Influence of Water Masses on the Biodiversity and Biogeography of Deep-Sea Benthic Ecosystems in the North Atlantic

Circulation patterns in the North Atlantic Ocean have changed and re-organized multiple
times over millions of years, influencing the biodiversity, distribution, and connectivity
patterns of deep-sea species and ecosystems. In this study, we review the effects
of the water mass properties (temperature, salinity, food supply, carbonate chemistry,
and oxygen) on deep-sea benthic megafauna (from species to community level) and
discussed in future scenarios of climate change. We focus on the key oceanic controls
on deep-sea megafauna biodiversity and biogeography patterns. We place particular
attention on cold-water corals and sponges, as these are ecosystem-engineering
organisms that constitute vulnerable marine ecosystems (VME) with high associated
biodiversity. Besides documenting the current state of the knowledge on this topic,
a future scenario for water mass properties in the deep North Atlantic basin was
predicted. The pace and severity of climate change in the deep-sea will vary across
regions. However, predicted water mass properties showed that all regions in the North
Atlantic will be exposed to multiple stressors by 2100, experiencing at least one critical
change in water temperature (C2C), organic carbon fluxes (reduced up to 50%), ocean
acidification (pH reduced up to 0.3), aragonite saturation horizon (shoaling above 1000
m) and/or reduction in dissolved oxygen (>5%). The northernmost regions of the North
Atlantic will suffer the greatest impacts. Warmer and more acidic oceans will drastically
reduce the suitable habitat for ecosystem-engineers, with severe consequences such
as declines in population densities, even compromising their long-term survival, loss of
biodiversity and reduced biogeographic distribution that might compromise connectivity
at large scales. These effects can be aggravated by reductions in carbon fluxes,
particularly in areas where food availability is already limited. Declines in benthic biomass
and biodiversity will diminish ecosystem services such as habitat provision, nutrient cycling, etc. This study shows that the deep-sea VME affected by contemporary
anthropogenic impacts and with the ongoing climate change impacts are unlikely to
withstand additional pressures from more intrusive human activities. This study serves
also as a warning to protect these ecosystems through regulations and by tempering
the ongoing socio-political drivers for increasing exploitation of marine resources.