A mix of silts and organic waste particles, tinier than grains of sand, give raw sewage its cloudy appearance. Sewage solids discharges have affected the harbor both directly, by clouding the water, and indirectly by the BOD and toxic contaminants associated with the solids. Water clarity in the black sludge plume was often zero.
Aesthetics are important, but water clarity also has a major impact on marine ecosystems.
Many important species of plants, including seagrasses, require adequate light. If poor water clarity means insufficient light penetrates to the seafloor, where seagrasses are rooted, the plants will die off. When this happens, the bottom sediments are no longer bound and stabilized by the roots, and can be more easily stirred up, further exacerbating poor water clarity. Natural turbidity is highest in spring through fall. Other natural sources of suspended solids include particles brought in by rivers, and sediments stirred up into the water by storms or normal wave action.