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Abstract

Background. The effluents of the mono-specific aquaculture contain high concentrations of dissolved nutrients and organic matter, which affect negatively water quality of the recipient aquatic ecosystems. A key feature of water quality is its transparency. Chromophoric dissolved organic matter (CDOM) determines most of the light transmission in the ultraviolet and blue bands in the aquatic ecosystems. A sustainable alternative to mono-specific aquaculture is the integrated multitrophic aquaculture that includes species trophically complementary named "extractive" species. Sea cucumbers are recognized as efficient extractive species, with a high potential to improve water quality, due to the consumption of particulate organic matter (POM). However, the effects of sea cucumbers on CDOM are still unknown. Methods. During one year, we biweekly monitored CDOM in two aquaculture tanks with different trophic structure. One of the tanks (-holothurian tank) only contained the primary species, Anemonia sulcata, whereas the other tank (+ holothurian tank) also contained individuals of Holothuria tubulosa and H. forskali. We routinely performed CDOM absorption spectra from 200 nm to 750 nm and determined quantitative (absorption coefficients at 325 nm) and qualitative (spectral slopes and molar absorption coefficients at 325 nm) optical parameters in the inlet waters, in the tanks, and in their corresponding effluents. Results. Absorption coefficients at 325 nm (a 325) and spectral slopes from 275 to 295 nm (S 275-295) were significantly lower in the effluents of the +holothurian tank (average: 0.33 and 16 μm-1 , respectively) than in the effluents of the −holothurian tank (average: 0.69 m-1 and 34 μm-1 , respectively), being the former similar to those found in the inlet waters (average: 0.32 m-1 and 22 μm-1 , respectively). This reduction in CDOM absorption appears to be mediated by the POM consumption by the holothurians. The reduction of POM concentration in the +holothurian tank may weaken the process of POM disaggregation into dissolved organic matter, which ultimately might have generated CDOM in the-holothurian tank. Discussion. Extractive species such as holothurians