The Trichoptera of Øvre Heimdalsvatn

Trichopteran larvae were sampled on soft substrata at all depths, on stony substrata in the exposed zone, and the adults in emergence traps, placed along the lake shore. Fourteen species were taken as larvae in the lake. On stony substrata and in the exposed zone Polycentropus flavomaculatus (Pictet) was dominant, followed by Limnephilus nigriceps (Zetterstedt) in terms of numbers. In terms of biomass L. nigriceps dominated followed by Potamophylax cingulatus (Stephens).
Species which usually inhabit running water made up for a larger part of the trichopteran fauna of the exposed zone.
The two most numerous species showed different habit preferences. P. flavomaculatus was most common on stable stony bottom, while L. nigriceps was most common on unstable stony bottom. The other species showed no significant preference. On unstable soft bottom at 3 m and below, Mystacides azureus (L.) dominated, accounting for about 90% of the total trichopteran fauna both in terms of weight and numbers.
The major trichopteran species in the lake were either detritus feeders, such as M. azureus, L. nigriceps and Potamophylax spp. or omnivores such as P. flavomaculatus and Molanna albicans (Zetterstedt).     

The Ephemeroptera of Øvre Heimdalsvatn

The species composition, distribution, seasonal variations in abundance, life cycles, emergence periods, length-weight relationships and biomass of the Ephemeroptera of Øvre Heimdalsvatn were investigated. Detritus was a significant factor in explaining the variation in total ephemeropteran numbers around the lake. Numbers in the exposed zone reached a maximum during July due to emergence activity. The ephemeropteran biomass at emergence, to which Leptophlebia vespertina (L.), L. marginata (L.) and Siphlonurus lacustris Eaton were the main contributors, was estimated to be 0 02g dw m⁻². Other species recorded from the lake were Baëtis macani Kimmins, B. rhodani Pictet, Siphlonurus aestivalis Eaton and Ameletus inopinatus Eaton. All species were univoltine, but differences in ability to grow during ice cover and in life cycle timing resulted in a succession in emergence during the ice free period.     

The post-glacial sediments of Øvre Heimdalsvatn

Two sediment cores, 143 cm and 268 cm in length, have been investigated. The oldest dating is 8240 ± 500 radiocarbon years B.P. The variation in the composition of the sediments is probably caused solely by climatic changes. The organic fraction is small, never exceeding 20%, reflecting the low primary production and rapid decomposition of allochthonous organic matter within the lake. The proportions of inorganic constituents indicate that the mineral fraction consists of clay and silt particles flushed into the lake from the drainage area.     

The lake ecosystem of Øvre Heimdalsvatn

The Norwegian subalpine lake. Øvre Heimdalsvatn, has a surface area of 0.78 km² and a maximum depth of 13 m. It is ice-covered for 7.5–8 months, has a marked spring spate and a mean annual renewal period of about two months. The water is poor in electrolytes. Intensive studies have been made by an interdisciplinary team of the lake's physical and chemical properties, primary production and secondary production under the auspices of IBP/PF from 1969 to 1973, Allochthonous material accounted for 1/3 of utilized plant input. The major lake predator, the brown trout, fed largely on benthic organisms and did not exploit the zooplankton biomass. On account of the long period of ice cover and the rapid rise in temperature after ice break, many organisms (both planktonic and benthic) showed synchronous development. Abiotic conditions, such as the nature of the spring spate and the temperature rise, strongly affect species and community development.     

The aquatic Coleoptera of Øvre Heimdalsvatn

The species composition, seasonal and spatial distribution, and biomass of the coleopteran Fauna of the exposed zone of Øvre Heimdalsvatn in 1972 are presented. Individual weight data are given for the 8 species present in the lake: Haliplus fulvus F., Hydroporus palustris L., Deronectes alpinus Payk., D. multilineatus , Flkstr., D. rivalis Gyll., Platambus maculatus L., Colymbetes dolabratus Payk. and Helophorus glacialis Villa. Deronectes alpinus occurred in highest densities, but P. maculatus , on account of its higher individual weight, was the major contributor to the coleopteran biomass.     

The benthos of Øvre Heimdalsvatn: Introduction

For the purpose of benthic studies Øvre Heimdalsvatn was divided into three depth zones. These were the exposed zone (from the shore down to 1–2 m depth), the macrophytic zone (1–2 m to 5–5.5 m depth) and the non-macrophytic zone (5–5.5 m to 13 m). The reasons for this division and the main characteristics of each zone, including the substrata present, are given.     

The Mollusca of the exposed zone of Øvre Heimdalsvatn

The molluscan fauna was surveyed monthly in 1972 during the ice free period. Densities were low and the most common species were the gastropods Gyraulus acronicus (Férussac) and Lymnaea peregra (Müll.).     

The Chironomidae of the exposed zone of Øvre Heimdalsvatn

A total of 67 species of chironomids were caught in emergence traps placed at the shoreline. Additional larval records increase the number of species to be at least 75, a high number of species possibly explained by the exposed nature of the lake, which also provides habitats for lotic forms.
The species composition and relative abundance of the four main groups of Chironomidac indicate that Øvre Heimdalsvatn is a subarctic lake, with a strong oligotrophic character. The emergence period starts in mid-June and lasts to mid-September. Diversity studies show similarity with mountain lakes in France and with the Bodensee, but the equilability in Øvre Heimdalsvatn is lower.
The chironomid faunal composition of Øvre Heimdalsvatn seems to be determined by abiotic ecological factors and not the history of distribution.     

Phytoplankton and pelagic primary productivity in Øvre Heimdalsvatn

A phytoplankton investigation was carried out in the subalpine, low-productive Norwegian lake Øvre Heimdalsvatn in 1969–70 and 1972. This paper describes the temporal and spatial distribution of the standing stock of phytoplankton, and phytoplankton primary productivity. The annual average primary productivity in 1972 was 4.0–4.9 mg C m⁻³ d⁻¹; the annual average standing stock varied from 120 mg m⁻³ (freshweight) in 1969–70, to 250 mg m⁻³ in 1972. Phytoplankton species composition and size distribution is discussed. Throughout the year the phytoplankton is dominated by small (ultraplankton) species; μ-algae (< 5 μm) showed cell concentrations up to 15 mill. cells 1⁻¹. The dominating group was chrysophytes; cryptophytes, dinoflagellates or green algae were at times abundant. A phytoplankton monthly budget and a diagram showing annual average carbon flow through the standing stock of phytoplankton are presented; the phytoplankton dynamics in Øvre Heimdalsvatn is compared to that of other low-productive lakes.     

The life cycle dynamics and production of zooplankton in Øvre Heimdalsvatn

The zooplankton in Øvre Heimdalsvatn was studied from May 1969 to Aug 1973 in order to obtain knowledge about their functioning and role in the lake ecosystem. The paper concentrates on the main species: Holopedium gibberum, Bosmina longispina, Cyclops scutifer. Heterocope saliens, Conochilus unicornis, Polyarthra vulgaris and Kellicottia longispina but Megacyclops gigus, Daphnia longispina and ciliated cells are also considered. The development of the populations was fairly synchronized and made it possible to determine life cycles, development times, growth, reproduction and production from field data. The total production of the multicellular zooplankton was 2–3 mg dry wt m⁻² yr⁻¹. The cladocerans dominated and their share of the total was 84–92% compared to the copepods 5–8% and the rotifers 3–8%. The ciliate production was not estimated, but their biomass exceeded that of the copepods and rotifers. The spring spate, June temperatures, food availability and invertebrate predation were found to be the most important regulatory factors for life cycle dynamics and production. Allochthonous organic matter was found to be a necessary food resource in addition to algae.     

An annotated check-list of microalgae from Øvre Heimdalsvatn and its tributaries

About one hundred genera with more than 300 species have been identified in water samples, net haul material, periphyton samples from artificial substrate, and benthos and sediment samples collected during 1969–1972. More than 40 taxa of lower rank (subspecies, varieties, forms) were recorded. The most prominent algal groups were diatoms with 138 taxa, and green algae with 122 taxa of which the majority was desmids. The material from the epipelic and epilithic communities in the lake is very scarce, while the list of plankton species is relatively complete, with the exception that a large number of small flagellates could not be identified to species level. The taxa which were identified, are presented in a list with additional notes on occurrence.     

Periphytic and drifting microalgae in a tributary stream of Øvre Heimdalsvatn

Samples of drifting and periphytic microalgae were collected during 1972 from a fast-flowing, stony stream (Brurskardsbekken) in the Jotunheimen mountain area, central southern Norway. The predominant algal groups in the drift and the periphyton were diatoms and green algae, while only a few species were recorded in both communities. A considerable number of species from Chrysophyceae, Cryptophyceae and other algal classes were also recorded in the drift samples. The species composition was in good agreement with microalgal communities earlier described from mountain areas in Scandinavia, although species which probably are new to Norway, were also recorded. A quantitatively important fraction of planktonic species in the drift is interpreted as a contribution from lacustrine habitats in the watercourse. A general change in the periphyton during the summer, from green algae to diatoms, was observed, Altitudinal differences in the periphyton included a delayed green algal maximum at higher altitudes compared to lower. In the zone around the upper birch limit, a transition in species composition as one goes up stream, described in other investigations, was not observed.     

The dynamics and exploitation of the brown trout population of Øvre Heimdalsvatn

The very dense trout population in the lake was substantially reduced by heavy fishing. The resulting changes in growth, age distribution and population density were examined. Fishing efforts with 32 mm nets gave the highest sustainable yield. With a yearly effort of 1600 nights with these nets the predicted sustained yield would be 5.7 kg ha⁻¹ of trout with a mean weight of 269 g.     

The fauna of the exposed zone of Øvre Heimdalsvatn: Methods, sampling stations and general results

Twenty stations were sampled monthly during the Ice free period (June-September) in 1972. Emergence traps, emptied daily, were also employed. The major macroinvertebrates in the exposed zone were Ephemeroptera, Gammarus lacustris , Trichoptera, Chironomidae, Plecoptera and Coleoptera, and these constituted over 90% of total numbers. Densities of G. lacustris , Ephemeroptera, Tipulidae and Plecoptera showed a positive correlation with detritus. No significant relationship was found for the other taxa. Nearly 70% of the fauna emerged, and did so during the period from June to September. Chironomidae, Ephemeroptera, Plecoptera, Trichoptera and Tipulidae accounted for nearly all emergence and their total average annual emergence was 372 cal m⁻². Of this total, Chironomidae constituted 28%, Ephemeroptera 28%, Plecoptera and Trichoptera 15% each and Tipulidae 4%.     

The geology of the Øvre Heimdalen valley

Øvre Heimdalsvatn is clearly influenced by the geological conditions, and the lake basin and catchment area are determined by the bedrock and the mountain structures. The distribution and characteristics of the loose deposits are decisive for the lake's water budget. The groundwater in the catchment area and the Quaternary deposits near the lake influence the amount and quality of the lake waters.     

The vegetation of the Øvre Heimdalen valley

The catchment area of Øvre Heimdalsvatn reaches from the upper half of the subalpine birch forest zone to the lowermost part of the high-alpine subzone within the alpine zone. The units dominating the area include mountain birch forest, tall herb meadow, mire vegetation, bilberry heath, snowbed communities, chionophobous heath vegetation, and areas without a continuous vegetation layer. It is shown that the potential natural vegetation corresponds closely to the actual vegetation distribution.     

The vertebrates of the Øvre Heimdalen valley

The small mammals of the Øvre Heimdalen valley were assessed by snap-trapping. Birds were surveyed in the shore zone of the lake. Numbers of domestic animals were collected from existing sources. These data are used in a discussion of the influence of the terrestrial fauna on the lake ecosystem. Aquatic production of food for terrestrial vertebrates, and the fertilizing effect of the terrestrial fauna are the main influences on the lake.     

Biology of the minnow Phoxinus phoxinus and its interactions with brown trout Salmo trutta in Øvre Heimdalsvatn, Norway

Minnows have recently been introduced into the mountain lakes of southern Norway. In Øvre Heimdalsvatn, an intensively studied lake, the minnow population has increased rapidly during the last decade. Studies were performed on feeding habits, habitat preferences, age, growth, energy content, densities, parasites, predation and spawning activities. Several aspects were found to be different from other studies, e.g. higher age of sexual maturation, lower growth rate, no feeding on plants, and low infection of parasites. The interaction with the only other fish, the brown trout, is discussed in relation to the minnow population. In 1978 the minnows constituted little more than 10% of production and food consumption of the trout population, and only limited food competition and predation was documented. Increasing competition is likely with a greater minnow population.     

The local climate of the Øvre Heimdalen valley

Owing to the high altitude of Øvre Heimdalen and the complex topography within the valley, climatic conditions are complicated. The valley is situated in a mountain region with a subarctic or tundra macroclimate, which exists over the lake itself and on the SSW-valley side. However, the favourable combination of slope angle and exposure of the NNE-valley side gives a radiation or continental type of local climate on this side.
The large net input of radiant energy onto the NNE-valley side during the summer results in higher air temperatures, greater precipitation and higher air humidity than elsewhere in the valley. This is clearly reflected in the luxuriant vegetation on this side. Thus, the climate of the Øvre Heimdalsvatn area is strongly influenced by terrain irregularities and shows how fundamentally different growth possibilities due to local climates may be over short distances in the uppermost valleys on the eastern side of the central mountain chain.