Habitat aging and degradation in terrestrialized floodplains: a need to rejuvenate processes for sustaining freshwater mussel populations

Progressive floodplain terrestrialization leads to the deterioration of floodplain ecosystems. Although the importance of hydrological connectivity between floodplain habitats and river channels for floodplain-dependent organisms has been emphasized, less attention has been paid to the temporal relationship between habitat degradation and aquatic organisms. In this study, we examined temporal changes in mussel population structures and the hydrological connectivity of pond-like floodplain water bodies (FWBs) in the terrestrialized floodplains of a lowland river in Japan. We tested the hypothesis that FWB habitats for mussels degrade over time, while newly formed FWBs contribute to providing recruitment sites for mussels. The habitat age for 53 FWBs was determined as of 2018, using historical aerial photographs. Inundation frequency, mussel abundance, and size structures (shell length) were also investigated in the FWBs in 2007 and 2018. Mussel abundance decreased with FWB aging (6–36 years), which coincided with a decrease in inundation frequency. In most existing FWBs, mussel abundance in all size classes decreased over the decade (2007–2018). In contrast, small individuals occurred in the newly formed FWBs at age 6, and in the existing FWBs where inundation frequency slightly increased over the decade (2007–2018), such FWBs were rare. In conclusion, the “young” and “rejuvenated” FWBs were found to be critically important for the recruitment and persistence of mussel populations. This suggests that the floodplain management strategy of “cyclic floodplain rejuvenation” could be applicable for mussel conservation in terrestrialized floodplains of lowland rivers.     

Distribution of chromobacteria in a lowland river

A selective medium was used to estimate the numbers of purple pigmented bacteria in the River Wey. Chromobacteria were never isolated from the springs at the source of the river, but they appeared within a short distance and their numbers generally increased over the first 30 km. Counts were always low but were much higher during the winter than during the summer.Records of water temperature and turbidity, as well as daily rainfall, were maintained during the sampling period. An unweighted multiple regression analysis of some of the results has shown that much of the variability in the counts of chromobacteria was correlated with turbidity (r=0.67,P<0.001). After turbidity was allowed for, a significant inverse correlation with temperature (r=0.60,P < 0.001) became apparent and these two factors together accounted for 64.7% of the variation in counts. Allowing for these two factors revealed a significant correlation with rainfall during the previous 4 days (r=0.27,P<0.05), and the three factors together accounted for 67.2% of the variability in counts.In an attempt to discover the source of chromobacteria, samples of silt, soil, and storm overflow effluents were studied. In general, chromobacteria were present in highest numbers in soil or silt containing substantial quantities of organic material.     

Connectivity and biocomplexity in waterbodies of riverine floodplains

1. In river corridors, water plays a key role in connecting various landscape patches. This `hydrological connectivity' operates on the four dimensions of fluvial hydrosystems: longitudinal, lateral, vertical, and temporal. The present review focuses on: (1) lateral connectivity that links the main course of a river with floodplain waterbodies; and (2) vertical connectivity, the exchanges between the surface and groundwater via infiltration into the alluvial aquifer and exfiltration of phreatic water from the hillslope aquifer.
2. The biocomplexity of fluvial hydrosystems results from interactions between processes operating at various spatial and temporal scales. Differences in the nature and intensity of hydrological connectivity contribute to the spatial heterogeneity of riverine floodplains, which results in high alpha, beta and gamma diversity. Differences in connectivity also provide complementary habitats that are required for the parts of life cycles and life-cycles of some species. Hydrological connectivity also produces antagonistic effects, even within the same waterbody.
3. Two temporal scales are distinguished in connectivity dynamics. River level fluctuations within years lead to a pulsing connectivity that drives the functioning of floodplain ecosystems, namely the exchange of organic matter and inorganic nutrients and the shift between production and transport phases. On the scale of decades to centuries, the interactions between various processes increase the biocomplexity of floodplains; for example, river dynamics, which create highly connected waterbodies, compensate for succession that tends towards disconnection. Alternatively, river-bed incision leads to the reduction of fluvial dynamics and to the disconnection of waterbodies, although river incision may increase vertical connectivity where waterbodies are supplied by the hillslope aquifer.     

Restoration strategies for river floodplains along large lowland rivers in Europe

1. Most temperate rivers are heavily regulated and characterised by incised channels, aggradated floodplains and modified hydroperiods. As a consequence, former extensive aquatic/terrestrial transition zones lack most of their basic ecological functions.
2. Along large rivers in Europe and North America, various floodplain restoration or rehabilitation projects have been planned or realised in recent years. However, restoration ecology is still in its infancy and the literature pertinent to river restoration is rather fragmented. (Semi-) aquatic components of floodplains, including secondary channels, disconnected and temporary waters as well as marshes, have received little attention, despite their significant contribution to biological diversity.
3. Many rehabilitation projects were planned or realised without prior knowledge of their potential for success or failure, although, these projects greatly contributed to our present understanding of river–floodplain systems.
4. River rehabilitation benefits from a consideration of river ecosystem concepts in quantitative terms, comparison with reference conditions, historical or others, and the establishment of interdisciplinary partnerships.
5. We present examples from two large European rivers, the Danube and the Rhine, in which the role of aquatic connectivity has been extensively studied. The Danube delta with its diversity of floodplain lakes across an immense transversal gradient (up to 10 km) serves as a reference system for restoration projects along lowland sections of large rivers such as the Rhine in the Netherlands.     

Terrestrial invertebrates inhabiting lowland river floodplains of Central Amazonia and Central Europe: a review

1. Amazonian terrestrial invertebrates produce high population densities during favourable periods and may suffer a drastic decrease during occasional floods and droughts. However, the monomodal, predictable flood pulse of the larger Amazonian rivers favours the development of morphological (respiratory organs, wing‐dimorphism), phenological (synchronization of life cycles, univoltine mode of life), physiological (flooding ability, gonad dormancy, alternating number of developmental stages), and behavioural adaptations (migration, temporal diving) with numerous interactions. 2. In lowlands of Central Europe, the flood pulse of large rivers is less predictable than in Central Amazonia and is superimposed by the seasonal light/temperature pulse (summer/winter regime). Some terrestrial invertebrates show physiological resistance against inundation or drought, phenologies fitting the normal annual rhythm of water level fluctuation (quiescence or diapause of eggs or adult invertebrates), high dispersal ability and migration. However, most species survive simply using a `risk strategy', combining high reproduction rates, dispersal and reimmigration following catastrophic events. 3. The diversity of species in terrestrial invertebrates is lower in lowland riverine ecosystems of Central Amazonia and Central Europe compared with the respective uplands because of flood stress in these systems. However, floodplains in Central Amazonia possess a greater number of endemic species in comparison with Central European floodplains because of long periods of fairly stable climatic conditions in comparison with large palaeoclimatic changes in Central Europe.     

Distribution and hydraulic significance of large woody debris in a lowland Australian river

The line-intersect technique was used to measure the loading of large woody debris in a 1.8 km reach of the Thomson River, Victoria (catchment area of 3540 km²). A debris census (measuring every item present) was done over 0.775 km of this reach. The transect technique over-estimated the actual loading revealed by the census. The loading of debris 0.01 m in diameter for the total 1.8 km reach was 0.0172 m³ m^–2, which is higher than that measured in many headwater streams in other parts of the world. The volume loading of debris measured from low level aerial photographs was only 4.8% of the value estimated by the line-intersect technique. The line-intersect estimates were biased due to non-random orientation of debris in the stream (causing estimated errors of +8% for volume loading and +16% for surface area loading). It is recommended that to avoid this problem, when using the line-intersect transect technique in lowland rivers, each line should comprise at least two obliquely-angled transects across the channel. The mean item of debris (0.1 m in diameter) had a trunk basal diameter of 0.45 m, a length of 7.4 m, and volume of 0.7 m³. The riparian trees and the in-channel debris were of similar dimensions. The debris tended to be close to the bed and banks and was oriented downstream by the flow at a median angle of 27°. Because of this orientation, most debris had a small projected cross-sectional area, with the median value being only 1 m². Thus, the blockage ratio (proportion of projected area of debris to channel cross-sectional area) was also low, ranging from 0.0002 to 0.1, with a median value of 0.004. The average item of debris, which occupied only 0.4% of the cross-section, would have minimal influence on banktop flow hydraulics, but the largest items, which occupied around 10%, could be significant. Judicious re-introduction of debris into previously cleared rivers is unlikely to result in a large loss of conveyance, or a detectable increase in flooding frequency.     

The effects of endangered freshwater pearl mussels on channel morphology and flow in a low-gradient sandy river

Hydrobiologia - Conservation of ecosystem engineers, which modulates the surrounding habitat by causing physical state changes in biotic or abiotic materials, is important for maintaining the...     

Biogeography of sulfate-reducing prokaryotes in river floodplains

In this study, a large-scale field survey was conducted to describe the biogeography of sulfate-reducing prokaryotes (SRPs) in river floodplains. Fingerprints obtained with three methods, i.e. 16S rRNA gene-based oligonucleotide microarray, dsrB-based denaturing gradient gel electrophoresis (DGGE) and polar lipid-derived fatty acid (PLFA) analyses, were used as a proxy to describe the SRPs community diversity. Each set of profiles was subjected to a combined multivariate/correlation analysis in order to compare SRP community profiles and to highlight the environmental variables influencing the SRPs distribution along environmental gradients. Floodplain soils harbored distinct SRP communities displaying biogeographic patterns. Nearly all profiles from the tidal sites consistently separated from the nontidal sites, independently from the screening method and the multivariate statistics used. The distribution of the microarray/DGGE/PLFA-based fingerprints in the principal component plots could be correlated to eight soil variables, i.e. soil organic matter, total nitrogen, total phosphorous and total potassium, and extractable ammonium, nitrate, phosphate and sulfate, as well as seven pore water variables, i.e. phosphate, sulfate, sulfide, chloride, sodium, potassium and magnesium ions. Indication of a salinity- and plant nutrient-dependent distribution of SRPs related to Desulfosarcina, Desulfomonile and Desulfobacter was suggested by microarray, DGGE and PLFA analyses.     

Macroinvertebrate distribution and production on the floodplains of two lowland headwater streams

SUMMARY.

• 1 Spatial and temporal distribution, abundance and production of the floodplain macroinvertebrate communities of two low-gradient headwater streams in Virginia, U.S.A., were studied over 1 year.
• 2 Spatial and temporal distribution patterns of invertebrates were affected by inundation patterns. Numbers, biomass and production were higher in low than high floodplain areas and on a floodplain continuously inundated over 9 months compared with a floodplain completely flooded only occasionally during storms.
• 3 The predominant species in terms of numbers and production on both floodplains was the harpacticoid copepod Attheyella illinoisensis Forbes. Other species with relatively high production were the sphaeriid clam Pisidium sp., the leptophlebiid mayfly Leptophlebia sp., the isopod Caecidotea racovitzai (Williams), and the chironomids Paratendipes sp. and Polypedilum spp.
• 4 Annual invertebrate production (dry weight) on the floodplain continuously inundated over 9 months was 6.1 g m^?2; production on the periodically inundated floodplain was 1.7 g m^?2. Collector-gatherers accounted for 58–61%, and predators 19–25%, of the production.
• 5 Annual invertebrate production on the floodplains, calculated on the basis of a linear metre of channel length, was 84–490 gm^?1, or about one to two orders of magnitude greater than probable production in the channels, suggesting the significance of floodplain invertebrates to stream system trophic dynamics.

     

Reproductive phenology of river and lake populations of freshwater mussels (Unionida: Hyriidae) in the River Murray

Gonadal cycles are described for Alathyria jacksoni and Velesunio ambiguus from the River Murray at Overland Corner (near Lock 3) and for V. ambiguus at Point Sturt, Lake Alexandrina. The study included periods of low flow (1982), minor flooding (1983) and intermediate flow (1986). In males, gametogenesis continued year-round with fertilisation in late winter. Female A. jacksoni brooded embryos in spring and released glochidia during spring and summer, whereas in female V. ambiguus, glochidia were present most of the year, with peak releases in spring and summer. In both species, the proportions of actively reproducing individuals varied. In the Murray, populations were influenced by variations in discharge. At Point Sturt, gonadal cycles in V. ambiguus were irregular, influenced by variable lake levels. Although both species are essentially gonochoristic, there were hermaphrodites in river and lake populations, and a female bias in the population at Point Sturt. Infections by parasitic trematodes at Point Sturt were rare, but occurred in 10% of A. jacksoni and 35% of V. ambiguus at Overland Corner. The river populations were also affected by gill damage due to unionicolid mites. The disparities in infections between river and lake mussels are unexplained, but did affect their reproductive activity.     

Filter and deposit: a potential role of freshwater mussels in ecosystem functioning associated with enhanced macroinvertebrate assemblage structure in a Neotropical river

Hydrobiologia - Mussels provide important ecological functions in freshwater ecosystems but the associations between Amazonian mussels, macroinvertebrate assemblage and habitat quality remain...     

Abundance and microhabitats of freshwater sponges (Spongillidae) in a Danubean floodplain in Austria

1. This study examined the abundance and distribution of freshwater sponges (Spongillidae) at 32 sites in a floodplain on the Danube within the ‘Donau‐Auen’ National Park east of Vienna, Austria. Ranked from abundant to rare, the species inventory comprised Ephydatia fluviatilis, Spongilla lacustris, Ephydatia mülleri, Eunapius fragilis and Trochospongilla horrida. 2. The presence of hard substratum was essential for the growth of sponges. Timber stands near the water and drifting dead wood increased the abundance of E. fragilis, E. fluviatilis and E. mülleri, whereas stony substrata were important for S. lacustris. A small fraction of E. fluviatilis was collected from macrophytes (Phragmites). 3. Based on the area colonised, the abundance of S. lacustris, E. fragilis and E. fluviatilis was highest (94.2–100% of the total) in floodplain waters where hydrological connectivity with the Danube was low (0–6 days year⁻¹), whereas E. mülleri and T. horrida made up 20.3–35.9% of the total at sites connected for up to 179 days year⁻¹. Moreover, the area colonised by T. horrida at a current velocity >0.20 m s⁻¹ was larger than in the remaining species. Sites with E. mülleri and T. horrida had a higher silicon concentration (0.9 mg L⁻¹) than sites where the remaining three species were collected (0.4–0.6 mg L⁻¹). 4. In most species, the length of macroscleres (the larger spicules) was positively correlated with conductivity and negatively with pH. With respect to aberrant macroscleres, hooks were observed most frequently, whereas the proportion of centrotylotes (ie with the one on more globular swellings along the spicule) was lowest. 5. Freshwater sponges have a great deal of potential as bioindicators and restoration measures that improve floodplain connectivity will favour rare species, such as E. mülleri and T. horrida, while impairing others (e.g. E. fragilis, S. lacustris and E. fluviatilis).     

Phytoplankton and zooplankton development in a lowland, temperate river

The longitudinal and seasonal patterns of plankton developmentwere examined over 2 years in a lowland, temperate river: theRideau River (Ontario, Canada). Following an initial decreasein phytoplankton and zooplankton biomass as water flowed fromthe headwaters into the Rideau River proper, there was an increasein chlorophyll a (chl a) and zooplankton biomass with downstreamtravel. At approximately river km 60, both phytoplankton andzooplankton reached their maximum biomass of 27 µg l^–1(chl a) and 470 µg l^–1 (dry mass), respectively.Downstream of river km 60, the biomass of both planktonic communitiesdeclined significantly despite increasing nutrient concentrationsand favorable light conditions. These downstream declines maybe due to the feeding activity of the exotic zebra mussel (Dreissenapolymorpha) which was at high density in downstream reaches(>1000 individuals m^–2). There was no evidence forlongitudinal phasing of phytoplankton and zooplankton, as increasesand decreases in chl a and zooplankton biomass appeared to coincide.Overall, chl a was best predicted by total phosphorus (R²=0.43),whereas zooplankton biomass was best predicted by chl a (R²=0.20).There was no evidence for significant grazing effects of zooplanktonon phytoplankton biomass.     

Annual movement and migration of adult pikeperch in a lowland river

The downstream migration of 46–66 cm radio-tagged adult pikeperch Stizostedion lucioperca , in the River Gudenaa, Denmark, from late September to late December, coincided with a reduction in the frequent occurrence of prey fish in the tagging area, and hence was interpreted as a feeding migration. The upstream migration from March to mid-April in spring took place prior to spawning which occurred from late April to June.     

Spatial patterns of invertebrate species richness in a river: the relationship between riffles and microhabitats

This study describes the pattern of invertebrate species richness in a river reach with large differences in habitat complexity at two, hierarchically nested, spatial scales. The aim was to determine whether the mass effect was likely to be increasing invertebrate species richness in epilithic microhabitats in this river. The mass effect is the process by which the species richness of a patch is increased when it acts as a ‘sink’ for species generated by ‘source’ patches. Microhabitat patch types in Mountain River, Tasmania, were distinguished on the basis of physical structure and orientation on the river bed. They were nested within two types of riffle with contrasting structural complexity: bedrock and boulder-cobble riffles. It was hypothesized that microhabitats with high species richness would act as source patches, contributing species to other microhabitats (sinks) and thereby increasing their species richness. Microhabitat sampling was carried out in four consecutive seasons and rarefaction was used to estimate riffle-scale species richness. Analysis of variance ( ANOVA ) was used to compare the identical microhabitats present in the contrasting riffle types, to detect evidence of the mass effect in either riffle type. The more structurally complex boulder-cobble riffles had higher species richness than did bedrock riffles. Amongst the microhabitats, the spaces beneath the cobbles had the most species. Microhabitats accounted for a higher percentage of the variation in species richness than did differences between riffles of the same type. No evidence was found for the operation of the mass effect in either riffle type. The majority of species found only in boulder-cobble riffles were unique to the beneath-cobble microhabitat and appeared to be unable to colonize other microhabitats, even as transients. In Mountain River, small-scale habitat characteristics appeared to be more important than larger-scale effects in determining microhabitat species richness.     

Efficacy of a nature-like bypass channel in a Portuguese lowland river

Throughout Europe in the last decade there has been a steady shift away from more technical fish pass designs to more nature‐like passes, such as nature‐like bypass channels. Upstream fish passage in a nature‐like bypass channel was investigated in a lowland river, the Lima River, for 117 days from March 2000 to May 2002. Fish passage was recorded using an automatic video recording system. Electrofishing samples within the bypass and below the weir were compared with species abundance found on the tape recordings. More than 7500 individuals of eight species passed through the bypass channel. Species composition was dominated by striped mullet (65.3%) and potamodromous species (34.3%), which used the bypass mainly at night. Of the environmental variables considered, bypass discharge explained most of the variation in the number of cyprinids, whereas water temperature was more important for diadromous species. Comparing species composition below the weir using passage recordings provided a useful tool to assess species efficacy of the bypass, although biological requirements should also be taken into account. This study proved the efficacy of the bypass for passage of almost all occurring species and life stages and also for providing suitable habitat for fish fauna, highlighting the use of these facilities for river restoration schemes.     

Strong observer effect on tree microhabitats inventories: A case study in a French lowland forest

Validating biodiversity indicators requires an analysis of their applicability, their range of validity and their degree of correlation with the biodiversity they are supposed to represent. In this process, assessing the magnitude of observer effect is an essential step, especially if non-specialist observers are involved. Tree microhabitats – woodpecker cavities, cracks and bark characteristics – are reputed to be easily detected by non-specialists as microhabitat observation does not require prior forestry or ecology knowledge. We therefore quantified the probabilities of true and false positive detections made by observers during inventories.Within two 0.5 ha plots in a forest reserve that has not been harvested for at least 150 years, 14 observers with various backgrounds visually inventoried microhabitats on 106 oak (Quercus petraea and Quercus robur) and beech (Fagus sylvatica) trees. We used parametric and Bayesian statistics to compare these observers’ recorded observations with results from an independent census.The mean number of microhabitats per tree varied widely among observers – from 1.4 to over 3. Only five observers reported a mean number of microhabitats per tree that was statistically equivalent to the reference census. The probability of true detection also varied among observers for each microhabitat (from to 0 to 1) as did the probability of false positive detection (from 0 to 0.7). These results show that microhabitat inventories are particularly prone to observer effects.Such strong observer effects weaken the usefulness of microhabitats as biodiversity indicators. If microhabitat inventories are to be developed, we recommend controlling for observer effects by (i) defining standard operating procedures and multiplying the number of observer training sessions and of consensual standardization censuses; (ii) using pairs of observers to record microhabitats whenever possible (though the efficiency of this method remains to be tested); (iii) planning fieldwork so that the factors of interest are not confused with observer effects; and (iv) integrating observer profiles into the statistical models used to analyze the data.     

Global diversity of freshwater mussels (Mollusca, Bivalvia) in freshwater

The term freshwater bivalve is very inclusive and not very informative. There are representatives of at least 19 families that have at least one representative living in freshwater. This suggests at least 14 different invasions of freshwater. At least nine families have small to large radiations in the freshwater environment: Corbiculidae, Sphaeriidae, Dreissenidae, and the unioniforme families: Hyriidae, Margaritiferidae, Unionidae, Etheriidae, Iridinidae, and Mycetopodidae. The unioniforme families contain at least 180 genera and about 800 species. This order is characterized by the unique parasitic larval stage on the gills, fins or the body of a particular host fish. This order of freshwater bivalves is suffering a very high rate of extinction, with about 37 species considered presumed extinct in North America alone. The level of endangerment and extinction facing these animals is primarily the result of habitat destruction or modification. Guest editors: E. V. Balian, C. Lévêque, H. Segers & K. Martens Freshwater Animal Diversity Assessment     

Development of European bitterling in the gills of freshwater mussels

The development of bitterling embryos within the unique environment of a freshwater mussel's gills requires a departure from typical cyprinid embryological development. Eggs are large (2·6×1·7 mm), illustrating the low risk of predation, and elliptical; a response to unionid gill morphology and a way of increasing the transfer rates of respiratory and excretory products to and from the tissues. The yolk sac develops elongated lateral processes during early ontogeny; these secure the embryo into the host's interlamellar space. Once the larva is capable of movement (8·2 mm) the lateral processes are lost and the larva becomes less dependent of the host's gills for the provision of oxygen. Hatching (3·3 mm) and pigmentation of the blood (6·4 mm) occur relatively early; this may increase the rate of oxygen supply to the tissues. Pigmentation of the eyes and appearance of the melanophores occurs relatively late (7·4 mm and 7·9 mm, respectively); embryos are not required to detect or avoid predators. Bitterling larvae generally emerge from the host once the yolk sac has been consumed (10·5 mm); this may mark a change in respiratory and nutritional requirements.