An animated introduction to ocean acidification

Today, I came across a really fun animation that introduces ocean acidification and its consequences on marine life. It is only short and rather basic, but I think it serves as a good introduction to my next blog post - Deadly Threat No. 2: Ocean Acidification. 

I hope you enjoy and I will be back in a couple of days!

Living in a warmer ocean

Following my last post where I introduced ocean warming, today I will be exploring some of the many impacts that rising ocean temperatures are having on marine life, now and in the future. 

Firstly, biological processes within marine species and organisms are significantly influenced by temperature. At the simplest level, variations in temperature influence molecular kinetic energy, which governs the rate of processes such as diffusion, membrane transport and enzyme reactions. A rise in temperature also sees an increase in metabolic rate, determining ‘life history traits, population growth, and ecosystem processes’ (Hoegh-Guldberg and Bruno, 2010). Accordingly, species tend to specialise to their local environmental conditions; they are able to acclimatise to small variations, but beyond their thermotolerance limits, their fitness reduces and risk of mortality increases (Hofmann and Todgham, 2010). Thus, with ocean temperatures rising, we are seeing ranges of species shift to higher altitudes and latitudes - environments that have become better suited to species’ windows of thermal tolerance. Those shifting and changing in abundance include algal, plankton and fish populations. Their migration ensures that they are still able to feed and reproduce (Parmesan and Yohe, 2003). Furthermore, a decrease in body size has been argued as an additional ecological response to ocean warming (Daufresne et al., 2009). Indeed, observed reductions in the size of individual phytoplankton have been attributed to ocean warming (Hoegh-Guldberg and Bruno, 2010).

As discussed last week, decreased overturning of the ocean, combined with increased water stratification, will reduce the supply of nutrients to the ocean's surface. As this delivery of nutrients drives primary productivity, we have recently seen a fall; since the early 1980s, annual primary productivity has reduced by at least 6% (Hoegh-Guldberg and Bruno, 2010). Within this decline, 70% has occurred at higher latitudes. These changes have had, and will continue to have, profound implications for the marine biosphere, carbon sinks and biogeochemistry of Earth. Furthermore, in areas where upwelling occurs today, the reductions in nutrient and dissolved inorganic carbon (DIC) concentrations will ultimately allow more CO₂ absorption (Riebesell et al., 2009).

A further response to ocean warming are the coral bleaching events that have also been occurring much more frequently since the 1970s. With anomalously high rates of change in temperature, corals become stressed. They expel the algae called zooxanthellae living in their tissues, causing them to become white in colour. However, a symbiotic relationship exists between these two; the zooxanthellae are a source of food to the coral, which in turn provides a safe place for the algae to live. Thus, upon expulsion, corals lose their food supply and if high temperatures persist for too long, they starve and eventually die. Alternatively, if normal temperatures return, corals can recover zooxanthellae and return to health. Due to the warming of our oceans, anomalously high temperatures are becoming more common. Coral bleaching events have, therefore, become more frequent and widespread, resulting in an increase in coral reef mortality (Pandolfi et al., 2011). They have also been associated with depressed growth and reproduction of surviving coral.

Bleached coral

The melting of polar ice comes with many ramifications for marine life. It leads to habitat loss, causing many animals problems in hunting and breeding. Polar bears, seals, penguins and whales are amongst those affected and consequently, they are facing serious decline. Reductions in polar ice are also now allowing predators into previously restricted areas of ocean, triggering cascading effects up the food chain (Bijma et al., 2013). Take, for example, the now annual sightings of killer whales in the previously confined Hudson Bay. A sea ice ‘choke point’ opened up about 50 years ago and this has led to an exponential increase in the distribution of killer whales in the region (Higdon and Ferguson, 2009).

Winter sea ice in the Antarctic is also influencing the abundance of krill (Krupnik et al., 2009). The timings, duration and extent of sea ice are said to impact food availability during critical stages in krill’s life cycle. In turn, this lowers their growth and survival rates. At the base of the pelagic food web, many species that are dependent on krill are consequently suffering.

Antarctic krill

Regarding sea level rise, many habitat-forming species such as corals, seagrass, mangroves and salt marsh grasses are already being affected. To survive, they need to be able to migrate into shallower waters but with sea level rising, many slower-moving species are being outpaced and are subsequently dying. Focusing on mangroves, their survival prospects vary with location (Hoegh-Guldberg and Bruno, 2010). Most at risk are those in coastal areas with steep inclines, or human infrastructure that limits landward movement. Elsewhere, mangroves can survive by shifting landwards, however, this unfortunately threatens other coastal habitats such as saltmarshes.

Sea level rise also jeopardises sea turtle rookeries (Feuntes et al., 2009). Rising water levels are expected to reduce available nesting areas, promoting nest infection and competition. Ultimately, some nesting beaches could be fully inundated, threatening the reproductive success of sea turtles.

Lastly, ocean warming will have repercussions on marine species that are dependent on ocean currents for migration, such as sea turtles (Hawkes et al., 2009). As baby hatchlings, they rely on ocean currents to transport them away from their nesting sites. However, with potential changes to these currents, it has been suggested that hatchlings may not disperse as widely as they do today. Though the exact consequences are difficult to estimate.

My apologies for how long this has ended up! I have not covered every single impact of ocean warming on marine life (it would be impossible in one blog post!) but I have tried to detail the main ones. To end things on a lighter note, I thought I would leave you with a fun clip from one of my favourite childhood movies. It’s from Finding Nemo and quite relevantly shows turtles and fish using the East Australian Current as a 'superhighway'!

Deadly Threat No. 1: It’s getting hot in here…

As I mentioned in my last post, anthropogenic forcing has caused atmospheric concentrations of carbon dioxide (CO₂) to increase. When observations began in 1958, CO₂ levels were approximately 315 ppm, but today, they have reached 393 ppm (CO2 Now, 2013). This increase in CO₂, alongside other greenhouse gases such as methane and nitrous oxide, has changed the radiation balance of the Earth. Now, more radiation is received than emitted at the Earth’s surface and consequently, we are seeing a rise in global average air temperatures (Tyrrell, 2011). Indeed, the IPCC report from Working Group 1, published last month, declared that ‘each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850’. 

However, not only has the atmosphere warmed, it has induced ocean warming. This is what Bijma et al. identifies as the first deadly threat (2013). With a heat capacity 1000 times that of the atmosphere, the ocean has become the primary heat sink for the planet. Between 1971 and 2010, ocean warming accounted for over 90% of the extra energy added to the climate system (IPCC, 2013). However, thanks to the ocean’s thermal inertia, a time lag exists in the global warming response to elevated greenhouse gases (Pierce et al., 2011). To explain this simply, imagine you are heating a saucepan of water. Even though the flame (atmosphere) has a temperature in the hundreds of degrees celcius, the water (ocean) takes time to warm up and boil. This means our children and our children's children, are already committed to climate change associated with the current radiation imbalance.

The latest statistics from the IPCC show that between 1971 and 2010, the upper 75m of the global ocean warmed by 0.11 [0.09 to 0.13] ^oC, per decade. The degree of warming decreased with depth, reducing to less than 0.02^oC per decade, at 500m (IPCC, 2013). This ocean warming has had and will continue to have a whole host of implications (see bold below). However, as they are all big issues in their own right, I do not feel like I can do every one justice in this blog post. Therefore, I will briefly introduce them, with the intention to cover each in greater detail in the future!

Impacts of Ocean Warming

Sea level rise - With an increase in temperature, seawater expands (a process called thermal expansion) and this causes a rise in sea level. Simultaneously, the warming of higher latitudes promotes the melting of polar and land-based ice, adding further water to the ocean (Tyrrell, 2011). Together, these increase sea level and pose various threats, such as: coastal inundation, shoreline erosion and more powerful storm surges. From 1901 to 2010, the IPCC report stated that sea level has risen by 0.19 [0.17-0.21]m, at a much larger rate than the mean over the previous two millennia (2013).

Changes to the thermohaline circulation – The thermohaline circulation describes the movement of water driven by variations in density (Barry and Chorley, 2010). As ocean temperatures increase, surface waters become less dense. This increases their buoyancy and, subsequently, inhibits sinking. Similarly, with the melting of polar ice, low-salinity water is added to starting points of the thermohaline circulation (locations where surface water normally sinks to great depth). This lowers the salinity of surface waters, decreasing their density, which again inhibits sinking. Together, these both lead to increased stratification of ocean layers. Lighter, warmer waters float on top of the denser, colder waters, preventing vertical mixing (Tyrrell, 2011). This has repercussions for both the global transport of heat and nutrient supply. 

Decreased surface oxygen concentrations – Oxygen’s solubility decreases in warmer water and, therefore, with ocean temperatures increasing, there is less oxygen in the surface seawater. This deoxygenation has many impacts on ocean biodiversity and as the third deadly threat, will be discussed in depth in a couple of weeks, so stay tuned.

Stronger storms – There are two factors contributing to this. Firstly, ocean warming is providing many more opportunities for hurricane formation. Hurricanes require a minimum sea surface temperature (SST) of about 26^oC and this is becoming more common as SSTs increase. Secondly, with the ocean warming, it's excess heat is being expelled through evaporation, further warming the atmosphere. This increases the air's water vapour capacity meaning there is more fuel for potentially larger storms (Trenberth, 2007).

So, it's clear that the warming of our ocean is having a multitude of impacts. Some of you may be wondering why I haven't discussed how these are affecting the ocean's marine life. Don't fret - I will be focusing on this next time!

'The deadly trio': an ocean of problems

Picture planet Earth from space - what comes to mind? For me and I’m guessing for many of you too, the dark blue ocean dominates. Covering around 70 per cent of the Earth’s surface, the ocean is the world’s largest ecosystem. Through its ability to absorb and release carbon dioxide whilst also transporting heat and water around the globe, it plays a fundamental role in the global climate. As a consequence, however, the ocean also bears the brunt of climate change (Tyrrell, 2011).

Since the Industrial Revolution, human’s impact on the Earth system has been ever increasing. Take carbon dioxide, for instance, over the last 300 years emissions have increased by 40 per cent. Humans have caused the Earth system to approach a number of planetary boundaries, transforming climate, ecosystems and biogeochemical cycling along the way. Resultantly, the Earth system is no longer dominated by natural processes, but by humanity. So much so that this time period of anthropogenic forcing has become a geological epoch in its own right - the Anthropocene (Zalasiewicz et al., 2011). However, quite when this started remains a highly contentious debate.

Due to the ocean’s inextricable link to the global climate and the fact that I have always found the ocean fascinating, I chose to focus this blog on how the Anthropocene is impacting our oceans (hence the title AnthropoSea!). In particular, I will be looking at ‘the deadly trio’. This is the name coined by Bijma et al. to describe the three major impacts that climate change is having on our oceans; ocean acidification, ocean warming and deoxygenation (2013). Both individually and together, this deadly trio is dramatically affecting the ocean’s biodiversity.

So, where will I be taking this blog? Over the course of the next 3 months, I hope to cover a number of topics. I will be exploring each of the deadly trio in depth before examining what and exactly how marine biodiversity is being affected. Any criticism that I find in literature or the news will also be debated. With the recent issue of the IPCC report, I will later discuss the future of our oceans and any solutions that have been suggested.

Whilst I have chosen to focus specifically on the deadly trio, it is still a very broad topic; each deadly threat has repercussions of its own and additional factors that then exacerbate them. I have done this to allow myself scope for manoeuvre and besides, as they are all heavily interlinked, it would be impossible to only focus on one.

 

That’s all for now, I hope you enjoy following my exploration into the AnthropoSea! I will leave you with this rather dramatic but poignant insight into humanity’s impact on Earth.