Effects of wind forcing on the trophic conditions, zooplankton biomass and krill biochemical composition in the Gulf of Tehuantepec

The trophic conditions, the zooplankton biomass and the krill biochemical composition in the Gulf of Tehuantepec were studied in relation to spatial and temporal changes in the physical environment, caused by Norte winds. These winds, which occur mostly from October to March, are intermittent, strong wind jets that blow offshore and normal to the coast at the head of the Gulf of Tehuantepec. Ekman pumping raises the thermocline in the east and lowers it in the west of the jet axis; in the central region of the gulf, vertical mixing brings cool, nutrient-rich waters to the surface, enhancing biological productivity. Data from a January 1989 survey show that mean zooplankton biomass was highest in the central region, but there was no significant difference between the three regions (east, central and west). Among euphausiids, Euphausia lamelligera was the dominant species, with 92%. Mean particulate protein, lipids and particulate organic matter ( POM = protein + carbohydrates + lipids ) did not show significant differences among the three regions; however, mean particulate carbohydrates were significantly different. For the entire area, low but significant linear regressions between (ArcSin) lipids in euphausiids and lipids in POM, and between POM and (ArcSin) lipids in euphausiids were obtained. Better regressions between zooplankton biomass and POM and other variables were obtained when stations were analyzed by hydrographic regions. When data were grouped into those taken before and after a strong Norte (Leg I and Leg II, respectively), a significant positive regression was obtained between (ArcSin) krill lipids and POM for Leg I; but for Leg II, the slope, although not statistically significant, was negative. POM is apparently utilized during Leg II, but the response in the lipid content of the animals is evident only after some time has passed (during Leg I), when the animals had assimilated the food surplus.