Ecologists have long been fascinated by cyclic population fluctuations, because they suggest strong interactions between exploiter and victim species. Nonetheless, even for populations showing high-amplitude fluctuations, it is often hard to identify which species are the key drivers of the dynamics, because data are generally only available for a single species. Here, we use a paleoecological approach to investigate fluctuations in the midge population in Lake Mývatn, Iceland, which ranges over several orders of magnitude in irregular, multigeneration cycles. Previous circumstantial evidence points to consumer-resource interactions between midges and their primary food, diatoms, as the cause of these high-amplitude fluctuations. Using a pair of sediment cores from the lake, we reconstructed 26 years of dynamics of midges using egg remains and of algal groups using diagnostic pigments. We analyzed these data using statistical methods that account for both the autocorrelated nature of paleoecological data and measurement error caused by the mixing of sediment layers. The analyses revealed a signature of consumer-resource interactions in the fluctuations of midges and diatoms: diatom abundance (as inferred from biomarker pigment diatoxanthin) increased when midge abundance was low, and midge abundance (inferred from egg capsules) decreased when diatom abundance was low. Similar patterns were not found for pigments characterizing the other dominant primary producer group in the lake (cyanobacteria), subdominant algae (cryptophytes), or ubiquitous but chemically unstable biomarkers of total algal abundance (chlorophyll a); however, a significant but weaker pattern was found for the chemically stable indicator of total algal populations (β-carotene) to which diatoms are the dominant contributor. These analyses provide the first paleoecological evaluation of specific trophic interactions underlying high amplitude population fluctuations in lakes.
1. Monitoring of the ecosystem of Lake Mý vatn, Iceland, since 1975 has revealed extreme fluctuations in important food web components, such as chironomids and cladocerans, with amplitudes of several orders of magnitude and a period of 5-8 years. This study uses sediment cores from the lake to examine if the food web fluctuations appear in the microfossil record of the sediment. 2. Dating was achieved by means of a combination of 137 Cs and volcanic tephra and was fine-tuned by wiggle-matching of chironomid microfossil and monitoring data. 3. Cladocera exuviae and chironomid egg capsules in the uppermost 34 cm of sediment were compared with the monitoring record that consisted of 30 years of window trap catches of flying chironomids and a 16-year record of chydorid Cladocera caught in activity traps. 4. The observed chironomid and cladoceran population fluctuations were reflected in the sediment record of chironomid eggs and of the exuviae of three of seven cladocerans: Alonella nana, Alona rectangula and Eurycercus lamellatus, which also had the most extreme fluctuations in the monitoring data (3-4 orders of magnitude). Chydorus sphaericus, and to some extent Alona quadrangularis and Acroperus harpae, showed regular fluctuations in the core that the monitoring did not reveal. Density of subfossil chironomid eggs correlated positively with that of larval head capsules but not with other microfossils. 5. This study shows a reasonably good correspondence between the fossil records of chironomids and cladocerans on the one hand and biomonitoring data on the other. The results pave the way for an extension of the food web history to much earlier time intervals of the ecosystem, allowing the study of long-term variation in the food web dynamics, including the impact of climatic variation and other external forcing. The results also indicate the usefulness of chironomid egg capsules in palaeolimnological studies.
. 2017. Midge-stabilized sediment drives the composition of benthic cladoceran communities in Lake M yvatn, Iceland. Ecosphere 8(2):e01659. 10. 1002/ecs2.1659Abstract. The importance of environmental disturbances as drivers of ecological communities depends not only on the magnitude of the disturbance, but also on the disturbance-specific sensitivity of the community. Organisms that alter the physical structure of their surroundings can affect the sensitivity of their habitat to environmental disturbance, and may alter the potential for disturbance to shape ecological communities. Such organisms therefore act as ecosystem engineers by indirectly modifying the resources available to other species. The benthos of shallow, eutrophic Lake M yvatn, Iceland, is frequently disturbed by wind events that lead to sediment resuspension. The impact of wind, however, depends on the abundance of midges (Chironomidae) whose larval tubes bind sediment and reduce wind-driven resuspension. Here, we investigate the long-term effect of fluctuations in midge abundance on the benthic cladoceran community using two lake sediment cores representing 30 and 140 years of deposition. In both cores, midge remains show a significant positive correlation with abundance of a large benthic surface-dwelling cladoceran, Eurycercus lamellatus, relative to the abundance of a small within-sediment-dwelling cladoceran, Alona rectangula. To experimentally investigate whether this shift could have been caused by midges acting as ecosystem engineers, we subjected cladoceran communities to sediment resuspension events within mesocosms. We found a significant decrease in abundance of the large epibenthic E. lamellatus relative to the abundance of small infaunal Alona spp. when subjected to disturbance. These findings show that physical alteration of benthic sediment and hence the sensitivity of the sediment to disturbance may explain the community shift in cladocerans observed with fluctuating midge abundance in Lake M yvatn.
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