Study on redox state and grain size of sediments in a mud flat of the Venice Lagoon

Abstract

An extensive investigation based on the redox potential and grain size distributions was made on the sediment of a Venice Lagoon mud flat subjected to excessive growth of macroalgae. Redox potential and grain size measurements are proved useful “tracers” for, respectively, oxygen bearing and consuming processes in the water-sediment column and hydrodynamical behavior inside the mud flat. Depth measurements and a considerable number of sites with respect to the size of the area studied are needed to obtain an outline of the behavior of the water body in response to stress conditions caused by human activities. With respect to the top 15-cm-thick sediment layer where EH variations occur, the mud flat is subdivisible into sectors with different characteristics. Positive or near-zero EH values were recorded in zones characterized by sparse macroalgae growth and a high content of coarse sediments (diameter ⩾44 µm). On the contrary, very negative EH values were found in zones affected by overabundant macroalgae bloom and with a higher presence of fine-grained sediment (diameter ⩽44 µm). The clear relationships between algae presence in the mud flat and both the redox potential and grain size characteristics of the sediment emphasize the hydrodynamics as a “critical factor” determining the variations of the environmental conditions in the ecosystem.