Eutrophication Phase Research Articles

Temporal Changes in the Physico chemical Composition of Lake Geneva from 1957 to the Present

This manuscript presents an exhaustive analysis of a physicochemical dataset collected from Lake Geneva since 1957, offering a detailed exploration of its long-term environmental evolution. This study encompasses an array of cycles and trophic states that the lake has undergone. A critical finding is the significant influence of human activities within the catchment area, notably the introduction of substantial quantities of nutrients, especially phosphorus, into the lake’s ecosystem. This anthropogenic impact culminated in a marked ecological decline between the 1960s and 1970s, characterized by a phase of eutrophication. This period saw a surge in algal blooms and a consequent decrease in water transparency. In the 1970s, strategic interventions were implemented to address this environmental challenge. These measures, including the enhancement of phosphorus treatment at sewage facilities in the catchment area and the nationwide ban on phosphated detergents in Switzerland, proved instrumental in reducing the influx of nutrients. Consequently, a significant reduction in the lake’s phosphorus levels was observed by the 1980s. The lake is currently undergoing a re-oligotrophication process, with phosphorus concentrations indicating a mesotrophic state. However, it is noteworthy that the biological components of the lake are exhibiting a slower recovery rate in adapting to reduced nutrient levels, in contrast to the more immediate response noted during the eutrophication phase. Despite the progress achieved, ongoing vigilance and efforts are essential to continuously monitor, identify, and manage potential sources of phosphorus pollution. This proactive approach is vital for maintaining the lake’s ecological balance and ensuring it remains free from significant algal proliferation.

Ecological assessment of eutrofication of lakes in «Shatskyі» biosphere reserve

The natural process of eutrophication of hydroecosystems is determined by many factors and is mostly long-term, almost irreversible. In the 20th and 21st centuries. Accelerated anthropogenic eutrophication of many waters took place, in particular the lakes of Shatsky group (Shatskyi National Natural Park, Volyn Region, Ukraine). Changes in water due to their enrichment with biogenic substances and chemical compounds accompany the increase in plant productivity and may be the result of «natural aging», anthropogenic influence, organic pollution. One of the concepts of the succession of lake ecosystems assumes that lakes go through different stages of trophicity (nutrition), starting with oligotrophy or dystrophy. The last stage (climax) of the successional series is intensive eutrophication, during which the lake is partially replaced by a swamp and, in the end, can become a dry land, meadow, shrub or forest vegetation. Eutrophication of lake ecosystems is accompanied by a change in transparency, chlorophyll a concentration, and oxygen concentration. An active phase of eutrophication of the limnoecosystems of Shatskyi biosphere reserve has been established. On the territory of the latter there are rivers and streams with a constant water flow, more than 114 km long, and also includes 31 lakes. The results of ecological studies of various types of water bodies of Shatsky Lakes, which were carried out in 2016–2020, are presented. The performed typification of Shatsky Lakes by the area of the water mirror proved that there are no very large lakes here, there are three large lakes (Svityaz, Pulemetske, Luky-Peremut). The analysis of the nature-reserved territory of Western Polissia proved that special attention should be paid to lakes of small and small sizes, as well as medium and shallow lakes (Lynovets, Somynets, Buzhnya Bay, etc.), which are not capable of active self-regulation. The optimal mechanism for monitoring the ecological state of water bodies (lakes) using the remote sensing method is proposed.

Eutrophication history and organic carbon burial rate recorded in sediment cores from the Mar Piccolo of Taranto (Italy)

During the second half of the twentieth century, coastal lagoons in densely populated regions experienced eutrophication due to excessive nutrient inputs. Detrimental effects, including hypoxia/anoxia and harmful algae blooms, have occurred in many Mediterranean lagoons, but their trophic evolution is poorly documented. The lack of adequate monitoring data can partly be offset by examining sedimentary records. In the Mar Piccolo, a lagoon comprising two basins near Taranto (Italy), eutrophication has followed population growth and pollution resulting from naval activities and massive industrialisation. Based on 210Pb-dated sediment cores, continuous in situ density profiles obtained with computed tomography, organic carbon (OC) and total nitrogen (TN) content and OC and TN isotopic signatures, this paper reconstructs the history of eutrophication, discusses the sources of organic matter and provides an estimate of the OC burial rate before and during the eutrophic period. OC burial increased in the period 1928–1935 and peaked in the decade 1960–1970. OC and TN content were still high in the surface sediments collected in 2013, despite partial diversion of sewage outfalls in the period 2000–2005. The divergent δ13C and δ15N signatures of the two basins during the eutrophic period suggest they were affected by different nutrient sources. The OC burial rate during the eutrophic phase (≈ 46 g m−2 y−1) was close to the world median value for lagoon sediments, and was about twice the burial rate recorded in the preceding oligotrophic phase.

Bryoherms from the lower Sarmatian (upper Serravallian, Middle Miocene) of the Central Paratethys

Bryozoan–serpulid–algal–thrombolite bioherms of up to 50 cm size are described from the Sarmatian (upper Middle Miocene) of the Central Paratethys. They occur on top of lower Sarmatian carbonate sediments of high-energy conditions and the individual bioherms settle on crests of ripples. The buildups are overlain and partly truncated by cross-bedded oolites of late Sarmatian age. Buildup growth starts with a Cryptosula/Hydroides (bryozoan/serpulid) pioneer community, followed by nodular Schizoporella (bryozoan) colonies overgrown by coralline algae/microbial mats and a thrombolite with calcareous algal filaments. All these constituents form a framestone fabric which is overall dominated by bryozoans labeling them as bryoherms. Inside the bioherms ecological successions of higher frequencies occur which are interpreted to reflect short-time environmental fluctuations such as nutrient availability, oxygenation (possible anoxia), salinity (possible brackish water), temperature and water level. The internal succession in individual bioherms is related to long-term environmental changes including general shallowing, increasing nutrient supply and decreasing water circulation and oxygenation. The described bioherms are most similar to modern bryostromatolites of the Coorong lagoon in S Australia and also similar to structures in the Netherlands. The widespread occurrence of bryoherms/bryostromatolites in the Central Paratethys suggests a phase of considerable eutrophication during the early Sarmatian.

Cryptic persistence and loss of local endemism in Lake Constance charr subject to anthropogenic disturbance.

In the welcome circumstance that species believed extinct are rediscovered, it is often the case that biological knowledge acquired before the presumed extinction is limited. Efforts to address these knowledge gaps, in particular to assess the taxonomic integrity and conservation status of such species, can be hampered by a lack of genetic data and scarcity of samples in museum collections. Here, we present a proof-of-concept case study based on a multidisciplinary data evaluation approach to tackle such problems. The approach was developed after the rediscovery, 40 years after its presumed extinction, of the enigmatic Lake Constance deep-water charr Salvelinus profundus. Targeted surveys led to the capture of further species and additional sympatric normal charr, Salvelinus cf. umbla. Since the lake had been subject to massive stocking in the past, an evaluation of the genetic integrity of both extant forms was called for in order to assess possible introgression. A two-step genomic approach was developed based on restriction site associated DNA (RAD). Diagnostic population genomic (single nucleotide polymorphism [SNP]) data were harvested from contemporary samples and used for RNA bait design to perform target capture in DNA libraries of archival scale material, enabling a comparison between extant and historic samples. Furthermore, life history traits and morphological data for both extant forms were gathered and compared with historical data from the past 60-120 years. While extant deep-water charr matched historical deep-water specimens in body shape, gill raker count, and growth rates, significant differences were discovered between historical and extant normal charr. These resulted were supported by genomic analyses of contemporary samples, revealing the two extant forms to be highly divergent. The results of population assignment tests suggest that the endemic deep-water charr persisted in Lake Constance during the eutrophic phase, but not one of the historical genomic samples could be assigned to the extant normal charr taxon. Stocking with non-endemic charr seems to be the most likely reason for these changes. This proof-of-concept study presents a multidisciplinary data evaluation approach that simultaneously tests population genomic integrity and addresses some of the conservation issues arising from rediscovery of a species characterized by limited data availability.

Geochemical characteristics of n-alkanes in sediments from oligotrophic and eutrophic phases of five lakes and potential use as paleoenvironmental proxies

Profiles of n-alkane proxies in sediment cores have been frequently used to detect historical trends in the nutrient status of lakes. However, the results obtained from these proxies are occasionally confusing because the situations in which a given n-alkane proxy might make a valid indication have tended thus far to be elusive. The current study compared geochemical characteristics of n-alkanes in sediments from the oligotrophic (before 1950) and relatively eutrophic phases (after 1980) of five representative lakes in order to assess their suitability and reliability as proxies for lake trophic state. The results from Lakes Liangzi, Changdang, Taibai and Erhai demonstrated that short-chain odd carbon n-alkane abundances were significantly higher (P < 0.05) in sediments deposited after 1980 than in those dating to before 1950, whereas TAR (terrigenous: aquatic ratios) and ACL15–33 (average chain length of C15–C33n-alkanes) were significantly lower (P < 0.05) in the more recent sediments. These results are compatible with generally accepted variation seen in lakes that have undergone eutrophication. In Lake Fuxian, however, short-chain odd carbon n-alkane abundances, TAR and ACL15–33 were all higher in sediments after 1980 than in sediments before 1950, although the variation was only significant (P < 0.05) for ACL15–33. These results, especially the unexpected trends in TAR and ACL15–33, mean these cannot be relied upon as proxies for eutrophication in this lake. The anomaly is likely to be related to the predominantly low phytoplankton productivity and OM input in Lake Fuxian, coupled with the availability of OM from submerged macrophytes in the relatively eutrophic phase. Interestingly and importantly, two new proxies CPI15–20 ((n-C15 + n-C17 + n-C19)/(n-C16 + n-C18 + n-C20) alkanes) and n-C17/n-C16 alkane, which are indicative of OM input from phytoplankton over non-photosynthetic bacteria, were significantly higher (P < 0.05) in post-1980 sediments than in those dating to before 1950 in all five studied lakes. These two are thus shown to be widely applicable for recording lake eutrophication. Their presence in Lake Fuxian can be explicitly attributed to the absence of influence from submerged macrophytes. Moreover, as ratio results of n-alkanes with similar carbon number, these two proxies are less influenced by processes of OM diagenesis due to comparable degradation of n-alkanes in sediments. The combined results suggest that these previously overlooked proxies may be of considerable value to studies of lake trophic history.

Vegetation changes over the last centuries in the Lower Lake Constance region reconstructed from sediment‐core environmental DNA

AbstractMany European lake ecosystems, including their respective catchment areas, underwent anthropogenic environmental changes over the last centuries. This has resulted in changes in the aquatic and terrestrial vegetation, but historical records on the composition of the past vegetation on centennial scale are scarce. In this study, we examined changes in the terrestrial and aquatic plant communities in and around Lower Lake Constance using metabarcoding of sedimentary DNA (sedDNA) of three cores from different sub‐basins covering the past, up to 300 years. We successfully identified an average of c. 3000 sequence variants (molecular operational taxonomic units ‐ MOTUs) and obtained a taxonomically annotated dataset of 127 species, 104 genera, and 72 families. We could detect major changes in the terrestrial and aquatic vegetation of the Lower Lake Constance region by examining the cores. For example, alpha diversity decreased in the last c. 100 years, and this decrease was more pronounced in the terrestrial than in the aquatic plant community. Unlike the terrestrial plant community, the current aquatic plant‐community composition partially resembles the community from before the 20th‐century eutrophication phase of the lake. In addition to changes that can be attributed to anthropogenic impacts, we also captured the effect of DNA sedimentation on the terrestrial DNA diversity representation in sediments during periods of extensive flooding and potentially as a consequence of extremely cold winters. With sedDNA from Lower Lake Constance, we provide a new local dataset to investigate and extend the historical changes of different shoreline habitats and to identify characteristic and invasive plant species. Such highly resolved datasets spanning the past centuries can provide detailed information on human environmental history in densely populated regions that have undergone severe changes in the recent past.

Century-Long Homogenization of Algal Communities Is Accelerated by Nutrient Enrichment and Climate Warming in Lakes and Reservoirs of the North Temperate Zone.

Anthropogenic pressures can threaten lake and reservoir ecosystems, leading to harmful algal blooms that have become globally widespread. However, patterns of phytoplankton diversity change and community assembly over long-term scales remain unknown. Here, we explore biodiversity patterns in eukaryotic algal (EA) and cyanobacterial (CYA) communities over a century by sequencing DNA preserved in the sediment cores of seven lakes and reservoirs in the North Temperate Zone. Comparisons within lakes revealed temporal algal community homogenization in mesotrophic lakes, eutrophic lakes, and reservoirs over the last century but no systematic losses of α-diversity. Temporal homogenization of EA and CYA communities continued into the modern day probably due to time-lags related to historical legacies, even if lakes go through a eutrophication phase followed by a reoligotrophication phase. Further, algal community assembly in lakes and reservoirs was mediated by both deterministic and stochastic processes, while homogeneous selection played a relatively important role in recent decades due to intensified anthropogenic activities and climate warming. Overall, these results expand our understanding of global change effects on algal community diversity and succession in lakes and reservoirs that exhibit different successional trajectories while also providing a baseline framework to assess their potential responses to future environmental change.

Diatom and geochemical paleolimnology reveals a history of multiple stressors and recovery on Lake Ontario

Among the Laurentian Great Lakes, Lake Ontario is the farthest downstream and has the longest history of environmental stress from human activities. Two sediment cores, spanning the last 300 years, from Lake Ontario were analyzed for diatom composition and geochemistry and compared with anthropogenic activities in the surrounding watershed. Despite some regional variation a clear, lake-wide record of cultural eutrophication and recovery is presented in three phases: (1) a largely pre-impact phase (1700s through ~ 1920) dominated by oligotrophic/mesotrophic diatoms and more inorganic sediment accumulation; (2) an accelerated eutrophication phase (1920s to 1980s) reflecting human population growth, watershed modifications and rapid industrial expansion as indicated by increases in nutrient-tolerant diatom taxa and sediment contaminants; and (3) a recovery phase (1980s to present) when diatom-inferred nutrient concentrations trended back to pre-impact levels in response to pollution abatement measures and novel stressors such as dreissenid invaders and climate change. Based on our long-term record, Lake Ontario’s ecosystem continues to evolve in response to contemporary stressors.

A comparison of n-alkane contents in sediments of five lakes from contrasting environments

In paleolimnology studies, the trajectories of n-alkane abundances and distributions in dated sediments are widely used indicators of changing environmental and ecological status in the overlying water over time. However, the factors contributing to the variable n-alkane profiles of lake systems are not yet fully understood. Here, a comparative study was conducted to examine natural and anthropogenic influences on the sediment n-alkane profiles of five representative lakes in China. Core samples from the deep oligo- mesotrophic lakes Fuxian and Lugu (FX-1 and LG-1 cores), the shallow eutrophic lakes Taibai and Changdang (TB-1 and CD-1 cores) and the alpine Lake Heihai (HH-1 core) were used. While there were striking differences in n-alkane abundances and distributions between the cores, the results suggested that the n-alkanes in all five studied lakes were derived mainly from biogenic sources, rather than from petroleum contamination. The short-chain (<C20) even carbon n-alkanes, assumed to derive from non-photosynthetic bacteria, were especially prevalent in FX-1, reaching 158.3–284.4 μg/g TOC, which might be a result of high levels of bacterial productivity in the deep (average 89.6 m) water column of Lake Fuxian. The contribution of bacterial production diminished in the other four lakes, with <C20n-alkanes declining to between 21.4 and 60.5 μg/g TOC on average in the studied cores. In the two shallow lakes, abundances of short-chain (<C20) odd carbon n-alkanes were greatly elevated in sediments corresponding to the eutrophic phase; for example, with maximum values of 136.3 μg/g TOC recorded in TB-1. This nutrient-driven effect was less apparent in deep lakes. The >C21n-alkane profiles of different cores varied significantly. Abundances of long-chain n-alkanes (C29, C31 and C33) in HH-1 were approximately 3–7 times greater than in other cores. Coupled with the lowest Paq (average 0.29) values recorded in the study, this indicates a predominantly terrestrial plant origin for the >C21n-alkanes, most likely the dense alpine meadow which surrounds the lake. In other cores, however, aquatic macrophytes appear to be more important contributors of the n-alkanes, a result that corresponds with the relatively low density of terrestrial plants in those catchments.

Paleoenvironmental change in a precession-paced succession across the onset of the Messinian salinity crisis: Insight from element geochemistry and molecular fossils

In marginal Mediterranean sub-basins, the early phase of the Messinian salinity crisis (MSC) is recorded by cyclic successions of gypsum and shales, which in deeper parts of the sub-basins make lateral transition into organic-rich shales, marls, and carbonates. The cyclic stacking pattern of the gypsum-bearing sequences is assumed to reflect periodic paleoenvironmental change induced by precession-driven climate perturbations, with the assumption that shales reflect humid climate (precession minima) while gypsum reflects arid conditions (precession maxima). However, this correlation has not been verified to date, mostly because of the scarcity of microfossils, the most commonly used tools for the reconstruction of precession-driven paleoenvironmental change. Such change can, instead, be reconstructed through the study of the deeper water counterparts of gypsum (marly and carbonate layers) with geochemical indicators (major and trace elements, molecular fossils), which provide insight on climate and aquatic productivity. We used this approach to study a section from the Piedmont Basin (NW Italy) where the onset of the MSC is archived in a sequence of organic-rich sediments. This sequence displays distinct lithological cyclicity, evidenced by the repetition of couplets of organic-rich shales and marls, either bioturbated (in the pre-MSC part of the section) or laminated (during the MSC). The influence of orbitally-driven (precession) climate oscillations is demonstrated by fluctuations of Ti/Al, Si/Al, Mg/Al, K/Al, Zr/Al, and Ba/Al ratios that are in phase with lithological cyclicity. These fluctuations are interpreted to reflect alternation of humid (shales, deposited during precession minima) and arid (bioturbated and laminated marls, deposited during precession maxima) phases, dominated by fluvial and aeolian transport of detrital material, respectively. The cyclicity of the element ratios is mirrored by changes in organic carbon content and molecular fossil inventory. In particular, the distribution of long-chain n-alkanes and their degree of preservation reveal that humid phases at times of precession minima were typified by the maximum input of degraded terrestrial organic matter driven by enhanced riverine runoff, which promoted water column stratification. Coeval increase in Ba content, a common paleoproductivity proxy, agrees with enhanced nutrient supply during humid periods, promoting phases of eutrophication in the basin. Lithological and geochemical changes are observed in MSC sediments deposited at times of precession maxima, evidenced by the replacement of pre-MSC bioturbated marls by laminated marls rich in filamentous fossils corresponding to the remains of probable colorless sulfide-oxidizing bacteria. Such changes reflect an intensification of water column stratification after the onset of the MSC, possibly related to the combined effect of persistent freshwater inflow and basin isolation, preluding the advent of gypsum precipitation.

Whole-genome sequencing of an extinct whitefish species reveals extensive introgression after speciation reversal

Event Abstract Back to Event Whole-genome sequencing of an extinct whitefish species reveals extensive introgression after speciation reversal David F. Frei1, 2*, Ole Seehausen1, 2 and Philine G. Feulner1, 2 1 Swiss Federal Institute of Aquatic Science and Technology, Switzerland 2 University of Bern, Switzerland A large proportion of contemporary biodiversity evolved through ecological speciation, which is the evolution of reproductive isolation as a consequence of adaptation to divergent selection between niches. If species evolved in response to divergent ecological selection, a change in the balance between gene flow and selection can erode reproductive isolation and lead to speciation reversal. Although a major part of the world’s species diversity is sensitive to hybridization-driven biodiversity dynamics, the concept of speciation reversal has received little attention. In Switzerland, cultural eutrophication of pre-alpine lakes caused a substantial loss of endemic whitefish diversity by a combination of speciation reversal and demographic decline. We made use of a collection of historical whitefish scales as source of DNA to sequence whole-genomes of the pre-eutrophication populations of endemic Lake Constance whitefish species, including the profundal Coregonus gutturosus that went extinct during eutrophication. We found significant introgression of the extinct species into three contemporary species that happened during the eutrophic phase, demonstrating the involvement of speciation reversal in the extinction process of C. gutturosus. We identified introgressed regions and show the genomic landscape of introgression after speciation reversal in genomes of extant whitefish species. In total, we detected a proportion of 11% of the genome of C. gutturosus distributed across six genomes of contemporary species, suggesting that introgression rescued considerable amounts of genetic variation of the extinct species from being lost. Currently, we are investigating whether this variation contains genes that may play a role in future adaptation to profundal habitats that currently have no whitefish anymore, although they can be occupied again due to recent re-oligotrophication. As a large fraction of contemporary biodiversity evolved through ecological speciation, the potential for speciation reversal when environments change is widespread. Our results highlight the importance of the conservation of ecological and evolutionary mechanisms maintaining species diversity and advance our understanding of speciation reversal as one evolutionary trajectory that lineages can follow through time. Acknowledgements We thank Eawag and the interreg project "SeeWandel: Leben im Bodensee – gestern, heute und morgen" for financial support. Thanks to the genetic diversity center (GDC) of ETH Zurich, Eawag and the University of Bern for providing the infrastructure and support to analyze our data. Keywords: ecological speciation, speciation reversal, Alpine whitefish, Introgression, Eutrophication Conference: XVI European Congress of Ichthyology, Lausanne, Switzerland, 2 Sep - 6 Sep, 2019. Presentation Type: Oral Topic: EVOLUTIONARY GENOMICS OF FISHES Citation: Frei DF, Seehausen O and Feulner PG (2019). Whole-genome sequencing of an extinct whitefish species reveals extensive introgression after speciation reversal. Front. Mar. Sci. Conference Abstract: XVI European Congress of Ichthyology. doi: 10.3389/conf.fmars.2019.07.00041 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 20 May 2019; Published Online: 14 Aug 2019. * Correspondence: Mr. David F Frei, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland, david.frei@eawag.ch Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers David F Frei Ole Seehausen Philine G Feulner Google David F Frei Ole Seehausen Philine G Feulner Google Scholar David F Frei Ole Seehausen Philine G Feulner PubMed David F Frei Ole Seehausen Philine G Feulner Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Reconstruction of trophic state shifts over the past 90 years in a eutrophicated lake in western Switzerland, inferred from the sedimentary record of photosynthetic pigments

Anthropogenic eutrophication can initiate vast and persistent ecosystem state changes in lakes. Such changes may be characterized by increased phytoplankton taxa variability, which can affect the effectiveness and time of lake recovery mechanisms. Lake Morat in Switzerland has undergone intense eutrophication in the twentieth century (phosphorous concentrations up to 150 μg L−1) caused by excessive nutrient loadings from agricultural intensification and urbanization. Phosphorous reduction measures since 1986, such as the ban of phosphates in detergents and decreased use of fertilizers in agriculture, have resulted in total phosphorous concentrations up to 20 μg L−1 today. Despite this drastic reduction of total phosphorous, total biomass production stays high. We investigate historical changes in the phytoplankton community during the eutrophication and re-oligotrophication periods by comparing historical limnological data with sediment pigment concentrations measured by HPLC and other geochemical proxies in a radiodated sediment core. For the last 90 years, we identified four major trophic state changes in Lake Morat. The first period (AD 1924–1937) is characterized by low pigment concentrations and nutrient inputs, with good oxygen conditions. This section represents trophic conditions before the intensive eutrophication phase. The second period (AD 1937–1970) revealed an abrupt increase in pigment concentrations with higher primary production, cyanobacteria dominance and reduced oxygen levels. Oscillaxanthin indicated a Planktothrix rubescens dominance (AD 1954–1970) with concentrations up to 800 nmol g−1 OM. Their decline after AD 1970 suggests the initiation of an intense eutrophication phase (AD 1970–1983) associated with the dominance of other cyanobacteria species, higher total phosphorous inputs, and intense anoxia. In the restoration period (AD 1983–2014), there was a shift in the phototrophic community from cyanobacteria to green algae dominance, yet some cyanobacteria species remain present. Rapid phytoplankton community changes were identified in the studied period, yet overall primary production response was low. Limnological data revealed a delay of phosphorous reduction due to phosphorous recycling from the sediments. The observation of complex lake ecosystem reactions to prolonged eutrophication and subsequent re-oligotrophication, as shown by the paleolimnological and limnological data in this study, emphasize the importance of careful lake management to revert eutrophication back to historical reference biomass values.

Plankton Community Respiration and ETS Activity Under Variable CO2 and Nutrient Fertilization During a Mesocosm Study in the Subtropical North Atlantic

The enzymatic electron transport system (ETS) assay is frequently used as a proxy of respiratory activity in planktonic communities. It is thought to estimate the maximum overall activity of the enzymes associated with the respiratory ETS systems in both eukaryotic and prokaryotic organisms. Thus, in order to derive actual respiration rates (R) from ETS it is necessary to determine empirical R/ETS conversion algorithms. In this study we explore the temporal development of R and ETS activity in natural plankton communities (from bacteria to large phytoplankton) enclosed in mesocosms, treated with different CO2 concentrations. The experiment lasted 30 days, during which abrupt changes in community structure and biomass occurred through a sharp transition from oligotrophy (phase I) to highly eutrophic conditions (phase II) after nutrient-induced fertilization (day 18). R and ETS did not show any response to CO2 under oligotrophic conditions, but R increased significantly more in the two high CO2 mesocosms after fertilization, coinciding with a sharp rise in large phytoplankton (mostly diatoms). R and ETS were significantly correlated only during the eutrophic phase. The R/ETS ranged more than threefold in magnitude during the experiment, with phase-averaged values significantly higher under oligotrophic conditions (0.7-1.1) than after nutrient fertilization (0.5-0.7). We did not find any significant relationship between R/ETS and community structure or biomass, although R correlated significantly with total biomass after fertilization in the four mesocosms. Multiple stepwise regression models show that large phytoplankton explains most of the variance in R during phases I (86%) and II (53%) and of ETS (86%) during phase II, while picophytoplankton contributes up to 73% to explain the variance in the ETS model during phase I. Our results suggest that R/ETS may be too variable in the ocean as to apply constant values to different communities living under contrasting environmental conditions. Controlled experiments with natural communities, like the present one, would help to constrain the range of variability of the R/ETS ratio, and to understand the factors driving it.

Algal palynomorphs as proxies of human impact on freshwater resources in the Great Lakes region

Abstract Management of freshwater resources requires an understanding of the response of lakes to human impact. The long sedimentary records in lake archives hold the key to accurate forecasting. The remains of algae in “pollen” slides record two distinct phases of cultural eutrophication and siltation/turbidity resulting from soil erosion in sediments from two lakes in southern Ontario, Canada: 1) agricultural settlements by Iroquoian (Wendat/Huron) people around the middle of the last millennium and 2) widespread land-clearing by European colonists in the mid-nineteenth century, followed by industrial expansion and urbanization in the Great Lakes watershed to the present day. The half-cells of benthic desmids were particularly sensitive to turbidity associated with land clearing. In contrast, planktonic algae adapted to eutrophic waters thrived in response to increased agricultural runoff and human and animal waste during both intervals in cores from Lake Simcoe and in the well-documented varved sediments from Crawford Lake. These under-utilized microfossils can be useful proxies of human impact, particularly where mineralized microfossils are sparse due to dissolution.

Effects of changing nutrient inputs on the ratio of small pelagic fish stock and phytoplankton biomass in the Black Sea

Significant increases in nitrogen and phosphorus inputs to the Black Sea in the second half of the 20th century caused eutrophication and drastically decreasing Si:N and Si:P ratios. Combined with climate change, overfishing of top predators and a huge outbreak of the non-indigenous ctenophore Mnemiopsis, the pelagic food web was strongly modified and its efficiency for channeling primary production to higher trophic levels substantially reduced. We used the ratio between small pelagic fish stock and phytoplankton biomass on the Danube shelf and in the open Black Sea to investigate long-term changes in food web functioning. The ratio had 1) highest values for the pre-eutrophication period when diatoms and copepods dominated the pelagic food web (‘muscle food chain’), 2) decreased during the eutrophication period with stronger prevalence of autotrophic pico- and nanophytoplankton, bacteria, heterotrophic nanoflagellates, microzooplankton, Noctiluca and jellyfish (‘jelly food chain’ with increased importance of the microbial loop), 3) lowest values during the ecological crisis (1989–1992), when small pelagic fish stocks collapsed, and 4) increased after 1993, indicating that the ecosystem went out of the crisis and exhibited a trend of recovery. However, in the last period (1993–2008) the ratio remained close to values observed in the middle eutrophication phase, suggesting that the ecosystem was far from fully recovered. Since early 2000s, fluctuating pelagic fish stocks, with a tendency to decreasing fish landing again, have been observed in the Black Sea. Additionally, the quality of food for the small pelagic fish has deteriorated due to warming trends and the legacy of eutrophication, giving support for the ‘jelly food chain’, exhibiting low energy transfer and prevalence of organisms with high respiration rate and low nutritional value.

Trophic state changes can affect the importance of methane-derived carbon in aquatic food webs.

Methane-derived carbon, incorporated by methane-oxidizing bacteria, has been identified as a significant source of carbon in food webs of many lakes. By measuring the stable carbon isotopic composition (δ13C values) of particulate organic matter, Chironomidae and Daphnia spp. and their resting eggs (ephippia), we show that methane-derived carbon presently plays a relevant role in the food web of hypertrophic Lake De Waay, The Netherlands. Sediment geochemistry, diatom analyses and δ13C measurements of chironomid and Daphnia remains in the lake sediments indicate that oligotrophication and re-eutrophication of the lake during the twentieth century had a strong impact on in-lake oxygen availability. This, in turn, influenced the relevance of methane-derived carbon in the diet of aquatic invertebrates. Our results show that, contrary to expectations, methane-derived relative to photosynthetically produced organic carbon became more relevant for at least some invertebrates during periods with higher nutrient availability for algal growth, indicating a proportionally higher use of methane-derived carbon in the lake's food web during peak eutrophication phases. Contributions of methane-derived carbon to the diet of the investigated invertebrates are estimated to have ranged from 0-11% during the phase with the lowest nutrient availability to 13-20% during the peak eutrophication phase.

Reconstructing the trophic history of the Black Sea shelf

In the last 50 years the Black Sea has undergone large changes driven by increasing anthropogenic pressures. We estimated the integrated annual primary production (APP) for different shelf regions during the early eutrophication phase (1963–1976) using chlorophyll a and winter nitrate concentrations as proxy observations of primary production to describe its seasonal variation. For comparison, APP was estimated during the period when eutrophication peaked (1985–1992). In the early eutrophication period APP was estimated at 64–89gCm−2yr−1 for most part of the shelf, except the shelf part influenced by the Danube River (the shallow waters off the Romanian and Bulgarian coasts) where APP was ∼126gCm−2yr−1. In these two different shelf parts, APP increased to 138–190 and 266–318gCm−2yr−1 during the peak eutrophication period. These spatial differences are attributed to the large nutrient inputs from the Danube River. The APP estimates provide new insight into the eutrophication history of the Black Sea shelf, documenting stronger signs of eutrophiction than observed in other enclosed seas such as the Baltic Sea. Since the peak eutrophication period APP is estimated to have decreased by approximately 15–20%.

Lake biota response to human impact and local climate during the last 200 years: A multi-proxy study of a subalpine lake (Tatra Mountains, W Carpathians)

Element content, loss-on-ignition, chironomid analysis and 210Pb dating were applied on a sediment core from a subalpine Tatra lake (Popradské pleso) to reveal the response of aquatic biota to eutrophication induced by human activities in the lake catchment. The lead dating indicates that the 0–8cm section of the core represents the past ca 200years, ending at ~1814AD. Comparing the key changes of the proxies with human activities that are historically well documented, four phases of the recent lake development were distinguished: (1) a pre-tourism phase, (2) a phase of increasing touristic activity and early cottage development, (3) a phase of eutrophication, and (4) a phase of post-eutrophication. Neither touristic activity, nor early cottage development around the lake (1st and 2nd phases) had considerable influence on the chironomid assemblage structure or organic content of the lake. The most significant change both in chironomid assemblage structure and loss-on-ignition occurred during the 3rd phase, when a big tourist hotel was built close by the lake and started contaminating it via direct wastewater input. However, the structure of the chironomid assemblage has not changed significantly over time and the dominating taxa remained the same during the whole period. Parallel with the nutrient signal of the paleo assemblage, a secondary signal has been identified as the ratio of rheophilic taxa on total abundance that did not correlate with the sediment's organic content, and is most likely driven by local climatic oscillations. Changes of most of metal elements concentrations reflected rather bigger scale changes of industrial activities than local scale human disturbances. Our results indicate that hydromorphological properties can moderate the impact of organic pollution on the lake biota.

Threshold quantification and short-term forecasting of Anabaena, Aphanizomenon and Microcystis in the polymictic eutrophic Lake Müggelsee (Germany) by inferential modelling using the hybrid evolutionary algorithm HEA

Forecasting models for Anabaena, Aphanizomenon and Microcystis have been developed for the hypertrophic phase from 1979 to 1990 and the eutrophic phase from 1997 to 2012 of the polymictic Lake Muggelsee by means of the hybrid evolutionary algorithm HEA. Comparisons of limnological parameters of the two phases revealed not only a distinct seasonal extension of N-limitation but also higher water temperatures that rose earlier and lasted longer between spring and autumn from 1997 to 2012. These differences were reflected by threshold conditions and sensitivity functions of the cyanobacteria-specific models evolved by HEA for the two phases. Seven-day-ahead forecasts matched well timings of peaking biomass observed for the three cyanobacteria but partially failed to predict accurate magnitudes, whereby coefficients of determination r 2 ranged between 0.48 and 0.76 for models in Phase I and between 0.42 and 0.69 in Phase II. The threshold conditions of the models quantified ranges of key predictor variables such as water temperature and transparency, concentrations of NO3-N and PO4-P that were symptomatic for sudden outbreaks of high biomass of the three cyanobacteria. Sensitivity functions extracted from 20 best performing models for each of the three cyanobacteria in both phases indicated different abundances between N-fixing Anabaena and Aphanizomenon compared to non-N-fixing Microcystis in response to strengthened N-limitation in Phase II.