Mondego Estuary Description

The Mondego River drains a hydrological basin of approximately 6670 km2 at the western coast of Portugal. Urban wastewater is still discharged into the Mondego without treatment, and the estuary supports industrial activities, desalination ponds, and aquaculture. Additionally, the lower Mondego River valley has about 15,000ha of farming fields (mainly rice paddies), with a significant loss of nutrients to the estuary (Marques, 1989).

The Mondego Estuary is located in a warm/temperate region with a basic mediterranean temperate climate. It consists of two arms, north and south (Figure 9.13) separated by an island. The two arms splits in the estuarine upstream area about 7 km from the sea, and join again near the mouth. These two arms of the estuary present very different hydro-graphic characteristics. The north arm is deeper (5-10m during high tide, tidal range about 2-3 m), while the south arm (2-4m deep, during high tide) is almost filled with silt in the upstream areas, directing most of the freshwater through the north arm. The water circulation in the south arm is controlled by tidal circulation and the relatively small fresh water input from the tributary, the Pranto River, which is controlled by a sluice located 3 km from the confluence with the south arm of the estuary. Due to differences in depth, the tidal excursion is longer in the north arm, causing daily changes in salinity to be much stronger, whereas daily temperature changes are highest in the south arm (Marques, 1989).

Seasonal intertidal macroalgae blooms (mainly of Enteromorpha spp.) have been reported in the south arm of the estuary for several years (Marques et al., 1993a, b) and Zostera noltii beds, which represent the richest habitat with regard to productivity and biodiversity, are

Figure 9.13 Map of Mondego River showing the field areas and the division of the river into the north and south arms.

being drastically reduced in the south arm of the Mondego estuary, presumably out competed by Enteromorpha (Rafaelli et al., 1991). The physical data are listed in Table 9.12.

Nutrient loading into the south arm of the estuary was estimated, assuming that the major discharge is through the Pranto River, from the Armazens channel (there is no freshwater discharge but, in each cycle, the tidal wave washes out the channel, where several industries discharge waste waters), and from the downstream communication of the south arm. The only way out of the system is the downstream communication (Figure 9.13). The nutrient inputs from the Pranto River and Armazens channel, and the exchanges (input vs. output) in the downstream communication of the south arm have been monitored from May 1993 to June 1994.

Table 9.12 Physical parameters of Mondego River

Physical parameters

Mondego River

Length and width of estuary (km2)

10, 0.3

Area (km2)

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