Spatial and Temporal Variability of Suspended Sediments Concentration in Ria de Aveiro Lagoon and Fluxes between the Lagoon and the Ocean INTRODUCTION Located in the transition between the Vouga river basin and the Atlantic Ocean, the Ria de Aveiro coastal lagoon is influenced by marine and fluvial processes that print their signature in the fine-grained sediment dynamics. Investigations about fine-sized matter in estuaries and coastal lagoons show that the suspended sediments concentration is extremely variable and may vary regionally, with season and with tidal amplitude and phase ( ., 1998; ., 2000). Seasonal fluctuations may result from variations in rivers discharge, wave and wind climate. Under tidal influence estuarine fine sediments are moving forward and backward, participating in processes of deposition and resuspension, which are dependent on tidal currents strength ( , 1996). Semidiurnal variations in the concentration of the Ria de Aveiro suspended sediments have already been reported by (1994) and (1995), but just for the northwestern channels of the lagoon. A two-dimensional depth-integrated transport model for cohesive suspended sediments has also been applied to simulate tidal evolution of suspended sediments in the lagoon by (2000) and indicated that sediments concentrations show spatial, semidiurnal and fortnightly variability. The purpose of this work is to study the suspended sediments concentration spatial and temporal variability in the central area of the Ria de Aveiro lagoon and to determine the magnitude and sign of water fluxes and suspended sediments between the lagoon and the ocean. Ria de Aveiro is a shallow well-mixed estuary-coastal lagoon, located in the northwest of Portugal. The lagoon is 45 km long and has a maximum width of 10 km (Figure 1), covering an area of 83 km and 66 km , at the highest and lowest tide, respectively. The average depth in the lagoon is about 1 m, with exception for the navigation channels where dredging operations maintain a depth of about 7 m ( ., 2000). This lagoon, which is connected with the Atlantic Ocean through a narrow entrance (1.3 km long, 350 m wide and 20 m deep), has a complex topography characterized by extensive intertidal zones and four main channels: S. Jacinto, Ílhavo, Mira and Espinheiro. FETTWIS RIDDERINKOF VAN LEUSSEN SILVA PEREIRA LOPES DIAS et al et al et et al al. SETTING 2 2 Journal of Coastal Research SI 39 718 - 723 ICS 2004 (Proceedings) Brazil ISSN 0749-0208 I.Abrantes†; J. M. Dias‡ and F. Rocha† † MIA (FCT) Dept. of Geosciences University of Aveiro 3810-193 Aveiro, Portugal isabel.abrantes@clix.pt. frocha@geo.ua.pt. ABRANTES, I.; DIAS, J. M. and ROCHA, F., 2006. Spatial and temporal variability of suspended sediments concentration in Ria deAveiro Lagoon and fluxes between the lagoon and the ocean. Journal of Coastal Research, SI 39 (Proceedings of the 8th International Coastal Symposium), 718 - 723. Itajaí, SC, Brazil, ISSN 0749-0208. The measurements of suspended sediment concentration in different parts of the Ria deAveiro coastal lagoon during four tidal cycles show a significant spatial and temporal variability. Fluxes of water and suspended sediments between the lagoon and the coastal ocean were estimated for each tidal cycle applying a hydrodynamic two- dimensional numerical model and using the values measured at the lagoon mouth. The hydrodynamics of the lagoon are essentially dominated by tidal forcing. Tides are predominantly semidiurnal and are present in the entire lagoon The differences in suspended sediments concentration within the lagoon seem to be largely determined by the magnitude of the currents and by the bottom properties. As in other estuarine systems, the suspended sediments concentration fluctuated with tidal amplitude. In almost all stations, higher mean values occur during spring tides due to the strength of the tidal currents. Anomalous higher mean values observed in stations close to the tidal inlet during winter neap tide suggest that wind-induced resuspension and horizontal advection from the adjacent coastal area is taking place during that period of the year. Semidiurnal variations are explained, especially during the summer, by tidal-current velocity asymmetry (ebb dominance). Seasonal variability, with winter higher suspended sediments concentration, is probably related with wind climate, biological activity and coastal wave regime. In general, the fluxes of water and of suspended sediments between the lagoon and the coastal ocean showed a seaward flux that is consistent with the ebb dominance. Under winter neap tide conditions external factors allowed retention of suspended sediments in the lagoon. ADDITIONAL INDEX WORDS: Resuspension, deposition, advection. ABSTRACT ‡ Dept. of Physics University of Aveiro 3810-193 Aveiro, Portugal jdias@fis.ua.pt. Figure 1. Ria the Aveiro with sampling stations and plots of SSC mean concentration (in the legend, between parentheses are indicated the station number). Journal of Coastal Research Special Issue 39, 2006 ,