A. bij de Vaate

Zebra mussels (Dreissena polymorpha): a new perspective for water quality management

In the evaluation of the role of lake restoration programmesin situ measurements of the filtration rate of the freshwater musselDreissena polymorpha have been performed in Lake Wolderwijd, The Netherlands. The filtration rate mainly depends on the suspended matter content of the water, and shows an inverse exponential relationship with this factor. The filtration activity is temperature indifferent between approx. 5 and 20 °C. At low temperatures the filtration rate drops abruptly, at high temperatures gradual inhibition occurs. The filtration rate shows a sigmoidal relation with the length of the mussel. The largestD. polymorpha have a diminished filtration rate compared to animals of smaller size. This might be a degenerative feature of the oldest mussels. In Lake Wolderwijd a population density of 675 per m2 is required to compensate phytoplankton growth by grazing. Manipulation of the size of the population can be executed by adding suitable substrates for the mussel.

Trophic relationships in the Rhine food web during invasion and after establishment of the Ponto-Caspian invader Dikerogammarus villosus

The Rhine ecosystem is highly influenced by anthropogenic stresses from pollution, intensive shipping and increased connectivity with other large European rivers. Canalization of the Rhine resulted in a reduction of heterogeneity to two main biotopes: sandy streambeds and riverbanks consisting of groyne stones. Both biotopes are heavily subjected to biological invasions, affecting the rivers food web structure. The Ponto-Caspian amphipods, Chelicorophium curvispinum and Dikerogammarus villosus, have exerted the highest impact on this food web. The filterfeeding C. curvispinum dominated the Rhine food web on the stones in 1998, swamping the stone substrata with mud. However, in 2001 it decreased in numbers, most likely due to top-down regulation caused by increased parasitic and predatory pressure of other more recently invaded Ponto-Caspian species. D. villosus showed a fast population increase after its invasion and particularly influenced the macroinvertebrate community on the stones by predaceous omnivory. This species seemed to have maintained its predatory level after its population established. Effects of these mass invaders on the macroinvertebrate community of sandy streambeds in the Rhine are unclear. Here, low densities of macroinvertebrates were observed with the Asiatic clam, Corbicula fluminea, as most abundant species. Stable isotope values of food webs from the stones and sand in 2001 were similar. Aquatic macrophytes are nearly absent and the food web is fuelled by phytoplankton and particulate organic matter, originating from riparian vegetation as indicated by similar δ13C values. Omnivores, filter-, deposit-, and detritus-feeders are the primary and secondary macroinvertebrate consumers and function as keystone species in transferring energy to higher trophic levels. Invaders comprise 90% of the macroinvertebrate numbers, and can be considered ecosystem engineers determining the functional diversity and food web structure of the Rhine by either bottom-up or top-down regulation.

Ecological aspects, explosive range extension and impact of a mass invader, Corophium curvispinum Sars, 1895 (Crustacea: Amphipoda), in the Lower Rhine (The Netherlands)

A few years after it invaded, the amphipod Corophium curvispinum Sars appeared to be the most numerous macroinvertebrate species in the River Rhine. From 1987 to 1991 the densities of this species on the stones of groins in the Lower Rhine at a depth of 0.5 m increased from 2 to 200000 specimens per m2. In the Lower Rhine and its branches the densities of C. curvispinum increased with increasing current velocities and with increasing water depths. So far, a maximum population density of 750000 specimens per m2 has been found in the Lower Rhine, which is many times the densities recorded elsewhere. Population parameters, densities and distribution of C. curvispinum were studied in the Lower Rhine and its branches, using artificial substrates and sampling stones from groins. The success of this immigrant is related to its competitive strategy, which shows several aspects of a r-strategy. In addition, the heavily eutrophicated Lower Rhine provides abundant food (phytoplankton, suspended organic matter) for this opportunistic filter-feeder. The increased salinity and water temperatures in the Lower Rhine resulting from industrial discharges have contributed to the current success of this southern species originating in brackish waters. The very high densities of C. curvispinum might have an enormous impact on the river ecosystem by changing food webs.A few years after it invaded, the amphipod Corophium curvispinum Sars appeared to be the most numerous macroinvertebrate species in the River Rhine. From 1987 to 1991 the densities of this species on the stones of groins in the Lower Rhine at a depth of 0.5 m increased from 2 to 200000 specimens per m2. In the Lower Rhine and its branches the densities of C. curvispinum increased with increasing current velocities and with increasing water depths. So far, a maximum population density of 750000 specimens per m2 has been found in the Lower Rhine, which is many times the densities recorded elsewhere. Population parameters, densities and distribution of C. curvispinum were studied in the Lower Rhine and its branches, using artificial substrates and sampling stones from groins. The success of this immigrant is related to its competitive strategy, which shows several aspects of a r-strategy. In addition, the heavily eutrophicated Lower Rhine provides abundant food (phytoplankton, suspended organic matter) for this opportunistic filter-feeder. The increased salinity and water temperatures in the Lower Rhine resulting from industrial discharges have contributed to the current success of this southern species originating in brackish waters. The very high densities of C. curvispinum might have an enormous impact on the river ecosystem by changing food webs.

Bioprocessing of polluted suspended matter from the water column by the zebra mussel (Dreissena polymorpha Pallas)

Measurements of pseudofaeces production of Dreissena polymorpha were carried out with the aim of developing a biological filter at the freshwater inlet of Lake Volkerak-Zoommeer, the Netherlands. Bioprocessing of polluted suspended matter by suspended cultures of D. polymorpha occurs by filtration and sedimentation of the suspended matter as pseudofaeces. The measurements were conducted under semi-natural conditions.Pseudofaeces production was mainly determined by the dry matter content of the water; the relation is linear. Temperature was of much less importance. This agrees with earlier investigations of the filtration rate of D. polymorpha. Even at the lowest temperature measured during the experiments (6.4 °C) no large decrease in activity was observed. The relation of pseudofaeces production with shell length was sigmoid in shape, in accordance with measurements of the filtration rate.The pseudofaeces produced was slightly more polluted than suspended matter, partly due to a finer grainsize. D. polymorpha from Lake IJsselmeer exposed for 217 days at the intended location of the filter showed bioaccumulation of toxicants, especially organic pollutants and Polycyclic Aromatic Hydrocarbons (up to 10-fold accumulation). The required number of D. polymorpha in the biological filter to treat the waterflow of 14 m3 s−1 entering Lake Volkerak-Zoommeer is 1.24 * 109. The purification efficiency of the filter, the reduction of the amount of toxicants, partly depends on the binding properties of the toxicants and is highest for those strongly bound to suspended matter.

Development of an Index of Trophic Completeness for benthic macroinvertebrate communities in flowing waters

The analysis of the trophic structure of benthic macroinvertebrate communities can be used in biological assessments of the condition of river ecosystems. Using the trophic, or functional approach, the Index of Trophic Completeness (ITC) was developed. The goal was to overcome the problems and drawbacks of using conventional diversity or biotic indices in biological assessments of rivers, such as limitation to distinct geographical regions or focus on species richness without regard for ecosystem functioning. Following an extensive review of the literature on the trophic characteristics of benthic macroinvertebrates, a large number of species (±300) were characterized according to a number of trophic criteria: plant:animal ratio in the diet, feeding mechanism, food size, food acquisition behaviour, and energy and substance transfers. On the basis of their trophic characteristics, the species could be divided into 12 trophic groups. After examination of data from geographically diverse rivers, it was concluded that any undisturbed riverine benthic macroinvertebrate community should be represented by members of each of these 12 trophic groups, with each group fulfilling a function in the benthic community. Being a community which plays a central role in the functioning of the aquatic ecosystem, the benthic invertebrates are expected to respond to disturbances to the hydrobiocoenose. The outcome of an ITC assessment is clearly presentable in the form of a pie graph with 12 wedges, each representing one of the 12 defined trophic groups. Functionally complete communities are represented by 12 wedges; a blank wedge indicates that a trophic group is not represented. This paper describes the preliminary developments in the ITC method, its potential as a biological assessment method in rivers in different geographical zones, and presents examples of trial mappings of Russian and European rivers. The application of the ITC to these rivers demonstrated the absence of ITC trophic groups at sites under the influence of anthropogenic activity.

Long-term developments in ecological rehabilitation of the main distributaries in the Rhine delta: fish and macroinvertebrates

Significant anthropogenic pressure in the Dutch part of the River Rhine is present from the 12th century. River engineering and water pollution were the main stress factors for flora and fauna. From the middle of the 20th century measures were taken to reduce water pollution. Recently, from 1987 onwards, these activities were put into a wider context of ecological river rehabilitation. Effects of improvements on fish and macroinvertebrates in the main distributaries in the Rhine delta are reviewed. The conclusions are that (a) most of the alterations in the Rhine delta are irreversible due to hard socio-economic boundary conditions (e.g. safety, navigation); (b) chances for the development of riverine biotopes have therefore to be found in the forelands and not in the main channels of the Rhine delta; (c) further reduction of pollutants, especially thermal pollution, is needed to help original species to colonise the Rhine delta again; (d) non-indigenous species clearly leave a mark on recolonisation possibilities of original species.

Decline of zebra mussel populations in the Rhine

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Interference competition between alien invasive gammaridean species

The relative abundances of gammaridean species in the river Rhine have profoundly changed since the invasion of Dikerogammarus villosus in 1994/1995. This study tested whether these changes in gammaridean dominance could have been determined by interspecific competition and unequal mortality, for example by intraguild predation (IGP). Single and two species tests have been carried out in aquariums provided with all substrata present in the main channel of the Rhine. Changes in substratum choice, increased swimming activity and increased mortality of a species were used as indicators of interspecific competition during interaction between gammaridean species. Interspecific competition and mortality between the most abundant invasive gammaridean species in the Rhine, viz. Gammarus tigrinus, Echinogammarus ischnus and Dikerogammarus villosus were tested. In single-species experiments, G. tigrinus and D. villosus showed similar preferences for a stony substratum, whereas E. ischnus mostly occupied the water column. The two-species aquarium experiments indicated direct interference competition for substratum and unequal mortality between G. tigrinus and D. villosus, with D. villosus being the stronger competitor. Competitive stress was influenced by population density, was size-dependent and varied between the different types of substratum due to substratum choice. G. tigrinus did not show any behaviour indicative of interference competition in the presence of E. ischnus, and neither did E. ischnus or D. villosus in the presence of any of the other gammarideans. Swimming in the water layer may already enable E. ischnus to minimise its encounters with the stone-dwelling D. villosus and G. tigrinus. To maximise the encounters between E. ischnus and D. villosus, a fish (Lepomis gibbosus) was added to occupy the water layer during the aquarium experiments. E. ischnus showed a higher mortality in the presence of both D. villosus and fish, probably due to increased stress, as shelter opportunities to escape the predators had been minimised. The study shows that interference competition between gammaridean species can explain the replacement of the North American invader G. tigrinus by D. villosus in the river Rhine. E. ischnus and D. villosus both Ponto-Caspian invaders did not show interference competition in our experiments and co-exist in the Rhine.

Biological invasions: concepts to understand and predict a global threat

Charles Elton was the modern founder of the science of biological invasions. He wrote that ‘biological invasions are so frequent nowadays in every conti- nent and island, and even in the oceans, that we need to understand what is causing them and try to arrive at some general viewpoint about the whole business ’ (Elton 1958). He tried to predict the outcome of global invasion processes and assumed that invasions would result in homogenization of regional floras and faunas. The prediction of homogenization was formulated earlier by Lyell (1832) who, in contrast to Elton (1958), did not consider the resulting human-caused extinctions to be a cause of concern because, in his opinion, this was a natural process (Wilkinson 2004). Interest in biological invasions has rapidly increased in recent decades and today biological inva- sions are a major concern in ecology and conservation. Particularly dramatic consequences of invasions have been reported from island ecosystems where endemic species suffered severely, but wetlands (marshes, lakes, rivers) and estuaries are also among the most affected systems (Moyle 1996;Williamson 1996;Ruiz et al.1997). On the background of accelerating invasion rates, sci- ence has become increasingly interested in understanding the underlying mechanisms of biological invasions to predict invasion processes and impacts. Following a brief overview on the nature and impacts of invasions, we review different concepts regarding determinants of invasion success. We also highlight promising research areas to cope with this major threat to bio- diversity in wetlands.

To conquer and persist : colonization and population development of the Ponto-Caspian amphipods Dikerogammarus villosus and Chelicorophium curvispinum on bare stone substrate in the main channel of the River Rhine

Macroinvertebrate communities on the stones in the Rhine are dominated by the Ponto-Caspian amphipods Chelicorophium curvispinum (since 1987) and Dikerogammarus villosus (since 1995), which have invaded the Rhine through canals connecting the large rivers of Europe. Colonization of bare stones suspended in the water of the Rhine main channel was studied. At the same time the macroinvertebrates drifting in the water layer were sampled. Macroinvertebrate populations on the newly colonized stones were followed for two months (June - August 2002). Bare stones were colonized from the water layer, with D. villosus and C. curvispinum most numerous from the start. Species richness was highest after one month. D. villosus and C. curvispinum continued to dominate the macroinvertebrate community on the stones throughout the experiment, representing 70-95 % of the total number of macroinvertebrates. In the first period week of colonization, especially juveniles of both amphipod species settled on the bare stones. After one week, the number of adults of D. villosus increased. After one month, ovigerous females of D. villosus became abundant on the newly colonized substrate. The numbers of adult C. curvispinum increased after one month and ovigerous females were present after two months. Newly settled populations resembled the amphipod populations present in the water layer, but started to deviate as colonization time increased, indicating that development of populations on stones became increasingly autonomous and less dependent on new colonization by amphipods from the water layer. Ovigerous females of both amphipods were much more abundant on the stones than in the water layer. Juvenile C. curvispinum were smaller on the stones than in the water layer, indicating that the stone substrate is important for reproduction of at least C. curvispinum. Most juveniles of this species first grow to a certain body length on the stones before they start drifting off and swimming in the water layer. C. curvispinum and D. villosus densities were positively correlated in the early stages of colonization, but showed an inverse relationship after longer colonisation time. As both dominant amphipods colonize new stone substrate from the water layer where they drift or swim, they may not depend on extra vectors, such as shipping, for dispersal through the connecting canals and within the river.