Restoring estuarine ecosystems

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In order to have or achieve a healthy and resilient estuarine ecosystem, a site-specific understanding of the physical components and processes as mentioned on the previous page should be kept in mind. Moreover, there are other ecological factors which can further stimulate the restoration of an estuarine ecosystem. Three ecological rules to enable successful restoration of the estuarine ecosystem are given:

1. the ecosystem will emerge from the physical and biological opportunities;
2. the most resilient ecosystem will emerge when the physical processes are as natural as possible;
3. (physical) processes may be dynamic but should be within the confounds of an equilibrium (e.g. a fluctuation tide).

With these principles as a basis, one can start thinking on how to enable the restoration of the estuarine ecosystem. This is a complex process, with the risk of unforeseen outcomes (such as invasive species and unwanted morphological consequences). Here, a step-wise process is provided, to maximize the potential of a restoration project in an estuary.

Step 1: system analysis

A system analysis should always be performed (System thinking). This analysis is an integrated assessment of all biological, hydrological, physical, and chemical processes. The analysis should start with an inventory of the current state of the estuary (E.g. Which species are present? Which are absent? Which physical processes are observed?). The analysis serves to gain insight into the (absence of) processes that have led to the current state of an estuary. This should provide insight into the physical processes, which habitats emerge from these processes, and thus which species are to be expected. The analysis should aim to combine all available data from monitoring, literature and historic development. Optimally, this is combined in a (conceptual) model of the estuarine system.

Step 2: determine physical boundary conditions

It is important to be aware of the physical boundary conditions within the system. During this step, one should keep an open mind and only limit the scope of the project by what is clearly impossible. For example, it is unlikely to it is possible to change the course of a river inlet in a highly-populated area, even though this might have been the natural dynamics of the system.

Step 3: set ecological goals

During this step, one should specify what kind of ecosystem is preferred. Often, we are tempted to focus on facilitating specific species. This has major disadvantages, as it is often challenging to facilitate the entire life cycle of the target species. Additionally, other species may benefit more from certain measures than the target species. It is therefore key to define principles: focus on ensuring a natural system based on natural biophysical processes and the most resilient ecosystem will emerge from it by itself.

Step 4: enhance natural dynamics and restore connections

Tidal fluctuation and Inundation

Free-flowing tides are an important factor in natural systems. This, in effect, also ensures a natural salinity gradient. Barriers, such as locks, strongly limit natural tidal fluctuations. If natural variations are absent, the opportunities for successful enhancement of estuarine ecological conditions are limited. However, one can still strive towards a well functioning ecosystem, by mimicking natural processes. When (re-)introducing tidal dynamics, one should predict the expected tidal range and the variance in this. Equilibrium in introducing these dynamics must be reached within a foreseeable future. This is especially important for sessile species (cannot move), as they cannot move with a changing water level. Animals and plants are equipped to a certain amount of inundation (flooding). Most seaweeds, for example, need to be submerged to grow but can survive a few hours above the waterline. Reeds, on the other hand, need to have their roots submerged, but cannot survive fully inundated. As vegetation is sessile , water level changes must be somewhat constant. Very little vegetation is able to survive long-lasting high or low water conditions (days). If these events do occur, the ecosystem is often reset to a more or less pioneer state. Extreme (unnatural) high or low water conditions, lasting more than a few days should thus be avoided. Sessile animals, such as mussels, have a similar trait, where they can survive for a few hours above water, but not for days or weeks. Therefore it is important to quickly reach the new dynamic state of conditions.

Salinity gradient

Similarly the salinity gradient will determine the successful settlement and long-term survival of a species. When exposed to a salinity outside of this range (say, for example low salinity), the species will most likely not survive. This will provide opportunities for freshwater species. However, once these are exposed to extreme salt conditions, high mortality is to be expected: very little species are adapted to both very high and very low salinity. It is therefore important that the salinity gradient is stable. A stable range in salinity gradient is also import for the diadromous fish, which use estuarine systems to transition between fresh- and saltwater life stage. Fresh- and saltwater inflow should therefore be mitigated to ensure a stable fluctuation. This can be achieved by managing the lock and pump activities.

The effect of variability in freshwater inflow should be considered, especially when the river is susceptible to high seasonal changes. These changes could result in long lasting highwater events, which could result in an unstable salinity gradient, and thus the disappearance of water plants and seaweeds.

Barriers

Diadromous fish and other migratory species need to be able to move freely over the course of the waterway. Man-made barriers should be limited as much as possible. Barriers not only limit opportunities for migration, but also often limit the dynamics in abiotic conditions. For example by blocking the tidal movements and limiting salinity gradients.

If barriers need to be implemented, it is recommended to explore how the change in (a)biotic conditions and impact of migrating species can be minimized. Impacts on fish migration can for example be limited by selection of fish-friendly solutions (see practical applications).

Around locks and pumps, the currents are often higher and animals have difficulties passing the locks and pumps. Even with open locks, fish experience challenges due to the large water level differences on both sides.

Implementing fish passages with low currents can allow fish and other organisms to pass barriers. These passages can be designed alongside the primary structure (lock or dike), without hindering its functions.

Gradual shores

Often estuaries have been embanked for flood protection or land reclamation purposes, which has resulted in very discrete land-water boundaries. There are means to restore this. The morphology and the slope of the banks are important for vegetation zonation. In general, two aspects are important:

• Slope: a typical, natural slope steepness for shoals is between 1:10 and 1:20 (see figure below). With the additions of irregularities (e.g. tidal pools), a wide range of niches occur on a single embankment. A wide range in niches can result in a wide range of flora and fauna. In the saline and brackish area, the slope steepness determines the type of shellfish species and weeds which are present;

• Substrate: the type of substrate of the embankments determines which species will emerge. Smooth substrates, such as concrete, do not facilitate the settlement of organisms. In general, seaweeds are able to adhere to rough stones, while most water plants need soil substrates to be able to root. Additionally, the integration of holes of varying sizes may function as shelter for migrating fish (Gotjé, Cleveringa, and De Jong 2016);
• A foreshore can also assist in the trapping of sediment and the protection of vegetation (see figure below). For more information on this, see integrating vegetated foreshores.

Step 5: Monitoring 

Finally, it important to continuously monitor the ecosystem. Due to the complex nature of the estuarine ecosystem, unforeseen interactions may occur, leading to unwanted outcomes. By monitoring the system, one can identify early warning signs and implement maintenance measures where needed. Monitoring also forms a basis in proving the potential of an innovative BwN design. For more information on the importance of adaptive management, monitoring and maintenance see Enabler 3.