Frontiers: The deep sea and climate

When we think about climate change and the ocean, many minds turn immediately to images of shallow-water corals, bleached white from the lack of zooxanthellae (internal, photosynthetic symbionts), driven away by heat and other types of stress.  However, the consequences of an increased atmospheric CO₂ reach much deeper into the ocean.  The global ocean has an average depth of 3800 meters and comprises 71% of the total area of Earth, making the deep-sea far and away the largest biome on this planet.  In terms of volume, the deep-sea pelagic—the water-column itself—contains over a billion cubic kilometers of seawater.  Less than 5% of the deep benthos (the seafloor) has been remotely sensed, and less than a hundredth of one percent has been observed directly, sampled, and studied.  Even so, species diversity in the deep-sea is among the highest known¹.

As a society, we still collectively get excited about the discovery of new species. And we should—such discoveries are essential to science.  The public being interested in new species is also quite importance for the continued funding of exploratory research.  Since 1840, 28 new habitat or entire ecosystems have been discovered in the deep ocean.  Not simply new species, but entirely new environments. Cold seeps, hydrothermal vents, brine pools, xenophyophore fields, just to name a few—these are all habitats that have only been known since the 1970s¹.

Year of discovery of new habitat/ecosystem in the deep sea since 1840 (Ramirez-Llodra at al. 2010)

However, the lack of taxonomists to classify and describe the new species in these novel habitats dampens the spirit of discovery somewhat—specimens languishing in collections, as of yet unidentified due to the lack of support for specialists, harkening back to the last, frustrating scene in Raiders of the Lost Ark.

Atmospheric carbon dioxide concentrations are predicted to exceed 500 ppmv before 2100²,a value not seen in the past few million years³.  This is contributing towards both warming and ocean acidification^4,5.  It is uncertain how benthic organisms and their associated ecosystems as a whole will react; particularly little is known regarding the effects of climate change in the deep-sea.

Food Supply and Temperature

Ecologically, an important characteristic of the abyssal seafloor is energy limitation.  Benthic production is dependent on the sinking of particulate organic carbon (POC) from the sunlit zone of the surface ocean, though massive, yet very spatially contained, food-falls in the form of large vertebrate and invertebrate carcasses provide organic matter in a more unpredictable fashion⁶.  Some habitats host chemosynthetic organisms, capable of using inorganic chemicals rather than sunlight as an energy source; however, most deep-sea ecosystems known are heterotrophically fueled by the sinking organic matter from surface waters.

Despite the isolation experienced by the deep-sea, the climate does have an effect in this seemingly remote environment.  For instance, global latitudinal species diversity gradients (LSDGs) are well known and numerous in terrestrial, freshwater, and shallow marine systems⁷ with species diversity usually peaking in the tropics.  In 1993, work done in the North and South Atlantic by Dr. Rex and colleagues showed an LSDG for the first time in the deep-sea⁸.  Species diversity in gastropods, isopods, and bivalves showed elevation near the equator and depression near the poles.  Though not universally seen, as habitats and depths sampled vary, this is very often the case¹. Temperature variation is likely not to be responsible for this gradient as temperature in the deep-sea, especially on the abyssal plains and deeper, varies little⁶ and probably does not vary consistently enough with latitude.  Rex and coauthors point out that phytodetrital accumulation, or the vertical particulate flux from the surface, islikely responsible for this large-scale pattern in the deep-sea.  Even outliers in this study (sites at high latitude with higher than expected diversity) could be related to certain regions’ uniqueness in surface primary production. These spatial patterns in the deep-sea also vary though time and are highly influenced by climatological mechanisms.  On geologic timescales, these LSDGs have been shown to weaken or vanish entirely during glacial periods⁹.  On shorter timescales, evidence predicting seasonality in the deep-sea has also been documented, mainly in respect to the vertical flux of organic material⁶.  Lateral particle transport (another process of food input to the deep-sea) from continental margins has been related to climate in the Mediterranean Sea. In this case, particularly cold winters increase the intensity of the downslope cascade of deepwater and serve as a reliable indicator of the recruitment success of a deepwater species of shrimp, allowing the persistence of a heavily exploited fishery¹⁰.

Shifts in deep-sea community structure are also tied to climate.  The analysis of deep-sea fossils (benthic ostracodes, in this case) over past periods of climatic events show that the even deep-sea is not isolated from the consequences of rapid climate change, resulting in community collapses with drastically different suites of species present¹¹.  In more contemporary timescales, deep-sea nematodes have been seen to be vulnerable to environmental shifts, as evidenced by a decadal data set from the Eastern Mediterranean in which an abrupt drop of 0.4 C between 1992 -1994 (temperature in the deep ocean is usually quite stable) resulted in lower functional diversity that did not recover when the temperature returned to normal¹².  In a long-term study, the abundance of mobile epibenthic megafauana was monitored in the Pacific at 4100 meters depth for 14 years (1989-2002); community shifts in holothuroids were apparent¹³.  Prior to 1996, no trends in abundance and no correlation to food supply were observed.  However, after a sampling hiatus from 1999-2000, a common holothuroid, Elpidia minutissima, once found at densities of 1 individual per m², was found to be significantly less abundant in 2001-2002.  Other species of holothuroids at the same site increased in abundance during this same period. These shifts were correlated to El Niño/La Niña indices, which then were shown to be related to POC flux.  Upwelling data showed that POC transport to the deep-sea at this particular site increased from 1998-2002, providing evidence that some species may have advantages over others when food input is high, and others when food input is low.

If climate and the deep ocean are coupled both spatially and temporally, how will deep-sea ecosystems be altered, if at all, by the high CO₂ world above?  Increased atmospheric pCO₂ may indirectly reduce the amount of POC falling into the deep-sea, leading to reduced food supplies for abyssal ecosystems.  Warming of the surface ocean will result in biogeochemical changes, as well as increased thermal stratification in the upper regions of the water column¹⁴.  These biogechemical changes are likely to include both the reduction of surface primary production and the shifting of diatom-dominated phytoplankton populations to picophytoplankton-dominated populations, representing a decrease in the total carbon export efficiency to the deep ocean¹⁵.  Already oceanic warming has decreased global ocean primary productivity by 6%¹⁶.  The relationship between decreased surface production and POC input is nonlinear, with abyssal POC fluxes having the potential to decline more steeply than surface production, a phenomenon likely to be exacerbated by the effects of ocean acidification on planktonic communities^14,15.  Deep-sea ecosystems are affected by climatic forcing on the biogeochemistry of the upper ocean and are likely to experience restructuring in the form of increased food limitation as rapid climate change continues1⁴.  However, unknowns abound.  For example, a recent synthesis of global carbon budgets point out that the estimated metabolic activity in the deep-sea actually exceeds the known  inputof organic material¹⁷.   This work points to the further quantification of lateral advection and subsets of slowly settling particles, both poorly known mechanisms of organic input, as means of balancing carbon estimates and collective metabolic activity.   Better knowledge of carbon budgets in the ocean will help put constraints on estimates of climate change impacts in the deep-sea.

Seawater Chemistry

A warming climate is not the only consequence of a high atmospheric CO₂ load.  The ocean acts as a carbon sink, absorbing nearly a third of the carbon in the atmosphere¹⁸.  As atmospheric carbon rises, chemical speciation in the ocean shifts.  This ocean acidification alters the carbonate chemistry of the sea, leading to both reduced carbonate ion concentrations and pH values¹⁹.  The average surface ocean water has decreased 0.1 pH units since the preindustrial era to its value of 8.10 today²⁰, and is expected to drop another 0.3-0.4 units by 2100²¹.  This also reduces the aragonite and calcite saturation state of seawater—a measure of how readily these types of calcium carbonate can precipitate—which makes this of particular concern for marine calcifiers.

The insidious arm of ocean acidification may reach the deep ocean yet.  Deep-sea corals are suspension-feeding cnidarians, which can create complex spatial structures at depths from ~50- ~4000 m, hosting assemblages of fish and invertebrates²². Some can produce large amounts of calcium carbonate, similar to their shallow, photosynthetic relatives and can serve as paleoceanographic proxies, recording glaciation cycles and circulation events.  Changes in the carbonate system are predicted to cause the aragonite saturation horizon (below which aragonite–a type of calcium carbonate begins to dissolve) to shoal, creating larger depth ranges experiencing aragonite undersaturation²³, potentially creating calcification issues much like those experienced in shallow water corals.  Areas of the Southern Ocean are expected to become undersaturated in respect to aragonite as soon as 2050 under the IPCC IS92a scenario (‘business-as-usual,’ ²³).  This is likely to have a substantial impact on deep-sea corals, resulting in reduced skeletal accretion and increased dissolution, as most records of deep-sea corals are found above the aragonite saturation horizon.  Even calcitic corals, which are less soluble then aragonitic species²⁴, will be affected as ocean acidification progresses.

Beyond affecting calcification, other physiological processes seem to be altered by shifting seawater chemistry as well, such as reduced reproductive success, at least in shallow-water studies.  Due to sampling expense and technological limitations, other physiological effects on deep-sea fauna are not well known.

To borrow a term from Dr. Jeremy Jackson, a “brave new ocean” is beginning to manifest²⁵.  In the deep-sea, organisms will be affected indirectly from the altered flux of organic material from the surface, though direct chemical effects are predicted (and may already be occurring).  Large-scale changes in the distributions of benthic organisms may occur in terms of latitude or depth (as with deep-sea corals) in these systems, yet few studies have addressed this.  While geoengineering approaches boast a potential quick fix compared to the seemingly more difficult task of lowering emissions, strategies such as deep-sea carbon injection may harm benthic organisms^26,27, while aerosol mitigation proposals cannot address the issue of ocean acidification²¹.  Carbon emission stabilization thresholds have been proposed²⁸, and this seems to be the most prudent plan.  Rapid climate change is impacting the benthos in such a way that restructured ecosystems could become widespread in the earth’s largest environment.

Image 1:  Lophelia pertusa in 450 meters of seawater, NOAA-OE on Flickr (cc).  Table 1 and image 2 are both from Ramirez-Llodra et al. 2010 (cc).

For a great primer on the deep sea, Ramirez-Llodra et al. 2010 is an excellent and thorough read.  You can download it here (open-access).

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