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'!