Natural variability and change in coastal ecosystems

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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.

This article is largely based on the review paper of Crossland et al. (2005[1]).


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 gradually increasing human impact [2]. What appears also is that the spatial scale of the variability differs by orders of magnitude [3].

Spatial: from global to local

At global level, the El Niño-Southern Oscillation (ENSO) cycle is well known as resulting in major disruptions of fisheries in South-American waters and many other ecological deviations world-wide every 4-7 years [4]. The North Atlantic Oscillation, similar to the El Niño phenomenon in the Pacific basin, is a periodic change in atmospheric pressure between Iceland and the Azores. It determines the strength of the prevailing westerlies in the North Atlantic [5]. 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 [6]. 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 among the main factors contributing to the ecological regulation of the diversity of planktonic organisms.[7] The similar geographical pattern evident between currents/water masses and the species associations suggests that the species groups may be used as environmental indicators 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. [8] Geographical changes in the diversity of calanoid copepods (planktonic crustaceans) have been studied in the North Atlantic and the North Sea based on data historically collected by the Continuous Plankton Recorder (CPR) survey. [9] 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 have been shifts in the abundance and distribution of a number of arctic-boreal plankton species in the North Sea region. [10] At the same time, species used to live in the North Sea were slowly establishing themselves in the Channel and the Atlantic. [11] 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. Phytoplankton biomass and physical properties such as temperature and salinity were thought to be homogenised by turbulent fluid motions in coastal seas such as the English Channel. However, recent studies in marine ecology have demonstrated, using advanced statistical methods, that phytoplankton biomass and physical properties such as temperature and salinity can be heterogeneously distributed at small spatial scales. Nutrient patches appear in tidally mixed coastal waters of the eastern English Channel, characterised by its megatidal regime.[12] These patches are thought to result from complex interactions between hydrodynamic conditions, biological processes related to phytoplankton populations, and bacterial production and nutrient recycling. This interpretation is supported by observations that the structure of temperature and salinity, regarded as passive scalars under purely physical control of turbulent motions and recorded simultaneously with nitrite concentrations, remained similar under all hydrodynamic conditions, whereas nutrient distributions showed stronger patchiness.[12]

Frontal zones form where water masses of different origin meet at sharp boundaries, such as shelf-break fronts, upwelling fronts, and tidal fronts. Boundaries between stratified and mixed waters are often characterised by high phytoplankton production and large numbers of associated zooplankton.[13] In turn, the plankton is consumed by foraging fish, themselves prey for seabirds and marine mammals. Links have also been suggested between these patterns and changes in short-term or large-scale weather conditions, including wind, winter and summer temperatures, and rainfall.[14] Spatial variability is thus governed by biological dynamics in relation to both internal and external forces that 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 in some regions and for some variables. [15] This is the case in parts of the North Atlantic, where some studies have reported small increases in significant wave height over recent decades, although these trends are spatially variable and not uniform across the basin [16]. 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. [17] 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. [17]

Despite an increasing number of examples for many areas around the world, cyclic behaviour in coastal seas is not undisputed. [18] 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. [19] Some studies have also explored possible links with solar activity. [20] However, very long datasets also indicate alternations of periods with clear cyclicities with periods with no patterns at all. [18] 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 climate models, global air temperature is projected to increase over the 21st century by roughly 1.5°C to more than 4°C, depending on future greenhouse gas emissions, with current policies leading to about 2.5–3°C warming by 2100 [15]. 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 may be, 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 of heat, salt, nutrients and organisms. [21] It has already been demonstrated that ocean pathways are exhibiting changes in relation to different regimes in atmospheric circulation. [22] The atmosphere does not respond as an isolated system and energy fluxes couple the ocean-atmosphere system. It would seem that the resulting changing climate patterns are currently leading to more extreme conditions in some regions. [15] Projected climate change could have other effects, including changes in precipitation regime, ocean currents, salinity (due to changes in river flow), and surface temperatures. [15] 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 and isostatic movements. See the article Sea level rise for details.

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. [23] 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. [24]

Effect of wave variability

On the open coast, an increase in high tide level and high waves will lead to widespread 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. Sea level rise will cause the tidal wave to move further landward with increased amplitude and current velocities. As a result, increased salinities in the inner estuary or the coastal lagoon, a landward migration of the estuarine turbidity maximum and an erosion of flats and marshes or mangrove swamps may be expected. [25] Possible consequences of climate change on the biology of coasts and estuaries include a partial loss and landward shift of estuarine habitats 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. Changes in the underwater light climate may affect the metabolism and productivity of plankton and benthic primary producers. [26]

A decrease in the sea water pH may impede shell formation and promote shell dissolution[27]. Acidification may also affect the speciation of metals in seawater and alter the bioavailable fractions of these metals (increase or decrease). These changes could possibly affect the availability and toxicity of metals on marine organisms.[28][29]. See the article Ocean acidification for further details.

Recent experiments have shown the sensitivity of certain algal species to changes in temperature, the length of the photo period and solar radiation intensity. [30] 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. [30] 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.[30]

Atmospheric influences

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. [31] Benthic organisms on tidal flats could be especially affected due to shallow water depth and emersion.

UV radiation, especially UV-B, can strongly affect life in the sunlit ocean upper layer. One of the biggest effects is on photosynthesis. UV-B can damage the machinery that phytoplankton, algae, and cyanobacteria use to turn sunlight into energy, reducing their ability to produce food and oxygen. Since phytoplankton form the base of marine food webs, this can affect the entire ecosystem[32]. UV-B also causes cells to produce harmful reactive oxygen species (ROS), which create additional stress and damage. Warmer temperatures can make this oxidative stress worse[32]. Another major impact is DNA damage. UV-B can break DNA strands and create mutations, making it harder for organisms to grow, reproduce, and survive. Although many organisms have repair systems and protective compounds, strong or prolonged UV exposure can overwhelm these defenses[32]. With an increase in UV radiations, it can be envisaged that certain species might encounter problems with the renewal of populations, leading to changes in biodiversity.

Trends on the long term are even more difficult to predict as they range from centuries to millennia. Wave and current regimes, climate, morphological processes and fluxes of chemicals and nutrients from land, the atmosphere and oceans exhibit strong natural variability, which is still imperfectly understood. It would be difficult to speak of “natural threats” on coastal ecosystems when they are due to the natural variability of environmental factors. However, in the last decades, 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.

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:

  1. The climate-meteorology, and
  2. (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.


Related articles


References

  1. ↑ Crossland, C.J., Baird, D., Ducrotoy, J.P. and Lindeboom, H.J. 2005. The coastal zone – a domain of global interactions. In: Coastal fluxes in the Anthropocene (eds. Crossland, C.J., Kremer, H.H., Lindeboom, H.J., Marshall-Crossland, J.I. and Le Tissier, M.D.A.). Springer-Verlag, Berlin: 1-37.
  2. ↑ Scheffer, M., Carpenter, S., Foley, J.A., Folke, C. and Walker, B. 2001. Catastrophic shifts in ecosystems. Nature 413: 591–596.
  3. ↑ Seuront, L., Schmitt, F. and Lagadeuc, Y. 1999. Universal multifractal analysis as a tool to characterize multiscale intermittent patterns: example of phytoplankton distribution in turbulent coastal waters. Journal of Plankton Research 21: 877–922.
  4. ↑ Bertrand, A., Lengaigne, M., Takahashi, K., Avadí, A., Poulain, F. and Harrod, C. 2020. El Niño Southern Oscillation (ENSO) effects on fisheries and aquaculture. FAO Fisheries and Aquaculture Technical Paper No. 660. Rome, FAO.
  5. ↑ Hurrell, J.W., Kushnir, Y., Ottersen, G. and Visbeck, M. 2003. An overview of the North Atlantic Oscillation. In: The North Atlantic Oscillation: climatic significance and environmental impact (eds. Hurrell, J.W., Kushnir, Y., Ottersen, G. and Visbeck, M.). Geophysical Monograph 134. American Geophysical Union, Washington, DC: 1–35.
  6. ↑ Hátún, H., Payne, M.R., Beaugrand, G., Reid, P.C., Sandø, A.B., Drange, H., Hansen, B., Jacobsen, J.A. and Bloch, D. 2009. Large biogeographical shifts in the north-eastern Atlantic Ocean: from the subpolar gyre, via plankton, to blue whiting and pilot whales. Progress in Oceanography 80: 149–162.
  7. ↑ Beaugrand, G., Edwards, M., Brander, K., Luczak, C. and Ibanez, F. 2008. Causes and projections of abrupt climate-driven ecosystem shifts in the North Atlantic. Ecology Letters 11: 1157–1168.
  8. ↑ Sherman, K. and Alexander, L.M. (eds.) 1986. Variability and Management of Large Marine Ecosystems. AAAS Selected Symposium 99. Westview Press, Boulder, Colorado.
  9. ↑ Beaugrand, G., Reid, P.C., Ibanez, F., Lindley, J.A. and Edwards, M. 2002. Reorganization of North Atlantic marine copepod biodiversity and climate. Science 296: 1692–1694.
  10. ↑ Beaugrand, G. 2009. Decadal changes in climate and ecosystems in the North Atlantic Ocean and adjacent seas. Deep-Sea Research II 56: 656–673.
  11. ↑ Beaugrand, G., Reid, P.C., Ibanez, F., Lindley, J.A. and Edwards, M. 2002. Reorganization of North Atlantic marine copepod biodiversity and climate. Science 296: 1692–1694.
  12. ↑ 12.0 12.1 Seuront, L. and Schmitt, F.G. 2005. Multiscaling statistical procedures for the exploration of biophysical couplings in intermittent turbulence. Part II. Applications. Deep-Sea Research II 52: 1531–1543.
  13. ↑ Pingree, R.D., Holligan, P.M., Mardell, G.T. and Head, R.N. 1978. The effects of vertical stability on phytoplankton distributions in the summer on the northwest European Shelf. Deep-Sea Research 25: 1011–1028.
  14. ↑ van der Molen, J. and Pätsch, J. 2022. An overview of Atlantic forcing of the North Sea with focus on 2001–2018. Journal of Sea Research 189: 102281.
  15. ↑ 15.0 15.1 15.2 15.3 Intergovernmental Panel on Climate Change 2021. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the IPCC. Cambridge University Press, Cambridge, UK and New York, NY, USA.
  16. ↑ Young, I.R., Ribal, A. and Zieger, S. 2019. Multiplatform evaluation of global trends in wind speed and wave height. Science 364: 548–552.
  17. ↑ 17.0 17.1 Croxall, J.P., Trathan, P.N. and Murphy, E.J. 2002. Environmental change and Antarctic seabird populations. Science 297: 1510–1514.
  18. ↑ 18.0 18.1 Southward, A.J. 1980. The western English Channel — an inconstant ecosystem? Nature 285: 361–366.
  19. ↑ Cronin, T.M. 2010. Paleoclimates: Understanding Climate Change Past and Present. Columbia University Press, New York.
  20. ↑ Cronin, T.M. 2010. Paleoclimates: Understanding Climate Change Past and Present. Columbia University Press, New York.
  21. ↑ Talley, L.D., Pickard, G.L., Emery, W.J. and Swift, J.H. 2011. Descriptive Physical Oceanography: An Introduction. 6th edition. Academic Press, Boston.
  22. ↑ Hátún, H., Payne, M.R., Beaugrand, G., Reid, P.C., Sandø, A.B., Drange, H., Hansen, B., Jacobsen, J.A. and Bloch, D. 2009. Large biogeographical shifts in the north-eastern Atlantic Ocean: from the subpolar gyre, via plankton, to blue whiting and pilot whales. Progress in Oceanography 80: 149–162.
  23. ↑ Southward, A.J. 1991. Forty years of changes in species composition and population density of barnacles on a rocky shore near Plymouth. Journal of the Marine Biological Association of the United Kingdom 71: 495–513.
  24. ↑ Poloczanska, E.S., Brown, C.J., Sydeman, W.J., Kiessling, W., Schoeman, D.S., Moore, P.J., Brander, K., Bruno, J.F., Buckley, L.B., Burrows, M.T. and others. 2013. Global imprint of climate change on marine life. Nature Climate Change 3: 919–925.
  25. ↑ Khojasteh, D., Glamore, W., Heimhuber, V. and Felder, S. 2021. Sea level rise impacts on estuarine dynamics: A review. Science of the Total Environment 780: 146470.
  26. ↑ Edwards, K.F., Thomas, M.K., Klausmeier, C.A. and Litchman, E. 2016. Phytoplankton growth and the interaction of light and temperature: a synthesis at the species and community level. Limnology and Oceanography 61: 1232–1244.
  27. ↑ Doney, S.C., Fabry, V.J., Feely, R.A. and Kleypas, J.A. 2009. Ocean acidification: the other CO2 problem. Annual Review of Marine Science 1: 169–192
  28. ↑ Whitby, H., Park, J., Shaked, Y., Boiteau, R.M., Buck, K.N. and Bundy, R.M. 2024. New insights into the organic complexation of bioactive trace metals in the global ocean from the GEOTRACES era. Oceanography 37: 142-155
  29. ↑ Cheriyan, E., Kumar, B.S.K., Gupta, G.V.M. and Bhaskara Rao, D. 2024. Implications of ocean acidification on micronutrient elements-iron, copper and zinc, and their primary biological impacts: A review. Marine Pollution Bulletin 199, 115991
  30. ↑ 30.0 30.1 30.2 Lüning, K. 1990. Seaweeds: Their Environment, Biogeography, and Ecophysiology. John Wiley & Sons, New York.
  31. ↑ Häder, D.-P., Kumar, H.D., Smith, R.C. and Worrest, R.C. 2007. Effects of solar UV radiation on aquatic ecosystems and interactions with climate change. Photochemical & Photobiological Sciences 6: 267–285.
  32. ↑ 32.0 32.1 32.2 Häder, D-P., Williamson, C.E., Wängberg, S-A., Rautio, M., Rose, K.C., Gao, K., Helbling, E.W., Sinhah, R.P. and Worrest, R. 2015. Effects of UV radiation on aquatic ecosystems and interactions with other environmental factors. Photochem. Photobiol. Sci. 14, 108


The main author of this article is Ducrotoy, Jean-Paul
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