Shallow lake trophic status linked to late Holocene climate and human impacts

Lake Mattamuskeet, North Carolina, USA is a large (162 km²) and shallow (mean depth = 1 m) coastal lake, which was significantly modified to support agricultural activities following European settlement in 1850. Paleolimnological proxies measured on a 400-cm sediment core collected from Lake Mattamuskeet reveal shifts in organic matter input and primary producer community structure in response to climatic and human impacts on the lake during the late Holocene. Stratigraphic changes in organic matter content, nutrients, metals, lignin phenols and photosynthetic pigments were used to divide the sediment core into three intervals. Interval I includes sediment deposited between A.D. 360–1584 and indicates a clear-water, sand-bottom state with low algal abundance. In addition, the lake catchment area experienced two significant fires during this interval that were recorded as charcoal layers in the core around A.D. 360 and A.D. 1435 (calibrated ¹⁴C AMS dates). Trophic structure changed with the onset of Interval II (A.D. 1584–1860) when total algal abundance increased, and the primary producer community was comprised primarily of diatoms, chrysophytes, cryptophytes and cyanobacteria. During this interval there was also an increase in terrestrial organic material input into the lake as well as a shift in plant type from woody gymnosperms to non-woody angiosperms as determined from lignin data. Sediment deposited in Lake Mattamuskeet following European settlement (Interval III, A.D. 1860-present) suggests a dramatic increase in organic-matter deposition, metals, primary-producer abundance and the onset of cyanobacterial dominance. Sedimentary evidence indicates that shallow-water primary producers can respond rapidly to climate change and human development.     

Response of the cladoceran community to trophic state change in Lake Apopka, Florida

A paleolimnological evaluation of cladoceran microfossils was initiated to study limnological changes in Lake Apopka, a large (125 km²), shallow (mean depth = 1.6 m), warm, polymictic lake in central Florida. The lake switched from macrophyte to algal dominance in the late 1940s, creating a Sediment Discontinuity Layer (SDL) that can be visually used to separate sediments derived from macrophytes and phytoplankton. Cladoceran microfossils were enumerated as a means of corroborating extant eutrophication data from the sediment record. Inferences about the timing and trajectory of eutrophication were made using the cladoceran-based paleo-reconstruction. The cladoceran community of Lake Apopka began to change abruptly in both total abundance and relative percent abundance just before the lake shifted from macrophyte to algal dominance. Alona affinis, a mud-vegetation associated cladoceran, disappeared before the SDL was formed. Planktonic and benthic species also began to increase below the SDL, indicating an increase in production of both planktonic and benthic species. Chydorus cf. sphaericus, an indicator of nutrient loading, increased relative to all other cladocerans beginning in the layer below the SDL and continuing upcore. Changes in the transitional sediment layer formed before the lake switched to phytoplankton dominance, including an increase in total phosphorus concentration, suggest a more gradual eutrophication process than previously reported. Data from this study supported conclusions from other paleolimnological studies that suggested anthropogenic phosphorus loading was the key factor in the hypereutrophication of Lake Apopka.     

Sediment records of phosphorus-driven shifts to phytoplanktondominance in shallow Florida lakes

Primary producer community structure (PPCS) in shallow lakes isinfluenced by phosphorus (P) load and water column P concentration.Theoretically PPCS may shift between phytoplankton and macrophyte states withintermediate P loading, but phytoplankton dominate when P loading exceeds acritical threshold. We analyzed sediment cores from five shallow, eutrophiclakes (size range: 0.6 to 125 km²) that arephytoplankton dominated to determine whether the development of the currentstate was associated stratigraphically with an increase in sediment total P(TP) and a shift in PPCS. We used sponge biogenic silica(BSi_Sponges) concentrations and total carbon to total nitrogenratios (TC:TN) as proxies for macrophyte abundance and sediment organic mattersource, respectively. Three stratigraphic groups of sediments were identifiedwith k-means cluster analysis. These samples were grouped by increasing TPconcentrations and decreasing age and identified as macrophyte, transitionaland phytoplankton sediments. Results show that as P loading increased in thelate 19th and early 20th centuries, the lakes producedsediments with an increasing contribution from phytoplankton. Four of our lakesmay represent a subset of shallow lakes because of their large size (30 to 125km²) and relatively rapid historic P enrichment. Inthese Florida lakes, PPCS shifted to phytoplankton dominance with nopaleolimnological record of lake-wide alternating stable states or of lake-widephytoplankton dominance before anthropogenic P enrichment.     

Historic primary producer communities linked to water quality and hydrologic changes in the northern Everglades

The northern Everglades Water Conservation Areas have experienced recent ecological shifts in primary producer community structure involving marl periphyton mats and dense Typha-dominated macrophyte stands. Multiple investigations have identified phosphorus (P) as a driver of primary producer community structure, but effects of water impoundment beginning in the 1950s and changes in water hardness [e.g., (CaCO₃)] have also been identified as a concern. In an effort to understand pre-1950, primary producer community structure and identify community shifts since 1950, we measured pigment proxies on three sediment cores collected in Water Conservation Area-2A (WCA-2A) along a phosphorus enrichment gradient. Photosynthetic pigments, sediment total phosphorus content (TP), organic matter, total organic carbon and nitrogen were used to infer historic primary producer communities and changes in water quality and hydrology regulating those communities. Excess ²¹⁰Pb was used to establish historic dates for the sediment cores. Results indicate the northern area of WCA-2A increased marl deposition and increased algal abundance ca. 1920. This increase in (presumably) calcareous periphyton before intensive agriculture and impoundment suggest canal-derived calcium inputs and to some extent early drainage effects played a role in initiating this community shift. The northern area community then shifted to Typha dominance around 1965. The areas to the south in WCA-2A experienced increased marl deposition and algal abundance around or just prior to 1950s impoundment, the precise timing limited by core age resolution. Continued increases in algal abundance were evident after 1950, coinciding with impoundment and deepening of canals draining into WCA-2A, both likely increasing water mineral and nutrient concentrations. The intermediate site developed a Typha-dominated community ca. 1995 while the southern-most core site WCA-2A has yet to develop Typha dominance. Numerous studies link sediment TP >650 mg P/kg to marsh habitat degradation into Typha-dominance. The northern and intermediate cores where Typha is currently support this previous research by showing a distinct shift in the sediment record to Typha dominance corresponding to sediment TP between 600 and 700 mg P/kg. These temporal and spatial differences are consistent with modern evidence showing water-column gradients in mineral inputs (including Ca, carbonates, and phosphorus) altering primary producer community structure in WCA-2A, but also suggest hydroperiod has an effect on the mechanisms regulating periphyton development and Typha dominance.     

A sediment record of recent nutrient loading and trophic state change in Lake Norrviken,Sweden

Human-induced perturbations in the Lake Norrviken catchment, Sweden, over the last 100+ years have left distinctive geochemical imprints in the sediments. Disposal of sewage, industrial, and agricultural run-off into the lake since the end of the nineteenth century changed the trophic status from eutrophic to hyper-eutrophic. The primary organic matter (OM) source in the lake is in situ algal material. Total organic carbon (TOC) concentrations increased near the mid-section of a short sediment core collected from the deepest part of the lake, reflecting elevated epilimnetic productivity and consequent hypolimnetic anoxia. Accompanying shifts to lighter stable organic C and total N isotopic compositions suggest that cyanobacterial productivity increased during this period. The distribution of pigments in the core indicates a shift in the phytoplankton community to a cyanobacteria-dominated system. Moreover, pigments confirm that N₂-fixing versus non-N₂-fixing phytoplankton varied depending upon the external inputs of N and P. Conditions in the lake improved after sewage input was diverted and the lake is currently mesotrophic.     

Biomonitoring past salinity changes in an athalassic subarctic lake

A short sediment core was taken from a small saline lake located on an intermontane plateau in the central Yukon Territory, Canada. In July 1990, chemical analyses indicated that, although the lake was shallow (Z_max=1.1 m), it was also chemically stratified, with hyposaline (9.9 to 10.0 g L⁻¹) surface waters and slightly mesosaline (22.0 g L⁻¹) deeper waters. The surface water was dominated by Na⁺ and HCO ₃ ⁻ . To our knowledge, this is the northernmost athalassic saline lake yet recorded. Quantification of algal (diatom, chrysophyte, and pigment) and invertebrate (chironomid, ceratopogonid, andChaoborus) fossils at four stratigraphic levels indicated that the lake sediments preserved numerous biological indicators that could be used to infer recent lake development. Many of the taxa are found in other athalassic salt lakes. The most striking stratigraphic change was a remarkable drop in the species richness of diatoms and invertebrates in the recent sediments, which parallels the elimination of species characteristic of less saline conditions. Halophilous taxa dominate the most recent sediments, indicating the development of more saline conditions. At the same time, a significant shift in chrysophyte cyst composition was observed. Fossil carotenoids and chlorophylls indicated a decrease in total algal abundance in recent sediments, as green and blue-green algae replaced diatoms and chrysophytes. Together, these paleolimnological data suggest a recent shift to drier conditions or increased evaporation in the central Yukon Territory.     

Sedimentary Steryl Chlorin Esters (SCEs) and Other Photosynthetic Pigments as Indicators of Paleolimnological Change Over the Last 28,000 Years from the Buguldeika Saddle of Lake Baikal

Detailed depth profiles of photosynthetic pigments in a sediment core (G-12) collected at the BDP93 site, the Buguldeika saddle, of south Lake Baikal, along with depth profiles of total organic carbon (TOC) and biogenic silica, were studied to elucidate the temporal changes of phytoplankton assemblages in the lake during the past 28 kyr. In addition to the quantification of carotenoids by high-performance liquid chromatography with photodiode-array detection (HPLC-PDA), steryl chlorin esters (SCEs) were analyzed by HPLC-PDA, HPLC-mass spectrometry (LC-MS) and sterols in SCEs by gas chromatography–mass spectrometry (GC–MS) to enrich the taxonomical information on the phytoplankton composition. Allochthonous input of organic matter from the Selenga River resulted in the higher TOC contents in core G-12 than in a previously reported core (G-6) collected at another site from the southern basin. The poorer correlation in core G-12 than in G-6 between TOC and chlorophyll-a-originating pigments, which are indicative of autochthonous production, also indicated a significant allochthonous input at the site. The abundance of lutein among the carotenoids detected, and the good correlation of total chlorophyll a and b shows that green algae represented a significant portion of the phytoplankton, accompanying the diatoms at the G-12 site, after the last glacial period. The presence of cryptomonads and cyanobacteria were confirmed from marker carotenoids in the sediment core. GC–MS analysis of sterols in SCEs detected marker sterols of diatoms, green algae, chrysophytes and dinoflagellates. The depth profiles of the measured indicators gave consistent features for temporal changes in phytoplankton assemblage at the G-12 site of Lake Baikal after the last glacial maximum. Notably, the profile of a chrysophyte-specific sterol in SCEs was consistent with the reported distribution of chrysophyte cysts during the Holocene. The presence of phytoplankton, such as green algae, diatoms and chrysophytes, in Lake Baikal during the late last glacial period was indicated by the analysis of sterols in SCEs. Sedimentary carotenoids and sterols in SCEs were found to give complementary information about phytoplankton composition. These molecular indicators allow us to reconstruct past lake phytoplankton assemblages responding to environmental changes with a time resolution as high as age–depth relationship in sediments attainable at present.     

北极新奥尔松地区湖泊沉积物色素含量变化及环境意义

大量的古气候资料和现代器测数据表明,最近100年来北极气候经历了明显的变暖过程,这必然会对脆弱的极地湖泊生态系统产生影响。以北极新奥尔松地区的湖泊沉积柱样为研究对象,对其中4种色素(叶绿素及其衍生物CD、总胡萝卜素TC、蓝藻叶黄素Myx和颤藻黄素Osc)进行了分析,结合其他物化指标如碳酸钙含量、TOC等,从历史演化的角度重点探讨了新奥尔松地区最近100年以来湖泊初级生产力演化过程。结果表明,在相对寒冷的小冰期,气候对湖泊中藻类的生长不利,湖泊生产力下降,沉积物中色素含量出现低值,生物硅含量降低;表层5cm(对应约1890AD)沉积物中有机质含量显著升高,各色素含量均明显增加,表明小冰期后气温上升,导致湖泊藻类生长迅速,湖泊生产力大大提高,但此时生物硅仍保持在相对较低的水平,这可能与其他藻类的竞争生长有关。最近100年,沉积物中Osc/Myx比值不断降低,表明该湖泊中蓝藻含量不断上升,暗示北极地区人类活动的增强可能导致湖泊营养水平增加。     

Late Holocene changes in cyanobacterial community structure in maritime Antarctic lakes

Despite the dominance of cyanobacteria in polar freshwater aquatic ecosystems, little is known about their past biodiversity and response to climate and environmental changes. We explored the use of light microscopy of microfossils, high performance liquid chromatography of the fossil pigment composition and denaturing gradient gel electrophoresis of fossil 16S rRNA genes to study past and present-day differences in cyanobacterial community structure in response to climate changes in two adjacent maritime Antarctic lakes with contrasting depths (4 and 26 m) and light climates. Light microscopy was of limited use because of degradation of cell structures. Fossil cyanobacterial pigment concentrations were below the detection limits of our method in several sediment samples in the deep lake, but abundant and diverse in the sediment core from the shallow pond, probably as a consequence of increased light availability and/or a more diverse and abundant benthic cyanobacterial flora. Total carotenoid and chlorophyll concentrations were highest in both lakes between ca. 2,950 and 1,800 cal yr BP, which coincides with the late Holocene climate optimum recognised elsewhere in maritime Antarctica. Cyanobacterial molecular diversity was higher in the top few centimeters of the sediments in both lakes. In deeper sediments, the taxonomic turnover of cyanobacteria appeared to be relatively small in response to past climate anomalies in both lakes, underscoring the broad tolerance of cyanobacteria to environmental variability. This, however, may in part be explained by the low taxonomic resolution obtained with the relatively conserved 16S rRNA gene and/or the preferential preservation of particular taxa. Our results highlight the potential of fossil DNA in lake sediments to study colonization and succession dynamics of lacustrine cyanobacteria and warrant further investigation of the factors that affect preservation of cyanobacterial DNA.     

Whole-basin,mass-balance approach for identifying critical phosphorus-loading thresholds in shallow lakes

Lake Lochloosa, Florida (USA) recently underwent a shift from macrophyte to phytoplankton dominance, offering us the opportunity to use a whole-basin, mass-balance approach to investigate the influence of phosphorus loading on ecosystem change in a shallow, sub-tropical lake. We analyzed total phosphorus (TP) sedimentation in the basin to improve our understanding of the forcing factor responsible for the recent shift to phytoplankton dominance. We measured ²¹⁰Pb activity, organic matter (OM), organic carbon (OC) and TP in short sediment cores from 20 locations to develop a comprehensive, whole-basin estimate of recent mass sedimentation rates (MSR) for bulk sediment, OM, OC and TP. The whole-basin sedimentation models provided insights into historic lake processes that were not evident from the limited, historic water quality data. We used Akaike’s Information Criteria to differentiate statistically between constant MSR and exponentially increasing MSR. An eightfold, exponential increase in TP accumulation over the past century provided evidence for the critical role of increased P loading as a forcing factor in the recent shift to phytoplankton dominance. Model results show increased TP retention and decreased TP residence time were in-lake responses to increased TP loading and the shift from macrophyte to phytoplankton dominance in Lake Lochloosa. Comparison of TP loading with TP retention and historic, diatom-inferred limnetic TP concentrations identified the TP loading threshold that was exceeded to trigger the shift to phytoplankton dominance.     

Algal-silica cycling and pigment diagenesis in recent alpine lake sediments: mechanisms and paleoecological implications

The quality and interpretability of the paleobiological record depends on the preservation of morphological and geochemical fossils. Siliceous microfossils and sedimentary pigments are often cornerstones in paleoecology, although the microbial and geochemical processes conducive to their preservation remain poorly constrained. We examined sediments from an alpine lake in Banff National Park (Alberta, Canada) where diatom frustules are completely dissolved within 50 years of deposition. Diatom dissolution, silica recycling, and diagenetic alteration of algal pigments were investigated, in conjunction with porewater geochemistry and microelectrode profiling of the sediment–water interface. Analysis of sediment trap material showed ~90% of biogenic silica (BSi) production is lost prior to burial. Silica flux calculations, based on dissolved silica (as H₄SiO₄) in pore-waters, show a further ~6% of total BSi is returned to the water column from the upper 4 cm of sediments, implying that only ~4% of total BSi is permanently archived in sediments. In situ sediment pH and O₂ profiles reveal that aerobic respiration by bacteria fully consumes oxygen by a depth of 4 mm into the sediment, with associated strong pH and redox gradients. During sedimentation and early diagenesis, diatoms undergo loss of extracellular polymeric substances that coat their frustules, promoting silica dissolution and leading to the loss of the microfossil record by a depth of 3.25 cm. Sedimentary pigments similarly undergo rapid degradation, but diatom-related carotenoids persist below the depth of silica dissolution. This work provides new insights on diagenetic processes in lakes, with broad implications for the interpretation of sedimentary proxies for algal production.     

Primary production in Lake La Cruz (Spain) over the last four centuries: reconstruction based on sedimentary signal of photosynthetic pigments

We analysed photosynthetic pigments in annually laminated sediment of meromictic Lake La Cruz, Spain, to cope with the timing and characterisation of primary productivity changes over the last four centuries. The photosynthetic pigments identified included chlorophyll a and b (and its derivatives) and specific carotenoids of different algal groups, such as zeaxanthin, lutein, alloxanthin, diadinoxanthin and diatoxanthin among others. Marker pigments of phototrophic sulfur bacteria were also observed, including bacteriochlorophyll a derivatives, homologue series of bacteriophaeophytins d and bacterial carotenoids okenone and chlorobactene. We investigated the diagenetic processes of pigment alteration in anoxic sediments and the possible implications for paleoproductivity reconstruction and interpretation. The lack of systematic down-core changes in diagenetic indicators suggests that variability of sedimentary pigment concentrations is the result of changes in lake productivity. The lower concentration of algal photosynthetic pigments in the bottom of sediment sequence corresponds to the onset of meromictic conditions. Before that, sediment was not continuously anoxic and the preservation of pigments was reduced. Regarding photosynthetic bacteria, green sulfur bacteria derivatives indicate that population growth was limited until the settled organic matter was sufficient to provide enough sulphide. Moreover, the presence of bacterioviridine, an oxidised derivative of bacteriochlorophyll a, suggest the competitive relation among photosynthetic bacteria. The high relative abundance of zeaxanthin indicates the dominance of picocyanobacteria in the primary productivity over the last centuries. Fluctuations of the pigment signal observed in the sediment sequence during the meromictic period were well correlated with fluctuations of solar activity.     

Multi-proxy Reconstruction of Trophic State,Hypolimnetic Anoxia and Phototrophic Sulphur Bacteria Abundance in a Dimictic Lake in Northern Germany over the past 80 Years

During monthly investigations from 1996 to 2000, a hypolimnetic layer of phototrophic sulphur bacteria (Chromatium spp.) were observed in Lake Dudinghausen, a small dimictic lake in northern Germany. This paleolimnological study was initiated to detect if the occurrence of sulphur bacteria was related to cultural eutrophication or reflected natural conditions. Therefore, diatoms, algal pigments, okenone, geochemical proxies, and ²¹⁰Pb and ¹³⁷Cs were used in four sediment cores to investigate historical changes in trophic development, hypolimnetic redox conditions, anoxia and phototrophic sulphur bacteria abundances. Fossil diatoms, pigments, the ratio of chlorophyll derivatives to total carotenoids and the ratio of chlorophyll a to its derivatives suggest two phases of eutrophication coupled with hypolimnetic anoxia over the last ~80 years: a first phase from about 1923–1932 and a second from 1952 to 1982. In the first phase the ratios of Fe–Mn as well as Fe–Ca increased, suggesting seasonal anoxia. However, hypolimnetic anoxia was only weak because low levels of okenone suggest no mass development of sulphur bacteria. In contrast, sulphur bacteria increased during the early stages of the second eutrophication phase, suggesting increased temporal and spatial hypolimnetic anoxia. Surprisingly, the ratios of Fe–Mn as well as Fe–Ca decreased during this time. Possibly Fe, Mn and Ca were equally reduced through the intense anoxia. In the final stage, sulphur bacteria decreased again. As these bacteria need both anoxic conditions and a certain amount of light, the increased nutrient load probably led to low Secchi depth and therefore insufficient light conditions. In more recent years, diatoms and pigments suggest a decrease in nutrient levels. A second mass development of sulphur bacteria occurred, probably due to improved light conditions and continued anoxia in the upper hypolimnion. We conclude that the recent development of phototrophic sulphur bacteria do not represent natural conditions in Lake Dudinghausen. Furthermore, the upper sediments contain a completely new diatom flora that never occurred in older sediments of Lake Dudinghausen. Therefore, nutrient levels may eventually reach natural conditions, however they may not represent biological background reference conditions.     

Sedimentary pigments as indicators of cyanobacterial dynamics in a hypereutrophic lake

Lac Saint-Augustin is an urban lake located on the outskirts of Quebec City, one of North America’s oldest cities. Anthropogenic inputs from land clearing, agriculture, highway development and urbanization in the surrounding catchment have resulted in strong impacts on the limnology of the lake throughout the past three centuries. In recent years, this lake has experienced severe eutrophication, including persistent cyanobacterial blooms. In winter 2011, a sediment core was extracted from the deepest area of the lake. A detailed paleopigment analysis was used to assess eutrophication processes in the lake and to determine the timing and appearance of cyanobacterial blooms and their subsequent variability. Extracted chlorophyll a, its degradation products and 11 carotenoid pigments were identified and quantified via reverse-phase high performance liquid chromatography to examine relative changes in the phytoplankton. The results revealed large variations in the phytoplankton community structure of Lac Saint-Augustin over the past 356 years. Chlorophyll a concentrations per unit organic matter (OM) increased significantly from the base of the core to present day, rising more than 15-fold from 18.4 µg (g OM)^?1 at the base of the core to 287 µg (g OM)^?1 in the most recent strata. Biostratigraphical analysis revealed three major periods of enrichment, with episodes of cyanobacterial abundance from the 1890s onwards. The greatest changes occurred in the most recent period (from the 1960s to the present) relative to earlier periods, with pigment increases for all phytoplankton groups. The cyanobacterial pigments canthaxanthin, echinenone and zeaxanthin (also a marker for green algae) showed concentrations in the surface sediments that were significantly above values at the bottom of the core, and these differences were large, even giving consideration to the lesser pigment degradation near the surface. Overall, the results indicate that cyanobacterial blooms are not a recent feature of Lac Saint-Augustin but began to occur soon after catchment modification 150 years ago. The pigment records also imply that cyanobacterial and associated algal populations have risen to unprecedented levels over the last few decades of ongoing development of the Lac Saint-Augustin catchment. This study highlights the utility of multiple pigment analysis of lake sediments for identifying the timing and magnitude of anthropogenic impacts.     

生物硅在东南极湖泊古初级生产力研究中的应用

本文对南极中山站区的莫愁湖和大明湖沉积物中生物硅、有机质等环境指标进行了分析,并在对比研究的基础上,探讨了生物硅在重建东南极湖泊沉积生态环境记录研究中的应用.研究结果表明,大量保存较好的硅藻出现在东南极淡水湖泊沉积物中,生物硅含量在深度剖面上表现出显著波动,与有机质等生物地球化学指标具有一致的垂向变化趋势,表明它们具有...     

Identifying sources of organic matter in sediments of shallow lakes using multiple geochemical variables

We analyzed ²¹⁰Pb-dated sediment cores from four relatively shallow lakes (z_max < 10 m) in the Upper Ocklawaha River Basin, Florida, USA to compare primary producer community structure before and after anthropogenic impacts. We measured physical and chemical sediment variables including density, organic matter (OM), water-soluble phosphorus, polyphosphate (Poly-P), total P (TP), total carbon to total nitrogen mass ratios of OM (TC:TN), biogenic silica (diatoms, sponge spicules), total amorphous silica, and stable carbon and nitrogen isotope ratios of bulk OM. Principal component analysis showed that diatom biogenic silica, TC:TN, Poly-P and TP displayed discernible stratigraphic changes associated with the shift in the primary producer community. We applied k-means cluster analysis to these variables to identify macrophyte-derived, transitional, and phytoplankton-derived sediments. Our approach provides an objective method for identifying sediment sources that may be applied to shallow lakes in other regions. The four study lakes shifted from a macrophyte-dominated state to a transitional state before major anthropogenic disturbances, and became phytoplankton-dominated after ~1950.     

Multi-proxy paleolimnological assessment of biogeochemical versus food web controls on the trophic states of two shallow,mesotrophic lakes

The frequency of nuisance algal blooms has been increasing during the last two decades in the shallow, headwater East Pond (Smithfield, Maine, USA). Meanwhile, the hydrologically linked North Pond has not experienced an increase in algal blooms, despite similar morphometry and higher external nutrient loads. Possible explanations for this difference include stronger trophic cascade effects from planktivorous white perch (Morone americana) in East Pond as well as differences in phosphorus (P) release from the sediments of these two lakes. We conducted a paleolimnological investigation of these two lakes to assess whether sedimentary evidence supported trophic cascade effects based on cladoceran ephippia size, diatom fossils, and fossil pigments or biogeochemical controls based on potential sedimentary P release as the primary driver of these increased algal blooms in East Pond. At the time of white perch introduction (~1930–1950), ephippia size increases in East Pond, although no changes are observed in either diatom abundance or trends in the algal pigments. Instead, algal pigments increase in recent decades (~1980 to present) along with an increase in diatom taxa with higher TP optima These results suggest that predation by white perch is not resulting in top-down effects on algal abundance in East Pond, as predicted by the trophic cascade hypothesis. While the P content of sediments from both lakes is relatively equal, the releasable P in the top 10 cm of sediment in East Pond constitutes a greater percentage of the P extracted. Also, North Pond sediments exhibit a greater capacity to permanently bury P via the mechanisms of sorption to Al(OH)_3(s) and a slower mineralization of organic P compounds. The results of this investigation suggest that the ultimate driver of the recent algal blooms in East Pond is internal P release from the sediments instead of trophic cascade effects.     

Eutrophication-induced changes in Lake Nakaumi,southwest Japan

Lake Nakaumi, southwest Japan, is an enclosed lagoon characterized by polyhaline and halocline conditions. Since the last century, its ecological state has been altered by eutrophication. We used a paleolimnological approach and studied multiple proxies, including chemical compounds, diatoms, foraminifera and molluscs, to infer the eutrophication history of the ecosystem. Eutrophication in Lake Nakaumi was associated with several factors, including increased nutrient loading, input of herbicides, and dike building since the 1920s. The ecological condition of this lake was divided into several stages that reflect the eutrophication process after the 1940s. A catastrophic “regime shift” from a clear state with aquatic vegetation to a turbid one with phytoplankton occurred in the early 1950s. Environmental degradation in the Honjo area, a part of Lake Nakaumi, was attributed primarily to physical changes caused by the construction of an enclosing dike. Eutrophication occurred almost simultaneously with the physical changes to the Honjo area in the 1970s. Until recently, no regime shift was observed in this area, though the core-top sediments show possible symptoms of incipient change.     

DNA-based methods in paleolimnology: new opportunities for investigating long-term dynamics of lacustrine biodiversity

We reconstructed 150 years of ecological change in a shallow boreal lake located on the east shore of the Baltic Sea by integrating different types of environmental evidence: paleolimnological records (XRF-measured elements, fossil pigments and Cladocera assemblages), information from historical limnological surveys and archival maps. We assessed the role of biomanipulation by liming and fish-removal in the disappearance of submerged macrophytes, such as Lobelia dortmanna L., and their replacement by persistent cyanobacterial blooms. The combination of different strands of evidence revealed that the shift from macrophyte to cyanobacterial dominance was part of a long-term ecological response to eutrophication and increased sediment load from catchment disturbances. The findings demonstrate that a gradual loss of wetlands and increase of ditches in a catchment had a more significant effect on the lake ecosystem, compared to the direct, but short-term impact of biomanipulation. The study highlights the importance of catchment land-use and disturbance by ditches in changing the ecology of boreal water bodies. Also, it illustrates that a thorough understanding of the long-term ecosystem dynamics and differentiation among responses to multiple anthropogenic impacts are essential preconditions for addressing the deterioration of habitats and change in aquatic environments.     

Natural and anthropogenic forcing of Holocene lake ecosystem development at Lake Uddelermeer (The Netherlands)

Lake Uddelermeer (The Netherlands) is characterized by turbid conditions and annual blooms of toxic cyanobacteria, which are supposed to be the result of increased agricultural activity in the twentieth century AD. We applied a combination of classic palaeoecological proxies and novel geochemical proxies to the Holocene sediment record of Lake Uddelermeer (The Netherlands) in order to reconstruct the natural variability of the lake ecosystem and to identify the drivers of the change to the turbid conditions that currently characterize this lake. We show that the lake ecosystem was characterized by a mix of aquatic macrophytes and abundant phytoplankton between 11,500 and 6000 cal year BP. A transition to a lake ecosystem with clear-water conditions and relatively high abundances of ‘isoetids’ coincides with the first signs of human impact on the landscape around Lake Uddelermeer during the Early Neolithic (ca. 6000 cal year BP). An abrupt and dramatic ecosystem shift can be seen at ca. 1030 cal year BP when increases in the abundance of algal microfossils and concentrations of sedimentary pigments indicate a transition to a turbid phytoplankton-dominated state. Finally, a strong increase in concentrations of plant and faecal biomarkers is observed around 1950 AD. Canonical Correspondence Analysis suggests that reconstructed lake ecosystem changes are best explained by environmental drivers that show long-term gradual changes (sediment age, water depth). These combined results document the long-term anthropogenic impact on the ecosystem of Lake Uddelermeer and provide evidence for pre-Industrial Era signs of eutrophication.