Data from: Disassembly of an epibenthic assemblage in a sustained severely hypoxic event in a northeast Pacific basin

As global ocean deoxygenation proceeds and the frequency of extreme
low-oxygen (hypoxic) events increases, seafloor ecosystems will inevitably
be affected. Our study examines how benthic community responses scale with
the severity and spatial extent of hypoxia. Saanich Inlet is a natural
model system for testing the effects of hypoxic conditions that determine
the benthic megafaunal community structure during annual deoxygenation and
reoxygenation. In 2016, an anomalously severe and widespread hypoxic event
occurred in the fjord after a decade of oxygen decline at a rate of
0.07 mL L−1 y−1 as measured by a cabled seafloor observatory. We use a
living ecological time-series generated from remotely operated vehicle
surveys to assess how the benthic megafaunal community disassembled in
response to this extreme hypoxic event. Three benthic surveys at similar
times in 2013 and 2016 reveal large increases in the area of seafloor
bathed in anoxic and hypoxic waters in the latter year. Both bottom oxygen
and species depth distributions shoaled from 2013 to 2016, accompanied by
a 56% overall decline in megafauna. The abundant habitat-forming
pennatulacean octocoral, Halipteris willemoesi, decreased in abundance by
92.3% from fall 2013 to fall 2016, and pandalid shrimp disappeared from
the community. Hypoxia-tolerant species experienced milder losses, and two
new species occurred in 2016 – one a predator on the coral. Co-occurrence
analyses revealed loss of significant community segregation from 2013 to
2016, and a re-mix of pairwise species co-occurrences in the latter year.
The loss of oxygenated habitat compressed species into narrower depth
ranges and led to disassembly probably through niche space constriction,
while decimation of some populations eliminated key community
associations. Community fragmentation and disruption of interactions is a
possible outcome for many marine benthic ecosystems as marine hypoxia
intensifies.