COMPARISON OF MIXED-SOFT AND TRADITIONAL HOMOGENEOUS SEDIMENTS – ARE THESE SEDIMENTS CHARACTERIZED BY DISTINCTIVELY DIFFERENT PARADIGMS?

During studies from 1989 through 1996 and in 2002 to evaluate treatment effects of high-pressure washing on bivalve assemblages and sediments on unconsolidated beaches oiled by the Exxon Valdez spill in Prince William Sound, we found that sedimentary components and the biota in mixed-soft (heterogeneous) sediments characteristic of these beaches do not respond in accordance with paradigms traditionally observed for homogeneous gravel, sand, or mud sediments. Relationships observed between mean grain size in mixed-soft sediments (ranging from 2 to 13 mm), and silt/clay, organics, and biodiversity in these sediments differ substantially from those generally observed in homogeneous sediments (e.g., 1.0 to 0.008 mm). For example, regression models show that organic nitrogen content in homogeneous sediments declined from ≈0.10% at a median grain size of ≈0.016 mm to zero at ≈0.5 mm. In contrast, a comparable range of concentrations was observed in mixed-soft sediments ranging from 2 to 15 mm. Similarly, regression shows that silt/clay content in homogeneous sediments declined from >90% at median grain sizes finer than 0.063 mm to zero at 0.5 mm but ranged up to 7% in the much coarser mixed-soft sediments. Similar patterns were observed for species richness and abundance.

Patchy shoreline washing provided an experimental design that allowed comparison of sedimentary and biological variables between washed (disturbed) and unwashed (undisturbed) sites. Based on this comparison, we deduced that larger clasts at the surface of coarse sediments at unwashed sites appear to form an organized surficial layer that protects underlying sediments and the infauna from erosion, physical disturbance, and predation. In contrast, this protection appears weakened at washed sites.

Although subsequent observation suggests that such organized sediments appear to be distributed worldwide, their presence has been poorly noted previously and paradigms describing their sedimentary and infaunal relationships have not been reported. Furthermore, the importance of clast organization and the resulting rich infaunal assemblages has not been recognized when considering significant sediment management decisions. Differences between the sedimentary and biological paradigms for these contrasting sediments will be demonstrated.