Environmental changes in man-made coastal dune pools since 1850 as indicated by sedimentary and epiphytic diatom assemblages (Belgium)

1. Diatom assemblages of man-made coastal dune wetlands between Blankenberge and Heist (Belgium), dating from 1852 to 1929 and sampled from herbarium specimens of macrophytes, were compared with more recent samples collected in the remaining calcareous dune marshes and pools in this area. 2. Overall, nutrient conditions inferred from the reference assemblages were fairly eutrophic for phosphorus. Only a minority of the historical assemblages pointed to presumably nitrogen-limited conditions. 3. Significant alterations in general assemblage composition were observed, including a marked decline of epiphytic species, and a decrease in the compositional variation in sediment diatom assemblages. These changes can be attributed mainly to an increased availability of nutrients and degradable organic matter since the mid 1970s. No changes in the salinity range seem to have occurred, suggesting fairly stable hydrological conditions. 4. Possible causes for eutrophication include increased atmospheric deposition of nutrients, but also more site-related phenomena such as guanotrophication, angling and, perhaps, effects of nature management on soil-nutrient cycling. Their relative importance needs to be established and further monitoring is necessary. 5. Measures are required to reduce nutrient levels of both permanently and periodically inundated sites and to promote small-scale habitat differentiation. Due to physical constraints, the latter will be possible only by mimicking the processes that act upon more natural dune systems in management practice.