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Kwadwo Omari - Phytoplankton Ecology

Kwame Nkrumah University, Ghana

Why is the Rhode River so Muddy?

The Rhode River which was once vegetated exhibited less underwater grasses in the 1960’s and sporadic occurrence since then.  Sea grasses are important as food sources and providing habitats for organisms as breeding grounds and protection from predators.  Their presence in the water much depends on the clarity of the water which is dependent upon the amount of chlorophyll and particulate matter.  Turbidity is introduced into the water column through run-off and bank erosion sediments.  Bottom sediments may be re-suspended through wave action and through a process called bioturbation.  Historically water clarity is highest in the winter (November-March) and lowest in the summer (June-August) for the Rhode River.  Additional factors during the year that affect sediment loading to the river are peaks of winds, run-off, and sediments.  The experimental approach was to enclose a volume of water by using short and long cylinders at two locations: Fox Point and Canning House Bay.  Turbidity and chlorophyll readings were taken every 30 seconds using an YSI probe.  The experiments were conducted in two parts; before and after anchor was dropped in the cylinders.  Water samples were collected and analyzed for total suspended solids (TSS) and fixed suspended solids (FSS).  Conclusions drawn from these experiments found that the TSS was comprised of 75% inorganic and 25% organic components.  Before the disturbance there was no significant difference between sites for time constant or baseline turbidity.  After the disturbance, time constant was significantly shorter and baseline turbidity higher at Fox Point.  The water column clearance time for rapidly settling particles was 0.6 hour.  The difference between sites was consistent with expectations for the sandy and muddy bottoms.  The comparisons between sites indicate that shallow muddy sites may be the source of turbidity for most of the river.  The susceptibility of the bottom to re-suspension appeared to progress throughout the summer period.  In the summer there is much biological activity and would be worth repeating this study during the winter months to better understand the re-suspension process in the Rhode River.

Funding was provided by the Smithsonian Institution Women’s Committee