Climate change and the coasts
Unbridled carbon dioxide emissionsCoasts are adversely affected by many stressors. These include not only local construction or pollution. In addition, coasts are increasingly facing global threats from climate change – sea-level rise, ocean acidification and ocean warming. These trends are primarily due to the still unrestrained burning of the fossil fuels natural gas, petroleum and coal, which adds large quantities of the greenhouse gas carbon dioxide (CO2) to the atmosphere. Since the beginning of the Industrial Revolution, the atmospheric CO2 concentration has risen from 280 parts per million (ppm) in 1800 to a level of 400 ppm today. This increase has resulted in gradual climate change, with the attendant consequences.
Climate inertiaDue to the inertia that is inherent in our climate system, many impacts of human-induced global warming are slow to become apparent. Even if we managed to completely stop all carbon dioxide emissions today, near-surface air temperatures would continue to increase for at least another hundred years. The sea level would even continue to rise for several centuries. What is the cause of this? One factor is that due to slow deep sea warming the ocean waters are only gradually expanding. At the same time, the continental ice sheets in Greenland and the Antarctic probably react very slowly to atmospheric warming. As a result, the melting of the glaciers is a long drawn-out process that will continue for thousands of years.
Increasing ocean warming will substantially alter the conditions faced by many marine organisms. These processes are likely to result in sustained changes to the composition of the oceans’ biotic communities (biocoenoses) and food webs. Such changes will be further amplified by ocean acidification, which effectively alters the chemistry of marine waters. This acidification is a result of the increasing uptake by seawater of carbon dioxide from the atmosphere. Simply put, when carbon dioxide dissolves in water it forms acid.
In recent years, the number of research projects investigating the impacts of climate change on the oceans has increased rapidly. Many of these studies primarily deal with the impacts on coasts and coastal waters. They also address the question of how far the impacts of ocean warming and ocean acidification are similar in coastal waters and the open sea, or whether they differ significantly between those two marine realms.
- 3.1 > In the ocean, a thermocline often forms between the warm surface water and cool water at greater depth. This distinct layer can be seen with the naked eye as the water’s density changes with the temperature and certain particles concentrate at the thermocline as can be seen here off the Thai island of Ko Phangan.
- OCEAN WARMING
Warmer water, increased stratificationWhile it is already possible to fairly accurately predict which coastal areas will be affected by a specific amount of sea-level rise, it is much harder to appraise the impacts of ocean warming. Enhanced stratification of ocean waters in future is however deemed a certainty. It will be more difficult for oxygen-rich layers at the water surface to mix with colder, deeper layers. This may result in a lack of oxygen at greater depths, as has already been observed in various marine regions of the world.
The stratification of waters is a natural process: during the summer months sea surface water warms up and forms a layer of water close to the surface that covers the heavier, colder deep water like a lid. The transition from the warm surface layer to the colder water below is quite abrupt, which is why the line separating warm and cold water is called the thermocline. These thermoclines range in thickness from only a few decimetres to many metres in different marine regions, with thermoclines in the open ocean with deep waters being considerably thicker than those in coastal areas.
At the thermocline, a warm and less dense water layer rests over a colder water body of higher density. The thermocline thus functions like a barrier. The greater the temperature differential, the greater the difference in density and the more stable the thermocline. Ultimately hardly any oxygen-rich surface water can be mixed into deeper layers by means of wave motion, with the lack of light in the deep ocean also precluding oxygen production through photosynthesis. As the decomposition of organic material by microorganisms in the deeper water layers continuously consumes oxygen this is a serious issue which results in oxygen deficiency in the deeper layers of many coastal seas.
Today, ocean warming further exacerbates oxygen deficiency in deep waters. This is due to the fact that biochemical processes generally run faster at higher temperatures as the biochemical substances involved are more reactive. This is also true for the metabolism of bacteria. Bacteria decompose the remains of dead plankton that has sunken into greater ocean depths and use oxygen in the process. The higher the temperatures, the faster the bacterial metabolism and the more oxygen will be used up.
Unique measurements spanning six decadesFor the German Baltic Sea, scientists have detected the current specific impacts of ocean warming by analysing a unique time series, the data points of which go all the way back to 1957. The scientists regularly measure the water’s temperature, nutrient and oxygen contents as well as other parameters at the same location in the Eckernförde Bay. The data show that the water’s nutrient content has decreased in recent years, very probably due to lower nutrient loads from terrestrial sources. Surprisingly, however, the deeper water layers are nonetheless affected by oxygen deficiency during the spring and summer months. At a depth of 25 metres, oxygen concentrations in the Eckernförde Bay have decreased significantly, with the lowest values found between May and September. At times oxygen is completely absent from the deep-water areas.
- 3.6 > Scientists have regularly studied the water at a certain location in the Eckernförde Bay on the Baltic Sea coast since 1957. Nowadays they use modern water samplers that take water samples at different depths.
- This is most likely caused by ocean warming which on the Baltic coast gives rise to two interconnected phenomena. Firstly the warming of the upper water layers results in a more pronounced thermocline which hampers oxygen transportation to greater depths during the summer months. Secondly this is accompanied by a biological phenomenon. Small filamentous algae that settle on macroalgae such as bladder wrack thrive particularly well in warmer waters. Normally such filamentous algae are grazed by small crustaceans. But when water temperatures increase, the crustaceans become more sluggish and hardly feed at all. This allows the filamentous algae to proliferate and ultimately overgrow the bladder wrack and other macroalgae. Bladder wrack, which is dependent on sunlight for photosynthesis, dies off, thus generating unnaturally large quantities of dead biomass which then sinks to deeper water layers where it is decomposed by bacteria. This increases the oxygen demand, with oxygen already being in short supply due to the more pronounced thermocline. These processes can give rise to oxygen-depleted zones, especially during July and August.
For several years now the scientists have observed a collapse of the biocoenoses in the water layers near the bottom of the Eckernförde Bay at the height of summer.
Cyanobacteria are a group of bacteria that are able to photosynthesise. For this reason they were originally considered to be plants and were called blue-green algae. The term “blue” refers to the fact that some types of cyanobacteria contain the bluish plant pigment phycocyanin instead of the green plant pigment chlorophyll.
- These observations in the Eckernförde Bay are congruent with measurements that have been analysed for the entire Baltic Sea. US American weather satellites have been measuring Baltic Sea surface temperatures several times per day since 1990, thus building up a very good set of temperature data. These data show that the Baltic Sea surface temperature has increased by 0.6 degrees Celsius per decade since 1990. This figure is based on annual averages, as the Baltic Sea is subject to strong seasonal fluctuations and also displays clear regional differences. Over the study period of 27 years the surface temperature has therefore increased by 1.62 degrees Celsius. The increasing temperatures particularly favour the growth of cyanobacteria. In calm summer weather periods during which the water heats up particularly swiftly, these algae rise to the sea surface where they form mats, primarily in the central Baltic Sea. Winds can wash such algal mats onto the beaches. From the human point of view this is a problem because many species of cyanobacteria produce toxic substances. Overly rapid growth of cyanobacteria can result in toxic carpets of Harmful Algal Blooms (HABs). Swimming is prohibited in affected coastal areas. Moreover, HABs can poison marine animals such as fish, thus resulting in potentially significant losses for coastal fisheries.
- 3.7 > In the Eckernförde Bay, the number of months per year in which the water at 25 metres of depth is oxygen-deficient has increased since the late 1950s. This is thought to be due to the warming of the Baltic Sea waters.
Corals under heat stressTropical coral reefs are one of the coastal ecosystems particularly at risk from ocean warming. Not only are they sensitive to a rise in water temperatures, but in many areas they suffer additional pressures, particularly as a result of the pollution of coastal waters with toxic substances, nutrients and suspended solids. While globally only approximately 1.2 per cent of the continental shelves are covered by coral reefs, these reefs are enormously species-rich. It has been estimated that they host more than 1 million species of fish, bivalves, corals and bacteria.
Coral bleaching – a symbiosis is failingCorals are marine animals in the Cnidaria phylum living in symbiosis with unicellular plants. These single-celled organisms, the zooxanthellae, reside in the tissue of corals. They are green-brown in colour and are able to photosynthesise. It is these organisms that provide corals with much of their colour. They also provide their hosts with sugars and in return they receive various nutrients. Coral bleaching occurs when this symbiosis fails and the zooxanthellae leave the corals, which as a result lose much of their colour. Recent research has been able to identify the various factors contributing to the failing of this symbiotic relationship. Ocean warming evidently plays a key role.
The optimum water temperature range for many tropical coral species is between 25 and 29 degrees Celsius. For many species, an increase of as little as 1 to 3 degrees Celsius can trigger bleaching. This appears to be caused by changes in the zooxanthellae’s metabolism. At higher temperatures, many metabolic processes, such as photosynthesis, run faster and result in the production of increased amounts of cell-damaging radicals, i.e. aggressive molecules, a proportion of which enters the corals from the zooxanthellae. As soon as the corals register an increase in the production of radicals they trigger a protective reaction, expelling the zooxanthellae into the water column. Bleaching is therefore a mechanism protecting corals from cell damage.
- 3.9 > Corals bleach when they come under stress – such as this stony coral in the Indonesian Raja Ampat Archipelago. The corals then expel the zooxanthellae, pigmented single-celled organisms with which they live in symbiosis.
- Coral bleaching is a natural and reversible phenomenon. Once the stressor abates, for example if water temperatures drop, the corals once again take up the zooxanthellae from the surrounding water into their tissues and recover. However, in many coral reefs bleaching now occurs much more frequently than in the past due to ocean warming in combination with other stressors. While in the past a reef may have experienced a bleaching event roughly once in twenty years, in many areas the phenomenon now tends to occur at intervals of only a few years, leaving the corals hardly any time to recover. Once the zooxanthellae have been expelled they can no longer provide the corals with sugars. The corals then begin to starve and weaken and become more susceptible to being attacked by pathogens such as bacteria.
Approximately 20 per cent of corals have been killed and a further 30 per cent are severely damaged as a result of ocean warming and other stressors. Moreover, a total of 60 per cent of all tropical coral reefs are locally at risk due to at least one of the following local aspects:
- destructive fisheries practices that destroy the reef, such as anchored boats or nets;
- coastal development (construction);
- pollution of marine waters due to riverine inputs of pollutants or suspended solids;
- local pollution of marine waters due to direct inputs of wastewater along the coast or from merchant vessels and cruise ships as well as destruction resulting from bottom-contact by ferries or tourist vessels.
Adaptation to warmingFortunately corals can adapt to rising ocean temperatures to a certain extent. Recent studies have shown that some coral species selectively incorporate other species of zooxanthellae following a bleaching event. This form of adaptation is called adaptive bleaching. The corals appear to prefer species of zooxanthellae that only moderately increase their metabolism under conditions of rising water temperatures and thus produce fewer radicals. However, these zooxanthellae tend to have a lower metabolic rate which means they also produce less sugar. If in the course of the year temperatures drop again, this may put the corals at a disadvantage, as the zooxanthellae will then be less productive and provide lower quantities of sugar due to their lower metabolic rate. Research is currently underway into the consequences this may have. The insufficient supply of sugars might slow down the corals’ growth. Moreover, adaptive bleaching has limits. If the water temperatures are constantly too high, the symbiosis may fail nevertheless, resulting in renewed bleaching. This may be due to the production of radicals in the zooxanthellae or to other metabolic processes that are as yet not fully understood.
In addition, eutrophication of coastal waters with nutrients from agricultural or aquacultural sources may contribute to the failure of the symbiosis. Nitrogen plays an important role in this context as it is a vital nutrient for zooxanthellae. If a lot of nitrogen is available, the zooxanthellae increase their metabolism and display strong growth. However, if in the course of this growth phosphorus, another important plant nutrient, is missing, problems may arise.
Phosphorus is an essential component of cell membranes. If it is in short supply during cell growth, insufficient amounts of phosphorus are integrated into the membranes, making them more permeable. As a result, increased amounts of free radicals can transfer from the zooxanthellae into the coral tissue, which in turn leads to the zooxanthellae being expelled, and to coral bleaching.
- 3.10 > Corals are basically colourless. Single-celled organisms (zooxanthellae) residing in the coral tissue are responsible for making them appear colourful. Zooxanthellae engage in photosynthesis and are of a greenish or reddish colour. If the coral comes under stress, for example due to elevated water temperatures or water pollution, it expels the zooxanthellae and bleaches as a result. Moreover, it now lacks the essential sugar compounds normally provided by the zooxanthellae. This weakens the coral.
- Efforts are now underway to restore dead coral reefs. To this end, fragments of living corals are attached to the dead ones in the hope that they will grow and reproduce. Experts have also been searching for particularly stress-resistant coral species suited to these efforts. The Red Sea appears to host particularly robust species. Due to the seasonal variations in water temperatures – just over 20 degrees Celsius in winter and often more than 30 degrees Celsius in summer – many corals in this region are adapted to fluctuations in water temperature and would therefore be suitable for the restoration of damaged reefs. However, it is important to consider that there are several hundred species of corals worldwide. Experts consider that probably only very few species will be suitable for reef restoration efforts in that they are sufficiently robust to exist in other oceanic regions with other environmental conditions. Even if reef restoration for purposes of coastal protection was to be successful, the reef’s species diversity will have been irretrievably lost upon its destruction.
Bleaching is not the only impact of ocean warming. There are numerous diseases that can cause corals to die. Bacterial infections in particular are on the increase, Acropora White Syndrome (AWS), for example, or the Black Band Disease (BBD), both of which quickly kill the Cnidaria upon infection. These diseases are therefore clearly more dangerous than bleaching, as the latter is reversible while the infections are generally lethal. These infectious diseases primarily affect reefs in the Caribbean where they can spread several metres in just a few days. It is thought that in such cases the corals are weakened by ocean warming and are not able to produce sufficient quantities of antibodies which would normally help them to keep the pathogens at bay. >