Spatial distribution of particulate organic matter (POM) and benthic invertebrates in a river-floodplain transect (Danube, Austria): importance of hydrological connectivity
Tockner, K.; Bretschko, G.
Sediment parameters (grain size distribution, organic matter content) and invertebrate communities were taken as 'functional describers' to investigate the importance of hydrological connectivity in a river-floodplain transect of a free-flowing section of the Austrian Danube. Bed-sediments with their associated fauna were sampled quantitatively using the freezing-core technique in combination with in situ electro-positioning. In sediments of floodplain water bodies, organic matter contents were one order-of-magnitude greater than in river sediments (TOC: 38.90 g.kg -1 ± 36.60, TN: 1.98 g.kg -1 ± 1.55). Contents and concentrations were positively correlated with the degree of isolation (expressed as accumulated water table fluctuations in meters per year), but decreased with sediment depth, whereby the degree of decrease was highest in the most isolated waters. The data presented here showed no clear relationships between inundation frequency and macroinvertebrate structure. Within sites, heterogeneity was as high as between channels; the highest number of macroinvertebrate taxa was found in plesiopotamic water bodies directly connected with the river for 45 days per year. Our results demonstrate an ongoing separation of floodplain waters from the main river and an increasing fragmentation of backwaters. Presently, macroinvertebrate abundances and diversities in backwaters are still high, although this may be transitional. Most of the backwaters accumulate organic matter that was previously exported into the river channel. Generally, temporal variability is replaced by spatial heterogeneity.