Matching thirty years of ecosystem monitoring with a high resolution microfossil record

Abstract

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.

Identifying consumer–resource population dynamics using paleoecological data

“…This time period corresponds closely to our data on midge abundances obtained through trapping adults that began in 1977, and Hauptfleisch et al. () validated the estimates of midge abundances obtained from sedimentary egg counts against the monitoring estimates of adult abundances. From a second core we assayed an array of pigments representing diatoms (diatoxanthin), cyanobacteria that occur in the water column and therefore are not a major component of midge food (echinenone), subdominant cryptophytes (alloxanthin) not heavily consumed by midges, and general indicators of algal abundance that exhibit either robust chemical stability (β‐carotene) or are highly labile (chlorophyll a ; Leavitt and Hodgson ).…”

“…Core KB‐1 was analyzed for loss on ignition (LOI) and arthropod microfossils (cladoceran exuviae and chironomid egg capsules) by Hauptfleisch et al. (). Core KB‐2 was used for the pigment analysis presented here.…”

“…The time scale of the cores was fine‐tuned by matching the profile of the sediment with known events including documented peaks in chironomid abundances (1979, 1987, 1992, 2000) and a tephra layer from an eruption in Grimsvotn volcano in 2004 (Hauptfleisch et al. ). We regressed the time of these events against core depth using a quadratic curve, and all results are subsequently presented in terms of this time scale.…”

Age of the Younger Laxá Lava and Lake Mývatn, Northern Iceland, Determined by AMS Radiocarbon Dating

“…Einarsson and Örnólfsdóttir 2004;Hauptfleisch et al 2010). This requires reliable sediment chronology, including an age estimation of the lake.…”

“…14 C dating is usually applied to establish a chronology of lake sediments (Björck and Wohlfarth 2004), but in Lake Mývatn a spatially variable freshwater 14 C reservoir effect (~10,000 14 C yr) (Ascough et al 2011) prevents 14 C dating of its sediments. A number of tephra layers in the sediment of Lake Mývatn enable the establishment of a chronological model for the microfossil records of the lake (Einarsson 1982;Einarsson et al 1988Einarsson et al , 1993Hauptfleisch et al 2010). An age estimation of the YLL by AMS 14 C dating would provide the maximum age of the sediment sequence deposited in Lake Mývatn and underpin the local tephrochronology by an independent state-of-the-art dating method.…”