In:
Biogeosciences, Copernicus GmbH, Vol. 16, No. 22 ( 2019-11-15), p. 4337-4356
Abstract:
Abstract. Thriving benthic communities were observed in the oxygen minimum zones along
the southwestern African margin. On the Namibian margin, fossil cold-water
coral mounds were overgrown by sponges and bryozoans, while the Angolan
margin was characterized by cold-water coral mounds covered by a living
coral reef. To explore why benthic communities differ in both areas, present-day environmental conditions were assessed, using conductivity–temperature–depth (CTD) transects and bottom
landers to investigate spatial and temporal variations of environmental
properties. Near-bottom measurements recorded low dissolved oxygen
concentrations on the Namibian margin of 0–0.15 mL L−1 (≜0 %–9 % saturation) and on the Angolan margin of 0.5–1.5 mL L−1 (≜7 %–18 % saturation), which were
associated with relatively high temperatures (11.8–13.2 ∘C and
6.4–12.6 ∘C, respectively). Semidiurnal barotropic tides were
found to interact with the margin topography producing internal waves. These
tidal movements deliver water with more suitable characteristics to the
benthic communities from below and above the zone of low oxygen.
Concurrently, the delivery of a high quantity and quality of organic matter was
observed, being an important food source for the benthic fauna. On the
Namibian margin, organic matter originated directly from the surface
productive zone, whereas on the Angolan margin the geochemical signature of
organic matter suggested an additional mechanism of food supply. A nepheloid
layer observed above the cold-water corals may constitute a reservoir of
organic matter, facilitating a constant supply of food particles by tidal
mixing. Our data suggest that the benthic fauna on the Namibian margin, as
well as the cold-water coral communities on the Angolan margin, may
compensate for unfavorable conditions of low oxygen levels and high
temperatures with enhanced availability of food, while anoxic conditions on
the Namibian margin are at present a limiting factor for cold-water coral
growth. This study provides an example of how benthic ecosystems cope with
such extreme environmental conditions since it is expected that oxygen
minimum zones will expand in the future due to anthropogenic activities.
Type of Medium:
Online Resource
ISSN:
1726-4189
DOI:
10.5194/bg-16-4337-2019
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2019
detail.hit.zdb_id:
2158181-2
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