Figure 2 shows the surface temperature over the shelf. There is only about 1° C variation in the sea surface temperature off Guatemalan. Figure 3 shows the hydrographic profile off Champerico. The transistion layer with a temperature drop from 25 - 16°C and a reduction in dissolved oxygen from 4 to 1 ml/l is found from abt 70 to abt 100m depth over the shelf, and the cool oxygen-depleted bottom water thus seems to cover a smaller part of the shelf than further south.
The sea surface temperature in the Tehuantepec Gulf shows a distinct decrease from the south east, abt. 28° C to the north west, 25 - 26° C. There is a tendency for decrease shorewards in the north which may be related to the prevailing northerly wind. The hydrographic profile over the shelf commensing a little to the east of the San Marcos bar is shown in Figure 3. The shelf is here about 60 nm wide, the outer half forming a deep platform 200 - 300 m deep. The lower part of the transition layer with an about 10° C drop in temperature and a reduction in oxygen content from 4 to 0.5 ml/l lifts from a depth of about 100m inshore to about 30 m some 40 n m offshore and drops again further seawards, This dome or ridge has been observed in previous hydrographic surveys of the Gulf and is explained as an effect of the existing current - and wind systems. The water near the bottom over the inner part of the shelf has a temperature exceeding 20° C and an oxygen content of more than ml/l down to about 70 m of depth. This is probably of significance for the distribution of fish and shrimp,
