Environmental effects related to the local absence of exotic fish

Given the extent of biological invasions in industrialized countries, our understanding of the determinants of overall patterns of biological invasions could gain most from consideration of why exotic species are absent from some areas, rather than from distribution patterns of exotic species. Fish communities were sampled at 381 sites representing 221 rivers in the Adour-Garonne stream system (116 000 km², SW France). Very few rivers were not colonized by exotic fish species, however, on a local basis, only 33% of the sampling sites hosted exotics. Using General Linear Modelling, we found that patterns of exotic fish (occurrence, number of species, proportion within assemblage) responded to both land-use and physical variables, whereas patterns of native fish only responded to the local meso-scale characteristics of each stream reach from headwaters to mouth. All fish communities were susceptible to invasion regardless of native species richness, and higher native species richness did not decrease the opportunity for establishment by exotic species. The likelihood that exotic fish are absent primarily increased with elevation and with lower human influence upon the land cover, while human-impacted landscapes (agricultural and urban areas) were more likely to host exotic fish and higher numbers of exotic species. In light of urban and agricultural development, our ability to detect responses of exotic species to landscape alterations using a combination of simple physical and land cover variables exemplifies a cost-effective technique for assessing areas at greater invasion risk in large stream systems

Dietary breadth and trophic position of introduced European catfish Silurus glanis in the River Tarn (Garonne River basin), southwest France

Although being a widely introduced and successfully established species, the European catfish Silurus glanis L. (the world’s third largest freshwater fish) remains poorly studied in its introduced areas. Here we studied the trophic ecology of non-native European catfish in a large river system in south-western France using stomach content and stable isotope analyses (SIA). We used fin samples for SIA of catfish and hence tested the validity of using fin tissue as a proxy for muscle in SIA. The mean δ15N and δ13C values analysed from fin tissues did not differ from those analysed from muscle tissue and reflected strong and consistent relationships (r2 = 0.95 for carbon and r2 = 0.98 for nitrogen). The δ15N values varied almost 5‰ among the analysed catfish individuals, while δ13C values varied >5‰. Total length of these catfish ranged from 200 to 2240 mm and was correlated with δ15N and especially with δ13C values. Although catfish length and δ15N values were positively correlated, the mean trophic positions of catfish increased only slightly from smaller individuals to larger ones (4.3 to 4.7). However, larger catfish were considerably 13C-enriched in their δ13C values compared to smaller individuals and had up to 4‰ higher δ13C values than their expected aquatic prey. This might indicate frequent consumption of mammals and/or non-aquatic birds by the larger sized individuals, which were found in the catfish stomachs

Freezing and chemical preservatives alter the stable isotope values of carbon and nitrogen of the Asiatic clam (Corbicula fluminea)

We tested the impacts of most common sample preservation methods used for aquatic sample materials on the stable isotope ratios of carbon and nitrogen in clams, a typical baseline indicator organism for many aquatic food web studies utilising stable isotope analysis (SIA). In addition to common chemical preservatives ethanol and formalin, we also assessed the potential impacts of freezing on δ¹³C and δ¹⁵N values and compared the preserved samples against freshly dried and analysed samples. All preservation methods, including freezing, had significant impacts on δ¹³C and δ¹⁵N values and the effects in general were greater on the carbon isotope values (1.3-2.2% difference) than on the nitrogen isotope values (0.9-1.0% difference). However, the impacts produced by the preservation were rather consistent within each method during the whole 1 year experiment allowing these to be accounted for, if clams are intended for use in retrospective stable isotope studies

Contribution of anadromous fish to the diet of European catfish in a large river system

Many anadromous fish species, when migrating from the sea to spawn in fresh waters, can potentially be a valuable prey for larger predatory fish, thereby efficiently linking these two ecosystems. Here, we assess the contribution of anadromous fish to the diet of European catfish (Silurus glanis) in a large river system (Garonne, southwestern France) using stable isotope analysis and allis shad (Alosa alosa) as an example of anadromous fish. Allis shad caught in the Garonne had a very distinct marine delta(13)C value, over 8 per thousand higher after lipid extraction compared to the mean delta(13)C value of all other potential freshwater prey fish. The delta(13)C values of European catfish varied considerably between these two extremes and some individuals were clearly specializing on freshwater prey, whereas others specialized on anadromous fish. The mean contribution of anadromous fish to the entire European catfish population was estimated to be between 53% and 65%, depending on the fractionation factor used for delta(13)C

Vulnerability of the North Water ecosystem to climate change

High Arctic ecosystems and Indigenous livelihoods are tightly linked and exposed to climate change, yet assessing their sensitivity requires a long-term perspective. Here, we assess the vulnerability of the North Water polynya, a unique seaice ecosystem that sustains the world’s northernmost Inuit communities and several keystone Arctic species. We reconstruct mid-to-late Holocene changes in sea ice, marine primary production, and little auk colony dynamics through multi-proxy analysis of marine and lake sediment cores. Our results suggest a productive ecosystem by 4400–4200 cal yrs b2k coincident with the arrival of the first humans in Greenland. Climate forcing during the late Holocene, leading to periods of polynya instability and marine productivity decline, is strikingly coeval with the human abandonment of Greenland from c. 2200–1200 cal yrs b2k. Our long-term perspective highlights the future decline of the North Water ecosystem, due to climate warming and changing sea-ice conditions, as an important climate change risk

Impacts of biomanipulation on lake ecosystem structure revealed by stable isotope analysis

Rehevöityneiden järvien ravintoketjukunnostus epäonnistuu useinJärvien ravintoketjukunnostusta eli biomanipulaatiota on käytetty paljon rehevöityneiden järvien kunnostuksessa. Biomanipulaatiossa pyritään poistamaan järvestä pieniä eläinplanktonia syöviä kalalajeja, lähinnä eri särkikaloja ja pieniä ahvenia, jolloin eläinplanktonin määrä ja yksilökoko kasvavat. Tällöin kasviplanktonia ravintonaan käyttävä eläinplankton pystyy paremmin kontrolloimaan kasviplanktonin määrää järvessä. Biomanipulaation onnistumisen kriteerinä voidaan siis pitää kasviplanktonin määrän vähenemistä ja siitä aiheutuvaa veden kirkastumista. - Varsin suoraviivaisesta teoriasta huolimatta saavutetut tulokset eivät aina ole hyviä, ja monesti onnistuneidenkin biomanipulaatioiden tulokset ovat olleet varsin lyhytkestoisia. Syitä epäonnistumiselle ei tarkkaan tiedetä, koska biomanipulaation vaikutuksia koko järven ekosysteemiin on varsin hankalaa ja työlästä tutkia, Syväranta kertoo. Syväranta tutki biomanipulaation vaikutuksia järviekosysteemin rakenteeseen ja toimintaan käyttämällä luonnossa esiintyviä hiilen ja typen vakaita isotooppeja. Tutkimus täydentää Jyväskylän Jyväsjärven tutkimushanketta, jossa biomanipulaation vaikutuksia järven kasvi- ja eläinplanktonlajistoon sekä kalakantaan on tutkittu pääasiassa perinteisin menetelmin. Näiden avulla voi kuitenkin olla vaikeata ja työlästä saada tietoa kalapopulaatioiden ekolokeromuutoksista, ravintoketjutason muutoksista eri lajeilla tai etenkin järven ranta- ja ulappa-alueen perustuotannon muuttuneesta merkityksestä ravintoketjun eri tasoille. Luonnossa esiintyvien vakaiden isotooppisuhteiden määritykseen perustuva tekniikka tuo uutta tietoa biomanipulaation vaikutuksista ekosysteemitasolla.Impacts of biomanipulation on lake ecosystem structure revealed by stable isotope analysis Eutrophication of aquatic ecosystems is an increasing problem, especially threatening small and shallow freshwater lakes. While the extreme inputs of nutrients (phosphorus and nitrogen) can nowadays be reduced and controlled rather efficiently, additional measures to reverse eutrophication may be needed. These often involve biomanipulation, typically mass removal of cyprinid fish. However, the success of biomanipulations has many times been limited or short lived. The reasons for this are not well known, perhaps because the more subtle impacts of biomanipulation on the ecosystem-wide processes of lakes have not been thoroughly studied. Natural abundance stable isotopes may provide a cost-effective tool to study such ecosystem level impacts of food web perturbations. In this thesis, I used stable isotope analysis (SIA) to study the impacts of biomanipulation on lake ecosystem properties in Jyväsjärvi, a lake recovering from severe eutrophication. Variability in stable isotope signatures within the studied lake was examined as a prerequisite for reliable SIA. Temporarily lower δ15N values of fish in one area of Jyväsjärvi resulted from extensive fish migration into Jyväsjärvi from the connected lake Päijänne. Stable isotope data indicated that the migration was considerably reduced following biomanipulation and the immigrant fish from Päijänne did not quickly recolonise Jyväsjärvi. Isotopes also indicated significant changes in feeding niche widths of perch and roach after fish removals. From a wider perspective, contribution from pelagic energy sources to higher trophic levels in the Jyväsjärvi ecosystem increased after biomanipulation, in contrast to expectation. To enhance the potential of SIA in ecological studies, I also tested the possibility of using archived sample materials in retrospective SIA, which could allow for longer time perspectives in such ecosystem studies. I found that typical archived freshwater samples do provide valid materials for SI

The influence of lipid content and taxonomic affiliation on methane and carbon dioxide production from phytoplankton biomass in lake sediment

The greenhouse gases methane (CH4) and carbon dioxide (CO2) are end products of microbial anaerobic degradation of organic matter (OM) in lake sediments. Although previous research has shown that phytoplankton lipid content influences sediment methanogenesis, current understanding on how OM quality affects methanogenesis is still limited. Such information is needed to more accurately assess how lake greenhouse gas emissions may change in response to anthropogenic activities. We cultured 11 phytoplankton species from five classes and studied how taxonomic identity, C : N ratio, lipid content, and fatty acid composition of phytoplankton biomass affects the CH4 and net CO2 production in anaerobic lake sediments with an incubation experiment that lasted > 100 d. The carbon‐normalized potential CH4 (0.09–0.23 μmol mg C−1 d−1) and net CO2 (0.09–0.28 μmol mg C−1 d−1) production rates were not related to phytoplankton taxonomic affiliation (e.g., class, species), C : N ratio, or fatty acid composition of algal biomass. Methane or net CO2 production potentials did not increase with higher lipid content (10–30%); however, total fatty acid content had a weak correlation with CH4 production potential. In contrast to previous research, our results suggest that lipid content is of minor importance in determining methanogenesis rates from the biomass of multispecies phytoplankton communities settling on sediments. The decrease in CO2 concentration and the correlation between stable carbon isotope signatures of CH4 and molar ratio of CH4 and CO2 at the end of the experiment may indicate that importance of hydrogenotrophic methanogenesis, which uses CO2 when other substrates become limiting, increased during the long incubation.peerReviewe

Periphyton support for littoral secondary production in a highly humic boreal lake

Steep stratification and poor light penetration in highly humic lakes typically restrict oxygenated littoral areas to narrow lake margins. However, in some instances, surrounding floating vegetation mats can sustain highly productive periphyton and more diverse invertebrate communities than pelagic areas. Little is known about how these littoral food webs function or the extent to which the pelagic and littoral food webs are coupled. We added 15N-labeled ammonium nitrate to the floating moss mat surrounding the littoral zone of Mekkojärvi, a small highly humic and fishless lake in southern Finland. Our goal was to increase the δ15N values of periphyton to investigate the diets of littoral invertebrates and possible pelagic–littoral coupling in the lake. We divided the lake in 2 with a plastic curtain and added European Perch (Perca fluviatilis) to 1 basin while the other remained fishless. δ15N of periphyton and most littoral invertebrates increased well above the natural abundance levels. δ15N of pelagic Daphnia generally did not increase, except for a sudden and transitory increase in the basin where fish were introduced. Only one perch of the 33 recaptured following their introduction showed clearly increased δ15N. The lowest δ13C values were found in pelagic invertebrates. Most littoral invertebrates had values closer to those of periphyton, which clearly contributed significantly to the diets of most littoral invertebrate groups and was an important basal resource in the littoral food web. Chironomids and ephemeropterans had surprisingly low δ13C values, which may reflect inclusion in their diets of highly 13C-depleted methane-oxidizing bacteria, which were known to contribute to the diets of Daphnia in the lake. Our results indicate that the pelagic and littoral habitats are not strongly coupled in the absence of fish but that zooplanktivorous fish may increase coupling by driving zooplankton into the littoral zone to seek refuge from predation.peerReviewe

Influence of littoral periphyton on whole-lake metabolism relates to littoral vegetation in humic lakes

The role of littoral habitats in lake metabolism has been underrated, especially in humic lakes, based on an assumption of low benthic primary production (PP) due to low light penetration into water. This assumption has been challenged by recent recognition of littoral epiphyton dominance of whole-lake PP in a small highly humic lake and of epiphyton as an important basal food source for humic lake biota. However, as these studies have mostly concerned single lakes, there is a need to test their wider generality. We studied the whole-lake PP and community respiration (CR) in eight small humic lakes in southern Finland during July 2015 using 14C incorporation to measure pelagic PP and the changes in dissolved inorganic carbon in light and dark in situ incubations to measure CR and littoral PP by epiphyton. Changes in O2 concentration in both pelagic and littoral surface water were measured periodically from each lake and, additionally, continuously with a data logger from one lake during the study period. The results revealed that the littoral dominated whole-lake net primary production (NPP) in five of the eight lakes, which was supported by observed O2 supersaturation in the littoral surface water in most of the lakes. Calculated pelagic:littoral ratios by area correlated negatively with both littoral NPP and littoral contribution to whole-lake NPP. Moreover, there was a significant positive relationship between littoral proportion of whole-lake NPP and the fraction of lake surface area covered by littoral aquatic vegetation. This demonstrates that increased aquatic littoral vegetation cover increases the overall importance of the littoral to whole-lake PP in highly humic lakes. Littoral NPP also correlated strongly with littoral O2 saturation, and the continuously measured O2 revealed substantial temporal variation in O2 saturation, particularly in the littoral zone. Whole-lake gross primary production:community respiration (GPP:CR) ratios revealed that accounting for littoral metabolism produced a marked shift towards lake metabolic balance, although all the eight lakes remained net heterotrophic. This study emphasizes that littoral metabolism needs to be accounted for when estimating whole-lake C fluxes in all lakes, even in highly colored humic waters.peerReviewe

Accounting for littoral primary production by periphyton shifts a highly humic boreal lake towards net autotrophy

1. The prevailing view that many humic lakes are net heterotrophic is commonly based on pelagicmeasurements alone. Poor light conditions in humic lakes are assumed to constrain littoral primaryproduction (PP), such that the littoral zone has been considered an insignificant contributor towhole-lake PP. However, that assumption is based on models and inferences from pelagic processeswhich do not take littoral zone structure into account. Many lakes have an extensive ring of aquaticvegetation lying near the water surface, which provides substratum for epiphytic algae under well-illuminated conditions.2. We measured both pelagic and littoral PP and community respiration (CR) in Mekkoj€arvi, a small,highly humic headwater lake, in southern Finland throughout the open water season in 2012. Weused a14C incorporation technique to measure pelagic PP, while littoral PP was determined usingchanges in dissolved inorganic carbon concentrations during in situ incubations. We then estimatedwhole-lake PP and CR for both the littoral and pelagic zones.3. We found that littoral PP usually dominated whole-lake PP, contributing >90% to total PP insummer. A mean pelagic production to respiration ratio (GPP : CR) of 0.4 clearly indicated strongnet heterotrophy, but a value of 2.2 for the littoral zone indicated strong autotrophy. For bothhabitats combined, the mean whole lake GPP : CR was 1.6, indicating net autotrophy.4. We suggest that littoral PP can contribute significantly to whole-lake PP even in highly humiclakes, and that the littoral contribution can shift some lakes which have been considered netheterotrophic to metabolic balance or even net autotrophy during the ice-free period. Small humiclakes like Mekkoj€arvi with rings of littoral vegetation are widespread, especially in the boreal zone,and at least for similar lakes, evaluating lake metabolism from pelagic measurements alone is likelyto be misleading.peerReviewe