Carbon fixation, flux and burial efficiency in two contrasting eutrophic lakes in the UK (Rostherne Mere & Tatton Mere)

Much of the current research into the processing and storage of carbon (C) in small lakes has focused on arctic and boreal lake systems, due to their global abundance. However this has led to an imbalance in the interpretation of lake functioning. Oligotrophic lakes are prevalent in the arctic and boreal zone, but are typically net heterotrophic due to loading of catchment-derived dissolved organic carbon (DOC) which alters their metabolic balance. In comparison, temperate lake systems tend to be more nutrient rich, typically due to anthropogenic activity, and would therefore be expected to exhibit the signs of net autotrophy, as a result of higher rates of gross primary production (GPP) and lower rates of catchment-derived DOC potentially subsidising respiration (R). In order to test the hypothesis that temperate, eutrophic lakes are net autotrophic (GPP > R) on an annual basis the C-dynamics of Rostherne Mere (maximum depth, zm, 31 m) and Tatton Mere (zm = 11 m), two monomictic Cheshire-Shropshire meres, were quantified over an 18 months period from 2010 2012. This monitoring study used high-resolution (hourly) oxygen (O2) sonde measurements, combined with high-resolution data from an automated on-lake monitoring buoy at Rostherne Mere (as part of the national UKLEON lake network) to calculate rates of epilimnion C-fixation. For both lakes, sediment traps were also used to determine water column C-flux and sediment core data to establish C-burial efficiency of these strongly stratifying lakes. Water column profiles of dissolved O2 and CO2 was also measured at 2 4 weekly intervals across both lakes. Particular attention was focused on: i) the long term C-storage of eutrophic, monomictic lakes; ii) up-scaling C-accumulation estimates from these two meres to the Cheshire-Shropshire meres region and all UK eutrophic waters; and iii) methodological sensitivity for estimating C-fixation, flux and burial efficiency and upscaling C-accumulation estimates. The results show that both lakes are net autotrophic on an annual basis, on average fixing 121 ± 2 g C m-2 yr-1 and sequestering 68 ± 4 g C m-2 yr-1, a C-burial efficiency of ~60%. If up-scaled to the Cheshire-Shropshire meres region, annual C-accumulation was estimated to be 506 ± 32 t C yr-1 or 0.05 ± 0.001 Mt C since 1900. From this, it was estimated that UK eutrophic waters could be sequestering 0.12 ± 0.01 Mt C yr-1 or 13.3 ± 0.2 Mt C since 1900. Annual UK CO2 emissions are ~128.85 Mt C yr-1, therefore UK eutrophic waters currently offset 0.09% of yearly UK CO2 emissions. Despite the finding that eutrophic, stratifying lakes have high C-fixation and sequestration values, lakes in other areas of the globe such as the arctic and boreal zones are typically a more important long term C-sink as they are far more abundant within the landscape and local soils are typically very poor within low C retention rates. Further investigation is needed into how lakes function on a regional and national scale, the importance of lake type and number when up-scaling C accumulation estimates and the potential impact on future C accumulation as a result of a changing environment and supra-regional policies in areas such as Europe

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

Diel surface temperature range scales with lake size

Ecological and biogeochemical processes in lakes are strongly dependent upon water temperature. Long-term surface warming of many lakes is unequivocal, but little is known about the comparative magnitude of temperature variation at Diel timescales, due to a lack of appropriately resolved data. Here we quantify the pattern and magnitude of Diel temperature variability of surface waters using high-frequency data from 100 lakes. We show that the near-surface Diel temperature range can be substantial in summer relative to long-term change and, for lakes smaller than 3 km2, increases sharply and predictably with decreasing lake area. Most small lakes included in this study experience average summer Diel ranges in their near-surface temperatures of between 4 and 7°C. Large Diel temperature fluctuations in the majority of lakes undoubtedly influence their structure, function and role in biogeochemical cycles, but the full implications remain largely unexplored

Reconstitution of Functionalized Transmembrane Domains of Receptor Proteins into Biomimetic Membranes

For integral membrane proteins, an assessment of their structures and interactions within a biomimetic lipid bilayer environment is critical for evaluating their cellular function. Hydrophobic sequences prevalent within transmembrane domains, however, make these proteins susceptible to aggregation and, thus, create difficulties in examining their structural and functional properties via canonical techniques. Working exclusively with single-pass transmembrane (TM) segments of bitopic membrane proteins, in the form of soluble peptides, bypasses many of the pitfalls of full-length protein preparations while allowing for the opportunity to examine the properties of TM domains within biomimetic membrane environments. In this study, peptides mimicking the TM domains of the epidermal growth factor receptor (EGFR) and CD4 co-receptor, both cell-signaling surface receptors, have been reconstituted into 1-palmitoyl-2-oleoyl-<i>sn</i>-glycero-3-phosphocholine (POPC) lipid bilayers. The formation of their native α-helical structures within vesicle membranes was observed from circular dichroism, and full partition of the peptides into the membrane was demonstrated by tryptophan fluorescence and neutron reflectivity (NR). Using an engineered planar lipid bilayer system ideal for surface characterization methods, such as surface plasmon resonance (SPR) and NR, the TM peptides, functionalized with a N-terminal biotin tag, proved capable of “activating” a membrane surface, as evidenced by the capture of streptavidin. On the basis of these initial assessments, we anticipate these membrane-bound peptides will provide a versatile platform for understanding the intricate roles of receptor TM domains in cell signaling

Relationship between the diel range in lake surface water temperature and surface area.

<p>Relationship between the observed (light violet circles) and theoretical (red circles) diel surface temperature range with lake area during summer, with the solid line illustrating the fitted generalised additive model with 95% confidence interval shown by the shaded region; lake surface areas where the diel temperature range changes significantly (P < 0.001) are shown with a red line.</p

Temporal variability in near-surface lake water temperature.

<p>(a) Seasonal variability in the diel temperature range for 96 Northern Hemisphere lakes with 95% confidence intervals (note that not all lakes had data for the whole year). (b) Individually normalized (zero-mean) summer average diel cycle for the lakes that had the highest (red) and lowest (blue) 10% of diel temperature ranges measured. The bold lines represent the mean diel cycle for the 10% considered and the horizontal black line indicates zero. For clarity, we excluded Jekl Bog, which had the highest diel cycle, from this figure. (c) Example of hourly-resolution near-surface lake water temperature variation at Jekl Bog (surface area 2.5 x 10³ m², red), and Sparkling Lake (surface area 6.2 x 10⁵ m², blue), both situated in Wisconsin, USA.</p

First derivatives of the fitted generalised additive model.

<p>The red line indicates those parts of the model fit that are statistically significantly changing and the shaded region shows the 95% confidence intervals.</p

Fitted splines for the Generalised Additive Model.

<p>The y-axis is the additive contribution of the spline to the fitted model over the range of the covariate. The smooth functions are subject to centring constraints and are plotted here on different scales for clarity. The shaded region is an approximate 95% confidence interval on the function; however, it excludes uncertainty in the model's constant term, β₀, hence the narrowness of the interval at the “middle” of the distribution for the smooths of altitude and latitude.</p

Summary output from the fitted statistical model.

<p>Summary of the model used to describe the influence of surface area (A₀), the percent transmission per metre (I_z), altitude above sea level (h), and latitude (φ), as shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0152466#pone.0152466.e003" target="_blank">Eq 3</a>, on the diel surface temperature range. EDF is the effective degrees of freedom for the spline representing each covariate. Ref. DF is the reference degrees of freedom used in the statistical test of “no effect” for each smooth. F is the test statistic and <i>p</i> the approximate <i>p</i>-value of the test. <i>I</i>_<i>z</i> is the percent transmission per meter.</p

Estimated ecological and biogeochemical consequences of the diel surface temperature range.

<p>Potential bias in estimates of CO₂ and O₂ solubility and rates of processes with Q₁₀ values of 2 or 4 for a diel temperature range of 1 (blue) or 7°C (red).</p