Initial observations of the impacts of infauna on portable free fall penetrometer measurements in sandy parts of Mobile Bay

Abstract

The seabed surface provides habitat for abundant and diverse fauna, whose burrowing activities have been shown to modify geotechnical properties of surface sediments. Whether these impacts affect geotechnical properties on larger scales of traditional measurements has not been well studied. This study represents an initial attempt to assess whether infaunal activity affects seabed properties on a scale relevant for, and therefore, detectable in portable free fall penetrometer measurements. Specifically, we examine sediment strength profiles of the upper 10–70 cm of sandy (poorly graded sand and muddy sand) seabed sediments in Mobile Bay, Alabama, USA, hypothesizing that infauna create heterogeneity in sediment structure that would lead to variability in PFFP vertical profiles as well as among replicate measurements at a site. Sediments were composed predominantly of sands, with only 17% of the sites featuring sand contents < 97% and median grain sizes ranging from 0.0987 to 0.3457 mm. Sediment strength generally decreased with a decreasing sand content, but variability was not explained by sand content alone. PFFP impacts in sandier sites (> 97% sand) were limited to the surface few cm, but considerable vertical and spatial variability in muddy sands and lower strength at sites with abundant burrowing infauna suggest that infaunal activities may affect PFFP measurements in these sediments.