Effects of dredged spoils on selected microbial activities in lake column simulators

Four Lake Column Simulators were filled with hypolimnetic water from Lake Ontario and inoculated with algae and fish. Various microbial processes were monitored before and after the addition of dredged spoils. Loading rates of dredged spoils of 0.33, 3.3 or 33 g day^?1 were used for each of three columns and one column with no addition of dredged spoils was maintained as a control. The effects of dredged spoils on microbial activities were most pronounced at the highest loading rate. ³²P-PO₄ was rapidly removed from the control water but the presence of suspended particles from dredged material increased both biological assimilation and physical adsorption processes. The influence of dredged spoils on ³²P-PO₄ uptake was most pronounced immediately following loading. Dredged spoils also increased ¹⁴C-acetate mineralization in water and the effects were again more pronounced immediately following loading. A 3 fold increase in heterotrophic activity during a 1 h incubation at 20°C was observed following addition of a 33 g day^?1 loading. Dredged spoils increased the levels of N₂-fixation in the columns as indicated by the C₂H₂-reduction assay procedure. Suspended dredged material reduced light penetration and decreased primary production in the columns. The increased availability of dissolved and particulate organic and inorganic nutrients from continuous addition of dredged spoils was responsible for a general stimulation of microbial activities in the epilimnetic and hypolimnetic regions of the simulated lake columns.