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No previous study has examined the large-scale distributional drivers of the entire global pool of 3,499 macrophyte species, despite the obvious importance of this for understanding the macroecology of these plants. To assess the hypothesis that natural rather than human-related transfer vectors act as the primary long-distance drivers of global movement of aquatic macrophytes, we analysed current macrophyte species distributions in relation to a set of human-related and natural transfer vectors. Most macrophytes (2,492 species: 71.2% of the global total) are endemic to a single ecozone, and generally lack the various functional adaptations needed for successful long-distance propagule transport. Such traits are, however, common in the 1,007 (28.8%) species native in > 1 ecozone. In total, 779 species (22.3%) are introduced, naturalised or invasive (I species) in one or more ecozones outwith their native range. The proportion of I species varies between ecozones and is best predicted by annual temperature and longitude. A migratory bird transfer vector and climatic variables strongly predict global native macrophyte species occurrence. Some native species of Miocene origin (or older) may have had their world distribution influenced by ancient vicariance events, while inter-ecozone hydrochory and Late Quaternary climate change are also relevant factors influencing a few species. [ABSTRACT FROM AUTHOR]

We analysed long-term monitoring data on submerged macrophytes and water chemistry from 666 Danish lakes > 1 hectare and mean depth < 3 m, encompassing a total of 1447 lake years. Our aim was to describe how plant cover (COV), plant volume inhabited (PVI) and species richness related to physical and chemical and environmental variables. Boosted regression tree (BRT) analyses revealed that chlorophyll a, Secchi depth and depth were the strongest predictors of COV and PVI. Chlorophyll had a strong negative effect up to 50 µg/l, whereas the changes related to Secchi depth and depth were more gradual and covered more of the gradient. Macrophyte species richness was best predicted by lake area and alkalinity, with chlorophyll a, nutrients and colour having significant but less marked effects. For chlorophyll a, 78% of the observed variance could be explained by the BRT model, with the most powerful predictors being both phosphorus and nitrogen, but with significant additional effects of plant cover and alkalinity. Our analyses revealed limited direct effect of nutrients on macrophyte abundance, but an indirect hierarchical effect of nutrients mediated through chlorophyll a with additional interactive effects by plant cover itself, alkalinity, mean depth and colour. [ABSTRACT FROM AUTHOR]

Macrophyte invasive alien species (IAS) fitness is often hypothesised to be associated with beneficial environmental conditions (environmental matching) or species‐poor communities. However, positive correlations between macrophyte IAS abundance and native plant richness can also arise, due to habitat heterogeneity (defined here as variation in abiotic and native biotic conditions over space and time). We analysed survey and palaeoecological data for macrophytes in satellite lakes along the Upper Lough Erne (ULE) system (Northern Ireland, UK), covering a gradient of eutrophication and connectivity to partition how environmental conditions, macrophyte diversity and habitat heterogeneity explained the abundance of Elodea canadensis, a widely distributed non‐native macrophyte in Europe. E. canadensis abundance positively correlated with macrophyte richness at both the within‐ and between‐lake scales indicating coexistence of native and invasive species over time. E. canadensis was also more prolific in highly connected and macrophyte‐rich lakes, but sparser in the more eutrophic‐isolated ones. Partial boosted regression trees revealed that in eutrophic‐isolated lakes, E. canadensis abundances correlated with water clarity (negatively), plant diversity (positively), and plant cover (negatively) whereas in diverse‐connected lakes, beta diversity (both positively and negatively) related to most greatly E. canadensis abundance. Dense macrophyte cover and unfavourable environmental conditions thus appear to confer invasibility resistance and sufficient habitat heterogeneity to mask any single effect of native biodiversity or environmental matching in controlling E. canadensis abundance. Therefore, in shallow lake landscapes, habitat heterogeneity variously enables the coexistence of native macrophytes and E. canadensis, reducing the often‐described homogenisation effects of invasive macrophytes. [ABSTRACT FROM AUTHOR]

To test hypotheses concerning the applicability of the Rapoport effect (RE: "species that occur at higher latitudes tend to have greater geographical range‐size than species which have ranges limited to latitudes closer to the equator") to aquatic macrophytes at global scale, we analysed the world latitudinal distribution and range‐size of 1,083 vascular aquatic macrophyte species, from 91 genera in 11 families. We targeted macrophyte families strongly associated with inland aquatic habitats (i.e. with a zero, or only very low, proportion of constituent species which occur also in non‐aquatic habitats), and which are distributed across a substantial latitudinal gradient, a necessary condition to test our hypotheses.The macrophyte species present in these families include plants from all the normally accepted life form‐defined functional groups of macrophytes, namely submerged, free‐floating, floating‐leaf rooted and emergent species, and represent the three major vascular taxonomic groups occurring as aquatic macrophytes (ferns/fern allies, monocots, and dicots). For the analysis, we used both latitude‐only and areal measures of macrophyte species geographic range‐size, within a 10 × 10° (latitude × longitude) grid of 238 grid cells, covering the six world ecozones (Palaearctic, Orient, Australasia, Nearctic, Neotropics, Afrotropics) that primarily contain inland freshwater and brackish macrophyte habitats.The results provide new insight into the relationships between global range‐size of macrophytes, latitude, and other potential spatio‐environmental and anthropogenic drivers acting upon these plants at world scale. In particular, the outcomes indicated that: (1) the range‐size versus latitude distribution of macrophytes shows evidence of a strong RE influence, with significantly greater species range‐size at higher latitudes; and (2) the β‐diversity pattern of species distribution along this latitudinal gradient is poorly explained by nestedness organisation, and species turnover is a more likely explanation of the observed changes in species distribution with latitude.Spatio‐environmental and anthropogenic variables other than latitude may also influence the observed global geographical pattern of macrophyte range‐size, although their importance as predictors varies between individual families. Extent of agricultural land use, altitude, and historic (post‐Quaternary) climate change velocity were all significant predictor variables for some families. However, interestingly, neither the area of land nor the area of waterbody present per grid cell were major influences on macrophyte range‐size distribution.Our finding of evidence for an RE, acting at global scale in aquatic macrophytes, contributes to increasing the generality of conclusions so far reached about the large‐scale factors that drive patterns of species range‐size at global scale. The study also provides a baseline for future macroecological work on aquatic plants, and potentially other freshwater organisms, particularly in the context of predicting how the world ranges of freshwater biota will respond to ongoing global environmental change. [ABSTRACT FROM AUTHOR]

Aim: To investigate if connectivity and zebra mussel (Dreissena polymorpha) occurrence can mitigate effects of eutrophication in a lowland lake landscape. Location: Upper Lough Erne, Northern Ireland, UK. Methods: Data on environment, macrophytes and invertebrates were assembled for three basins of a large central lake and its satellite floodplain lakes via field surveys and palaeolimnological analyses. Space–time interaction analyses of palaeoecological data were compared pre‐1950 and post‐1950. Multivariate analyses examined how connectivity, environment and zebra mussels influenced contemporary lake communities, and explain their divergence from historical communities in the past. Results: Pre‐1950, we found high community variation across sites and low within‐lake variation in macrophytes, but progressive eutrophication accentuated within‐lake community variation after 1950. Partitioning analysis showed larger effects of connectivity than nutrient enrichment on contemporary macrophyte composition, while local effects structured invertebrate communities. Three clusters of lakes were revealed according to variation in macrophyte composition, isolation from the central lake and nutrient enrichment: Group 1– the central lake and six nearby lakes were meso‐eutrophic (TP = 66.7 ± 47.6 μg/L; TN = 0.79 ± 0.41 mg/L) and had the highest zebra mussel abundances and organismal biodiversity; Group 2– Eight eutrophic (TP = 112±36.6 μg/L; TN = 1.25 ± 0.5 mg/L) and connected lakes; Group 3– Seven isolated and hypertrophic (TP = 163.2 ± 101.5 μg/L; TN = 1.55 ± 0.3 mg/L) lakes. Pre‐1950 palaeolimnological data for macrophytes and invertebrates for 5 lakes and a basin in the central lake most resembled extant lake communities of Group 1. However, palaeo‐records revealed that macrophytes and invertebrates subsequently converged towards those of Groups 2 and 3. Main conclusions: Our study reveals that the central "mother" lake acts as a hub for preserving biodiversity via shared hydrological connectivity with satellite lakes and high zebra mussel abundances. These may buffer the impoverishing effects of eutrophication and sustain unexpectedly high biodiversity in the short term. Such protective buffering, however, cannot be relied upon indefinitely to conserve biodiversity. [ABSTRACT FROM AUTHOR]

Submerged macrophytes play a key role in north temperate shallow lakes by stabilizing clear-water conditions. Eutrophication has resulted in macrophyte loss and shifts to turbid conditions in many lakes. Considerable efforts have been devoted to shallow lake restoration in many countries, but long-term success depends on a stable recovery of submerged macrophytes. However, recovery patterns vary widely and remain to be fully understood. We hypothesize that reduced external nutrient loading leads to an intermediate recovery state with clear spring and turbid summer conditions similar to the pattern described for eutrophication. In contrast, lake internal restoration measures can result in transient clear-water conditions both in spring and summer and reversals to turbid conditions. Furthermore, we hypothesize that these contrasting restoration measures result in different macrophyte species composition, with added implications for seasonal dynamics due to differences in plant traits. To test these hypotheses, we analyzed data on water quality and submerged macrophytes from 49 north temperate shallow lakes that were in a turbid state and subjected to restoration measures. To study the dynamics of macrophytes during nutrient load reduction, we adapted the ecosystem model PCLake. Our survey and model simulations revealed the existence of an intermediate recovery state upon reduced external nutrient loading, characterized by spring clear-water phases and turbid summers, whereas internal lake restoration measures often resulted in clear-water conditions in spring and summer with returns to turbid conditions after some years. External and internal lake restoration measures resulted in different macrophyte communities. The intermediate recovery state following reduced nutrient loading is characterized by a few macrophyte species (mainly pondweeds) that can resist wave action allowing survival in shallow areas, germinate early in spring, have energy-rich vegetative propagules facilitating rapid initial growth and that can complete their life cycle by early summer. Later in the growing season these plants are, according to our simulations, outcompeted by periphyton, leading to late-summer phytoplankton blooms. Internal lake restoration measures often coincide with a rapid but transient colonization by hornworts, waterweeds or charophytes. Stable clear-water conditions and a diverse macrophyte flora only occurred decades after external nutrient load reduction or when measures were combined. [ABSTRACT FROM AUTHOR]

This study assessed the hypothesis that spatial and environmental drivers of river macrophyte diversity and community composition differ in relative importance in calcareous river systems located in warm regions of America versus Africa., We collected aquatic vegetation and spatio-environmental data, during 2006-11, from >200 hardwater rivers, and associated floodplain waterbodies, located up to 30° north or south of the Equator, in México, Trinidad, Brazil, Argentina, U.S.A. (Florida), South Africa, Botswana and Zambia., Species rarefaction procedures were used to assess the impacts of differing sampling effort in the two continents upon estimation of γ-diversity ('species pool'). We then used a cluster analysis approach (two-way indicator species analysis: TWINSPAN) to classify samples into groups based upon species composition. Variation in species richness, community composition and six spatial and environmental variables, among samples making up these groups, were compared using ANOVA and Kruskal-Wallis procedures. Regression trees and redundancy analysis were used to infer the relative importance of spatial and environmental drivers in explaining variation in local species richness and species community composition between the two continents. Sorensen's index ( Cs) was calculated to estimate species turnover (β-diversity) between African and American samples., In total, 378 macrophyte taxa were recorded, with no significant difference in mean macrophyte α-diversity between African and American sites, but with evidence for high species turnover between the two continents ( Cs = 0.17). Rarefaction analysis confirmed the existence of a larger macrophyte species pool in the hardwater rivers sampled in Africa compared to America. TWINSPAN classification identified seven sample end-groups, only one of which contained a mix of sites from both continents. PERMANOVA and non-metric multidimensional scaling ordination analysis confirmed significant differences in community composition present in these sample-groups. There were substantial differences between the sample-groups for α-diversity, and for spatial and environmental variables., The high species turnover between Africa and America may be accounted for by geographical segregation, along with differences in aquatic habitat characteristics, and varying long-distance dispersal capacities of individual species. The relative importance of spatial and physicochemical drivers (latitude, pH, altitude, alkalinity and electrical conductivity but not flow) differed between the continents in influencing variation in both macrophyte diversity and community composition. Latitude was a significant, although nonlinear and rather complex, spatial driver of macrophyte α-diversity in both American and African hardwater rivers. Water chemistry variables varied in relative importance as drivers of macrophyte α-diversity for African and American sites individually, and for all sites combined, but pH and/or electrical conductivity were more important than alkalinity in each case. In all three cases, altitude was consistently the third most important driver of α-diversity. Spatial and environmental variables played important roles in structuring macrophyte community composition in warm-water calcareous rivers in both America and Africa, with latitude being the strongest individual driver. Thus, this spatial variable, which is a surrogate for numerous enviro-climatic variables, appears to be of importance in determining macrophyte distributions at large spatial scales, for the ecosystem type examined here. [ABSTRACT FROM AUTHOR]

Shallow lakes may play an important role for the nitrogen (N) balance in drainage basins by processing, transferring and retaining N inputs. An increase in the frequency of storm-induced short-term N pulses and increased water temperatures are both likely outcomes of climate change, potentially affecting the N processing in lakes., An experiment with a K15 NO3− pulse addition (increase in NO3− concentration from c. 0.1 to 2 mg/L) was carried out in 12 mesocosms with relatively low (applies to Danish lakes) total N ( TN) and total phosphorus ( TP) concentrations ( c. 0.3 mg N L−1 and 0.04 mg P L−1) to assess the effects of an N pulse on N processing and storage in shallow lake ecosystems. The mesocosms have a hydraulic retention time of approximately two and a half months, and at the time of the experiment, they had been adapted to contrasting temperatures for a period of 10 years: ambient, T3 (heating according to the Intergovernmental Panel on Climate Change 2007 A2 scenario, +3.7-4.5°C, depending on season) and T5 (heating with A2 + 50%, +4.9-6.6°C)., Macrophytes and filamentous algae retained up to 40% and 30% of the added 15N, respectively, reflecting their high biomass in the mesocosms. Macrophytes and filamentous algae constituted between 70% and 80% of the biomass of all primary producers during the experiment in the T3 and ambient treatments and between 20% and 40% in T5. By comparison, less than 1% of the added 15N diffused to the sediment and less than 5% was lost to the atmosphere as N2 gas. Snails represented the long-term storage of 15N, retaining up to 6% of the tracer and with detectable enrichment 100 days after tracer addition., We found no significant differences among the temperature treatments in the 15N turnover after pulse dosing. However, a larger percentage of 15N was stored in macrophytes in the ambient and T3 mesocosms, reflecting higher biomasses than in T5 where filamentous algae were more abundant. Macrophytes and filamentous algae rather than temperature were therefore key controllers of N processing during the summer N pulse in these shallow, relatively low TP lakes. [ABSTRACT FROM AUTHOR]

Shallow lakes are a key component of the global carbon cycle. It is, therefore, important to know how shallow lake ecosystems will respond to the current climate change. Global warming affects not only average temperatures, but also the frequency of heat waves (HW). The impact of extreme events on ecosystems processes, particularly greenhouse gas ( GHG) emissions, is uncertain., Using the world's longest-running shallow lake experiment, we studied the effects of a simulated summer HW on the fluxes of carbon dioxide ( CO2), methane ( CH4) and nitrous oxide (N2O). The experimental mesocosms had been exposed to different temperature treatments and nutrient loading for 11 years prior to the artificial HW., In general, there was an increase in total GHG emissions during the 1-month artificial HW, with a significant increase in CO2, CH4 and N2O being observed in the shallow lake mesocosms. No significant effect of the HW on CO2 emissions could be traced, though, in the mesocosms with high nutrient levels. Furthermore, the data suggested that in addition to the direct effect of increased temperature on metabolic processes during the HW, biotic interactions exerted a significant control of GHG emissions. For example, at low nutrient levels, increased CO2 emissions were associated with low macrophyte abundance, whereas at high nutrient levels, decreased phytoplankton abundance was linked to increased emissions of CO2 and CH4., In contrast to the observable heat-wave effect, no clear general effect of the long-term temperature treatments could be discerned over the summer, likely because the potential effects of the moderate temperature increase, applied as a press disturbance, were overridden by biotic interactions. This study demonstrates that the role of biotic interactions needs to be considered within the context of global warming on ecosystem processes. [ABSTRACT FROM AUTHOR]

Fresh waters make a disproportionately large contribution to greenhouse gas ( GHG) emissions, with shallow lakes being particular hot spots. Given their global prevalence, how GHG fluxes from shallow lakes are altered by climate change may have profound implications for the global carbon cycle. Empirical evidence for the temperature dependence of the processes controlling GHG production in natural systems is largely based on the correlation between seasonal temperature variation and seasonal change in GHG fluxes. However, ecosystem-level GHG fluxes could be influenced by factors, which while varying seasonally with temperature are actually either indirectly related (e.g. primary producer biomass) or largely unrelated to temperature, for instance nutrient loading. Here, we present results from the longest running shallow-lake mesocosm experiment which demonstrate that nutrient concentrations override temperature as a control of both the total and individual GHG flux. Furthermore, testing for temperature treatment effects at low and high nutrient levels separately showed only one, rather weak, positive effect of temperature ( CH4 flux at high nutrients). In contrast, at low nutrients, the CO2 efflux was lower in the elevated temperature treatments, with no significant effect on CH4 or N2O fluxes. Further analysis identified possible indirect effects of temperature treatment. For example, at low nutrient levels, increased macrophyte abundance was associated with significantly reduced fluxes of both CH4 and CO2 for both total annual flux and monthly observation data. As macrophyte abundance was positively related to temperature treatment, this suggests the possibility of indirect temperature effects, via macrophyte abundance, on CH4 and CO2 flux. These findings indicate that fluxes of GHGs from shallow lakes may be controlled more by factors indirectly related to temperature, in this case nutrient concentration and the abundance of primary producers. Thus, at ecosystem scale, response to climate change may not follow predictions based on the temperature dependence of metabolic processes. [ABSTRACT FROM AUTHOR]

Highly calcareous (marl) lakes are infrequent but important freshwater ecosystems, protected under the EU Habitats and Species Directive. Chara lakes have been considered resistant to eutrophication owing to the self-stabilising properties of charophyte meadows. However, the opposite is suggested by the large-scale biodiversity declines in marl lake taxa in Europe, and evidence of charophyte sensitivity to eutrophication. We combined contemporary, palaeolimnological and archival methods to investigate the eutrophication of Hawes Water, a shallow marl lake in north-west England ( U.K.)., Changes in aquatic macrophyte and invertebrate communities were reconstructed through the analysis of historical macrophyte surveys and sedimentary plant and animal macrofossils in two dated sediment cores from the littoral and deep zones of the lake. In addition, chlorophyll and carotenoid pigments were analysed to track changes in primary production from benthic and pelagic areas. Substantial changes in macrophyte communities were detected over centennial timescales, suggesting high ecosystem sensitivity considering the presently moderate phosphorus concentrations in Hawes Water (mean annual total phosphorus 20 μg L−1)., Two apparent periods of threshold-like change were identified from the sediment record: (i) changes in cyanobacteria (aphanizophyll + myxoxanthophyll to canthaxanthin + zeaxanthin) and potentially in nutrient stoichiometry, reductions in the maximum macrophyte colonisation depth and water clarity, reduced charophyte and Potamogeton diversity, and increases in Nymphaeaceae; and (ii) severe reductions in light availability inferred from subdecadal doubling in phytoplankton abundance, substantial increases in Daphnia abundance and the extinction of charophytes from higher water depths., Further, change in both the littoral and deeper water has confined key marl lake taxa to smaller niches. In the littoral, increasing siltation and reed and Nymphaeaceae densities caused extinction of Littorella uniflora in the early 1900s and have reduced the evenness of Characeae with suspected imminent extinction of two highly localised Chara spp. In the deeper water, upslope creep of maximum colonisation depth has reduced habitat for intermediate-depth marl lake taxa leading to the loss of four Potamogeton and one Chara species, and replacement of these taxa by Nuphar lutea., The large changes in macrophyte community composition and increased incidences of turbid water have reduced the distinctive and valued marl lake features of Hawes Water, indicating that marl lakes can, as a habitat type, be highly sensitive to eutrophication. The persistence of abundant generalist macrophyte species at considerable water depth may be a feature of high-alkalinity lakes in clearwater, macrophyte-dominated states, but is a distinct eutrophication response in marl lakes rather than an indication of resistance to eutrophication. [ABSTRACT FROM AUTHOR]

Zooplankton community response to the combined effects of nutrients and fish (hereafter N + F) at contrasting temperatures was studied in a long-term experiment conducted in 24 shallow lake mesocosms with low and high nutrient levels. We found a positive effect of N + F on zooplankton biomass, chlorophyll- a and turbidity. In contrast, zooplankton species and size diversity decreased with added N + F, as did submerged macrophyte plant volume inhabited (PVI). The community composition of zooplankton in high N + F mesocosms was related to chlorophyll- a and turbidity and to macrophyte PVI in the low N + F mesocosms. Macrophytes can protect zooplankton from fish predation. Compared to N + F effects, temperature appeared to have little effect on the zooplankton community. Yet analysis of community heterogeneity among treatments indicated a significant temperature effect at high N + F levels. The results indicate an indirect temperature effect at high N + F levels that can be attributed to temperature-dependent variation in fish density and/or chlorophyll- a concentration. [ABSTRACT FROM AUTHOR]

Ponds have significant conservation value due to their potentially high contribution to local and regional diversity. However, most ponds are located in anthropogenically influenced areas, and their biodiversity is constantly threatened by human activities. Thus, knowledge of the effect of pond management on biodiversity is essential for designing effective conservation strategies., Here, we study the main drivers of diversity of three functional groups of primary producers (phytoplankton, filamentous green algae and submerged macrophytes) in 87 ponds distributed across a large region (c. 90000 km2) in Southern Spain. We hypothesised that spatial effects would increase with increasing propagule size. However, given the regional extent of the study, we anticipated that both spatial and environmental controls would be significant., We determined α -, β - and γ-components of biodiversity for each functional group and pond type (embankment ponds, excavated ponds and artificial ponds) and assessed the influence of environmental and spatial drivers on diversity with generalised additive models ( GAMs). Redundancy analyses ( RDAs) with variation partitioning were used to determine the relative contribution of environmental and spatial predictors of the community assembly. Spatial variables were calculated by applying distance-based Moran's eigenvector maps (db- MEM)., Both α - and β -diversities of phytoplankton and filamentous green algae varied significantly with pond type. Generally, environmental predictors of diversity were more important than spatial variables. Assemblage structure was controlled by water chemistry and eutrophication, with a marked influence of pond type. Spatial variables included broad-scale variation for the three groups of primary producers, which were also strongly influenced by the management regime., Limited management activities, as occurs at embankment ponds, promoted the local richness of phytoplankton and contributed importantly to the regional diversity of macrophytes. Moreover, the relative contribution of environmental and spatial variables was similar between embankment ponds and natural ecosystems, that is, dispersal limitation increasing with propagule size. Excavated and artificial ponds also contributed importantly to regional diversity by enhancing phytoplankton and filamentous green algal β-diversity. However, spatial patterns in the latter pond type did not meet our expectations, most likely due to the intensive disturbance from pond management hampering the development of macrophytes. [ABSTRACT FROM AUTHOR]

Aquatic macrophytes are commonly used to assess the ecological condition of lakes. Little is known, however, about long-term macrophyte dynamics in shallow lakes. In the absence of historical data, the remains of macrophytes (fruits, seeds and vegetative fragments) found in lake sediments may provide just such information. In order to interpret confidently past change in aquatic plant communities from their sedimentary remains, it is vital to establish the similarity between the contemporary and fossil assemblages., We investigated the relationship between present lake vegetation and plant macrophyte remains in surface sediments. Thirty-five shallow lakes, spanning around six degrees of latitude and mostly located in the semi-arid Mediterranean climatic zone of Turkey, were sampled for aquatic plants, surface sediment plant remains and a range of other key environmental variables., Around 50% of the taxa recorded in the modern vegetation were represented in the sediment. Sedimentary macrofossils of some taxa were under- or over-represented relative to their frequency in the modern vegetation, for example Potamogeton spp. and Characeae, respectively. Despite this disparity, there was good agreement between the assemblage composition of the modern and sedimentary samples. Furthermore, conductivity and trophic state (as indicated by total nitrogen, total phosphorus and chlorophyll- a) were the environmental variables most clearly correlated with both the contemporary and macrofossil assemblages in these lakes., We conclude that aquatic macrophyte macrofossils can be used as reliable indicators of ecological status and to determine qualitative changes in assemblages of aquatic plants consequent to environmental change (e.g. in lake trophic status and/or salinity). This may be especially useful for lakes in arid and semi-arid Mediterranean regions, which are particularly vulnerable to hydrological constraints under climate change. [ABSTRACT FROM AUTHOR]

ABSTRACT [ABSTRACT FROM AUTHOR]

1. Eutrophication has a profound effect on the biological structure and function of shallow lakes, altering the composition and abundance of submerged macrophyte and fish assemblages. Relatively little is known, however, about decadal to centennial-scale change in these important aspects of shallow lake ecology. 2. Established palaeolimnological inference models are limited to reconstructing a single variable. As macrophyte and zooplanktivorous fish abundance exert dual and interacting controls on cladoceran assemblages a single variable inference model may contain significant error. To obviate this problem, we applied a new cladoceran-based multivariate regression tree (MRT) model to cladoceran subfossil assemblages from dated cores from a small shallow lake (Felbrigg Lake, U.K.) to assess long-term change in fish and submerged macrophyte abundance. Plant macrofossil, chironomid and mollusc subfossil assemblages were also analysed to track changes in biological structure and function and to evaluate the inferences of the MRT model. 3. Over the 200+ year period covered by the sediment cores, there was good agreement in the timing and nature of ecological change reflected by the plant macrofossil, mollusc, chironomid and cladoceran data. The sediment sequence was divided into three dated zones: c. 1797–1890, c. 1890–1954 and c. 1954–present. Prior to 1890 plant-associated mollusc, cladoceran and chironomid assemblages indicated a species-rich macrophyte community; a scenario confirmed by the plant macrofossil data. From c. 1890 to 1954 macrophyte-associated species of all three invertebrate groups remained abundant but the proportion of pelagic cladocerans rose. Post-1954 mollusc and chironomid assemblages changed to sediment associated detrital feeders and the proportion of pelagic cladoceran taxa increased further. 4. The cladoceran-based MRT model indicated a long period of stability, c. 1790–1927, characterised by abundant submerged macrophytes and zooplanktivorous fish. From c. 1927 to 1980, the MRT model inferred a decline in zooplanktivorous fish density (ZF) but relative stability in August macrophyte abundance. From 1980 to 2000, an increase in zooplanktivorous fish was inferred tallying well with available data on the fish population (since the 1970s), which indicated extirpation of perch in the 1970s and a subsequent increase in the rudd population. The model inferred little change in August macrophyte abundance until post- c. 1980 at which point it indicated a decline. The surface sediment assemblage was placed in MRT group A, where submerged plants are absent or very rare in late summer in good agreement with current conditions at the site. 5. The MRT model, applied here for the first time, appears to have successfully tracked changes in macrophyte abundance and ZF over the last 200 years at Felbrigg Lake. The inferences agreed with historical observations on the fish community and the supporting palaeolimnological data. Given that multiple structuring forces shape most biological communities, the application of a model capable of allowing for this represents a significant advance in palaeolimnology. [ABSTRACT FROM AUTHOR]

1. Shallow lakes are often cited as classic examples of systems that exhibit trophic cascades but, whilst they provide good model systems with which to test general ecological theory and to assess long-term community change, their food web linkages have rarely been resolved, so changes associated with the structure and dynamics of the ecological network as a whole are still poorly understood. 2. We sought to redress this, and to demonstrate the potential benefits of integrating palaeolimnological and contemporary data, by constructing highly resolved food webs and stable isotope derived measures of trophic interactions and niche space, for the extant communities of two shallow U.K. lakes from different positions along a gradient of eutrophication. The contemporary surface sediment cladoceran and submerged macrophyte assemblages in the less enriched site, Selbrigg Pond, matched the palaeolimnological assemblages of the more enriched site, Felbrigg Hall Lake, in its more pristine state during the 1920s. Thus, Selbrigg was a temporal analogue for Felbrigg, from which the consequences of long-term eutrophication on food web structure could be inferred. These data represent the first steps towards reconstructing not only past assemblages (i.e. nodes within a food web), but also past interactions (i.e. links within a food web): a significant departure from much of the previous research in palaeolimnology. 3. The more eutrophic food web had far fewer nodes and links, and thus a less reticulate network, than was the case for the more pristine system. In isotopic terms, there was vertical compression in δ15N range (NR) and subsequent increased overlap in isotopic niche space, indicating increased trophic redundancy within Felbrigg. This structural change, which was associated with a greater channelling of energy through a smaller number of nodes as alternative feeding pathways disappear, could lead to reduced dynamic stability, pushing the network towards further simplification. These changes reflected a general shift from a benthic-dominated towards a more pelagic system, as the plant-associated subweb eroded. 4. Although these data are among the first of their kind, the palaeo-analogue approach used here demonstrates the huge potential for applying food web theory to understand how and why these ecological networks change during eutrophication. Furthermore, because of the rich biological record preserved in their sediments, shallow lakes represent potentially important models for examining long-term intergenerational dynamics, thereby providing a means by which models and data can be integrated on meaningful timescales – a goal that has long proved elusive in food web ecology. [ABSTRACT FROM AUTHOR]

1. Submerged macrophyte and phytoplankton components of eutrophic, shallow lakes have frequently undergone dynamic changes in composition and abundance with important consequences for lake functioning and stability. However, because of a paucity of long-term survey data, we know little regarding the nature, direction and sequencing of such changes over decadal–centennial or longer timescales. 2. To circumvent this problem, we analysed multiple ( n = 5) chronologically correlated sediment cores for plant macro-remains and a single core for pollen and diatoms from one small, shallow, English lake (Felbrigg Hall Lake, Norfolk, U.K.), documenting 250 years of change to macrophyte and algal communities. 3. All five cores showed broadly similar stratigraphic changes in macrophyte remains with three distinct phases of macrophyte development: Myriophyllum–Chara–Potamogeton ( c. pre-1900), to Ceratophyllum–Chara–Potamogeton ( c. 1900–1960) and finally to Zannichellia–Potamogeton ( c. post-1960). Macrophyte species richness declined from at least 10 species pre-1900 to just four species at the present day. Additionally, in the final Zannichellia–Potamogeton phase, a directional shift between epi-benthic and phytoplankton-based primary production was indicated by the diatom data. 4. Based on macrophyte–seasonality relationships established for the region, concomitant with the final shift to Zannichellia–Potamogeton, we infer a reduction in the seasonal duration of plant dominance (plant-covered period). Furthermore, we hypothesise that this change in species composition resulted in a situation whereby macrophyte populations were seasonally ‘sandwiched’ between two phytoplankton peaks in spring and late summer as observed in the contemporary lake. 5. We suggest that eutrophication-induced reductions in macrophyte species richness, especially if the number of plant-seasonal strategies is reduced, may constrict the plant growing season. In turn, this may render a shallow lake increasingly vulnerable to seasonal invasions of phytoplankton resulting in further species losses in the plant community. Thus, as part of a slow (over perhaps 10–100s of years) and self-perpetuating process, macrophytes may be gradually pushed out by phytoplankton without the need for a perturbation as required in the alternative stable states model of plant loss. [ABSTRACT FROM AUTHOR]

1. To correctly interpret chironomid faunas for palaeoenvironmental reconstruction, it is essential that we improve our understanding of the relative influence of ecosystem variables, biotic as well as physicochemical, on chironomid larvae. To address this, we analysed the surface sediments from 39 shallow lakes (29 Norfolk, U.K., 10 Denmark) for chironomid head capsules, and 70 chironomid taxa (including Chaoborus) were identified. 2. The shallow lakes were selected over large environmental gradients of aquatic macrophytes, total phosphorus (TP) and fish communities. Redundancy analysis (RDA) identified two significant variables that explained chironomid distribution: macrophyte species richness ( P < 0.001) and TP ( P < 0.005). Generalised linear models (GLM) identified specific taxa that had significant relationships with both these variables. Macrophyte percentage volume infested (PVI) and species richness were significant in classifying the lake types based on chironomid communities undertwinspan analysis, although other factors, notably nutrient concentrations and fish communities, were also important, illustrating the complexities of classifying shallow lake ecosystems. Lakes with plant species richness >10 all had relatively diverse (Hill’s N2) chironomid assemblages, and lakes with Hill’s N2 >10 all had TP <250 μg L−1 and total fish densities <2 fish per m2. 3. Plant density (PVI), and perhaps more importantly species richness, were primary controls on the distribution of chironomid communities within these lakes. This clearly has implications for palaeoenvironmental reconstructions using zoobenthos remains (i.e. chironomids) and suggests that they could be used to track changes in benthic/pelagic production and could be used as indicators of changing macrophyte habitat. 4. Measuring key biological gradients, in addition to physicochemical gradients, allowed the major controls on chironomid distribution to be assessed more directly, in terms of plant substrate, food availability, competition and predation pressure, rather than implying indirect mechanisms through relationships with nutrients. Many of these variables, notably macrophyte abundance and species richness, are not routinely measured in such studies, despite their importance in determining zoobenthos in temperate shallow lakes. 5. When physical, chemical and ecological gradients are considered, as is often the case with palaeo-reconstructions rather than training sets chosen to maximise one gradient, complex relationships exist, and attempting to reconstruct a single trophic variable quantitatively may not be appropriate or reliable. [ABSTRACT FROM AUTHOR]

Eutrophication is commonly implicated in the reduction in macrophyte species richness in shallow lakes. However, the extent to which other more nuanced measures of macrophyte diversity, such as assemblage heterogeneity, are impacted concurrently by eutrophication over space and time and the joint influences of other factors (e.g., species invasions and connectivity) remains relatively poorly documented. Using a combination of contemporary and paleoecological data, we examine how eutrophication influences macrophyte assemblage heterogeneity and how nutrient enrichment interacts with watercourse connectivity, lake surface area, and relative zebra mussel abundance over space (within and among lakes) and time (decades to centuries) at the landscape scale. The study system is the Upper Lough Erne, Northern Ireland, UK, which is composed of a large main lake and several smaller satellite lakes that vary in their hydrological connectivity to the main lake. By applying homogeneity analysis of multivariate dispersions and partial redundancy analysis, we demonstrate that contemporary lake macrophyte heterogeneity and species richness are reduced in lakes with intensified eutrophication but are increased in lakes with greater zebra mussel abundance and lake surface area. Watercourse connectivity positively influenced assemblage heterogeneity and explained larger proportions of the variation in assemblage heterogeneity than local environmental factors, whereas variation in species richness was better related to local abiotic factors. Macrophyte fossil data revealed within‐ and among‐lake assemblage homogenization post‐1960, with the main lake and connected sites showing the highest rates of homogenization due to progressive eutrophication. The long‐term and contemporary data collectively indicate that eutrophication reduces assemblage heterogeneity over time by overriding the importance of regional processes (e.g., connectivity) and exerts stronger pressure on isolated lakes. Our results suggest further that in connected lake systems, assemblage heterogeneity may be impacted more rapidly by eutrophication than species richness. This means that early effects of eutrophication in many systems may be underestimated by monitoring that focuses solely on species richness and is not performed at adequate landscape scales. [ABSTRACT FROM AUTHOR]

Shallow lakes produce and emit substantial amounts of methane (CH 4). Part of the CH 4 produced in lakes is consumed by methane-oxidizing bacteria (MOB) present in the sediment and water column, thus reducing the overall CH 4 emissions. However, the role of aquatic plants as habitat for CH 4 oxidation by MOB is poorly understood. In this study, we compared CH 4 oxidation rates and MOB abundance associated with different types of aquatic plants (periphyton, filamentous algae, and both above-ground macrophytes and their rhizosphere). The plants were collected from shallow lake mesocosms exposed to experimental nutrient enrichment and warming treatments for 17 years prior to this study. Incubations of all sampled plants showed CH 4 oxidation, with above-ground macrophyte tissue and filamentous algae having the highest rates of up to 0.25 µmol CH 4 h−1 g−1 dw. Oxidation rates associated with macrophytes were species dependent, with consumption rates on rhizospheres of Potamogeton crispus higher than those on Elodea canadensis. The increase in nutrients and dissolved CH 4 in the water tended to increase MOB abundance and activity for all plant types, while no effect of long term warming was detectable. Our results showed that MOB associated with periphyton, filamentous algae and macrophytes oxidize CH 4 in shallow lakes at different rates across species or plant types. We also found that high macrophyte biomass is associated with reduced CH 4 concentration in the water. This study shows that CH 4 oxidation occurs on many plant surfaces but that oxidation rates alone cannot explain the reduced CH 4 emissions at higher plant biomass. [ABSTRACT FROM AUTHOR]

• A new ploidy database provides cytotype information for 1574 macrophyte species. • Polyploids occupy larger geographical ranges than haploid/diploid macrophytes. • Differing latitudinal patterns exist between haploid/diploid and polyploid species. • Temperature and evapotranspiration drive global macrophyte cytotype patterns. • Polyploidy in macrophytes may be an adaptation to survive in harsh environments. Despite the increasing evidence that ploidy is a factor of importance for environmental adaptation, little work has hitherto been done about cytotype patterns in aquatic plants. We tested a set of hypotheses concerning the drivers of ploidy in macrophyte communities at worldwide scale, and also how ploidy state may be related to other attributes of aquatic plants such as community diversity, species range size and invasiveness. We built a global ploidy database with 1574 species (representing 45.0 % of the known total global pool) and allocated ploidy state to species occurring in 238 10 × 10° gridcells worldwide. We used Boosted Regression Trees Analysis to analyse the relationship of 16 global-scale spatial, landscape, environmental and biotic variables with ploidy state. Our results demonstrate that (1) Polyploid and mixed ploidy species occupied larger geographical ranges than haploid/diploid species. (2) There were contrasting latitudinal patterns between haploid/diploid and polyploid species with polyploidy increasing in importance with distance from the equator, while species richness measures also influenced ploidy state. (3) Climatic factors, especially temperature and evapotranspiration, are congruent with macrophyte species ploidy state, across the planet. Our data support the assertion that increased chromosome number is an adaptation associated with plant survival of harsh environmental conditions, especially in environmentally-stressed high latitudes. Our findings increase the generality of knowledge of the large-scale factors that drive cytotype patterns in vascular plant species at global scale. The ploidy database provides a resource to support further studies about the effects of varying environmental conditions on ploidy state in aquatic plants, at differing scales. [ABSTRACT FROM AUTHOR]