V. Nutrient analysis

20 S. Air. T. Nav. Antarkt., Deel15, 1985

The South African SffiEX I Cruise to the Prydz Bay region, 1984:

V. Nutrient analysis

The micro-nutrients silicate, phosphate, nitrate, nitrite, and ammonia were analysed on an ongoing basis during the 51- BEX I cruise to provide a chemical background for the phys- ical and biological studies undertaken concurrently. Prelim- inary results are presented and discussed.

Tydens die S!BEX 1-vaart is deurlopend analises gedoen van die mikrovoedingstowwe silikaat, fosfaat, nitraat, nitriet en ammoniak in die seewater om die chemiese agtergrond te verskaf vir fisiese en biologiese ondersoeke wat gelyktydig daarmee ingestel is. Die voorlopige resultate word voorgele en bespreek.

Introduction

Analysis of the micro-nutrients silicate, phosphate, nitrate, nitrite and ammonia was undertaken on the SIBEX I cruise of S.A. Agulhas with a view primarily to providing a chemi- cal background for both the physical and biological studies.

In addition the silicate distribution proved useful in delimit-.

ing the extent and distribution of the Bottom and Deep Water masses identified in the grid area. This has been ex- amined elsewhere by Orren et al (this volume).

Methods

Analyses of nutrients were done on board using a Technicon Autoanalyzer II system (Mostert 1983).

Statistical analyses of data were performed using North Western University's SPSS package. Computations and graph plotting was done on Rhodes University's CDC Cyber 825.

Variation of nutrients relative to the following thermal zones was also examined:

Surface mixed layer all stations Winter Water stations

Midwater mixing stations (2,3,4, 13, 16,29,41) (42)

0-80 m 80-120 ¹¹¹ 80-400 m 120-400 ¹¹¹ Deep Water at Winter Water stations below 120 m midwater mixing station below 400 m other stations below 800 m Analysis of 20 replicates of a single sample established the variation of the determinations (Table 1).

Results and Discussion

Ammonia

Ammonia determination was plagued by contamination of samples during sampling, storage and analysis mainly by faults in the ship's air-circulating system. Ammonia was therefore largely ignored during data analysis.

C. Duncombe Rae Dept. of Zoology and Entomology Rhodes University, Grahamstown 6140

Table I

Standard deviations and coefficients of variation for the five nutrients determined from analysis of 20 replicates of a single sample.

Mean (r.tmol.l-¹)

Std Deviation (n=20) Coefficient of

Variation

A

Silicate

(Si)

85.56 1.58 1.8%

Ph os- ph ate

(P)

2,44 0.05 2.0%

Nitrate Nitrite Ammonia

(N) (N) (N)

37.64 0.39 0.23 0.05 0.013 0.08 1.9% 3.3% 35%

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Fig. 1. Distribution of silicate concentrations in (a) the surface mixed layer

(b) the Winter Water (c) the mid water mixing layer

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S. Afr. J. Antarct. Res., Vol. 15, 1985

Silicate

Figure 1 a, b, c, shows the distribution of silicate concen- trations in the upper three thermal water types suggested by Brundrit (this volume). Silicate concentrations increase to- wards the south. No distinct discontinuities related to frontal effects appear to be evident, except perhaps between sta- tions 4 and 5 and stations 11 and 12.

Figure 2 shows the plots of nutrients vs salinity for the en- tire data set. It seems that the silicate concentrations sep- arated into three types possibly associated with different water masses. Nitrate and phosphate appear to separate into two types. To examine this possibility the nutrient concen- trations were plotted against salinity according to the ther- mal water types: surface mixed layer, Deep Water and Win- ter Water.

From this plot silicate distribution in the deep water ap- pears to show further division into two types. This possibility deserves further investigation.

21

Nitrate

The distribution of nitrate is uniform in the surface mixed layer and generally increases to a maximum around 200 m, decreasing slightly to the bottom.

Nitrite

Nitrite concentration is uniformly around 0.30 ~mol!e in the surface mixed layer, decreasing in the region of the Win- ter Water and the midwater layer to less than the limit of de- tection in the Deep Water. This pattern persists throughout the grid.

Phosphate

The pattern of phosphate distribution follows that of ni- trate; constant and low in the surface mixed layer reaching a maximum in the Winter Water and the mid-water mixing layers, and decreasing slightly in the Deep Water.

Analysis of this data set is continuing.

SILICATE -Si

150 PHOSPHATE- P NITRATE-N

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30

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3533 31. 3533 31. 35

SALINITY (%o)

Fig. 2. Silicate. phosphate and nitrate vs salinity for all data showing the division into two or three types.

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SALINITY (%o)

Fig. 3. Nutrients vs salinity for the surface mixed layer (0-80 m).

22

SILICATE -Si 150

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0

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S. Afr. T. Nav. Antarkt., Deel15, 1985

NITRATE - N 1.2

z

0 1~~-L~~L-~~~22~~~-L~~~~~

()

31. 3533 31. 3533 31. 35

SALINITY (%o)

Fig. 4. Nutrient vs salinity for the Winter Water (80-400 m).

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31. 35 33 31. 35 33

SALINITY (%o)

Fig. 5. Nutrients vs salinity for the Deep Water (400 m).

References

MOSTERT, S.A. 1983. Procedures used in South Africa for the automatic photometric determination of micro-nutrients in sea water. S.Afr. J. mar. Sci. 1: 189-198.

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