The influence of submarine canyons on the structure and dynamics of megafaunal communities

Abstract

Submarine canyons are considered potential hotspots of biodiversity and abundance, which
makes them an important focus for scientific investigation and for policy-makers who seek to
manage their resources. This study investigates the structure and dynamics of megafaunal
communities in relation to inherent environmental parameters within submarine canyons;
understanding the distribution and resilience of hotspot ecosystems is important in creating
ecologically-sustainable resource-management plans. Data were collected from Nazaré,
Setúbal, Lisbon and Cascais Canyons (Portuguese margin) and a channel and adjacent slope on
the Pakistan margin, using passively-towed camera platforms and a remotely operated vehicle.
Photographic data were analysed from a range of canyon habitats; abundance and distribution
of megafauna were recorded and diversity indices calculated. Environmental variables were
used to interpret patterns of species distribution, abundance and diversity. Community
analyses were used to assess changes in community structure within and among canyons, and
the adjacent slope.

Portuguese margin: The canyons were ranked in order of decreasing activity: Nazaré,
Lisbon, Setúbal and Cascais Canyons. Expected species richness conformed to Connell’s
intermediate disturbance hypothesis, reflecting the activity of the canyons. Megabenthic
abundance did not exceed that reported in studies of nearby slope communities, suggesting
that the canyons are not hotspots of megabenthic abundance. Suspension-feeders dominated
all canyons though were most abundant in Nazaré Canyon, which was linked to observed
higher activity. All canyons showed a higher proportion of suspension-feeders than are
reported in studies from nearby slope environments. There was high variability in habitat type
within and between the canyons, which was reflected in the megabenthic assemblages.

Pakistan margin: Observations of turbidity and current velocity indicated that the
Pakistan margin channel is relatively inactive. Megafaunal abundance was significantly lower,
and diversity higher, within the channel than on the adjacent slope. Suspension-feeders
dominated all sites, though were significantly more numerous on the slope. The sites were
located below the base of the OMZ core, and communities displayed OMZ edge-effects with a
peak in abundance at ~ 1000 m WD, high dominance and a high rate of species turnover.
Community analysis suggested that the communities were more heavily influenced by the
proximity to the OMZ than by the channel, which indicated that either the influence of the
OMZ masks any potential channel-induced patterns, or that the channel is too small to
influence megafaunal communities.

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