Boudewijn Goddeeris

Body size and dispersal mode as key traits determining metacommunity structure of aquatic organisms

Relationships between traits of organisms and the structure of their metacommunities have so far mainly been explored with meta-analyses. We compared metacommunities of a wide variety of aquatic organism groups (12 groups, ranging from bacteria to fish) in the same set of 99 ponds to minimise biases inherent to meta-analyses. In the category of passive dispersers, large-bodied groups showed stronger spatial patterning than small-bodied groups suggesting an increasing impact of dispersal limitation with increasing body size. Metacommunities of organisms with the ability to fly (i.e. insect groups) showed a weaker imprint of dispersal limitation than passive dispersers with similar body size. In contrast, dispersal movements of vertebrate groups (fish and amphibians) seemed to be mainly confined to local connectivity patterns. Our results reveal that body size and dispersal mode are important drivers of metacommunity structure and these traits should therefore be considered when developing a predictive framework for metacommunity dynamics.

Oxygen concentration profiles in sediments of two ancient lakes: Lake Baikal (Siberia, Russia) and Lake Malawi (East Africa)

Oxygen concentration profiles have been measured, by means of with microelectrodes in sediments of Lake Baikal and Lake Malawi, along transects allowing to give a survey of two major ancient Rift lakes: Lake Baikal (Eastern Siberia) and Lake Malawi (East Africa), along depth transects in the constitutive basins of the lakes and/or of relevant depths with regard to oxygen (including including the deepest point, 1680 m, in Lake Baikal). Sediment oxygen penetration depths (SOPs) display very different patterns, depending on the lake in the two lakes. In Lake Baikal, SOPs are variable, show no significant relationship with bathymetric depth and are surprisingly deep on Akademichesky ridge (> 50.0 mm), emphasizing the distinctive feature of this region in the lake. While the Selenga river is an important source of eutrophication, the similarity of SOP-values in the Selenga shallow with those of most other sites suggests either a dilution of organic material by allochthonous matter, or a strong south-to-north transport of particles. In Lake Malawi, available oxygen is restricted to a maximum of three millimetres of the sediment, and there is a negative relationship with bathymetric depth, as a result of a steady decline of oxygen concentration with depth through the water column. Amongst the few parameters known to affect SOPs, the oxygen consumption by the sediment seems the most significant in both lakes. SOP-values furthermore confirm differences in the trophic status of Baikal and Malawi, respectively. The importance of oxygen as a factor likely to create ecological segregation for benthic organisms is discussed. Lake Malawi offers possibilities of bathymetric segregation but no vertical segregation in the sediment. In contrast, no bathymetric segregation related to oxygen is possible in Lake Baikal, but vertical segregation in the sediment is very likely.

On the relationship between vertical microdistribution and adaptations to oxygen stress in littoral Chironomidae (Diptera)

Animals that dwell at different depths in the sediment, are adapted to different respiratory environments. It is possible that animals that occur deep in the sediment have a higher hemoglobin concentration than surface-dwelling animals. To test this hypothesis, hemoglobin concentrations and weights of eight chironomid species that dwell in the littoral zone were measured. High hemoglobin concentration and weight both seemed to contribute to an ability to cope with low oxygen concentrations, and determined the vertical distribution of chironomids in the sediment. A multiple regression equation, including these factors, was derived. It may be used to predict the median depth of occurrence for species that were not included in this study. High sensitivity of small animals to oxygen stress is discussed from a theoretical point of view.

Global diversity of dipteran families (Insecta Diptera) in freshwater (excluding Simulidae, Culicidae, Chironomidae, Tipulidae and Tabanidae)

Today’s knowledge of worldwide species diversity of 19 families of aquatic Diptera in Continental Waters is presented. Nevertheless, we have to face for certain in most groups a restricted knowledge about distribution, ecology and systematic, particularly in the tropical environments. At the same time we realize a dramatically decline or even lack of specialists being able, having the time or the opportunity to extend or even secure the present information. The respective families with approximate numbers of aquatic species are: Blephariceridae (308), Deuterophlebiidae (14), Nyphomyiidae (7), Psychodidae (∼2.000), Scatopsidae (∼5), Tanyderidae (41), Ptychopteridae (69), Dixidae (173), Corethrellidae (97), Chaoboridae (∼50), Thaumaleidae (∼170), Ceratopogonidae (∼6.000), Stratiomyidae (∼43), Empididae (∼660), Lonchopteridae (2), Syrphidae (∼1.080), Sciomyzidae (∼190), Ephydridae (∼1.500), Muscidae (∼870). Numbers of aquatic species will surely increase with increased ecological and taxonomical efforts.

The life-cycle of the asexual ostracod Darwinula stevensoni (Brady & Robertson, 1870) (Crustacea, ostracoda) in a temporate pond

The life-cycle of the ancient asexual ostracod Darwinula stevensoni was studied during 1 year in a eutrophic pond in Belgium. The reproductive period of this species started in March and was effectively completed by September of the same year. All changes in population structure took place during the spring and summer months and a rapid turnover of the instars was observed. The life-cycle of Darwinula stevensoni appears to take one year or less in Belgium and this is considerably shorter than the 4 years which had been reported previously from subarctic populations. The difference to the present study is most likely temperature-related. Maximal densities of D. stevensoni were observed in June and July and attained 105 ind. m−2. During winter, densities were lower with a mean of 104 ind. m−2. Consequently, the calculated population size of each month was high throughout the year. Together with the low mutation rate, such a large population size could effectively counteract the stochastic loss of mutation-free genotypes as predicted by Mullers ratchet. D. stevensoni is a brooder; the maximum number of embryos and juvenile instars (up to third stage) found within a single female was 11.

Caged midge larvae (Chironomus riparius) for the assessment of metal bioaccumulation from sediments in situ

First-stage larvae of the midge Chironomus riparius were exposed in small enclosures at 19 sites located in three different river basins in Flanders (Belgium). Sediments were sampled and sieved at 200 microm at all exposure sites. A layer of approximately 2 cm of sediment was placed in each cage and 100 midge larvae were added. Cages were placed in watercourses where resident midge larvae were present. Accumulation of Cd, Cr, Cu, Pb, Ni, and Zn was determined after four weeks of exposure when larvae had reached the fourth stage. Comparing metal levels between caged and resident larvae revealed no significant differences. A significant correlation between metal levels in caged and resident larvae was found when all sites were considered. However, such correlation was low (r2 = 0.28) for Pb. The highly significant r2 values found for Cu and Ni probably were due to only one site. Metal levels in tissue were related to levels in water and sediment, taking into account some sediment characteristics (particle size distribution and organic carbon) and oxygen level in the water. To determine the relative importance of these different sediment factors contributing to the variation in metal accumulation by the chironomids, nonlinear regression models were constructed. With the models used, 56.1, 32.2, and 57.4% of the variation for Cd, Pb, and Zn, respectively, could be described. None and 26.9% of the variation could be described for Cu and Ni, respectively. Among the environmental factors, organic carbon and oxygen levels in water were important in describing the accumulation of metals.

The hemoglobin concentration ofChironomus cf.Plumosus l. (Diptera: Chironomidae) larvae from two lentic habitats

Hemoglobin concentrations ofChironomus cf.plumosus larvae were measured in two different habitats of the same pond. Larger larvae have higher hemoglobin concentrations than small larvae. There is strong indication that the animals of poorly oxygenated deep water, have higher hemoglobin concentrations than the animals from the well-oxygenated littoral zone.

On the spatial distribution and respiratory environment of benthic macroinvertebrates in ponds

The influence of oxygen distribution in a shallow water on the spatial distribution of the macrobenthic fauna has been investigated. We studied both the bathymetric and the vertical distribution in the sediment in order to assess the biological importance of oxygen microstratification. Oxygen concentrations in the water column were measured with a Clark Au-Ag electrode (WTW EO 196). Oxygen concentrations at the sediment-water interface and in the sediment were measured directly in core samples with Clark-style micro-electrodes (Diamond 737GC). Our results indicate that the composition of the macrobenthic community is well correlated with the bathymetric distribution of oxygen in the water column. However, some burrowing chironomid species appear to be dependent on the oxygen at the sediment-water interface. Our conclusion is that the respiratory environment of benthic animals (cf. Brundin, 1951) is broader than expected, very specific, and linked to their vertical distribution in the sediment.

On the reliability of Ponar grab samples for the quantitative study of benthic invertebrates in ponds

We present observations on the variability of sediment penetration depth by the Ponar grab sampler, which lead us to question the reliability of grab samples in the quantitative study of freshwater benthos. Penetration depth of the Petite Ponar grab depends on substrate type, and correlates with the amount of organic carbon, the water content and the granulometry of the sediment. Since these factors can also influence faunal composition and vertical distribution in the sediment, it is important to study the performance of the sampler before a biological explanation for the observed pattern is given. At the site studied, a case study was performed, in which variable grab penetration did not influence biological interpretation because the penetration depth of the grab followed that of the organisms under study.