Laboratory of Aquatic Ecology, K.U.Leuven, Charles de Bériotstraat 32, B-3000 Leuven, Belgium

Corresponding author : Gerald Louette. Current address : Research Institute for Nature and Forest, Kliniekstraat 25, B-1070 Brussel, Belgium. Tel. : +32 2 558 18 59, Fax : +32 2 558 18 05, E-mail address : gerald.louette@inbo.be

KEY WORDS : Cladocera, faunistics, new record, non-indigenous species, Red List

The four crustacean orders of the cladocerans represent, together with copepods and rotifers, the most common zooplankton taxa in all types of lentic freshwater bodies (1). They exhibit a parthenogenetic (clonal) reproduction mode during periods of favourable environmental conditions, and produce sexual dormant eggs (ephippia) when conditions deteriorate (2). As such they are capable of remaining dormant in the habitat for decades (3). Because of their capacity for rapid population growth, some pelagic members of the group (especially the large-bodied Daphnia and Diaphanosoma) are able of keeping water bodies in a clear water state by grazing down the phytoplankton (4). Most species feed on bacteria, protists, periphyton, and detritus (many chydorids and macrothricids), some are parasitic (e.g. Anchistropus on the polyp Hydra) or predacious on small-sized zooplankton (e.g. Leptodora and Polyphemus) (5; 6). Cladocerans themselves are a main food source for fish, macro-invertebrates, and amphibians (7).

So far, several authors have provided species lists and updates on the occurrence of cladocerans in Belgium (summarized in 8; 9; 10; 11). However, there are virtually no published data on the geographic distribution and the frequency of occurrence of these species in Flanders. A notable exception is the monograph of Luyten (12), dating from the first half of the 20^th century, and reporting on the occurrence of 56 cladocerans in 35 sites, spread over Flanders. Furthermore, data on the current status of cladocerans from regions in Europe are almost nonexistent (apart from [13] who provided a Red List of Cladocera from Carinthia).

In this paper, we present contemporary data on the frequency of occurrence and geographic distribution of inland cladocerans in Flanders (Belgium), and compare our results with the observations of (12). We try to identify major trends in the occurrence of species, and indicate hot spots of rare species. Finally, we also report the occurrence of two cladocerans new to the Belgian fauna.

During the period 2000-2005, we collected zooplankton samples from 64 different sites that are evenly spread over Flanders (Fig. 1, App. 1). In each site (defined as an area of ca 28km²) we sampled multiple types of water bodies (ditches, temporary pools, ponds, lakes, and canals) once in summertime. During the entire survey, 605 different water bodies were sampled, with an average of 9.5 (SE 0.9) water bodies per site.

Cladoceran samples were obtained with a tube sampler (diameter 75mm, length 2m), taking an integrated sample of the water column at random sites in the water body. The collected water was filtered through a plankton net (mesh size 64µm) and preserved in formaldehyde (4%) saturated with sucrose. When water bodies were too shallow (e.g. temporary pools with a water depth of less than 30cm), samples were taken by a plankton dip net (mesh size 64µm). In 13% (80 out of 605) of the water bodies, samples were taken from both the active and dormant cladoceran community. The dormant egg bank was sampled using a hand corer (diameter 52mm, length 1m). Eggs were isolated from the surficial 3cm of the sediment applying the sugar flotation technique, and hatched under simulated summer conditions (see [14] for protocol details). Cladocerans in the samples were identified to species level following the key of (6), with the exception of the genera Chydorus and Bosmina, which were identified to the genus level.

For each taxon, we calculated the frequency of occurrence (% of the sites) to obtain an idea of the representation of each species in Flanders. We distinguished between six categories using the ACFOR scale : Abundant (>75%), Common (75%-51%), Frequent (50%26%), Occasional (25%-6%), Rare (5%-1%), and Not observed (not detected in any of the investigated sites; the species may be extinct). The same categorization was performed on the dataset of (12).

SPECIES LIST AND FREQUENCY
OF OCCURRENCE

Diaphanosoma brachyurum (Liévin, 1848) Frequent Occasional Sida crystallina (O.F. Müller, 1776) Frequent Occasional

Ceriodaphnia dubia Richard, 1894 Occasional Not observed Ceriodaphnia laticaudata P.E. Müller, 1867 Common Occasional Ceriodaphnia megops Sars, 1862 Occasional Occasional Ceriodaphnia pulchella Sars, 1862 Abundant Frequent Ceriodaphnia quadrangula (O.F. Müller, 1785) Frequent Occasional Ceriodaphnia reticulata (Jurine, 1820) Frequent Occasional Ceriodaphnia rotunda Sars, 1862 Not observed Not observed Ceriodaphnia setosa Matile, 1890 Not observed Rare Daphnia ambigua Scourfield, 1946 ** Frequent – Daphnia atkinsoni Baird, 1859 Rare Not observed Daphnia cucullata Sars, 1862 Frequent Frequent Daphnia curvirostris Eylmann, 1887 * Occasional – Daphnia galeata Sars, 1864 * Common – Daphnia hyalina Leydig, 1860 Rare Not observed

Complete list of all different cladocerans that have been reported for Belgian water bodies. For each species, the current status in Flanders as revealed by the present study, and the status during Luyten’s days (12) is presented. Species where no status is assigned are indicated by (–).

Daphnia longispina O.F. Müller, 1785 Occasional Frequent Daphnia magna Straus, 1820 Frequent Occasional Daphnia obtusa Kurz, 1874 * Common – Daphnia parvula Fordyce, 1901 ** Frequent – Daphnia pulex Leydig, 1860 Common Occasional Megafenestra aurita (Fischer, 1849) Occasional Not observed Moina brachiata (Jurine, 1820) Occasional Occasional Moina macrocopa (Straus, 1820) Occasional Not observed Moina micrura Kurz, 1874 Occasional Not observed Moina weismanni Ishikawa, 1896 ** Not observed – Scapholeberis mucronata (O.F. Müller, 1785) Abundant Frequent Scapholeberis rammneri Dumont & Pensaert, 1983 * Occasional – Simocephalus exspinosus (Koch, 1841) Occasional Occasional Simocephalus serrulatus (Koch, 1841) Rare Not observed Simocephalus vetulus (O.F. Müller, 1776) Abundant Frequent

Bosmina coregoni Baird, 1857 – Occasional Bosmina longirostris (O.F. Müller, 1785) Abundant Common Bosmina longispina Leydig, 1860 – Not observed

Acantholeberis curvirostris (O.F. Müller, 1776) Occasional Occasional Drepanothrix dentata (Eurén, 1861) Rare Occasional Ilyocryptus acutifrons Sars, 1862 Not observed Rare Ilyocryptus agilis Kurz, 1878 Occasional Occasional Ilyocryptus sordidus (Liévin, 1848) Frequent Frequent Lathonura rectirostris (O.F. Müller, 1785) Not observed Occasional Macrothrix hirsuticornis Norman & Brady, 1867 Occasional Not observed Macrothrix laticornis (Jurine, 1820) Occasional Rare Macrothrix rosea (Jurine, 1820) Rare Occasional Streblocerus serricaudatus (Fischer, 1849) Rare Not observed

Acroperus harpae (Baird, 1835) Frequent Common Alona affinis (Leydig, 1860) * Frequent – Alona costata Sars, 1862 Frequent Frequent Alona elegans Kurz, 1875 Not observed Not observed Alona guttata Sars, 1862 Common Frequent Alona intermedia Sars, 1862 Rare Not observed Alona phreatica Dumont, 1983 * Not observed – Alona protzi Hartwig, 1900 Not observed Not observed Alona quadrangularis (O.F. Müller, 1785) Occasional Common Alona rectangula Sars, 1862 Common Frequent Alona rustica Scott, 1895 Not observed Not observed Alonella excisa (Fischer, 1854) Occasional Frequent Alonella exigua (Lilljeborg, 1853) Occasional Occasional Alonella hamulata (Birge, 1879) ** Rare – Alonella nana (Baird, 1843) Frequent Frequent Alonopsis elongata (Sars, 1861) Rare Occasional Anchistropus emarginatus Sars, 1862 Not observed Occasional Camptocercus lilljeborgi Schoedler, 1862 Not observed Rare

TABLE 1

Complete list of all different cladocerans that have been reported for Belgian water bodies. For each species, the current status in Flanders as revealed by the present study, and the status during Luyten’s days (12) is presented. Species where no status is assigned are indicated by (–).

Note : (12) did not yet identify some taxa as separate species (*); some species were not yet present in Flanders (nonindigenous species, **).

In order to search for trends in the frequency of occurrence of cladocerans over the past 70 years, we made an attempt to compare our results with those of (12) (Table 1). We are fully aware that the interpretation of this comparison should be done with care, as there are differences among both datasets in geographic location and number of sites, number and type of water bodies sampled ([12] sampled in most cases only one water body per site), and frequency of sampling in the habitats. Table 1 illustrates that many species show no or only minor shifts between categories of the ACFOR scale. Only a limited number of species is nowadays more widespread than before (i.e. mostly daphniids such as Ceriodaphnia and Moina, Daphnia pulex, Megafenestra aurita, S. mucro-nata, and S. vetulus). Their increase in frequency of occurrence may be attributed to the greater number of small water bodies incorporated in our survey, but may also be related to an increased nutrient load in many water bodies during the last decades (15; 16). The increased nutrient load has lead to a higher production of organic material, and as such to an increased availability of food sources. This may explain why (12) found Daphnia magna only twice during his survey, whereas it is now frequently observed. Other species of non-indigenous origin (e.g. Daphnia ambigua, Daphnia parvula, and Pleuroxus denticulatus) were only introduced in Europe many years after Luyten’s study (6), and are now frequently observed. For a subset of species, a comparison of the frequency of occurrence cannot be made, as (12) did not yet identify them as separate species (Daphnia curvirostris probably identified as D. pulex; Daphnia galeata probably identified as Daphnia longispina; Daphnia obtusa probably identified as D. pulex; Scapholeberis rammneri probably identified as S. mucronata; and Alona affinis probably identified as Alona quadrangularis).

The relatively large proportion of species (17%) that were not observed during our intensive survey may indicate the loss of specific habitats. For instance, the degradation of clear, weakly buffered, and oligotrophic water bodies may explain the disappearance of some species. Other species were only recently recorded as isolated cases, and are relict species (e.g. Eurycercus glacialis reported by [17]) or non-indigenous species (Moina weismanni reported by [9]). More detailed research in the Campine region would probably result in the rediscovery of some species that were not observed during our study.

Hot spots for cladocerans, identified as sites in which 2 or 3 rare species were observed, are sites which contain clear, weakly buffered, and oligotrophic to mesotrophic water bodies such as Zonhoven (De Teut), and Genk (De Maten and Het Wik) (Fig. 1). The site of Damme is another hot spot, as it contains many small, turbid and often temporary habitats, which are frequented by migrating waterfowl (see further).

ACKNOWLEDGMENTS

REFERENCES