6. Historical image of ES value
In historical times, the TIDE estuaries have played a different but not necessarily less important role. Major shifts in physical aspects (tidal amplitude, saline penetration, marsh formation, embankments,…) have occurred during the last centuries, and the demand for ecosystem services in historical times (food provision, recreation, navigation) is entirely different from todays’.
The goal of this “hindcasting” exercise is thus to project the historic habitat configuration (and estimation of historical ES demand) on todays’ estuaries and get a tentative idea of the gained and lost values of ecosystem services. As data availability on habitats (compatible with supply survey categories) was limited, this exercise was only performed for the Weser and the Scheldt.
The supply of a certain service by a habitat can be multiplied by its surface to get a qualitative assessment of changes in ES supply caused by shifts in habitat surfaces. However, the surface-supply relationship is not the same for all habitats and services. Differences exist in the quantity of this relationship: e.g. one hectare of tidal flat will not supply the same ‘amount of benefit’ for nutrient capture as of sedimentation regulation. Also, surface-supply curves might be linear, exponential, or saturated: e.g. more deep water will increase navigation service, but after a certain amount is reached and demand is met, the service will not further increase. Therefore ES calculations based on surfaces should be interpreted as an indication and interpreted with caution. This is mainly the case for services like water for navigation, wave reduction and water current reduction, which strongly depend on the form of the habitat (length-width, orientation along river, presence of bottlenecks,…).
Using the habitat surface areas and the ES supply scores per habitat, ES supplies were was calculated. This supply was weighed with the demand scores of the appropriate time period and zone. This yields a “total value indicator” per ES, which contains supply, surface area and demand aspects.
- TVESx total value indicator of ESx
- AHy the surface area of habitat y in hectares
- SHy the average supply score of habitat y over the salinity zones.
- DESx the average demand score of ESx over the salinity zones. For 1930 or earlier, historical demand scores were applied.
This indicator can be compared between historical time steps. A higher value can thus be generated by a higher surface, a higher supply score or a higher demand for the ES.
Habitat categories could be reconstructed based on historical physiotope maps, but not all habitats were available. For the merged “intertidal’ category, intertidal flat supply scores were used (assuming minor relative surface importance of intertidal steep habitat), for the merged subtidal moderately deep and deep habitat, the supply scores were averaged. The TV change over time gives an indication of the shift in total ES value.
Weser Estuary 1950-2005
Habitat shifts In the Weser were derived from the report “Shallow water areas in North Sea estuaries
”) and summed according to the habitats used in the ES supply survey.
Table 4: Habitat surface evolution in the Weser estuary in ha (“Shallow water areas in North Sea estuaries”). For 1950, only data for four categories are available. 2005 habitats were merged accordingly.
||Intertidal flat + steep
||Subtidal deep + moderately deep
In the Weser, the total value is lower for most services today than in the scenario with habitat surfaces of 1950. Only six services (water quality regulation, water for navigation, …) seem to have higher Total Value in the current habitat configuration. A remarkable higher value of the water quality regulation, is caused by increase in surface of marsh and subtidal shallow habitat. However, the total bundle supply does not seem so different in 2005 compared to the 1950 hindcasting scenario. As can be observed from the Scheldt hindcasting excercice, it could be that main shifts in habitat surfaces (and ES supply) occurred at earlier stages.
Scheldt Estuary 1880-1930-1960-2001-2010
Table 5: Habitat surface evolution in the Scheldt estuary from 1880 till 2010. For subtidal categories in 1880, only the total surface is known (see “Shallow water areas in North S ea estuaries”), separate surfaces were estimated (italics) based on ratios of 1930. Historical data are only available for freshwater, oligo- and mesohaline zones.
||intertidal flat + steep
||subtidal moderately deep
For the Scheldt estuary, a general decrease in all ecosystem service values can be observed along the different hindcasting scenarios from 1880 till 2010. Most remarkable is the general negative for all services. This decrease is mainly driven by a decrease in habitat surfaces, but the major shift observed from 1930-1960 is also influenced by the demand values used in the formula (see section 6.2). For 1880 and 1930, historical demand values were used in the calculation, yielding a higher total value for some ecosystem services in before 1930.
Because of this higher demand, some services’ value indicators have increased from 1960 to 2001 (water for navigation, water for industrial use, transport of excess nutrients and drainage of river water), but even these values apparently declined during the last decennium.
Compared to the Weser, the total bundle of values decreases from the mid-nineties up till today. As can be observed in the habitat surface areas, a continuous decrease of all habitat types is observed in favor of deep water habitat. However, the ongoing and planned restoration of about 1500 ha of tidal habitats (mainly marshes and intertidal flats) could probably reverse this trend.
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