Scottish Marine and Freshwater Science Volume 1, Number 19

High levels of suspended sediment caused by anthropogenic activity can be detrimental to the healthy functioning of the marine ecosystem. However, before a proper evaluation can be made, such elevated levels of suspended sediment should be set against the natural pattern of sedimentation in the marine environment. To understand the pattern of natural sedimentation in a hydrographically dynamic environment, this study measured suspended sediment concentrations, settled sediment levels, near-bed current speeds and wind speeds in the Firth of Lorn special area of conservation between December 2008 and March 2009. Background sediment concentrations were found to vary within the 2 – 13 μl l-1 range, with sediment concentrations rapidly increasing above 17 μl l-1 during strong gales from the south. Maximum concentrations of suspended sediment reach 60 μl l-1, with minimum levels typically 1 μl l-1. The study suggests that a prerequisite for high concentrations in suspended load are high levels of settled sediments. Northeast residual currents correspond with periods of high particle suspension with a possible explanation of this being that such flow patterns transport sediment to the site. Tidal currents in the Firth of Lorn caused regular 5 μl l-1 and 1 μl l-1 fluctuations in suspended sand (particle size 50 – 500 μm) and suspended silt (particle size 4 – 58 μm) respectively. Overall, the study suggests that tide and wind driven currents can explain much of the patterns observed in sedimentation, but features specific to the site, such as the islands and sand banks in close proximity, also influence the suspended load.