Estuarine biodiversity as an indicator of groundwater discharge

Abstract

Communities located in the interface between marine/brackish and freshwater habitats are likely to be early responders to climatic changes as they are exposed to both saline and freshwater conditions, and thus are expected to be sensitive to any change in their environmental conditions. Climatic effects are predicted to reduce the availability of groundwater, altering the hydrological balance on estuarine-aquifer interfaces. Here, we aimed to characterise the estuarine faunal community along a gradient dependent on groundwater input, under a predicted climatic scenario of reduction in groundwater discharge into the estuary. Sediment macrofauna was sampled along a salinity gradient following both the wet and dry seasons in 2009. Results indicated that species abundance varied significantly with the salinity gradient created by the groundwater discharge into the estuarine habitat and with sampling time. The isopode Cyathura carinata (Krøyer, 1847) and the polychaetes Heteromastus filiformis (Claparède, 1864) and Hediste diversicolor O.F. Muller, 1776 were associated with the more saline locations, while oligochaeta and Spionidae were more abundant in areas of lower salinity. The polychaete Alkmaria romijni Horst, 1919 was the dominant species and ubiquitous throughout sampling stations. This study provides evidence for estuarine fauna to be considered as a potentially valuable indicator of variation in the input of groundwater into marine-freshwater interface habitats, expected from climatic pressures on aquifer levels, condition and recharge rates. For instance, a reduction in the abundance of some polychaete species, found here to be more abundant in freshwater conditions, and increasing Oligochaeta found here on higher salinities, can potentially be early warnings of a reduction in the input of groundwater into estuaries. Estuarine benthic species are often the main prey for commercially important fish predators such as in our case study, making it important to monitor the aquatic habitat interfaces taking into consideration the estuarine macrobenthos and groundwater availability in the system.

Introduction

Freshwater ecosystems have been important sources for the development of environmental monitoring programmes (De Pauw et al., 1992), but these solutions are hardly applicable to the very variable tide-related characteristics of the brackish estuarine environment such as salinity variation. Hence, there is a necessity of developing rapid tools for monitoring the marked estuarine environmental gradients such as salinity, particularly for habitats which are subject to groundwater influence (McKenna and Martin, 2004), as this key resource is under severe climatic and human pressure (Danielopol et al., 2003).

The evaluation of shifts in the species presence on an ecosystem is a valid strategy for environmental monitoring (Van Hoey et al., 2010), because the life cycle of organisms integrates alterations on the environmental characteristics in a relatively wide period of time. This approach overcomes the limitations of an evaluation of the environmental condition singly based on physical–chemical parameters, which does not considers the consequences of environmental alterations on the ecosystem (Cairns et al., 1993). Management plans which include biological indicators are advantageous because they incorporate the conjugated action of environmental conditions and make the impact easily detectable. Biological indicators also allow evaluating factors not directly measurable, such as biological complexity and ecological value and, no expensive laboratorial chemical analysis are required such as for chemical indicators (Ambrogi and Forni, 2004).

Biological indicators can be defined as the presence or abundance of some taxa which may be considered to represent biodiversity of a large assemblage, of species living in a particular habitat or the presence or abundance of other taxa (MacNally and Fleishman, 2002). They are expected to be useful in identifying areas that warrant special protection and specific conservation measures, and are based on the presence of specific taxonomic groups. Within an aquatic ecosystem, the benthic fauna is often used as indicator of environmental changes and considered to be: (i) key precursor of the implementation of the Water Framework Directive to the very recent implementation of the Marine Strategy Framework Directive (Van Hoey et al., 2010) and; (ii) the most adequate descriptor of estuarine habitats because organisms are mostly sedentary and thus, reliable local indicators over time of environmental conditions (Warwick, 1993).

Groundwater is currently considered a key resource under severe threat due to human consumption, pollution and climatic pressures (Danielopol et al., 2003). Groundwater is also regarded as the primary provider for human freshwater demands such as agriculture and industry (Santos et al., 2001, Danielopol et al., 2003). In southern Portugal, where the present study was conducted, groundwater represents 60% of freshwater human usage and nearly 80% of agricultural demand. These demands are likely to increase in the future as a result of global warming (Santos et al., 2001).

Estuaries often interface with coastal aquifers and receive groundwater discharge, implying that communities therein are subject to changes in the salinity due to both the marine and freshwater influences. The groundwater discharge into estuarine habitats may be reduced by the climatic-driven pressures on its availability, and thereby induce changes the ecosystem stability. The interface or border habitats such as estuaries and wetlands are considered to be highly vulnerable to alterations in variables such as salinity, sediment and nutrient availability (Bates et al., 2008). This is the case of the estuary associated with the Arade River at the south coast of Portugal, a system of ecological and economic importance. The estuary permanently receives groundwater from the Querença-Silves aquifer, the largest and most productive aquifer in the south of Portugal. If the average temperature rises in the near future as predicted in climate change scenarios for this region (Santos et al., 2001), more drought periods will occur. In these scenarios, the groundwater withdraws will rise and its output to adjacent habitats such as estuaries will be reduced and ecosystem stability be altered (Ranjan et al., 2006). Coastal aquifers such as the Querença-Silves are also highly vulnerable to the threat of saline intrusion due to sea level rise (Ranjan et al., 2006). Hence, the fauna of associated habitats such as the Arade estuary will likely be exposed to increased salinities, becoming important to understand how benthic estuarine species respond in the present day to the groundwater-estuarine interaction. Invertebrate communities of transitional waters (freshwater-saltwater ecotone) are highly influenced by freshwater discharge, showing marked seasonal variation mainly related to salinity fluctuations (Chainho et al., 2006). Alterations in the benthic community will likely have cascade trophic consequences for their predators, which in the studied estuary correspond mainly to commercially important fish species (Cabral, 2000, Gonçalves et al., 2004). The salinity variation related to the groundwater discharge into the estuary and the benthic species tolerances to salinity levels have an important role in determining prey availability for their predators.

Identification of bioindicators demands establishing a causal relationship and this ideally requires in situ manipulative tests (Goodsell et al., 2009). To accomplish this in the context of the present study, differing groundwater availability would have to be supplied to faunal communities and their response (e.g. presence-absence, abundance, etc) measured. Such manipulations would be logistically challenging and are out of scope of the present work. Instead, the aim of the present study was to identify taxonomic groups and/or species of estuarine sediment which can potentially be monitored to detect a reduction in groundwater availability. It was expected that due to discriminating tolerances, benthic species were distributed accordingly to the salinity gradient originated by the freshwater output into the estuary. The null hypotheses tested were that there are no differences: a) in the identity and abundance of species between near and far locations in relation to the point of groundwater input to the sampled estuarine habitat and, b) between the end of the dry and wet sampling times.