Hilmar J. Malmquist

Impacts of climate warming on lake fish community structure and potential effects on ecosystem function

Fish play a key role in the trophic dynamics of lakes, not least in shallow systems. With climate warming, complex changes in fish community structure may be expected owing to the direct and indirect effects of temperature, and indirect effects of eutrophication, water-level changes and salinisation on fish metabolism, biotic interactions and geographical distribution. We review published and new data supporting the hypotheses that, with a warming climate, there will be changes in: fish community structure (e.g. higher or lower richness depending on local conditions); life history traits (e.g. smaller body size, shorter life span, earlier and less synchronised reproduction); feeding mode (i.e. increased omnivory and herbivory); behaviour (i.e. stronger association with littoral areas and a greater proportion of benthivores); and winter survival. All these changes imply higher predation on zooplankton and macroinvertebrates with increasing temperatures, suggesting that the changes in the fish communities partly resemble, and may intensify, the effects triggered by eutrophication. Modulating factors identified in cold and temperate systems, such as the presence of submerged plants and winter ice cover, seem to be weaker or non-existent in warm(ing) lakes. Consequently, in the future lower nutrient thresholds may be needed to obtain clear-water conditions and good ecological status in the future in currently cold or temperate lakes. Although examples are still scarce and more research is needed, we foresee biomanipulation to be a less successful restoration tool in warm(ing) lakes without a strong reduction of the nutrient load.

The Arctic Charr Salvelinus alpinus in Thingvallavatn

The coexistence of four morphs of arctic charr in Thingvallavatn, Iceland, was demonstrated by investigations of morphology, habitat use, diet, endoparasitic fauna, life history, time and place of spawning, early ontogeny, and population genetics. Head morphology in embryos, juveniles and adults revealed the presence of two charr morphotypes, each including two morphs: (1) a benthic morphotype including large benthivorous (LB-) and small benthivorous (SB-) charr, featuring overshot mouth and relatively large pectoral fins, and (2) a pelagic morphotype including planktivorous (PL-) and piscivorous (PI-) charr, featuring termimal mouth and relatively small pectoral fins

Biodiversity of macroinvertebrates on rocky substrate in the surf zone of Icelandic lakes

Sinee 1992 an EcoJogieal Survey of lcelandie Lakes (ESIL projeet) has been eondueted, where key bioJogieal, hydrogeoJogical and physieo-chemieal faetors are assessed and reeorded. The aim of the projeet is to estabJish a database with standardized eeoJogieal information for alJ major types of lcelandic Jakes. The database offers a multitude of uses, espeeialJy for pattern analysis and eomparative studies within the field of environmental and evoJutionary eeoJogy. Other appJieations relate to the inereasing demand for knowJedge on the status of water quality and biodiversity, e.g. for environmental impaet assessment. The eeoJogieal impJieations of bedroek geoJogy have, until reeentJy, not been studied systematieally in lcelandic freshwaters. The present study of the ESIL projeet and a study of lcelandie rivers (GfsLASON & ADALSTEINSSON 1996) are the first made in this eontext. However, GARDARSSON (1979) proposed an eeoJogieaJ classifieation of lcelandie freshwater systems, partly dedueed from hydrogeoJogieal features of eatehment areas. Also, the importanee of the struetural eompJexity of Java roeks and Java bottom has been advoeated in the eontext of the evoJutionary eeology of fish and invertebrates in Lake ThingvalJavatn (e.g. SNORRASON 1982, MALMQUIST et al. 1992, )óNASSON 1993, SNORRASON et al. 1994). In this paper we analyse the taxonomie diversity and abundanee of macroinvertebrates on roeky substrate in the Jittoral surf zone of 35 freshwater Jakes. This is done in relarion to Jake altitude, area, mean depth, eleetroJytie eonduetivity, temperature, and microhabitat strueture of the roeky substrate, a feature strongJy Jinked to bedroek geoJogy. By this we aim to identify bioJogical eommunity strueture in the surf zone o f Icelandie Jakes within the eontext o f key environmental variabJes.

Threespine Stickleback Gasterosteus aculeatus in Thingvallavatn: Habitat and Food in a Lake Dominated by Arctic Charr Salvelinus alpinus

In the large and deep Lake Thingvallavatn, Iceland, threespine stickleback were restricted to the Nitella opaca vegetation found in the deep littoral zone (10-20 m). The stickleback matured during their third year, and reached an average asymptotic length of 57 mm. The diet consisted mainly of benthic cladocerans, ostracods and chironomid larvae. Some copepods were also eaten by fish of age 1 and older in August and September. There were some differences in food choice between the size groups of stickleback present, as older fish ate more large zoobenthos, like snails and water bugs

Short-and long term niche segregation and individual specialization of brown trout (Salmo trutta) in species poor Faroese lakes

Trophic niche divergence is considered to be a major process by which species coexistence is facilitated. When studying niche segregation in lake ecosystems, we tend to view the niche on a one-dimensional pelagic-littoral axis. In reality, however, the niche use may be more complex and individual fidelity to a niche may be variable both between and within populations. In order to study this complexity, relative simple systems with few species are needed. In this paper, we study how competitor presence affects the resource use of brown trout (Salmo trutta) in 11 species-poor Faroese lakes by comparing relative abundance, stable isotope ratios and diet in multiple habitats. In the presence of three-spined sticklebacks (Gasterosteus aculeatus), a higher proportion of the trout population was found in the pelagic habitat, and trout in general relied on a more pelagic diet base as compared to trout living in allopatry or in sympatry with Arctic charr (Salvelinus alpinus). Diet analyses revealed, however, that niche-segregation may be more complex than described on a one-dimensional pelagic-littoral axis. Trout from both littoral and offshore benthic habitats had in the presence of sticklebacks a less benthic diet as compared to trout living in allopatry or in sympatry with charr. Furthermore, we found individual habitat specialization between littoral/benthic and pelagic trout in deep lakes. Hence, our findings indicate that for trout populations interspecific competition can drive shifts in both habitat and niche use, but at the same time they illustrate the complexity of the ecological niche in freshwater ecosystems.

Variability in the functional role of Arctic charr Salvelinus alpinus as it relates to lake ecosystem characteristics

This study investigated how dietary habits vary with lake characteristics in a fish species that exhibits extensive morphological and ecological variability, the Arctic charr Salvelinus alpinus. Iceland is a hotspot of geological activity, so its freshwater ecosystems vary greatly in physical and chemical attributes. Associations of dietary items within guts of charr were used to form prey categories that reflect habitat-specific feeding behavior. Six prey categories were defined and dominated by snails (Radix peregra), fish (Gasterosteus aculeatus), tadpole shrimp (Lepidurus arcticus), chironomid pupae, pea clam (Pisidium spp.), and the cladoceran Bosmina sp.. These reflected different combinations of feeding in littoral stone, offshore benthic, and limnetic habitats. Certain habitat-specific feeding strategies consistently occurred alongside each other within lakes. For example, zooplanktivory occurred in the same lakes as consumption from offshore habitats; piscivory occurred in the same lakes as consumption from littoral benthic habitats. Redundancy analyses (RDA) were used to investigate how lake environment was related to consumption of different prey categories. The RDA indicated that piscivory exhibited by Arctic charr was reduced where brown trout were abundant and lakes were shallow, greater zooplanktivory occurred at lower latitudes and under decreased nutrient but higher silicon dioxide concentrations, and benthic resource consumption was associated with shallower lakes and higher altitudes. This study showed that trends previously observed across fish species were supported at the intraspecific level, indicating that a single species with flexible dietary habits can fill functional roles expected of multiple species in more diverse food webs.