Freshwater Ecology

Climate change and freshwater ecosystems: impacts across multiple levels of organization

by Dan Perkins

Fresh waters are particularly vulnerable to climate change because (i) these fragmented habitats contain many species... more Fresh waters are particularly vulnerable to climate change because (i) these fragmented habitats contain many species with limited abilities to disperse as their environment changes, (ii) water temperature and availability are climate-dependent, and (iii) many systems are already exposed to a plethora of anthropogenic stressors that could amplify the effects of climate change. Most studies of potential climate change effects in aquatic systems have focused on individuals or species populations, rather than the higher levels of organisation (i.e. communities, food webs, ecosystems). We propose that an understanding of the connections between these different levels, which are all ultimately based on individuals, can be used to develop a more coherent theoretical framework, via an integration of metabolic scaling, foraging theory, and ecological stoichiometry, to predict the ecological consequences of climate change. For instance, individual basal metabolic rate scales with body size, which is also a strong driver of food web structure and dynamics, and temperature, which determines many ecosystem process rates and key aspects of foraging behaviour. In addition increasing atmospheric CO2 is predicted to alter molar the CNP ratios of detrital inputs, which could lead to profound shifts in the stoichiometry of elemental fluxes between consumers and resources at the base of the food web. The different components of climate change (temperature, hydrology and atmospheric composition) not only affect multiple levels of biological organisation, but they also interact with the numerous other stressors that fresh waters are exposed to, and future research needs to address these potentially important synergies.

Global change and food webs in running waters

by Dan Perkins

Riverine habitats are vulnerable to a host
of environmental stressors, many of which are
increasing in... more Riverine habitats are vulnerable to a host
of environmental stressors, many of which are
increasing in frequency and intensity across the
globe. Climate change is arguably the greatest threat
on the horizon, with serious implications for freshwater
food webs via alterations in thermal regimes,
resource quality and availability, and hydrology. This
will induce radical restructuring of many food webs,
by altering the identity of nodes, the strength and
patterning of interactions and consequently the
dynamics and architecture of the trophic network as
a whole. Although such effects are likely to be
apparent globally, they are predicted to be especially
rapid and dramatic in high altitude and latitude
ecosystems, which represent ‘sentinel systems’. The
complex and subtle connections between members of
a food web and potential synergistic interactions with
other environmental stressors can lead to seemingly
counterintuitive responses to perturbations that cannot
be predicted from the traditional focus of
studying individual species in isolation. In this
review, we highlight the need for developing new
network-based approaches to understand and predict
the consequences of global change in running waters.

Environmental Warming and Biodiversity–Ecosystem Functioning in Freshwater Microcosms

by Dan Perkins

Predicting the effects of global warming on biodiversity–ecosystem functioning
(B–EF) relationships is... more Predicting the effects of global warming on biodiversity–ecosystem functioning
(B–EF) relationships is complicated by potential interactions among abiotic
and biotic variables at multiple levels of organisation, including adaptation
within regional species populations and changes in community composition
and species richness. We investigated the capacity for assemblages of
three freshwater invertebrate consumer species (Asellus aquaticus, Nemoura
cinerea and Sericostoma personatum) from temperate (southern England)
and boreal (northern Sweden) regions to respond to expected shifts in temperature
and basal resources, and quantified rates of a key ecosystem process (leaflitter
decomposition). Predictions of assemblage metabolism, derived from
allometric-body size and temperature scaling relationships, accounted for
approximately 40%of the variance in decomposition rates. Assemblage species
composition accounted for further variance, but species richness per se had no
discernible effect. Regional differences were evident in rates of leaf decomposition
across temperature and resourcemanipulations, and in terms of the processing
efficiency of temperate and boreal consumers of the same species (i.e. after
correcting for body size and metabolic capacity), suggesting that intraspecific
variation among local populations could modulate B–EF effects. These differences
have implications for extrapolating how environmental warming and
other aspects of climate change (e.g. species range shifts) might affect important
drivers of ecosystem functioning over large biogeographical scales.

Consistent temperature dependence of respiration across ecosystems contrasting in thermal history

by Dan Perkins

Ecosystem respiration is a primary component of the carbon cycle and understanding the mechanisms that determine its... more Ecosystem respiration is a primary component of the carbon cycle and understanding the mechanisms that determine its temperature dependence will be important for predicting how rates of carbon efflux might respond to global warming. We used a rare model system, comprising a network of geothermally heated streams ranging in temperature from 5 °C to 25 °C, to explore the nature of the relationship between respiration and temperature. Using this ‘natural experiment’, we tested whether the natal thermal regime of stream communities influenced the temperature dependence of respiration in the absence of other potentially confounding variables. An empirical survey of 13 streams across the thermal gradient revealed that the temperature dependence of whole-stream respiration was equivalent to the average activation energy of the respiratory complex (0.6–0.7 eV). This observation was also consistent for in-situ benthic respiration. Laboratory experiments, incubating biofilms from four streams across the thermal gradient at a range of temperatures, revealed that the activation energy and Q10 of respiration were remarkably consistent across streams, despite marked differences in their thermal history and significant turnover in species composition. Furthermore, absolute rates of respiration at standardised temperature were also unrelated to ambient stream temperature, but strongly reflected differences in biofilm biomass. Together, our results suggest that the core biochemistry,which drives the kinetics of oxidative respiratory metabolism, may be well conserved among diverse taxa and environments, and that the intrinsic sensitivity of respiration to temperature is not influenced by ambient environmental temperature.

Metabolic rate of late Holocene freshwater fish: evidence from d13C values of otoliths

by William Patterson

Measuring submerged macrophyte standing crop in shallow rivers: a test of methodology

by Kevin Wood

Wood, K.A., Stillman, R.A., Clarke, R.T., Daunt, F. & O'Hare, M.T. (in press). Measuring submerged macrophyte standing crop in shallow rivers: a test of methodology. Aquatic Botany

High natural variability in space and time can make accurate measurements of macrophyte standing crop difficult.... more High natural variability in space and time can make accurate measurements of macrophyte standing crop difficult. Accuracy of such measurements could be improved by quantifying the relationships between the different methods of measuring standing crop which are available to researchers. In this study we compare cover, volume, and biomass as measures of standing crop. Percentage cover, percentage volume, and dry weight biomass estimates were positively related (R2(adj) range = 54–96%), but these relationships were significantly different between sites, and to a lesser extent between months. Biomass was related (R2(adj) range = 18–73%) to stand height. Furthermore, cover, volume and biomass indicated different seasonal trends in standing crop at the two study sites. Our study presents a suite of standing crop measures that exhibit close congruence, can be measured efficiently and minimise destructive sampling in situ, attributes which will aid in the design and implementation of future macrophyte measurement protocols for shallow rivers.

Niche segregation in two closely related species of stickleback along a physiological axis: Explaining multidecadal changes in fish distribution from iron-induced respiratory impairment.

by Wilco Verberk

Aquatic Ecology in press, doi: 10.1007/s10452-012-9395-y

Acute exposure to iron can be lethal to fish, but long-term sublethal impacts of iron require further study. Here we... more Acute exposure to iron can be lethal to fish, but long-term sublethal impacts of iron require further study. Here we investigated whether the spatial and temporal distribution (1967–2004) of two closely related species of stickleback matched the spatial distribution of iron concentrations in the groundwater.
We used the ‘Northern Peel region’, a historically iron-rich peat landscape in The Netherlands as a case study. This allowed us to test the hypothesis that niche segregation in two closely related species of stickleback occurred along a
physiological axis. Patterns in stickleback occurrence were strongly associated with spatial patterns in iron concentrations before 1979: iron-rich grid cells were avoided by three-spined stickleback (Gasterosteus aculeatus , Linnaeus 1758) and preferred by nine-spined stickleback (Pungitius pungitius , [Linnaeus, 1758]).
After 1979, the separation between both sticklebacks became weaker, corresponding to a decreased influence of local groundwater on stream water quality. The way both species changed their distribution in the field provides a strong indication that they differ in their susceptibility to iron-rich conditions. These observed differences correspond with differences in their respiration physiology, tolerance of poor oxygen conditions
and overall life-history strategy documented in the literature.
Our results exemplify how species can partition niche along a non-structural niche axis, such as sublethal iron-rich conditions. Other fish species may similarly segregate along concentration gradients in iron, while sublethal concentrations of other metals such as copper
may similarly impact fish via respiratory impairment and reduced aerobic scope.

Patterns of elevational beta diversity in micro- and macroorganisms

by jianjun wang

Wang, Jianjun. Soininen, Janne. Zhang, Yong. Wang, Beixin. Yang, Xiangdong. Shen, Ji. Patterns of elevational beta diversity in micro- and macroorganisms, Global Ecology and Biogeography, In press

Keywords
Bacteria, beta diversity, Beta-sim, biogeography, China, diatoms, elevational gradient, human activities, macroecology, macroinvertebrates, species turnover, streams

Aim  While ecologists have long been interested in diversity in mountain regions, elevational patterns in beta... more Aim  While ecologists have long been interested in diversity in mountain regions, elevational patterns in beta diversity are still rarely studied across different life forms ranging from micro- to macroorganisms. Also, it is not known whether the patterns in turnover among organism groups are affected by the degree to which the environment is modified by human activities.

Location  Laojun Mountain, Yunnan Province, China.

Methods  The beta diversity patterns of benthic microorganisms (i.e. diatoms and bacteria) and macroorganisms (i.e. macroinvertebrates) in a stony stream were simultaneously investigated between elevations of 1820 and 4050 m. Data were analysed by using a distance-based approach and variation partitioning based on canonical redundancy analysis.

Results  Analyses of community dissimilarities between adjacent sampling sites showed comparable small-scale beta diversity along the elevational gradient for the organism groups. However, bacteria clearly showed the lowest elevational turnover when analyses were conducted simultaneously for all pairwise sites. Variation partitioning indicated that species turnover was mostly related to environmental heterogeneity and spatial gradients including horizontal distance and elevation, while purely human impacts were shown to be less important.

Main conclusions  The elevational beta diversity at large scales was lower for bacteria than for eukaryotic microorganisms or macroorganisms, perhaps indicative of high dispersal ability and good adaptability of bacteria to harsh environmental conditions. However, the small-scale beta diversity did not differ among the groups. Elevation was the major driver for the turnover of eukaryotic organisms, while the turnover of bacteria was correlated more with environmental variation.

Relationships between nutrient enrichment and the phytoplankton community at an andean oligotrophic lake: a multivariate assessment

by Francisco E. Fonturbel

Co-authored with Gabriel J. Castaño-Villa
Ecología Aplicada 10(2): 75-81

Phytoplankton is one of the groups most sensitive to eutrophic conditions, and its disturbance has a potential... more Phytoplankton is one of the groups most sensitive to eutrophic conditions, and its disturbance has a potential negative bottom-up effect on lentic ecosystems. In this research we used a multivariate statistics approach (Canonical Correspondence Analysis with a Monte Carlo permutational test) to assess the relationships between nutrient enrichment and the phytoplankton community diversity. Four locations with different levels of eutrophication were sampled at the Bolivian sector of Lake Titikaka. Phytoplankton richness ranged from eight to 16 genera, varying significantly among sites, and its diversity was significantly correlated to nitrogen, phosphorus, and pH (Monte Carlo test, p < 0.05). Phosphorus was determined to be the limiting nutrient in the ecosystem. Community structure assessment showed a non-aggregated distribution of genera among study sites, with few abundant genera, and a BDG analysis pointed to a log-series distribution, suggesting a non–fully random niche partition. The methodological approach used here allowed a rapid assessment of the nutrient enrichment effect considering phytoplankton and nutrients as a whole, which is a more powerful approach than studying single-nutrient or single group relationships with univariate procedures.

Captive breeding of the endangered freshwater pearl mussel, Margaritifera margaritifera (L.)

by Carlos Garcia de Leaniz

Thomas,G.R.; Taylor,J.; Garcia de Leániz,C. (2010). Endangered Species Research 12: 1-9.

The freshwater pearl mussel (Margaritifera margaritifera L.) ranks among the most endangered aquatic invertebrates in... more The freshwater pearl mussel (Margaritifera margaritifera L.) ranks among the most endangered aquatic invertebrates in the world. It is also one the slowest growing and longest living known invertebrates, which make its conservation particularly problematic.  Yet, only recently have captive breeding programs for the freshwater pearl mussel begun. Here we review the four main in-situ and ex-situ strategies for the conservation of freshwater pearl mussels, namely (i) protecting and restoring critical mussel habitats, (ii) translocating adult mussels from healthy populations to unpopulated habitats, (iii) releasing artificially infected fish hosts, and (iv) and stocking of cultured juveniles. Because water pollution, habitat loss and collapse of fish hosts appear to have been influential in the decline of many freshwater mussel populations, in-situ conservation efforts should be primarily directed to tackling these problems. In this context, restoration of river connectivity, reduction of silt loads, and improvements in water quality at the watershed level are likely to yield the best results.  Ex-situ captive breeding could help safeguard critically endangered populations, as adult mussels can be kept relatively easily in flow-through or re-circulating systems, but rearing methods are still primitive and need to be optimised. Areas in particular need of research include the formulation of algal diets for maintaining broodstock and juveniles in captivity, and the development of optimal rearing and density conditions.  Likewise, the extent to which the species is host specific and can encyst and transform successfully on different salmonid species and/or populations is still unclear and needs to be further investigated, as it will largely dictate the success of reintroduction programs. Finally we note that much more information is needed on the fate of stocked juvenile mussels, and the degree of genetic structuring at various spatial scales before translocation programs can be recommended.  Captive breeding of freshwater mussels cannot be viewed as a substitute for habitat restoration. Keywords: freshwater pearl mussel, Margaritifera margaritifera, captive breeding, host specificity, juvenile culture.

Les stratégies de reproduction et les premiers stades de vie des poissons du fleuve Sinnamary (Guyane Française) : revue bibliographique

by Luis Tito de Morais

Ponton, D. & Tito de Morais, L. (1994). Les stratégies de reproduction et les premiers stades de vie des poissons du fleuve Sinnamary (Guyane Française) : revue bibliographique. Revue d’Hydrobiologie tropicale, 27, 441–465.

The Petit-Saut hydroelectric dam built by Électricité de France on the Sinnamary River, French Guiana, was closed on... more The Petit-Saut hydroelectric dam built by Électricité de France on the Sinnamary River, French Guiana, was closed on January 1994. It strongly modified different components of a system which had evolved for years wilhout human intervention. The fish species that will survive downstream from the dam or in the reservoir will adapt their reproductive strategies to the new environmenlal conditions. Their early life stages will also be affected. The aim of the present work was to present an extensive review of the data available in the scientific or aquaria hobby literature on the reproductive strategies and early stages of the genus represented in lhe River. Available data appear scarce, helerogeneous and biased towards taxa with fishery or hobby interests. However, these data provide a basic frame for research programs on the reproductive strategies and early stages of the fish in the Sinnamary River.

Zonation longitudinale des peuplements ichtyques avant mise en eau de la retenue de Petit-Saut (Guyane française)

by Luis Tito de Morais

Tito de Morais, L. & Lauzanne, L. (1994). Zonation longitudinale des peuplements ichtyques avant mise en eau de la retenue de Petit-Saut (Guyane française). Revue d’Hydrobiologie tropicale, 27, 467–483.

Effort de pêche et production exploitée dans les petites retenues du Nord de la Côte d'Ivoire

by Luis Tito de Morais

Da Costa, K.S., Traoré, K. & Tito de Morais, L. (1998). Effort de pêche et production exploitée dans les petites retenues du Nord de la Côte d’Ivoire. Bulletin Français de la Pêche et de la Pisciculture, 71, 65–78.

The fishing-effort and the fish production have been studied in four agropastoral dams in the north of the Ivory Coast... more The fishing-effort and the fish production have been studied in four agropastoral dams in the north of the Ivory Coast : Sambakaha (25 ha) and Gboyo (16 ha) in the region of Ferké, Kiémou (6 ha) and Korokara serpent (15 ha) in the region of North Korhogo.The studies have shown that the fishing is mostly done by Bozos from Mali. The fishermen density by dam, from 4 to 17 fishermen.km2, is well above FAO norms (2 fishermemkm2). The catch and the effort are highly variable from dam to dam. The fish production varies from 70 to 224 kg.ha-'.y-', which is in the same order of magnitude as the values registered in other small dams of the West African region. Yet, catch values are under FAO potential catches given for similar water bodies (between 100 and 750 kg-ha-y),indicating that there is a possibility of fish production increase. Total estimated catches in the small dams in the north of the Ivory Coast represent from 16 to 46 % of total freshwater catches. Catches include economically important fishes (Oreochromis niloticus, Heterotis niloticus, Chrysichthys spp, etc.) and marketing values indicate that the fishing activity is an important source o currency for the villages.

Caractérisation des peuplements ichtyologiques de deux fleuves côtiers ouest-africains soumis à des aménagements hydroagricoles et hydroélectriques

by Luis Tito de Morais

Da Costa, K.S., Gourène, G., Thys van den Audenaerde, D.F.E. & Tito de Morais, L. (2000). Caractérisation des peuplements ichtyologiques de deux fleuves côtiers ouest-africains soumis à des aménagements hydroagricoles et hydroélectriques. Vie et Milieu, 50, 65–77.

The relationship between fish species richness and water quality-related variables are studied in the Agnébi and Bia... more The relationship between fish species richness and water quality-related variables are studied in the Agnébi and Bia rivers. Both are coastal rivers in eastern Côte d’Ivoire submitted to the impacts of small (Agnébi) and large (sia) reservoirs. Up-stream fish species composition is similar in both rivers whereas it differs in the median and lower course, mainly due to the presence of the large dams of Ayamé I and II. The presence of the dams on the river Bia explains the longitudinal gradient of species richness observed. Such gradient is much less effective in the Agnébi river where the small reservoirs are located to the upper-most part of the affluents of the river. Water quality is similar in both rivers. Yet the ordination of the water related variables by principal component analysis indicates a difference between the upper zone in the Agnébi and the median zone in the Bia river. Main discriminant factors are conductivity, total dissolved solids and hardness of water to a less extent. There was a reduced seasonal rhythmic variability in some water quality related variables, mainly nitrogen compounds and water temperature. A Co-inertia analysis was performed to relate fish abundance and water quality variables. Fishes are distributed into five groups according to their affinities to specific water conditions

River eutrophication: Irrigated vs. non-irrigated agriculture through different spatial scales

by Laura Monteagudo

Co-authored with Jose Luis Moreno and Felix Picazo
Doi:10.1016/j.watres.2012.02.035

The main objective of this study was to determine how spatial scale may affect the results when relating land use to... more The main objective of this study was to determine how spatial scale may affect the results when relating land use to nutrient enrichment of rivers and, secondly, to investigate which agricultural practices are more responsible for river eutrophication in the study area. Agriculture was split into three subclasses (irrigated, non-irrigated and low-impact agriculture) which were correlated to stream nutrient concentration on four spatial scales: large scale (drainage area of total subcatchment and 100 m wide subcatchment corridors) and local scale (5 and 1 km radius buffers). Nitrate, ammonium and orthophosphate concentrations and land use composition (agriculture, urban and forest) were measured at 130 river reaches in south-central Spain during the 2001–2009 period. Results suggested that different spatial scales may lead to different conclusions. Spatial autocorrelation and the inadequate representation of some land uses produced unreal results on large scales. Conversely, local scales did not show data autocorrelation and agriculture subclasses were well represented. The local scale of 1 km buffer was the most appropriate to detect river eutrophication in central Spanish rivers, with irrigated cropland as the main cause of river pollution by nitrate. As regards river management, a threshold of 50% irrigated cropland within a 1 km radius buffer has been obtained using breakpoint regression analysis. This means that no more than 50% of irrigation croplands should be allowed near river banks in order to avoid river eutrophication. Finally, a methodological approach is proposed to choose the appropriate spatial scale when studying river eutrophication caused by diffuse pollution like agriculture.

Oxygen limits heat tolerance and drives heat hardening in the aquatic nymphs of the gill breathing damselfly Calopteryx virgo (Linnaeus, 1758).

by Wilco Verberk

Journal of Thermal Biology 37: 224-229.

Thermal limits in ectotherms may arise through a mismatch between oxygen supply and demand. At higher temperatures,... more Thermal limits in ectotherms may arise through a mismatch between oxygen supply and demand. At higher temperatures, the ability of their cardiac and ventilatory activities to supply oxygen becomes insufficient to meet their elevated oxygen demand. Consequently, higher levels of oxygen in the environment are predicted to enhance heat tolerance, while reductions in oxygen are expected to reduce thermal limits.

Here, we extend previous research on thermal limits and oxygen limitation in aquatic insect larvae and report critical upper temperatures in nymphs of the damselfly Calopteryx virgo (Linnaeus, 1758) exposed to different levels of oxygen. In addition, we explore the potential for a mechanistic link between oxygen conditions and thermal plasticity by exposing nymphs to two consecutive extreme heat events, using different levels of oxygen in the second exposure.

As predicted, hypoxia severely lowered critical temperatures. However, thermal tolerance was not improved under hyperoxia. Damselfly nymphs may be precluded to take advantage of hyperoxia if oxygen uptake and delivery is controlled locally near the caudal gills where most of the gas exchange occurs. The same asymmetrical effects of hypoxia and hyperoxia on heat tolerance in terrestrial insects could be similarly explained if tracheal opening and/or ventilation are not centrally regulated. Prior exposure to hypoxia enhanced critical thermal maxima in subsequent heat exposures and hyperoxia negated this hardening effect, indicating potential for oxygen-driven heat hardening in these aquatic insects.

Our study provides broad confirmation for oxygen limitation as a key mechanism setting upper thermal limits, pointing to a vital role for heat shock proteins in reducing oxygen requirements by slowing down rates of protein denaturation.

A conceptual model for freshwater mussel (family: Unionidae) remain preservation in zooarchaeological assemblages

by Steve Wolverton

Wolverton et al. 2010

Expectations for survival of vertebrate remains have been well developed and intensely studied in the... more Expectations for survival of vertebrate remains have been well developed and intensely studied in the zooarchaeological taphonomic literature. Taphonomic studies of shellfish remains focus on marine species and on variables relevant to remains from paleontological contexts (e.g., fossil marine beds). In this paper we develop a conceptual framework from which to derive expectations concerning the preservation of freshwater mussel remains focusing on two parameters, shell microstructure and shell shape. Shell size does not influence survivorship. Our model is validated through application to late Holocene zooarchaeological mussel assemblages from north Texas. Taphonomically robust species are important regarding zooarchaeological and biogeographic interpretations based on mussel paleofaunas, and fragile species are important indicators of whether or not an assemblage is well preserved.

Migratory benthic fishes may induce regime shifts in a tropical floodplain pond

by Roger Paulo Mormul

Freshwater Biology 2012 - accepted and awaiting online first version

Alternative states are a widely recorded phenomenon in shallow lakes, which may shift between turbid and clear water... more Alternative states are a widely recorded phenomenon in shallow lakes, which may shift between turbid and clear water conditions. Here we investigate whether such shifts in a tropical floodplain pond may be related to the effect of the flood pulse regime on the community structures of fish and macrophytes. Using a long-term data set, we demonstrate how benthic fish migration together with colonization by submerged plants affected the transition from a turbid to a macrophyte-dominated state in a floodplain pond without top-down control. In our study, the turbid state occurred mostly during low water phases (in contrast to the pattern in temperate regions) and was largely characterized by high values for the biomass of benthic fish, chlorophyll-a and total phosphorous. During the period of rising water levels, the migration of benthic fish out of the pond occurs simultaneously with the establishment of submerged plants, while water turbidity decreases along with phytoplankton and nutrient concentrations, inducing a clear-water phase. However, when submerged plants are absent and fish migration is low, a transient state is generated. We suggest that, different from the temperate ponds and shallow lakes, where the main driving mechanisms establishing alternative states are related to cascading effects via the food chain, in the tropical ponds and shallow lakes, resuspension of sediments by benthic fish may play the most significant role in establishing alternative states. However, the effect of the flood pulse regime plays an important role in the temporal dynamics of fish community structure by controlling benthic fish migration.

Water brownification may increase the invasibility of a submerged non-native macrophyte

by Roger Paulo Mormul

Biological Invasions 2012 - online first

Environmental conditions and human activities play a significant role in structuring novel assemblages of native and... more Environmental conditions and human activities play a significant role in structuring novel assemblages of native and non-native species. Ongoing and future climatic change may alter the performance of native and non-native species and their biotic interactions. In the northern hemisphere, expected climate changes include warmer temperatures and higher precipitation, the latter of which may increase dissolved organic carbon (humic) concentrations, resulting in browner water in aquatic ecosystems (brownification). We tested the effects of elevated temperature (3ºC) and brownification on native and non-native aquatic plant production in mesocosms over 56 days. Elodea canadensis, an aquatic invasive plant, had higher relative growth rate in terms of both length and weight, as well as higher weight to length ratio when grown in brown vs. clear water; E. canadensis did not respond to temperature treatments. Different functional groups of native producers (phytoplankton, periphyton, macrophytes) showed different relationships to temperature and brownification treatments, with the macrophyte response being most notable because it was opposite to that of E. canadensis. Native macrophytes decreased in biomass in browner water, where they represented about 40% of total biomass compared to 85% in clear water. In regression analyses, E. canadensis length RGR was best predicted only by water color treatment, but biomass RGR and biomass per length were inversely correlated with native macrophyte biomass, which is consistent with competition. Our results unexpectedly showed water brownification to have more influence on lake invasion than climate warming at this temperature regime. Two pathways emerged for climate to interact with biological invasions in structuring novel communities: directly, if non-native species respond positively to climate change, and indirectly through species interactions, for instance, because water brownification impairs growth of native macrophytes and reduces biotic resistance to invasion.

Selectivity and competitive interactions between two benthic invertebrate grazers (Asellus aquaticus and Potamopyrgus antipodarum): experimental study using 13C- & 15N-labelled diatoms

by Jonathan Grey

Freshwater Biology 2005

1. Tracer experiments with two diatoms labelled with 13C (Nitzschia palea) and 15N (Fragilaria crotonensis), were... more 1. Tracer experiments with two diatoms labelled with 13C (Nitzschia palea) and 15N (Fragilaria crotonensis), were conducted to investigate feeding selectivity and interspecific competition between the grazers Asellus aquaticus (Isopoda, Crustacea) and Potamopyrgus antipodarum (Hydrobiidae, Gastropoda). Conventional methods, such as cell counts and estimated biovolume, were used first to detect feeding preferences within the different grazer treatments. 2. The results revealed a significant decline in algal biovolume in all grazer treatments and no indications of active selectivity were observed. In contrast to conventional methods, measurements based on isotope signatures showed strong differences in tracer uptake, thus indicating different degrees of assimilation and digestion by the two grazers. 3. The selectivity index Q, which provides information on the uptake ratio of 13C to 15N, showed a significant time effect for both grazer species and a significant difference between single- and mixed-grazer treatments for P. antipodarum. Thus, this technique enabled the direct quantification of the uptake by grazers and, therefore, served as an ideal tool for the detection of passive selectivity. 4. Our results indicate a shift in feeding preferences related to between-species competition and a potential divergence of trophic niches when species coexist.