Search

Abstract Ecosystem engineers can have diverse and conflicting effects on their ecosystems, and the balance between these effects can depend on the physical environment. This context dependence means that environmental variation can produce large differences in engineer effects through space and time. Here, we explore how local variability in environmental conditions can lead to large spatiotemporal variation in the effect of tube‐building midges on benthic ecosystem metabolism in a shallow subarctic lake. Using field experiments, we found that midge engineering increases both gross primary production (GPP) and respiration (RESP) in the sediment. Gross primary production and RESP have opposing influences on net ecosystem production, and the net effect of midges on the benthic ecosystem depends on the balance between their effects on GPP and RESP. Variation in light mediates this balance—under high light conditions, primary producers are able to exploit the structural benefits provided by midges, while in the dark, the elevation of respiration from midge engineering predominates. Benthic light levels vary spatially and temporally due to episodic cyanobacterial blooms that prevent almost all light from reaching the benthos. By quantifying the nonlinear relationship between midge engineering and light, we were able to project ecosystem‐wide consequences of natural variation in light conditions across the lake. Our results illustrate how the sign and magnitude of ecosystem‐wide effects of ecosystem engineers can vary through space and time.

Lake Myvatn, Iceland, is one of the most biologically productive lakes in the northern hemisphere, despite seasonal ice cover. Hydrothermal and groundwater springs make up the dominant source to this lake, and we investigate their Mg isotope ratio to assess the effect of mid-ocean ridge hydrothermal springs, which are the primary modern sink of seawater magnesium. We also examine a time series in the only outflow from this lake, the Laxa River, to assess the effects of seasonal primary productivity on Mg isotopes. In the hydrothermal waters, there is a clear distinction between cold waters (largely unfractionated from primary basalt) and relatively hot waters, which exhibit over 1‰ fractionation, with consequences for the oceanic mass balance if the hydrothermal removal of Mg is not fully quantitative. The outflow Mg isotopes are similar to basalts (δ26Mg = −0.2 to −0.3) during winter but reach a peak of ∼0‰ in August. This fractionation corresponds to calcite precipitation during summer in Lake Myvatn, preferentially taking up light Mg isotopes and driving the residual waters isotopically heavy as observed, meaning that overall the lake is a CO2 sink.

Numbers of moulting diving ducks at Myvatn, north Iceland, were monitored over a period of 25 years; aquatic insects (Chironomidae and Simuliidae), a major food resource, were monitored with window traps for 23 years. The response of the duck populations to changes in the food situation during the moulting period varied between species. Histrionicus histrionicus and Melanitta nigra invariably go to the sea to moult. Three species (Aythya fuligula, Clangula hyemalis and Mergus serrator) have shown long term variation in moult migration habits. Aythya marila and Bucephala islandica stay for moulting, their numbers supplemented by birds from elsewhere. A. marila is a generalist feeder and therefore at low risk of starving, the strategy of Bucephala islandica is to move within the water system where the food situation tends to alternate between the lake and the river. Numbers of Bucephala islandica males moulting on Lake Myvatn were strongly and positively correlated with chironomid numbers and those moulting on the river Laxá with Simulium vittatum, the main food resource there. We did not find convincing evidence that numbers of moulting A. marila and C. hyemalis responded to variation in the food supply during the study period. This applied also to A. fuligula in the South Basin of Myvatn, but numbers in the North Basin were positively associated with chironomid numbers. M. serrator moulting on Myvatn showed negative correlations with the chironomids, perhaps reflecting a negative association between chironomids and its main food, Gasterosteus aculeatus. Apart from safety considerations for a flightless bird, the choice of a moulting site is apparently influenced by the local food conditions on one hand and by the opportunities and risks involved in migrating to distant moulting sites with an unknown food situation on the other hand. [ABSTRACT FROM AUTHOR]

Ecological features of Lake Myvatn and the outflowing River Laxá show a wide range of spatial and temporal variations. The physical division of the lake into three main basins and the variation in chemical composition and temperature of the artesian springs feeding this shallow productive lake have large spatial effects. Variation in groundwater characteristics depends on percolation time and proximity to geothermal sources. Variation in precipitation is evened out by the porous volcanic soil and bedrock and the spring-water discharge is therefore very stable. A pulse of volcanic activity in 1975–1984 (the Krafla Fires) heated the groundwater entering the North Basin of the lake and changed its chemistry. Although much reduced, these effects have not disappeared yet, but overall the impact of the volcanic activity on the biota seemed minimal. Recycling of nutrients through internal loading is important and occurs on various time scales. In winter, when the lake is ice-covered, the topmost 5-cm layer of sediment pore water has a hundredfold concentration of nutrients relative to the overlying lake water. The nutrients are released during the ice-free period by sediment resuspension, diffusion, bioturbation and recycling. In spring, resuspension events sometimes lead to spikes in dissolved phosphorus and nitrogen, but there is little evidence of any major desorption of nutrients from suspended particles during such events later in the summer. In contrast to the stable groundwater, the biota show more or less regular fluctuations with no straightforward correlation with external signals. The most prominent fluctuations, those of the chironomid Tanytarsus gracilentus seem to be driven by interactions between the species and its sediment resources. Fluctuations in other invertebrates could be a consequence of the Tanytarsus cycles due to the large impact this species has on the benthic environment of this detritus-driven ecosystem. Temporal variation in epibenthic chironomids and Cladocera translates into variable production of vertebrate predators (Arctic charr, Salvelinus alpinus, and ducks), body condition and mortality of fish and sometimes into return rates of migrating adult ducks. The waterfowl show large temporal variation on a centennial scale, e.g., the invasion of the tufted duck (Aythya fuligula) which arrived by the end of the 19th century and has by now outnumbered other species. Fluctuations of Cyanobacteria (Anabaena) and the fish Gasterosteus aculeatus (three-spined stickleback) harmonize with the cycles in the benthic community. Palaeolimnological studies indicate that primary production in the South Basin became increasingly benthic as the lake depth was reduced by sedimentation (around 2 mm year−1). Other trends include a decrease in Tanytarsus and Daphnia and an exponential increase in green algae (Cladophorales, Pediastrum) and associated organisms. [ABSTRACT FROM AUTHOR]

In situ paired light and dark-stirred benthic flux chambers were used to estimate dissolved oxygen flux across the sediment–water interface in Lake Mývatn, Iceland. Three sampling stations were selected, each station reflecting a specific sedimentary environment, benthic communities, and water depth. During this study the phytoplankton density was low. Spatial and seasonal variations of bottom DO concentration and DO flux have been observed during this study. The oxygen consumption rate at all study sites had a mean of −89 (± 44) mmol m−2 d−1 while the oxygen production rate due to benthic algae had a mean of 131 (± 103) mmol m−2 d−1. There was a strong correlation (r = 0.91) between oxygen consumption rate and temperature. This was presumably because of the temperature influence on rates of microbial and macrobenthic processes. The mean benthic primary production rate at all study sites was 1216 (± 957) mg C m−2 d−1 between June 2000 and February 2001. Annual gross benthic primary production was estimated from the gross mean daily benthic DO production (P) and Redfield's C:O2 ratio of 106:138 to be 420 g C m−2 y−1 at station HO, 250 g C m−2 y−1 at B2 and 340 g C m−2 y−1 at station 95. Thus, the mean gross benthic primary production was estimated as 1151 mg C m−2 d−1 at station HO, 685 mg C m−2 d−1 at station B2, and 932 mg C m−2 d−1 at station 95. [ABSTRACT FROM AUTHOR]

In July 1999 and May 2001 a group of limnologists and other scientists involved in the research of Lake Myvatn gathered in the village of Skutustadir by the lake for two symposia to discuss and sum up the results based on the research and monitoring that has been conducted there over the past quarter of a century. The ensuing papers now appear in this issue of Aquatic Ecology under the heading ‘The Ecology of Lake Myvatn and the River Laxá – Temporal and Spatial Variation’. [ABSTRACT FROM AUTHOR]

Benthic Cladocera were studied with a modified type of an activity trap in Lake Myvatn in 1990–1992. After feasibility experiments, the operation time and the distance of the traps from the benthic substrate were adjusted in order to minimize the effects of diurnal variation and the trapping of planktonic organisms. The trap catches of Eurycercus lamellatus were positively correlated with their abundance at the bottom as estimated by grab sampling (r = 0.910, P < 0.001). The usefulness of the activity trap was demonstrated by: (1) a lake-wide survey of the benthic Cladocera; (2) a study of the seasonal variation in the size distribution, abundance and sex ratio of E. lamellatus; and (3) a study of the seasonal succession of Chydorus sphaericus and Alona affinis. The variation of benthic Cladocera among 21 trap sites distributed on a 1–2-km scale across the lake exceeded the within-site variation. The sampling sites could be divided into five main groups based on cluster analysis. Eurycercus lamellatus was the most common species in the mat of filamentous green algae (Cladophorales). Alonella nana dominated the area of spring water inflow in the north basin and Macrothrix hirsuticornis the area of spring water inflow in the southeastern part of the lake. In other parts of the lake either Chydorus sphaericus or Alona quadrangularis tended to dominate. The size distribution and sex ratio of E. lamellatus was followed at two sites through one summer. In early summer most individuals were females less than 1.45 mm long. Around mid-summer they had grown to 0.69–3.1 mm. By the end of August the size distribution had become bimodal, with a large number of small males and a smaller number of much larger females. The seasonal succession in the abundance of E. lamellatus, A. affinis and C. sphaericus was followed at four sites over two seasons. With some exceptions the abundance of a species followed a similar seasonal trajectory on the different stations in any one year. There was, however, a marked difference between the two years (1991 and 1992), probably related to different temperatures. [ABSTRACT FROM AUTHOR]

Aquatic dipterans dominate the macrozoobenthos of Lake Myvatn and the River Laxá and are important in the food web. As benthic sampling is time-consuming and expensive, a window trap was developed in order to facilitate the monitoring of the zoobenthos resource. Four window traps were operated at Myvatn and Laxá during 1977–1996. The results yielded population indices for about 20 species of chironomids and one simuliid. Numbers of the most abundant midge species showed close correlations between 2 trap sites on the lake shore, 5 km apart; Cricotopus tibialis did not show such synchrony. At the River Laxá, the dominant blackfly (Simulium vittatum) showed correlation between 2 trap sites, at the outlet and 3 km downstream; while the chironomids, apart from Micropsectra atrofasciata, did not. The dominant midge of Lake Myvatn, Tanytarsus gracilentus showed 3 cyclic oscillations during the 20-year period. The fluctuations were tracked by Micropsectra lindrothi, Procladius islandicus, Orthocladius consobrinus, Psectrocladius barbimanus and Chironomus islandicus. A group of non-cyclic species included Cricotopus sylvestris and Orthocladius oblidens. [ABSTRACT FROM AUTHOR]