A palaeolimnological study of the anthropogenic impact on dissolved organic carbon in South Swedish lakes

During the past three decades, increases have been observed in dissolved organic carbon (DOC) concentrations and colour in the surface waters of lakes and rivers in parts of Europe and North America, raising concern about the effects on the quality of aquatic environments with consequences for biodiversity, resource availability and recreational use. Various hypotheses have been put forward to explain the recent increases in DOC concentration and numerous studies have been published linking them to declining anthropogenic atmospherically deposited sulphur. Others have argued that increases in DOC content are a consequence of changes in climate, land use or land management practices. The work presented in this thesis concentrates on identifying the major forcing mechanisms behind observed increases in DOC concentration in the upland area of southern Sweden during recent decades, by comparing variations in the total organic carbon (TOC) concentration in lake water inferred from lake sediments, in response to changes in land use, sulphur deposition and climate during the past eight centuries. Two small lakes with different catchment properties were selected for the study; one dominated by woodland with abundant peat deposits, and another located nearby with patches of agricultural land in an otherwise mainly forested terrain. A number of palaeolimnological methods were applied to the sediment sequences; decadal-scale variations in TOC concentration in the lakes were reconstructed based on visible-near infrared spectroscopy (VNIRS) of sediment successions, high-resolution (20-y) pollen-based reconstructions of local land use were quantified using the Landscape Reconstruction Algorithm (LRA) and the model Local Vegetation Estimates (LOVE), geochemical records provided further information on environmental changes in the lakes and their catchment areas, and changes in pH in the lakes were inferred from diatom analysis. Comparisons were made with population density data and climate records. The results obtained with the LRA and LOVE models revealed a dynamic land-use pattern, with agricultural expansion from AD 1500 to the end of the 1800s, when population growth and the related increase in the exploitation of the surrounding land had a major impact on catchment erosion and input of terrestrial inorganic and organic matter to the lakes. Evidence also exists of a period of agricultural expansion around AD 1200-1300, followed by partial abandonment of the landscape, which can probably be attributed to the Black Death pandemic. A transition from traditional to modern land use following the industrial revolution took place during the past century, and a concurrent shift in most of the proxy records at around AD 1900 suggests a marked change in external forcing mechanisms common to both lakes, related to a major decrease in population density and the introduction of modern land use. The results revealed generally high TOC concentrations in the lakes prior to AD 1900, with second-order variations associated mainly with changes in the intensity of agricultural land use. The TOC concentrations in the lakes started to decrease around AD 1900, and unusually low TOC concentrations were recorded in the period AD 1930-1990, followed by a recent increase. The variation in sulphur emissions, with an increase in the early 1900s to a peak around AD 1980 followed by a significant decrease, was probably the main driver of lake-water TOC dynamics during the past century. This demonstrates that declining atmospheric sulphur deposition is the most probable driver of the increase in TOC concentration during the past three decades and that these lakes may be recovering to their naturally high-TOC pre-depositional states. The results also demonstrate regional versus local forcing of environmental change and indicate broadly similar regional sensitivities to anthropogenic impact, although responses were site-specific due to the different properties of the catchment areas. Given the reduction in atmospheric sulphur emission during recent decades, it is likely that previously suppressed or masked effects of changes in land use, land management and climate during the past century will become progressively more important drivers of TOC concentrations in lake water in the future. Long-term records of environmental history on decadal to millennial time scales enabled the assessment of ecosystem variability and responses to past anthropogenic disturbance, and may be a useful tool for the development of future environmental management strategies

A palaeoecological study of Holocene lake sediments above the highest shoreline in the province of Västerbotten, northeast Sweden

En sedimentborrkärna från sjön Svartkälstjärn, belägen 260 m ö.h. nära Vindeln i Västerbotten, provtogs for analys av frekvensen pollen, sporer och kolpartiklar samt beräkning av ackumulationshalter. Syftet var att rekonstruera den lokala miljöutvecklingen under Holocen med betoning på skogssammansättning, brandfrekvens, markerosion, akvatisk produktion samt mänsklig påverkan. Som komplement användes andra tillgängliga data från samma borrkärna såsom magnetisk susceptibilitet, organisk halt (TC) och kol/kväve kvoten (C/N). Kronologin baserades på 14C-datering och analys av antropogent cesium (137Cs). Studien visar att området kring Svartkälstjärn har varit helt täckt av skog sedan 8500 kalenderår fore nutid ( cal BP). Tall och björk dominerade initialt medan inslaget av lövträd blev större kring 7000 cal BP. Pollenförekomst och påtagliga polleninfluxvärden av ek, alm och lind kan indikera förekomst av dessa ädla lövträd under mellersta delen av Holocen. Klimatet blev gradvis kallare och fuktigare kring 4500 cal BP och en tydlig ökning av vitmoss-sporer antyder att våtmarker troligen upptog en större areal i sjöns omedelbara närhet. Granen etablerade sig i området kring 3300 cal BP och inom 500 år var gran det dominerande trädslaget kring sjön. Kolpartikelfrekvensen ökade generellt kring 3000 cal BP följt av tre perioder av högre ackumulation (vid 2800 cal BP, 1700 cal BP och i recent tid). Den mänskliga påverkan på skogsmiljön är påtaglig under de senaste 500 åren. Markerosionen ökade markant och ormbunkssporer upptar ca 55% av den totala pollenfrekvensen i den översta delen av borrkärnan. Resultaten tyder på omfattande mänsklig aktivitet i sjöns närområde i form av avverkning, tjärbränning och dikning. Historiska dokument från trakten ger ytterligare belägg för denna utveckling.A 2.1-m sediment sequence retrieved from Lake Svartkälstjärn, located at 260 m a.s.l. in the province of Västerbotten, Sweden, was subsampled for pollen and spore analysis, charcoal analysis, and influx calculations. The aim was to establish the local environmental development during the Holocene with focus on forest composition, fire frequency, soil erosion, aquatic production, and human impact. Comparisons with additional environmental proxies previously obtained from the same sequence, such as magnetic susceptibility, elemental carbon content (TC), and C/N-ratios of the sediments, contributed to the conclusions. The chronology was established through radiocarbon dating and the identification of the late 20th century 137Cs peak. The study shows that the area around Lake Svartkälstjärn has been occupied by continuous forest since c. 8500 cal BP. Forests with abundant pine and birch initially colonized the area, followed by the domination of deciduous trees, primarily birch, around 7000-3000 cal BP. Noteworthy pollen influx values of oak, elm, and linden may suggest the local presence of these thermophilous tree species during mid-Holocene. The climate gradually became colder and moister around 4500 cal BP and increased frequencies and influx values of bog-moss spores indicate the presence of marginal mires. Spruce established around 3300 BP and within 500 years spruce was the dominating tree around the lake. The record of charcoal particles exhibits a general increase from c. 3000 cal BP with three periods of increased accumulation (around 2800 cal BP, 1700 cal BP and in recent time). The human influence on vegetation was significant during the past c. 500 years. Soil erosion increased distinctly and fern spores constitute 55 % of the total pollen assemblage near the core top. These results suggest an extensive anthropogenic impact on the local forest ecosystem, such as felling, burning, and ditching in the vicinity of the lake. Independent evidence of sub-recent human-induced environmental change is provided by historical accounts.Populärvetenskaplig sammanfattning: En sedimentsekvens från sjön Svartkälstjärn i Västerbotten provtogs för pollen- och sporanalys samt kolpartikelanalys. Sedimentens ålder bestämdes med 14C-dateringar och spår av radioisotopen 137Cs, som indikerar modern tid då de återspeglar provsprängningarna av kärnvapen under senare delen av 1900-talet. Dateringarna visar att sedimentationen i sjön började för ca 10 000 år sedan, d.v.s. vid gränsen mellan den senaste istiden (Weichsel) och vår mellanistid (Holocen). Syftet med studien var att genom att räkna samt identifiera pollen från olika nivåer på borrkärnan, försöka återspegla vegetationsutvecklingen under Holocen. Stora förändringar av intressen var t.ex. när granen invandrade, skogsbrandfrekvens och mänsklig aktivitet. För en återspegling av brandfrekvensen räknades kolpartiklar i sedimenten. Som hjälp i tolkningarna användes även annan tillgänglig data från borrkärnan såsom magnetisk susceptibilitet, organisk halt (TC) och kol/kväve-kvoten (C/N). Dessa data ger information om jorderosion vid sjön och hur hög den organiska produktionen var samt om det organiska materialet härrör från sjön eller land. Resultaten delades in i fem olika perioder (pollenzoner) efter skogssammansättning. Den första zonen (10 000-8600) uppvisar ett tundraliknande landskap med gräs, örter och småbuskar. Tall och björk är de första träden som etablerar sig kring sjön. Blandskog av tall och björk dominerar under den andra pollenzonen (8600-7000). Al invandrade och blev ett vanligt inslag, asp och ask förekom troligen. Den tredje zonen (7000-3200) inleds med en dominans av lövträd kring sjön. Klimatet var gynnsamt och den organiska produktionen når maximala värden. Hassel och alm invandrar, pollenförekomsten av ek och lind kan betyda att träden fanns i regionen. I mitten av zonen blir klimatet fuktigare och kallare, vitmossa blir vanligare och kring sjön finns troligen våtmarker. Lövträden minskar i omfattning och tall tillsammans med björk dominerar. Den fjärde zonen (3200-500) markeras av granens invandring på bekostnad av björken. Även tallen visar en minskande trend och inom 500 år dominerar troligen granen i skogen. Busk- och örtvegetationen ökar åter och indikerar kallare klimat med öppna fuktigare marker. Brandfrekvensen ökar under denna period men sätter inga synliga spår i pollenresultaten. Den femte sista pollenzonen (500-nutid) är en zon med stor mänsklig påverkan på landskapet. Kommersiell skogsavverkning tog fart och industriell tillverkning av bl.a. tjära och pottaska krävde att stort antal träd. Trädfrekvensen minskar i pollenregistret och erosionen ökar kraftigt i närområdet

Boreal forest dynamics in north-eastern Sweden during the last 10,000 years based on pollen analysis

A pollen record obtained from a 2.2-m sediment succession deposited in a small lake in the province of Vasterbotten, north-eastern Sweden, reveals the presence of continuous forest cover since 8,500 calendar years before present (cal B.P.). Forest with abundant Pinus (pine) and Betula (birch) initially colonized the area, followed by a dominance of deciduous trees, primarily Betula, from ca. 8,000 to ca. 3,200 cal B.P. Pollen accumulation rates of Quercus (oak), Ulmus (elm) and Tilia (linden) suggest the possible local presence of these thermophilous tree species during this period. The climate gradually became colder and moister around 3,500 cal B.P. and an increased abundance of Sphagnum spores indicates paludification. Picea (spruce) became established around 3,200 cal B.P. and less than 500 years later this was the dominant tree species around the lake. The fire frequency as inferred from charcoal particles exhibits a general increase from ca. 3,000 cal B.P. with subsequent charcoal accumulation maxima at around 2,800 cal B.P., 1,700 cal B.P. and in recent time. The human influence on vegetation was significant during the last 200-300 years. Soil erosion increased substantially and fern spores amount to ca. 55% of the total pollen assemblage in the uppermost samples. These results suggest an extensive anthropogenic impact on the local forest ecosystem, with abundant logging, burning and ditching in the vicinity of the lake. Independent evidence of sub-recent human-induced environmental change is provided by historical accounts. Complementary information on catchment soil development and aquatic nutrient status was provided by records of magnetic susceptibility and elemental carbon, and nitrogen contents obtained from the same sediment core

Lake ecosystem responses to catchment disturbance and airborne pollution: an 800-year perspective in southern Sweden

Sediment sequences spanning the last 800 years from two small lakes in the south Swedish uplands were explored for assessment of effects of changing human population, local land-use practices and airborne pollution on lake-ecosystem functioning and resilience. Variations in nutrient cycling and deposition of lithogenic elements were studied, using a multi-proxy stratigraphic approach. Carbon and nitrogen elemental and isotopic analyses were applied in combination with records of hydrocarbons (n-alkanes) to investigate the sources and depositional conditions of sediment organic matter. Changes in fluvial and airborne delivery of inorganic matter were based on X-ray fluorescence measurements. The results reveal that population growth and related increases in land-use pressure had a major impact on catchment erosion and input of terrestrial organic matter to the lakes from the 1500s to the end of the 1800s. Evidence also exists of a brief period of catchment disturbance at ca. 1200-1300, followed by recovery, likely connected to the Black Death pandemic. At ca. 1900 synchronous shifts in most of the proxy records suggest a marked change in external forcing common to the two lakes related to a major decrease in population density and the introduction of modern forestry following the industrial revolution. Interestingly, the two sediment records exhibit generally coherent trends in C/N ratio, organic C content and delta C-13, both before and after 1900, indicating broadly similar sensitivities of the lake ecosystems to human impact. In contrast, deviating trends in total N content, delta N-15 and lithogenic element concentrations (K, Ti, Rb and Zr) reflect site-specific responses to local disturbances during the last century due to different nutrient conditions and catchment properties. Our companion sediment records highlight the importance of understanding long-term human impact on watersheds and demonstrate how regional versus local forcing of lake ecosystems, as well as site-specific responses related to catchment characteristics can be reconstructed

Recent brownification of South Swedish lake waters – an effect of climate change or land use?

International audienceDissolved organic carbon (DOC) content, especially terrestrial humic matter (DHM), and water colour of many lakes and streams have increased substantially during the last decades (Evans et al. 2004, Roulet and Moore 2006). Brownification not only reduces the quality of these waters as drinking water supplies and for recreational purposes, but lake biodiversity is also expected to be effected. The aim of this project is to evaluate the potential underlying causes of the recent brownification of South Swedish lakes. Amultiproxy approach is applied (fig 3.) for reconstruct on of the biogeochemical and biological changes within selected lakes, and the vegetation/land use changes within their catchments for the past c. 1000 years. Here we present the study area, main methods, preliminary results and links to Fredh et al. projec