RIA FORMOSA   - PORTUGAL

CEMAS - University Fernando Pessoa
Praça 9 de Abril, 349
4249-004 Porto, Portugal

Ria Formosa (Portugal). Ria Formosa is a shallow mesotidal lagoon located at the south of Portugal (Algarve coast fig 1) with a wet area of 10 500 ha (Fig. 2). The lagoon has several channels and an extensive intertidal area mostly constituted by sand, muddy-sand flats and salt marshes (Bettencourt, 1994). Due to the slope, the intertidal area is exposed to the atmosphere for several hours, over each semi-diurnal tidal period. Several sporadic streams drain the watershed during rainfall and an important river (Gilão) supplies fresh water to the lagoon over the year.

Figure 1: Location of the Ria Formosa - Portugal

The sources of treated and untreated domestic waste, as well as agricultural run-off may contribute to episodic eutrophication and changes in the N/P ratios. Salinity remains close to 36 ppt, except during sporadic and short periods of winter run-off (Falcão, 1997). This ecosystem is considered a nursery for a large number of coastal species (Monteiro, 1989), and its extensive inter-tidal areas being used as growth bank of clams, fish farms and salt ponds.

Figure 2: Detailed map of Ria Formosa

The natural biogeochemical cycles of the Ria Formosa are essentially regulated by tidal exchanges with the seawater and by the exchanges with the sediment interface. The tidal amplitude varies from 1 to 3.5 meters meaning that a rather intense exchange of water mass (50 – 75%) occurs during each tide (Sprung, 1994) contributing to the nutrient balance in the ecosystem.

Benthic-pelagic coupling is of particular importance because the water volume that overlies a given area of the bottom is relatively small. The influence of the bottom on the regeneration of nutrients that enrich water column should not be negligible mainly concerning ammonium, phosphate and silicate (Lerat et al., 1990; Kristensen, 1993). Studies developed in this coastal lagoon, demonstrated that its sediments are apparently capable of supplying most of the daily N and P requirements of phytoplankton in the overlying water (Falcão & Vale, 1998).

Other authors demonstrate that rates of net nitrogen mineralization were relatively low during most of the year with a particularly active period from June to August, possibly due to an effect of temperature on soil microbial activity (Cartaxana et al., 1999). This rate of net nitrogen mineralization agrees with the fluxes sediment-water of ammonium and phosphates obtained during summer in Ria Formosa. These fluxes were one order of magnitude higher than those obtained in winter period (Falcão & Vale, 1998). The sharp release/consume of nutrients during the warmer periods may explain the high primary productivity obtained, during spring/summer (20-30 mg C. m^-3. h^-1 ), in this ecosystem (Falcão, 1997).

Figure 3: Ria Formosa-Tavira inlet

This ecosystem is considered a nursery for a large number of coastal species, and its extensive intertidal areas being used for clam and aquaculture ponds. The main economic activities are tourism, fisheries and aquaculture (mainly clams and oysters). The economical annual revenue is the main source of income for approximately 5000 families.

Figure 4: Clam culture (bivalve hand collection, 5000 tons/y)

References

Bettencourt, P., 1994.

Les environnementssedimentaires de la côte sotavento (Algarve, sud Portugal) et leur évolution holocéne et actuelle.Thèse en Geologie Marine. Université de Bordeaux I: 494p

Falcão, M., 1997.

Dinâmica dos nutrientes na Ria Formosa: interacção da laguna com as suas interfaces na reciclagem do azoto, fósforo e sílica. Tese de Doutoramento em Ciências do Mar, pela Universidade do Algarve.

Falcão, M. & Vale, C., 1998.

Sediment-water exchanges of ammonium and phosphate in inter-tidal and sub-tidal areas of a meso-tidal coastal lagoon (Ria Formosa). Hydrobiologia, 373/374: 193-201. 

Kristensen, E., 1993.

Seasonal variations in benthic community metabolism and nitrogen dynamics in a shallow, organic-poor danish lagoon. Est., Coast. and Shelf Sci., vol. 36: 565-586.

Lerat, Y., P. Lasserre & P. Corre, 1990.

Seasonal changes in pore water concentrations of nutrients and their diffusive fluxes at the sediment-water interface. J. Exp. Mar. Biol. Ecol., vol. 135: 135-160.

Monteiro, C.C., 1989.

La faune icthyologique de la lagune Ria Formosa (sud Portugal)-répartition et organisation spatio-temporelle des communautés: application à l'aménagement des ressources.  Thèse de doctorat. Université des Sciences et techniques du Languedoc: 219p.

Sprung, M., 1994.

Macrobenthic sedentary production in the inter-tidal zone of the Ria Formosa - a lagoon in southern Portugal. Est., Coast. and Shelf Sci., vol. 38: 539-558.

Contact Persons

Dr. Manuela Falcão,

INIAP-IPIMAR - National Research Institute for Fisheries and Sea,

CRIPSul- Av. 5 de Outubro 8700-305 Olhão, Portugal.

E-mail: mfalcao@ipimar.ualg.pt

Dr. P. Duarte,

CEMAS- University Fernando Pessoa,

Praça 9 de Abril  349 - 4249-004 Porto,

Portugal.

E-mail: pduarte@ufp.pt