Results

In-situ and remote observations: 

The continental shelf of the Yucatan Peninsula is a shallow region characterized by a very smooth slope (1 m/3000 m off Campeche ), where the mobility of the vessels is limited by draft restrictions. The sampling stations during this survey fell on depths ranging between 2 to 20 m depth, which captured the thermohaline variations of the first ~30 Km of water. Figure \ref{554331} depicts the T/S diagram of the campaign, where two water types were found: (1) Caribbean Tropical Surface Water (CTSW), also called Yucatan Sea water (off Yucatan state, \citealp{Enriquez_2013}) or Gulf Common Water (off the Gulf west coast, \citealp{Vidal_1994}), and (2) Caribbean Subtropical Under Water (CSUW). The former occupied most of the study region, presenting a modification of the CTSW having saltier values than the reported for the region (\citealt*{mclellan1967}\citealp{Vidal_1994}\citealp{Merino_1997}\citealp{Aldeco_Ram_rez_2009}\citealp{Enriquez_2013}). Waters located in the first 5 m showed very warm temperatures (29.6\(\pm\)1 °C) with large salinity and density ranges, influenced mainly by evaporation processes between the lower atmosphere and the surface ocean, and to a lesser extent by nearshore freshwater inputs from the Champoton river, coastal lagoons, and submarine groundwater discharges located near the coast, particularly in the northern part of the study area between Lerma and Isla Arena (20-21°N, 91°W). 
 On the other hand, water temperatures below 26 °C were seen between 7 to 16 m depth, located on the farthest sampling stations to the shore. At these depths, saltier Caribbean Subtropical Under Water-type signature was found. This water is commonly found in the Yucatan Channel below 250 m depth, and its presence over the Yucatan shelf is attributed to upwelling processes occurring at the northeastern side of the Yucatan Peninsula (\citealp{Merino_1997}, \citealt{Jouanno_2018}), next to the Yucatan Channel (> 600 Km to the east from the study region). This result illustrated a two-layer distribution of coastal waters on the deepest parts of the study region, represented by the CTSW above the CSUW, with local processes (evaporation/freshwater inputs) modifying their characteristics. It was found that evaporation processes greatly exceeded precipitation (or freshwater inputs) as both water-types showed saltier values, even though the survey was carried in July, which is at the middle of the rainy season defined from May to November each year.