Browsing by Author "Silva, A. J."
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- Acoustic detection of bubbles in a pond covered by the seagrass Cymodocea nodosaPublication . Felisberto, Paulo; Silva, J. P.; Silva, A. J.; Jesus, Sergio; Olivé, Irene; Santos, Rui; Quental-Ferreira, H.; Pousao-Ferreira, P.; Cunha, M. E.This paper describes two experiments conducted in a pond covered by the seagrass Cymodocea nodosa at the Aquaculture Research Station of the Portuguese Institute for the Sea and Atmosphere in Olhao, Portugal, aiming at developing acoustic methods to assess oxygen production of seagrasses. The first experiment was carried out in July covering two days, when warm water and high photosynthetic rates give a high probability of oxygen supersaturation in water. The second experiment was carried out in late October, covering a period of 10 days, when seagrass productivity was expected to be lower than in July given the low irradiance and photoperiod. In the July experiment the high attenuation of low frequency pulses and broadband water pump noise (< 20 kHz) in the afternoon is ascribed to bubbles formation during oxygen supersaturation conditions. This hypothesis is coherent with the significant increase of the backscattering level, as measured by an acoustic backscatter system operating at 0.5, 1, 2, 4 MHz. Both, the attenuation of low frequency signals and backscattering level are correlated with oxygen supersaturation in water as measured by an optode. In the October experiment, when only water pump noise was acquired, the acoustic variability that can be related to photosynthetic activity was much weaker, nevertheless the attenuation shows a diurnal pattern correlated with the dissolved oxygen. The results suggest a significant release of oxygen as bubbles during photosynthesis, and therefore the potential contribution of acoustic methods to assess oxygen production of seagrass ecosystems.
- Nutrient concentrations in coastal waters: impact of the Guadiana RiverPublication . Cravo, Alexandra; Madureira, M. A.; Rita, F.; Silva, A. J.; Bebianno, Maria JoãoWater samples were collected over an area of 900 km(2) adjacent to the outflow of the Guadiana River in January 1999 to characterize the concentrations and spatial distribution of nutrients (ammonium, nitrite, nitrate, phosphate and silicate) in the water column. The purpose of the study was to characterize a winter situation of low discharge in terms of nutrient concentrations in the coastal area adjacent to the Guadiana outflow, before the reduction of the river flow due to the completion of the Alqueva Dam. The results show that the maximum influence of the Guadiana outflow was close to the mouth of the Guadiana River, where the highest nutrient concentrations and the minimum of salinity were registered. At the surface, the nutrient concentrations decreased gradually as the distance from the coast increased. The influence of the Guadiana outflow at the surface, despite being small, was evident in the area of about 90 km(2) that extends westwards from the mouth of the river. In this area, the increase in N compounds was more significant than in P and Si. The vertical influence of the outflow of the Guadiana River, until over the isobath lower than 30 m, was reflected in nutrient concentrations that decreased with the increase in depth. However, when the depth of the water column was greater than 30 m, the nutrient concentrations increased with the increase in depth. It is expected that with the completion of the Alqueva Dam, the outflow of the Guadiana River will be reduced even further. Nutrient concentrations will also be reduced and the influence of the river in the coastal zone will be even smaller. This could have a negative impact on the nutrient biogeochemical cycles and on the overall productivity of the area.