Oceans under climate change
WOR 7 The Ocean, Guarantor of Life – Sustainable Use, Effective Protection | 2021

Biodiversity under assault

Biodiversity under assault fig. 2.26: © Rich Reid Photo

Biodiversity under assault

> Climate-induced changes in the ocean are now affecting marine biological communities at all levels. As a result, many marine creatures are being forced to abandon their ­traditional territories. Predator-prey relationships are changing and ocean productivity is falling. More­over, the impacts of climate change are reinforcing each other and weakening the resistance of marine species to other anthropogenic stress factors. There is no longer any question that climate change is one of the driving forces behind the extinction of marine species.

The limits of endurance

The oceans make up the largest and most species-rich habitat on Earth. They are home to an estimated 2.2 million different species, although only a few more than 200,000 have been identified and scientifically des­cribed. Most of them have adapted to the living conditions in their native waters over long periods of time. These in­clude the prevailing temperatures, oxygen content, acidity of the water, the natural rhythms at which nutrients or food is available or even abundant, and critical environmental compo­nents such as ocean currents, which are important for many species in transporting their spawn or larvae over long distances or distributing them over wide areas. Under ­these familiar conditions, marine organisms grow best, live longest, and reproduce at rates that guarantee the survival of the populations.
However, these physical and chemical foundations of life in the ocean are currently changing at a pace that has not been seen in the world’s oceans for the past 50 to 300 million years. The impacts of climate change can now be observed in all seas and at all depths, and they pose numerous risks to marine ecosystems. Most of the long-term scientific observations of the impacts of climate ­change on marine biological communities have been carried out in the northern hemisphere. Researchers have been studying climate-induced changes in the North Sea, the Mediterranean Sea, and the ocean regions around Labrador and Newfoundland for several decades. With the notable exception of some Australian observations, there have only been a small number of long-term biological studies from the equatorial regions or the seas of the southern hemisphere. This is why researchers must also rely heavily on numerous laboratory and field experiments, as well as on model simulations and historical species-distribution data in order to obtain a realistic picture of the effects of climate change on marine life.
In order to understand how and why marine organisms react to climate change, it is necessary to realize that most marine inhabitants, with the exception of birds and mammals, are cold-blooded animals. These are creatures whose body temperatures are determined by the temperature of their surroundings. As a rule, the temperature requirements of a species thus correspond to the water temperatures that prevail in its native habitat throughout the year, including the total range of seasonal variation. This means that every cold-blooded ma­rine dweller has absolute upper and lower temperature limits at which it can continue to live and grow. ­Scientists refer to the range between these two limits as the thermal tolerance window, or thermal niche.
This window varies in size depending on the species. Species in temperate latitudes like the North Sea generally have a wider temperature window. This is because of the more strongly pronounced seasonality in these ­areas. Animals living here must be able to survive through warm summers as well as cold winters. For organisms in the tropics or polar regions, by contrast, the temperature windows are two to four times narrower than those for the North Sea inhabitants. On the other hand, they have had to adapt to generally more extreme living conditions. Species of Antarctic icefish, for example, can live in water as cold as minus 1.8 degrees Celsius. Their blood contains antifreeze proteins. Due to their low metabolism and the abundance of oxygen available in their habitat, they are also able to survive without the red blood pigment haemoglobin. For this reason, their blood is less viscous and they require less energy to pump it through their bodies.
But icefish live at the extreme boundary. If the temperature rises by just a few degrees Celsius, the animals quickly find themselves at their physical limit. At this point they are no longer able to generate sufficient energy to maintain all of their bodily functions. The reason for this is that the energy requirements of the cold-blooded organisms increase exponentially with every degree of warming. There is a corresponding increase in oxygen demand because energy cannot be generated without respiration. For species with a narrow thermal window this increase is especially drastic. In other words, marine organisms can only survive increasing temperatures in their habitat if they are able to supply their bodies with more oxygen. If that is no longer possible, their cardiovascular systems collapse. Scientists therefore also refer to this as the oxygen- and capacity-limited thermal tolerance of living organisms.
2.23 > The scaleworm Peinaleopolynoe orphanae is one marine species that biologists discovered and described in 2020. The deep-sea dweller from the east Pacific has a carapace of pink to gold-coloured iridescent plates.
fig. 2.23 Greg Rouse/Scripps Oceanography
To make matters worse, the size of the thermal tolerance window can change over the course of a fish, mussel or starfish’s life. In the early life stages, as an embryo in the egg or as a larva, cold-blooded animals are, as a rule, more sensitive to heat than in the later stages of development. This sensitivity is further exacerbated when the animals are exposed to decreasing pH values (acidification) and declining oxygen concentrations at the same time. In this situation, the stress factors often act in concert to amplify the overall effect.
It has recently become known that abnormally warm water temperatures are more dangerous for fish in the embryonic stage and during the mating season. The reason for this difference in thermal tolerance can be explained by the anatomy of fishes. Fish embryos, for example, do not have gills or a cardiovascular system with which to increase their oxygen supply. Furthermore, fish preparing to mate have to produce egg and sperm cells. This in­creased body mass must also be supplied with oxygen, which is why the cardiovascular systems of animals getting ready to spawn are already under a condition of heightened stress even at lower temperatures.
2.24 > Every organism has a temperature range within which it can survive. The size of an organism is limited by the ability to supply its body with increasing amounts of oxygen as the temperature rises in order to maintain sufficient energy production. Fish are able to do this much better as adult animals than they can in the larval stage or when spawning.
fig. 2.24 after Dahlke et al., 2020
It therefore follows that fish will suffer particularly from climate change during their reproductive phase, because the water temperature in the spawning area is crucial to their reproductive success. This is true for marine species as well as for the fish in lakes and rivers. A recent study by German marine biologists analysed temperature tolerance data for almost 700 fish species throughout their life stages and compared them with the new climate scenarios (Shared Socioeconomic Pathways, SSPs) of the Intergovernmental Panel on ­Climate Change. The results indicated that if global ­warming can be limited to the Paris climate target of 1.5 degrees Celsius by the year 2100, only about ten per cent of the fish species studied would be forced to leave their traditional spawning grounds as a result of the water being too warm. But if greenhouse emissions remain at high or very high levels, the warming will be as much as 4.4 degrees Celsius or more. This would force up to 60 per cent of the fish species to leave their traditional spawning grounds.

More rapid species turnover than on land

All organisms, like fish, will react to changes in their environment by first attempting to adapt their indivi­dual behaviour to the new conditions. Scientists refer to this kind of adaptation as acclimatization. The organisms ramp up respiration and metabolic processes, pump more blood or water and nutrients through their bodies, or eat more if necessary. If they are unable to do this, they must migrate to areas where more familiar environmental conditions prevail. But all of these measures require the organisms to generate additional energy. If they can do that, they have a relatively good chance of survival. If they do not have the necessary reserves, however, the individuals will soon reach their capacity limits and face the risk of death.
As a rule, however, those which are able to acclima­tize over the short or moderate term have a chance to reproduce sexually and enable the species to adapt genetically to the new conditions through multiple generations. This essentially means that the organisms produce offspring whose genetic makeup may be modified in such a way that the subsequent generation is better able to cope with the new environmental conditions than their parents’ and grandparents’ generations. This kind of adaptation is ­called genetic or evolutionary adaption.
Comparing biological communities on land with those in the sea reveals fundamental differences that are important within the context of climate change. These include:
  • The primary producers in the sea (phytoplankton) have much shorter reproduction cycles than the trees or grasses on land. While trees in some cases can live for centuries, the worldwide stocks of plankton are renewed about 45 times every year, which is approximately once every eight days. In theory, this capacity enables plankton to adapt genetically to changing environmental conditions more readily than plants on land are able to.
  • The proportion of cold-blooded organisms in the sea is comparatively high, which means that species diversity and distributional patterns in the ocean are determined to a large extent by temperature. On land, by contrast, other factors, such as the amount of precipitation or geographic barriers play a greater role.
  • Unlike land animals, the ocean dwellers have vir­tually no option to retreat into caves or other cool, shady locations during heatwaves. They are completely defenceless against the warm water temperatures, and must therefore take flight sooner.
  • Tropical marine species, as a rule, live in regions that are naturally so warm that they are already near the upper tolerance limits of the individual species, so that only a very small increase in temperature is sufficient to exceed their heat threshold.
  • Compared to land animals, it is easier for mobile aquatic creatures like fish to follow their preferred temperatures into cooler regions because there are relatively few obstacles, such as undersea mountain chains, deep trenches or strong currents (e.g., the Antarctic Circumpolar Current) to be overcome, and these often do not really present a significant impediment. Many terrestrial creatures or species living in lakes, rivers or ponds, on the other hand, are more likely to encounter geographical barriers, which now increasingly include land areas used by humans, that make it difficult for them to move further or relocate their habitat.
2.25 > Sea ice and near-freezing water temperatures are not at all problematic for the Antarctic blackfin icefish (Chaenocephalus aceratu). It is perfectly adapted to life in the Southern Ocean. Compared to fish in the temperate latitudes, however, its thermal tolerance window is very narrow.
fig. 2.25 Paulo Oliveira/Alamy Stock Foto
For all of these reasons, heat-driven species shifts in the sea are occurring much faster than on land. For scientists, however, it is not always easy to clearly determine whether the reactions of an individual species or biotic community are exclusively related to climatic changes such as rising water temperatures, acidification or oxygen depletion, or whether anthropogenic stress factors like fishing, resource extraction and marine pollu­tion also play a role. It is an irrefutable fact, however, that ­marine communities that are already under stress react more sensitively to climate change than those that are not overfished or exposed to high levels of pollution or nu­trient overload.

fig. 2.26 © Rich Reid Photo

2.26 > Because their prey fish are migrating northward to warmer waters, Arctic seabirds like the ivory gull (Pagophila eburnea) now have to fly further out to sea than in the past. This consumes precious energy and causes the offspring to go hungry more often.

Fleeing from the heat

The most evident response of marine organisms to rising water temperatures is the relocation of their habitats to ­areas where the animals and plants find their preferred ambient temperatures. The shift can occur either in an active or passive way. It is active when fish, crustaceans and other mobile marine life migrate under their own power to new habitats to escape from adverse environmental conditions. Passive relocation, on the other hand, occurs when the spores, eggs or larvae of a species are carried by changing ocean currents, for example, into an area that was not previously inhabited by that species, but where it is able to recolonize and reproduce because the environmental conditions are suitable. However, researchers also consider it to be a relocation of habitat when a species reproduces and grows only in the cooler part of its traditional distributional area, but dies out in the parts where temperatures are rising. In this situation, the boundaries of its colonies have, of course, also been shifted.
The flight from heat induced by humankind’s emissions began more than half a century ago. The habitats of marine organisms have been shifting poleward since the 1950s. Populations that live north of the equator are migrating northward, while those south of the equator are moving southward. Biodiversity in the warm tropics has been declining significantly since that time. Scientists have recognized this trend across all groups of organisms, from single-celled plankton to large fishes. They are even able to reconstruct the rate of this shift over time. So far, it has been occurring at around 51.5 kilometres per decade for mobile species. For organisms that live on the sea floor it is somewhat slower. Their distribution boundaries are moving by an average of 29 kilometres every ten years. Comparing the migration statistics of all groups of organisms on land with those in the ocean, the marine organisms are shifting their habitats about six times faster toward the poles than the organisms on land. However, these numbers should not obscure the fact that all organisms, plant or animal, will react differently to ocean warming, even those that are closely related.
Scientists are observing particularly strong migratory movements from the tropics, where species are fleeing to the north or south in large numbers due to rising water temperatures. As a consequence, researchers are recording an increase in biodiversity in the marginal regions of the tropics where the climate refugees are now competing with endemic species for food and living space. The newcomers often have an advantage, because water temperatures are also rising in the subtropical marine regions. The result is a turnover in the subtropical communities toward a more tropical marine assemblage. Researchers refer to this phenomenon as tropicalization.
In marine regions where geography tends to prevent migration to higher latitudes, for example, in the Mediterranean Sea or the Gulf of Mexico, rising temperatures in the upper layers of the water column are driving the mo­bile marine inhabitants to greater depths. Because the deep water is generally cooler than the water in the overlying layers, these species usually do not have to migrate very far in order to reach their preferred temperature conditions. But it is uncertain whether these species will be able to find sufficient food in the deeper waters. For algae and other water plants, moreover, the light conditions deteriorate with increasing depth.
A successful flight from warming waters, therefore, does not depend on the individual mobility of a species alone. Rather, there is a combination of climatic and other environmental factors that determine the extent to which marine organisms can change their habitat. These factors include, among others:
  • local temperature and oxygen gradients;
  • marine currents that transport eggs or larvae to new regions;
  • the shape and depth of the sea floor (bathymetry) for those species that spend a part or all of their life on the bottom;
  • the availability of nutrients or food, suitable spawning sites, or hard substrates to settle on;
  • the presence of new or familiar predators; and
  • stressors introduced by humans such as fishing, ­shipping, resource mining and marine pollution.
Particularly limited retreat options are available to cold-loving species or those dependent on sea ice like the Ant­arctic icefish, or the polar cod, a key species in the ­marine Arctic ecosystem. Not only does it have a comparably low thermal tolerance and is therefore rarely found in ­regions where the water temperature is above three degrees Celsius, but its offspring are also dependent on the Arctic sea ice. The ice offers protection to the young fish and abundant food in the form of ice algae and zooplankton. But the area of summer sea ice on the Arctic Ocean has shrunk by around 40 per cent since the beginning of satellite measurements in 1979. This, for one thing, is reducing the area of the habitat for young polar cod. For another, there is less food available for the young fish, which is why scientists anticipate that their growth will be retarded and that their average body size will decrease.

Heat-driven upheaval of ecosystems

Because individual marine organisms react to rising temperatures in unique ways and at their own speed, a broad restructuring of the biological communities is occurring in many places. Long-established predator-prey relationships are collapsing and processes that have been running smoothly for millennia are no longer in sync. A striking example of this is the ocean’s changing spring season, which now occurs much earlier in the year in the high and middle latitudes than it did a few decades ago. This means that algae are blooming earlier every year in response to temperature, by an average of 4.4 days every decade.
However, the erstwhile exploiters of these algal blooms, such as fish, mussels and many other sea creatures, are having a very difficult time in adjusting their reproductive rhythms at this rapid pace. As a result, their offspring may just be ready to start foraging when the algal blooms have already ended.
Seabirds are facing similar problems, as their prey species migrate poleward or produce too few offspring because they missed the algal bloom, putting their populations at risk. The birds have to fly much further out to sea or spend more time on the sea to obtain enough prey. As a result, they are not successful enough in hunting to feed their hungry offspring, and the young animals face starvation.
Due to rising water temperatures, researchers are observing a decline in the reproductive success of ­northern fulmars, manx shearwaters and kittiwakes, among other bird species in the northeast Atlantic. In the Southern Ocean, by contrast, the breeding success of the wandering and Laysan albatrosses has im­proved as a result of climate change. The birds are benefitting from the strengthening and the southward shift of the westerly winds over the Southern Ocean, and from the temperature-driven migration of species toward the pole. As long-distance gliders, albatrosses depend upon the wind to reach their fishing grounds. Because the west winds are now stronger and the fishing grounds of the albatross have shifted closer to the continent of Antarctica, the hunting efficiency of the birds has been enhanced, which is a great benefit for their offspring.
Marine reptiles such as turtles and snakes are primarily affected by global warming during their most vulnerable life stage, as embryos in the egg. The ambient temperature of their clutches determines not only the sex of the young, but also their size, their stage of development at the time of hatching, and their general fitness. During the incubation of turtle eggs, if the sand is just one to four degrees Celsius warmer than normal (29 degrees Cel­sius for a male-female ratio of 50:50), more females will hatch from the eggs and much fewer males, or possibly none, a pattern that will eventually lead to extinction of the species.
2.27 > The sex and general condition of freshly hatched leatherback sea turtles depends on the ambient temperature of the turtles’ clutch of eggs. If the sand is one to four degrees Celsius warmer than normal during the incubation period, hardly any males will hatch.
fig. 2.27 mauritius images/nature picture library/Jürgen Freund

A further aspect of carbon dioxide

Ocean warming is the most widespread climatic stress factor for biological communities in the oceans today, and it is thus also the major force driving changes in the ­species distributions in the sea. But there are other responses acting parallel to this that are altering the living conditions in all of the ocean basins. One of these, increasing ocean acidification, has been in the spotlight of modern research over the past two decades, which has helped us to understand the concept that different marine organisms react differently to carbon dioxiderich water.
Among the organisms that benefit from acidification are the picophytoplankton. These, the smallest of the phytoplankton species, are only 0.2 to two micrometres (thousandths of a millimetre) in size, but in many marine regions they are the most abundant primary producers, in part because they propagate copiously even in waters with low nutrient content. Various field experiments on ocean acidification have now shown that these tiny algae use elevated carbon dioxide levels in seawater as a growth accelerator. They have been found to grow faster and produce more biomass.
On the losing side, however, are many of those ­marine dwellers, like fish, that must breathe in the water. Carbon dioxide-rich seawater creates elevated carbon dioxide levels in the body fluids of these animals. This impairs the transport of substances through the cell membrane, among other effects. The sensory responses of fish held in carbon dioxide-rich water for research purposes were also altered. Their hearing and vision were impaired and they were less skilful at dodging predators. Cod larvae raised in water very rich in carbon dioxide showed evidence of damage to vital organs such as the liver, kidneys and pancreas. In addition, the mortality rates of the young animals doubled during the critical phase between hatching from the egg and the formation of functioning gills.
2.28 > 2.28 > One consequence of ocean acidification is that the calcareous shells of foraminifera that lived about 150 years ago (lower row, warmer colours) were as much as 76 per cent thicker than those of the same species today (upper row).
fig. 2.28 Quantifying the Effect of Anthropogenic Climate Change on Calcifying Plankton, Author: Fox, L., Stukins, S., Hill, T. et al., Publication: Scientific Reports, Date: January 31, 2020, Sci. Rep. 10, 1620 (2020), https://doi.org/10.1038/s41598-020-58501-w, © 2020, Springer Nature
In similar experiments, however, herring proved to be much more resistant to the changing pH levels in seawater. One explanation for this might be found in the natural lifestyle of the fish. Herring usually spawn near the seabed, where microorganisms constantly decom­pose sinking biomass, and where carbon dioxide concentrations are therefore naturally higher than at the sea surface. The animals are thus presumably better adapted to a wider range of pH values than fish species like cod that spawn near the surface.
Ocean acidification poses a particular threat to carbonate-secreting organisms such as clams, corals and echinoderms, or planktonic species such as coccoliths (calcareous algae), foraminifera and conchs. All of these need calcium carbonate to build their shells and skeletons. With increasing acidification, however, the concentra­tion of carbonate in the sea decreases, and the concentration in their body fluids decreases unless the organism has a good system of acid-base regulation. For the organisms, this means that when the water is more acidic, they have to exert more effort to form their calcareous shells and skeletons. The long-term results can include decreases in either the shell thickness, the size, weight or effi­ciency of the marine organisms. With increasing acidification, moreover, it becomes more likely that the carbon ­dioxide-rich water will corrode mussel shells, snail shells or coral skeletons, damaging or even completely dis­solving them.
Research findings by British scientists, who recently compared zooplankton samples from the legendary Challenger expedition (1872 to 1876) with sample material from the Tara Oceans expedition of 2009 to 2016, reveal the extent of pressure on marine organisms due to in­creasing acidification. They discovered that foraminifera collected in the eastern Pacific more than 150 years ago possessed calcareous shells that were as much as 76 per cent thicker than those retrieved in the past ­decade from the same ocean region.
But it is not only the tiny marine inhabitants that are affected. In a laboratory study, a joint German-South African research team discovered that even ocean predators like sharks are suffering from the increasing acidity. If the animals remain in waters with a pH value of 7.3 or lower for several weeks (8.2 is the normal ocean value), their small, tooth-shaped skin plates as well as their teeth are affected. Over the long term, this could impair the swimming ability of these predators as well as their ­hunting yield.
A mean pH value of 7.3 in the ocean is actually not expected to be seen until the year 2300, and then only if the present carbon dioxide emission levels continue unchecked. In upwelling areas, however, like those off the southern and western coasts of South Africa or off the US Pacific coast, researchers are already observing occasional occurrences of these depressed values today. These are mostly cold, nutrient-rich water masses that are also oxygen-deficient and rich in carbon dioxide with a pH value between 7.4 and 7.6, which rise up from greater depths under certain wind and current conditions and end up in the coastal areas. There, because of their high nutrient content, they frequently support large plankton blooms. When the plankton die, microbes break down the plant remains and enrich the already carbon dioxide-rich waters with additional carbon dioxide. Under these conditions the pH value of the water, at least off the coast of South Africa, has even been known to drop to extreme lows of 6.6 for periods of a few days.

Less oxygen, less energy

Most multicellular marine organisms require oxygen to produce the energy necessary to carry out their life processes. Birds and marine mammals like whales or sea lions breathe it in with air from the atmosphere. The others extract oxygen for respiration from the water, which generally contains more oxygen in cold regions than in warm areas. But since the 1950s the amount of dissolved oxygen in sea water has been decreasing in the wake of climate change, both in the open ocean and in coastal waters.
As a result, oxygen minimum zones (OMZs) have now expanded into all the world’s oceans. These are generally water layers between the depths of 100 and 1000 metres with oxygen concentrations very far below the hypoxic threshold of around 70 micromoles per kilogram of water. In the Indian and Pacific Oceans these minimum zones have an oxygen content of less than 20 micromoles per kilogram of water. In the At-lantic Ocean the values are commonly below 45 micromoles.
Just how dangerous these kinds of environmental conditions are to life becomes apparent when we consider that many marine organisms have difficulty supplying their bodies with sufficient oxygen even at concentrations of 60 to 120 micromoles per kilogram of water. Large animals in particular like sharks and tuna, with extremely high energy needs, cannot survive in ocean waters where the oxygen concentration is less than 70 micromoles per kilogram of water, so they avoid these areas
When oxygen, that crucial elixir of life, is only present at such low concentrations, marine life suffers at all levels, from the individual cell processes to the interactions of the total ecosystem. Especially for animals, productive capacity and survival prospects are diminished. For example, oxygen deficiency reduces the reproductive success of many species. Organisms in the oxygen-poor zones are often no longer able to mate and produce offspring, which results in a collapse of the stocks. Animals that are frequently exposed to short phases of oxygen deficiency exhibit a weakened immune system and become less able to defend themselves against disease and parasites. Their growth is also significantly impaired, which is why researchers expect, among other things, that the number of large predators will decline over the long term.
Most mobile ocean dwellers will flee their traditional habitats when the oxygen concentration falls below the threshold value for their species. This reaction may lead the animals to congregate in the marginal zones, where they compete for food and become easier prey for fishermen. The high catch volumes of Peru’s fishing fleet can thus be attributed in part to an oxygen minimum zone within the Peru Current at intermediate depths that prevents the huge schools of Peruvian anchovetas (Engraulis ringens) from migrating to greater depths. Instead, the fish remain near the surface where they are more easily and efficiently caught.
The expansion of oxygen minimum zones in deeper waters also has a confining effect on mako sharks (Isurus oxyrinchus), blue marlins (Makaira nigricans) and sailfish (Istiophorus albicans). These predators of the open ocean are actually known for their habit of diving down to great depths to pursue fish and squid. In the eastern tropical Pacific, however, these hunting forays do not extend nearly as deep as they do in the western Atlantic. The reason is that the predators run out of oxygen in the deeper waters due to the more pronounced oxygen minimum zone in the eastern Pacific. In the western Atlantic there is sufficient oxygen content at ­greater depths. Scientists studying the hunting behaviour of blue marlin in the eastern tropical Atlantic found that an expanding oxygen minimum zone in deeper water was forcing the fish to reduce the depth of their dives by around one metre every year. Over the time frame of the study (1960 to 2010), the habitat area of these predators shrank by 15 per cent.
When oxygen-deficient zones form at the sea floor, or if they extend down to this depth from above, the ­creatures living on the bottom may have to leave their burrows and hiding places and climb to the highest accessible point in the area, in hopes that the shallower water layers contain more oxygen. This response makes them easier prey for predators, but the animals are usually forced to take this risk because the alternative is death by suffocation.
As the habitats for species with high oxygen requirements shrink with the worldwide decline in oxygen concentration in the oceans, organisms with lower oxygen needs are actually benefitting from the hypoxic zones. On the one hand, these are well suited as places of ­refuge because potential enemies, generally predators with higher oxygen requirements, cannot pursue them there. On the other hand, they offer feeding advantages for some species. The warty comb jellyfish (or sea walnut, Mnemiopsis leidyi), for example, is found in the Chesapeake Bay, the largest estuary in the USA, and it can tolerate much lower oxygen concentrations than the fish with which it normally competes for food. If large areas of the Chesapeake Bay now become low-oxygen zones in the summer, the jellyfish can still hunt when their feeding competitors have long since left the area.

fig. 2.29 Science Photo Library/Frans Lanting, Mint Images

2.29 > The warty comb jellyfish Mnemiopsis leidyi benefits from diminishing oxygen in the ocean. Unlike many fish it is able to hunt in oxygen-poor waters and thus avoid the dis­advantage of feeding competition.

The deadly trio and its ramifications

For now, the consequences of climate change are still considered to be a relatively small factor of environmental stress compared to the more direct impacts of humans on the sea. But some scientists are convinced that climate change will soon become the driving force behind global species extinction.
In the ocean, it especially influences the biological communities where physical and chemical changes are occurring simultaneously, and their interactions are amplifying the impacts of the deadly trio of rising tem­peratures, increasing acidification and declining oxy­gen content. Under these conditions, if the species assemblages are affected at the same time by extreme events such as tropical cyclones, marine heatwaves, or flooding with its associated coastal erosion, the extent of damage will increase many times over. In these cases, local mass mortality can no longer be ruled out. There is an in­- creasing danger that the ecosystems will reach a threshold or a tipping point beyond which recovery of the commu­nities is impossible, and the changes will thus become irreversible.
Hurricane Dorian, for example, left a path of destruction in the near-coastal coral reefs of the Bahamas as it ploughed across the northern Caribbean in early September 2019 with maximum wind speeds of 290 kilometres per hour.
After two days of heavy storms, 25 to 30 per cent of the shallow-water coral reef was severely damaged. Winds and waves had overturned coral colonies and the fragile reef structures had been battered by uprooted trees. Sediments stirred up by the storm buried entire ­segments of the reef under a layer of sand, threatening to suffocate the corals. In addition, the reefs showed signs of bleaching at many sites, which researchers attributed to temperature shock, among other possible causes. More­over, after the storm many of the fish species crucial to the reef’s survival had disappeared. In their initial damage analysis, scientists were not able to determine whether the fish had retrea-ted into deeper waters or if they were injured or killed during the hurricane. It is certain, however, that the reef will require several decades to recover from the storm, assuming that climate change allows it to recover at all.
2.30 > Storm consequences: In early September 2019, as Hurricane Dorian raged over the Bahamas, this large coral colony (patch reef) in the Great Abaco Barrier Reef broke apart.
fig. 2.30 Will Greene/Perry Institute for Marine Science
The disastrous interplay between ocean warming, acidification, oxygen loss and extreme events now threatens marine biodiversity in all corners of the world’s oceans. Scientists refer to these as the cascading impacts of climate change. If one or more species within a food web reacts to the pressures of climate change, a chain reaction is started that triggers changes at all levels of the ecosystem. Increasingly, completely new species communities or combinations are emerging. It is uncertain to what extent these will be able to contribute to the ecosystem services of the sea.
Another significant change is the decreasing amount of animal biomass in the sea. Model calculations aligned with the Representative Concentration Pathways (RCPs) used by the Intergovernmental Panel on Climate Change have estimated that the world ocean will lose around 4.3 per cent of its animal biomass by the year 2100 compared to the period from 1986 to 2005, even if humankind is able to limit global warming to less than two degrees Celsius (RCP2.6).
But if greenhouse gas emissions continue to increase strongly, and the global average temperature rises by more than four degrees Celsius by 2100 compared to pre-industrial times (RCP8.5), the animal biomass in the sea will decrease in response to climate change by about 15 per cent. The losses will be particularly high in the tropical seas and the temperate latitudes.
Furthermore, climate stressors amplify the detrimental effects of direct human interference in marine communities. Diminished fish stocks, for example, react much more sensitively to the effects of climate change than ­healthy abundant stocks. The same is true for coral reefs that have already been damaged by untreated sewage, or for benthic communities that are disrupted by frequent bottom trawling. When extreme climate events such as heatwaves or abrupt decreases in oxygen content strike these already vulnerable communities, they will have ­little or no chance of fully recovering from them.
2.31 > Scientists have found that biomass production in large regions of the ocean will drop by more than 20 per cent if the world warms by more than four degrees Celsius (B) by 2100. If the target of 1.5 degrees is met (A), the losses will be much less.
fig. 2.31 after IPCC, 2019


fig. 2.31 after IPCC, 2019


fig. 2.31 after IPCC, 2019

Extra Info Coral reefs and kelp forests – no chance under extreme temperatures Open Extra Info

Many coastal biological communities are facing an additional climate problem. Due to rising sea levels, sea turtles, seabirds and many other shore dwellers are losing their traditional nesting places or even their entire habitats.
The prognosis for mangrove forests, one of the hotspots of marine biodiversity, is particularly critical. ­According to current research, they are able to compensate for a sea-level rise of up to six or seven millimetres per year. They achieve this by storing large amounts of sediment in their dense root systems. In a sense, the forests build their own platforms and climb upward with rising sea levels. But if the regional water level rises faster than the mangroves are able to collect sediment, the trees will drown. New mangroves then can only grow closer to the shore, which means that over longer time periods the forests will migrate landward if there is room available there. Otherwise, they will disappear.
The gravity of the situation is highlighted by the projections of sea-level rise in tropical and subtropical coastal areas. If anthropogenic greenhouse gas emissions maintain their present levels, the rate of rise will increase to six or seven millimetres per year within the next 30 years, thus reaching the critical threshold value for mangrove forests. But if warming can be limited to 1.5 degrees Celsius in accordance with the Paris climate ­target, many of the coastal and estuarine forests could have a future.
Knowing about these climate-related threshold values for marine biological communities is crucially important, because it allows us to consider key biological knowledge in making decisions about managing the ocean and its resources. With regard to mangroves, for example, it is now well understood that no further measures should be allowed that would prevent the coastal forests from ­collecting sufficient sediment material. Currently, however, this is happening in numerous river channels and estuaries, for example with the construction of dams or the dredging of gravel and sand. Wells drilled for groundwater, oil or natural gas in coastal areas are equally harmful. Their extraction leads to subsidence of the coastal lands and thus amplifies the effect of sea-level rise. The lands of the Mekong Delta in Vietnam, for example, are sinking today by six to 20 millimetres per year as a result of human intervention. Under these conditions, the outlook for mangroves is extremely dire.

Less bounty from the sea

Climate change is altering the distribution and productivity of marine organisms on a global scale, and thus adversely affecting the important ecosystem services of the ocean. Where coral reefs, kelp and mangrove forests are dying out, for example, waves are rolling up onto the shores unchecked and accelerating their erosion. Tourists who once came in droves to marvel at the bio-diversity of these habitats are now losing interest, and culturally or spiritually inspiring sites are losing their magic.
The decline of marine biodiversity is particularly noticeable in the reduction of biomass in the ocean that is available to humans. The primary victims, therefore, are commercial fishing and aquaculture businesses as well as the many small-scale fishermen who fish for their own consumption. They are all being increasingly faced with the following challenges in the wake of climate change:
  • decreasing nutrient concentrations in the surface waters of the low and middle latitudes due to enhanced stratification, which lowers primary production in these regions. This results in decreasing food supplies for fish, mussels, crabs and other sea-food;
  • a shift in fishery productivity toward the poles with a corresponding decline in fish stocks in the lower ­latitudes;
  • decreasing reproductive success of numerous fish ­species;
  • the loss of important fish spawning sites, particularly coral reefs, kelp and mangrove forests;
  • decreasing individual body size in various species;
  • due to temperature changes, a greater susceptibility to disease and parasites for species raised in aquaculture; and
  • increasing frequency of harmful algal blooms and oxygen-poor zones in coastal regions where aquaculture is carried out.
A shift in the distributional areas of many species due to climate change also presents difficulties for effective ­fishery management in fixed sectors, as well as for the protection of rare species in designated marine protected areas. For example, if large schools of fish leave their traditional habitats due to rising water temperatures or diminishing oxygen concentrations, they may cross a boun­dary into adjacent fishing sectors. This would result in a decrease in stocks in the former sector and an increase in the new sector. The monitoring of this kind of boundary-crossing population changes and incorporating them into the fishery management plans of the various sectors in a timely manner currently poses enormous challenges for scientists and fishery managers. Mastering these will only be possible when the observation systems are improved and all of those involved are able to cooperate across sector boundaries.
The situation is similar for marine protected areas. In the future, these areas will only achieve their conservation purpose if their boundaries migrate together with the species in need of protection, or if the protected areas are interconnected. Here, as elsewhere, climate change is forcing human societies to develop new solutions, to constantly re-evaluate decisions, and to adapt those where necessary.
Minimizing the impacts of climate change on the ­oceans must become a top priority in policy-making. Humankind still has some time left, but we must act without delay if we are to preserve the oceans and their unique biological communities. Textende