THE POSITIVE EFFECTS OF GLOBAL WARMING ON ESTUARIES

ABSTRACT

With the general trend of global warming which has lead to increasing sea level rise, the negative impacts for estuaries & the people living in close proximity have become clear. The subject material, however, highlights the beneficial aspects these changes may bring about. The possibility of harnessing increased tidal range as a renewable energy source is worth researching & modelling in detail. Opportunities also exist for agriculture & aquaculture, as well as economic benefits of safer navigable passages. A greater understanding of estuaries as an ecosystem may lead to the realisation of more benefits, which can be utilised in the future.

INTRODUCTION

In the last century there has been a trend of Overall global warming, this has reached a peak in recent decades & is currently increasing rapidly. There are many views as to what causes this warming, including Milankovitch variables & anthropogenic influences. As global warming continues so eustatic sea level rises through both increased volume of water as the ice sheets melt, & through thermal expansion. Estuaries are extremely sensitive to changes in sea level as this can drastically alter the dynamic balance, which is tentatively retained; though estuaries never achieve equilibrium. Once initiated global warmnig shows a positive feedback & thus cannot be reversed, therefore it is important to realise the benefits of inundation for estuaries.

EFFECTS OF SEA LEVEL RISE ON ESTUARIES

As the sea level rises estuaries will widen & deepen & tidal penetration upstream will be extended.Global warming leads to a more dynamic atmosphere with an increase in the amount of water vapour circulating within the hydrologic cycle. Schnieder & Rosenburg (1989) predict an increase in average global precipitation of 7- 15% with a 5-10% increase in evapotranspiration. A warmer atmosphere produces a warmer hydrosphere, causing thermal expansion of the ocean’s surface layer, which accounts for the greatest proportion of sea level rise. An increase of 1°C throughout the depth of the ocean would cause a sea level rise of ∼1m. In the tropics it is predicted that the top 100m would expand by 10cm. Estuaries are more common during sea level high stands, ‘If sea level rises, estuaries will start to form in the upper valleys of present day rivers’ Dyer (1998).

Despite the obvious adverse effects for the human population of a sea level rise, over two billion people living on the edge of estuaries, it is important to look at the benefits for the estuaries & the marine life involved. In the Clyde estuary spring tides & strong westerly winds can cause water levels up to 5m higher than normal high tide water level. The estuary is seen to widen & deepen increasing in size by800 Ha with sea water penetrating further upstream. This can be used to indicate what would happen in a sea level highstand.

The Bay of Fundy provides an historic record of similar events. In the past three thousand years mean high water level has risen by approximately 7.5m. During rapid incursion the higher salt marsh species, (mainly Scirpus maritimus, Spartina patens) were replaced by low marsh species (mostly Spartina alterniflora). In slower periods oftransgression the higher salt marsh species returned. Sediment underlying the marsh is 98% inorganic, past accretion having dependedon an offshore external sediment supply. Assuming is still available there is no reason that the salt marshes in the Bay of Fundy should not be able to maintain themselves throughout another accelerated sea level rise. Raised beaches, fossil cliff lines & wave cut platforms are other evidence of previous sea level rises. Indeed in tropical ecosystems reefs require a warm climate so warmer temperature means more reefs so more lagoon estuaries. Sea level may have been up to 35m higher during some parts of the Pleistocene, Goudie (1990). These examples show that estuaries have previously recovered to their present form following transgression. Regionally the greatest increases in sea level occur at low latitudes with increases of 30cm recorded & smaller rises of 10-17cm for mid latitudes.

Thus latitudinal position is important, estuaries between 20°N & 20°S most affected by sea level rises. Effects also depend on individual estuaries, their shape, size, the geology of the drainage basin feeding the estuary & its sediment supply. The use of the land adjacent & the anthropogenic uses of the estuary are all critical factors.

PHYSIOLOGICAL BENEFITS OF SEA LEVEL RISE ON ESTUARIES

An eustatic sea level rise would lead to the widening & deepening of the channels of most estuaries; this may regenerate those which have shallowed due to increased sedimentation of anthropogenic origin. This is not only beneficial to safer navigation, but in commercial waterways deep channels are needed to maintain the thoroughfare of increasingly large vessels into harbours thus boosting the local economy. Less dredging would be needed & this would protect the benthos allowing them to recover from previous episodes of waterway maintenance. Penetration of the seawater further inland will lead to salinisation of fresh water habitats, inter-tidal mud flats would be drier for shorter periods at low tide & vegetation fringing the edges of the estuary would change to more salt tolerant species benefiting the xerohytic species.

Estuaries are predicted to adapt, sediments from fresh water tributaries compensating for the increased water in the hydrologic cycle, through greater sedimentation from increased surface runoff. Thus sand & mud deposits may grow compatibly with sea level rise at the edges of the estuary, preserving the mud banks & natural wave defences may maintain themselves, retaining safe havens & feeding areas for seabirds. In salt marshes on the East Coast of America sedimentation has kept up with sea level rise due to the trapping of sediment by vegetation during periods of flooding. The flooding of land close to the estuary during the wet months may replenish nutrients stripped from the soil by intensive agriculture. Aquaculture may also flourish in the shallow environments created at the edges of the estuary; kelp farms could possibly be set up. The harnessing of tidal power is also an option. Estuaries are high-energy environments, large volumes of water passing through narrow channels creating high current velocities. Barrages are often coupled with the disadvantage of reduced tidal range, but in a sea level highstand this could be used to stabilise the increased tidal range allowing the estuary to adapt at its slower pace. Because the estuary is deeper the problem of increased sedimentation caused by the barrage may not pose such a problem. This is a clean way of producing power & may reduce the pollution of the estuary previously caused by other power sources. La Rance tidal power station in Northern France is an example of this.

DISCUSSION

Evidence of past sea level high stands is abundant in the geological record, unpublished research from Chick, (1999), shows pollen samples, dendrochronology, drift, & ratios which support this. Inundation & irrational flooding of estuarine areas may deter people from living too close thus reducing anthropogenic harm to the area. This has made humans more aware of estuaries, leading to greater research concerning their conservation, which can only be a good thing. The environmental pressure on organisms may serve to promote evolution, inhibited by environmental equilibrium, also allowing opportunistic species to flourish. Indeed secondary effects of climate changes in such disturbance regimes favour some species over others & merely lead to a change in the species composition of ecosystems. Increased CO2 rates due to global warming may increase the productivity & efficiency of water use in some plant species.

The controlled flooding of an area could serve as a test to see whether species will migrate, as well as the manual movement of such species to check resilience & resistance. This would enable us to gather more information as to the likely consequences of sea level rise in an estuary. The awareness of our own fallibility may serve to better educate us to the needs of the surrounding environment. Most negative affects of a rise in sea level are from a human viewpoint. The acceptance of a sea level rise may be our only option. Stone, (1974) suggests that natural landscapes should have legal rights. Concrete sea defences hinder the natural migration of species away from the rising water. A balance of the economic costs of development against the ecological affects to a coastal ecosystem home to plants & creatures other than mankind should be struck.

CONCLUSION

The issue of global warming has been one of media interest & scientific enigma for many years. Much has been made of the negative impacts of this phenomenon to our planet, but little has been said of the positive impacts. In terms of estuaries these can be categorised into the following fields. Primarily the harnessing of tidal power as a renewable energy source may reduce the need for other forms of power which have detrimental effects on the environment, as well as stabilise an increasing tidal range, allowing more time for adaptation in the estuary.

A sea level rise will widen & deepen estuaries benefiting navigation hence boosting local economy. Agriculture & aquaculture could also benefit from the creation of new habitats & deposition of nutrients through flooding. Increased sedimentation could aid in retaining their ecosystem with the rising sea level, thus preserving many habitats & acting as a natural coastal defence. Possibly the most important influence of global warming on estuaries for the future is the research it has provoked into this ecosystem. A further understanding of the impacts of a global warming on estuaries will enable more effective conservation in the future & the realisation of the importance of such habitats.