2000.Phenolic contents of submerged, emergent and floating leaves of aquatic and semiaquatic macrophyte species: why do they differ? -Oikos 91: 307 -310.The phenolic content of leaves of 40 aquatic and semi-aquatic plant species, collected from stands distributed over the Netherlands was investigated. Species with submerged leaves showed a significantly lower phenolic content than species with emergent or floating leaves; the latter two showing no significant differences. Possible explanations for the observed differences are discussed in relation to defence strategies.

From 1997 until 1999 the extent and the ecological effects of zinc, copper, lead, and cadmium pollution were studied in different reaches of the South American Pilcomayo River. A comparison of metal concentrations in water, sediment, and chironomid larvae, as well as the diversity of macroinvertebrate species, was made between sites near the origin of the Pilcomayo River, with hardly any mining activities, sites in the Potosí region, with intensive mining, and sites located 500 km or further downstream of Potosí, in the Chaco plain. Samples were also collected in an unpolluted river (Cachi Mayu River) and in the Tarapaya River, which is strongly contaminated by mine tailings (1000 tons a day). The upper parts of the Pilcomayo River are strongly affected by the release of mine tailings from the Potosí mines where mean concentrations of lead, cadmium, copper, and zinc in water, filtered water, sediment, and chironomid larvae were up to a thousand times higher than the local background levels. The diversity of the benthic macroinvertebrate community was strongly reduced in the contaminated parts; 97% of the benthic macroinvertebrates consisted of chironomid larvae. The degree of contamination in the lower reaches of the river, however, was fairly low because of sedimentation processes and the strong dilution of mine tailings with enormous amounts of clean sediment from erosion processes. Analysis of sediment cores from the Ibibobo floodplain, however, reveal an increase of the heavy metal concentrations in the lower reaches since the introduction of the contaminating flotation process in the mine industry in 1985.

The relationship between the nitrogen and the free amino acid content in Stratiotes aloides L. was investigated for 11 healthy stands in the Netherlands. In the shoots, the mean free asparagine and free arginine levels were strongly correlated with the mean total N concentrations and the ammonium concentrations in the water layer. The percentage of total N present as free amino acids varied from 5.4 % to 27.5 % between the stands. In the stands with the highest total N levels, up to 92 % of the nitrogen in the free-amino acid pool consisted of asparagine and arginine. The results reveal that the accumulation of these "N-efficient" free amino acids was caused by increased ammonium availability in the environment. The accumulation of specific free amino acids is regarded as an early indication for N-stress in still healthy S. aloides stands.

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