Abstract
Pollutants and oil spills tend to concentrate in cohesive mixed-muddy sediments in low energy depositional environments such as estuaries. It is important to understand the critical bed shear stress, erodibility, and effect of bioturbation on these sediments to provide a baseline and predictive tool for potential movement of pollutants, and oil spills. The objective is to investigate seasonal variations in erodibility of mixed-muddy, cohesive sediments at a single site near Beautiful Island in the upper Caloosahatchee River estuary, Fort Myers, Florida. The investigation site is in a quiet area, sixty meters from the river bank, out of the main channel and current, and conducive to deposition of mixed-muddy cohesive sediments. FGCU’s U-GEMS laboratory erosion chamber is used to evaluate critical bed shear stress, and erodibility of sediment cores retrieved over three consecutive winter-dry, spring-transitional, and fall-wet seasons in 2013, 2014 and 2015. An additional objective is investigating seasonal variations in bioturbation, and its effect on erodibility through placing, coring, and counting of colored tracer grains redistributed by benthic fauna in the river bed. Twenty-one cores have been recovered with multiple cores per winter-dry, spring-transitional, and fall-wet seasons for erodibility, and twenty-four cores for bioturbation. Seasonal variations in erodibility are evident, with more erodibility in winter-dry and spring-transitional seasons, less in fall-wet seasons. Bioturbation follows a similar seasonal pattern and may influence erodibility. Changes in cohesive clay and silt and organic biomass content are affecting erodibility, whereas changes in salinity and fresh water releases from Lake Okeechobee are associated with reduced bioturbation, and by inference benthic macro-fauna. Increasing micro-algae and biofilms with higher nutrient loads from fresh water releases may also be suppressing erodibility.
