Linking land and lake: Using novel geochemical techniques to understand biological response to environmental change

The exploitation of lakes has led to large-scale contemporary impacts on freshwater systems, largely in response to catchment clearance. Such clearance is causing changes to carbon dynamics in tropical lakes which may have significance for wider carbon budgets, depending on the changes in carbon sequestration and mineralisation driven by changing roles of terrestrial and aquatic carbon in lakes over time. Despite increasing awareness of the pivotal role of carbon source in carbon dynamics, discriminating the source of carbon from a palaeolimnological record is rarely undertaken. Here we use novel geochemical techniques (brGDGTs, n-alkanes, Rock-Eval pyrolysis), paired with traditional analyses (diatoms, pollen), to elucidate changing sources of carbon through time and ecosystem response. Environmental changes at Lake Nyamogusingiri can be divided into three phases: Phase I (CE 1150-1275), a shallow and productive lake, where a diverse terrestrial environment is, initially, the main carbon source, before switching to an aquatic source; Phase II (CE 1275-1900), variable lake levels (generally in decline) with increasing productivity, and carbon is autochthonous in source; Phase III (CE 1900-2007), lake level declines, and the carbon is of a mixed source, though the terrestrially derived carbon is from a less diverse source. The organic geochemical analyses provide a wealth of data regarding the complexity of aquatic response to catchment and with-in lake changes. These data demonstrate show that small, tropical lake systems have the potential to bury high quantities of carbon, which has implications for the disruption of local biogeochemical cycles (C, P, N, and Si) both in the past, and the future as human and climate pressures increase

Deciphering long-term records of natural variability and human impact as recorded in lake sediments: a palaeolimnological puzzle

Global aquatic ecosystems are under increasing threat from anthropogenic activity, as well as being exposed to past (and projected) climate change, however, the nature of how climate and human impacts are recorded in lake sediments is often ambiguous. Natural and anthropogenic drivers can force a similar response in lake systems, yet the ability to attribute what change recorded in lake sediments is natural, from that which is anthropogenic, is increasingly important for understanding how lake systems have, and will continue to function when subjected to multiple stressors; an issue that is particularly acute when considering management options for aquatic ecosystems. The duration and timing of human impacts on lake systems varies geographically, with some regions of the world (such as Africa and South America) having a longer legacy of human impact than others (e.g., New Zealand). A wide array of techniques (biological, chemical, physical and statistical) is available to palaeolimnologists to allow the deciphering of complex sedimentary records. Lake sediments are an important archive of how drivers have changed through time, and how these impacts manifest in lake systems. With a paucity of ‘real-time’ data pre-dating human impact, palaeolimnological archives offer the only insight into both natural variability (i.e., that driven by climate and intrinsic lake processes) and the impact of people. While there is a need to acknowledge complexity, and temporal and spatial variability when deciphering change from sediment archives, a palaeolimnological approach is a powerful tool for better understanding and managing global aquatic resources

The future of tropical water resources: using palaeolimnology to inform sustainable management

Lakes are an important resource. They provide vital ecosystem services and employment for many communities worldwide. Maintaining lakes as ecosystem providers without damaging the lake ecosystems themselves, against a background of increasing human use of landscapes and climate change, requires careful and informed management. Key to such management is an understanding of how lakes will respond to ongoing and future changes in their catchments. Long-term monitoring, through regular measurements of lake chemistry for example, can help provide this understanding but such data are rare, particularly for tropical lakes. Using a palaeolimnological approach can provide an alternative to long term monitoring. This paper compares the information that monitoring data and lake sediment records can bring to an understanding of lake change in western Uganda. Water chemistry data show a general pattern to lakes with higher Chlorophyll-a and TP values over the last 15 years, although not all lakes follow this pattern. Sediment cores from Lakes Kamunzuka and Nyungu both show changes in diatom flora through the latter half of the twentieth century and increases in dry mass accumulation rate between c. 1980 and 2000. This study highlights the importance of a co-ordinated monitoring approach to provide the data needed to benchmark management decisions. The importance of understanding each lake on its own merits, from a monitoring or palaeolimnological perspective is also highlighted. Combined, these approaches provide an approach to inform management decisions to sustain lake ecosystems in a healthy state, for the benefit of all users