**4. Hydrographic setting**

 The surface circulation of the GoM is dominated by the warm and saline waters that flow in through the Strait of Yucatan, forming the Loop Current (LC), and then exit at the Florida Strait [26]. In its passage through the Gulf Basin, anticyclonic gyres are formed from the LC, that later collide with the upper slope of the northwestern Gulf [27]. The speed of these vortexes (~6 km day<sup>−</sup><sup>1</sup> ) and their residence time (~9–12 months) determine the distribution of physicochemical properties of the water masses, the circulation field, and the transport that controls the exchange of water masses between the continental shelf and the oceanic region [28, 29].

 On the inner continental shelf on the west coast, in the province called "continental shelf and slope of the NW Gulf of Mexico" that goes from the south of Veracruz to the north of the Rio Bravo [30], the circulation is primarily toward the south from September to March and to the north from May to August. This circulation pattern produces temperature and salinity changes and coastal upwellings [31–33]. During the autumn and winter, cold fronts generated intense flows to the south that are alternated with periods of relative calm and flows to the north that coincide with high chlorophyll-*a* values at the surface. The summer-autumn conditions are less variable but are strongly affected by the passage of eddies and meteorological disturbances (tropical storm or hurricane); under these conditions, the lowest chlorophyll-*a* concentrations are recorded at the surface [26, 34]. Few are the studies on the dynamic conditions on the Tamaulipas coastline. The water masses on the platform are different from those on the slope or in the deep-sea. The *The Hazards of Monitoring Ecosystem Ocean Health in the Gulf of Mexico: A Mexican Perspective DOI: http://dx.doi.org/10.5772/intechopen.81685* 

 circulation in the outer shelf and on the slope is often affected by the presence of cyclonic and anticyclonic eddies. When these are absent or weak, the circulation is toward the north. During the summer there is a semipermanent upwelling in the area, and during the winter there is advection of cold water and low salinity different to that of offshore waters [31, 32, 35]. On the slope, there is a strong influence of cyclonic and anticyclonic eddies generated in the east by the LC. These events do not have a seasonal periodicity or occur in the slope region. During the winter, the strong winds from the north (northerlies) maintain a homogenized water column, while in the summer the water column is stratified [26].

## **5. Oceanographic conditions**

 During M-I, toward the end of June, the studied area endured in its surface waters the effects of Hurricane Alex. The instability caused by this meteorological phenomenon produced strong turbulence in the water column along the coastal zone. Also, due to the unusual discharge from the Rio Bravo, salinity values were diluted in neritic waters, the concentrations of nutrients were high, and the zooplankton biomass exhibited a shift toward the north. There were abnormal values of oxygen and Chl-*a*. The interpretation of the hydrographic conditions and concentrations of nutrients indicated ascending conditions of the subsurface water in the northern sector and a sinking process of water in the southern sector. The upward motion of subsurface water took place mainly on the edge of the continental shelf, causing processes of fertilization in the euphotic zone.

 During M-II, hydrographic conditions presented greater instability in the surface water, with a significant injection of water coming from the continental shelf of Louisiana to Texas. The structure of the water masses in the oceanic area was similar to the one described during M-I. The water column did not present a marked stratification in neritic waters, and the mixed layer was slightly deeper. On this occasion, no processes of upwelling of deep water were recognized nor intrusion of oceanic waters on the continental shelf.

The processes of convection of water masses that govern the concentrations of oxygen, Chl-*a*, and nutrients in the water column helped to maintain values of these variables within the normal ranges for neritic and oceanic waters of the GoM.

 The concentrations of dissolved oxygen reported for the Gulf of Mexico vary from 2.4 to 5.4 mL/L [36, 37]. In this study, the oxygen remained relatively constant during the three oceanographic cruises, registering an average of 4.3 ± 0.8 mL/L. The highest values were recorded at the surface (<4.0 mL/L) and the lowest between 200 and 500 m (<3.0 mL/L). This layer corresponds to the Tropical Atlantic Central Water (TACW) located between 250 and 400 m. The minimum oxygen (2.6–2.9 mL/L) in M-II and M-III was recorded between 100 and 600 m depth.

In the case of the nutrients, the values showed a slight impoverishment (nitrates, 29.3–37.9 μM; silicate, 3.5–8.2 μM; phosphates, 1.9–3.4 μM). This fact emphasized the prevailing oligotrophic conditions (Chl-*a* < 0.25 ± 0.14 μg/L) reflected in the plankton components.

 In M-III, the analysis of the density and the flotation frequency data, particularly at the isobaths of 500, 1500, and 2000 m, made it possible to distinguish the North Atlantic Subsurface Waters (NASW). Other identified water masses in the Gulf were as follows: North Atlantic Common Water (NACW), Tropical Atlantic Central Water (TACW), North Atlantic Intermediate Water (NAIW), and North Atlantic Deep Water (NADW) (**Figure 3**).

 As indicated earlier, the region where intensive water mixing occurs is near the surface of coastal waters. The salinity and density values indicated the intrusion

#### **Figure 3.**

*T-S diagrams showing the profiles obtained during M-I (in red), M-II (in blue), and M-III (in black). North Atlantic subsurface waters (NASW), North Atlantic common water (NACW), tropical Atlantic central water (TACW); North Atlantic intermediate water (NAIW), and North Atlantic deep water (NADW).* 

 of fresh water from the river discharge onto the Tamaulipas continental shelf. The degree of water mixing of the water column was estimated by calculating the Brunt-Väisälä frequency (N). The results of this procedure revealed a significant stratification in the three oceanographic campaigns (N > 0). The average value for each campaign was 6.6 cycles/s for M-I, 4.12 cycles/s for M-II, and 5.0 cycles/s for M-III. Furthermore, in M-I the highest N values corresponded to the depth of 30 m, which coincide with the thermocline's depth recorded between 10 and 35 m. In M-II, the highest N value was recorded at 75 m and in M-III at 50–70 m. In both instances, the thermocline depth was of 70–90 m and 50–70 m, respectively (**Figure 4**).

During M-II and M-III, the oxygen, nutrients (nitrates, phosphates, and silicates) and the Chl-*a* concentrations maintained values that fall within the known ranges reported for the GoM (0.05–2.5 μM PO4, 0–35 μM NO3; <0.29 ± 0.31 μg/L Chl-*a*) [36, 37].

#### **Figure 4.**

*Density and Brunt-Väisälä profiles for the three oceanographic cruises: M-I, M-II, and M-III. The dotted lines depict individual profiles, and the black line is the average. The long dash line is the Brunt-Väisälä profile associated with the density average.* 

*The Hazards of Monitoring Ecosystem Ocean Health in the Gulf of Mexico: A Mexican Perspective DOI: http://dx.doi.org/10.5772/intechopen.81685* 
