Ecohydrological Conceptualisation of the Fortescue Marsh Region
Status: Final
September 2015
Project No.: 83501069
Page 11
Our ref: FM-EcoConcept_v8.docx
Ecological water requirements of wetlands
Wetlands are permanently or persistently inundated or saturated by surface or groundwater. They may
include subterranean habitats. Such areas require special attention owing to their potentially hi gh level
of connectivity with other landscape elements, restricted occurrence, and propensity to support unique
or unusual ecosystems.
Water can converge and collect in wetlands from many different pathways. Transit times along these
pathways can vary spatially and temporally in response to inputs and the physical structure of the
catchment, resulting in dynamic and complex hydrological regimes (Neumann, 2013). In flat terrain
where flow rates are lower, such as in lake chain systems, water is exposed for a longer time to climatic
and biogeochemical processes which can alter its physicochemical properties. In steeper terrain where
flow rates are greater, such as in rivers or headwater streams, there may be little modification of water
properties owing to the shorter transit times. The properties of groundwater contributing to a wetland
environment may similarly be modified based on residence times in aquifer systems.
The level of persistence of aquatic environments has an important effect on their biotic asse mblages,
and their functional importance as refuges (Davis et al., 2013). Longer hydroperiod allows more species
to colonise and greater habitat complexity to develop (Boulton and Jenkins, 1998; Sheldon et al., 2002).
In ephemeral wetlands the transition between wet and dry periods is important for driving biotic and
abiotic exchanges, ecosystem succession processes, and maintaining ecosystem integrity (Boulton and
Lloyd, 1992; Boulton and Jenkins, 1998; Junk et al., 1989).
Wetland ecosystems in the central Pilbara tend to be strongly influenced by episodic, intense, rainfall
events and rapid surface water movement from source areas (Pinder et al., 2010). Many surface water
features, such as river pools and clay pans, are intermittent or ephemeral. The persi stence of
waterbodies is dictated by flood frequency; the rate of water loss following flood events by drainage,
evapotranspiration and infiltration/percolation into deep groundwater systems. Bank storage and/or
perched groundwater may be important for prolonging waterbody persistence, where permitted by local
area geomorphology. Different hydrological regimes associated with flooding and flow frequency, flood
duration, salinity and source of water may support different species assemblages. Different stages of the
hydrological regime may also be important for species life cycles, such as the utilisation of flooded
wetlands by waterbirds for breeding.
Permanent pools usually occur where bedrock structures impede hyporheic groundwater flow, where
springs discharge groundwater or where flow has scoured pools that are sufficiently deep to encounter
the watertable (Pinder et al., 2010). These wetlands are uncommon, typically supporting unusual or
unique flora and fauna assemblages and potentially functioning as re fuges for a range of aquatic
species.
Subterranean habitats in groundwater are defined by the types of voids and interstitial spaces within
host rocks and groundwater chemistry (Halse et al., 2014). Both attributes are influenced by the geology
of the aquifer, the amount of landscape weathering, and local chemical and hydrological processes
(Reeves et al., 2007).
Connectivity with the surface influences the supply of oxygen and organic matter into subterranean
ecosystems, highlighting the importance of recharge dynamics for these habitats as associated with
cyclonic recharge events. Depth to groundwater is considered to constrain the complexity and
abundance of stygofauna communities in the Pilbara (Halse et al., 2014). The response of stygofau na to
fluctuations in the watertable is poorly understood; however, their persistence over geological time
suggests they have capacity to adapt to dynamic habitat availability.
1.3.3
Ecohydrological conceptualisation approach
Taking into account the ecohydrological principles discussed in Section 1.3.2, a landscape
ecohydrological conceptualisation was developed through the definition of ecohydrological units (EHUs)
for the study area. Each EHU represents a landscape element with broadly consistent and distinctive
ecohydrological attributes (MWH, 2014).
In summary the spatial definition of the EHUs is based on interpretation of land system mapping units
developed by the Department of Agriculture (Van Vreeswyk et al., 2004), surface drainage networks,
groundwater systems, inferred vegetation water use behaviour based on vegetation mapping (structure
and dominant species) and Landsat NDVI (Normalized Difference Vegetation Index).
Nine EHUs are recognised within the studystudy area as listed below: