Browsing by Author "Silva, J. P."
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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.
- Tank and field experiments of short-range acoustic propagation through a seagrass canopyPublication . Felisberto, Paulo; Silva, J. P.; Jesus, Sergio; A.J. Silva; Silva, A.T.; Silva, J.; Santos, R.; Ferreira, H.; Pousao, P.Aiming at monitoring the bubbles released during seagrass photosynthesis in different conditions, three experiments were made: two in an outdoor tank and one in the field. The outdoor tank experiments were conducted in winter and summer conditions with Cymodocea nodosa plants transplanted from a nearby meadow. The field experiment took place in September 2018 on a Posidonia oceanica meadow, where bubbles formation due to plants photosynthesis has been reported. In all the experiments continuous wave pulses in the band 1020 kHz were used to estimate the diel change of effective sound speed and the attenuation over the plants. Additionally, high frequency backscatter images at 0.5, 1, 2 and 4 Mhz were obtained to figure out bubbles signature. The results suggest that even in winter, the tank conditions, without water movement, allows for the formation of a small amount bubbles of small radii. In summer tank conditions, the bubbles formation due to photosynthesis of seagrasses significantly increases, but also algae may contribute for the bubbles formation in the environment. The field experiment was carried out after the peak of a bad weather event. There was no evidence of bubbles formation, what can be related to water movement and leaves stirring, that prevents bubbles formation.