## 4 Discussion
The pH, the redox potential and the total organic matter content are used to evaluate the environmental quality of water bodies associated with fish cultured in cage rafts located in coastal waters of temperate countries. A team led by Hargrave described that the enrichment of organic matter in sediments produced anoxia and decreased the pH and redox potential in sediments associated with salmon farming [83; 84]. These indicators were included in the regulatory frameworks of salmon-producing countries such as Chile, which established thresholds to allow the continuity of the operation of production centers when the sediments under and in the vicinity of cages have a pH value greater than 7.1, a redox potential greater than \(-\)50 mV and the organic matter content did not exceed 9%, in accordance with Resolution No. 3612 of 2009 [85]. Although these indicators are practical for monitoring, since they allow (except for OM content) in situ recording with easily acquired sensors, their usefulness would require multiple verification tests (lines of evidence) in addition to the opinion of mangrove experts once they were established, since tropical mangrove systems behave differently from temperate and cold-water environments with higher dissolved oxygen contents. The capacity to store carbon in mangrove systems varies between different environmental factors that combine the origin of the sediments (terrigenous or carbonated) with different geomorphic settings (deltas, estuaries, lagoons and open coasts). Terrigenous deltaic and estuarine mangroves corresponding to the studied area are the ones that accumulate the most carbon [86]. The loss of carbon would be proportional to the loss of total organic matter following the Van Bemmelen factor 1.724 [87]. Nitrogen is associated with deforestation processes and the removal of previous soils when shrimp ponds were built. The decrease of OM in shrimp ponds and mangroves receiving shrimp effluents has been described for Brazil [14], Saudi Arabia [88] and southern Ecuador [89], and the increase of carbon in mangroves by shrimp effluents has been described in China [90]; changes in the composition of mangrove sediments receiving shrimp farm effluents decrease the diversity of the bacterial microbiome [91]. In the Matang Mangrove Forest Reserve system of Malaysia, it was described that one year after the mangrove filling, the content of carbon and nitrogen in soils and fallen wood decreased by half. After 40 years of reforestation, the same sites had 26% less carbon and 15% less nitrogen compared to control forests [92]. These results allow us to notice the slow recovery of mangroves that takes several decades. The assessment and monitoring of sediment quality in tidal creeks receiving shrimp farm effluent can support environmental protection and decision making for sustainable development in coastal areas, since sediment quality often shows essential information on long-term aquatic ecosystem health. Chemistry and the data of benthic assemblages combined produced a better description of the quality and impacts of the evaluated environments [55]. The multivariate analyses of this study confirmed that sediment quality variables have a greater influence on the structuring of sector groupings, as well as on the distribution of benthic creatures shared among the compared sectors. A canonical correspondence analysis integrating both sampling periods shows ten species decreasing when the total nitrogen content and REDOX of the sediment decrease; four polychaetes (_Euclynene_ sp., two types of Nephyidae and _Arabella_ sp.) and one bivalve _Polymesoda__inflata_ would be more related to this trend, and only one polychaete _Cossura__rostrata_ increased its population when the organic compounds and the pH level increased; however, two sampling campaigns of 20 sites are not enough to conclude on this regard, as there are considerable interannual variations in the wild productivity along the Ecuadorian coast [93].