Subterranean ecosystems - Universidade de Évora

IAH 2007 XXXV Congress - Groundwater and Ecosystems Lisbon, Portugal
Expanding concepts within eco-hydrology to accommodate the
hydrodiversity of groundwater dependent ecosystems
(or how do aquifers make ecosystems lazy, inefficient and vulnerable?)
ABSTRACT
Christine COLVIN
Natural Resources and the Environment, CSIR, Stellenbosch, South Africa.
E-mail: ccolvin@csir.co.za
Whilst hydrogeology studies the flow of water through the lithosphere, ecology studies the biologically mediated
flow of energy through the biosphere. We currently lack a generic conceptual framework to examine the waterenergy
interface of groundwater linked ecosystems. Eco-hydrology has emerged as a trans-disciplinary area of
study over the last 2 decades in an attempt to consider water links to ecosystems more holistically. However,
currently two domains are evident within this field: soil-moisture driven terrestrial systems and surface-water
driven aquatic systems. Groundwater dependent ecosystems (GDEs) exist in a diverse range of habitats,
spanning both aquatic and terrestrial systems, and their study encourages thinking towards a more inclusive and
holistic earth-system model. Much of the previous (valuable) work on GDEs has considered individual case
studies (eg Doñana wetlands) or types of habitats (in-aquifer systems), and studies have adopted a bi-disciplinary
approach, linking hydrogeology to ecology, ecophysiology or hydrology. Predominantly bi-disciplinary studies
have resulted in some confusing and conflicting applications of terminology.
This paper proposes some clarifications for terminology, particularly groundwater dependent
ecosystems and aquifer dependent ecosystems. It proposes that ecosystem water requirements should be
considered in terms of hydrotopes at the landscape and ecosystem scale. Hydrotopes may be delineated in
specific habitats with characteristic spatial and temporal patterns of water provision from precipitation, soil
moisture, surface water, groundwater, tidal and oceanic sources. The Trigger-Transfer-Reserve-Pulse (TTRP)
model of landscape ecology, is incorporated and expanded to account for the variable storage-transfer functions
of different hydrological pathways. The interplay of (typically) soil-moisture/ surface water/ groundwater
transfers to an ecosystem define the hydrodiversity and hydroperiodicity of an ecosystem. Special attention is
given to aquifer dependent ecosystems (ADEs) in water controlled (semi-arid and arid) environments, with an
illustration of hydrotope typesetting examples in Southern Africa. Semi-arid zone ADEs typically represent
keystone ecosystems at ecotones with characteristic hydrotopes of biologically mediated aquifer discharge.
Linear and discrete spatial discharge patterns of fractured and alluvial aquifer systems contribute to ecosystem
heterogeneity and ‘patchiness’. The larger (volume) and more sustained hydrodiversity of aquifer fed
hydrotopes sustain relatively high productivity during seasonal periods of relative dormancy in neighbouring
soil-moisture dependent hydrotopes. Both terrestrial and aquatic ADEs in these environments are believed to
have greater hydrological resistance to disturbances in water inputs and contribute resilience to the surrounding
ecosystem. However, ADEs have a relatively low resilience (ability to recover) following changes to the aquifer
flow regime beyond natural minima and maxima, and are more vulnerable to irreversible alteration by disruption
of aquifer flow. Typically ADEs subsidise neighbouring non-ADEs through trophic cascades. Linked to their
smoothed hydroperiodicity, it is expected that ADEs have a lower water efficiency in primary productivity
(gC.H2Omm -1 ) may show net mean annual productivity equivalent to their more ‘flashy’ episodic neighbours,
who have high productivity during the shorter periods of water availability.
In a nutshell – ADEs in water controlled environments have a greater hydrodiversity and characteristic
smoothed, longer hydroperiod due aquifer-fed baseflow, than neighbouring hydrotopes, reliant predominantly on
surface water or soil-moisture. The additional water budget available to ADEs, slowly released from aquifer
reserves, effectively lubricates slower and more prolonged productivity. Whilst ADEs are typically net exporters
of ecological services to surrounding non-ADEs, they tend to be lazy, inefficient and vulnerable!
Keywords: ecohydrology, groundwater dependent ecosystems, hydrotopes, hydrodiversity.
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