Natural variability and change in coastal ecosystems

Coastal ecosystems, especially those of transitional waters, are dynamic, varying in both time and space. This article provides an introduction to the scales of variability as a basis for understanding the problems that managers face because of the constant change taking place within coastal systems.

Natural spatial and temporal variability is considered here, at different geographical scales and at different levels of coastal and marine ecosystems. The way in which natural variation is influenced by issues relating to climate change, water catchments and human activity is also discussed.

Temporal and spatial scales of variability

Coastal and marine ecosystems are not in a steady state, but exhibit continuous changes in production and species composition at different trophic levels of the food webs. Awareness and scientific understanding of this variability has increased during the last decades. Long-term data sets on phytoplankton, zooplankton, macrofauna, fish and birds have been collected around the world. Up until recently, these data sets have been mainly applied to demonstrate the effects of human use on the ecosystem. However, when the various data sets are combined, it is striking that certain changes are very sudden and not gradual, as one would expect from a gradual increasing human impact. What appears also is that the spatial scale of the variability differs by orders of magnitude.

Spatial: from global to local

At global level, the El Niño-Southern Oscillation (ENSO) cycle is well known as resulting in the nearly complete failure of fisheries in South-American waters and many other ecological deviations world-wide every 4-7 years. The North Atlantic Oscillation, similar to the El Niño phenomenon in the Pacific basin, is a periodic change in atmospheric pressure between Iceland and Portugal. It determines the strength of the prevailing westerlies in the North Atlantic. This in turn affects ocean surface currents and hence the movement of water towards North-Western Europe, in particular into the North Sea. It is expected that the flow of the North Atlantic Current, as well as the Continental Slope Current along the European Shelf Break, determining the rate of heat transferred towards Europe may have a large influence on diversity. Thus, a modification in the path or strength of these currents could imply marked changes in the north-eastern path of the North Atlantic Drift Province, the European Shelf Break and the North Sea.

Present-day patterns in pelagic biodiversity are the result of the interaction of many factors acting at different scales. Temperature, hydrodynamics, stratification and seasonal variability of the environment are likely to be the main factors contributing to the ecological regulation of the diversity of planktonic organisms. The similar geographical pattern evident between currents/water masses and the species associations suggest that the species groups may be used as an environmental indicator to evaluate long-term changes in the marine environment related to climate change and other increasing human-induced influences. Changes are visible in biologically distinct areas of seawater and coasts recognised by scientists as large marine ecosystems. Geographical changes in the diversity of calanoid copepods (planktonic crustaceans) has been studied in the North Atlantic and the North Sea based on data historically collected by the Continuous Plankton Recorder (CPR) survey. Detectable year-to-year or decadal changes in the pelagic diversity of this region may be expected to have already occurred, or to be possible in the future with climate change. Recently, there has been an increase in the abundance of a number of arctic-boreal plankton species, moving southward into the North Sea. At the same time, species used to live in the North Sea were slowly establishing themselves in the Channel and the Atlantic. Considering the possible relationship between diversity and ecosystem function, a fluctuation in the diversity of this key group could be accompanied by major changes in the structure and functioning of ecosystems, because of changes taking place in the food webs.

At an even smaller scale, local patterns arise. The high turbulence, phytoplankton biomass and purely passive scalars such as temperature and salinity are basically regarded as homogenised by turbulent fluid motions in coastal seas such as the English Channnel. However, recent studies have demonstrated, on the basis of innovative statistical analyses in marine ecology, that these parameters were heterogeneously distributed at a small scale. Nutrient patches appear in tidally mixed coastal waters of the eastern English Channel, characterised by its megatidal regime. The observed patches could be the result of complex interactions between hydrodynamic conditions, biological processes related to phytoplankton populations, and the productive efficiency of bacterial populations. This hypothesis is supported by observations that the structure of temperature and salinity-regarded as passive scalars under purely physical control of turbulent motions (recorded simultaneously to the nitrite data) remained similar under all hydrodynamic conditions. Around such water patches, frontal zones appear. Fronts also form when water masses of distinctly different properties meet at a sharp boundary, forming shelf break fronts, upwelling fronts, and tidal fronts. The resulting boundary, which limits stratified and mixed waters, is characterised by high phytoplankton production and high numbers of associated zooplankton. In turn, the plankton is consumed by foraging fish, themselves the prey of seabirds and marine mammals. Links with changes of short-term or large-scale weather patterns, wind, winter and/or summer temperatures or rainfall, have been suggested. Spatial variability is thus governed by biological dynamics in relation to internal and external forces which regulate ecosystems.

Temporal: from long term to cyclic

Time is the other dimension to consider when looking at the natural variability of coastal ecosystems. On the long term, it would seem that changing climate patterns are currently leading to more extreme conditions. This is the case in the Atlantic where a steady increase of the significant wave heights in the last 30 years by about 2 to 3 cm per year has occurred. A shift in storm frequencies or wind directions might cause changes in sediment water exchange or mixing. Possible causes of the observed phenomena also include changes in water or nutrient fluxes from the landward or seaward side, and internal processes in the marine ecosystem. However, to prove the real cause-effect relationships in the complex coastal ecosystem is often very difficult, if not impossible. Most likely different causes can have similar effects, and the (local) human disturbances are another complication in the analyses.

In temperate regions the occurrence of cold winters strongly influences the species composition of intertidal benthic communities. With milder winters, changes in their biodiversity might be expected. Actually, in the mid-term, recent changes in Antarctic seabird populations may reflect direct and indirect responses to regional climate warming. The best long-term data for high-latitude Antarctic seabirds (Adelie and Emperor penguins and snow petrels) indicate that winter sea-ice has a profound influence on their life cycles. However, some effects are inconsistent between species and areas, some in opposite directions at different stages of breeding, and others remain paradoxical. The combination of fisheries driven changes and those caused by global warming may produce rapid shifts rather than gradual change.

Despite an increasing number of examples for many areas around the world, cyclic behaviour in coastal seas is not undisputed. Is it really the result of complex physical-biological interactions or just a statistical feature of datasets? In freshwater systems and tree-rings cyclicity has been well documented. Growing evidence for this behaviour in sediments, corals and shellfish growth has been reported for marine systems. The number of papers suggesting links with solar activity is increasing. However, very long datasets also indicate alternations of periods with clear cyclicities with periods with no patterns at all. Whether predictable cyclicity really exists is a major scientific question.

Long-term data series can help to answer questions on the variability of coastal ecosystems, and the continuous collection of data, often hampered by limited funding, should be strongly supported. These long-term data sets, in combination with the results of experimental laboratory and field studies, are necessary to understand trends in coastal marine ecosystems and whether they are due to local disturbances (for example pollution) and to global climatic (oceanic) changes.

Increased variability due to climate change

The earth climate is subject to natural cycles. Some occur over short time scales or may span decades, centuries or millennia and environmental change rather than stability has characterised the last 2 million years. However, the rate and duration of warming in the 20th century was greater than in any of the previous centuries. Humans are just changing the rate of change and the scale and character of local changes can usually be determined by management practices. Based on computer models, the global air temperature is expected to increase by an additional 1.4 to 5.8 °C over the next 100 years. Such climate changes relate to increases in greenhouse gases and are part of a global change. Human activities, such as the burning of fossil fuels, have been held responsible for such dramatic increases. They affect the earth’s energy balance, which in turn influences the atmospheric and oceanic circulation patterns, hence the weather. However there are large regional variations, including cooling in some areas. Whatever the changes maybe, they push ecological systems to their limits and put them under stress whether they are natural or under large anthropogenic influence. In such circumstances, natural variability is increased and its effects more dramatic.

Variability in oceans

Oceans are divided in two main important layers of water separated by a halocline, with surface water circulating at the very top. Surface currents have a large capacity for transport and may even surpass the atmospheric circulation in importance. It has already been demonstrated that ocean pathways are exhibiting changes in relation to different regimes in atmospheric circulation. The atmosphere does not respond as an isolated system and energy fluxes couple the ocean-atmosphere system. For example, surface winds mobilize the currents and sea water stores latent heat. As a consequence, the ocean stores more energy than the atmosphere. This is why atmospheric circulation patterns can be described by differences in sea-level air pressure. It would seem that the resulting changing climate patterns are currently leading to more extreme conditions, i.e. storminess. This is the case in the Atlantic where a steady increase of the significant wave heights in last 30 years by about 2 to 3 cm per year has occurred. Projected climate change could have other effects, including changes in precipitation regime, ocean currents, salinity (due to changes in river flow), and surface temperatures. Changes in temperature and precipitation patterns will affect run-off and flow in rivers, while the physical responses of estuaries and coasts to sea level rise will depend upon a combination at local level of eustatic movements (the shear increase of sea water volume in the oceans) and isostatic movements (due to the tilting of landmasses in relation to the melting of the ice cap). Estimates of the magnitude of the sea level rise hazard have been based on a doubling of CO₂ leading to a sea level rise of 49 cm to the year 2100.

Changes in temperature

Changes in temperature force species to move to more favorable habitats. Since 1950 the north/south balance between two barnacle species with overlapping geographical ranges but otherwise similar habitats around South West England has shown both predictable and unexpected changes. Mid-term changes in population numbers were correlated with 10-11 year cycles in mean sea temperature. Quick responses were noted in relation to abnormal heat or cold. A decline of the southern species coincided until about 1975 with a secular cooling trend, when its numbers began to rise. Presently, the southern species of barnacle and other taxons have been described moving northward.

Effect of wave variability

On the open coast, an increase in the occurrence of waves linked to an increase in high tide level will lead to wide spread erosion with a landward migration of the high tide mark and a flattening of the shore. If the shore is protected by embankments, it is expected that the intertidal profile will become steeper with a shrinking of intertidal communities. Associated to such a threat, a key factor for estuaries is the variable salinity as sea level rise has now become a main issue when biogeochemical cycles might be affected, due to an increase in wave and in tide return intervals, provoking a decrease in the frictional drag on the tidal wave. The wave will move further landward with increased amplitude and current velocities with, as a result, increased salinities in the inner estuary or the coastal lagoon, a landward migration of the turbidity maximum and an erosion of flats and marshes or mangrove swamps. Such a variability of climatic conditions has to be taken into account to understand any potential changes in the biota. The erosion of salt marshes will mean that, if the space is available, there will be a longshore migration of flats with an establishment of new marshes upstream. However, whatever the local conditions, on the long term, a reduction of existing salt marshes is expected with a negative effect on biological diversity and changes in benthos communities with attached managerial issues. Possible consequences of the climate change on the biology of coasts and estuaries include a move of high energy habitats in the outer parts and a change in biochemical cycles.

Long term outlook

The possible effects of predicted climate changes need to be considered on the long-term. The direct effects of increased CO₂ on living organisms will bear upon carbon fixation pathways, in particular photosynthesis. An increase in primary production might be expected but a change in cloud albedo may have a negative effect on the metabolism of plankton. Because of a possible decrease in the sea water pH, shell dissolution and a lack of availability of essential metal ions could have a negative effect on growth and the morphology of coastal organisms. Recent experiments have shown the sensitivity of certain algal species to changes in temperature, the length of the photo period and solar radiation intensity. Inadequate timing (e.g. appearance of juveniles) could mean alteration of growth and survival with tolerance varying with age and development stage of the individual. In particular, the seasonality of red seaweeds (Rhodophyta) was studied in relation to growth and reproduction. Adequate timing of life history events (e.g. appearance of juveniles in spring) appeared more important than maximal growth and reproduction of adults during the season with the most favourable temperatures.

Atmospheric constitutents

Any interference by atmospheric constituents (gasses, vapour etc.) and the scattering and absorption by particles will lead to a depletion of the original spectrum of solar radiations, in particular ultra-violet (UV), which are solar electromagnetic radiations of wavelength between 100 and 400 nm. In coastal waters, due to high concentrations of suspended matter and yellow substance, the transmission of UV-B (280 – 315 nm) will be limited to surface layers between 0.5 and 1 m. On tidal flats, effects on benthic organisms due to shallow water depth and emersion will be much larger in space. Decreased fecundity, growth and development (including an inhibition of photosynthesis in plants), survival and mobility will be linked to an inhibition of phototactic and photophobic responses and a decreased photorepair ability. The tolerance of marine macroalgae to ultraviolet and visible radiation is to be related to the content of amino acids. Under exposure to UVA and UVB radiation for various periods in the laboratory, the rate of the initial decrease was greater. The tolerance of macroalgae to UV radiation depended on their age, germinating spores being the most sensitive growth phase. With an increase in UV radiations, it can be envisaged that certain species might encounter problems with the renewal of populations, leading to dramatic changes in biodiversity.

Their variation in time and seasonality is experienced every year by human societies whose culture is adapted to such cyclic events. However year to year changes take place and are difficult to predict. Trends on the long term are even more difficult to predict as they range from centuries to millennia. Changing wave and current regimes, climate, morphological processes and fluxes of chemicals and nutrients from land, the atmosphere and oceans result from a high natural variability, which is still imperfectly understood. On a spatial scales, there are many ways of defining geographical units as coastal ecosystems range from small estuaries to fjords or large bays. It would be difficult to speak of “natural threats” on coastal ecosystems when they are due to the changeability of environmental factors. However, in the last decades, with their increasing technological capabilities, humans have accelerated the rate of change, increasing their influence on already highly variable ecosystems. Impacts originate locally and regionally, but recently the global context has become primordial to understand because of the climatic change which is taking place.

Catchment / watershed influences

Coastal ecosystems, and estuaries in particular, are under strong influence of their watershed. Estuaries are semi-enclosed basins receiving water directly from a riverine basin. They are permanently connected to the sea whose waters are diluted by fresh water drainage and run-off. Estuaries maintain exceptionally high levels of biological productivity and play important ecological roles, including water purification, ‘exporting’ nutrients and organic materials to outside waters through tidal circulation; providing habitats for a number of commercially or recreationally valuable fish species; and serving the needs of migratory and oceanic species which require areas for breeding and/or sanctuary for their young. Influences come both from the ocean and the adjacent land. In megatidal estuaries, differential degrees of sediment mobility have shown crucial effects on the zonation of the tidal flat macrofauna. Particle fluxes to mid water depths in the adjacent sea are mainly controlled by fluvial discharge and primary production. Fluvial discharge could be responsible for the higher lithogenic flux during autumn and winter, while high primary production could play a key role in generating biogenic particles during spring and summer. The benthic fauna responds to changes in particulate fluxes of mineral and organic matter from the photic layer. In turn, changes in the secondary production due to estuarine benthos have an impact on food chains, in particular fish. Consecutively, changes in the biodiversity of fish may affect the productivity of coastal systems. Therefore, the intensive exploitation of fish communities often leads to substantial reductions in the abundance of target species, with ramifications for the structure and stability of the ecosystem as a whole. Changes in the mean trophic level of fish communities have been shown using commercial landings, survey data and estimates of trophic level derived from the analysis of nitrogen stable isotopes. Long-term changes in the trophic level in fish communities were finally connected to fish market price distribution, having an impact on landings and hence the fishing effort.

Natural variability vs. human induced variability

Coastal ecosystems are exposed to human activities such as fish- and shrimp farming, industrial and domestic pollution, dredging and industrial and agricultural land reclamation. Mangroves are also subject to clear-cutting and over logging. Such disturbances increase the natural variability of natural systems. To understand better how humans induce more variability a combination of different modelling approaches aimed at combining the respective benefits of global system analyses (trophic models of biomass flows), detailed process studies (simulation packages) and a risk assessment combination both on ecological and socio-economical aspects is used. In this way, it is possible to integrate the constant advances in scientific knowledge of all the participant research disciplines. This dynamic interplay between theory and empirical study forms the basis for efficient, transdisciplinary work.

Coral reefs

Coral reef. Source: USGS

Coral reefs are undergoing change with widespread impacts from human use and “natural” influences of global climate dynamics. The fundamental structure of reefs depends on corals and algae using carbon dioxide and accumulating calcium carbonate (calcification). Recent findings show that changes in the global carbon dioxide are having significant effects on reef calcification by effect on the calcium carbonate saturation state. Local human impacts add up to the climate change, both directly (e.g. over fishing and poison/dynamite fishing, limestone mining, land reclamation, pollution) and indirectly (e.g. tourism, coastal and catchment developments). As a result of 30% decrease in reef calcification over 300 years of industrialisation has been described. Effects take place at whole reef system scale and at the level of organism physiology and reproduction.

Salt marshes

Salt marsh. Source: USGS

Salt marsh is an important coastal habitat influenced by both global and local environmental factors. The spatial distribution of marshes depends upon a delicate balance between sediment accretion and erosion forces. Intertidal sedimentary areas, such as the Wadden Sea and tidal estuaries, are highly influenced by the dynamics in erosion and sedimentation and consequently by the variation in particle load and thus turbidity. A key factor for estuaries is variable salinity but sea level rise, a consequence of the global change, is now becoming a main issue. Sediment drift tend to increase from high towards low tide level, while abundance of nearly all species decrease possibly due to changes in sediment stability. In relation to sea-level change, two main trends have been noted in megatidal estuaries. In deposition areas, colonisation occurred whereas most marshes were affected by increased hydrodynamism in peripheral zones. In spatial terms these two situations can occur close to each other as was observed for the mainland salt marshes in the Dutch Wadden Sea. Understanding and interpreting such contradictory findings needed further ecological considerations. As a highly productive system, salt marshes are characterised by relatively high below-ground biomasses. For example, in Spartina marshes two to four times more biomass is tied up as below-ground roots and rhizomes. In both seagrass and salt marsh systems macroalgae, benthic microalgae, phytoplankton and epiphytes contribute to the total production. However, the growth of epiphytes is boosted by the water nutrient content. In areas with high nutrient inputs of anthopogenic origin, epiphytes might hamper the growth of seagrasses and marsh plants.

Spatial scale of influence

In order to discriminate between global and local influences, in-depth knowledge of natural processes is essential to acquire as well as relevant institutional, cultural, economic, social and political frameworks through a transdisciplinary approach. Suitable models should be developed and used to analyse causal linkages within the ecosystem, to forecast the effects of acute or chronic interference on used resources, and to answer wider, management-related questions (i.e. restoration of damaged habitats, potential for aquaculture, etc).

Conclusion

Whatever the domain, the variability of ecosystems appears a major feature. This has to be considered as a significant contribution to a better understanding of the ocean as well as a base for a sustainable management of the marine systems. In effect, knowledge of the fundamental mechanisms and their variability are essential to establish an efficient management of the environment. As far as the forcing functions are considered, two main factors control the variability of coastal systems:

The climate-meteorology, and
(Directly or indirectly), the anthropogenic activities, through the use of mineral and living resources.

These factors have an impact on the quality (biogeochemistry) of the environment and on the dynamics, thus influencing the biological performance of the system. Distant activities may affect natural patterns of variable systems, and scientists need to understand this from a holistic point of view.

As a result, the spatial distribution of main wind trajectories, river runoff, currents and fluxes through straits, have a paramount influence on the observed characteristics of sub-basins and their gradients over entire basins. When disturbing such processes, human activities might affect natural patterns of ecosystem or accelerate the rate of change disturbing them. Sea-level changes, as a consequence of glacial mass balance deficits, affect the physical and chemical characteristics of coastal environments and the marine plant and animal communities which inhabit them. The structure and organisation of communities, conditioned in each coastal environment by a combination of abiotic factors, also depend upon biological characteristics such as recruitment and productivity rates. Keystone species may control the outputs of local biodiversity through large indirect effects, disproportionately large relative to their abundance, and hence have an impact on the local natural variability, notably by changing local habitats and the biogeochemical cycles involved in their maintenance.

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