Acoustic detection of bubbles in a pond covered by the seagrass Cymodocea nodosa

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.