Patterns of morphological changes and hybridization between sympatric whitefish morphs (Coregonus spp.) in a Swiss lake: a role for eutrophication?

Whitefish, genus Coregonus, show exceptional levels of phenotypic diversity with sympatric morphs occurring in numerous postglacial lakes in the northern hemisphere. Here, we studied the effects of human-induced eutrophication on sympatric whitefish morphs in the Swiss lake, Lake Thun. In particular, we addressed the questions whether eutrophication (i) induced hybridization between two ecologically divergent summer-spawning morphs through a loss of environmental heterogeneity, and (ii) induced rapid adaptive morphological changes through changes in the food web structure. Genetic analysis based on 11 microsatellite loci of 282 spawners revealed that the pelagic and the benthic morph represent highly distinct gene pools occurring at different relative proportions on all seven known spawning sites. Gill raker counts, a highly heritable trait, showed nearly discrete distributions for the two morphs. Multilocus genotypes characteristic of the pelagic morph had more gill rakers than genotypes characteristic of benthic morph. Using Bayesian methods, we found indications of recent but limited introgressive hybridization. Comparisons with historical gill raker data yielded median evolutionary rates of 0.24 haldanes and median selection intensities of 0.27 for this trait in both morphs for 1948-2004 suggesting rapid evolution through directional selection at this trait. However, phenotypic plasticity as an alternative explanation for this phenotypic change cannot be discarded. We hypothesize that both the temporal shifts in mean gill raker counts and the recent hybridization reflect responses to changes in the trophic state of the lake induced by pollution in the 1960s, which created novel selection pressures with respect to feeding niches and spawning site preferences.

Eutrophication causes speciation reversal in whitefish adaptive radiations

Species diversity can be lost through two different but potentially interacting extinction processes: demographic decline and speciation reversal through introgressive hybridization. To investigate the relative contribution of these processes, we analysed historical and contemporary data of replicate whitefish radiations from 17 pre-alpine European lakes and reconstructed changes in genetic species differentiation through time using historical samples. Here we provide evidence that species diversity evolved in response to ecological opportunity, and that eutrophication, by diminishing this opportunity, has driven extinctions through speciation reversal and demographic decline. Across the radiations, the magnitude of eutrophication explains the pattern of species loss and levels of genetic and functional distinctiveness among remaining species. We argue that extinction by speciation reversal may be more widespread than currently appreciated. Preventing such extinctions will require that conservation efforts not only target existing species but identify and protect the ecological and evolutionary processes that generate and maintain species.

Spring temperature variability and eutrophication history inferred from sedimentary pigments in the varved sediments of Lake Żabińskie, north-eastern Poland, AD 1907–2008

Varved lake sediments are excellent natural archives providing quantitative insights into climatic and environmental changes at very high resolution and chronological accuracy. However, due to the multitude of responses within lake ecosystems it is often difficult to understand how climate variability interacts with other environmental pressures such as eutrophication, and to attribute observed changes to specific causes. This is particularly challenging during the past 100 years when multiple strong trends are superposed. Here we present a high-resolution multi-proxy record of sedimentary pigments and other biogeochemical data from the varved sediments of Lake Żabińskie (Masurian Lake District, north-eastern Poland, 54°N–22°E, 120 m a.s.l.) spanning AD 1907 to 2008. Lake Żabińskie exhibits biogeochemical varves with highly organic late summer and winter layers separated by white layers of endogenous calcite precipitated in early summer. The aim of our study is to investigate whether climate-driven changes and anthropogenic changes can be separated in a multi-proxy sediment data set, and to explore which sediment proxies are potentially suitable for long quantitative climate reconstructions. We also test if convoluted analytical techniques (e.g. HPLC) can be substituted by rapid scanning techniques (visible reflectance spectroscopy VIS-RS; 380–730 nm). We used principal component analysis and cluster analysis to show that the recent eutrophication of Lake Żabińskie can be discriminated from climate-driven changes for the period AD 1907–2008. The eutrophication signal (PC1 = 46.4%; TOC, TN, TS, Phe-b, high TC/CD ratios total carotenoids/chlorophyll-a derivatives) is mainly expressed as increasing aquatic primary production, increasing hypolimnetic anoxia and a change in the algal community from green algae to blue-green algae. The proxies diagnostic for eutrophication show a smooth positive trend between 1907 and ca 1980 followed by a very rapid increase from ca. 1980 ± 2 onwards. We demonstrate that PC2 (24.4%, Chl-a-related pigments) is not affected by the eutrophication signal, but instead is sensitive to spring (MAM) temperature (r = 0.63, pcorr < 0.05, RMSEP = 0.56 °C; 5-yr filtered). Limnological monitoring data (2011–2013) support this finding. We also demonstrate that scanning visible reflectance spectroscopy (VIS-RS) data can be calibrated to HPLC-measured chloropigment data and be used to infer concentrations of sedimentary Chl-a derivatives {pheophytin a + pyropheophytin a}. This offers the possibility for very high-resolution (multi)millennial-long paleoenvironmental reconstructions.

Cichlid Fish Diversity Threatened by Eutrophication That Curbs Sexual Selection

Cichlid fish species of Lake Victoria can interbreed without loss of fertility but are sexually isolated by mate choice. Mate choice is determined on the basis of coloration, and strong assortative mating can quickly lead to sexual isolation of color morphs. Dull fish col- oration, few color morphs, and low species diversity are found in areas that have become turbid as a result of recent eutrophication. By constraining color vision, turbidity interferes with mate choice, relaxes sexual selection, and blocks the mechanism of reproductive isolation. In this way, human activities that increase turbidity destroy both the mechanism of diversification and that which maintains diversity.

The recent eutrophication of Baldeggersee (Switzerland) as assessed by fossil diatom assemblages

Diatom analyses with an annual resolution were carried out on varves of the hypertrophic Baldeggersee (Central Swiss Plateau) for the timespan ad 1885 to 1993. They reveal seven major changes in the dominant planktonic diatoms. As a result of progressive nutrient enrichment, Baldeggersee changed in the 1910s from a Cyclotella to a Tabellaria fenestrata dominated assemblage, and eventually in the 1950s to a Stephanodiscus parvus dominated diatom assemblage. The timing and direction of diatom-assemblage changes in the varved sediment compare well with sedimentological and limnological observations. Partitioning of the variance in the diatom data revealed that TP is a stronger explanatory variable than temperature for these changes. A diatom-inferred total phosphorus (TP) reconstruction indicates three major steps in eutrophication, occurring at 1909, the mid-1950s and the mid-1970s. Comparison with TP measurements in the water column demonstrates that the diatom-TP inference model used is able to hindcast past TP concentrations reliably. The major steps in eutrophication led to decreases in diatom diversity and also resulted in a progressive increase of calcite grain-size. The lake restoration programme established since 1982 shows no direct impact on the composition of the diatom assemblages. However, the decrease in phosphorus loads since the mid-1970s is reflected in the diatom assemblages and in decreasing diatom-inferred TP concentrations.

Stable isotope response to lake eutrophication: Calibration of a high-resolution lacustrine sequence from Baldeggersee, Switzerland

Stable isotope analyses of discrete seasonal layers from a 108-yr annually laminated freeze-core from Baldeg-gersee, a small, eutrophic lake in central Switzerland, provide information on the climatological and environmental factors, including lake eutrophication, that control oxygen and carbon isotopic composition of epilimnic biologically induced calcite precipitate. During the last 100 yr, Baldeggersee has undergone major increases in productivity and eutrophication in response to nutrient loading from agriculture and industrialization in the lake's watershed. Calibration of the isotopic signal in Baldeggersee to historical limnological data quantitatively links evidence of isotopic depletion in the sedimented calcite to trophic state of the lake. δ18O values from the spring/summer “light” sediment layers steadily diverged to more depleted values in response to historical eutrophication: measured δ18O values were up to 21.5‰ more negative than calculated equilibrium δ18O values. Evidence for 13C depletion in the calcite, relative to equilibrium values, is more difficult to ascertain because of an overall dominance of isotopic enrichment in the dissolved inorganic pool as productivity in Baldeggersee increases. A positive association exists between the degree of oxygen-18 depletion and the calcite crystal size. Thus, large amorphous calcite grains can be used as a proxy for recognizing apparent isotopic nonequilibrium in sediment sequences from highly productive lacustrine environments from all geologic time scales. In contrast to the light layers, the oxygen isotopic composition of the calcite in the late summer/fall “dark” sediment layers is unaffected by the apparent isotope nonequilibrium. Oxygen and carbon isotope values from the dark laminae in the Baldeggersee sediment therefore provide environmental and climatological proxies that can be calibrated with known environmental and regional climate data for the last century.

Life cycle complexity, environmental change and the emerging status of salmonid proliferative kidney disease

P>1. Proliferative kidney disease (PKD) is a disease of salmonid fish caused by the endoparasitic myxozoan, Tetracapsuloides bryosalmonae, which uses freshwater bryozoans as primary hosts. Clinical PKD is characterised by a temperature-dependent proliferative and inflammatory response to parasite stages in the kidney.;2. Evidence that PKD is an emerging disease includes outbreaks in new regions, declines in Swiss brown trout populations and the adoption of expensive practices by fish farms to reduce heavy losses. Disease-related mortality in wild fish populations is almost certainly underestimated because of e.g. oversight, scavenging by wild animals, misdiagnosis and fish stocking.;3. PKD prevalences are spatially and temporally variable, range from 0 to 90-100% and are typically highest in juvenile fish.;4. Laboratory and field studies demonstrate that (i) increasing temperatures enhance disease prevalence, severity and distribution and PKD-related mortality; (ii) eutrophication may promote outbreaks. Both bryozoans and T. bryosalmonae stages in bryozoans undergo temperature- and nutrient-driven proliferation.;5. Tetracapsuloides bryosalmonae is likely to achieve persistent infection of highly clonal bryozoan hosts through vertical transmission, low virulence and host condition-dependent cycling between covert and overt infections. Exploitation of fish hosts entails massive proliferation and spore production by stages that escape the immune response. Many aspects of the parasite's life cycle remain obscure. If infectious stages are produced in all hosts then the complex life cycle includes multiple transmission routes.;6. Patterns of disease outbreaks suggest that background, subclinical infections exist under normal environmental conditions. When conditions change, outbreaks may then occur in regions where infection was hitherto unsuspected.;7. Environmental change is likely to cause PKD outbreaks in more northerly regions as warmer temperatures promote disease development, enhance bryozoan biomass and increase spore production, but may also reduce the geographical range of this unique multihost-parasite system. Coevolutionary dynamics resulting from host-parasite interactions that maximise fitness in previous environments may pose problems for sustainability, particularly in view of extensive declines in salmonid populations and degradation of many freshwater habitats.

The impact of plant diversity and fertilization on fitness of a generalist grasshopper

In many environments land use intensification is likely to result in a decrease in species richness and in an increase in eutrophication. Although the importance of both factors for higher trophic levels such as insect herbivores is well documented, their impact has rarely been studied in combination. Herbivorous insects have a strong impact on the functioning of ecosystems and it is therefore important to understand how they are affected by eutrophication in high or low diversity environments. We used a grassland biodiversity experiment to investigate the combined effect of fertilization and plant diversity loss on the fitness of the generalist grasshopper Chorthippus parallelus by rearing grasshopper nymphs for four weeks in cages on unfertilized or fertilized (NPK) subplots across a species richness gradient from 1 to 60 plant species. Survival, the number of oothecae, body mass and the number of hatchlings were measured separately for each cage. Plant diversity had no effect on any of the grasshopper fitness measures, neither in unfertilized nor in fertilized plots. NPK-fertilization reduced grasshopper survival but increased body mass of males and reproductive success of the surviving females. Fertilization effects were not mediated by plant community structure, productivity or composition, suggesting that higher food plant quality was one of the main drivers. There was no interaction between plant diversity and fertilization on any of the measures. In conclusion, an increase in eutrophication, in both species-rich and species-poor grasslands, could lead to higher reproductive success and therefore higher abundances of herbivorous insects including insect pests, with fertilization effects dominating plant diversity effects.