Jan Simons

Clear Water Associated with a Dense Chara Vegetation in the Shallow and Turbid Lake Veluwemeer, The Netherlands

The presence of submerged aquatic macrophytes in lakes is affected by the underwater light climate. Lakes with clear water can show abundant macrophyte vegetation, whereas lakes with turbid water usually have a poor submerged vegetation (Moss, 1990; Scheffer et al., 1993). Moreover, macrophytes improve their own light climate by enhancing the water transparency.

Macroinvertebrate communities in relation to submerged vegetation in two Chara-dominated lakes

Relationships between macroinvertebrates and the presence ofsubmerged vegetation were studied in two shallow eutrophiclakes inThe Netherlands, Lake Veluwemeer and Lake Wolderwijd. A shiftfromturbid water with sparse macrophyte cover (Potamogetonperfoliatus, Potamogeton pectinatus) towards clearwaterwith a dense cover of submerged vegetation (Chara spp.)hasbeen observed in the lakes over the past 10 years. RelativelylargeChara meadows (300–500 ha) have recently developed inbothlakes. The composition of macroinvertebrate fauna wasdetermined atsites varying in cover and dominant vegetation type bysamplingsediment and water during 1992 and 1994. Macrophyte biomass,sampling year and vegetation type were the major determinantsofmacroinvertebrate community composition. Valvatapiscinalis,Bithynia tentaculata, Gammarus tigrinus and Chironomussp.characterized the sites with high charophyte biomass, whereasPotamopyrgus antipodarum, Cladotanytarsus sp., Stictochironomus sp. dominated the samples with lowcharophytebiomass. Chara vegetation was different from Potamogeton sp. by showing lower densities of the midgelarvaeEinfeldia dissidens and Cricotopus gr. sylvestris.Seasonal variations in densities of the dominant molluscspecies(V. piscinalis, P. antipodarum) were closelyrelated tothe development of Chara biomass as well as toperiphytoncover on charophytes. Thus, changes of the light climate inbothlakes, which have led to an increase in colonization bysubmergedvegetation (particular Chara meadows), indirectly had alargeimpact on macroinvertebrate communities.

The role of characean algae in the management of eutrophic shallow lakes.

Characeae (charophytes or stoneworts) are anatomically highly developed green algae. The family is divided into two tribes, the Chareae and Nitelleae, which together contain six genera and about 200 species worldwide (Moore 1986). They grow mainly in alkaline freshwater lakes and ponds. The algae are fixed to the sediment by rhizoids, and several species tend to cover the sediment in dense mats, which often are referred to as meadows. Eutrophication has led to a decline in the charophyte vegetation in many lakes. Over the past decade, numerous restoration projects have been carried out to reduce the negative effects of eutrophication. The main purpose of such projects in shallow lakes has been to change the state of turbid water with dominance by phytoplankton into an alternative stable state with clear water and dominance by macrophytes, including charophytes (Scheffer et al. 1993, Moss et al. 1996). Biomanipulation, mostly comprising a temporary strong reduction of the fish stock, may help to induce the switch to clear water, but a fast return of macrophytes seems crucial for the stabilization of the clear water state (Meijer et al. 1994). Charophytes often play an important role in such projects because they are notoriously rapid colonizers (Crawford 1977, Simons et al. 1994, Beltman and Allegrini 1997). Furthermore, dominance by charophytes can be of special value in recreational waters because the plants normally do not reach the water surface. Such meadows cause less nuisance to swimmers or boats than stands of canopy-forming angiosperms. This minireview presents some ecological aspects of charophytes and their implications for management of shallow lakes. The first section presents the

Competition between Chara aspera and Potamogeton pectinatus as a function of temperature and light

Abstract Two experiments were carried out to study the interaction between Chara aspera Deth. ex Willd. and Potamogeton pectinatus L. The purpose of the first experiment was to assess the effect of temperature on the rate of emergence and the second was designed to study the effect of light on the competition during the established phase. P. pectinatus tubers sprouted after about 4 days at 16°C and 9 days at 10°C, which was sooner than either oospores or bulbils of C. aspera (about 15 days at 16°C and 27 days at 10°C). When the irradiance was 36 μ mol m −2 s −1 , the ash-free dry weight biomass of both species was about 80% lower than at 416 μ mol m −2 s −1 and the biomass was not affected by neighbouring plants. However, in the higher light treatment the presence of P. pectinatus suppressed the biomass of C. aspera by maximally 63%. The individual biomass of P. pectinatus at high initial plant densities was reduced by maximally 70% by other plants of the same species, but was not affected by C. aspera . It is concluded that P. pectinatus rather than C. aspera has an advantage in the phase of emergence during spring. Furthermore, under higher light conditions and high inorganic carbon concentrations P. pectinatus may be a better competitor for light than C. aspera , because of its canopy placement near the water surface.

Propagule bank buildup of Chara aspera and its significance for colonization of a shallow lake

Several aspects of the propagule bank dynamics including germination, burial and storage characteristics, and the relationship between the distribution of the propagule bank and cover of an expanding Chara vegetation (dominated by C. asperaDeth. ex Willd.) was studied in the shallow lake Veluwemeer. The density of oospores in the sediment was positively correlated with the number of years that Chara was present at that particular site. After six years of Chara presence, at least 1.7 × 106 oospores m−2 had accumulated in the sediment. Oospores in sediment were evenly distributed in the 15 cm top layer, which was very similar to the foraging depth of Bewicks Swans (Cygnus columbianus bewickii Yarr). Burial in the sediment may be an important mechanism by which oospores are stored. On the other hand, at shallow areas about 50% of the biomass was consumed enhancing the potential dispersal of oospores by water birds. In a laboratory experiment, 100% of the tested bulbils of C. aspera emerged and thus may be important for short time survival of established vegetation. In contrast, oospore germination varied between 1 and 15% depending on light level and burial depth. The low germination and the high accumulation of oospores suggest that oospores are adapted to long time survival in a dormant state. Charophytes colonized Veluwemeer step by step in the course of about nine years. Not all the sites with suitable light conditions were colonized at the same speed. C. aspera established a dense vegetation only at sites with high oospores densities (> c. 1 × 104 m−2). The results indicate that the high density of oospores needed for attaining complete vegetation cover may be an important limiting factor during colonization.

Playing the waves: Lars von Trier's Game Cinema

Dogma95 werd toegejuicht als een Europees alternatief voor de Hollywoodblockbuster. Voor veel critici en filmliefhebbers is Dogma95 gaan staan voor de begrippen waarmee de kunstzinnige film gebruikelijk in verband wordt gebracht: low budget en realisme. Von Triers benadering van filmen voert film over de grenzen van de traditionele filmesthetiek en plaatst de praktijk van het filmmaken en film zelf in het hart van wat vandaag de dag het nieuwe-mediagenre bij uitstek is geworden: games en gaming. Dogma95 is geen uitzonderlijke episode in de loopbaan van Von Trier, zoals het dat wel was in het werk van zijn mede-oprichters,maar vormde de meest expliciete formulering van Von Triers spelopvatting van filmmaken, die hem bij het concipieren en produceren van al zijn films heeft geleid. Von Trier herdefinieert filmmaken als een aan regels gebonden activiteit, introduceert spelvormen en structuren in zijn films, en trekt ontnuchterende lessen uit de economische en evolutionaire speltheorie. Von Triers films kunnen dan ook beter begrepen worden vanuit het perspectief van gamesstudies en speltheorie, dan vanuit de traditionele filmtheorie en esthetiek.

A Framework for Responsive Environments

In this paper, we define Responsive Environments as adaptive venues that possess context awareness, deliver ubiquitous computing and natural interaction. They also yield a pre-determined User Experience. We propose a framework for the development and assessment of such environments and we discuss applying the framework to some examples. Highlighting benefits and usefulness of the framework.