Resistance of Icelandic lake ecosystems to two centuries of agricultural pressures despite growing instability

Abstract In the early nineteenth century, Iceland entered the global market, which caused a large expansion and modernisation of agricultural practices, including rising livestock numbers, higher crop density and the introduction of artificial fertilizers and pesticides. However, the impact of these pressures on Icelandic ecosystems over the past two centuries are largely unknown. This study applies paleolimnological analyses to a sediment core taken from Vestra Gíslholtsvatn, a small lake in southern Iceland surrounded by three farms that broadly reflect national agricultural trends, to assess how changes in agricultural practices have influenced lake-ecosystem dynamics over the past 250 years. Geochemical proxies such as total organic carbon and total nitrogen show increasing trends alongside decreasing δ 13 C values, suggesting increased terrestrial and/or algal organic matter influx, which coincide with the introduction of artificial fertilizers. Additionally, Chironomidae head capsules were analysed to investigate changes in the lake ecosystem. The chironomid community shows no sustained change in the assemblage, with a continuous dominance of Heterotrissocladius grimshawi -type, which favours cold and oligotrophic waters, and Psectrocladius sordidellus -type, which prefers macrophyte-rich littoral habitats. Only Chironomus spp. shows some short term increases possibly in response to artificial fertilizer usage. Despite the relative stability in the chironomid assemblage, statistical analyses of the chironomid record reveal increasing skewness, kurtosis and amplitude of the rate of change towards the top of the core. These results possibly represent early warning signals of ecosystem instability. In spite of these signals, no ecological tipping point was reached within the studied period, indicating that impacts from the agricultural intensification were insufficient to drive regime shifts in the lake ecosystems. Our results highlight the resistance of Icelandic lake ecosystems to the agricultural pressures of the past two centuries, despite growing instability.