Difference between revisions of "Testpage3"

From Coastal Wiki
Jump to: navigation, search
(2 intermediate revisions by the same user not shown)
Line 1: Line 1:
−
Estuarine turbidity maximum
+
{{Review
 +
|name=Job Dronkers|AuthorID=120|
 +
}}
  
  
−
{{Definition|title=Turbidity maximum
+
'''Resilience and resistance'''
−
|definition= Convergence zone of suspended sediment transport, where turbidity levels are high due to high suspended sediment concentrations. }}
 
  
  
 +
{{Definition|title=Resistance
 +
|definition= The capacity to weather a disturbance without loss (Lake 2013<ref name=L>Lake, P.S. 2013. Resistance, Resilience and Restoration. Ecological Management and Restoration 14: 20-24</ref>). }}
  
−
==Suspended sediment concentration in the estuarine turbidity maximum==
 
  
−
The presence of a turbidity maximum is a common feature of many [[#Estuary|estuaries]]. The term turbidity maximum suggests that it corresponds to a particular location in an estuary. In fact, it is a broad zone where concentrations of fine suspended sediment are much higher than in the upstream river or in the adjacent sea, where concentrations of fine suspended sediment are generally below 100 mg/l and often even below 10 mg/l. The suspended sediment concentration (SSC) in the turbidity maximum can be very high, up to several g/l averaged over the water column. A typical example of a turbidity maximum is shown in Fig. 1. The suspended sediment concentration in the turbidity maximum depends on the supply of fine sediments from fluvial and marine sources; erosion of ancient mud deposits can also play a role. The most important factor, however, is the trapping efficiency of the estuary, i.e. the efficiency of hydrodynamic processes to retain fine sediments that have entered the estuary and to prevent their escape to the sea. These processes will be discussed in the next section.
+
{{Definition|title=Resilience
 +
|definition=(1) the capability to anticipate, prepare for, respond to, and recover from significant multihazard threats with minimum damage to social well-being, the economy, and the environment (sometimes called 'socio-ecological resilience')(Olsen et al. 2019<ref name=O>Olsson, S., Melvin, A. and Giles, S. (eds.) 2019. Climate change and ecosystems. Procs. Sackler Forum on Climate Change and Ecosystems, Washington, DC, November 8-9, 2018, organized by the National Academy of Sciences and The Royal Society</ref>);
  
 +
(2) the capability of a (socio-)ecological system to remain within a stability domain when subjected to environmental change, while continually changing and adapting yet remaining within critical thresholds (sometimes called 'general resilience') (Folke et al. 2010<ref name=F>Folke, C., Carpenter, S. R., Walker, B., Scheffer, M., Chapin, T. and Rockstrom, J. 2010. Resilience thinking: integrating resilience, adaptability and transformability. Ecology and Society 15(4): 20</ref>; Scheffer 2009<ref>Scheffer, M. 2009. Critical transitions in nature and society. Princeton University Press, Princeton, New Jersey, USA</ref>; Brand and Jax 2007<ref name=BJ>Brand, F.S. and K. Jax. 2007. Focusing the meaning(s) of resilience: resilience as a descriptive concept and a boundary object. Ecology and Society 12(1):23</ref>);
  
−
[[Image:TurbidityMaximumLoireEstuary.jpg|thumb|700px|center|Figure 1: Turbidity maximum in the Loire estuary at low fluvial discharge. Suspended sediment concentration (SSC) is expressed on g/l. Top panel: Spring tide. Bottom panel: Neap tide. Source: Christine Bertier (2011)<ref name=B> Bertier, C. 2011. Dynamique et suivi du bouchon vaseux dans l’estuaire de la Loire. Séminaire Technique sur le transport sédimentaire: Principes et expériences sur le bassin Ligrien, Vierzon 24 Novembre 2011</ref>.]]
+
(3) the capacity to experience shocks while retaining essentially the same function, structure, feedbacks, and therefore identity (sometimes called 'ecological resilience') (Brand and Jax 2007<ref name=BJ/>; DEFRA 2019<ref name=DEFRA>Haines‐Young, R. and Potschin. M. (eds.) 2010. The Resilience of Ecosystems to Environmental Change (RECCE). Overview Report, 27 pp. Defra Project Code: NR0134</ref>), which is closely related to the concept of 'ecosystem resistance': the amount of disturbance that a system can withstand before it shifts into a new regime or an alternative stable state (Holling 1973<ref>Holling, C.S. 1973. Resilience and stability of ecological systems. Annual Rev. Ecol. Syst. 4: 1–23. doi: 10.1146/annurev.es.04.110173.000245</ref>; Gunderson 2000<ref>Gunderson, L.H. 2000. Ecological Resilience - in Theory and Application. Annual Review of Ecology and Systematics 31:425-439.</ref>);
  
 +
(4) the capacity of an ecosystem to regain its fundamental structure, processes, and functioning (or remain largely unchanged) despite stresses, disturbances, or invasive species (e.g., Hirota et al., 2011<ref>Hirota,M., Holmgren,M., Van Nes, E. H, and Scheffer,M. 2011. Global resilience of tropical forest and savanna to critical transitions. Science 334: 232–235. doi: 10.1126/science.1210657</ref>; Chambers et al., 2014<ref>Chambers, J. C., Bradley, B. A., Brown, C. S., D’Antonio, C., Germino, M. J., Grace, J. B., et al. 2014. Resilience to stress and disturbance, and resistance to Bromus tectorum L. invasion in the cold desert shrublands of western North America. Ecosystems 7: 360–375. doi: 10.1007/s10021-013-9725-5</ref>; Pope et al., 2014<ref>Pope, K. L., Allen, C. R., and Angeler, D. G. 2014. Fishing for resilience. T. N. Am. Fisheries Soc. 143: 467–478. doi: 10.1080/00028487.2014.880735</ref>; Seidl et al., 2016<ref>Seidl, R., Spies, T. A., Peterson, D. L., Stephens, S. L., and Hick, J. A. 2016. Searching for resilience: addressing the impacts of changing disturbance regimes on forest ecosystem services. J. Appl. Ecol. 53 : 120–129. doi: 10.1111/1365-2664.12511</ref>), which can be measured by the time needed to recover its original state (sometimes called 'engineering resilience'<ref name=L>Lake, P.S. 2013. Resistance, Resilience and Restoration. Ecological Management and Restoration 14: 20-24</ref>).
 +
}}
  
−
==Formation of the turbidity maximum==
 
−
 
−
The formation of a turbidity maximum is due to hydrodynamic processes that promote upstream (landward) transport of fine suspended sediment. These typically estuarine processes are<ref>Burchard, H. and Baumert, H. 1998. The Formation of Estuarine Turbidity Maxima Due to Density Effects in the Salt Wedge. A Hydrodynamic Process Study. J. Phys. Oceanography 28: 309-321</ref>:
 
  
−
(1) Tidal asymmetry, generated by the non-linearity of tidal propagation in estuarine channels. This nonlinear tidal propagation results in shorter flood and longer ebb periods, with enhanced maximum flood currents and reduced maximum ebb currents, see [[Tidal asymmetry and tidal basin morphodynamics]]. Due to the strong dependence of sediment resuspension on current strength, the suspended sediment load carried by the flood currents is much higher than the suspended load carried by the ebb currents. This sediment import mechanism is sometimes designated by the term 'tidal pumping'.
 
  
−
[[Image:EbbFloodProfiles.jpg|thumb|350px|left|Figure 2: Schematic representation of along-channel velocity (left) and vertical profiles of salinity (right, relative to dept-averaged value) for maximum flood and ebb flow, representative for observations in Rotterdam Waterway, see [[Estuarine circulation]] Fig. 5. The left figure shows ebb-dominant flow near the water surface and flood-dominant flow in the lower part of the water column. The right figure shows stronger stratification during ebb than during flood.]]
+
==Introduction==
 +
Coastal and marine ecosystems are affected by environmental disturbance at a variety of spatio-temporal scales. The organisms inhabiting these systems are adapted to such disturbance, either by being tolerant of these conditions or by playing a role in one or more of the successional stages that follow during ecosystem recovery.
  
−
(2) Estuarine circulation, the residual flow pattern in an estuary induced by the density difference between seawater and river water, see Fig. 2. The residual upstream flow along the bed of the main channel carries a higher load of suspended sediment than the compensating seaward flow near the surface, see [[Estuarine circulation]].
+
If all species in the system were tolerant to a particular perturbation, very little would change at the ecosystem level, and we could call the system resistant to this disturbance. However, often a disturbance, such as a temporary very low oxygen level, affects a substantial proportion of the organisms dramatically, either causing them to die, or forcing them to rapidly migrate to more favorable parts of the environment. Such an adverse disturbance could locally defaunate a certain volume in the pelagic or a certain area of hard or soft substrate. Such destruction at a local scale does not mean the end of local functioning. Usually organisms are available at a larger spatial scale that can re-colonize the affected area, according to their particular tolerances and abilities to favorably affect their local environment.  
  
−
(3) Stratification. Ebb flow in estuaries is typically more stratified than flood flow, mainly as a consequence of convective overturning during flood when saline seawater is advected over less dense estuarine water<ref>Jay, D. A. and Musiak, J. D. 1994. Particle trapping in estuarine tidal flows. J. Geophys. Res. 99: 445–461</ref><ref>Prandle, D. 2004. Salt intrusion in partially mixed estuaries. Est.Coast.Shelf Sci. 59: 385-397</ref><ref>Burchard, H. and Schuttelaars, H.H. (2012) Analysis of Tidal Straining as Driver for Estuarine Circulation in Well-Mixed Estuaries. J. Phys. Oceanograph. 42: 261-271</ref>. This implies that estuarine outflow is mainly concentrated in the upper layer of the water column, with lower suspended sediment concentrations than the vertical average. Flood flow therefore carries more sediment than ebb flow.
+
The term resilience has been defined in different ways, illustrated in the definition above. According to DEFRA (2019<ref name=DEFRA/>) there is limited consensus in the literature about how resilience can be characterized and assessed. The term resilience is sometimes used to represent some kind of normative proposition about what kinds of ecosystem characteristics are desirable or necessary in the context of sustainable development, reflecting particular cultural and philosophical assumptions<ref name=DEFRA/>. However, the resistance of an ecosystem (see the definition above) to changing conditions and the rate of recovery following some disruptive event are generally considered major components of resilience that can in principle be expressed in quantitative terms.  
  
 +
Other attributes such as the capacity of ecosystems to transform and adapt in the face of environmental change (i.e. system's ability to re-organize itself) are more difficult to translate to practice. According to Dawson et al. (2010<ref name=D>Dawson, T.P., Rounsevell, M.D.A., Kluvankova‐Oravska, T., Chobotova V. and Stirling, A. 2010. Dynamic properties of complex adaptive ecosystems: implications for the sustainability of services provision. Biodiversity and Conservation 19: 2843‐2853</ref>), resilience concerns the response of ecosystems to changing environmental conditions and must be looked at alongside other additional dynamic features, namely durability, robustness and stability. These concepts can be defined as<ref name=D/>:
 +
* Durability:  ability to cope with a chronic stress, but the source of this stress is endogenous;
 +
* Robustness: ability to recover or maintain the systems' social-ecological functions in the face of an external and chronic driver;
 +
* Stability:  system’s tolerance to transient and endogenous shocks or disruptions.
  
−
Further upstream in the estuary the residual flow is dominated by river discharge. A convergence zone of suspended sediment transport therefore exists where downstream suspended transport by river flow is of the same order as the upstream suspended sediment transport by estuarine circulation and tidal asymmetry. In this convergence zone, fine suspended sediment accumulates and forms a turbidity maximum. The convergence zone is generally situated around the location of maximum seawater intrusion. In estuaries with strong tides, tidal asymmetry is the major mechanism for upstream sediment transport<ref>Brenon, I. and LeHir, P. 1999. Modelling the Turbidity Maximum in the Seine Estuary (France): Identification of Formation Processes. Estuarine, Coastal and Shelf Science (1999) 49, 525–544</ref><ref>Scully, M.E. and Friedrichs, C.T. 2007. Sediment pumping by tidal asymmetry in a partially mixed estuary. Journal of Geophysical Research 112, c07028
+
Both resistance and resilience cause an ecosystem to remain relatively unchanged when confronted to a disturbance, but in the case of resistance no internal re-organization and successional change is involved. In contrast, resilience implies that the system is internally re-organizing, perhaps through a mozaic of patches that are at different stages of re-assembly. System responses to changing environmental conditions are displayed schematically in Fig. 1, corresponding to different resilience characteristics.
−
York river (USA)</ref><ref>Toublanc, F., Brenon, I. and Coulombier, T. 2016. Formation and structure of the turbidity maximum in the macrotidal Charente estuary (France): Influence of fluvial and tidal forcing. Estuarine, Coastal and Shelf Science 169: 1-14</ref><ref name=MS>Van Maanen, B. and Sottolichio, A. 2018. Hydro- and sediment dynamics in the Gironde estuary (France): Sensitivity to seasonal variations in river inflow and sea level rise. Continental Shelf Research 165: 37–50</ref>. In these cases, the turbidity maximum is shifted in upstream direction relative to the location of maximum seawater intrusion. In microtidal estuaries, the turbidity maximum is mainly due to estuarine circulation and stratifcation processes; the location of the turbidity maximum is determined by the seawater intrusion length<ref>Restrepo, J.C., Schrottke, K., Traini, C., Bartholomae, A., Ospino, S., Ortíz, J.C., Otero, L. and Orejarena, A. 2018. Estuarine and sediment dynamics in a microtidal tropical estuary of high fluvial discharge: Magdalena River (Colombia, South America). Marine Geology 398: 86–98</ref>.
 
  
−
[[Image:FluidMudLoireEstuary.jpg|thumb|400px|left|Figure 3: Fluid mud in the Loire estuary during neap tide and low fluvial discharge. Source: Christine Bertier (2011)<ref name=B></ref>. ]]
+
[[Image:ResilienceTrajectories.jpg|thumb|900px|center|Figure 1. Schematic representation of the trajectories of a (socio-)ecological system in a plane defined by the system state (fundamental structure, processes, and functioning - vertical axis) and the change of environmental conditions (horizontal axis), for different resilience characteristics (a, b, c, d). The initial state corresponds to the position on the graph at the vertical axis (zero change in environmental conditions). In all situations the ecosystem is assumed to collapse irreversibly (down to the horizontal axis) when the change in environmental conditions is much greater than the systems' resistance. The angle <math>\alpha</math> represents the rate at which the system recovers when the change in environmental conditions is reduced (small <math>\alpha</math> means slow recovery, large <math>\alpha</math> means fast recovery). Panel a: Resilience characterized by high resistance (definition 3) and slow recovery (definition 4). Panel b: Resilience characterized by low resistance and fast recovery. Panel c: Resilience characterized by a shift to an alternative stable system state. Panel d: Low resilience, characterized by low resistance and slow recovery.]]
  
  
 +
When considering the potential effect of a certain type of disturbance it is thus useful to ask two questions:
 +
# Will the species of this system be able to tolerate it (implying resistance), and if not,
 +
# Is recovery possible through a successional trajectory, back to the same, or at least a desirable, ecosystem state (implying resilience)?
 +
Resistance breaks down when uni-directional ongoing change acts faster than the organisms' ability to adapt their tolerances. If uni-directional ongoing change is this fast (even if gradual), the system will not be sufficiently resilient either, as full recovery through succession will then not be possible. Recovery from sudden and local disturbance is often possible through recolonization, but the rate of recovery will depend crucially on the spatial extent of disturbance. For example, recovery from anoxia could take 5 to 8 months at the scale of square meters (Rossi et al. 2009<ref name=R>Rossi, F., Vos, M. & Middelburg, J.J. 2009. Species identity, diversity and microbial carbon flow in reassembling macrobenthic communities. Oikos 118: 503-512.</ref>), but could take 5 to 8 years at the scale of a whole bay (Diaz & Rosenberg 1995<ref>Diaz, R.J. & Rosenberg, R. 1995. Marine benthic hypoxia: a review of its ecological effects and the behavioural responses of benthic macrofauna. Oceanogr. Mar. Biol. Annu. Rev. 33:245-303.</ref>).
  
 +
According to definition (4), the speed at which an ecosystem returns to its former state following a (minor) disturbance can be considered a measure of resilience. The idea is that a system with a short return time is more resilient than one with a long return time. Such resilience measured as (1 / the return time to a stable equilibrium) has also been called ''engineering resilience''. It has however a long history of use among ecologists (Pimm 1982<ref>Pimm, S.L. 1982. Food Webs. The University of Chicago Press.</ref>, DeAngelis 1992<ref>DeAngelis, D.L. 1992. Dynamics of Nutrient Cycling and Food Webs. Chapman and Hall, London.</ref>, Vos et al. 2005<ref>Vos, M., Kooi, B.W., DeAngelis, D.L. & Mooij, W.M. 2005. Inducible defenses in food webs. In: Dynamic Food Webs. Multispecies Assemblages, Ecosystem Development and Environmental Change. Eds. P.C. de Ruiter, V. Wolters & J.C. Moore. Academic Press. Pp. 114-127.</ref>). Resilience is also used in a way that more closely resembles the definition of resistance. ''Ecological resilience'' was defined as the amount of disturbance that an ecosystem could withstand without changing self-organized processes and structures (definition 3).
  
−
Suspended sediment settles to the channel bed during periods of slack water, especially in the neap tidal phase of the fortnightly tidal cycle. [[Dynamics of mud transport#Flocculation|Flocculation]] plays an important role as an accelerator of the sedimentation process<ref>Guo, C., He, Q., Guo, L. and Winterwerp, J.C. 2017. A study of in-situ sediment flocculation in the turbidity maxima of the Yangtze Estuary. Estuarine, Coastal and Shelf Science 191: 1-9</ref>. In estuaries with a strong turbidity maximum, a fluid mud layer forms on the channel bed when the fine suspended sediment concentration exceeds a few tens of g/l <ref name=W>Winterwerp, J.C. 2016 Fine sediment transport by tidal asymmetry in the high-concentrated Ems River: indications for a regime shift in response to channel deepening. Ocean Dynamics 61: 203–215</ref> (see Fig. 3 and [[Dynamics of mud transport]]). Under spring tidal conditions, the fluid mud layer is (partly) resuspended and contributes to high turbidity levels, as illustrated in Fig. 1. Fine sediments that have been mixed into the near surface flow can escape from the turbidity maximum if the river discharge is sufficiently high.  
+
Resilience of coastal systems largely depends on biodiversity, which is a major requirement for allowing ecosystems to adapt to changing conditions. The human impact on the environment through pollution, fisheries, sediment erosion / deposition and global climate change has brought about much faster change than would occur under natural conditions, putting severe stress on many ecosystems. Without genetic diversity, natural selection cannot occur and if natural selection is limited, adaptation is impossible. Preservation of biodiversity and, more specifically, genetic diversity is therefore of paramount importance for successful adaptation to our rapidly changing environments. However, biodiversity may not always protect ecosystems from major abiotic disturbances (Folke et al. 2004<ref>Folke, C., Carpenter, S., Walker, B., Scheffer, M., Elmqvist, T., Gunderson, L. & Holling, C.S. 2004. Regime Shifts, Resilience, and Biodiversity in Ecosystem Management. Annual Review of Ecolog and Systematics 35:557-581.</ref>).
  
 +
==Resilience through recolonization==
  
 +
To understand resilience of ecosystems it is essential to understand what drives succession within these ecosystems. Succession determines how, and how fast, communities return to their original state, or perhaps enter a new state. Many aspects of succession can be understood in terms of trade-offs between the ability to be either a good early (re)colonizer, or a good competitor. Succession involves a gradual replacement of colonizer/competitor species according to the degree to which they tolerate, facilitate or inhibit certain environmental conditions and other species (Rossi et al. 2009<ref name=R/>). The extent to which processes of (re)colonization and succession can take place largely determines the recovery of ecosystems after major disruption and is therefore an essential characteristic of the resilience of ecosystems.
  
 +
In this context, it is important to consider the spatial component of ecosystem resilience. Diversity of structurally and functionally connected landscapes, rich in resources and species, promotes the flow or movement of individuals, genes, and ecological processes. Below certain thresholds of connectivity the capacity to regain structure and function after perturbation is lost (Holl and Aide, 2011; Rudnick et al., 2012;McIntyre et al., 2014; Rappaport et al., 2015; Ricca et al., 2018). Chambers et al. (2019<ref name=CAC>Chambers, J.C., Allen, C.R. and Cushman, S.A. 2019. Operationalizing Ecological Resilience Concepts for Managing Species and Ecosystems at Risk. Front. Ecol. Evol. 7:241. doi: 10.3389/fevo.2019.00241</ref>), based on Allen et al. (2016<ref> Allen, C. R., Angeler, D. G., Cumming, G. S., Folk, C., Twidwell, D., and Uden, D. R. 2016. Quantifying spatial resilience. J. Appl. Ecol. 53, 625–635. doi: 10.1111/1365-2664.12634</ref>), have therefore introduced the concept of  'spatial resilience', which is a measure of how spatial attributes, processes, and feedbacks vary over space and time in response to disturbances and affect the resilience of ecosystems. Self-organization through strong feedbacks at multiple scales and high levels of functional diversity and redundancy, stabilizes the system with respect to disturbances within the range of historic variability.
  
 +
When creating Marine Protected Areas, the sources of populations at all stages of succession should be protected, to preserve 'ecological memory' to the fullest possible extent. This includes protecting not only 'high quality' habitats that harbour healthy mature communities, but also 'low quality' and disturbed habitats that are required for those species that contribute to early recovery of perturbed areas (Rossi et al. 2009<ref name=R/>). The selection of Marine Protected Areas thus involves evaluating
 +
the number, size, and spatial configuration of habitat fragments and degree of connectivity required to support restoration of ecosystems and conservation of focal habitats and species<ref name=CAC/><ref name=O/>.
  
 +
==Resistance to changes in abiotic and biotic factors==
  
−
==Tidal flat accretion==
+
Community composition and ecosystem function may change very little under environmental change when the organisms can adapt to such change or tolerate it for some time (when the change is only temporary). However, all organisms have bounds to what they can temporarily or permanently tolerate, and when change exceeds some of these limits, the community composition and ecosystem functioning is likely to change.
  
−
[[Image:LandVanSaeftinghe.jpg|thumb|300px|right|Figure 4: View of the Land van Saeftinghe, a large salt marsh adjacent to the turbidity maximum of the Westerschelde estuary. The marsh level is close to the highest astronomical tide; flooding occurs only occasionally. Photo credit Anita Eijlers.]]
+
It is unlikely that communities can be resistant to ongoing gradual change, such as global warming. Acclimation and phenotypic plasticity do not suffice to maintain the system as it is. Genetic adaptation could allow community members to track such abiotic environmental change, but it is more likely that the area where the community is functioning will be invaded by species that function well at higher temperatures. The original species will thus have to deal with new competitors and predators, in addition to the changed abiotic factor. To some extent the original community can track the preferred temperature range, by moving spatially to greater depths or to alternative geographic areas. But these new areas are likely to differ in other ecological aspects such as water pressure, light climate and perhaps speeds of water flow etc.
  
 +
==Adaptation and the consequences of mortality at different trophic levels==
  
 +
External disturbance interacts with internal mechanisms that shape community structure. To understand how an increased mortality of top-predators will affect the entire food chain, it is essential to understand how processes of mutual adaptation within food chains already give shape to existing patterns such as trophic structure (how biomass in ecosystems is partitioned between trophic levels).
  
 +
Abundances at different trophic levels (such as algae, herbivores, carnivores and top-predators) and their responses to increased mortality (as under environmental change) depend critically on different mechanisms of adaptation within food chains and on the importance of population density at each of these trophic levels. However, different types of adaptation to living in a food chain context (balancing the need to acquire resources with the need to avoid predation) can often have similar consequences. For example, micro-evolution of behaviour, species replacement and induced defenses at a middle trophic level may all have similar effects on trophic level abundances in disturbed food chains (Abrams and Vos 2003<ref>Abrams, P.A & Vos, M. 2003. Adaptation, density dependence and the responses of trophic level abundances to mortality. Evolutionary Ecology Research 5: 1113-1132</ref>).
  
  
−
Part of the fine sediments of the turbidity maximum settles on the tidal flats and marshes bordering the main estuarine channel. This reduces the concentration of suspended sediment in the turbidity maximum. The turbidity maximum is advected along the estuary with the tide. It therefore covers a wide estuarine zone. [[Tidal flat]]s situated in this zone are subject to fast accretion. The landward part of these tidal flats that are not subject to strong wave action, will rapidly grow to a level where flooding only occurs during the highest spring tides <ref>Allen, J.R.L. 2000. Morphodynamics of Holocene salt marshes: a review sketch from the Atlantic and Southern North Sea coasts of Europe. Quaternary Science Reviews 19: 1155-1231</ref><ref>Temmerman, S., Govers, G., Meire, P. and Wartel, S. 2003. Modelling long-term tidal marsh growth under changing tidal conditions and suspended sediment concentrations, Scheldt estuary, Belgium. Marine Geology 193: 151-169</ref><ref>French, J. 2006. Tidal marsh sedimentation and resilience to environmental change: Exploratory modelling of tidal, sea-level and sediment supply forcing in predominantly allochthonous systems. Marine Geology 235: 119–136</ref><ref>Deloffre, J., Verney, R., Lafite, R., Lesueur, P., Lesourd, S. and Cundy, A.B. 2007. Sedimentation on intertidal mudflats in the lower part of macrotidal estuaries: Sedimentation rhythms and their preservation. Marine Geology 241:19–32</ref><ref>Boyd, B.M., Sommerfield, C.K., Elsey-Quirk, T. 2017. Hydrogeomorphic influences on salt marsh sediment accumulation and accretion in two estuaries of the U.S. Mid-Atlantic coast. Marine Geology 383: 132–145</ref>, as illustrated in Fig. 4. The growth process is stimulated by the development of marsh vegetation.  The concentration of suspended sediment in the turbidity maximum greatly increases when it is no longer possible to store fine sediment on the mud flats, because of their elevation or because they are reclaimed <ref>Van Maren, D.S., Oost, A.P., Wang, Z.B. and Vos, P.C. 2016. The effect of land reclamations and sediment extraction on the suspended sediment concentration in the Ems Estuary. Marine Geology 376: 147–157</ref>. Fast siltation also takes place in adjacent harbour basins that are situated in the high turbidity zone (see [[Siltation in harbors and fairways]]).
+
==Related articles==
−
 
+
:[[Integrated Coastal Zone Management (ICZM)]]
−
 
+
:[[Thresholds of environmental sustainablility]]
−
 
+
:[[Sustainability indicators]]
−
 
 
−
 
 
−
==Influence of river discharge==
 
−
 
 
−
[[Image:DischargeLoireGirondeTurbidityMax.jpg|thumb|400px|left|Figure 5: Position of the turbidity maximum in the Loire and Gironde estuaries for different fluvial discharges. For the Loire estuary, the mean fluvial discharge <math>\small Q_R=825 m^3/s</math> and for the Gironde, <math>\small Q_R=1000 m^3/s</math>. The turbidity maximum is further displaced seawards in the Loire estuary than in the Gironde estuary when the fluvial discharge equals twice the mean discharge. The Gironde estuary is therefore a more efficient sediment trap than the Loire estuary. The turbidity in the Gironde estuary is higher than in the Loire estuary. Data from Sottolichio and Castaing (1999) <ref>Sottolichio, A. and Castaing, P. 1999. A synthesis on seasonal dynamics of highly concentrated structures in the Gironde estuary. Comptes Rendus Acad. Sci. Earth Planet. Sci. 329: 795–800</ref> and Christine Bertier (2011)<ref name=B></ref>).]]
 
−
 
 
−
 
 
−
 
 
−
 
 
−
The turbidity maximum can be flushed out of the estuary when the river discharge is sufficiently high<ref>Jalon-Rojas, I., Schmidt, S., Sottolichio, A. and Bertier, C. 2016. Tracking the turbidity maximumzone in  the Loire Estuary (France)based on a long-term, high-resolution and high-frequency monitoring network. Continental Shelf Research 117: 1–11</ref><ref>Pritchard, M. and Green, M. 2017. Trapping and episodic flushing of suspended sediment from a tidal river. Continental Shelf Research 143: 286–294</ref>, see Fig. 5. This happens under spring tidal conditions, if the suspended sediment transport by river flow dominates transport by estuarine circulation and tidal asymmetry over the entire estuary <ref name=MS></ref>. In estuaries with such high peak discharges, the turbidity maximum does not become as high as in estuaries where the peak discharges are lower.
 
−
 
 
−
 
 
−
==Infill of estuaries==
 
−
 
 
−
If the peak discharge is not sufficient to flush the turbidity maximum to the open sea, most of the fine sediments will remain in the estuarine mouth zone. When the river discharge decreases, these sediments are re-imported into the estuary. The estuary then acts as trap for fine sediments. In this case a gradual infill of the estuary takes place. A mud bed forms when the fine sediment concentration exceeds 1000 g/l  (see [[Dynamics of mud transport]]). The infill stops when the estuarine depth and width are reduced up to a point where peak river discharges can flush the fine sediments far enough into the sea. In this way the estuary evolves towards a dynamic equilibrium.
 
  
  
 +
==References==
 +
<references/>
  
−
==Influence of dredging==
 
  
−
Many estuaries are dredged for navigation purposes. Channel dredging opposes the evolution towards an equilibrium state by strengthening estuarine circulation and reducing the fluvial flushing efficiency. The enhanced fine sediment trapping leads to very high concentrations in the turbidity maximum and requires a corresponding increase of maintenance dredging <ref name=W></ref>. A strong increase of the turbidity maxima has been observed in several estuaries (e.g., Ems, Loire) after deepening of the estuarine entrance channel<ref>Van Maren, D.S., Kessel, T., Cronin, K. and Sittoni, L. 2015. The impact of channel deepening and dredging on estuarine sediment concentration. Continental Shelf Research 95: 1–14</ref>. The observed strong increase in turbidity is also related to reclamation of tidal flats<ref>Winterwerp, J., Wang, Z.B., van Braeckel, A., Holland, G. and Koesters, F. 2013. Man-induced regime shifts in small estuaries - II: A comparison of rivers. Ocean Dynamics 63: 1293-1306</ref>.
+
<br>
−
 
  
 +
{{author
 +
|AuthorID=11928
 +
|AuthorFullName=Vos, Matthijs
 +
|AuthorName=Matthijs}}
  
−
==Environmental impact==     
+
[[Category:Coastal and marine ecosystems]]
−
 
+
[[Category:Integrated coastal zone management]]
−
High turbidity prevents sunlight from penetrating into the water column and therefore suppresses oxygen production by fytoplankton. At the same time, oxygen is consumed by mineralization of the degradable organic components of the fine sediments trapped in the turbidity maximum, as a result of bacterial activity or chemical oxidation <ref>Morris, A. W., Loring, D. H., Bate, A. J., Howland, R. J. M., Mantoura, R.F. C. and Woodward, E. M. S. 1982. Particle dynamics, particulate carbon and the oxygen minimum in an estuary. Oceanol. Acta 5: 349-353</ref><ref>Uncles, R.J., Joint, I. & Stephens, J.A. 1998. Transport and retention of suspended particulate matter and bacteria in the Humber-Ouse Estuary, United Kingdom, and their relationship to hypoxia and anoxia. Estuaries 21: 597–612</ref><ref name=E>Etcheber, H., Taillez, A., Abril, G., Garnier, J.A., Servais, P., Moatar, F. and Commarieu, M-V. 2007. Particulate organic carbon in the estuarine turbidity maxima of the Gironde, Loire and Seine estuaries: origin and lability. Hydrobiologia 588: 245–259</ref>. This is illustrated in Fig. 6 for the Loire estuary. High turbidity causes oxygen depletion and mortality of estuarine organisms. This is a serious problem in many estuaries, particularly during warm dry periods, when rapid mineralization occurs while mixing in the water column is suppressed due to density stratification (both salinity and fine sediment concentrations contribute to stratification at small river discharge <ref>Winterwerp, J.C. 2006. Stratification effects by fine suspended sediment at low, medium, and very high concentrations. Journal of Geophysical Research 111, c05012</ref>).
 
−
 
 
−
[[Image:LoireOxygen.jpg|thumb|600px|center|Figure 6: Dissolved oxygen concentration in the Loire estuary at Cordemais (20 km from the mouth) for the years 2007-2011. Each year severe oxygen depletion occurs during the summer months. Oxygen depletion in the Loire estuary is stronger than in the more turbid Gironde estuary due to the larger fraction of degradable organic matter<ref name=E></ref>. Source: Syvel Bulletin 2 (2011), GIP Loire Estuary. ]]
 
−
 
 
−
 
 
−
 
 
−
==References==
 
−
<references/>
 

Revision as of 15:04, 24 August 2020



Resilience and resistance


Definition of Resistance:
The capacity to weather a disturbance without loss (Lake 2013[1]).
This is the common definition for Resistance, other definitions can be discussed in the article


Definition of Resilience:
(1) the capability to anticipate, prepare for, respond to, and recover from significant multihazard threats with minimum damage to social well-being, the economy, and the environment (sometimes called 'socio-ecological resilience')(Olsen et al. 2019[2]);

(2) the capability of a (socio-)ecological system to remain within a stability domain when subjected to environmental change, while continually changing and adapting yet remaining within critical thresholds (sometimes called 'general resilience') (Folke et al. 2010[3]; Scheffer 2009[4]; Brand and Jax 2007[5]);

(3) the capacity to experience shocks while retaining essentially the same function, structure, feedbacks, and therefore identity (sometimes called 'ecological resilience') (Brand and Jax 2007[5]; DEFRA 2019[6]), which is closely related to the concept of 'ecosystem resistance': the amount of disturbance that a system can withstand before it shifts into a new regime or an alternative stable state (Holling 1973[7]; Gunderson 2000[8]);

(4) the capacity of an ecosystem to regain its fundamental structure, processes, and functioning (or remain largely unchanged) despite stresses, disturbances, or invasive species (e.g., Hirota et al., 2011[9]; Chambers et al., 2014[10]; Pope et al., 2014[11]; Seidl et al., 2016[12]), which can be measured by the time needed to recover its original state (sometimes called 'engineering resilience'[1]).
This is the common definition for Resilience, other definitions can be discussed in the article


Introduction

Coastal and marine ecosystems are affected by environmental disturbance at a variety of spatio-temporal scales. The organisms inhabiting these systems are adapted to such disturbance, either by being tolerant of these conditions or by playing a role in one or more of the successional stages that follow during ecosystem recovery.

If all species in the system were tolerant to a particular perturbation, very little would change at the ecosystem level, and we could call the system resistant to this disturbance. However, often a disturbance, such as a temporary very low oxygen level, affects a substantial proportion of the organisms dramatically, either causing them to die, or forcing them to rapidly migrate to more favorable parts of the environment. Such an adverse disturbance could locally defaunate a certain volume in the pelagic or a certain area of hard or soft substrate. Such destruction at a local scale does not mean the end of local functioning. Usually organisms are available at a larger spatial scale that can re-colonize the affected area, according to their particular tolerances and abilities to favorably affect their local environment.

The term resilience has been defined in different ways, illustrated in the definition above. According to DEFRA (2019[6]) there is limited consensus in the literature about how resilience can be characterized and assessed. The term resilience is sometimes used to represent some kind of normative proposition about what kinds of ecosystem characteristics are desirable or necessary in the context of sustainable development, reflecting particular cultural and philosophical assumptions[6]. However, the resistance of an ecosystem (see the definition above) to changing conditions and the rate of recovery following some disruptive event are generally considered major components of resilience that can in principle be expressed in quantitative terms.

Other attributes such as the capacity of ecosystems to transform and adapt in the face of environmental change (i.e. system's ability to re-organize itself) are more difficult to translate to practice. According to Dawson et al. (2010[13]), resilience concerns the response of ecosystems to changing environmental conditions and must be looked at alongside other additional dynamic features, namely durability, robustness and stability. These concepts can be defined as[13]:

  • Durability: ability to cope with a chronic stress, but the source of this stress is endogenous;
  • Robustness: ability to recover or maintain the systems' social-ecological functions in the face of an external and chronic driver;
  • Stability: system’s tolerance to transient and endogenous shocks or disruptions.

Both resistance and resilience cause an ecosystem to remain relatively unchanged when confronted to a disturbance, but in the case of resistance no internal re-organization and successional change is involved. In contrast, resilience implies that the system is internally re-organizing, perhaps through a mozaic of patches that are at different stages of re-assembly. System responses to changing environmental conditions are displayed schematically in Fig. 1, corresponding to different resilience characteristics.

Figure 1. Schematic representation of the trajectories of a (socio-)ecological system in a plane defined by the system state (fundamental structure, processes, and functioning - vertical axis) and the change of environmental conditions (horizontal axis), for different resilience characteristics (a, b, c, d). The initial state corresponds to the position on the graph at the vertical axis (zero change in environmental conditions). In all situations the ecosystem is assumed to collapse irreversibly (down to the horizontal axis) when the change in environmental conditions is much greater than the systems' resistance. The angle [math]\alpha[/math] represents the rate at which the system recovers when the change in environmental conditions is reduced (small [math]\alpha[/math] means slow recovery, large [math]\alpha[/math] means fast recovery). Panel a: Resilience characterized by high resistance (definition 3) and slow recovery (definition 4). Panel b: Resilience characterized by low resistance and fast recovery. Panel c: Resilience characterized by a shift to an alternative stable system state. Panel d: Low resilience, characterized by low resistance and slow recovery.


When considering the potential effect of a certain type of disturbance it is thus useful to ask two questions:

  1. Will the species of this system be able to tolerate it (implying resistance), and if not,
  2. Is recovery possible through a successional trajectory, back to the same, or at least a desirable, ecosystem state (implying resilience)?

Resistance breaks down when uni-directional ongoing change acts faster than the organisms' ability to adapt their tolerances. If uni-directional ongoing change is this fast (even if gradual), the system will not be sufficiently resilient either, as full recovery through succession will then not be possible. Recovery from sudden and local disturbance is often possible through recolonization, but the rate of recovery will depend crucially on the spatial extent of disturbance. For example, recovery from anoxia could take 5 to 8 months at the scale of square meters (Rossi et al. 2009[14]), but could take 5 to 8 years at the scale of a whole bay (Diaz & Rosenberg 1995[15]).

According to definition (4), the speed at which an ecosystem returns to its former state following a (minor) disturbance can be considered a measure of resilience. The idea is that a system with a short return time is more resilient than one with a long return time. Such resilience measured as (1 / the return time to a stable equilibrium) has also been called engineering resilience. It has however a long history of use among ecologists (Pimm 1982[16], DeAngelis 1992[17], Vos et al. 2005[18]). Resilience is also used in a way that more closely resembles the definition of resistance. Ecological resilience was defined as the amount of disturbance that an ecosystem could withstand without changing self-organized processes and structures (definition 3).

Resilience of coastal systems largely depends on biodiversity, which is a major requirement for allowing ecosystems to adapt to changing conditions. The human impact on the environment through pollution, fisheries, sediment erosion / deposition and global climate change has brought about much faster change than would occur under natural conditions, putting severe stress on many ecosystems. Without genetic diversity, natural selection cannot occur and if natural selection is limited, adaptation is impossible. Preservation of biodiversity and, more specifically, genetic diversity is therefore of paramount importance for successful adaptation to our rapidly changing environments. However, biodiversity may not always protect ecosystems from major abiotic disturbances (Folke et al. 2004[19]).

Resilience through recolonization

To understand resilience of ecosystems it is essential to understand what drives succession within these ecosystems. Succession determines how, and how fast, communities return to their original state, or perhaps enter a new state. Many aspects of succession can be understood in terms of trade-offs between the ability to be either a good early (re)colonizer, or a good competitor. Succession involves a gradual replacement of colonizer/competitor species according to the degree to which they tolerate, facilitate or inhibit certain environmental conditions and other species (Rossi et al. 2009[14]). The extent to which processes of (re)colonization and succession can take place largely determines the recovery of ecosystems after major disruption and is therefore an essential characteristic of the resilience of ecosystems.

In this context, it is important to consider the spatial component of ecosystem resilience. Diversity of structurally and functionally connected landscapes, rich in resources and species, promotes the flow or movement of individuals, genes, and ecological processes. Below certain thresholds of connectivity the capacity to regain structure and function after perturbation is lost (Holl and Aide, 2011; Rudnick et al., 2012;McIntyre et al., 2014; Rappaport et al., 2015; Ricca et al., 2018). Chambers et al. (2019[20]), based on Allen et al. (2016[21]), have therefore introduced the concept of 'spatial resilience', which is a measure of how spatial attributes, processes, and feedbacks vary over space and time in response to disturbances and affect the resilience of ecosystems. Self-organization through strong feedbacks at multiple scales and high levels of functional diversity and redundancy, stabilizes the system with respect to disturbances within the range of historic variability.

When creating Marine Protected Areas, the sources of populations at all stages of succession should be protected, to preserve 'ecological memory' to the fullest possible extent. This includes protecting not only 'high quality' habitats that harbour healthy mature communities, but also 'low quality' and disturbed habitats that are required for those species that contribute to early recovery of perturbed areas (Rossi et al. 2009[14]). The selection of Marine Protected Areas thus involves evaluating the number, size, and spatial configuration of habitat fragments and degree of connectivity required to support restoration of ecosystems and conservation of focal habitats and species[20][2].

Resistance to changes in abiotic and biotic factors

Community composition and ecosystem function may change very little under environmental change when the organisms can adapt to such change or tolerate it for some time (when the change is only temporary). However, all organisms have bounds to what they can temporarily or permanently tolerate, and when change exceeds some of these limits, the community composition and ecosystem functioning is likely to change.

It is unlikely that communities can be resistant to ongoing gradual change, such as global warming. Acclimation and phenotypic plasticity do not suffice to maintain the system as it is. Genetic adaptation could allow community members to track such abiotic environmental change, but it is more likely that the area where the community is functioning will be invaded by species that function well at higher temperatures. The original species will thus have to deal with new competitors and predators, in addition to the changed abiotic factor. To some extent the original community can track the preferred temperature range, by moving spatially to greater depths or to alternative geographic areas. But these new areas are likely to differ in other ecological aspects such as water pressure, light climate and perhaps speeds of water flow etc.

Adaptation and the consequences of mortality at different trophic levels

External disturbance interacts with internal mechanisms that shape community structure. To understand how an increased mortality of top-predators will affect the entire food chain, it is essential to understand how processes of mutual adaptation within food chains already give shape to existing patterns such as trophic structure (how biomass in ecosystems is partitioned between trophic levels).

Abundances at different trophic levels (such as algae, herbivores, carnivores and top-predators) and their responses to increased mortality (as under environmental change) depend critically on different mechanisms of adaptation within food chains and on the importance of population density at each of these trophic levels. However, different types of adaptation to living in a food chain context (balancing the need to acquire resources with the need to avoid predation) can often have similar consequences. For example, micro-evolution of behaviour, species replacement and induced defenses at a middle trophic level may all have similar effects on trophic level abundances in disturbed food chains (Abrams and Vos 2003[22]).


Related articles

Integrated Coastal Zone Management (ICZM)
Thresholds of environmental sustainablility
Sustainability indicators


References

  1. ↑ 1.0 1.1 Lake, P.S. 2013. Resistance, Resilience and Restoration. Ecological Management and Restoration 14: 20-24
  2. ↑ 2.0 2.1 Olsson, S., Melvin, A. and Giles, S. (eds.) 2019. Climate change and ecosystems. Procs. Sackler Forum on Climate Change and Ecosystems, Washington, DC, November 8-9, 2018, organized by the National Academy of Sciences and The Royal Society
  3. ↑ Folke, C., Carpenter, S. R., Walker, B., Scheffer, M., Chapin, T. and Rockstrom, J. 2010. Resilience thinking: integrating resilience, adaptability and transformability. Ecology and Society 15(4): 20
  4. ↑ Scheffer, M. 2009. Critical transitions in nature and society. Princeton University Press, Princeton, New Jersey, USA
  5. ↑ 5.0 5.1 Brand, F.S. and K. Jax. 2007. Focusing the meaning(s) of resilience: resilience as a descriptive concept and a boundary object. Ecology and Society 12(1):23
  6. ↑ 6.0 6.1 6.2 Haines‐Young, R. and Potschin. M. (eds.) 2010. The Resilience of Ecosystems to Environmental Change (RECCE). Overview Report, 27 pp. Defra Project Code: NR0134
  7. ↑ Holling, C.S. 1973. Resilience and stability of ecological systems. Annual Rev. Ecol. Syst. 4: 1–23. doi: 10.1146/annurev.es.04.110173.000245
  8. ↑ Gunderson, L.H. 2000. Ecological Resilience - in Theory and Application. Annual Review of Ecology and Systematics 31:425-439.
  9. ↑ Hirota,M., Holmgren,M., Van Nes, E. H, and Scheffer,M. 2011. Global resilience of tropical forest and savanna to critical transitions. Science 334: 232–235. doi: 10.1126/science.1210657
  10. ↑ Chambers, J. C., Bradley, B. A., Brown, C. S., D’Antonio, C., Germino, M. J., Grace, J. B., et al. 2014. Resilience to stress and disturbance, and resistance to Bromus tectorum L. invasion in the cold desert shrublands of western North America. Ecosystems 7: 360–375. doi: 10.1007/s10021-013-9725-5
  11. ↑ Pope, K. L., Allen, C. R., and Angeler, D. G. 2014. Fishing for resilience. T. N. Am. Fisheries Soc. 143: 467–478. doi: 10.1080/00028487.2014.880735
  12. ↑ Seidl, R., Spies, T. A., Peterson, D. L., Stephens, S. L., and Hick, J. A. 2016. Searching for resilience: addressing the impacts of changing disturbance regimes on forest ecosystem services. J. Appl. Ecol. 53 : 120–129. doi: 10.1111/1365-2664.12511
  13. ↑ 13.0 13.1 Dawson, T.P., Rounsevell, M.D.A., Kluvankova‐Oravska, T., Chobotova V. and Stirling, A. 2010. Dynamic properties of complex adaptive ecosystems: implications for the sustainability of services provision. Biodiversity and Conservation 19: 2843‐2853
  14. ↑ 14.0 14.1 14.2 Rossi, F., Vos, M. & Middelburg, J.J. 2009. Species identity, diversity and microbial carbon flow in reassembling macrobenthic communities. Oikos 118: 503-512.
  15. ↑ Diaz, R.J. & Rosenberg, R. 1995. Marine benthic hypoxia: a review of its ecological effects and the behavioural responses of benthic macrofauna. Oceanogr. Mar. Biol. Annu. Rev. 33:245-303.
  16. ↑ Pimm, S.L. 1982. Food Webs. The University of Chicago Press.
  17. ↑ DeAngelis, D.L. 1992. Dynamics of Nutrient Cycling and Food Webs. Chapman and Hall, London.
  18. ↑ Vos, M., Kooi, B.W., DeAngelis, D.L. & Mooij, W.M. 2005. Inducible defenses in food webs. In: Dynamic Food Webs. Multispecies Assemblages, Ecosystem Development and Environmental Change. Eds. P.C. de Ruiter, V. Wolters & J.C. Moore. Academic Press. Pp. 114-127.
  19. ↑ Folke, C., Carpenter, S., Walker, B., Scheffer, M., Elmqvist, T., Gunderson, L. & Holling, C.S. 2004. Regime Shifts, Resilience, and Biodiversity in Ecosystem Management. Annual Review of Ecolog and Systematics 35:557-581.
  20. ↑ 20.0 20.1 Chambers, J.C., Allen, C.R. and Cushman, S.A. 2019. Operationalizing Ecological Resilience Concepts for Managing Species and Ecosystems at Risk. Front. Ecol. Evol. 7:241. doi: 10.3389/fevo.2019.00241
  21. ↑ Allen, C. R., Angeler, D. G., Cumming, G. S., Folk, C., Twidwell, D., and Uden, D. R. 2016. Quantifying spatial resilience. J. Appl. Ecol. 53, 625–635. doi: 10.1111/1365-2664.12634
  22. ↑ Abrams, P.A & Vos, M. 2003. Adaptation, density dependence and the responses of trophic level abundances to mortality. Evolutionary Ecology Research 5: 1113-1132



The main author of this article is Vos, Matthijs
Please note that others may also have edited the contents of this article.
Citation: Vos, Matthijs (2020): Testpage3. Available from http://www.coastalwiki.org/wiki/Testpage3 [accessed on 20-04-2024]
Retrieved from "https://www.coastalwiki.org/w/index.php?title=Testpage3&oldid=77730"