Janine Gibert

Subterranean Ecosystems: A Truncated Functional Biodiversity

T biodiversity varies among habitats is a basic tenet of ecology. Many hypotheses have been advanced to explain these variations: ecosystem stability and complexity (e.g., Pimm 1984), ecosystem predictability, habitat heterogeneity (e.g., Tilman 1982), and disturbance (the intermediate disturbance hypothesis; Grime 1973). Whatever the cause of its fluctuations, there is evidence that, at a certain threshold, biodiversity is critical to the maintenance of ecosystems. The link between biodiversity and ecosystem function depends on the dissipation of energy, and productivity might be the ultimate factor that controls species richness at local and, to some degree, at regional scales (Ricklefs and Schluter 1993). Ecosystem function accounts for the relative stability of biodiversity ratios between similar habitats of different continents (Caley and Schluter 1997). In contrast, absolute measures of biodiversity in similar habitats of different regions may differ greatly, which has been interpreted as a result of historical processes (Ricklefs and Schluter 1993). Biodiversity patterns of subterranean terrestrial and aquatic ecosystems are in line with these general observations. However, the features of this environment (absence of light, limited variations in temperature, paucity of food, high physical fragmentation) (Ginet and Decou 1977, Camacho 1992) provide unique opportunities to explore biodiversity issues and to test some of the general hypotheses listed above. Subterranean habitats predominate on the continental and ocean margins. Considering the continental earth, 97% of all unfrozen freshwater is subsurface, whereas lakes and rivers represent less than 2%. Terrestrial subterranean habitats encompass the whole unsaturated zone (vadose zone) of underground, most evident in karstic areas (caves, fissures, cracks, etc.), which represent nearly 4% of the rock outcrops of the world. Because they develop in rocks or sediments that protect them against surface environmental changes, these subterranean ecosystems, in contrast to most surface ecosystems which are short-lived (rivers, wetlands, or forests), may persist relatively unchanged for millions of years. In the last two decades groundwater ecology has developed rapidly, forming an important branch of limnology (Stanford and Simon 1992, Gibert et al. 1994, Stanford and Valett 1994, Danielopol et al. 1999). Recent literature has focused on general characteristics of subterranean ecosystems and interac-

Phylogeography of a subterranean amphipod reveals cryptic diversity and dynamic evolution in extreme environments

Extreme conditions in subsurface are suspected to be responsible for morphological convergences, and so to bias biodiversity assessment. Subterranean organisms are also considered as having poor dispersal abilities that in turn generate a large number of endemic species when habitat is fragmented. Here we test these general hypotheses using the subterranean amphipod Niphargus virei. All our phylogenetic analyses (Bayesian, maximum likelihood and distance), based on two independent genes (28S and COI), revealed the same tripartite structure. N. virei populations from Benelux, Jura region and the rest of France appeared as independent evolutionary units. Molecular rates estimated via global or Bayesian relaxed clock suggest that this split is at least 13 million years old and accredit the cryptic diversity hypothesis. Moreover, the geographical distribution of these lineages showed some evidence of recent dispersal through apparent vicariant barrier. In consequence, we argue that future analyses of evolution and biogeography in subsurface, or more generally in extreme environments, should consider dispersal ability as an evolving trait and morphology as a potentially biased marker.

A Perspective on the Permeability of the Surface Freshwater-Groundwater Ecotone

The ecotone concept is used to consider exchanges of matter and energy between groundwater and surface water systems. Ecotones control these exchanges by acting as a combination of three types of filter: the contrast between permanent darkness and day-night alternation produces a photic filter; the matrix of interstitial underground systems creates a mechanical filter which slows down water velocity; biological and chemical processes, which often occur simultaneously, define a biochemical filter. The permeability of these filters changes according to water velocity. Thus, for ecotones between a surface system and a porous aquifer with large substrate pore size, dynamics of permeability are governed by hydrology, whereas between a surface system and a partially clogged aquifer with small substrate pore size, they are governed by aerobic and anaerobic processes. According to the degree of permeability, ecotones can be classified as either permanent or temporary sinks for the two adjacent systems. In the latter case, matter fluxes are retained in the ecotone, often undergoing transformation before being released to one of the ecosystems.

Response of invertebrate assemblages to increased groundwater recharge rates in a phreatic aquifer

Abstract Low organic matter supply and reduced spatiotemporal heterogeneity severely constrain biodiversity in groundwater. At the aquifer scale, spatial variation in the groundwater recharge rate is considered a key factor in generating groundwater patches with distinctly different food supplies and spatiotemporal heterogeneity. Our study addressed the role of groundwater recharge in sustaining biodiversity in a phreatic aquifer by testing differences in the density and species richness of invertebrate assemblages among 11 reference sites and 13 sites artificially recharged with storm water. The vertical distribution pattern of invertebrate assemblages was examined using well clusters at one shallow water-table recharge site and one nearby reference site. Groundwater recharge elevated dissolved organic C (DOC) concentrations in groundwater and increased spatiotemporal physicochemical heterogeneity. The thickness of the vadose zone (VZT) controlled DOC input to groundwater at recharge sites but did not reduce spatiotemporal heterogeneity. The higher density and richness of invertebrate assemblages at the well-cluster recharge site than at the well-cluster reference site also was controlled by VZT, suggesting that organic matter supply was a primary factor determining biodiversity patterns in groundwater. Invertebrate density increased and species composition shifted with increasing depth below the groundwater table at the shallow water-table well-cluster site. Some taxa, including several epigean species, preferentially occurred near the water table, whereas others, including several hypogean species, colonized deeper groundwater layers.

Distribution of Dissolved Organic Carbon and Bacteria at the Interface between the Rhône River and Its Alluvial Aquifer

To understand the efficiency of interstitial habitats in the elimination of organic matter as it moves from surface water to groundwater (bank filtration), we studied spatial and temporal variations of sediment organic matter concentration, biodegradable (BDOC) and refractory (RDOC) fractions of the dissolved organic carbon, bacterial abundances, and microbial enzymatic activity in the first metre of sediment of the Rhône River immediately downstream of a large city. The study area was fed most of the year by the surface water inflow (downwelling area), because of groundwater pumping wells located ∼ 80 m from the river. Decreasing gradients from surface water to deep sediments and from the river to the shore were observed in most of the cases for the four variables. The decrease in RDOC concentrations did not vary seasonally (this decrease is probably due to physical process, such as adsorption on fine mineral particles), whereas decreases in BDOC concentrations only occurred when microbial enzymatic activities were high; BDOC is rapidly assimilated by microbial communities. Physical and biological processes together make this first metre of sediment an efficient filter for organic matter.

Groundwater/surface water ecotones : biological and hydrological interactions and management options

Part I. Introduction Part II. Function of Groundwater/Surface Water Interfaces Part III. Malfunction of Groundwater/Surface Water Interfaces Part IV. Management and Restoration of Groundwater/Surface Water Interfaces Part V. Conclusion.

Incorporating ecological perspectives in European groundwater management policy

Implementation of the European Union (EU) Water Framework Directive (WFD 2000) and its planned supplement ‘Directive on the Protection of Groundwater against Pollution’ (EU GWD [European Union Groundwater Directive] 2003) demands an initial characterization of all groundwater bodies by national and regional authorities. The main criteria considered in the Directives to define the groundwater (GW) status are quality (chemical) and quantity, but there is the obvious omission of ecological perspective in these. A directive for a comprehensive policy dealing with GW protection at the level of the EU is a prerequisite for human welfare. Additionally, recognition of GW-dependent ecosystems (GDEs) in the EU is equally important for their sustainable management. GW management and policy should recognize the ecological functions of GW and their interactions with GDEs. As they can stretch across national and regional boundaries, political recognition of their importance and the necessity for ecological consistency of management plans have to be emphasized. During the last three decades, a number of researchers have focused their attention on the dynamics and functioning of GW ecosystems and this work could serve as a basis for identifying impacts of changes in key attributes of GDEs. Detailed aspects of GW ecosystems have been comprehensively dealt with (Gibert et al . 1994; Wilkens et al . 2000; Griebler et al . 2001; Danielopol et al . 2003). Changes in key attributes of GW ecosystems have had consequences for the environment in the EU and USA (Klijn & Witte 1999; Winter 1999; Sophocleous 2002; Winter et al . 2003).

Obligate groundwater fauna of France : diversity patterns and conservation implications.

We examined taxonomic and geographic patterns of the obligate groundwater fauna (i.e. stygobiotic fauna) by assembling in a distributional data base all species occurrences reported from France since 1805. A simulated annealing algorithm was used to identify conservation targets. Until the 60s, biological surveys were restricted to caves but the proportion of sampling sites in unconsolidated sediments increased from 1 to 16% over the last 40 years. A total of 380 species and subspecies in 40 families were collected, 70% of which being restricted to France. As observed in other temperate regions, the stygobiotic fauna was dominated by crustaceans (65% of species) and molluscs (22%). The cumulative number of species did not level off over time, clearly showing that biodiversity was underestimated. Temporal trends in the cumulative number of obligate groundwater and surface water species suggested that groundwater comprised more crustaceans than surface freshwater. Endemism was high although the geographic range size of species increased as distributional data accumulated. Of 380 species, 156 were known from a single 400-km2 cell, among which 73% were located in the southern third of France. The distribution map of species richness changed dramatically over time, indicating that the location of richness hotspots was sensitive to sampling effort. Less than 2% of the French landscape was needed to capture 60% of known species. Thus, a large proportion of species could be protected by focusing habitat conservation efforts on a few complementary species-rich aquifers located in distinct regions.

Ecosystem engineering at the sediment-water interface: bioturbation and consumer-substrate interaction.

In soft-bottom sediments, consumers may influence ecosystem function more via engineering that alters abiotic resources than through trophic influences. Understanding the influence of bioturbation on physical, chemical, and biological processes of the water–sediment interface requires investigating top-down (consumer) and bottom-up (resource) forces. The objective of the present study was to determine how consumer bioturbation mode and sediment properties interact to dictate the hydrologic function of experimental filtration systems clogged by the deposition of fine sediments. Three fine-grained sediments characterized by different organic matter (OM) and pollutant content were used to assess the influence of resource type: sediment of urban origin highly loaded with OM and pollutants, river sediments rich in OM, and river sediments poor in OM content. The effects of consumer bioturbation (chironomid larvae vs. tubificid worms) on sediment reworking, changes in hydraulic head and hydraulic conductivity, and water fluxes through the water–sediment interface were measured. Invertebrate influences in reducing the clogging process depended not only on the mode of bioturbation (construction of biogenic structures, burrowing and feeding activities, etc.) but also on the interaction between the bioturbation process and the sediments of the clogging layer. We present a conceptual model that highlights the importance of sediment influences on bioturbation and argues for the integration of bottom-up influence on consumer engineering activities.

Groundwater Macrocrustaceans as Natural Indicators of the Ariège Alluvial Aquifer

The macrocrustacean fauna from an alluvial aquifer in the French Pyrenees was investigated using 15 wells scattered over the floodplain. Wells were sampled at three contrasting periods for faunal, phy-sical and chemical variables to investigate relationships between amphipod, isopod distributions and environmental factors. The assemblages, dominated by stygobite amphipod species, showed an overall persistence between the sampling dates. Groundwater quality exhibited between-site variations related to agriculturally-induced pollution. Low correlations existed between water quality and fauna distribution patterns. At the scale of the aquifer, groundwater contamination did not influence the macrocrustacean distribution. The spatial distribution of amphipods and isopods was clearly related to hydrogeology and hydrodynamics: strong relationships were found between the assemblages composition and the hydrological context of the stations (wells from the alluvial plain and from the alluvial terrace). With respect to spatial and temporal scales and providing adequate sampling methods, macrocrustaceans then constitute natural indicators of groundwater hydrodynamics in the Ariege aquifer.