Rapid Ecological Shift Following Piscivorous Fish Introduction to Increasingly Eutrophic and Warmer Lake Furnas (Azores Archipelago,Portugal): A Paleoecological Approach

Lake ecosystems are nowadays often subjected to multi-stressors, such as eutrophication, climate change, and fish manipulations, the effects of which can be difficult to disentangle, not least from the usual short-term limnological time-series that are available. However, multi-proxy paleoecological approaches may offer such opportunities, especially in the study of remote island lakes characterized by being species poor and buffered somewhat against the recent climate change. We used a paleoecological multi-proxy approach to determine the relative importance of nutrient loading, meteorological forcing, and fish species introduction for recent lake ecosystem development in Lake Furnas on the island of São Miguel, the Azores. The lake was stocked with cyprinids in the late nineteenth century and recently also with piscivorous fish, and has been affected by increasing agricultural activities in its catchment. We analyzed marker pigments, cladoceran remains, and subfossil diatoms in a 46-cm core representing the last 40 years. Remains of large-bodied cladocerans were virtually absent until the introductions of piscivorous pike and pikeperch in 1980 and 1982, respectively, after which the zooplankton community composition changed abruptly. First Ceriodaphnia sp. appeared (ca. 1980), followed by Daphnia a few years later. Carotenoids from cyanobacteria (myxoxanthophyll, aphanizophyll) were regularly present in the lake sediment with a major shift occurring around 1994, from N₂-fixing to non-fixing groups. This shift coincided with the onset of anoxia in the lake water, evidenced by the presence of pigments from Chlorobiaceae (BChl-e homologues and isorenieratene), and with a rapid decrease in benthic-tychoplanktonic diatoms and an increase in cyanobacteria, chlorophytes, dinoflagellates, and cryptophytes. The composition of microbial and algal assemblages changed rapidly after Daphnia appearance, and the covariance between fish stocking, nutrient loading, and enhanced temperatures captured most of the variability in algae accumulation, and thus likely in lake primary production as well. Thus, lake production apparently did not respond strongly to specific changes in temperature, food-web structure, or nutrient input, but rather to the combined effects of all the three forcing mechanisms, emphasizing the role of multi-stressors in lake ecosystem functioning. Our study demonstrates the sensitivity of these remote species-poor lakes to increased nutrient loading, introduction of non-native species, such as fish and climate change.