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Groundwater-Dependent Ecosystems (or GDEs) are ecosystems that rely upon groundwater for their continued existence. Groundwater is water that has seeped beneath Earth's surface to reside within the pore spaces in soil and rock fractures. This process can create water tables and aquifers, which are large storehouses for groundwater. An ecosystem is a community of living organisms interacting with the nonliving aspects of their environment (such as air, soil, water, and even groundwater). With a few exceptions, the interaction between various ecosystems and their respective groundwater is a vital yet poorly understood relationship, and their management is not nearly as advanced as in-stream ecosystems.

Role of Geochemistry in GDEs
Geochemistry and geochemical methods help scientists to identify GDEs and to better understand how nutrients are introduced to, and cycled, in GDEs. Organisms in GDEs adapt to local geochemistry to gather nutrients. Moreover, because groundwater is devoid of sunlight, some organisms can rely on chemosynthesis. To determine the origins of the chemicals being cycled, geochemists use dissolved gases, trace elements, and stable isotopes to track elements in groundwater. Geochemists also use reactive tracers or isotopic compositions to determine the geochemical variables, such as pH and Redox conditions.

Isotopes
Examining the composition of stable isotopes in the water found in soil, rivers, groundwater, and xylem (or vein systems) of vegetation, shows spatial changes over time in the use of groundwater by the vegetation in it's respective ecosystem. Using mass spectroscopy, masses in a sample are sorted and measured along with groundwater depth and vegetative rooting patterns, showing spatial changes over time in the use of groundwater by the vegetation in its respective ecosystem.

Plant
Groundwater-dependent ecosystems can also be inferred through plant water use and growth. Groundwater reliance, in areas with high rainfall, can be seen by monitoring the water use made by the plants of the ecosystem in relation to the water storage in the soil of the area. If the use of water in the vegetation exceeds that of the water being stored in the soil it is a strong indication of groundwater utilization. In areas of prolonged drought the continuation of water flow and plant growth are highly indicative of a groundwater reliant area.

Remote sensing/Geographical Information Systems (GIS)
Remote Sensing is the scanning of Earth by satellite or aircraft to obtain information. GIS is a system designed to capture, store, analyze and manage geographic data. Together the data collected (such as elevation and bore holes measuring groundwater levels) can very accurately predict where groundwater-dependent ecosystems are, how extensive they are, and can guide field expeditions to the right areas for further confirmation and data collection on the GDEs.

Biodiversity in GDEs
Groundwater ecosystems are host to unique, high diversity fauna. Groundwater organisms from karst systems, hyporheic zone of streams and rivers, and other groundwater habitats represent thousands of described species. Stygofauna are organisms included in groundwater biodiversity, which are of particular interest to scientists because of their adaptation to extreme, low-nutrient environments. Biodiversity within groundwater habitats are commonly attributed and arranged by a variety of regional processes, impacting local groundwater communities. Many studies aim at understanding biodiversity in GDEs and describe highly sensitive ecological communities to shifts in regional processes, driven by, for example, climate change. In addition to regional processes fueling groundwater biodiversity, stygofauna also contributes to groundwater ecosystems by cycling organic matter and other nutrients. Organisms from these environments are poorly understood and underrepresented in scientific research studies. Attempts to bridge this informational gap include research studies looking at groundwater management from an ecological perspective.

Classification of Ecosystems in Groundwater
Due to the numerous diverse ecosystems, and their individual dependency on groundwater, there is some uncertainty distinguishing between GDEs and groundwater-using ecosystems. Each ecosystem expresses a varying degree of groundwater dependency. An ecosystem can be directly or indirectly groundwater dependent, and variation in groundwater use throughout the seasons. There are a variety of methods for classifying different types of GDEs according to their geomorphological setting and/or to their groundwater flow mechanism (deep or shallow), as well as from various ecohydrological characteristics control the rate of groundwater recharge.

Terrestrial
Arid to humid terrestrial environments with no standing water but deeply rooted vegetation relies upon groundwater to support the producers of their ecosystem. The deeply rooted vegetation requires the groundwater to maintain a consistent or semi-consistent level to allow for their continued health and survival.

Springs
Springs, arguably, rely the most heavily on the continued contribution of groundwater because they are a natural discharge from relatively deep groundwater flows rising to the surface. Springs are often in association with uniquely adapted plants and animals.

Wetlands
Wetlands require a shallow discharge of groundwater, it flows as a seepage into depressions in the land surface, in some instances wetlands feed off of perched groundwater which is groundwater separated from the regular water table by an impermeable layer. Marshes are a type of wetland and though not directly reliant on groundwater they use it as an area of recharge. Bogs, are also a type of wetland that is not directly reliant on groundwater but uses the presence of groundwater to provide the area with recharge as well as buoyancy.

Rivers
Rivers collect groundwater discharge from aquifers. This can happen seasonally, intermittently, or constantly, and can keep an area's water needs stable during a dry season.

Environmental Threats
Groundwater ecosystem threats are more easily recognizable as the community structure of these ecosystems become more understood. Globally, there are several known threats to groundwater ecosystems including anthropogenic chemical contamination, improper groundwater management, and climate change.

Pollutants
Increases in populated areas pose a significant threat from pollution to aquatic ecosystems. In many cases, groundwater can become polluted from toxins and nutrients after water seeps down to the water table. Polluted groundwater can have many different effects on related ecosystems. In the case of an estuary in Cape Cod, Massachusetts, an influx of nitrogen from septic tank fields intercepted the groundwater flow path. Increased levels of nitrogen can lead to eutrophication and cause an abundance of plant growth, as well as result in death for a variety of aquatic organisms.

Groundwater Management
The extraction of groundwater in both large and smaller amounts lowers the areas water table, and in too large of quantities can even collapse parts of the aquifer and permanently damage the quantity of water the aquifer can store. In addition, urbanization of land has significant effects on groundwater recharge, deforestation a nd limits the amount of surface area viable for water to actually infiltrate and contribute to the groundwater. These impacts, known and unknown, push the need for sustainability-driven groundwater management.

Climate Change
The influence of global climate change on the hydrological cycle affects groundwater ecosystems by exacerbating climatic concerns. Predicted changes in climate will affect the magnitude and timing of groundwater recharge, which will impact GDEs by perturbing what can be stable environmental conditions in groundwater systems.