Data for: Direct and indirect ecosystem responses to vehicle compaction of soft sediments

In northeastern Pacific tidal flats, a native bioturbator, ghost shrimp
Neotrypaea californiensis, engineers soft-sediment habitat and interacts
antagonistically with bivalve shellfish. Vehicle compaction has been used
in pest control of ghost shrimp, but this disturbance lacks quantitative
evidence of its efficacy and environmental impacts. Through three large
(~10 ha) experiments in Grays Harbor, Washington, USA, we tested the
direct and indirect impacts of compaction by a tracked vehicle
(MarshMaster) on ghost shrimp density, sediment conditions, and infauna.
We also examined how oyster survival (cultch seeded with Pacific oysters,
Magallana gigas) and waterbird usage of tidal flats responded
post-compaction. In Experiment 1, compaction occurred on 13 Aug 2022 and
19 May 2023, and sampling occurred seasonally over three years, including
immediately post-compaction. In Experiment 2, compaction occurred on 2 Aug
2023, and sampling occurred seasonally over one year. Due to minimal
treatment effects at this site, a third experiment was set up on 0.16-ha
beds that were compacted with 0, 1, 3, or 5 compaction passes on 24 Jul
2024 and followed for a year. Compaction pushed <20% of ghost
shrimp to the surface, where they were vulnerable to predation and damage,
yet did not significantly reduce subsurface densities within 1-2 days.
Rather, declines in shrimp density and shifts to smaller size classes
appeared at later sample timepoints and were more pronounced with more
compaction passes. All compaction experiments resulted in firmer sediment
for at least a year, even in the experiment where shrimp densities were
unaffected by a single compaction pass. Where compaction briefly reduced
shrimp densities below 50 m-2, sediment increased in mud and organic
content and infauna increased in abundance, suggesting that these changes
were mediated through reduced bioturbation rather than a direct impact of
compaction. Similarly, multivariate responses of infauna appeared only in
the experiments where compaction reduced shrimp densities. Habitat use by
waterbirds was more influenced by tidal stage than by compaction;
statistically, only dunlin (Calidris alpina) foraged more on compacted
than on reference beds. Finally, although survival of outplanted oyster
seed improved with compaction at one site, it remained too low (~34-40%
yr-1) for viable farming.