Luc Denys

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.

A comparison of national approaches to setting ecological status boundaries in phytobenthos assessment for the European Water Framework Directive: results of an intercalibration exercise

The European Union (EU)’s Water Framework Directive (WFD) requires that all Member States participate in intercalibration exercises in order to ensure that ecological status concepts and assessment levels are consistent across the EU. This paper describes one such exercise, performed by the countries in the Central/Baltic Geographical Intercalibration Group stretching from Ireland in the west to Estonia in the east and from the southern parts of Scandinavia to the northern regions of Spain and Italy (but excluding alpine regions, which were intercalibrated separately). In this exercise, methods used to measure ecological status of rivers using benthic diatoms were compared. Ecological status is estimated as the ratio between the observed value of a biological element and the value expected in the absence of significant human impact. Approaches to defining the ‘reference sites’, from which these ‘expected’ values were derived, varied from country to country. Minimum criteria were established as part of the exercise but there was still considerable variation between national reference values, reflecting typological differences that could not be resolved during the exercise. A simple multimetric index was developed to compare boundary values using two widely used diatom metrics. Boundary values for high/good status and good/moderate status set by each participant were converted to their equivalent values of this intercalibration metric using linear regression. Variation of ±0.05 EQR units around the median value was considered to be acceptable and the exercise provided a means for those Member States who fell significantly above or below this line to review their approaches and, if necessary, adjust their boundaries.

Identification versus counting protocols as sources of uncertainty in diatom-based ecological status assessments

In 2009, seventeen analysts participated in a pan-European diatom ring-test (intercalibration), analyzing nine samples from seven countries following the European standard EN 14407. The objective of this exercise was to agree on practical conventions on diatom identification to facilitate future intercalibration work and to assess the extent to which national differences in sample analysis (counting protocol and identification conventions) contribute to variability in EU-level comparisons of diatom-based methods. Differences in the reported taxa lists were large, but not a major source of variation in values of a common metric (the phytobenthos Intercalibration Common Metric, ICM). Therefore, every country can apply its own identification conventions for national assessments, and still be fairly confident that the ICM reflects the national classification of its streams. Part of the index variation was due to differences in counting protocols and care should be taken when handling broken valves, girdle views and small taxa. More work at both national and European level is needed to provide a harmonized way of using diatoms for ecological status assessments in the future.

Comparing aspirations: Intercalibration of ecological status concepts across European lakes for littoral diatoms

Eleven European countries participated in an exercise to harmonise diatom-based methods used for status assessment in lakes. Lakes were divided into low, medium and high alkalinity types for this exercise. However, it was not possible to perform a full intercalibration on low alkalinity lakes due to the short gradient and confounding factors. Values of the Trophie Index were computed for all samples in order that national datasets could all be expressed on a common scale. Not all participants had reference sites against which national methods could be standardised and, therefore, a Generalised Linear Modelling approach was used to control the effect of national differences in datasets. This enabled the high/good and good/moderate status boundaries to be expressed on a common scale and for deviations beyond ±0.25 class widths to be identified. Those countries which had relaxed boundaries were required to adjust these to within ±0.25 class widths whilst the intercalibration rules allowed those countries with more stringent boundaries to retain these. Despite biogeographical and typological differences between countries, there was broad agreement on the characteristics of high, good and moderate status diatom assemblages, and the exercise has ensured consistent application of Water Framework Directive assessments around Europe.

Palaeolimnological aspects of a Late-Glacial shallow lake in Sandy Flanders, Belgium

A summary account is given of the development of a small Late-Glacial lake at Snellegem-St. Andries, Belgium. Sedimentation, hydrology, water quality and biotic succession clearly depended on climatic conditions and catchment processes (soil stability and leaching, vegetation). Special attention is drawn to a period of low water level near the end of the Allerod and the abundance of Fragilaria in certain periods.

Middle-Holocene alluvial forests and associated fluvial environments: A multi-proxy reconstruction from the lower Scheldt, N Belgium

Analyses of pollen, plant macrofossils (seeds, fruits, wood and mosses), molluscs, diatoms and vertebrate (mainly fish) remains allowed a detailed reconstruction of a middle-Holocene alluvial forest and its associated hydrological conditions. The use of multiple proxies resulted in a taxonomically more detailed and environmentally more comprehensive understanding of terrestrial as well as aquatic habitats. The results demonstrate possible biases in palaeoecological reconstructions of alluvial and estuarine environments drawn from single proxies. Many locally occurring woody taxa were underrepresented or remained undetected by pollen analyses. Seeds and fruits also proved to be inadequate to detect several locally important taxa, such as Ulmus and Hedera helix. Apparently brackish conditions inferred from diatoms, pollen and other microfossils conflicted strikingly with the evidence from molluscs, fish bones and botanical macroremains which suggest a freshwater environment. Brackish sediment (and the microfossil indicators) is likely to have been deposited during spring tides or storm surges, when estuarine waters penetrated more inland than usual. Despite the reworking and deposition of estuarine and saltmarsh sediment well above the tidal node at such events, local salinity levels largely remained unaffected.

Redundancy in the ecological assessment of lakes: Are phytoplankton, macrophytes and phytobenthos all necessary?

Although the Water Framework Directive specifies that macrophytes and phytobenthos should be used for the ecological assessment of lakes and rivers, practice varies widely throughout the EU. Most countries have separate methods for macrophytes and phytobenthos in rivers; however, the situation is very different for lakes. Here, 16 countries do not have dedicated phytobenthos methods, some include filamentous algae within macrophyte survey methods whilst others use diatoms as proxies for phytobenthos. The most widely-cited justification for not having a dedicated phytobenthos method is redundancy, i.e. that macrophyte and phytoplankton assessments alone are sufficient to detect nutrient impacts. Evidence from those European Union Member States that have dedicated phytobenthos methods supports this for high level overviews of lake condition and classification; however, there are a number of situations where phytobenthos may contribute valuable information for the management of lakes.

Opaline concretions in Weichselian Late-glacial lake marl from Flanders, northern Belgium

Macroscopic opal-A concretions were observed in lake marl deposited in a small Flemish lake (Belgium) during the Allerød biozone of the Weichselian Late-glacial (ca. 12–11 ka BP). The silica from these concretions was derived within the profile, by the leaching of siliceous microfossils – mainly diatom frustules. Formation of the concretions probably resulted from pH- and/or evaporation related precipitation of the silica at a lower stratigraphic level, presumably corresponding more or less to a former low position of the groundwater table. The presence of these concretions is probably related to alternatingly wet and dry local conditions during the middle and later part of the Allerød.

Dispersal of the non-native invasive species Crassula helmsii (Crassulaceae) may involve seeds and endozoochorous transport by birds

Reproductive abilities are essential to assess pathways of introduction, modes of dispersal, and possibilities for effective on-site remediation of invasive species, as well as to identify areas at risk and develop adequate biosafety protocols. Crassula helmsii (Kirk) Cockayne, an amphibious plant from New Zealand and Australia, was introduced in Great Britain in the early 1900s (Swale & Belcher, 1982) and now occurs throughout most of the British Isles. It was not recorded in continental Atlantic Europe until the 1980s (Margot, 1983) and it is still spreading rapidly there, especially in the Low Countries. Due to its proliferous growth and ensuing negative consequences (Robert et al., 2013), control of this highly invasive species is drawing considerable attention; efforts have so far met with little success. Dispersal of C. helmsii in Europe is believed to depend exclusively on the distribution of vegetative propagules (stem fragments or turion-like apical parts) by water, man, or animals. Overwintering also occurs in a vegetative state. As with many aquatic plants, minute fragments with a single node allow regrowth. Vaughan (1978) mentioned that the species ‘seems to set good fruit’ in Britain, but Dawson & Warman (1987) considered it likely that seeds from British plants are unviable, reporting that some were retrieved from soil samples but that these did not germinate. Germination experiments with UK material at CEH Dorset were unsuccessful (Brunet, 2002), whilst Delbart (2011) recovered only aborted seeds from three populations in southern Belgium. Recent reviews reiterate dependence on vegetative parts for dispersal, overwintering, and regrowth after management, noting uncertainty about seed viability in Europe (e.g. EPPO, 2007; Lansdown, 2012; Minchin, 2008; Willby, 2008) and current management strategies are entirely based on this presumption (Delbart et al., 2011). The rapid regrowth after sod-cutting at sites in Flanders, Belgium, where care was taken to remove even the smallest fragments, and observations of water birds grazing on stands of Crassula, led us to (re)consider (1) reproduction by means of seeds, and (2) the possibility of endozoochorous transport of vegetative propagules. Dense tufts of flowering C. helmsii were collected from the coastal dune nature reserve at D’Heye, Bredene, Belgium (51u149540N, 2u599190E) in October 2013. Plants were put into plastic bags and transferred to dark storage at 4uC for eight weeks, after which, wilted flowers with seemingly well-developed brown fruits (follicles) were removed by cutting the pedicel just below their base. Dissection of one hundred follicles yielded 30 mature seeds with a maximum of two per follicle. According to the literature, a single flower may produce two to five seeds, each c.500 mm long (EPPO, 2007). The elliptical seeds in our case were slightly smaller, 385–425 mm, and presented a characteristic rugulate surface texture (Fig. 1). 300 flowers were sown in a shallow tray with fine sand and covered with 2–3 mm of sand. Another 1000 were distributed in a similar tray but mixed with the substrate to a depth of 3–4 cm. The trays were placed in a growing chamber with 14 hours of fluorescent illumination at 18uC and 10 hours of darkness at 12uC. The substrate was kept moist by allowing demineralised water to be soaked up from below. The first seedling appeared on the twenty-fifth day in both trays. Emergence ceased completely after 43 days. Overall, 21 plants developed, 86% emerging within 32 days after sowing. The germination percentage was approximately ten times higher (18%) when seeds were at or very close to the surface than when distributed to a depth of a few centimetres; germination was epigeal. Given the small size of seeds and abundant flowering from early summer up to late autumn, we conclude that reproduction from seeds should not be dismissed as a means for site colonisation by C. helmsii or in its reestablishment after control in continental *Corresponding author: luc.denys@inbo.be

Actinoptychus splendens (Shadbolt) Ralfs (Bacillariophyceae): a biostratigraphic marker for the later part of the Holocene coastal deposits along the southern North Sea

Actinoptychus splendens is a conspicuous and widely distributed marine diatom. At present it is common in the coastal waters of the southern North Sea, but it appears to be absent in the older Holocene deposits along the Dutch and Belgian coasts. Its first postglacial appearance here can be dated at between 4400 and 4100 BP. By 3800 BP it is well established in the region. Although generally not abundant, it can be found in nearly every sample of younger marine or brackish deposits. The large amount of material investigated from the area, as well as the high preservation potential and easy recognition of the species, preclude any other explanation for its apparent absence along southern North Sea shores prior to ±4100 BP, than a late natural immigration. This appears to be quite exceptional, all the common species which generally occur in association with A. splendens being present from the onset of Holocene sedimentation. A. splendens may become an important marker taxon in the Holocene stratigraphy of the coastal plains along the southern North sea.