139736eo.pdf (20MB) - Japan Oceanographic Data Center

Nitrate - NitrogenSimilarly to phosphate the nitrate concentrations are quite uniform and very high in the wholecolumn, from 200 to 1000 m; values are consistently above 25 pM dm-3 N-NO3, with deep maximaof 33 pM dm-3 in the western and 39 pM dm-3 in the eastern Gulf. Waters in the lower part of theeuphotic layer and down to 100 m, however, show more pronounced oscillations, with an averagerange of 3 -15 pM dm-3 (except during summer upwellings), indicating a dynamic relationship withthe primary production. In the upper euphotic layer nitrate temporal dynamics is much moreexpressed. Although locally and rarely nitrate may approach depletion, with concentrations below 0.5pM dm-3, the oscillations in the surface layer, which average 8.6 - 3.0 pM dm-3, do not indicatelimitation of a balanced primary production. In blooming conditions such surface nitrate levels ofcourse would be depleted; however, this appears to happen only during the summer when upwellingcan supply enormous amounts of nitrate to the surface layers, resulting in surface concentrations of 3 -6 pM dm-3 in the Gulf generally and well above 20 ~ M d min - upwelling ~ centres (maximum recordedsurface concentration is 33.8 pM dm-3!).Nitrite and AmmoniaBoth nitrate and ammonia have been measured regularly from all collected nutrient samples inthe Gulf of Aden. However, since these samples have been preserved deep frozen, we consider theresults to be only semi-quantitative.Nitrite concentrations average 0.05 to 0.5 pM dm-3 N-N02. Extremely low concentrationsbelow 0.03 pM dm-:’ or below the detection limit,are found often (about 30% of measurements),usually (but not always) in the upper euphotic layer. Highest concentrations, in the range of 0.8 - 1.8pM dm-3, are found during the summer in the whole column of surface layer down to 200 m,restricted to upwelling centres. High concentrations, in the range 0.6 -1.2 pM dm-3, also have beenrecorded during the rest of the year, with irregular spatial distributions generally associated with thelayer just below the euphotic layer (50-100 m). This may correspond to the upper nitrite maximum asdescribed for the Arabian Sea previously (SEN GUPTA and NAQVI, 1984).Recorded ammonia concentrations average 0.4 - 1.0 pM dm-3 N-NH4 (65% of all measurements),with the most usual concentrations of about 0.5 pM dm-3. Minima (less than 0.2 pM dm-3 orbelow the detection limit)are rarely observed. Considering the rather modest concentrations, there isno indication for any regularity of a spatial-temporal distribution of ammonia. However, highammonia concentrations (extremely high values being 12.0 - 21.5 pM dm-3 ) are found as a Pule in theupper layers (surface - 200 m) on the edge of the shelf during the winter, and in the upwelling centresduring the summer. Both cases may indicate active bacterial desemination, or massive animalexcretion of ammonia whose concentrations exceed its assimilation by phytoplankton and/or itsnitrification.SilicateSilicate concentrations throughout the euphotic layer never fall below about 0.5 pM dm-3 Si- Si (OH),,and usually they are well above 2.0 pM dm-3. Therefore the silicate is not considereda potentially limiting nutrient, even though diatoms predominate phytoplankton communities.However, the spatial and temporal distribution of Si, as shown in Figure 12, provides an additionalexample of upwelled deep intermediate waters (500 m) into very surface layers. It shows also the“diluting” effect of the inflowing Red Sea waters in the western part of the Gulf, where in the deeplayers, (800-1,000 m) maximum silicate concentrations are about 47 pM dm-3, while in correspondingdepths of the eastern waters they are above 70 pM dm-3.In conclusion it appears that the Gulf of Aden, considering its nutrient balance, presents aunique oceanic environment where nutrients, namely phosphorus and nitrogen, are never limitingfactors of primary production, except perhaps during inshore blooms. Observed oscillations inphytoplankton standing crops seem to originate from physical (circulation, spring overheating ofeuphotic layers) and biological (grazing) components of the ecosystem.263