12.4: Assault on the Oceans - Chemical and Physical Changes

Acidification, Deoxygenation and Marine Heat Waves

As we humans have changed the chemistry of the atmosphere by emitting increasing amounts of carbon dioxide and other gases, we have also been changing the chemical composition and physical properties of the world’s oceans. Three major changes in the oceans are taking place globally in response to this: acidification, deoxygenation and an overall warming trend with focal areas of markedly higher temperatures than were the recent norm, all of which have ominous implications for the organisms that live there.

Ocean Acidification and the Decalcification of Shelled Marine Life

Only about half of the carbon dioxide we have emitted over recent decades has remained in the atmosphere; of the other half, about 30% has been absorbed and stored in the oceans and 20% incorporated into the bodies of terrestrial biota, holding down the amount of global temperature rise that would otherwise have occurred (Feely et al., 2004). When \(\ce{CO2}\) dissolves in seawater it forms carbonic acid, which releases hydrogen ions, making the water slightly less alkaline and more acidic. Acidity or alkalinity is measured in pH, a logarithmic scale on which 7.0 indicates neutrality. Ocean acidification doesn’t mean that the seas are “turning acid”—they are slightly alkaline, presently with a pH of 8.6—but rather that their pH is moving downwards, toward the acid side of the scale. Ocean acidification should perhaps be called ocean decalcification, however, because the most sinister effect of reducing the availability of carbonate ions in the oceans is that it will make it harder for many different types of shelled organisms to form the calcium carbonate that mineralizes them and, if carbon emissions continue to rise as they have been, this will threaten the survival of a large percentage of the organisms making up the base of oceanic food webs, with ramifications that will reverberate throughout marine ecosystems (Hardt & Safina, 2008).

Calcium carbonate (\(\ce{CaCO3}\)) can crystallize in three different forms, each with a different solubility; it takes the form of aragonite in corals and pteropods ^[9] as well as many larger molluscs, as magnesian calcite in coralline algae, and as calcite in coccolithophores and foraminifera. Aragonite and magnesian calcite are about 50% more soluble than calcite, so the organisms utilizing these forms are likely to be the most vulnerable in the near future. A combination of temperature, pressure and depth determine whether or not the ocean water is saturated with the calcium ion, Ca⁺⁺, a state in which the mineral will tend to be deposited, or undersaturated, in which it will tend to dissolve; a definite horizontal boundary, known as the saturation horizon, exists at a certain depth for each crystalline form, below which the shells and other calcified parts of the bodies of these marine organisms will start to dissolve, according to the reaction ^[10]

\[\ce{CO2}+\ce{CaCO3}+\ce{H2O} \rightarrow 2 \ce{HCO3} {^-} + \ce{Ca}{^{++}} \nonumber \]

The saturation horizons for all these forms of calcium carbonate are becoming shallower by tens to hundreds of meters, squeezing calcifying marine organisms into an ever-shrinking available habitat between the saturation horizon and the surface (Hardt & Safina, 2008). Moreover, even in waters above the saturation horizon, as the degree of carbonate ion supersaturation decreases, the rate at which these animals are able to calcify their body parts decreases; nearly all reef-building corals are showing “a marked decline” in calcification under these conditions (Feely et al., 2004). ^[11] A modeling study of calcium carbonate saturation under several emissions scenarios, “a new shallow aragonite saturation horizon emerges suddenly” in many places in the Southern Ocean between now and 2100 (Negrete-Garcia, 2019), potentially affecting shelled pteropods, cold-water corals, sea urchins, molluscs, coralline algae, and some foraminifera; this habitat contraction could occur as suddenly as within one year’s time, and occurred even under an emission-stabilizing scenario, just at a later time. “’That inevitability,” said one of the co-authors, Nicole Lovenduski, in an interview for the University of Colorado at Boulder (2019), “along with the lack of time for organisms to adapt, is most concerning.”

It is the rapidity of these anthropogenic changes, “potentially unparalleled” in the last 300 million years (Honisch et al., 2012), that has scientists extremely worried; “analog events” of relatively rapid CO₂ release—but far less rapid than the one now underway–include the Paleocene-Ecocene Thermal Maximum (PETM) of 56 million years ago, which resulted in the largest extinction of deep-sea foraminifera in 75 million years, the Triassic-Jurassic (T-J) mass extinction of 200 million years ago, when CO₂ levels doubled over 20,000 years, causing an almost total collapse of coral reefs, and the Permian-Triassic extinction of around 250 million years ago, the most severe extinction event since multicellular life evolved. An examination of the reef-building corals that survived the Cretaceous-Tertiary (K-T) mass extinction of 66 million years ago and those that are presently classified as “of least concern” under the conditions being imposed by the mounting Anthropocene extinction event (Dishon et al., 2020) shows similar “survival” traits possessed by both groups, providing “alarming evidence that reef communities are currently in the process of transforming into disaster communities akin to previous extinction events.” ^[12]

Ocean Deoxygenation

As if ocean acidification isn’t enough to worry about, our brave new Anthropocene is ushering in yet another grave concern: ocean deoxygenation—also a result of our unchecked emission of carbon, but in this case due to the ocean temperature increase it is causing. Many people are aware of the sudden “fish kills” that occur when a pulse of nitrogen- and phosphorus-enriched water, usually from agricultural runoff into surface waters and their outflow tracts, stimulates an algal bloom which then dies and decomposes, lowering the oxygen concentration in the water to a point that fish and other animals are unable to tolerate, but fewer are aware of the growing problem in the open oceans. The open ocean is believed to have lost about two percent of its dissolved oxygen since 1950, and has developed a number of “oxygen-minimum zones” (OMZs) that have expanded by millions of square kilometers over recent decades, now occupying a combined total area around the size of the European Union (see Breitburg et al., 2018).

Warming reduces the solubility of oxygen in water and increases stratification of ocean waters, reducing ventilation, the movement of oxygen from the surface into the interior of the ocean, and often limiting the input of nutrients as well, thereby reducing photosynthesis and thus the production of oxygen in the water. Moreover, just as the amount of oxygen available in seawater is decreasing, the metabolic processes of living organisms that consume oxygen are increasing with rising temperatures, putting the squeeze on many different types of marine life. Species vary in their oxygen requirements and their responses to low oxygen concentration, but alterations in their interactions, feeding habits, and therefore marine food webs are known to be occurring and expected to increase. Lowered oxygen concentration in the water column limits the extent of diel vertical migration, the movement of zooplankton and fish deeper into the ocean in the morning and toward the surface in the evening, compressing their available vertical habitat, reducing suitable habitat for deep-ocean organisms, and restricting some species to shallower waters where they are more vulnerable to predation and fishing pressure.

One of the most serious consequences of ocean deoxygenation is its potential to impair the vision of many marine organisms. The retina, containing photoreceptor cells, is the tissue with the highest metabolic demand in the bodies of terrestrial vertebrates, and hence of highest vulnerability; the need for oxygen is especially high in organisms with “fast” vision, where visual pigments need to regenerate rapidly, including not only fish but cephalopods like the octopus and squid and arthropods that depend on high-speed feeding and escape behavior, all of which may become subject to “visual hypoxia” after a much smaller drop in oxygen concentration than what would be metabolically limiting (McCormick & Levin, 2017). Hypoxia is also thought to be an important factor in the death of corals and their accompanying reef inhabitants. The evidence of increasing ocean deoxygenation as the climate warms is so alarming that a group of scientists and conservationists recently called for awareness of the problem to “extend to all facets of society, beyond the pages of scientific journals” (Earle et al., 2018), and the Kiel Declaration on Ocean Deoxygenation, calling for more marine and climate protection, was issued by over 300 scientists in September of 2018.

Marine Heat Waves

Another global-warming-related phenomenon that has recently emerged into common scientific parlance is the occurrence of “marine heat waves,” defined as strings of 5 or more days in which the ocean temperatures in a certain area are in the top 10% of temperatures recorded there over the past three decades. One such “marine heat wave” developed in the Gulf of Alaska in late 2013, a patch of exceptionally warm water a third the size of the continental United States that became nicknamed “the Blob” (see Cornwall, 2019). By the summer of 2015 it had doubled in size to over four million square kilometers, stretching from the waters off Baja California to the Aleutian Islands. with waters up to 2.5 ^°C above normal. A little over a year later, marine food webs all along the western coast of North America were collapsing, with dozens of whales and tens of thousands of seabirds dying and more than 100 million Pacific cod suddenly vanishing.

The disaster apparently began with a ridge of high pressure that held winter storms at bay in the fall of 2013, reducing the effect of winds that usually brought deeper, colder water to the surface in the Gulf, and with them the nutrients the winds typically churn up, leading to a decline in the phytoplankton biomass. The decline in marine plant matter led to a decline in copepods and krill, zooplankton that formed the prey base for small forage fish like capelin and sand lance, which were staples for many seabirds. Only 166 humpback whales returned to Glacier Bay from their tropical calving grounds in the summer of 2015, down 30% from 2013, and all calves born that year were lost, while the bodies of 28 humpbacks and 17 finback whales subsequently washed up along the shoreline from Alaska to British Columbia. Thousands of young California sea lions were stranded on beaches when their mothers were forced to forage farther and farther from the shore in search of food, as many as half a million common murres died of starvation in early 2016, and the cod population dropped by 70% over 2015-2016, finally ‘crashing’ in 2017. It seems likely that what was being witnessed was a crumbling of the marine food web from the bottom upward.

The arrival of cooling La Nina winds at the end of 2016 finally broke the heat wave, stirring up the waters and reversing some of the effects of ‘the Blob.’ But by 2018, only two of five murre colonies seem to be returning to normal breeding levels; only 99 humpbacks returned to Glacier Bay, accompanied by only one calf; and cod numbers were projected to be even lower than the year before. There are some hopeful signs, with some rebounding of copepods and krill and with them forage fish and tiny cod, but the effects of this rebound will have to work their way up food chains. Meanwhile, marine heat waves are becoming more common, the number of days with a marine heat wave present somewhere around the globe having doubled since 1982. Without a major effort to slow down planetary warming, Blob-like temperatures could become typical for the northeast Pacific and perhaps elsewhere by 2050, pushing marine organisms and ecosystems to the limits of their defaunated, already-diminished resilience (Cornwall, 2019).

Plastic, Microplastic and Nanoparticulate Pollution

Macroplastics

The amount of plastic produced since 1950 now exceeds six billion tonnes (Chen, 2014), accelerating rapidly over the last decade; annual global production is now said to be around 320 million tonnes annually, with less than 10% ever recycled and about 40% of plastic waste resulting from single-use packaging (see Lavers et al., 2020); as a result of increasing production combined with inadequate ways of dealing with disposal, it is accumulating in the environment and persisting for long periods of time, entangling or blocking the digestive tracts of seabirds, marine mammals, sea turtles and many other species. As one example of a potential population-level impact, significant entrapment of hermit crabs was discovered in plastic debris, with as many as 500,000 crabs dying on the beaches of the uninhabited but “very polluted” Cocos Islands (Lavers et al., 2020); hermit crabs depend on shells retrieved from other animals, and are attracted to the odor of dead conspecifics, which helps them locate empty shells as they become available, but with the addition of this type of anthropogenic waste to their environment, “the very mechanism that evolved to ensure that hermit crabs could replace their shells has resulted in a lethal lure”—one single container was found to contain 526 dead and dying crabs. Since ingested plastic can potentially cause a variety of lethal and sublethal effects, ranging from the toxicity of its component monomers and plasticizers, chemical pollutants adsorbed to plastic surfaces, and micro- and nano-sized fragments interfering with nutrient absorption, entering living tissues, and accumulating at higher trophic levels in marine food webs, there has been a call to recognize plastic as a “persistent marine pollutant” like the persistent organic pollutants (POPs) whose production is largely phased out (Worm et al., 2017).

In round numbers, the amount of plastic washing into the ocean is somewhere between five and 20 million tonnes per year (see Lebreton et al., 2018); a portion of this is swept into the sea and may enter an oceanic gyre, a rotating circular current that traps it in an “accumulation zone” resembling a giant floating island. There are five major ocean gyres, circling in the North and South Pacific, North and South Atlantic, and Indian Oceans, each with its own floating patch of garbage, the North Pacific being the largest. The plastic that ends up in the ocean and along shorelines has to get there somehow, of course, and most of it comes down via riverine systems. According to Schmidt, Krauth, and Wagner (2017), 88% to 95% of all that plastic waste is thought to be coming from just 10 rivers; eight of these plastic-loaded rivers are in Asia and two in Africa, with the Yangtze River in China alone responsible for more than half of this waste stream, dumping an estimated 1.5 million tonnes into the Yellow Sea annually (for a comparison graphic, see Patel, 2019).

The Great Pacific Garbage Patch (GPGP) is a mass of largely plastic debris floating in a 1.6 million square kilometer area in the North Pacific Ocean off the coast of North America; it can be seen from the air, and is often pointed out by commercial pilots to interested passengers. Lebreton and colleagues (2018) estimated its total mass to be at least 79,000 tonnes; these scientists collected, classified and quantified the buoyant plastic pieces and particles composing it. Megaplastics, large pieces like fishing gear, were calculated to make up 42,000 tonnes; macroplastics, like crates and plastic bottles, 20,000 tonnes; mesoplastics, in the size range of bottle caps, 10,000 tonnes; and microplastics, 0.05-0.5 cm in diameter, 6,400 tonnes. The microplastics were generally fragments of larger plastic items, dispersed in an estimated 1.7 trillion pieces—in other words, microplastics made up around eight percent of the total mass, but 94% of the total number of pieces.

Microplastics

All of the mega-, meso-, and macroplastic pieces accumulating in the oceans are problematic enough, but the microplastic pieces and smaller ones have the scientists particularly worried; they are plastic particles less than 5mm in size (the size of “a grain of rice down to a virus”), ^[13] generally formed as breakdown products of larger plastic pieces, and are now being discovered to be widely distributed in the air, water, and land around us (A. Thompson 2018a, 2018b, 2019). Extremely high concentrations of microplastic particles were recently found in Arctic sea ice by Ilka Peeken and colleagues (2018), and their findings suggest a larger circulation of them throughout the planet’s oceans, with the sea ice serving as a temporary sink; they speculate that large amounts of microplastics are likely to be released from sea ice as the Arctic meltdown accelerates. Fortunately, so far the concentration of microplastics in the Southern Ocean surrounding Antarctica appears to be much lower, although their presence there at all indicates that marine plastic pollution is ubiquitous—“plastic-free ocean environments are increasingly rare” (Isobe, Uchiyama-Matsumoto & Tokai, 2017). There are disturbing indications that this accumulating mass of microplastics is entering marine food webs. Richard Thompson and colleagues reported finding microscopic plastic particle concentrations steadily increasing in collections of plankton samples dating from the 1960s through the 1990s; these authors demonstrated that microplastic particles were rapidly ingested by various components of marine food webs (R. Thompson et al., 2004). More recently, a group of researchers (Cozar et al., 2014) discovered a “gap” in the expected number of plastic fragments below a few millimeters in size, indicating what appears to be a massive loss of plastic from the surface of the open ocean; the size range of these “lost” plastic particles corresponds with that of zooplankton in the oceans, and plastic particles within this size range are commonly found in the stomachs of small, mesopelagic fish, the most abundant predators of zooplankton in the open ocean and in turn an important part of the prey base for upper trophic levels of the marine community. But perhaps the most serious threat is to the ocean’s large filter-feeders, including the “brainy” morbulids, manta rays and devil rays, as well as whale sharks and baleen whales (Germanov et al., 2018); supporting their large bodies on tiny zooplankton, they must swallow hundreds to thousands of cubic meters of seawater daily, and therefore must be taking in microplastics both directly from the water and indirectly from their contaminated prey. According to lead author Elitza Germanov, “It is vital to understand the effects of microplastic pollution on ocean giants, since nearly half of the morbulid rays, two thirds of filter-feeding sharks, and over one quarter of baleen whales are listed by the IUCN as globally threatened species and are prioritized for conservation” (see Gaworecki, 2018).

Revealing a major source of microplastic contamination in North America, a study of municipal wastewater treatment plant effluent from 17 facilities across the US found that, on average, each is releasing over four million microparticles per day, leading researchers to estimate that somewhere between 3 and 23 billion particles of microplastic are being released in US waterways through municipal wastewater per day overall (Mason et al., 2016), polluting lakes and rivers before making it into the oceans. High levels of microplastics, mostly in the form of fibers shed from synthetic fabrics, were also found in treated wastewater in Paris, as well as substantial levels in the River Seine (Dris et al., 2015). Not all microplastic particles that end up in rivers, lakes and oceans are from the breakdown of larger-sized pieces of plastic, however; many facial cleansers, cosmetics, toothpaste, and other personal care products contain intentionally produced plastic particles, most less than 1 millimeter in size, that escape wastewater treatment plants and can reach the oceans (Fendall & Sewall, 2009); one study estimated between 4,000 and 95,000 microbeads could be released in a single use of a facial scrub (Napper, 2015).

Nanoparticulates

If we don’t know much about what the microplastics are doing to our bodies, there’s an even bigger unknown out there: microplastics may eventually degrade all the way down into ‘nanoplastics,’ plastic pieces in the ‘nano’ size range of a few billionths of a meter, several millionths of the size of a “microparticle.” This is getting down to the size range of single atoms and molecules, and particles in this size range often have unusual properties that can be quite different from their properties in the larger size ranges, properties with largely unknown effects on living systems. So far, scientists have not found a good way to quantify the amount of nanoparticulate plastic in the oceans and surface waters, although they assume that, the smaller the particle, the more of them are going to be out there; they are just beginning to attempt assessing the effects that anthropogenic nanoparticulates have on living organisms, but they do know that particles this small can easily penetrate living tissues. Antarctic krill have been shown capable of ingesting microplastics (less than 5mm in diameter) and breaking them down into nanoplastics (less than 1 micrometer in size) through digestive fragmentation, a process possibly shared by other zooplankton (Dawson et al., 2018). The breakdown of larger pieces of plastic is not the only source of nanoparticulate contamination of aquatic and marine ecosystems, however; sunscreens containing engineered nanoparticles of titanium dioxide and zinc oxide are polluting beaches, with the potential to harm marine and aquatic organisms.

Dr. Jerome Labille discovered that almost 70 kilograms of sunblock cream was deposited at one small beach in the south of France visited by about 3000 people daily, amounting to more than 1.8 tonnes over the summer season, and releasing around almost 2 kg of titanium dioxide daily, or over 50 kg for the summer, much of it expected to accumulate on the littoral zone, affecting seaside wildlife of various kinds (AAAS Eurekalert! 17 Aug. 2018). Titanium dioxide and zinc oxide have long been used as sunblockers in traditional, ‘bulk’ formulations and are considered inert and harmless, but questions about the safety of their ‘nano’ formulations have been raised; they reportedly can cause adverse effects in living organisms, largely through the generation of reactive oxygen species (ROS), resulting in cellular damage and possible genotoxicity and nanoparticle-sized titanium dioxide (nTiO₂) has been classified as “possibly carcinogenic to humans” via inhalation (see Skocaj et al., 2011). Since so little is yet known about the effects, and there are problems with informed consent, monitoring and controlling the material after release to the public, and the proportionality of hazards versus benefits, Jacobs, van de Poel and Osseweijer (2010) have called the marketing of nTiO₂ an ethically undesirable “social experiment.” In the marine environment, nTiO₂ has been found to bioaccumulate in the gills and digestive glands of clams, suggesting “a potential risk for filter-feeding animals” (Ilaria, 2018). Both inorganic (titanium and zinc oxides) and various organic sunscreens have been found to have deleterious effects on phytoplankton, which carries out the preponderance of photosynthesis going on in the oceans and thus make up the base of virtually the entire oceanic food web (Tovar-Sanchez et al., 2013).