in the lakes were not in balance: the species numbers between the 1930s and 1980s increased more strongly in the southern than northern lakes.
5. We suggest that the inverse relationship between latitude and plant species TR in Finland is most probably caused by human influence on lakes, especially eutrophication and immigration of new species in southern latitudes. In addition, although species richness per lake did not decrease towards the north, the total species pool probably does, which means that in the north there are fewer species that can actually immigrate.