Table 4 Criteria used for the determination of the dispersal types

Active dispersal

This category groups those organisms that perform vertical diel migrations by swimming up into the water column position in/on or near the sediment (i.e. emerging benthos Alldredge and King 1985; Kaartvedt 1986; Mees and Jones 1997; Kringel et al. 2003). This was corroborated with analyses of our own data collected using emergence traps in Bolsico and Colorado (A. Pacheco, M. Oliva & M. Thiel unpublished data). Emergence traps were constructed from 5 l cylindrical plastic buckets (21 cm diameter and 24 cm height) with an inverted funnel inside. The inner part of the cylinder lid was removed and replaced with a 0.5-mm mesh. The inner funnel (bottom diameter 21 cm and top 4.5 cm) was translucent plastic. The distance that emerging organisms had to swim was 14 cm (length of the funnel), and the area of sediment from which animals could emerge was 1,090 cm². Two metal bars extending 20 cm into the sediment were attached at two opposite sides of the trap to fit it in place. During 5 days, five replicate emergence traps were installed in the bottom shortly after sunset and collected the next day shortly after dawn. During sampling, traps were carefully removed by SCUBA divers, and a rubber plug was inserted into the inner funnel for transport to the boat. Onboard, lids were removed, and the interior part was washed with seawater dripping through a 0.3-mm mesh. The collected organisms were then deposited in plastic bags sealed with marked rubber bands and preserved in a 10% formalin–methanol solution stained with Bengal Rose. In the laboratory, organisms were identified and counted to the lowest taxonomical level. The experiment was repeated during 5 consecutive days in early May in Bolsico and during early July in Colorado in 2011. The main taxa in this category are amphipods, cumaceans and ostracods

Swimming/crawling

This category was assigned to those organisms with full swimming or crawling abilities (see also supplementary material in Pacheco et al. 2011). However, this category differs from active dispersal as the taxa considered here may not swim as often as in regular intervals that occur with swimming during diel vertical migrations, but rather in response to disturbances, for example sediment disturbance due to biotic or abiotic factors, predation or interference competition (Jensen 1985). The main taxa in this category are lancelets, gastropods and some polychaetes

Passive dispersal

These are organisms with poor swimming abilities, but they may take advantage of the bottom hydrodynamics for flotation and thus be passively dispersed. These organisms may also show some adaptation for such dispersal, for example mucus threads in bivalves (Armonies 1992, 1994; Valanko et al. 2010). These organisms are commonly eroded together with sediment during relatively strong bottom flow. Consequently, these organisms colonize the experimental containers during resuspension events. This criterion was further corroborated by the preliminary results of sampling organisms using water column drift traps. These traps were constructed by cutting out the bottom of a plastic jar (16 cm diameter and 25 cm long) that was surrounded by a conical mesh sleeve (0.5 mm mesh size) that extended 40 cm from the bottom part of the jar to the end of the cone mesh. In order to keep the opening of the trap vertically positioned and perpendicular to the bottom, a 0.5-l plastic bottle was tied to one side, and a 10-cm steel bar anchor was installed at the opposite side. The anchor was buried in the sediment to keep the trap in a fixed position. Both flotation and anchoring devices were attached to the trap using snap hooks which allowed the trap to freely rotate in its vertical axis, always self-adjusting into the current direction without contact with the sediment. Traps were elevated five cm from the bottom. Traps were exposed for three consecutive days and then sampled. This design used five replicate traps and sampling was repeated five times. This experiment was conducted in early May in Bolsico and during early July in Colorado in 2011. The main taxa in this category are bivalves and polychaetes