Relationships between penetration resistance of sediments and abundance of benthic macrofauna on an intertidal sandy beach

Human activities, such as trampling and off-road vehicle, compact soil and affect the distribution of benthic macrofauna. As an effective indicator for the degree of compaction, soil penetration resistance (PR) increases as the degree of compaction increases. PR had applied extensively to measure compaction in many fields. However, it’s still poorly understood whether PR is also a good indicator for the effect of human activities on benthic macrofauna. This study investigated the relationships between PR and macrofauna on an intertidal sandy beach of recreational area. The abundance of macrofauna and sediments properties including PR were measured where sediment properties were uniform except for PR. Macrofauna were grouped into Polychaeta, Bivalvia, Decapoda and Amphipoda. Amphipoda was further classified into large (> 4 mm in body length), middle (1~4 mm) and small (< 1 mm) size. Abundance of each macrofauna group was analyzed with different degree of PR. The results showed that PR exhibited higher variance than other sediment properties, ranging from 0.2 to 2.2 MPa of PR average on 0~15 cm depth. Regression analysis between PR average and max abundance of Polychaeta, Bivalvia, large Amphipoda and small Amphipoda showed similar trend (r2 = 0.89, 0.80, 0.54, 0.94, p < 0.05), the higher the PR average, the lower the max abundance, which can decreased 78 to 100% as PR increased from lowest to highest. Further regression analysis between PR average and max abundance of middle Amphipoda on 5~10 cm and 10~15 cm depth layers also showed the same trend (r2 = 0.48, 0.62, p < 0.05) as described above. This trend implies the increase of PR is a stress for benthic macrofauna such as Polychaeta, Bivalvia and Amphipoda. The increase of PR represents an increase of resistance to activities in sediments and decrease of porosity in sediments, the former may increase resistance of burrowing by Polychaeta, Bivalve and Amphipoda, and the latter may decrease pore space accessible for small Amphipoda and lower permeability to oxygen in sediments. This study suggests that PR is a good indicator to evaluate the stress to benthic macrofauna, and this would apply to monitor benthic ecology and manage human activities on an intertidal sandy beach.