ALBERT

Notes: Summary The succession of the dune slack vegetation around an artificial lake was studied by means of successive mapping. A succession scheme was prepared on the basis of comparison of the maps made in various years. This scheme was discussed in terms of systems theory, with reference to a kinematic graph prepared on the basis of the transformations. The succession derived from the vegetation maps was compared with that derived from the permanent plots; the results of this comparison indicated that only a small proportion of the total number of succession lines could be detected with the aid of the permanent plots. In addition, the information that could be derived from the permanent plot analysis depended on the degree of change of the vegetation in the area; more information could be derived from drier areas which showed the least degree of change. The best method for the investigation of the succession proved to be a combination of a restricted number of permanent plots and successive mapping.

Notes: Summary For vegetation analyses of permanent quadrats the author formerly used a modified scale of Braun-Blanquet with smaller intervals than in the original. For calculations of difference- and change quotients etc., on the basis of coverage, the symbols of this scale have to be converted to values proportional to the real coverage percentages. A conversion in simple terms is not possible; so the calculations are inconvenient. For an efficient analysis of permanent quadrats a scale is needed that fulfils some requirements. The decimal scale fulfils these. For the analysis of small sub-quadrats of a permanent quadrat a coarse scale is proposed. This scale is directly comparable with the finer decimal scale.

Notes: Summary Besides describing and photographing there are two other methods of registrating vegetation changes: analysis of permanent quadrats (p.q.'s) and successive mapping. Small changes can only be detected by p.q.'s, but not by, successive mapping. A disadvantage of p.q.'s compared with successive mapping is their spatial limitation. By studying p.q.'s valuable information about vegetation succession can be obtained, provided the following points are taken into account: - The relev\'es of the different years must be comparable and differences according to seasonal aspects must be excluded. - Since differences in time are mostly smaller than those in space, a fine scale is needed for p.q.-analysis. The decimal scale (Londo 1975a) for coverage and abundance is very suitable and has the advantage that calculations are easy. For very large quadrats determination of the coverage is less practical; in those cases the method of Tansley for global estimation of dominance and abundance is more suitable. For detailed studies of small quadrats the pointquadrat method may be useful as well. - When it is not possible to make a relev\'e of all p.q.'s every year, it is important to study at least some of them yearly, because extremes in environmental factors can cause changes in the vegetation which remain traceable many years later. These changes may be incomprehensible when the relev\'e of the extreme year is missing. - For determining the dimensions of new p.q.'s one has to take into account changes in the minimum area of the plant communities expected during the succession. Research through permanent quadrats can be done for several purposes. - The purpose can be concerned with vegetation science itself, notably the syndynamics of plant communities. Most statements about vegetation succession are concluded from spatial studies and little is known which is really based on succession studies. It is clear that the most reliable information can be obtained from analysing p.q.'s and successive mapping. When also environmental factors are studied, p.q.'s can contribute much to synecology as well. Research-in-time is especially needed and suitable in cases of communities which are bound to great changes in one or more environmental factors. Also information about community stability and diversity can be obtained in this way. When space-time relationships are to be studied the spatial distribution of the p.q.'s is very important. It became clear that relatively few p.q.'s carefully located in one or some transects gave much more information than many p.q.'s distributed at random. Especially transects across the vegetation zonation gave much information about pattern and process. Thus from a study of duneslack vegetation a negative correlation could be observed between the species diversity and the relative vegetation changeability. - P.q.-studies can also be done with respect to autecology and population dynamics: how is the behaviour of plant species in space and time in different vegetation types and in connection with their environmental factors? Permanent transects as well as series of p.q.'s arranged in two dimensions can provide valuable information in this respect. - The last category is concerned with applied p.q.-studies. Firstly it is done on behalf of the management of nature reserves. It is necessary to study the influence of the management measures on the vegetation types as well as the influences from the environment on the reserve. Further applied p.q.-studies are done to obtain information about the ecological side-effects of human activities a.o. air pollution, infiltration of river water in dune areas, embankment of salt marshes, management of forests, etc. The applied and the purely scientific studies by p.q.'s can be descriptive or experimental. Studies on p.q.'s are mostly not important for only one but for more of the above mentioned aspects together.

Notes: Summary Samples were collected monthly from the bottom of the littoral of three freshwater dune lakes on the West coast of the Netherlands. Although there was great similarity in the diatom flora of these waters differences were also found. It appeared that Quackjeswater and Breede Water had the greatest number of species in common. The relation between the differences in diatom flora and the differences in the chemical constitution of the water is not clear. It was the first time thatNavicula lagerstedtii andN. toulaae were observed in the Netherlands.