Resource Directed Measures for Protection of Water Resources: Estuarine Ecosystems
WATER RESOURCE PROTECTION AND ASSESSMENT POLICY IMPLEMENTATION PROCESS
RESOURCE DIRECTED MEASURES FOR PROTECTION OF WATER RESOURCES: ESTUARINE ECOSYSTEMS COMPONENT
SECTION D: PROCEDURE FOR RAPID DETERMINATION OF RDM FOR ESTUARIES
Senior Authors:Ms Susan Taljaard, Environmentek, CSIR
Dr Janine Adams, University of Port Elizabeth
Dr Anton Bok, Anton Bok and Associates
Editors:Dr H MacKay, Department Of Water Affairs and Forestry, 19 September 1999
Ms Barbara Weston, Department of Water Affairs and Forestry, 21 September 1999
Ms Lizette Guest, Guest Environmental Management
Version 1.0:
Date:24 September 1999
M:\f_rdm_october\estuaries\version 1.0\est_sectiond_version1.0.doc
Editor’s note:
- This section must be reviewed for clarity and readability.
Section D:Rapid Determination of RDM for Estuarine Ecosystems
D.1Flow chart of the rapid RDM determination process
A flow chart of events for the Rapid determination of RDM as it applies to the estuarine component is given in Figure D.1.
Figure D.1: Flow chart of events for the determination of Rapid RDM for estuarine ecosystems
The Rapid determination of the RDM on estuaries can be done at a 1-2 day workshop using a group of estuarine specialists with the following expertise on the particular system:
- Hydrology and estuarine physical dynamics
- Estuarine water quality
- Flora (microalgae and macrophytes)
- Fauna (invertebrates, fish and birds).
D.2Data requirements for the rapid determination of RDM
The rapid determination is generally based on available information. It is therefore important that a desktop assessment of available information on the different abiotic and biotic components is conducted prior to the workshop. In particular, available information on rare and endangered species, species with limited populations, and habitat diversity is required.
In terms of the hydrology, simulated monthly runoff data for present and reference conditions needs to be available prior to the workshop. [1]
D.3Generic Methodology
D.3.1Procedures to delineate the geographical boundaries (Step 1)
For the purposes of the Rapid Determination, the geographical boundaries of an estuary are defined by the mouth (seaward boundary) and the extent of tidal influence (upper boundary). The lateral boundaries along the banks are taken as the 5m above mean sea level (MSL) contour.
D.3.2Procedures to describe the present status and estimate reference condition (Step 3)
The reference condition of an estuary is a qualitative and quantitative description of the natural, unimpacted abiotic and biotic characteristics and functioning of the system. Although the Rapid Determination of RDM is primarily a desktop assessment, some knowledge or at least expert opinion is required to determine reference conditions. This information is used to define parameters such as Present Ecological Status (which is based on a comparison between present status and reference condition), Ecological Importance and Ecological Sensitivity. In turn, these parameters are used in the determination of the desired Ecological Management Class and in the determination of the water quantity component of the Reserve.
D.3.3Determine the Present Ecological Status, Ecological Importance and Ecological Management Class (EMC) (Steps 4 & 5)
For the Rapid Determinationof RDM in estuaries the ecological management class is primarily based on (also refer to A.2.2 step 5):
- present ecological status,
- ecological importance, and
- modifying determinants (e.g. if the estuary is part of a national park or a RAMSAR site).
Procedures to Determine Present Ecological Status
For the Rapid determination of RDM, available information and expert opinion on the reference condition and present status are required. This information is used to rate the present ecological status of the estuary in terms of the extent to which the components listed in Table D.1 have changed from the reference conditions to present conditions. Table D.2 indicates the scoring guidelines for assessing the present ecological status and confidence.
Table D.1: Variables used to assess present ecological status of estuaries
COMPONENT / BRIEF DESCRIPTION OF CHANGE (if any) / SCORE(CONFIDENCE)
Habitat-related or abiotic components:
% of natural MAR currently abstracted
Changes in seasonal river inflow patterns
Changes in mouth condition (i.e. frequency and duration of mouth closure)
Changes in water quality (referring to system variables, nutrients and toxic substances)
Changes in natural in-stream habitat (i.e. estuary bed modification, channel modification, infilling, migration barriers, bridges, weirs, bulkheads, training walls, canalisation, jetties, marinas)**
Changes in riparian habitat (i.e. flood plain development, bank vegetation, agriculture, grazing, industry, housing, infrastructure, recreational development)
Biotic components:
Plants, including microalgae and macrophytes (i.e. change in botanical importance score)
Benthic invertebrates
Fish (i.e. change in estuarine health index score for fish)
Birds
MEAN ECOLOGICAL INTEGRITY SCORE (averaging the 9 scores above. If no information is available for one of the 9 components, then it is not a divisor
**May need to distinguish between sedimentation/erosion and other ’man-made’ modifications, such as bridges, causeways etc.
Table D.2: Scoring guidelines for the present ecological status
FOR EACH OF THE ABOVE COMPONENTS, IF.... / SCORENatural, unmodified or if current status approximates natural condition / 5
Largely natural with few modifications, some loss of natural habitat / 4
Moderately modified, but with some loss of natural habitat / 3
Largely modified, large loss of natural habitats and basic ecosystem functions has occurred / 2
Seriously modified, the losses of natural habitats and basic ecosystem functions are extensive / 1
Critically modified, modifications have reached a critical level and the system has been modified completely with an almost complete loss of natural habitat / 0
The confidence levels of the scores (in brackets) are as follows:
IF... / CONFIDENCE LEVELSufficient, long-term data sets were available to describe the reference condition and present status with relative accuracy / High
(> 80%)
Sufficient shorter-term data sets were available to describe the present status, but the reference condition had to be derived. / Medium
(40-80%)
Only limited data sets were available from which to determine both the reference condition and present status / Low
(<40%)
Procedures to Determine Ecological Importance
Ecological importance is an expression of the importance of an estuary to the maintenance of ecological diversity and functioning on local and wider spatial scales i.e. the nearshore marine environment and the lower catchment. The following parameters will be used to rate the ecological importance of an estuary:
- Rare, endangered species or those with limited populations
- Habitat diversity (richness)
- Rarity of an estuary type or any unique estuarine features
Using the information gathered on the reference condition and present status of the estuary, the ecological importance of the estuary is rated as shown in Tables D.3 and D.4.
Table D.3: Components used to rate the ecological importance of an estuary
COMPONENT / BRIEF MOTIVATION / SCORE (CONFIDENCE)Rare/endangered/limited populations / Useful sources for this include the South African Red Data Books that are suitable for assessment on a National scale. Examples of unique or limited populations include the mangrove fern and red mangrove Rhizophora mucronata.
Habitat diversity (richness) / The number of habitat types in an estuary should be tallied using the following list:
1. open surface water area
2. sand flats
3. mudflats
4. submerged macrophyte beds
5. intertidal salt marsh
6. supratidal salt marsh
7. reeds and sedges
8. mangroves
9. lagoon swamp forest
10. rocks
11. deep channels
Rarity of an estuary or unique estuarine features / Unique features include canyons e.g. Msikaba, ravines or gorges, physical features such as the hole in the wall, small estuaries that remain permanently open e.g. Lupatana.
Certain estuaries are unique or rare on a National scale, for example there are only two permanently open estuaries in the cool temperate zone. All estuarine lakes and bays are considered unique.
Input to sea / Estuaries are rated according to size and their potential input of sediments and nutrients to the coastal zone.
MEAN ECOLOGICAL IMPORTANCE SCORE (averaging the 4 scores above)
Table D.4: Scoring guidelines for ecological importance
COMPONENT / SCORING GUIDELINERare/endangered/limited populations
(There are two possible scores only, i.e. 5 or 1) / If rare/endangered biota/limited populations are present / 5
If absent / 1
Habitat diversity (richness) / < 4 habitat types (low score) / 1
4 - 7 habitat types (medium score) / 3
8 - 11 habitat types (high score) / 5
Rarity of an estuary or unique estuarine features
(There are two possible scores only, i.e. 5 or 1) / If rare/unique features are present / 5
If absent / 1
Input to sea / Large runoff to the sea with significant input of nutrients and/or sediment to the sea, e.g. Tugela / 5
Medium run-off to the seas with some input of nutrients and/or sediment to the sea, e.g. Mtamvuma / 3
Small runoff to the sea with no significant input of nutrient and/or sediment to the sea, e.g. Diep / 1
The confidence levels of the scores (in brackets) are as follows:
IF... / CONFIDENCE LEVELImportance scores are based on sufficient, long-term data sets on the system / High (> 80%)
Importance scores are based on sufficient shorter-term data sets on the system / Medium (40-80%)
Importance scores are based on limited or data sets on the system / Low (<40%)
Identification of Modifying Determinants
If an estuary has special international importance (i.e. RAMSAR site) or national importance (i.e. part of a National Park, Nature Reserve, Municipal Park, Private Reserve or Conservancy), modifying determinants can be used to weight the importance rating given to an estuary.
For estuaries listed as RAMSAR sites an EMC of A should be allocated, even if the class allocated according to present ecological status and ecological importance score is lower. The management plan for the estuary should be designed to take the estuary into an A ecological management class.
For estuaries which are part of a National Park, Nature Reserve, Municipal Park, Private Reserve or Conservancy an EMC of no less than a B should be allocated, even if the present ecological status category and ecological importance score are lower. The management plan for the estuary should be designed to take the estuary into a B ecological management class.
Procedures to Set the Ecological Management Class based on present ecological status and ecological importance
For the Rapid Determination, the EMC for an estuary is primarily determined by its present ecological status (as described in D.3.3). Scores for present ecological status are linked to EMC classes as shown in Table D.5
If the mean ecological importance score is higher than the mean present ecological status score then the EMC is determined by the mean ecological importance score, where the relation between score count and EMC category will be the same as above, e.g. a mean ecological importance score of > 4 = Class A
The present ecological status of an estuary is the primary determinant of the ecological management class of an estuary in a rapid RDM determination, but a high importance score might modify this. An unimpacted estuary must remain at its present status for the purposes of the rapid reserve determination even though the estuary may not be of high ecological importance.
If any of the two modifying determinants (i.e. RAMSAR site or part of a national park, etc) applies to an
estuary, then the EMC is set according to this.
The class set according to the modifying determinant overrides any previous class allocated, for example:
- RAMSAR sites:An EMC of A should be allocated
- Part of a National Park, Nature Reserves, Municipal Park, Private Reserve or Conservancy: An EMC of no less than a B should be allocated.
Procedures for the inclusion of other important factors such as socio-economic importance in the allocation of an EMC will need to be included in the intermediate and comprehensive determinations. However, where details on such aspects are available at the Rapid determination stage, these should be noted.
For estuaries, sensitivity of the system to reduced inflow has not been considered as a criterion for determining the EMC. Sensitivity of the estuary to reductions and changes in flow is rather used as a criterion in quantifying the Reserve, the argument being that the more sensitive a system is to reduced flows (and the consequences thereof), the more conservative the determination of the Reserve should be.
Table D.5: Mean present ecological status score in relation to the ecological management class (EMC)
MEAN PRESENT ECOLOGICAL STATUS SCORE / RECOMMENDEDEMC CLASS / DESCRIPTION
> 4 / A / Unmodified, natural - the natural abiotic template should not be modified
The characteristics of the resource should be completely determined by unmodified natural disturbance regimes. There should be no human induced risks to the abiotic and biotic maintenance of the resource
> 3 and <= 4 / B / Largely natural with few modification - only a small risk of modifying the natural abiotic template and exceeding the resource base should be allowed
The risk to the well-being and survival of intolerant biota (depending on the nature of the disturbance) may be slightly higher than expected under natural conditions
> 2 and <= 3 / C / Moderately modified - a moderate risk of modifying the natural abiotic template may be allowed.
Risks to the well-being and survival of intolerant biota (depending on the nature of the disturbance) may generally be increased with some reduction of resilience and adaptability at a small number of localities
> 1 and <= 2 / D / Largely modified - a large risk of modifying the abiotic template and exceeding the resource base may be allowed.
Risks to the well-being and survival of intolerant biota (depending on the nature of the disturbance) may generally be allowed to increase substantially with resulting low abundance and frequency of occurrence
<=1 / Not acceptable, should be upgraded to at least Class D
The procedures to set the EMC for the Rapid determination of RDM for estuaries are summarised in Figure D.2.
Figure D.2: Summary of procedures to set the EMC for the Rapid determination of RDM
D.3.4Determination of the water quantity component of the Reserve (Step 6)
Approach and Assumptions
There are fundamental differences between rivers and estuaries. These include:
- In rivers, flow normally occurs in one direction only. Estuaries have more complex dynamics because of tidal influence and the intrusion of seawater.
- Estuaries are strongly dependent on extreme conditions, the most crucial of which are river baseflows and major floods. Changes in baseflow and major flood patterns can have a significant effect on the functioning of an estuary.
For most South African estuaries only limited or, in some instances, no measured data would be available during the Rapid RDM determinations. Therefore, in most cases, Reserve (water quantity) determination for different EMC’s will have to be based on our generic understanding of estuarine water processes and their sensitivity/reaction to changes in river inflow. River inflow into an estuary can broadly be divided into:
- average seasonal base flows, referring to the typical river inflow patterns measured on a monthly scale. Average seasonal base flows play an important role in determining mouth conditions and also have a marked influence on the natural cycles observed in aspects such as salinity distribution patterns and the subsequent biological composition and diversity in an estuary.
- floods and high seasonal flows, referring to events of high inflow, which are usually represented by the 1:2, 1:5, 1:20 or 1:50 and 1:100 flood peaks (flow rates and volume). This component of river inflow is primarily required to maintain the long-term sediment erosion/deposition equilibrium in an estuary.
For the rapid determination of the Reserve it is proposed that where there is a lack of certainty in terms of an estuary’s response to changes in seasonal base flows (which is expected to be the case in most estuaries during this phase), a conservative approach must be adopted in terms of seasonal base flow reductions, i.e.:
For rapid Reserve determination it will be assumed that the seasonal base flow component of the MAR will NOT be included in the allowable reduction from natural MAR. In doing so, the sensitivity of systems to reduction in seasonal base flows (and the associated consequences on the biota) will be covered by default.
If the sediment erosion/deposition equilibrium in an estuary is disturbed, this can either lead to siltation, resulting in the estuary becoming shallower, or erosion of important sediment habitats (if river sediment transport is trapped by dams). In estuaries floods and high seasonal flows primarily influence the sediment erosion/deposition equilibrium. Floods can alter important features within an estuary, such as the bathymetry (e.g. depth of the channel or the size of intertidal areas) and sediment composition (e.g. sand or mud).
Currently, the most limiting factor in estimating the water quantity requirements of estuaries is the inability to quantify the role of floods and seasonal high flows (magnitude and frequency) in maintaining the sediment erosion/deposition equilibrium, due to a lack of data and understanding of such processes. The need for further investigations has been identified and is currently being addressed. In previous Estuarine Freshwater Requirement studies (Appendix E1) this specific requirement was addressed by stating that the occurrence and magnitude of major floods should not be reduced. Based on the above uncertainties and where no information is available, it is proposed, that for the rapid RDM determinations, the following conservative approach is adopted:
Due to the uncertainties in quantifying the influence of floods in estuaries, floods of the magnitude of the 1:5 year flood and greater will not be included in the allowable reduction from natural MAR.
Based on the above assumptions, Figure D.3 indicates the remaining flow component from which water could theoretically be allocated, since seasonal baseflows and floods greater that the 1:5 year flood should not be reduced without more detailed investigations being carried out.
Figure D.3.: Schematic illustration of the remaining flow component from which water could theoretically be allocated.
Sensitivity to Reductions and Changes in River Flows
The parameters (primarily physical parameters) that will assist in obtaining some idea of an estuary’s sensitivity to reductions and changes in river inflow must be identified. This information can then be used in deriving an estimated annual flow (expressed as % of natural MAR) that would be required to maintain the estuary in its allocated EMC.