Heterogeneity in Esthwaite Water, a Small, Temperate Lake : Consequences for Phosphorus Budgets.

Eutrophication through phosphorus enrichment of lakes is potentially damaging to lake ecosystems, water quality and the ecosystem services which they provide. Traditional approaches to managing eutrophication involve quantifying phosphorus budgets. An important shortcoming of these approaches is that they take little account of the inherent heterogeneity of lakes. Furthermore, most studies of lake heterogeneity have been carried out in large lakes, a situation which reflects neither small lakes' importance in biogeochemical cycling nor their significant contribution to the global sum of lake environments. This thesis reports investigations into heterogeneity in Esthwaite Water, UK, a small, temperate, eutrophic lake. The overarching aim of the work was to improve understanding of phosphorus fluxes and budgets in this type of environment. Heterogeneity, and its governing physical mechanisms were elucidated in the lake's surface waters and bed sediments. In addition, the effects that this heterogeneity had on phosphorus supply to phytoplankton from the main stream inflow and internal sediment sources were examined. The research is presented as four studies, which address surface water heterogeneity, bed sediment heterogeneity, and phosphorus supply via inflowing streams and from internal sediment storage. Significant heterogeneity was found in the surface water, despite the lake being small and the physical forcing, relatively weak. Assumptions about the physical processes contributing to sediment heterogeneity based on models of large or shallow lakes were found not to be applicable. Taking account of bed sediment heterogeneity was found to be important for the accurate calculation of burial rates of both total phosphorus and organic carbon, as ignoring it led to discrepancies up to 110%. Soluble reactive phosphorus (SRP) dispersal in the shallow transition zone at the mouth of the main inflowing stream, and was found to be the result of not only hydrological dilution but also biological uptake. Ignoring SRP heterogeneity in this zone led to up to an 18% underestimate of lake-wide averaged concentration during the growing season. SRP pathways in the lake were both spatially and temporally heterogeneous, resulting in large seasonal and inter-annual variations in phosphorus supply. Internal and external supplies were of similar magnitude during the summer but internal anoxic sources dominated in the late summer and autumn. Inter-annual variation in the hypolimnetic build-up of phosphorus associated with differences in lake stability and mixing strongly affected the internal phosphorus supply. Overall, the thesis concludes that spatial and temporal heterogeneity is a characteristic of this lake at many scales, despite the relative weakness of the governing physical forcing, and that it affects significantly not only the nature of the lake at specific locations, but also lake-wide averaged parameter values. Specifically, different phosphorus sources have distinctly different patterns of variability, which need to be taken into account when calculating phosphorus budgets. Finally, the importance of particular physical processes for phosphorus budgets is likely to differ between large and small lakes owing to the influence of basin morphometry and therefore understanding derived about these budgets in large lakes cannot simply be assumed when considering small lakes

Evaluating the use of lake sedimentary DNA in palaeolimnology:A comparison with long‐term microscopy‐based monitoring of the phytoplankton community

Palaeolimnological records provide valuable information about how phytoplankton respond to long-term drivers of environmental change. Traditional palaeolimnological tools such as microfossils and pigments are restricted to taxa that leave sub-fossil remains, and a method that can be applied to the wider community is required. Sedimentary DNA (sedDNA), extracted from lake sediment cores, shows promise in palaeolimnology, but validation against data from long-term monitoring of lake water is necessary to enable its development as a reliable record of past phytoplankton communities. To address this need, 18S rRNA gene amplicon sequencing was carried out on lake sediments from a core collected from Esthwaite Water (English Lake District) spanning ~105 years. This sedDNA record was compared with concurrent long-term microscopy-based monitoring of phytoplankton in the surface water. Broadly comparable trends were observed between the datasets, with respect to the diversity and relative abundance and occurrence of chlorophytes, dinoflagellates, ochrophytes and bacillariophytes. Up to 20% of genera were successfully captured using both methods, and sedDNA revealed a previously undetected community of phytoplankton. These results suggest that sedDNA can be used as an effective record of past phytoplankton communities, at least over timescales of <100 years. However, a substantial proportion of genera identified by microscopy were not detected using sedDNA, highlighting the current limitations of the technique that require further development such as reference database coverage. The taphonomic processes which may affect its reliability, such as the extent and rate of deposition and DNA degradation, also require further research

Modelling lake cyanobacterial blooms:disentangling the climate-driven impacts of changing mixed depth and water temperature

1. Climate change is already having profound impacts upon the state and dynamics of lake ecosystems globally. A specific concern is that climate change will continue to promote the growth of phytoplankton, particularly blooms of toxic cyanobacteria, via lake physical processes including warming surface waters and shallowing of the mixed layer. These two mechanisms will have different impacts on lake phytoplankton communities, but their inter-connectedness has made it difficult to disentangle their independent effects. 2. We fill this knowledge gap by performing 1666 numerical modelling experiments with the phytoplankton community model, PROTECH, in which we separated the independent effects on lake phytoplankton of temperature change and changes in the depth of the surface mixed layer. Given the large global abundance of small lakes (< 1 km2) and the importance of their ecosystems in global processes and budgets, we used a small meso-eutrophic lake as an example study site for the modelling experiments. 3. Increasing the lake temperature and positioning the mixed layer at a shallower depth had different ecological impacts, with warming typically resulting in more biomass and a dominance of cyanobacteria. 4. The response to mixed depth shallowing depended on the original depth where mixing occurred. As anticipated, where the original mixed depth was moderate (4–6 m) and there was a simultaneous increase in water temperature, cyanobacterial biomass increased. However, when the same absolute difference in shallowing and temperature increase were applied to a deeper mixed depth (9–13 m), lower cyanobacterial biomass resulted, owing to poorer conditions for low-light tolerant cyanobacteria. 5. Our study shows that the response of cyanobacterial blooms to climate-induced warming and shallowing of mixed layers in lakes around the world will not be universal, but rather will be system-specific, depending upon the average mixed layer depth of the lake in question and the light affinity of the dominant cyanobacteria species

Northern Hemisphere atmospheric stilling accelerates lake thermal responses to a warming world

Climate change, in particular the increase in air temperature, has been shown to influence lake thermal dynamics, with climatic warming resulting in higher surface temperatures, stronger stratification, and altered mixing regimes. Less-studied is the influence on lake thermal dynamics of atmospheric stilling, the decrease in near-surface wind speed observed in recent decades. Here we use a lake model to assess the influence of atmospheric stilling, on lake thermal dynamics across the Northern Hemisphere. From 1980-2016, lake thermal responses to warming have accelerated as a result of atmospheric stilling. Lake surface temperatures and thermal stability have changed at respective rates of 0.33 and 0.38°C decade-1, with atmospheric stilling contributing 15 and 27% of the calculated changes, respectively. Atmospheric stilling also resulted in a lengthening of stratification, contributing 23% of the calculated changes. Our results demonstrate that atmospheric stilling has influenced lake thermal responses to warming

Wide-spread inconsistency in estimation of lake mixed depth impacts interpretation of limnological processes

The mixed layer, or epilimnion, is a physical concept referring to an isothermal layer at the surface of a water body. This concept is ubiquitous within limnology, is fundamental to our understanding of chemical and ecological processes, and is an important metric for water body monitoring, assessment and management. Despite its importance as a metric, many different approaches to approximating mixed depth currently exist. Using data from field campaigns in a small meso-eutrophic lake in the UK in 2016 and 2017 we tested whether different definitions of mixed depth resulted in comparable estimates and whether variables other than temperature could be assumed to be mixed within the layer. Different methods resulted in very different estimates for the mixed depth and ecologically important variables were not necessarily homogenously spread through the epilimnion. Furthermore, calculation of simple ecologically relevant metrics based on mixed depth showed that these metrics were highly dependent on the definition of mixed depth used. The results demonstrate that an idealised concept of a well-defined fully mixed layer is not necessarily appropriate. The widespread use of multiple definitions for mixed depth impairs the comparability of different studies while associated uncertainty over the most appropriate definition limits the confirmability of studies utilising the mixed depths

Adaptive forecasting of phytoplankton communities

The global proliferation of harmful algal blooms poses an increasing threat to water resources, recreation and ecosystems. Predicting the occurrence of these blooms is therefore needed to assist water managers in making management decisions to mitigate their impact. Evaluation of the potential for forecasting of algal blooms using the phytoplankton community model PROTECH was undertaken in pseudo-real-time. This was achieved within a data assimilation scheme using the Ensemble Kalman Filter to allow uncertainties and model nonlinearities to be propagated to forecast outputs. Tests were made on two mesotrophic lakes in the English Lake District, which differ in depth and nutrient regime. Some forecasting success was shown for chlorophyll a, but not all forecasts were able to perform better than a persistence forecast. There was a general reduction in forecast skill with increasing forecasting period but forecasts for up to four or five days showed noticeably greater promise than those for longer periods. Associated forecasts of phytoplankton community structure were broadly consistent with observations but their translation to cyanobacteria forecasts was challenging owing to the interchangeability of simulated functional species

Modelling lake cyanobacterial blooms: disentangling the climate‐driven impacts of changing mixed depth and water temperature

1. Climate change is already having profound impacts upon the state and dynamics of lake ecosystems globally. A specific concern is that climate change will continue to promote the growth of phytoplankton, particularly blooms of toxic cyanobacteria, via lake physical processes including warming surface waters and shallowing of the mixed layer. These two mechanisms will have different impacts on lake phytoplankton communities, but their inter‐connectedness has made it difficult to disentangle their independent effects. 2. We fill this knowledge gap by performing 1666 numerical modelling experiments with the phytoplankton community model, PROTECH, in which we separated the independent effects on lake phytoplankton of temperature change and changes in the depth of the surface mixed layer. Given the large global abundance of small lakes (<1 km2) and the importance of their ecosystems in global processes and budgets, we used a small meso‐eutrophic lake as an example study site for the modelling experiments. 3. Increasing the lake temperature and positioning the mixed layer at a shallower depth had different ecological impacts, with warming typically resulting in more biomass and a dominance of cyanobacteria. 4. The response to mixed depth shallowing depended on the original depth where mixing occurred. As anticipated, where the original mixed depth was moderate (4–6 m) and there was a simultaneous increase in water temperature, cyanobacterial biomass increased. However, when the same absolute difference in shallowing and temperature increase were applied to a deeper mixed depth (9–13 m), lower cyanobacterial biomass resulted, owing to poorer conditions for low‐light tolerant cyanobacteria. 5. Our study shows that the response of cyanobacterial blooms to climate‐induced warming and shallowing of mixed layers in lakes around the world will not be universal, but rather will be system‐specific, depending upon the average mixed layer depth of the lake in question and the light affinity of the dominant cyanobacteria species

Annual water residence time effects on thermal structure: a potential lake restoration measure?

Innovative methods to combat internal loading issues in eutrophic lakes are urgently needed to speed recovery and restore systems within legislative deadlines. In stratifying lakes, internal phosphorus loading is particularly problematic during the summer stratified period when anoxia persists in the hypolimnion, promoting phosphorus release from the sediment. A novel method to inhibit stratification by reducing residence times is proposed as a way of controlling the length of the hypolimnetic anoxic period, thus reducing the loading of nutrients from the sediments into the water column. However, residence time effects on stratification length in natural lakes are not well understood. We used a systematic modelling approach to investigate the viability of changes to annual water residence time in affecting lake stratification and thermal dynamics in Elterwater, a small stratifying eutrophic lake in the northwest of England. We found that reducing annual water residence times shortened and weakened summer stratification. Based on finer-scale dynamics of lake heat fluxes and water column stability we propose seasonal or sub-seasonal management of water residence time is needed for the method to be most effective at reducing stratification as a means of controlling internal nutrient loading

Evaluating the use of lake sedimentary DNA in palaeolimnology: a comparison with long-term microscopy-based monitoring of the phytoplankton community

Palaeolimnological records provide valuable information about how phytoplankton respond to long-term drivers of environmental change. Traditional palaeolimnological tools such as microfossils and pigments are restricted to taxa that leave sub-fossil remains, and a method that can be applied to the wider community is required. Sedimentary DNA (sedDNA), extracted from lake sediment cores, shows promise in palaeolimnology, but validation against data from long-term monitoring of lake water is necessary to enable its development as a reliable record of past phytoplankton communities. To address this need, 18S rRNA gene amplicon sequencing was carried out on lake sediments from a core collected from Esthwaite Water (English Lake District) spanning ~105 years. This sedDNA record was compared with concurrent long-term microscopy-based monitoring of phytoplankton in the surface water. Broadly comparable trends were observed between the datasets, with respect to the diversity and relative abundance and occurrence of chlorophytes, dinoflagellates, ochrophytes and bacillariophytes. Up to 20% of genera were successfully captured using both methods, and sedDNA revealed a previously undetected community of phytoplankton. These results suggest that sedDNA can be used as an effective record of past phytoplankton communities, at least over timescales of <100 years. However, a substantial proportion of genera identified by microscopy were not detected using sedDNA, highlighting the current limitations of the technique that require further development such as reference database coverage. The taphonomic processes which may affect its reliability, such as the extent and rate of deposition and DNA degradation, also require further research

Can reductions in water residence time be used to disrupt seasonal stratification and control internal loading in a eutrophic monomictic lake?

Anthropogenic eutrophication caused by excess loading of nutrients, especially phosphorus (P), from catchments is a major cause of lake water quality degradation. The release of P from bed sediments to the water column, termed internal loading, can exceed catchment P load in eutrophic lakes, especially those that stratify during warm summer periods. Managing internal P loading is challenging, and although a range of approaches have been implemented, long-term success is often limited, requiring lake-specific solutions. Here, we assess the manipulation of lake residence time to inhibit internal loading in Elterwater, a shallow stratifying lake in the English Lake District, UK. Since 2016, additional inflowing water has been diverted into the inner basin of Elterwater to reduce its water residence time, with the intention of limiting the length of the stratified period and reducing internal loading. Combining eight years of field data in a Before-After-Control-Impact study with process-based hydrodynamic modelling enabled the quantification of the residence time intervention effects on stratification length, water column stability, and concentrations of chlorophyll a and P. Annual water residence time was reduced during the study period by around 40% (4.9 days). Despite this change, the lake continued to stratify and developed hypolimnetic anoxia. As a result, there was little significant change in phosphorus (as total or soluble reactive phosphorus) or chlorophyll a concentrations. Summer stratification length was 2 days shorter and 7% less stable with the intervention. Our results suggest that the change to water residence time in Elterwater was insufficient to induce large enough physical changes to improve water quality. However, the minor physical changes suggest the management measure had some impact and that larger changes in water residence time may have the potential to induce reductions in internal loading. Future assessments of management requirements should combine multi-year observations and physical lake modelling to provide improved understanding of the intervention effect size required to alter the physical structure of the lake, leading to increased hypolimnetic oxygen and reduced potential for internal loading