Impact of changes in flow of freshwater on estuarine and open coastal habitats and the associated organisms

Abstract

Freshwater is scarce in many regions of the world. In some areas, water entitlements currently exceed the available water supply yet few proposals for regulating quantities extracted are scrutinised in terms of possible impacts or undergo any form of rigorous impact assessment. In addition, there is little understanding of the potential impacts. There is a growing need to understand better the impact of altered flows of fresh water on estuarine and open coastal marine systems. There is a perception that fresh water is lost when it enters the marine environment. We argue that freshwater-saltwater dynamics have profound influences on coastal ecosystems. The purpose of this paper is to review the nature of freshwater discharges and the effects of fresh water on the physical aspects of estuaries as well as estuarine and marine flora, fauna and habitats. Although the review focuses on decreased flows to marine systems, major increases in flow can also have a major impact on estuarine and coastal systems. Freshwater runoff is a function of numerous environmental variables, depending primarily on climate (precipitation and evaporation) and the physical characteristics of the drainage basin. Anthropogenic activities in catchments may result in diversions and reductions in freshwater flow, alterations of timing and rates of flow to estuarine and coastal systems, and/or adverse water quality conditions with major changes in nutrient loading. Sediment loads, pH, temperature, salinity, clarity, oceanography and nutrients are affected. Perturbations in coastal systems can be freshwater pulses (i.e. storms or opening of floodgates) or press scenarios (i.e. persistent flow of low variation from rivers or industry). Impacts on organisms can also be categorised as pulse events (where there is a rapid but not sustained change), or press events (where changes are sustained over long periods of time). Changes to freshwater input affect habitats and organisms within estuaries. The effects include mortality, changes in growth and development, and in some cases movement of organisms. Major mortalities are most likely during pulse events of freshwater input. There is considerable descriptive and small-scale experimental evidence to suggest that a variety of organisms may be affected by changes to freshwater input. Much of the experimental evidence focuses on single factor experiments and rarely have there been multifactorial experiments (an exception is seagrasses). In addition, there have been no large-scale experiments (e.g. size of sample unit 10's to 100's of metres), although it is acknowledged that such experiments will be difficult. We suggest that any changes in water management (e.g. removal of water for irrigation) should be treated as manipulative experiments and that estuarine and marine systems are monitored together with reference or control locations (where there has been no change) to determine the impact of such changes. At the large scale, finding suitable control locations could be difficult. Data should be collected multiple times prior to and after the change has occurred. In the first instance, objective scientific evidence should be used for decision-making and when that is not available, we suggest that the principle of precautionary action should be adhered to. In conclusion, freshwater flows have a great impact on physical and biological aspects of coastal environments. The impacts of fresh water on marine environments, as well as terrestrial environments, should therefore, be considered by managers.