Benthic contribution to whole-lake production in southwest Greenland: A palaeoecological perspective

Arctic aquatic ecosystems are typically strongly nutrient-limited and are sensitive to change over a range of timescales. These systems are recognised as being increasingly affected by recent environmental and climatic changes, both in terms of primary productivity and whole ecosystem function. Biological (primary producers, such as diatoms) and geochemical proxy data from sedimentary archives can be used as valuable tools to infer past ecological response to environmental and climate changes at high temporal resolution and give insight into how these critical ecosystems might respond in the future. 

Sediment cores are usually taken from the deepest part of the lake, where sediment focussing leads to greater sediment accumulation. However, due to uneven sediment redistribution, this often leads to quantitative and qualitative underrepresentation of the littoral communities. This has major implications for the interpretation of diatom records based on a single, central sediment core, especially since nutrient-poor conditions and high water transparency in many Arctic lakes result in a relatively high contribution of benthic primary production to whole-lake productivity. Therefore, this thesis criticises the representativeness of using one sediment core for whole-lake diatom production reconstructions in the Arctic. 

This thesis provides a bridge between contemporary research on algal habitat variability in the Arctic and how this is represented in the sedimentary archive as a means to improve our understanding of both recent and longer-term ecological change in Arctic lakes. For this, the spatial distribution of periphytic diatoms was analysed, as well as the diatom assemblage in surface sediments and multiple (3-5) sediment cores in three lakes in southwest Greenland. Furthermore, a correction was calculated for benthic, planktonic and total whole-lake diatom productivity, as well as for relative abundances of individual taxa, and these improved inferences were used to separate local from regional drivers of lake ecosystems in southwest Greenland.

The results indicated that the benthic diatom community was substantially underrepresented in the central sediment core, the degree of which varied spatially (between the lakes) and temporally. The abundance of macrophytes, lake morphometry and turbulence had a substantial impact on taphonomy (sediment focusing and dissolution). Therefore, inferences based on a single, central core did not include, for example, a doubling in benthic production from ca. 1.5 to 3 * 106 frustules cm-2 yr-1, as observed in one of the study sites (SS906) at ca. AD 1980, but the diatom accumulation rates remained around 0.5 * 106 frustules cm-2 yr-1 in the central core during that time. This highlights the relevance of considering whole-lake production when looking at key ecological variables or ecosystem function. The corrected central cores reflected the productivity changes in the whole-lake well and showed that the ecological changes observed in the study sites were driven by past environmental drivers, such as climate and catchment variability. However, differences between the lakes and offset in the timing of environmental change and ecological response, stress the importance of local variability. With many Arctic freshwater ecosystems showing (dramatic) ecological change over the last 150 years, it is now more relevant than ever to be able to disentangle the different drivers.