Remotely driven thermocline oscillations and denitrification in the eastern South Pacific: The potential for high denitrification rates during weak coastal upwelling

Abstract

During February–March 1985 denitrification in the ocean off Peru was enhanced in four environments despite relatively weak coastal upwelling winds. They were (i) bottom waters over the shelf, (ii) a deep layer (∼ 300 m) north of ∼ 11°S, (iii) a near surface layer (∼ 40 m) with exceptional nitrite concentrations, and (iv) at relatively shallow depths south of ∼ 11°S. The exceptional 1985 observations may have been related to an unusually shallow thermocline that in turn could have represented a large-scale response to the 1982–1983 el Niño, which was the largest in over 100 years. The super-elevation of the thermocline noted in 1985, however, represented only about a 10% increase in the range of thermocline depth observations in the previous data, suggesting that the 1985 data may represent an extreme development of a recurring situation.

During February–March 1985 the average primary production rate was ∼ 2g C m−2day−1, despite the extremely weak upwelling winds (average speed < 4 m−1s), presumably because the elevated thermocline placed high nutrient waters in close proximity to the photic zone. The thermocline waters off Peru are typically low in oxygen, and the low wind speeds apparently restricted the flux of oxygen from the atmosphere into the ocean, since waters with oxygen concentrations of almost zero (< 0.1 ml l−1 ∼ < 4.5 × 10−6M) were sometimes found within 20 m of the sea surface. Since respiration rates decrease with depth and since denitrification is favored by low oxygen tensions, this situation could lead to extremely high denitrification rates in the water column even though primary production might be lower than the average value (∼ 3 g C m−2day−1) due to the weak upwelling winds.

Thermocline shoalings like those observed in 1985 are believed to be remotely forced by changes in equatorial winds, and shallow thermocline depths may be a common condition during austral summer. Thus, the February–March 1985 data may shed light on an important and recurring remote influence on denitrification rates in the eastern South Pacific.

A major point that emerges from the 1985 observations is that remotely forced thermocline oscillations may have a significant effect on denitrification in the eastern South Pacific Ocean and may, at times, counteract the effect of weak upwelling winds. These data also lend further support to the idea that large temporal variations in the marine denitrification rate can occur in response to relatively small changes in circulation and stratification.