Nutrient limitation of periphyton growth in arctic lakes in south-west Greenland

Many arctic lakes are oligotrophic systems where phototrophic growth is controlled by nutrient supply. Recent anthropogenic nutrient loading is associated with biological and/or physico-chemical change in several lakes across the arctic. Shifts in nutrient limitation (nitrogen (N), phosphorus (P), or N ? P) and associated effects on the growth and composition of algal communities are commonly reported. The Kangerlussuaq region of south-west Greenland forms a major lake district which is considered to receive little direct anthropogenic disturbance. However, long-range transport of pollutant N is now reaching Greenland, and it was hypothesised that a precipitation gradient from the inland ice sheet margin to the coast might also deliver increased N deposition. In situ nutrient bioassays were deployed in three lakes across the region: ice sheet margin, inland (close to Kangerlussuaq) and the coast (near Sisimiut), to determine nutrient limitation of lakes and investigate any effects of nutrients on periphyton growth and community composition. Nutrient limitation differed amongst lakes: N limitation (ice sheet margin), N and P limitation (inland) and N ? P co-limitation (coast). Factors including variation in N supply, ice phenology, seasonal algal succession, community structure and physical limnology are explored as mechanisms to explain differences amongst lakes. Nutrient limitation of arctic lakes and associated ecological impacts are highly variable, even across small geographic areas. In this highly sensitive region, future environmental change scenarios carry a strong risk of significantly altering nutrient limitation; in turn, potentially severely impacting lake structure and function

Lake eutrophication and its implications for organic carbon sequestration in Europe

The eutrophication of lowland lakes in Europe by excess nitrogen (N) and phosphorus (P) is severe because of the long history of land-cover change and agricultural intensification. The ecological and socio-economic effects of eutrophication are well understood but its effect on organic carbon (OC) sequestration by lakes and its change overtime has not been determined. Here, we compile data from ~90 culturally impacted European lakes [~60% are eutrophic, Total P (TP) >30 μg P l-1] and determine the extent to which OC burial rates have increased over the past 100-150 years. The average focussing corrected, OC accumulation rate (C ARFC) for the period 1950-1990 was ~60 g C m-2 yr-1, and for lakes with >100 μg TP l-1 the average was ~100 g C m-2 yr-1. The ratio of post-1950 to 1900-1950 C AR is low (~1.5) indicating that C accumulation rates have been high throughout the 20th century. Compared to background estimates of OC burial (~5-10 g C m-2 yr-1), contemporary rates have increased by at least four to fivefold. The statistical relationship between C ARFC and TP derived from this study (r2 = 0.5) can be used to estimate OC burial at sites lacking estimates of sediment C-burial. The implications of eutrophication, diagenesis, lake morphometry and sediment focussing as controls of OC burial rates are considered. A conservative interpretation of the results of the this study suggests that lowland European meso- to eutrophic lakes with >30 μg TP l-1 had OC burial rates in excess of 50 g C m-2 yr-1 over the past century, indicating that previous estimates of regional lake OC burial have seriously underestimated their contribution to European carbon sequestration. Enhanced OC burial by lakes is one positive side-effect of the otherwise negative impact of the anthropogenic disruption of nutrient cycles

The influence of climate change on the restoration trajectory of a nutrient-rich deep lake

Nutrient reduction in impacted lowland freshwater systems is ecologically and culturally important. Gaining a greater insight into how lakes respond to lowering nutrient loads and how climate-driven physical limnology affects present and future cycling of available nutrients is important for ecosystem resource management. This study examines the nutrient decline in a hypereutrophic freshwater lake (Rostherne Mere, Cheshire, UK) 25 years after sewage effluent diversion, a uniquely long-term analysis of a recovering nutrient-rich deep lake. Using nutrient, phytoplankton, climate and catchment hydrological monitoring, the contemporary lake system is compared to previous studies from 1990 to 2002. Nutrient change since point source load diversion showed annual average and maximum phosphorus (P) concentrations decreased significantly for the first 10 years (1992: ~ 600 µg P L−1; 2002: ~ 200 µg P L−1), but have since stabilised due to a substantial legacy sediment P internal load. Dissolved inorganic nitrogen (DIN) concentrations have not substantially changed since diversion, resulting in the alteration of the DIN/SRP ratio from a system characterised by N limitation (N:P ~ 5), to one predominantly P limited (N:P > 20). Nutrient changes over this time are shown to drive ecological change, especially in the cyanobacterial and algal communities. Furthermore, very high-resolution monitoring of lake inflow and outflow (every 5 min during 2016) shows that water residence time at this lake is significantly shorter than previously estimated (~ 0.8 years compared to previous estimates of ~ 1.6–2.4 years). Together with long-term data demonstrating that the stratification period at Rostherne Mere has increased by 40 days over the last ~ 50 years (due to later autumnal mixing), we show that a rapid rate of epilimnetic flushing together with a long stratification period substantially reduces the available epilimnetic P during the summer cyanobacterial bloom. This is of growing importance for many such lakes, given widespread climate-driven lengthening of stratification and a national trend of decreasing summer rainfall (decreasing seasonal flushing) but more intense summer storm events (resulting in short-term flushing events).</div

The landscape–atmosphere continuum determines ecological change in alpine lakes of SE Tibet

© 2017 Springer Science+Business Media, LLC Remote alpine regions were considered to be largely unimpacted by anthropogenic disturbance, but it is now clear these areas are changing rapidly. It is often difficult to identify the causal processes underpinning ecological change because the main drivers (direct and indirect climate forcing, land use change and atmospheric deposition) are acting simultaneously. In addition, alpine landscapes are morphometrically complex with strong local environmental gradients creating natural heterogeneity which acts as a variable filter to climate and anthropogenic forcing, emphasizing the need for analyzing responses at multiple sites. The eastern margin of Tibet is a hotspot of global biodiversity and is affected by both atmospheric N and dust deposition, whereas regional climate warming is comparatively recent. Here we use 210 Pb and 137 Cs dated sediment records from nine alpine lakes, and statistical measures of diatom ecological change (turnover and PCA axis 1 scores) to determine regional scale patterns in community response to global environmental change forcing over approximately the last 150 years. The study lakes showed contrasting ecological responses with increased nutrient input as the primary driver of change, mediated by lake morphology and catchment characteristics. Turnover rates of diatom composition, although low, are significantly associated with lake volume, lake area, altitude and DOC

The historical dependency of organic carbon burial efficiency

Many studies have viewed lakes as quasi-static systems with regard to the rate of organic carbon (OC) burial, assuming that the dominant control on BE is sediment mineralization. However, in systems undergoing eutrophication or oligotrophication (i.e., altered nutrient loading), or climatic forcing, the changes in primary production will vary on both longer (> 10 yr) and shorter (seasonal) timescales, influencing the rate of OC accumulation and subsequent permanent burial. Here, we consider the extent to which permanent OC burial reflects changing production in a deep monomictic lake (Rostherne Mere, UK) that has been culturally eutrophied (present TP>200 μg L-1), but has undergone recent reductions in nutrient loading. We compare multi-year dynamics of OC fluxes using sediment traps to longer-term burial rates estimated from two 210Pb-dated sediment cores. The recent sediment record demonstrates that most of the autochthonous OC is preserved (∼95% of OC captured in the deep trap and 86% of the NEP in the contemporary system), contrary to widely held assumptions that this more labile, algal-dominated OC component is not well preserved in lake sediments. A revised method for calculating BE for lakes which have undergone changes in primary productivity in recent decades is developed, which reduces some of problems inherent in existing approaches using historical sediment records averaged over the last 25-150 yr. We suggest that an appreciation of lakes in all biomes as ecosystems responding dynamically to recent human impact and climate change (for example) can improve up-scaled regional and global estimates of lake OC burial

Expressions of climate perturbations in western Ugandan crater lake sediment records during the last 1000 yr

Equatorial East Africa has a complex, regional patchwork of climate regimes, with multiple interacting drivers. Recent studies have focussed on large lakes and reveal signals that are smoothed in both space and time, and, whilst useful at a continental scale, are of less relevance when understanding short-term, abrupt or immediate impacts of climate and environmental changes. Smaller-scale studies have highlighted spatial complexity and regional heterogeneity of tropical palaeoenvironments in terms of responses to climatic forcing (e.g. the Little Ice Age [LIA]) and questions remain over the spatial extent and synchroneity of climatic changes seen in East African records. Sediment cores from paired crater lakes in western Uganda were examined to assess ecosystem response to long-term climate and environmental change as well as testing responses to multiple drivers using redundancy analysis. These archives provide annual to sub-decadal records of environmental change. The records from the two lakes demonstrate an individualistic response to external (e.g. climatic) drivers, however, some of the broader patterns observed across East Africa suggest that the lakes are indeed sensitive to climatic perturbations such as a dry Mediaeval Climate Anomaly (MCA; 1000–1200 AD) and a relatively drier climate during the main phase of the LIA (1500–1800 AD); though lake levels in western Uganda do fluctuate. The relationship of Ugandan lakes to regional climate drivers breaks down c. 1800 AD, when major changes in the ecosystems appear to be a response to sediment and nutrient influxes as a result of increasing cultural impacts within the lake catchments. The data highlight the complexity of individual lake response to climate forcing, indicating shifting drivers through time. This research also highlights the importance of using multi-lake studies within a landscape to allow for rigorous testing of climate reconstructions, forcing and ecosystem response

Large increases in carbon burial in northern lakes during the Anthropocene

Northern forests are important ecosystems for carbon (C) cycling and lakes within them process and bury large amounts of organic-C. Current burial estimates are poorly constrained and may discount other shifts in organic-C burial driven by global change. Here we analyse a suite of northern lakes to determine trends in organic-C burial throughout the Anthropocene. We found burial rates increased significantly over the last century and are up to five times greater than previous estimates. Despite a correlation with temperature, warming alone did not explain the increase in burial, suggesting the importance of other drivers including atmospherically deposited reactive nitrogen. Upscaling mean lake burial rates for each time period to global northern forests yields up to 4.5 Pg C accumulated in the last 100 years—20% of the total burial over the Holocene. Our results indicate that lakes will become increasingly important for C burial under future global change scenarios

A landscape perspective of Holocene organic carbon cycling in coastal SW Greenland lake-catchments

Arctic organic carbon (OC) stores are substantial and have accumulated over millennia as a function of changes in climate and terrestrial vegetation. Arctic lakes are also important components of the regional C-cycle as they are sites of OC production and CO2emissions but also store large amounts of OC in their sediments. This sediment OC pool is a mixture derived from terrestrial and aquatic sources, and sediment cores can therefore provide a long-term record of the changing interactions between lakes and their catchments in terms of nutrient and C transfer. Sediment carbon isotope composition (δ13C), C/N ratio and organic C accumulation rates (C AR) of14C-dated cores covering the last ∼10,000 years from six lakes close to Sisimiut (SW Greenland) are used to determine the extent to which OC dynamics reflect climate relative to lake or catchment characteristics. Sediment δ13C ranges from −19 to −32‰ across all lakes, while C/N ratios are 20 (mean = 12), values that indicate a high proportion of the organic matter is from autochthonous production but with a variable terrestrial component. Temporal trends in δ13C are variable among lakes, with neighbouring lakes showing contrasting profiles, indicative of site-specific OC processing. The response of an individual lake reflects its morphometry (which influences benthic primary production), the catchment:lake ratio, and catchment relief, lakes with steeper catchments sequester more carbon. The multi-site, landscape approach used here highlights the complex response of individual lakes to climate and catchment disturbance, but broad generalisations are possible. Regional Neoglacial cooling (from ∼5000 cal yr BP) influenced the lateral transfer of terrestrial OC to lakes, with three lakes showing clear increases in OC accumulation rate. The lakes likely switched from being autotrophic (i.e. net ecosystem production > ecosystem respiration) in the early Holocene to being heterotrophic after 5000 cal yr BP as terrestrial OC transfer increased

Impacts of forestry planting on primary production in upland lakes from north-west Ireland

Planted forests are increasing in many upland regions worldwide, but knowledge about their potential effects on algal communities of catchment lakes is relatively unknown. Here, the effects of afforestation were investigated using palaeolimnology at six upland lake sites in the north-west of Ireland subject to different extents of forest plantation cover (4–64% of catchment area). 210Pb-dated sediment cores were analysed for carotenoid pigments from algae, stable isotopes of bulk carbon (δ13C) and nitrogen (δ15N), and C/N ratios. In lakes with >50% of their catchment area covered by plantations, there were two- to sixfold increases in pigments from cryptophytes (alloxanthin) and significant but lower increases (39–116%) in those from colonial cyanobacteria (canthaxanthin), but no response from biomarkers of total algal abundance (β-carotene). In contrast, lakes in catchments with <20% afforestation exhibited no consistent response to forestry practices, although all lakes exhibited fluctuations in pigments and geochemical variables due to peat cutting and upland grazing prior to forest plantation. Taken together, patterns suggest that increases in cyanobacteria and cryptophyte abundance reflect a combination of mineral and nutrient enrichment associated with forest fertilization and organic matter influx which may have facilitated growth of mixotrophic taxa. This study demonstrates that planted forests can alter the abundance and community structure of algae in upland humic lakes of Ireland and Northern Ireland, despite long histories of prior catchment disturbance

Shifts in the source and composition of dissolved organic matter in Southwest Greenland lakes along a regional hydro-climatic gradient

Dissolved organic matter (DOM) concentration and quality were examined from Arctic lakes located in three clusters across south-west (SW) Greenland, covering the regional climatic gradient: cool, wet coastal zone; dry inland interior; and cool, dry ice-marginal areas. We hypothesized that differences in mean annual precipitation between sites would result in a reduced hydrological connectivity between lakes and their catchments and that this concentrates degraded DOM. The DOM in the inland lake group was characterized by a lower aromaticity and molecular weight, a low soil-like fluorescence, and carbon stable isotope (δ 13 C-DOC) values enriched by ~2‰ relative to the coastal group. DOC-specific absorbance (SUVA 254 ) and DOC-specific soil-like fluorescence (SUVF C1 ) revealed seasonal and climatic gradients across which DOM exhibited a dynamic we term “pulse-process”: Pulses of DOM exported from soils to lakes during snow and ice melt were followed by pulses of autochthonous DOM inputs (possibly from macrophytes), and their subsequent photochemical and microbial processing. These effects regulated the dynamics of DOM in the inland lakes and suggested that if circumpolar lakes currently situated in cool wetter climatic regimes with strong hydrological connectivity have reduced connectivity under a drier future climate, they may evolve toward an end-point of large stocks of highly degraded DOC, equivalent to the inland lakes in the present study. The regional climatic gradient across SW Greenland and its influence on DOM properties in these lakes provide a model of possible future changes to lake C cycling in high-latitude systems where climatic changes are most pronounced