Alteration of biogenic structure and physical properties by tube-building chironomid larvae in cohesive sediments

From earlier research it has been suggested that chironomid larvae may significantly affect the physical properties of cohesive sediments, leading to chemical alteration in lake ecosystems. The present study was aimed to evaluate the effect of Tanytarsini larvae on the physical properties of cohesive sediments, both in controlled environments and in natural environments. For this the sediment surface was examined by low temperature scanning electron microscopy (LTSEM) and direct measurement of the shear strength. The larvae are able to modify the surface sediment significantly, e.g., by secretion of silk, which forms a matrix at the sediment surface in the vicinity of the larval tubes. The shear strength increased with greater sediment depth in all the experimental treatments, except where the densities were high. Considerably higher shear strength was observed in the natural sediment than in the experimental set up. A very significant density-dependent effect was observed within the top 1-cm by tube building Tanytarsini larvae. The relationship between larval densities and shear strength was not significant for 2 deeper sections (1-3 cm). The tube length was significantly greater in experimental containers with the low density (4960 ind. m^sup -2^) than in containers with high larval densities (19840 and 39680 ind. m^sup -2^).[PUBLICATION ABSTRACT]