Estuaries are some of the most productive areas on Earth. Many species rely on estuaries for their survival. They provide feeding grounds, nursery habitats, and breeding grounds. Humans also depend on estuaries for recreational use, food, and job opportunities.
Some 22 of the 32 largest cities in the world are located on estuaries (Ross 1995). Human activities are causing greater degradation of estuaries and their resources, making them amongst the most threatened ecosystems on Earth (NOAA 2013). The degradation of estuaries reduces the ability of the ecosystem to carry out many functions.
The functioning of estuarine resources include the distributions and abundances of estuarine biota based on how they interact and respond to estuarine conditions and the consequences of those interactions on community structure, food web interactions, rate of primary and secondary production, and material cycling (Alber 2002). Changes in estuarine resources’ functioning occur primarily from changes in freshwater inflows.
Salinity determines the habitat dynamics and in turn the distributions of organisms. This is because differing freshwater inflows in an estuary cause shifting isohalines that affect the vegetation and organism allocation (Alber 2002). The effect of changing locations of intertidal habitats caused by shifting isohalines can lead to implications on the suitability of the new location for benthic organisms. Changes in spatial distribution of critical habitat are therefore important to evaluating changes in freshwater inflows (Sklar and Browder 1998; Alber 2002).
As discussed earlier, although most of the estuarine organisms have a wide range of salinity tolerance, most occur within a focused salinity range depending on the organism’s life history stages. Organisms capable of horizontal and vertical movement such as blue crabs and fish species are also affected by changes in the estuarine salinity structure. Changes in species composition, distribution, abundance, and survival of estuarine organisms are linked to freshwater inflows (Montagna et al. 2013). Also linked to freshwater inflows are migration patterns, spawning habitats, and fish recruitment (Drinkwater and Frank 1994: referenced in Alber 2002).
The timing of freshwater inflows is important for estuarine resources because the delivery of freshwater flows triggers cues in the life histories of many organisms. For instance, shellfish and fish are cued to high spring inflows but changes in inflows can affect spawning and nursery cycles (Alber 2002). In Texas, a study done on the impact of salinity variability on estuarine organisms, a negative correlation was found between the standard deviation of salinity and the density of benthos that showed frequent salinity fluctuations lead to increases in physiological stress (Montagna and Kalke 1992).
The impacts of changes in freshwater inflows and the sediment, organic matter, and nutrients carried in the flows due to upstream activity can affect primary production, secondary production, nutrient cycling and the trophic structures in the estuary (Alber 2002). The relationship between freshwater inflows is a complex and dynamic one, with different trophic levels consequently affecting the other. Next, a brief overview of the trophic system will be given and information regarding primary, secondary, and nutrient cycling will be provided.