Adaptation by macrophytes to inorganic carbon down a river with naturally variable concentrations of CO2

The productivity and ecological distribution of freshwater plants can be controlled by the availability of inorganic carbon in water despite the existence of different mechanisms to ameliorate this, such as the ability to use bicarbonate. Here we took advantage of a short, natural gradient of CO2 concentration, against a background of very high and relatively constant concentration of bicarbonate, in a spring-fed river, to study the effect of variable concentration of CO2 on the ability of freshwater plants to use bicarbonate. Plants close to the source, where the concentration of CO2 was up to 24-times air equilibrium, were dominated by Berula erecta. pH-drift results and discrimination against 13C were consistent with this and the other species being restricted to CO2 and unable to use the high concentration of bicarbonate. There was some indication from stable 13C data that B. erecta may have had access to atmospheric CO2 at low water levels. In contrast, species downstream, where concentrations of CO2 were only about 5-times air-equilibrium were almost exclusively able to use bicarbonate, based on pH-drift results. Discrimination against 13C was also consistent with bicarbonate being the main source of inorganic carbon for photosynthesis in these species. There was, therefore, a transect downstream from the source of increasing ability to use bicarbonate that closely matched the decreasing concentration of CO2. This was produced largely by altered species composition, but partly by phenotypic changes in individual species

Dissipation and mixing during the onset of stratification in a temperate lake, Windermere

Acoustic Doppler Current Profilers and chains of temperature sensors were used to observe the spring transition to stable stratification over a 55 day period in a temperate lake. Observations of the flow structure were complemented by measurements of dissipation, based on the Structure Function method, near the lake bed and in the upper part of the water column. During complete vertical mixing, wind-driven motions had horizontally isotropic velocities with roughly equal barotropic and baroclinic kinetic energy. Dissipation was closely correlated with the wind-speed cubed, indicating law of the wall scaling, and had peak values of ~1 x 10-5.5 W kg-1 at 10 m depth during maximum wind forcing (W~ 15 m s-1). As stratification developed, the flow evolved into a predominantly baroclinic regime dominated by the first mode internal seiche, with root mean square (rms) axial flow speeds of ~2-3 cm-1; ~ 2.5-times the transverse component. At 2.8 m above the bed, most of the dissipation occurred in a number of strong maxima coinciding with peaks of near-bed flow. In the pycnocline, dissipation was low most of the time, but with pronounced maxima (reaching ~1 x 10-5 W kg-1) closely related to the local velocity shear. The downward diffusive heat flux across the pycnocline over 27.5 days accounted for ~ 70% of the temperature rise in the water column below. Total lake kinetic energy increased by a factor of 3 between mixed and stratified regimes, in spite of reduced wind forcing, indicating less efficient damping in stable conditions

Dissolved organic nutrient uptake by riverine phytoplankton varies along a gradient of nutrient enrichment

The concentration of dissolved organic matter (DOM) in freshwaters is increasing in large areas of the world. In addition to carbon, DOM contains nitrogen and phosphorus and there is growing concern that these organic nutrients may be bioavailable and contribute to eutrophication. However, relatively few studies have assessed the potential for dissolved organic nitrogen (DON) or dissolved organic phosphorus (DOP) compounds to be bioavailable to natural river phytoplankton communities at different locations or times. Temporal and spatial variations in uptake, relative to environmental characteristics were examined at six riverine sites in two contrasting catchments in the UK. This study also examined how the uptake by riverine phytoplankton of four DON and four DOP compounds commonly found in rivers, varied with concentration. Total nitrogen (TN) and phosphorus (TP) concentrations, the proportion of inorganic nutrient species, and nutrient limitation varied temporally and spatially, as did the potential for DON and DOP uptake. All eight of the DOM compounds tested were bioavailable, but to different extents. Organic nutrient use depended on the concentration of the organic compound supplied, with simple compounds (urea and glucose-6-phosphate) supporting algal growth even at very low concentrations. DON use was negatively correlated with the TN and ammonia concentration and DOP use was negatively correlated with soluble reactive phosphorus (SRP) and dissolved organic carbon (DOC) concentration. The evidence indicates that DOM in rivers has been overlooked as a potential source of nutrients to phytoplankton and therefore as an agent of eutrophication

Phytoplankton community responses in a shallow lake following lanthanum-bentonite application

The release of phosphorus (P) from bed sediments to the overlying water can delay the recovery of lakes for decades following reductions in catchment contributions, preventing water quality targets being met within timeframes set out by environmental legislation (e.g. EU Water Framework Directive: WFD). Therefore supplementary solutions for restoring lakes have been explored, including the capping of sediment P sources using a lanthanum (La)-modified bentonite clay to reduce internal P loading and enhance the recovery process. Here we present results from Loch Flemington where the first long-term field trial documenting responses of phytoplankton community structure and abundance, and the UK WFD phytoplankton metric to a La-bentonite application was performed. A Before-After-Control-Impact (BACI) analysis was used to distinguish natural variability from treatment effect and confirmed significant reductions in the magnitude of summer cyanobacterial blooms in Loch Flemington, relative to the control site, following La-bentonite application. However this initial cyanobacterial response was not sustained beyond two years after application, which implied that the reduction in internal P loading was short-lived; several possible explanations for this are discussed. One reason is that this ecological quality indicator is sensitive to inter-annual variability in weather patterns, particularly summer rainfall and water temperature. Over the monitoring period, the phytoplankton community structure of Loch Flemington became less dominated by cyanobacteria and more functionally diverse. This resulted in continual improvements in the phytoplankton compositional and abundance metrics, which were not observed at the control site, and may suggest an ecological response to the sustained reduction in filterable reactive phosphorus (FRP) concentration following La-bentonite application. Overall, phytoplankton classification indicated that the lake moved from poor to moderate ecological status but did not reach the proxy water quality target (i.e. WFD Good Ecological Status) within four years of the application. As for many other shallow lakes, the effective control of internal P loading in Loch Flemington will require further implementation of both in-lake and catchment-based measures. Our work emphasizes the need for appropriate experimental design and long-term monitoring programmes, to ascertain the efficacy of intervention measures in delivering environmental improvements at the field scale

Global data set of long-term summertime vertical temperature profiles in 153 lakes

peer reviewedClimate change and other anthropogenic stressors have led to long-term changes in the thermal structure, including surface temperatures, deepwater temperatures, and vertical thermal gradients, in many lakes around the world. Though many studies highlight warming of surface water temperatures in lakes worldwide, less is known about long-term trends in full vertical thermal structure and deepwater temperatures, which have been changing less consistently in both direction and magnitude. Here, we present a globally-expansive data set of summertime in-situ vertical temperature profiles from 153 lakes, with one time series beginning as early as 1894. We also compiled lake geographic, morphometric, and water quality variables that can influence vertical thermal structure through a variety of potential mechanisms in these lakes. These long-term time series of vertical temperature profiles and corresponding lake characteristics serve as valuable data to help understand changes and drivers of lake thermal structure in a time of rapid global and ecological change. © 2021, The Author(s)

Options for the remediation of Windermere: PROTECH modelling of the effects of different management scenarios

1. The purpose of this report was to establish the effect of reduced nutrient loading from the two wastewater treatment works (WwTW) at Ambleside and Tower Wood on the amount and types of phytoplankton in the two basins of Windermere. A secondary objective was to assess the likely impact of reduced grazing pressure by zooplankton on phytoplankton amount. 2. The nutrient loads from the catchment and the WwTWs were estimated in previous reports for the whole lake (Maberly 2008, 2009) but basin-specific loads were calculated here. The modelling work was carried out using the algal lake model PROTECH based on the year 1998. The model produced a good representation of the seasonal changes in phytoplankton chlorophyll a and also successfully simulated the types of algae present. 3. The mean contribution of direct discharge of SRP from WwTW between 1997 and 2007 was 30% of the total load in the North Basin but 52% of the total load in the South Basin of Windermere. In 1998, the year used for the modelling exercise, this difference was even greater at 15% and 62% for the North and South Basin respectively. 4. The differential contribution of the WwTWs to their respective basin translated across to the responsiveness of each basin to reductions in SRP loading from the WwTW. In the North Basin, even complete removal of the SRP load from the WwTW at Ambleside only caused an 11% decrease in annual mean phytoplankton chlorophyll a. In contrast, in the South Basin complete removal of the SRP load from the Windermere WwTW would cause a 54% decrease in annual mean phytoplankton chlorophyll a. However, these differences are consistent with the observed minimal reduction in winter concentrations of SRP and TP in the North Basin following tertiary treatment in 1992, while substantial reductions have been recorded in the South Basin. 5. Further removal of SRP from the Ambleside WwTW, while beneficial, will not be sufficient to cause a marked further improvement in water quality in the North Basin. More effort will be needed to tackle other sources of phosphorus including smaller point sources and diffuse sources from the catchment. In contrast, severely reducing the SRP load from WwTW discharging to the South Basin should have a further benefit in reducing phytoplankton. 6. The limited modelling of the effect of zooplankton grazing on phytoplankton did not show a large effect but a more sophisticated zooplankton grazing module is needed (and is currently being developed) before we can be confident about the magnitude of this effect. Further work addressing the effect of climate change will also need to be included in future models and the forecasts could be made more robust by modelling additional years

Insights from long-term studies in the Windermere catchment: external stressors, internal interactions and the structure and function of lake ecosystems

1. Lakes provide essential ecosystem goods and services on which humans depend, and are integral to many global biogeochemical cycles, yet are sensitive to environmental perturbation operating at global, regional and local scales, many resulting from human influence. 2. The complex web of external stressors and internal interactions that control the biological structure and ecological function of lakes requires a ‘systems approach’, where different trophic levels are studied and different approaches, including long-term monitoring are taken. 3. This Special Issue celebrates the ecological insights that have been gained from long-term monitoring and research on the lakes in the catchment of Windermere in the English Lake District. The research was started by the Freshwater Biological Association in the 1930s and has been continued by the Centre for Ecology & Hydrology since 1989 to produce a valuable record of lake response to changing conditions extending over 65 years. 4. We introduce a conceptual diagram of lake response and feedbacks to a hierarchy of conditions at different scales and interactions within the lake, and illustrate it using the contributions of papers in the literature and this Special Issue. 5. Long-term monitoring is often threatened at times of economic austerity, yet its continuation as part of a programme of research is a key activity that can produce mechanistic understanding and insights, and data to produce and validate models, and hence forecast future responses. These scientific disciplines interact synergistically to help us understand the whole system and safeguard the ecosystem services provided by lakes