Convergence in floodplain pond communities indicates different pathways to community assembly

Disturbance can strongly influence ecosystems, yet much remains unknown about the relative importance of key processes (selection, drift, and dispersal) in the recovery of ecological communities following disturbance. We combined field surveys with a field experiment to elucidate mechanisms governing the recovery of aquatic macroinvertebrates in habitats of an alluvial floodplain following flood disturbance. We monitored macroinvertebrates in 24 natural parafluvial habitats over 60 days after a major flood, as well as the colonization of 24 newly-built ponds by macroinvertebrates over 45 days in the same floodplain. We examined the sources of environmental variation and their relative effects on aquatic assemblages using a combination of null models and Mantel tests. We also used a joint species distribution model to investigate the importance of primary metacommunity structuring processes during recovery: selection, dispersal, and drift. Contrary to expectations, we found that beta diversity actually decreased among natural habitats over time after the flood or the creation of the ponds, instead of increasing. This result was despite environmental predictors showing contrasting patterns for explaining community variation over time in the natural habitats compared with the experimental ponds. Flood heterogeneity across the floodplain and spatial scale differences between the experimental ponds and the natural habitats seemingly constrained the balance between deterministic and stochastic processes driving the ecological convergence of assemblages over time. While environmental selection was the dominant structuring process in both groups, biotic interactions also had a prominent influence on community assembly. These findings have profound implications towards understanding metacommunity structuring in riverscapes that includes common linkages between disturbance heterogeneity, spatial scale properties, and community composition.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00027-023-00957-9.

Beaver effects on macroinvertebrate assemblages in two streams with contrasting morphology

Beaver populations are increasing throughout Europe and especially in Switzerland. Beaver are major ecological engineers of fluvial systems, dramatically influencing river morphology, ecohydrology and, consequently, aquatic and terrestrial biota. This study compared macroinvertebrate assemblages and trophic structure at two beaver complexes with contrasting topography in Switzerland over an annual cycle. One complex (Marthalen) was in a low gradient open basin, whereas the other complex (Flaach) flowed through a higher gradient ravine-like basin. Both complexes were embedded in an overall agricultural landscape matrix. Water physico-chemistry differed between the two complexes with nitrogen, phosphorus, and DOC being higher at Marthalen than at Flaach. Both complexes showed strong seasonality in physico-chemistry, but retention of nutrients (N, P) was highest in summer and only at Marthalen. Both complexes also showed strong seasonality in macroinvertebrate assemblages, although assemblages differed substantially between complexes. At Marthalen, macroinvertebrate assemblages were predominantly lentic in character at 'pool' sites within the complex. At Flaach, lotic macroinvertebrate assemblages were common at most sites with some lentic taxa also being present. Dietary shifts based on carbon/nitrogen stable isotopes occurred in spring and summer among sites at both complexes (autochthonous resource use increasing over allochthonous resource use downstream), although being most pronounced at Marthalen. In contrast, similar resource use across sites occurred in winter within both complexes. Although beaver significantly influenced fluvial dynamics and macroinvertebrate assemblage structure at both complexes, this influence was most pronounced at Marthalen where beaver caused the system to become more wetland in character, e.g., via higher hydraulic residence time, than at Flaach. We conclude that topography can shape beaver effects on fluvial systems and resident biota.

Influence of ultraviolet radiation on UVR-absorbing compounds in freshwater algal biofilms and Scenedesmus vacuolatus cultures

There is considerable concern about increasing levels of solar ultraviolet radiation (UVR) and the potential harmful effects of UVR on living organisms. Microcosm studies were conducted with algal biofilms and Scenedesmus vacuolatus cultures to examine the effects of increased UVR on freshwater algae. Because algae have evolved various strategies to minimize the potential damage induced by UVR, the sensitivity of an UVR ratio was evaluated using absorbance characteristics of algal extracts to short- and long-term responses of algae to UVR exposure. The effects of UVR on the photosynthetic activity of periphyton also were determined by fluorometry. Results showed various patterns: (1) a short-term decrease in the UVR ratio between UVR-absorbing compounds and chlorophyll-a that reflect damage from UVR, and (2) a long-term increase in the UVR ratio as communities change their photosystems by increasing UVR absorbance capabilities, and thereby cell protection. Results include (1) validation of the UVR ratio, (2) allowing assessment of different UVR exposure-induced effects to algae, and (3) providing mechanistic information on cellular strategies used by algae to reduce UVR-induced damages.

Genetic differentiation of Baetis alpinus Pictet (Ephemeroptera: Baetidae) in fragmented alpine streams

The interpretation of low FST values as evidence of high levels of gene flow among habitat fragments may be confounded by population genetic structures that are indicative of historical rather than present-day levels of gene flow. We examined the genetic structure of 23 populations of Baetis alpinus (Insecta: Ephemeroptera) living in alpine streams fragmented by lakes ( approximately 10 000 years old), reservoirs ( approximately 100 years old), and in nonfragmented streams, to examine if lakes act as barriers to gene flow and to investigate the temporal resolution of allozyme markers. Estimates of gene flow indicated little or no genetic divergence along four nonfragmented reference streams and across two lakes and two reservoirs (FST=0.004-0.041), but marked differentiation across four lakes (FST=0.092-0.362) and across one reservoir that was a lake enlarged by a dam (FST=0.075). Differentiation was unrelated to distance between fragments, but occurred only in lakes found in valleys that have been ice-free throughout the Holocene. We suggest that standing water bodies act as barriers to gene flow in B. alpinus and that low FST values observed between fragments separated by reservoirs do not indicate high levels of gene flow but rather show that genetic differentiation was not detectable within the first 100-1000 years of habitat fragmentation.

Development of macroinvertebrate-based index for bioassessment of Idaho rivers

Theoretical constructs, such as the river continuum concept, predict that the composition of benthic fauna in rivers will be different from that of headwater streams. There exists a need to modify, for use on larger rivers, the bioassessment techniques commonly used on small streams. Using aquatic macroinvertebrates and the "reference condition" approach, we developed and tested a multimetric index for use on the rivers of Idaho. Reference sites were selected to represent the best current conditions (i.e., least impacted) among Idaho rivers. The index performed well in distinguishing reference sites from sites displaying some form of anthropogenic impairment. Individual metrics used in the index included: number of EPT taxa, total number of taxa, percent dominant taxon, percent Elmidae, and percent predators. The index we developed for Idaho rivers was essentially a modification of a framework designed for small streams, suggesting that techniques, including data analysis, currently used for streams can be adapted for use on larger rivers. Adapting these methods for use in rivers is primarily a matter of (1) selecting metrics relevant to the rivers of interest; (2) expanding the field sampling to encompass the greater habitat area and, potentially, heterogeneity of rivers; and (3) selecting an appropriate form of data analysis. The approach we describe here should be applicable to geographic regions other than Idaho.

Tumor necrosis factor alpha regulates CXC chemokine receptor expression and function

The alpha chemokine family is central to the participation of neutrophils in the acute inflammatory response. These substances interact with neutrophils through two cell surface receptors, CXCR-1 and CXCR-2 (formally known as IL-8R-1 and IL-8R-2). We investigated the possible regulatory effects of tumor necrosis factor alpha (TNFalpha) pretreatment on CXCR-1 and CXCR-2. To this end, we examined these receptors with flow cytometry, radioligand binding, Northern blot analyzes, calcium mobilization, and chemotaxis experiments on human neutrophils. In flow cytometry experiments, TNFalpha pretreatment substantially decreased cell surface CXCR-2 receptor levels but showed partial recovery at 120 min. On the other hand, CXCR-1 receptor levels had a sharp decline at 15 min and maintained that level to 120 min. Northern blot analyzes showed that mRNA levels of both IL-8 receptors were essentially unchanged after 45 min of TNFalpha pretreatment, but declined markedly following 2 h of pretreatment. Chemotaxis experiments on cells treated with TNFalpha for 5-120 min showed a substantial down-regulation of chemotaxis to IL-8 and GROalpha. This was noted to be much greater than the decline in cell surface receptors. Calcium mobilization experiments revealed minimal inhibition of the IL-8-induced increase in calcium after pretreatment with TNFalpha, but the response to NAP-2 was substantially inhibited. The data demonstrate differential regulation of the IL-8 receptor.

Loss of rubella antibody from immune globulin treated with kaolin

Sera and immune globulin (IG) preparations are customarily treated with kaolin before titration of their rubella hemagglutination-inhibiting (HI) antibody in order to rid them of nonspecific inhibitors of hemagglutination. The treatment was shown in this investigation to have no adverse effect on the antibody level of the sera but was found to remove considerable amounts of gamma-globulin from IG preparations. Evidence of this removal was obtained by serological tests, by spectrophotometric determination of protein concentration and by disc electrophoresis. In contrast to kaolin, heparin-manganese chloride (MnCl(2)) treatment of IG preparations had essentially no effect on the level of antibody globulin by all the criteria used. Heparin-MnCl(2)-treated IG lots were in these respects similar, if not identical, to their untreated counterparts. Since nonspecific inhibitors associated with the beta-lipoprotein fraction of serum are removed by the method employed to fractionate the IG samples, it seems unnecessary to treat the latter in any way for the HI test. No difficulty was encountered in this investigation with several untreated IG lots.