Natural variability and change in coastal ecosystems

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Conceptual illustration of natural background variability, anthropogenic pressures and observed ecosystem change. AI-generated with ChatGPT/DALL·E based on this article.

Coastal ecosystems, especially those of transitional waters, are naturally dynamic. Their physical, chemical and biological properties vary over tidal, seasonal, interannual, decadal and longer time scales, and over spatial scales from centimeters to ocean basins. This natural background variability is not a disturbance in itself, but an intrinsic property of coastal systems.

Understanding this variability is essential for detecting anthropogenic change. Human pressures such as nutrient enrichment, pollution, fishing, habitat modification, coastal engineering, species introductions and climate change are superimposed on naturally variable systems. Without knowledge of the natural range of variability, short-term changes may be misinterpreted as degradation or recovery, while gradual anthropogenic change may remain undetected.

This article focuses on natural variability as a basis for monitoring and management. More detailed treatments of specific pressures and processes are provided in related Coastal Wiki articles, including Sea level rise, Ocean acidification, Coastal pollution and impacts, What causes eutrophication?, Effects of global climate change on European marine biodiversity, Disturbances, biodiversity changes and ecosystem stability, Biodiversity, ecosystem functioning and ecosystem function and Resilience and resistance.

This article is partly based on the review by Crossland et al. (2005[1]), with additional emphasis on monitoring and management implications.


Temporal and spatial scales of variability

Coastal and marine ecosystems are not in a steady state. Production, species composition, community structure, food-web interactions and biogeochemical cycling vary continuously. Long-term data sets on phytoplankton, zooplankton, benthic macrofauna, fish, seabirds and coastal habitats show that natural variability can be large, and that changes are not always gradual. Abrupt shifts can occur when thresholds are crossed, when key species or functional groups change, or when several drivers interact[2].

Natural variability occurs at many spatial scales. At basin scale, climate modes such as the El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) influence coastal ecosystems through changes in temperature, winds, currents, stratification, nutrient supply and food-web structure[3][4]. ENSO is a coupled tropical Pacific ocean–atmosphere mode that periodically alters winds, upwelling, sea temperature and nutrient supply, with major consequences for fisheries and marine food webs along the west coast of South America and elsewhere. The NAO is mainly an atmospheric pressure pattern over the North Atlantic that affects westerly winds, storm tracks, surface currents, winter temperatures and the inflow of oceanic water towards the European shelf. Such large-scale variability can influence local coastal ecosystems and must therefore be considered when interpreting regional monitoring data.

Regional-scale variability is also important. Changes in the strength or position of ocean currents, shelf-break fronts, upwelling systems and river plumes can influence nutrient supply, plankton communities, fish recruitment and the distribution of marine organisms. In the north-east Atlantic, for example, changes in circulation and hydrography have been linked to large biogeographical shifts in plankton and higher trophic levels[5]. Data from the Continuous Plankton Recorder (CPR) survey have shown changes in North Atlantic and North Sea plankton communities, including shifts in the distribution and abundance of calanoid copepods[6]. More recent analyses confirm that plankton changes may appear as gradual trends, cycles or abrupt regime shifts, often related to hydrographic variability and large-scale climate forcing[7]. Because plankton respond rapidly to temperature, nutrients, stratification and hydrodynamics, plankton indicators are useful for detecting change, but only if natural seasonal and interannual variability is well characterized.

At smaller scales, coastal ecosystems are shaped by local hydrodynamics, sediment properties, light climate, salinity gradients, nutrient patches, habitat structure and biological interactions. Phytoplankton biomass, nutrients, temperature and salinity may be patchy even in tidally energetic coastal waters[8]. Frontal zones between mixed and stratified waters can concentrate phytoplankton, zooplankton, fish, seabirds and marine mammals[9]. Such spatial heterogeneity shows why monitoring sites must be chosen carefully and why local observations cannot always be extrapolated to a whole coastal system. More detailed examples of spatial variability are given in articles such as Estuarine ecosystems, Spatial and temporal variability of salt marshes, Coral reefs, Mangroves and Seagrass meadows.

Temporal variability includes tides, storm events, seasonal cycles, interannual variability, decadal fluctuations and long-term trends. Seasonal changes in light, temperature, river discharge, stratification and nutrient supply strongly influence primary production, plankton blooms, benthic activity, recruitment and migration. Interannual and decadal variability may be related to climate modes, changes in ocean circulation, freshwater inflow, storminess or biological interactions. Around south-west England, for example, long-term observations showed changes in the relative abundance of northern and southern barnacle species, with responses to both decadal temperature variation and abnormal warm or cold periods[10]. This example illustrates that range shifts and community changes may reflect a combination of short-term anomalies, decadal variability and long-term warming.

In polar and subpolar systems, sea-ice variability can strongly influence food webs. Long-term observations of Antarctic seabirds indicate that winter sea-ice conditions affect breeding, survival and population trends of species such as Adélie penguins, emperor penguins and snow petrels, although responses differ among species, regions and life-history stages[11]. This shows that the same environmental driver can produce different ecological responses depending on species traits and ecosystem context.

Because natural variability occurs across many scales, the interpretation of ecological change depends strongly on the length, frequency and spatial coverage of monitoring. Short time series can be misleading, especially when observations coincide with unusual years, storms, heatwaves, floods, cold winters, hypoxia events or recruitment failures.

Natural variability and anthropogenic pressures

Natural variability can either mask, amplify or mimic the effects of human pressures. For example, eutrophication effects depend not only on nutrient inputs, but also on river discharge, residence time, stratification, light availability, grazing and sediment nutrient recycling. Pollution impacts depend on hydrodynamic dilution, sediment trapping, bioavailability and food-web transfer. The ecological effects of fishing, habitat loss or coastal engineering depend on recruitment variability, disturbance history, habitat connectivity and the presence of functionally important species.

Several different drivers can produce similar ecological responses. Declines in oxygen, changes in benthic fauna or shifts in plankton composition may result from nutrient enrichment, warming, stratification, organic loading, altered circulation or combinations of these factors. Conversely, a strong pressure may produce weak or delayed ecological change if natural variability, dispersal or functional redundancy temporarily buffers the system. This complicates attribution of observed change to a particular cause.

Transitional waters provide clear examples of this problem. In the Venice Lagoon, long-term changes in zooplankton communities have been interpreted in relation to rising temperature, sea level rise, increasing marine influence, morphological change and the arrival of alien species such as the ctenophore Mnemiopsis leidyi[12]. In Apalachicola Bay, 20 years of monitoring showed that spatial variation in nekton communities was strongly related to salinity, while seasonal variation was related to temperature; this helped assess the ecological consequences of altered freshwater inflow[13]. These examples show that long-term monitoring can help separate natural salinity and temperature variability from anthropogenic pressures such as altered freshwater inflow, morphological change, species introductions and climate-related change.

Coastal ecosystems are also affected by multiple stressors. Combined effects may be additive, synergistic or antagonistic, so that the combined response cannot always be predicted from the effects of single pressures. For details, see, for example, Disturbances, biodiversity changes and ecosystem stability, Coastal pollution and impacts and What causes eutrophication?.

Climate-related change as one component of anthropogenic change

Natural climate variability has always influenced coastal ecosystems, but human-induced climate change is now altering the mean state, variability and extremes of the coastal environment. Important climate-related drivers include ocean warming, marine heatwaves, sea level rise, acidification, changes in precipitation and river runoff, changes in stratification, oxygen loss, storminess and wave climate. These drivers interact with local pressures such as pollution, eutrophication, habitat loss, fishing and coastal engineering. For a broader treatment, see Ocean acidification, Effects of global climate change on European marine biodiversity and Resilience and resistance.

Ocean and coastal waters are often vertically structured into a surface mixed layer, an underlying stratified layer and deeper waters. Stratification is controlled by density differences caused by temperature and salinity and is therefore described by thermoclines, haloclines or, more generally, pycnoclines. In coastal seas and estuaries, this structure is modified by tides, wind mixing, river discharge, freshwater buoyancy and exchange with the open ocean. Changes in stratification can affect nutrient supply, oxygen conditions, plankton blooms and benthic–pelagic coupling, see Possible consequences of eutrophication.

Warming can shift species distributions, phenology, recruitment success and food-web interactions. In temperate regions, cold winters can strongly influence the species composition of intertidal benthic communities, while milder winters can favor warm-water species. Abrupt temperature anomalies, including marine heatwaves and cold spells, may cause mortality, recruitment failure, harmful algal blooms or rapid range shifts. Marine heatwaves provide a recent example of episodic extremes superimposed on longer-term climate change. They can affect kelp forests, seagrasses, coral reefs, plankton phenology and fisheries, and may interact with oxygen depletion, acidification, pollution or local habitat degradation[14]. Because marine heatwaves may be short-lived but ecologically important, they highlight the need for monitoring programs that combine long-term time series with event-based observations.

Sea level rise affects coastal ecosystems mainly through changes in inundation, salinity, erosion, sedimentation and habitat space. Intertidal flats, salt marshes, mangroves and lagoons may migrate landward where accommodation space and sediment supply are sufficient. Where migration is blocked by embankments, cliffs or urban infrastructure, coastal squeeze can lead to narrowing or loss of intertidal habitats. Local impacts depend on relative sea level rise, sediment availability, wave climate, tidal range, storm surges and human interventions.

Ocean uptake of anthropogenic CO2 lowers seawater pH and carbonate ion availability, making calcification more difficult for many organisms such as corals, molluscs, echinoderms, foraminifera and calcareous algae. Sensitivity differs among species and life stages, and acidification interacts with warming, deoxygenation and pollution. See Ocean acidification for details.

Changes in cloud cover, turbidity, dissolved organic matter and suspended sediment influence the underwater light climate. This affects phytoplankton, benthic algae, seagrasses and shallow-water organisms. In shallow or clear waters, enhanced UV-B exposure can damage photosynthetic systems and DNA, although effects depend strongly on water depth, turbidity, exposure time and species-specific protection and repair mechanisms.

Detecting anthropogenic change against a variable background

Detection of anthropogenic change requires separating a signal from natural variability. This is difficult when ecological responses are nonlinear, delayed, spatially heterogeneous or affected by several pressures at the same time. It is especially difficult when monitoring records are short compared with natural cycles or when reference conditions are poorly known.

Several principles are important:

  • Monitoring should cover the natural range of seasonal and interannual variability. Single surveys or short records are rarely sufficient to establish trends.
  • Physical, chemical and biological variables should be monitored together, because biological change often reflects interacting drivers such as temperature, salinity, nutrients, oxygen, turbidity, hydrodynamics and habitat structure.
  • Spatial replication is needed to distinguish local disturbance from regional or basin-scale variability.
  • Indicators should include not only species richness, but also abundance, dominance, community composition, functional traits and key ecosystem functions.
  • Event-based monitoring is important after storms, floods, heatwaves, hypoxia events, harmful algal blooms or pollution incidents, because episodic disturbances can cause long-lasting ecological change.
  • Assessment should consider multiple pressures rather than attributing change to a single cause too quickly.

Examples from long-term observations illustrate these principles. ENSO can strongly affect upwelling, productivity and fisheries in the tropical Pacific[3], while the NAO influences winds, storm tracks, currents and ecosystem conditions in the North Atlantic. CPR data show that plankton communities in the North Atlantic and North Sea can undergo gradual shifts, cyclic fluctuations and abrupt regime changes[7]. At smaller scales, nutrient and phytoplankton patches in tidally mixed waters and enhanced production at frontal zones show that local variability can be strong even within apparently well-mixed coastal seas. Long-term observations of barnacles near Plymouth[10]> and seabirds in Antarctica[11] further show that biological communities may respond rapidly to temperature anomalies, winter severity or sea-ice conditions. Recent examples from Venice Lagoon[12] and Apalachicola Bay[13] demonstrate that long-term monitoring can help separate natural salinity and temperature variability from anthropogenic pressures such as altered freshwater inflow, morphological change, species introductions and climate-related change.

Long-term data sets, such as plankton time series, benthic monitoring, habitat mapping, fish surveys and remote sensing, are essential. They should be combined with experiments, process studies and models that help identify causal mechanisms. Monitoring strategies should therefore be designed not only to describe change, but also to test whether observed changes are consistent with expected responses to specific pressures.

Implications for monitoring and management

Management of coastal ecosystems requires knowledge of both natural variability and anthropogenic pressures. Natural variability defines the background against which change must be assessed; anthropogenic pressures determine how far the system is pushed beyond this background. Effective management should therefore ask:

  • What is the natural range of variability for the ecosystem or indicator considered?
  • At what spatial and temporal scale does the relevant variability occur?
  • Which pressures are likely to affect the observed variable?
  • Are observed changes within the expected natural range, or do they indicate a trend, threshold response or regime shift?
  • Which indicators are most sensitive to the pressure of concern, and which are most robust to natural noise?
  • Can management actions reduce local pressures and increase resilience to large-scale change?

These questions are consistent with the ecosystem-based management and adaptive-monitoring approach of the Marine Strategy Framework Directive, in which monitoring, indicators, targets and measures are used to assess whether human pressures remain compatible with good environmental status and whether management responses need adjustment[15][16]. They also reflect the pressure–state–impact–response logic commonly used in marine environmental management[17]

Because coastal systems are open and connected, local observations should be interpreted within their catchment, coastal-cell, regional-sea and oceanographic context. River-basin management, marine spatial planning, fisheries management, pollution control, habitat restoration and coastal defense planning all need to account for natural background variability. This is consistent with the regional and ecosystem-based approach of the EU Marine Strategy Framework Directive.

Models can help analyze causal linkages and explore scenarios, but they require good observational data and should be used with an understanding of uncertainty. Adaptive management is therefore often appropriate: monitoring results are used to evaluate whether measures are effective and to adjust management when new information becomes available.

Conclusion

Natural variability is a defining feature of coastal ecosystems. It occurs at many spatial and temporal scales and affects physical conditions, biogeochemical cycling, species distributions, food webs, habitats and ecosystem services. Human pressures, including but not limited to climate change, are superimposed on this variable background.

Understanding natural variability is essential for detecting anthropogenic change, attributing causes, designing monitoring programs and evaluating management measures. The most useful monitoring approaches combine long-term observations, spatial replication, event-based measurements, pressure indicators, functional ecological indicators and process understanding. Without such knowledge, natural variability may be mistaken for human impact, or human-induced change may be overlooked as natural fluctuation.


Related articles

Estuarine ecosystems
Sea level rise
Ocean acidification
Ocean circulation
Spatial and temporal variability of salt marshes
Coral reefs
Mangroves
Seagrass meadows
Lagoon
Coastal squeeze
Effects of global climate change on European marine biodiversity
Coastal pollution and impacts
What causes eutrophication?
Possible consequences of eutrophication
Disturbances, biodiversity changes and ecosystem stability
Biodiversity, ecosystem functioning and ecosystem function
Resilience and resistance
Marine habitats and ecosystems
Marine Strategy Directive


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. ↑ 3.0 3.1 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.
  4. ↑ 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.
  5. ↑ 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.
  6. ↑ 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.
  7. ↑ 7.0 7.1 Bode, A. 2024. Synchronized multidecadal trends and regime shifts in North Atlantic plankton. ICES Journal of Marine Science 81: 575–589.
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ 10.0 10.1 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.
  11. ↑ 11.0 11.1 Croxall, J.P., Trathan, P.N. and Murphy, E.J. 2002. Environmental change and Antarctic seabird populations. Science 297: 1510–1514.
  12. ↑ 12.0 12.1 Camatti, E., Pansera, M., Bergamasco, A., Bianchi, F., Acri, F., Bernardi Aubry, F., Bastianini, M., Pugnetti, A., Socal, G., Berto, D., Facca, C., Pastres, R., Sfriso, A. and Franzoi, P. 2023. Natural or anthropogenic variability? A long-term pattern of the zooplankton communities in an ever-changing transitional ecosystem. Frontiers in Marine Science 10: 1176829.
  13. ↑ 13.0 13.1 Garwood, J.A., Allen, D.M., Pine, W.E., O’Connor, J.P., Johnson, M.W. and Walters, C.J. 2023. Using long-term ecological monitoring to evaluate how climate and human-induced disturbances impact nekton communities in a Northern Gulf of Mexico estuary. Hydrobiologia 850: 2385–2404.
  14. ↑ Capotondi, A., et al. 2024. A global overview of marine heatwaves in a changing climate. Communications Earth & Environment 5: 650.
  15. ↑ European Parliament and Council. 2008. Directive 2008/56/EC establishing a framework for community action in the field of marine environmental policy (Marine Strategy Framework Directive).
  16. ↑ Cochrane, S.K.J. et al. 2010. Marine Strategy Framework Directive Task Group 1 Report: Biological diversity. JRC Scientific and Technical Reports.
  17. ↑ Patrício, J., Elliott, M., Mazik, K., Papadopoulou, K.-N. and Smith, C.J. 2016. DPSIR—Two Decades of Trying to Develop a Unifying Framework for Marine Environmental Management? Frontiers in Marine Science 3: 177.


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