Period Of Eutrophication Research Articles

Population dynamics of resident fish in a coastal lagoon under eutrophication-mediated habitat changes

Coastal lagoons, as transitional ecosystems, are crucial for providing ecosystem services and preserving fish communities, yet they face increasing pressures from anthropogenic activities. The Mar Menor coastal lagoon in the western Mediterranean exemplifies this challenge, as it suffers from severe eutrophication driven by agricultural and urban influences. In this study, we assess changes in the population dynamics of three resident fish species (Apricaphanius iberus, Pomatoschistus marmoratus, and Syngnathus abaster) in the context of human-induced impacts in the Mar Menor. Using long-term monitoring data (the reference period of 2002-2004 vs. the eutrophication period of 2015-2019), we analyze temporal variations in their population dynamics and their relationships with eutrophication-mediated structural changes in shallow waters. Our results suggest that these resident species, which are closely associated with shallow areas, show population dynamics that reflect habitat alterations, underscoring their potential as indicators of eutrophication impacts. This study enhances our understanding of the dynamics within transitional ecosystems and provides a basis for integrated management strategies aimed at promoting effective fish conservation.

First lacustrine application of the diatom‐bound nitrogen isotope paleo‐proxy reveals coupling of denitrification and N2 fixation in a hyper‐eutrophic lake

AbstractPast changes in the input/output, and internal cycling, of bioavailable nitrogen (N) in marine and lacustrine environments can be reconstructed by analyzing the N isotopic composition (δ15N) of organic matter in the sedimentary record. To verify, and eliminate, potential biases of bulk sedimentary δ15N (δ15Nbulk) signatures by diagenetic alteration and external N inputs, we applied, for the first time, the diatom‐bound N isotope (δ15Ndb) paleo‐proxy to lake sediments. By comparing δ15Nbulk and δ15Ndb in a sedimentary record from eutrophic Lake Lugano (Switzerland), we demonstrate that changing redox conditions influence the degree of N‐isotopic alteration of the bulk sediment, emphasizing the need for caution when interpreting δ15Nbulk in paleolimnological studies. Furthermore, in combining δ15Ndb measurements with X‐ray fluorescence scanning and state‐of‐the‐art molecular biomarker analyses, we reconstruct nutrient cycling and paleoenvironmental conditions in the lake over the past ~ 125 yr. Coeval with the period of severe eutrophication in Lake Lugano in the 1960s, our proxy data indicate that export production, δ15Ndb, and the concentration of heterocyst glycolipids (a biomarker for N2‐fixing cyanobacteria) increased simultaneously. Together, these data suggest that the rise in δ15Ndb is likely the result of enhanced water‐column denitrification in response to increased phytoplankton productivity. We hypothesize that greater export production during eutrophication led to anoxic conditions in the hypolimnion as a result of enhanced organic matter remineralization, raising water‐column denitrification. Enhanced N loss and remobilization of phosphorous (P) from the sediments under anoxic conditions lowered the N : P ratio in the lake, fostering cyanobacterial N2 fixation in surface waters.

An abundant future for quagga mussels in deep European lakes

Quagga mussels have expanded their range across the northern hemisphere in recent decades owing to their dispersal abilities, prolific reproduction rates, and broad ecological tolerances. Their remarkable capacity to filter particulates from the water column has had profound effects on inland aquatic ecosystems. In the North American Great Lakes, quagga mussel populations have increased inexorably since the late 1980’s, but it remains unclear whether quagga mussels will follow a similar trajectory in Europe where they have appeared more recently. Here we apply knowledge from a long-term quagga population monitoring effort in the North American lakes to predict future quagga populations in deep European lakes, where quaggas are quickly becoming a conspicuous part of the underwater landscape. We predict that quagga mussel biomass in Lakes Biel, Constance, and Geneva may increase by a factor of 9–20 by 2045. Like in North America, this increase may be characterized by a shift to larger individuals and deeper depths as the population matures. If realized, this rapid expansion of quagga mussels would likely drive the largest aquatic ecosystem change in deep European lakes since the eutrophication period of the mid-20th century.

Using sedimentary prokaryotic communities to assess historical changes in the Gippsland Lakes

Abstract The Gippsland Lakes is the largest estuarine system in Australia. Over the last 2 centuries, this unique aquatic ecosystem has suffered substantial modifications mostly associated with anthropogenic impacts, including the creation in 1889 of an artificial channel to the ocean after European arrival, creating chronic salinisation in the system. However, the biological impacts of this historic shift are unclear. Here, we use shotgun metagenomics of environmental DNA from historical sediments of Lake King and Lake Victoria to detect ancient microbial DNA and track past ecological changes in prokaryotic communities. Sedimentary prokaryotic communities changed with core depth, organic matter levels and European arrival. Specifically, we observed an increase in sulfate‐reducing bacteria (e.g., Desulfobacterales) and organic matter enrichment in the upper layers of the sediment cores of the Gippsland Lakes, which could reflect a period of eutrophication caused by higher rates of organic deposition and marine intrusion post‐European arrival. However, while some species presented authentic ancient DNA signals, the shifts in community composition probably reflect changes in indigenous sediment‐dwelling microorganisms in response to geochemical changes of the sediment. Overall, these observations suggest that historic waterway management practices may influence microbial systems today and that sedimentary microbial communities can change in response to both natural biogeochemical dynamics along the depth profile and past environmental conditions.

Shallow waters as critical habitats for fish assemblages under eutrophication-mediated events in a coastal lagoon

Eutrophication is a major driver of the degradation of transitional waters worldwide, especially in environments with a restricted connection to the sea, such as coastal lagoons. In recent decades, intensive agriculture and urban water inputs around the Mar Menor coastal lagoon (Western Mediterranean) have disturbed this originally oligotrophic aquatic system. The nutrient input into the lagoon has triggered its eutrophication, leading to dystrophic crises and mass mortality events for aquatic biota, transforming it into one of the most eutrophication-impacted transitional waters in the Mediterranean basin. In this study, we applied a fish-based indicator to assess the ecological quality of shallow waters under different eutrophication-mediated environmental stress scenarios (from pre-eutrophic reference periods to critical eutrophic periods), as well as to explore the role of confinement (i.e., water renewal time) and shoreline anthropogenic pressure as factors modulating the indicator response. Despite the high magnitude of the eutrophication impact on the lagoon, the ecological quality of the shallow waters decreased only slightly after the mass mortality events. The level of confinement also had slight effects on the ecological quality of the most confined shallow areas in the summer during eutrophic periods. Hence, shallow waters could play a critical role as refuge habitats, both for fish assemblages and other aquatic taxa, by buffering euxinic conditions during eutrophication processes.”. In fact, shallow waters could act as critical habitats, allowing for the recolonisation of aquatic biota from more impacted areas in the lagoon. This attribute further reinforces the need to properly manage and protect the shoreline areas of transitional waters, particularly under eutrophication scenarios.

Can Zn isotopes in sediments record past eutrophication of freshwater lakes? A pilot study at Lake Baldegg (Switzerland)

In this study, the speciation and isotopic composition of Zn were traced across the sediments of a freshwater lake that experienced one hundred years of strong eutrophication, in order to assess the potential of sedimentary Zn isotopes to record such an environmental disturbance. The results indicate that the sedimentary Zn isotope signal varied with the change from pre-eutrophic to eutrophic conditions in the investigated lake. The average δ66ZnJMC value of the dominantly allochthonous lithogenic sediments deposited during the pre-eutrophic period was +0.27‰ ±0.05‰ (i.e. similar to the δ66ZnJMC value of +0.28‰ ±0.05‰ proposed for Bulk Silicate Earth), while enhanced autochthonous biochemical sedimentation during the eutrophic period resulted in significantly lower δ66ZnJMC values down to +0.04‰ ±0.06‰. Synchrotron-based X-ray absorption spectroscopy data revealed a concomitant change in Zn speciation from a dominant fraction of Zn in clay minerals during the pre-eutrophic period to a major fraction of Zn in ZnS during the eutrophic period. A linear regression relating the sedimentary Zn isotope signal to the fraction of Zn in ZnS indicated δ66ZnJMC values of +0.27‰ ±0.06‰ and 0.00‰ ±0.08‰ for Zn in clay minerals and in ZnS, respectively. The enrichment of light Zn in ZnS in the eutrophic sediments is tentatively attributed to enhanced biological uptake of light Zn in the water column, which resulted in an enhanced flux of organic-bound Zn towards the sediments and further transformation of organic Zn into ZnS upon biomass mineralization during early diagenesis. This hypothesis is in agreement with the fractionation towards lighter Zn reported for both biological uptake of Zn and ZnS precipitation. The results of this study emphasize the potential of sedimentary Zn isotopes to register past eutrophic periods in freshwater lakes, and thus to serve as a probe of paleo-environmental conditions and/or past land use at the catchment scale.

Introgression from extinct species facilitates adaptation to its vacated niche.

Anthropogenic disturbances of ecosystems are causing a loss of biodiversity at an unprecedented rate. Species extinctions often leave ecological niches underutilized, and their colonization by other species may require new adaptation. In Lake Constance, on the borders of Germany, Austria and Switzerland, an endemic profundal whitefish species went extinct during a period of anthropogenic eutrophication. In the process of extinction, the deep-water species hybridized with three surviving whitefish species of Lake Constance, resulting in introgression of genetic variation that is potentially adaptive in deep-water habitats. Here, we sampled a water depth gradient across a known spawning ground of one of these surviving species, Coregonus macrophthalmus, and caught spawning individuals at greater depths (down to 90 m) than historically recorded. We sequenced a total of 96 whole genomes, 11-17 for each of six different spawning depth populations (4, 12, 20, 40, 60 and 90 m), to document genomic intraspecific differentiation along a water depth gradient. We identified 52 genomic regions that are potentially under divergent selection between the deepest (90 m) and all shallower (4-60 m) spawning habitats. At 12 (23.1%) of these 52 loci, the allele frequency pattern across historical and contemporary populations suggests that introgression from the extinct species potentially facilitates ongoing adaptation to deep water. Our results are consistent with the syngameon hypothesis, proposing that hybridization between members of an adaptive radiation can promote further niche expansion and diversification. Furthermore, our findings demonstrate that introgression from extinct into extant species can be a source of evolvability, enabling rapid adaptation to environmental change, and may contribute to the ecological recovery of ecosystem functions after extinctions.

Thirty-Four-Year Record (1987–2021) of the Spatiotemporal Dynamics of Algal Blooms in Lake Dianchi from Multi-Source Remote Sensing Insights

Lake Dianchi is one of the most eutrophic lakes in China. The decline in water quality and the occurrence of massive algal blooms pose a significant threat to the health and environmental safety of the water ecosystem, making Lake Dianchi a key concern for algal bloom management in China. Obtaining the spatiotemporal dynamics of algal blooms for the longest time possible is crucial to algal bloom management and future prediction. However, it is difficult to acquire a long-term record of algal blooms from a single sensor in order to cover a more extended period of eutrophication in the lake due to the limitation of the spatial and temporal resolution of the sensors. In this study, Landsat and Moderate-Resolution Imaging Spectroradiometer (MODIS) images were combined with the Floating Algae Index (FAI) to reconstruct a unified time series of bloom areas to analyze the algal bloom dynamics in Lake Dianchi in recent decades. Regarding the interannual variation, the bloom area showed an increasing trend from 1987 to 2021, with larger bloom areas in 1991–1992, 2000–2003, 2012–2013, and 2020–2021. In terms of seasonal characteristics, the bloom area was significantly more prominent in the rainy season compared with the dry season during the year. The spatial distribution of the bloom frequency showed a pattern of higher frequencies in the north and lower frequencies in the south. From 2000 to 2021, the initial bloom time and bloom duration showed a trend of delaying and then advancing and decreasing and then increasing, respectively. We analyzed the importance of long-term records of algal blooms and found that the percentage of rainy season images is an essential factor in reconstructing time series based on different sensors. In addition, the relationship between environmental factors and algal blooms was analyzed. The results show that wind speed and air temperature were the main meteorological factors controlling the interannual variation in algal blooms in Lake Dianchi. Water quality factors such as nutrients have less of an influence on the variation in algal blooms because the algal growth demand has been met. Environmental management measures taken by local governments have led to improvements in the lake’s trophic state, and continued strengthening of environmental pollution control is expected to curb the algal blooms in Lake Dianchi. This study provides a long-term record of algal blooms in Lake Dianchi, which provides essential reference information for a comprehensive understanding of the development process of algal blooms in Lake Dianchi and its sustainable development.

Sedimentary DNA records long‐term changes in a lake bacterial community in response to varying nutrient availability

AbstractMicrobial communities play important roles in lake ecosystems and are sensitive to environmental change. However, our understanding of their responses to long‐term change such as eutrophication is limited, as long‐term lake monitoring is rare, and traditional paleolimnological techniques (pigments and microfossils) are restricted to a low taxonomic resolution, or organisms with well‐preserved structures. Sedimentary DNA (sedDNA) is a promising technique to reconstruct past microbial communities in sediments, but taphonomic processes and the ability of sedDNA to record bacterial pelagic history accurately are largely unknown. Here, we sequenced the 16S rRNA gene in triplicate sediment cores from Esthwaite Water (English Lake District) which has concurrent long‐term monitoring and observational data. The sediment record spanned 113 years and included an episode of increased nutrient availability from the 1970s, followed by a more recent decline. Trends in bacterial community composition were broadly similar among the three sediment cores. Cyanobacterial richness in the sediment cores correlated significantly with that of cyanobacteria in a 65‐year microscopy‐based monitoring record, and some known pelagic bacterial taxa were detected in the sediment. sedDNA revealed distinct shifts in community composition in response to changing lake physicochemical conditions. The relative abundance of cyanobacteria closely reflected nutrient enrichment, and Proteobacteria, Bacteroidetes, and Verrucomicrobia were relatively more abundant in recent sediments, while Chloroflexi, Firmicutes, Acidobacteria, Nitrospirae, Spirochaetes, and Planctomycetes declined in more recent sediments. Following lake restoration efforts to reduce nutrient enrichment, the relative abundance of cyanobacteria returned to pre‐1970 levels, but the bacterial community did not fully recover from the period of intense eutrophication within the time scale of our study. These results suggest that sedDNA is a valuable approach to reconstruct lake microbial community composition over the 100‐year time scale studied, but an improved understanding of DNA deposition and degradation is required to further the application of sedDNA in paleolimnology.

Properties of sediment dissolved organic matter respond to eutrophication and interact with bacterial communities in a plateau lake

Sediment dissolved organic matter (DOM) in inland waters is commonly affected by environmental changes. However, knowledge about how sediment DOM responds to eutrophication and the associations between sediment DOM and bacterial communities requires further investigation. We selected a sediment core from Dianchi Lake (China) that was dated from 1864 to 2019 by the activity of radionuclides (210Pb and 137Cs). δ13CDOC changes fit well with the historical record that heavy eutrophic status in Dianchi Lake were observed since 1980s. Large amounts of dissolved organic carbon (DOC), chromophoric (CDOM) and fluorescent (FDOM) DOM accumulated at the top of the sediments during the eutrophication period (1982-present). The additional algae sources with a higher degradation rate altered the composition, aromaticity and humification of DOM. After long-term mineralization, the remaining DOM became more and more recalcitrant and kept a relatively stable level at older sediments. A co-occurrence network analysis revealed that Proteobacteria, Chloroflexi, Acidobacteriota, Bacteroidota and Desulfobacterota were the most abundant species at the phylum level and clustered into three primary modules. Different microbes shared unique preferences for niches, causing a heterogeneous bacterial distribution at different depths. We conducted Spearman's correlation and redundancy analysis (RDA) to explore potential interactions between bacterial community and sediment DOM. The richness and diversity of bacterial communities were positively related to DOM content, suggesting abundant DOM can produce more available resources for bacteria. RDA results showed some specific species might modify DOM composition and structure. This study suggests that sediment DOM properties were regulated by source transformation during eutrophication, and emphasizes the importance of microbial role on sediment biogeochemical process.

Biophilic elements in core sediments as records of coastal eutrophication in the Seto Inland Sea, Japan

This study presents vertical profiles of biophilic elements in core sediments from the Seto Inland Sea, Japan. We determined total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), biogenic silica (BSi), and phaeopigments in sediments from 80-cm-long cores collected from Hiuchi-Nada (Site A) and Harima-Nada (Site B). The vertical profiles are generally similar: all concentrations decrease downward from the top layer and show maximum values in the top layers for each parameter at both study areas. Among these biophilic elements, the correlation between BSi and TOC is particularly high, suggesting that diatoms are the main source of TOC in the sediments. The degradation rates at Site A in increasing order, as estimated by a steady-state diagenetic model (one-G model), are TOC ≈ BSi < TP < TN ≈ phaeopigments. However, these rates differ substantially from those at Site B, suggesting that the degradation rates might differ among sedimentation processes and environments. Using the one-G model and molecular composition (C:N) ratios, we estimate the origin (terrestrial and marine) for the amount of carbon before decomposition and followed its variation over the past approximately 200 years. We found that marine carbon is a major component and tends to be higher during the eutrophication period (1960–1990) at Site B, although not so much at Site A. At Site B the marine carbon content increases by about 25% during the eutrophication period, but at Site A no remarkable increase can be confirmed. These results are consistent with previous studies showing that the trend in primary production in the Seto Inland Sea since the eutrophication period has not been regionally homogeneous.

Temporal changes in nutrients in a deep oligomictic lake: the role of external loads &lt;em&gt;versus&lt;/em&gt; internal processes

The impact of climate change on stratification and mixing patterns has important effects on nutrient availability and plankton dynamics in deep lakes. We demonstrate this in a long-term study of Lake Maggiore, a deep oligomictic lake located in the subalpine lake district in Northern Italy. Studies on physical, chemical and biological features of the lake have been performed continuously since the 1980s. The lake recovered from eutrophication in response to a reduction of catchment nutrient loads and reached a stable oligotrophic status by the end of the 1990s, with average total phosphorus concentrations in the water column around 10 µg L-1. However, both reactive and total phosphorus have slightly increased since 2010, leading to a shift in the lake trophic state towards mesotrophy. The increase in phosphorus has been limited to the hypolimnetic layers, concentrations being fairly stable or decreasing in the epilimnion. Reactive silica also progressively increased in the hypolimnion, while nitrate and total nitrogen concentrations have steadily decreased in both deep and surface layers, especially in the summer period. These changes were assessed in relation to catchment loads, atmospheric deposition and climate-related variations in stratification and mixing patterns and in nutrient retention. Long-term changes in primary production, represented by chlorophyll levels, and biovolume of the main algal groups were also considered. During the eutrophication period and until the 1990s, in-lake phosphorus concentrations were tightly related to external loads; successively, phosphorus and its vertical distribution up the water column became more controlled by internal processes, in particular by stratification and mixing regime. An increase of thermal stability and a reduced frequency and intensity of deep mixing events has fostered oxygen depletion and phosphorus and silica accumulation in the hypolimnion. Another consequence of reduced deep mixing events, has been a reduction in nutrient replenishment of the upper layers at spring mixing. External loads are still the main driver of change for nitrogen compounds: the decrease in the atmospheric load of nitrogen that occurred in the Lake Maggiore area over the last decade, as an effect of reduced nitrogen emissions, has caused decreasing concentration of inorganic nitrogen in the lake. However, the phytoplankton community changes observed might also play a role in nitrogen dynamics, particularly in the nitrate minima observed during summer in recent years.

Limnological changes in Lake Victoria since the mid‐20th century

Abstract Lake Victoria experienced a strong degradation of water quality between the 1960s and the 1990s and, as a consequence of eutrophication, the dominant phytoplankton group changed from diatoms to N2‐fixing cyanobacteria and there was a 2‐ to 10‐fold increase in chlorophyll‐a. The goal of this study is to determine whether the 2018–2019 physical (light, stratification) and ecological (nutrient, chlorophyll‐a, phytoplankton composition) conditions in Lake Victoria changed from the 1990s. Samples were collected in 2018–2019 in nearshore and offshore waters (Uganda), during three contrasting seasons: heavy rains (March), low rains (October), and dry (June), which corresponded to distinct water column mixing regimes, respectively, late‐stratified, early‐stratified, and mixed regimes. At each station (48 nearshore and 25 offshore), we measured vertical profiles of temperature, oxygen, phytoplankton biomass and composition, inorganic nutrients, and particulate organic carbon, particulate nitrogen (N), and phosphorus (P). Chlorophyll‐a concentrations in 2018–2019 were 10.3 ± 7.1 and 2.8 ± 1.1 µg/L in the nearshore and offshore surface waters, respectively, close to those measured in the 1960s before eutrophication, but distinctly lower than those measured in the 1990s (71 ± 100 and 14 ± 6 µg/L). The phytoplankton of Lake Victoria in 2018–2019 still appears dominated by diatoms and cyanobacteria. However, we observed more non‐heterocystous filamentous and coccal/colonial cyanobacteria taxa that are better adapted to mixing conditions than gas‐vacuolated heterocystous taxa, which were dominant in the 1990s. Particulate N was significantly lower in 2018–2019 than in the 1990s, indicative of less efficient N fixation. The dissolved silica concentrations in 2018–2019 were significantly higher with the concomitant reappearance of Aulacoseira spp., which was not observed in the 1990s, presumably due to low dissolved silica concentrations. As data from long‐term monitoring are absent, the reasons for the lower chlorophyll‐a concentrations in 2018–2019 compared to the 1990s are unclear. However, climatic controls (El Niño/La Niña conditions) may be an important factor influencing the historical trend in chlorophyll‐a. Higher wind in 2018–2019 promoted vertical mixing, resulting in a deeper thermocline and surface mixed layers, which eventually lowered phytoplankton production in comparison to the 1990s. In contrast, the thermocline and surface mixed layers in the 1990s were shallower, enabling phytoplankton to stay suspended in the upper well illuminated water, allowing greater productivity. The lake in 2018–2019 is still P saturated, suggesting that another episode of high chlorophyll‐a concentrations could develop if less windy conditions occur in future, or if continued warming of surface waters eventually overcomes the mixing from present windy conditions. This study gives insights about the present ecological functioning of Lake Victoria and emphasises the impacts of variations in climate on lake physics that changes the light environment for phytoplankton. A possible less windy period in the future resulting from a new El Niño phase or from climate change, will probably lead to another episode of eutrophication in Lake Victoria. As in 2018–2019 the lake was still saturated by nutrients, there is need to reduce the nutrient concentrations (especially P) to prevent future destructive eutrophic periods caused by reduced mixing.

No evidence for reduced growth in resident fish species in the era of de-eutrophication in a coastal area in NW Europe

Coastal areas in north-western Europe have been influenced by elevated nutrient levels starting in the 1960s. Due to efficient measures, both nitrate and phosphate levels decreased since the mid-1980s. The co-occurring declines in nutrient loadings and fish productivity are often presumed to be causally linked. We investigated whether four resident fish species (twaite shad, bull-rout, thick-lipped grey mullet and eelpout), that spend the majority of their life in the vicinity of the coast, differed in growth between the historic eutrophication period compared to the recent lower nutrient-level period. Based on Von Bertalanffy growth models of length at age, and the analysis of annual otolith increments, we investigated the difference in sex-specific growth patterns and related these to temperature, eutrophication level (Chlorophyll a), growth window and fish density. In all four species, annual otolith growth rates during the early life stages differed between the two periods, mostly resulting in larger lengths at age in the recent period. All species showed significant correlations between increment size and temperature, explaining the observed period differences. The lack of an effect of total fish biomass provided no evidence for density dependent growth. A correlation with chlorophyll was found in bull-rout, but the relationship was negative, thus not supporting the idea of growth enhanced by high nutrient levels. In conclusion, we found no evidence for reduced growth related to de-eutrophication. Our results indicate that temperature rise due to climate change had a greater impact on growth than reduced food availability due to de-eutrophication. We discuss potential consequences of growth changes for length-based indicators used in management.

A new intermittent regime of convective ventilation threatens the Black Sea oxygenation status

Abstract. The Black Sea is entirely anoxic, except for a thin (∼ 100 m) ventilated surface layer. Since 1955, the oxygen content of this upper layer has decreased by 44 %. The reasons hypothesized for this decrease are, first, a period of eutrophication from the mid-1970s to the early 1990s and, second, a reduction in the ventilation processes, suspected for recent years (post-2005). Here, we show that the Black Sea convective ventilation regime has been drastically altered by atmospheric warming during the last decade. Since 2009, the prevailing regime has been below the range of variability recorded since 1955 and has been characterized by consecutive years during which the usual partial renewal of intermediate water has not occurred. Oxygen records from the last decade are used to detail the relationship between cold-water formation events and oxygenation at different density levels, to highlight the role of convective ventilation in the oxygen budget of the intermediate layers and to emphasize the impact that a persistence in the reduced ventilation regime would bear on the oxygenation structure of the Black Sea and on its biogeochemical balance.

Anthropogenic impact on the historical phytoplankton community of Lake Constance reconstructed by multimarker analysis of sediment-core environmental DNA.

During the 20th century, many lakes in the Northern Hemisphere were affected by increasing human population and urbanization along their shorelines and catchment, resulting in aquatic eutrophication. Ecosystem monitoring commenced only after the changes became apparent, precluding any examination of timing and dynamics of initial community change in the past and comparison of pre- and postimpact communities. Peri-Alpine Lake Constance (Germany) underwent a mid-century period of eutrophication followed by re-oligotrophication since the 1980s and is now experiencing warm temperatures. We extended the period for which monitoring data of indicator organisms exist by analysing historical environmental DNA (eDNA) from a sediment core dating back some 110years. Using three metabarcoding markers-for microbial eukaryotes, diatoms and cyanobacteria-we revealed two major breakpoints of community change, in the 1930s and the mid-1990s. In our core, the latest response was exhibited by diatoms, which are classically used as palaeo-bioindicators for the trophic state of lakes. Following re-oligotrophication, overall diversity values reverted to similar ones of the early 20th century, but multivariate analysis indicated that the present community is substantially dissimilar. Community changes of all three groups were strongly correlated to phosphorus concentration changes, whereas significant relationships to temperature were only observed when we did not account for temporal autocorrelation. Our results indicate that each microbial group analysed exhibited a unique response, highlighting the particular strength of multimarker analysis of eDNA, which is not limited to organisms with visible remains and can therefore discover yet unknown responses and abiotic-biotic relationships.

Mercury dynamics in a changing coastal area over industrial and postindustrial phases: Lessons from the Venice Lagoon

During the industrial period, significant amounts of mercury (Hg) were discharged into the Venice Lagoon. Here, a spatially explicit model was implemented to reconstruct the temporal evolution of the total mercury (HgT) and methylmercury (MeHgT) concentrations in lagoon water and sediments over two centuries (1900–2100), from preindustrial to postindustrial phases. The model simulates the transport and transformations of particulate and dissolved Hg species. It is forced with time-variable Hg inputs and environmental conditions, including scenarios of future atmospheric deposition, reconstructed according to local and global socioeconomic scenarios. Since 1900, ~36 Mg of HgT and ~380 kg of MeHgT were delivered to the lagoon, and stored in the sediments. The deposition of Hg from the water to the seafloor increased during a period of eutrophication (1980s); however, the reverse fluxes increased during a period of high sediment resuspension caused by the unregulated fishing of Manila clams (1990s). In the current postindustrial phase, the lagoon sediments have acted as a secondary source to the lagoon waters, delivering Hg (~38 kg y−1) and MeHg (~0.07 kg y−1). The MeHg inputs from the watershed (~0.28 kg y−1) appear to be higher than the secondary fluxes from the sediments. The estimated HgT export to the Adriatic Sea is ~56 kg y−1. Since HgT and MeHgT outputs slightly exceed inputs, the concentrations are slowly decreasing. While the decreasing trend is maintained in all scenarios, the future level of atmospheric deposition will affect Hg concentrations and sediment recovery times. Though limited by inherent simplifications, this work results show that the reconstruction of historical dynamics using a holistic approach, supported by data, can improve our understanding of the pollutants distribution and the quantification of local emissions. Downscaling from trends predicted at the global scale taking into account for regional differences seems useful to investigate the pollutants fate.

Paleolimnological reconstruction of the centennial eutrophication processes in a sub-tropical South American reservoir

Reservoirs hold a detailed record of the changes in the input of sediments and nutrients over decades to centuries. Paleolimnological multi-proxy analysis makes it possible to reconstruct baseline conditions to infer early evidence of environmental change. Our study aims to reconstruct historical human impacts derived from urban development on the San Roque reservoir (Córdoba, Argentina) related to the centennial dynamics of sedimentary and eutrophication processes.A paleolimnological record, dated by 210Pb and 137Cs, made it possible to identify two stages during the environmental evolution of the San Roque reservoir. Physical processes, such as fluvial discharge and water level variation, dominantly ruled stage 1 (Unit C) during the initial infilling of the reservoir. Nutrient load and eutrophication processes controlled stage 2 (Units B and A). Stage 1 (77-55 cm; AD 1921 to 1965) occurred before and after the second dam was built and while the level of water increased by ≈ + 8 m; it displayed a high variation in mean grain size and maximum values of magnetic susceptibility. Stage 2 (AD 1965–2017) records a new reservoir base level and the maintenance of high water levels and comparatively more stable conditions. Regarding the eutrophication process throughout stage 2, three sub-stages were defined: a) Sub-stage I (AD 1965–1985) is a period of incipient eutrophication; b) Sub-stage II (AD 1985–2005) is an interval of increase of eutrophication as shown by the increase in several organic proxies related to the abundance of phytoplankton. Echinenone, zeaxanthin and myxoxanthophyll indicate that cyanobacteria increased concentration by three-to four-fold in comparison with the previous sub-stage. c) Sub-stage III (AD 2005–2017) shows the transition to a hypereutrophic state. Diatomea (fuco and diato), dinophyceae (diadino), chlorophyta (lut) and cryptophyta (allo) groups show a comparatively higher contribution.Our results mostly highlight that during the last century the main drivers of changes in the environmental state of the San Roque reservoir were trophic, fluvial and hydrometeorological. These results might provide tools for anticipating future scenarios for water management plans under increasing anthropic pressure.

Impacts of a deep reactive layer on sedimentary phosphorus dynamics in a boreal lake recovering from eutrophication

Using biogeochemical analyses of sediments and porewaters, we investigate the legacy of a brief, intense period of eutrophication on sedimentary phosphorus (P) cycling in a boreal lake (Enonselkä basin, Lake Vesijärvi, Finland). Point-source sewage inputs in the twentieth century caused deoxygenation of the lake and accelerated the focusing of iron (Fe) and manganese (Mn) oxides into deeper areas. Early diagenesis under Fe–Mn-rich conditions now favors rapid burial of P in these areas, likely as a combination of both oxide-bound P phases and authigenic manganous vivianite. A new P budget for Enonselkä basin shows that P burial causes an annual drawdown of 1.2% (± 0.2%) of the surface sediment P inventory, supporting a long-term trend towards recovery since the construction of a wastewater treatment plant in the mid-1970s. However, remineralization of organic matter and associated dissolution of Fe–Mn oxides continues to regenerate P from a deep reactive layer (20–60 cm depth) deposited at the height of past eutrophication, leading to an upwards diffusive flux of dissolved phosphate towards the surface sediments. The magnitude of this flux is similar to that of external P loading to the lake. The combined incoming fluxes of P are likely to retard the complete recovery from eutrophication by decades, despite ongoing restoration actions.

Aliphatic Hydrocarbons in Recent Dated Sediment Cores of Imo River, SE Nigeria: Environmental/Historical Implications

Four recent sediment cores (0-30 cm long) from Afam (AF), Mangrove (MG), Estuary (ES) and illegal Petroleum refinery (PT) sites of the Imo River, Southeastern Nigeria were analyzed to characterize the sources and distribution of organic matter (OM), as well as examine their historical trends of deposition and assess human-induced changes in the last ca. 5 decades using biomarker approach. Radionuclides 210Pb and 137Cs were used to assign approximate dates to each section of the cores. Evaluation of proxy parameters such as carbon preference index (CPI, 2.01 - 2.19), carbon number maximum (Cmax, 29, 31) and atomic C/N (16.51-31.32) for the most recent top layers (0-5 cm) revealed greater wash-in of land-derived organic matter (OM), attributable to the recent rise in water height. The bottom layer (PT1, 25-30 cm,) of the PT core deposited ca. 1964-1972 exhibited a CPI of 0.97 and pristane/phytane (Pr/Ph, 3.75), suggesting that oil bunkering/illegal refinery activity had begun in the region ca. 8 years after the first commercial discovery of oil in Nigeria in 1956. The occurrence in high abundance of heptadecane in the middle layer (ES4, 10-15 cm) of the ES almost corresponded with the period of eutrophication that blocked the waterway in the late 1980s. Measurement of a marked unresolved complex mixture at the near-top layer (AF5, 5-10 cm) of the AF indicated that the heaviest contamination by petroleum hydrocarbons occurred at ca. 1997-2005. This time frame coincided with the period of intensive bunkering and oil pipeline vandalism by Niger Delta militant groups.