6.4.1.3 Estuaries and lagoons

Global mean sea-level rise will generally lead to higher relative coastal water levels and increasing salinity in estuarine systems, thereby tending to displace existing coastal plant and animal communities inland. Estuarine plant and animal communities may persist as sea level rises if migration is not blocked and if the rate of change does not exceed the capacity of natural communities to adapt or migrate. Climate change impacts on one or more ‘leverage species’, however, can result in sweeping community level changes (Harley et al., 2006).

Some of the greatest potential impacts of climate change on estuaries may result from changes in physical mixing characteristics caused by changes in freshwater runoff (Scavia et al., 2002). A globally intensified hydrologic cycle and regional changes in runoff all portend changes in coastal water quality (Section 6.3.2). Freshwater inflows into estuaries influence water residence time, nutrient delivery, vertical stratification, salinity and control of phytoplankton growth rates. Increased freshwater inflows decrease water residence time and increase vertical stratification, and vice versa (Moore et al., 1997). The effects of altered residence times can have significant effects on phytoplankton populations, which have the potential to increase fourfold per day. Consequently, in estuaries with very short water residence times, phytoplankton are generally flushed from the system as fast as they can grow, reducing the estuary’s susceptibility to eutrophication³ and harmful algal blooms (HABs) (Section 6.4.2.4). Changes in the timing of freshwater delivery to estuaries could lead to a decoupling of the juvenile phases of many estuarine and marine fishery species from the available nursery habitat. In some hypersaline lagoonal systems, such as the Laguna Madre of Mexico and Texas, sea-level rise will increase water depths, leading to increased tidal exchange and hence reduced salinity (cf. Quammen and Onuf, 1993).

Increased water temperature could also affect algal production and the availability of light, oxygen and carbon for other estuarine species (Short and Neckles, 1999). The propensity for HABs is further enhanced by the fertilisation effect of increasing dissolved CO₂ levels. Increased water temperature also affects important microbial processes such as nitrogen fixation and denitrification in estuaries (Lomas et al., 2002). Water temperature regulates oxygen and carbonate solubility, viral pestilence, pH and conductivity, and photosynthesis and respiration rates of estuarine macrophytes⁴. While temperature is important in regulating physiological processes in estuaries (Lomas et al., 2002), predicting the ecological outcome is complicated by the feedbacks and interactions among temperature change and independent physical and biogeochemical processes such as eutrophication (cf. Section 6.2.4).

Decreased seawater pH and carbonate saturation (Mackenzie et al., 2001; Caldeira and Wickett, 2005) has at least two important consequences: the potential for reducing the ability of carbonate flora and fauna to calcify; and the potential for enhanced dissolution of nutrients and carbonate minerals in sediments (Andersson et al., 2003; Royal Society, 2005; Turley et al., 2006). As these potential impacts could be significant, it is important to improve understanding of them.

The landward transgression of natural estuarine shorelines as sea level rises has been summarised by Pethick (2001), who adopted a mass balance approach based on an equilibrium assumption resulting in landward retreat of the entire estuarine system. In this view, sea level rise of 6 mm causes 10 m of retreat of the Blackwater estuary, UK, and only 8 m of retreat for the Humber estuary, UK, due to the steeper gradient of the latter. The Humber estuary will also likely experience a deepening of the main channel, changes in tidal regime and larger waves that will promote further erosion around the margins (Winn et al., 2003). In Venice Lagoon, Italy, the combination of sea-level rise, altered sediment dynamics, and geological land subsidence has lowered the lagoon floor, widened tidal inlets, submerged tidal flats and islands, and caused the shoreline to retreat around the lagoon circumference (Fletcher and Spencer, 2005). In situations where the area of intertidal environments has been reduced by embanking or reclamation, the initial response will be a lowering of remaining tidal flats and infilling of tidal channels. Depending on tidal characteristics, the availability of marine sediment, and the rate of sea-level rise, the remaining tidal flats may either be further drowned, or their relative level in the tidal frame may be maintained, as shown by several tidal basins in the Dutch Wadden Sea (Dronkers, 2005).

A projected increase in the intensity of tropical cyclones and other coastal storms (Section 6.3.2) could alter bottom sediment dynamics, organic matter inputs, phytoplankton and fisheries populations, salinity and oxygen levels, and biogeochemical processes in estuaries (Paerl et al., 2001). The role of powerful storms in structuring estuarine sediments and biodiversity is illustrated in the stratigraphic record of massive, episodic estuary infilling of Bohai Bay, China during the Holocene, with alternating oyster reefs and thick mud deposits (Wang and Fan, 2005).