Oceans in the news: November

Here are a couple of ocean related articles that have been in the news recently!

17th November – ‘Emissions of CO2 driving rapid oceans 'acid trip'’ (BBC, 2013)

This article focuses on ocean acidification and follows suitably on from some of my latest posts. Following the 2012 Third Symposium on the Ocean in a High-CO₂ World, the article discusses the Summary for Policy Makers report that has just been released and that presents a summary of all the ocean acidification research that was raised at the symposium.

The report states that due to anthropogenic emissions of CO₂, the world’s oceans are acidifying at an unprecedented rate and this is happening quicker than at any point in the past 300 million years. Unsurprisingly, this has had many implications on marine life, including a 30% species loss in some ocean ecosystems. Furthermore, it has been proposed that by 2100, acidification could increase by 170%.

I would definitely advise giving the article a quick read over as it provides a good introduction to some of the main points in the report and to ocean acidification in general. If you have some more time on your hands though, take a glance at the full report here!

21^st November – ‘English seas get new marine conservation zones’ (BBC, 2013)

27 new marine conservation zones (MCZs) will be created around the English coast, in order to protect sea-life from dredging and bottom-trawling.

These are two anthropogenic fishing methods detrimental to ocean habitats and ecosystem health. Bottom-trawling is an extremely invasive method and consists of a net that is dragged across the ocean floor, often removing corals and sponges with it. Dredging, on the other hand, consists of a rakelike device that scrapes across the ocean bed. Both pose a major threat to any bottom-dwelling organisms and can dramatically alter biological communities. For instance, the disturbance of coral and sponges from bottom-trawlers can destroy many species’ habitats that they use for breeding, shelter and feeding and normally results in reduced population numbers (The Ocean Conservancy, 2002). This is just one example of how these two fishing methods can impact marine ecosystems; there are many, many more.

Amongst those species protected by this governmental scheme are seahorses, coral reefs and oyster beds. Even though it is a step in the right direction towards protecting these species from anthropogenic fishing activities, the number of MCZs proposed is nowhere near the 127 recommended by scientists for an ‘ecological coherent’ network. Hopefully more can be done in the next three years where there are plans to designate two more phases of MCZs.  

25^th November – ‘New widget counts global warming happening at 4 Hiroshima atomic bombs per second’

This is a really cool new widget that tries to put global warming into terms that the general public can more easily visualize. Its aimed to try and debunk the myth that global warming has stopped or paused. Now I know it is not directly ocean related, but the changes we are seeing in our seas are all related to global warming! Check it out!

Deadly Threat No. 3: Ocean deoxygenation

Principally, seawater’s oxygen concentration is determined by both the diffusion of atmospheric oxygen across the air-sea interface and the consumption through microbial respiration (Falkowski et al., 2011). The resultant oxygen concentration naturally varies with time and space, but in no way accounts for the decline in oxygen content that we are seeing today. Referred to as ocean deoxygenation, it is the third ‘deadly’ threat that the ocean faces and comes in two forms.

The first form of oxygen depletion, occurring in the central North Pacific Ocean and tropical oceans, relates to the impacts of global warming and regional freshening. In both of these open basins, observations over the past 50 years have shown falling oxygen levels (Keeling et al., 2010). Behind this decline are a number of factors connected to climate change (Bijma et al., 2013), such as:

1. The decreased solubility of oxygen in a warming ocean.
2. The increase in respiration rates with rising ocean temperatures.
3. The reduced ventilation in high-latitude areas linked to increased ocean stratification. The latter inhibits the exchange of oxygen between surface and subsurface waters, ultimately reducing the supply of oxygen to the ocean’s interior (Keeling et al., 2010).

Currently, though, it remains uncertain as to whether this deoxygenation in the open ocean is a long-term trend related to climate change (the factors listed above), the outcome of natural cyclical processes, or a combination of both (Bijma et al., 2013).

Elsewhere, on the western coast of North America, anoxic (lacking oxygen) waters have been found on the inner shelf, adjacent to the upwelling zone. These continental margin upwelling systems are naturally prone to deoxygenation, due both to the low oxygen source waters and the high flux of nutrients that promote high phytoplankton biomass that then settles to the seabed where it is decomposed by respiring bacteria. However, for the western coast of North America, it was discovered that the upwelling of oxygen depleted waters was due to an increase in the frequency and strength of upwelling inducing winds; events symptomatic of climate change (Chan et al., 2008).

The second form of oxygen depletion refers to the rapid rise in coastal hypoxia from anthropogenic eutrophication. This results from nutrients found in agricultural fertilisers and sewage pollution that find their way into the ocean through urban runoff, causing blooms of algae and plankton (Bijma et al.,2013). Eventually these die and fall to the seabed, where they are decomposed by oxygen consuming bacteria. This lowers the oxygen content of the water, depleting the supply available for marine life. It is for this reason that these hypoxic areas have become known as marine dead zones – there is simply not enough dissolved oxygen to support marine life. Since the introduction of industrially produced nitrogen fertiliser in the late 1940s, oxygen depleted zones have spread, but with a lag of 10 years between their use and the incidence of hypoxia. Indeed, over the past 50 years, the number of dead zones has approximately doubled each decade (Diaz and Rosenberg, 2008). This correlates to human’s continued settlement in coastal regions.

Figure 1 shows the location of the ocean's dead zones. Those areas most affected include the Baltic Sea, which harbours seven of the world’s 10 largest marine dead zones, and also the northern Gulf of Mexico (National Geographic, 2013).   Inland seas and estuaries are also extremely prone to coastal hypoxia; some examples include the Kattegat, the Black Sea and Chesapeake Bay (IPSO, 2013).

Figure 1. Map of aquatic dead zones around the world (NASA, 2010). Click here to see the high-resolution version.

However, it is important to note that the research into ocean deoxygenation is still in its infancy. Prior to 1960, it is extremely rare to find accurate measurements of oceanic oxygen concentrations, making it hard to accurately discern patterns (Falkowski et al., 2011). Large gaps in data also make it difficult to make reliable predictions for the future. Keeling et al. have predicted that by 2100, the ocean’s oxygen content will have declined by between 1 and 7% - notice the uncertainty (2010). This will undoubtedly have profound consequences on the ocean’s biology, but also its biogeochemical cycling. This post has ended up rather long though, so I shall save these for next time!

The biological implications of ocean acidification

A couple of posts ago, I explored the chemistry behind ocean acidification but did not touch on any of the implications on marine life. Hence, I will be covering this today, focusing mainly on calcification rates and acidosis. If you missed my explanation of ocean acidification, you can find it here.

Firstly, marine calcifiers such as reef-forming corals, protozoans, molluscs, crustaceans, echinoderms and some algae are greatly affected by the decrease in carbonate ions (CO_­3^2-) associated with CO₂ dissolution (Bijma et al., 2013). They use CO₃^2- to build and maintain their calcium carbonate shells and skeletons, but this becomes increasingly difficult with falling CO₃^2- concentrations. Consequently, many calcifiers have shown reduced rates of calcification in response to the increasing partial pressure of CO₂ (Fabry et al., 2008).

For tropical reef-building corals, it is predicted that calcification rates will have reduced by 20-60% once CO₂ concentrations reach double that of preindustrial levels. A change of this magnitude could slow growth rates, making reefs much more vulnerable to erosion. Furthermore, with the associated reductions in CO₃^2- ions, it is likely that much weaker skeletons will form and thus, allow erosional processes to occur at quicker rates than previously (Guinotte and Fabry, 2008). A report issued on Friday, following last year’s Third Symposium on the Ocean in a High-CO₂ World, delivers some extremely worrying facts. They state with high confidence that ‘If CO₂ emissions continue on the current trajectory, coral reef erosion is likely to outpace reef building sometime this century’. Take ocean warming into account and the prospect is even more dire – erosion could potentially outpace growth by mid-century, once CO₂ levels reach 560 ppm. If this happens, there would be consequences for coral biodiversity, with many species losing their habitat (IGBP et al., 2013).

Coral reef near Fiji

However, it is not all doom and gloom for tropical corals. Fine and Tchernov found that two species of scleractinian corals were able to survive under acidic conditions (pH values of 7.3-7.6) (2007). Initially, their skeletons dissolved leaving the coral polyps exposed, but when pH returned back to normal, the polyps were able to recalcify with no lasting effects. This offers a glimmer of hope for corals’ future in a high CO₂ world. Still, we have to remain cautious as the study did not consider the effects of predation on the naked polyps and there has been discrepancy about its representativeness (Guinotte and Fabry, 2008).

As for cold-water corals, 70% will experience unsaturated waters by 2100 (high confidence), with this beginning as early as 2020 for some (IGBP et al., 2013). As highly productive ecosystems with a rich biodiversity, this would impact the many deep-water organisms that rely on them as feeding grounds, habitat and nursery areas (Guinotte and Fabry, 2008).

Many plankton and zooplankton species are also affected by changes to the carbonate chemistry of the ocean (see Figure 1). This is potentially detrimental considering they form integral components of the marine food chain. Amongst those most sensitive to ocean acidification are pteropods – the major planktonic producers of aragonite. In waters that are unsaturated with respect to aragonite, it has been observed that pteropods are unable to maintain their shells and consequently, have begun dissolving (Orr et al., 2005). This is occurring in the Southern Ocean today, impacting predators such as pink salmon (IGBP et al., 2013). The future for pteropods that are endemic to polar regions depends on their ability to transition to lower latitudes, where warmer, carbonate-rich waters await (Orr et al., 2005).

Figure 1. Photos showing scanning electron microscopy photographs of the coccolithophorid Gephyrocapsa oceanica under different carbon dioxide concentrations; normal in the top image (300 ppm) and elevated in the bottom (780-850 ppm). The distinct structural malformations can be clearly seen under higher carbon dioxide concentrations (Riebesell et al., 2000). 

However, once again, it is not all bad. Whilst the majority of marine calcifiers do see decreasing calcification rates under acidic waters, some organisms actually benefit; these include some fleshy algae, seagrasses and phytoplankton groups (although further research is still required to fully understand the underlying mechanisms) (IGBP et al., 2013). It appears that under higher CO₂ concentrations, these experience increased photosynthesis and, therefore, growth (Gattuso and Hansson, 2011).

Lastly, for many marine animals, including invertebrates and some fish, the acidification of ocean waters can result in acidosis. This is where high environmental CO₂ levels cause an increase in carbonic acid in the bloodstream. Consequently, the blood's pH is lowered, affecting many cellular processes. Acidosis can lead to metabolic and behavioural depression, lowered resistance and asphyxiation (Howard et al., 2012). Compared to less mobile organisms, fish appear to be more resistant to ocean acidification as they do not have extensive calcium carbonate shells (IGBP et al., 2013). However, particularly in larval fish, acidic seawater has led to impaired sensory performance, altered behaviour and decreased growth rates, all of which impact predator-prey relationships (Howard et al., 2012). Overall though, this area remains insufficiently explored and further research is required to fully understand the physiological mechanisms behind individual animal responses to elevated CO₂ levels.

To round things up, whilst a few organisms can tolerate or actually benefit from acidic waters, many respond negatively to ocean acidification. Survival, growth, abundance and larval development for many organisms is reduced, alongside their ability to form and maintain shells and skeletons. These impacts have repercussions on marine food webs and biodiversity, which, in turn, ultimately affects society. It has been suggested that over generations, acclimatization and evolutionary adaption may occur that mitigates or even compensates for ocean acidification. To support this, there are a few examples of limited adaption from the paleo-record, but knowledge in this area remains patchy, not helped by the fact that every species differs in its potential to adapt (Wittmann and Pörtner, 2013). What is most problematic, however, is that the rate of ocean acidification today is unprecedented; 30-100 times faster than at any point in the recent geological past (Bijma et al., 2013). It is, therefore, hard to imagine how marine life could possibly ever adapt quick enough to these rapid changes in ocean chemistry.

Coastlines in an ice-free world

Yesterday, I was pointed to some fascinating maps created by National Geographic that depict the world’s new coastlines if all the ice on earth (five million cubic miles) were to melt. The scenario sees sea levels rise by 216 feet, causing drastic changes to Earth’s shorelines. The maps show both the present-day and new shorelines, as well as providing an informative timeline of ice history and it’s distribution on Earth. Click here to see the maps.

From the depicted rise in sea level, Europe sees some of the biggest changes; the Netherlands disappear and so too does the majority of Denmark. Eastern England, western France and northern Italy also suffer submergence. Those cities lost to the ocean include London, Venice, Stockholm and Amsterdam. 

Map of Europe following a 216 foot sea level rise (National Geographic, 2013)

Cross the Atlantic Ocean, over to North America, and the entire Atlantic seaboard is engulfed, losing cities such as New York, Norfolk and Charleston. The Gulf coast and Florida also disappear, meaning Houston, New Orleans and Miami are no more. Moving down to South America, two huge new Atlantic inlets are formed from the Amazon and Paraguay river basins.

Over in Asia, much of China, India and Bangladesh are flooded, again saying goodbye to large cities such as Shanghai, Dhaka and Hong Kong.

Australia is also heavily impacted. Much of its coastal strip has been lost to the rising ocean, proving catastrophic for the 80% of Australians living in this region. Additionally, a new sea has formed in the centre of the continent. 

As for Antarctica, it is virtually unrecognisable - not surprising considering the East-Antarctica ice sheet alone contains four-fifths of Earth's ice. 

Map of Antarctica following a 216 foot sea level rise (National Geographic, 2013)

Of all the continents, Africa is least affected. However, the increase in temperature due to the continual carbon emissions has made Africa less inhabitable by this stage. 

According to National Geographic, the time taken to reach this ice-free planet will ‘probably’ exceed 5,000 years, although ‘no one really knows’. If it does happen, however, it will be the first time that Earth has been ice-free in more than 30 million years. 

The maps are interesting to look at and do help increase the public’s awareness of future sea level rise; however, they have received criticism based on their scientific grounds (Bump, 2013). For one, people have questioned where they have ‘plucked’ 216 feet from, having been unable to find this figure anywhere else. Resultantly, in comparison to other studies, it has been suggested that National Geographic have underestimated this figure. However, the U.S. Geological Survey have concluded this argument, stating that the most recent data projects a sea level rise of 206 feet; thus National Geographic have slightly overestimated. More importantly though, the quoted rise in sea level has not taken into account the thermal expansion of water that occurs under rising ocean temperatures (I mentioned thermal expansion in one of my earlier posts, 'Deadly Threat No. 1: It's getting hot in here'). This will contribute ‘between 0.20-0.63m per °C of global mean temperature increase’ (IPCC, 2013). Thus, with rising global temperatures, 217 feet is a vast underestimate for future sea level rise and this has to be kept in mind when viewing the maps.

Deadly Threat No. 2: Ocean acidification

Following on from my introductory video a few days ago, today I will be discussing ocean acidification. As the second ‘deadly’ threat, it stems from the ocean’s ability to absorb atmospheric carbon dioxide (CO₂). Thus far, it is estimated that the ocean has absorbed about 30% of the emitted anthropogenic CO₂ (Bijma et al., 2013). Whilst this has benefitted humanity by diminishing the rise in atmospheric CO₂ and, therefore, reduced the rate of global warming, it has been at the expense of the ocean’s chemistry.

The CO₂ exchange between atmosphere and ocean is governed by the differences in CO₂ concentration. With today’s atmospheric CO₂ concentration higher than that of the ocean, the sea is absorbing CO₂ in attempts to reach equilibrium. As the equation below shows, when this atmospheric CO₂ dissolves into seawater, it undergoes hydration to form carbonic acid (H₂CO₃). This then dissociates into a hydrogen ion (H⁺) and a bicarbonate ion (HCO₃^-), before the latter once again dissociates into another H⁺ and carbonate ion (CO₃^2-).

CO₂(aq) + H₂O ⇄ H₂CO₃ ⇄ H⁺ + HCO­₃^- ⇄ 2H⁺ + CO₃^2- 

Figure 1 makes this look a little more friendly!

Figure 1. Ocean acidification. Source.

For typical ocean surface conditions, about ninety per cent of the total dissolved inorganic carbon (DIC) occurs as HCO₃^-, nine per cent as CO₃^2- and one per cent as H₂CO₃ or CO₂(aq) (Feely et al., 2009). However, with rising atmospheric CO₂ levels, the ocean is absorbing larger amounts of CO₂ and consequently, the concentration of CO₂(aq), HCO₃^- and H⁺ is increasing. Due to pH = –log₁₀[H⁺], we are seeing a reduction in the ocean’s pH - notice the progression from less acidic conditions on the left-hand side of Figure 1 to more acidic conditions (where the H⁺ are) on the right-hand side. Since the pre-industrial period, the ocean surface layers have acidified by 0.1 pH units (IPCC, 2013). Indeed, over the last 30 years, there has been a steady decrease of 0.02 pH units per decade (Bijma et al., 2013). Figure 2 displays the trends in atmospheric CO2 and ocean pH that have been observed. This level of acidification is 'unprecedented', causing the ocean to be more acidic than it has ever been in the last 300 million years (Laffoley et al., 2013). In the future, ocean acidification is predicted to continue, following the trend in atmospheric carbon dioxide (IPCC, 2013). This will undoubtedly have serious consequences for marine life. 

Figure 2. Observations of CO2 (parts per million) in the atmosphere and pH of surface seawater from Mauna Loa and Hawaii Ocean Time-series (HOT) Station Aloha, Hawaii, North Pacific (IGBP, IOC, SCOR, 2013) .

H⁺ also reacts with CO₃^2- to produce additional HCO₃^-. Thus, alongside an increase in H⁺, the dissolution of CO₂ in seawater decreases the concentration of CO₃^2-. This affects marine calcifiers that build their shells and skeletons out of CaCO₃ (more on specific calcifiers next time) (Guinotte and Fabry, 2008).

The saturation state of seawater (Ω) determines whether a mineral will precipitate (form) or dissolve and, therefore, affects calcification (Barker and Ridgwell, 2012). It is expressed as:

The subscript 'seawater' refers to the in-situ concentration of the mineral and 'saturation' refers to the concentration when the mineral is saturated. The denominator in the equation is also known as the apparent solubility product, K’_sp (Feely et al., 2009). It is a function of temperature, salinity and pressure and differs between calcium carbonate minerals. When conditions are supersaturated (Ω > 1), calcification is favoured and when conditions are undersaturated (Ω < 1), dissolution is preferred. Today, most global surface waters are supersaturated with respect to CaCO₃, meaning there are more than enough CO₃^2- ions for shell-building, although a few locations are close to undersaturation. However, with progressing ocean acidification, [CO₃^2-]_seawater will reduce, causing Ω to decrease (Barker and Ridgwell, 2012). This has the potential to decrease calcification rates for a number of species (Doney et al., 2009). The continued decrease in [CO₃^2-]_seawater  also means that saturation horizons (Ω = 1) are shoaling (getting shallower), increasing the vulnerability of marine calcifiers. High latitude surface waters are the first locations to experience this shoaling due to the increased solubility of CO₂ in colder waters (Barker and Ridgwell, 2012).

To sum up, ocean acidification is reducing seawater pH, carbonate ion (CO₃^2-) concentrations and saturation states for important minerals required by marine calcifiers. These reductions all have direct impacts on a wide range of marine life and I will be focusing on these next time (I promise there will be less chemistry!).