Dissolved boron and nutrients in the mixing plumes of major tropical rivers

Netherlands oTournalof Sea Research 12 (3]4): 345-354 (1978)

DISSOLVED BORON AND NUTRIENTS MIXING PLUMES OF MAJOR TROPICAL

IN THE RIVERS

by K. A. FANNING and V. I. MAYNARD (Department of Marine Science, University of South Florida, St. Petersburg, Florida, USA) CONTENTS I. II. III. IV. V.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . .

345 347 349 353 353

I. I N T R O D U C T I O N Reactions between suspended riverine sediment and solutes in river plumes are potentially important chemical processes at the boundaries of the ocean. The possible inorganic removal of dissolved silica (SiO ~) in river plumes has prompted opposing conclusions (e.g. LIss & SPENCER, 1970; FANNING • PILSON, 1973; BOYLE et al., 1974). Graphs of dissolved silica against salinity or another conservative variable are used to help decide the behaviour of silica during mixing, but interpretation of these curves can be complicated by uncertainty about the silica concentrations of end members or by different rates of mixing in different parts of the plume. A similar inorganic uptake of dissolved boron onto fine-grained riverine sediment has been both proposed (L~.vINSON & LUDWICK, 1966) and criticized (LANDERGREN & CARVAJAL,1969). Numerous laboratory studies have demonstrated that natural minerals can remove boron from solution (e.g. HARDER, 1961; FLEET, 1965; LERMAN, 1966; COUCH & GRIM, 1968). Clay minerals, especially illite, are the best adsorbents. Uptake of boron by clays seems to be enhanced by increased ionic strength or salinity, by higher temperature, by higher boron concentration in solution, and by longer exposure times. In addition, the initial uptake seems to be a surface reaction because it can be modelled by a Freundlich isotherm. There has been a report of actual uptake of boron in a river plume, that of the River Alde, Suffolk, England (LIss & POmTON, 1973). This river is small with a low suspended load (10 to 80 mg.l-1), and it discharges into a cold part of the ocean. Graphs of dissolved boron against salinity were curved and fell below a straight line of mixing

346

K.A.

FANNING

& V. t. M A Y N A R D

between the fresh and saline water types. LIss & POINTON claimed 25 to 30% of the boron in the river plume was removed by uptake on suspended sediment, which had a higher concentration toward the saline part of the estuary due to tidal stirring. To our knowledge, few studies of the behaviour of boron in other river plumes have been published. HOSOKAWA, OHSHIMA & KONDO ',1970) found conservative behaviour for boron in the plume of the Chikugogawa River in Japan. Some work has been done on the Mississippi (R.C. HARRISS, personal communication). But extensive surveys of boron concentrations have not been reported tor tile plumes of major rivers- i.e. those with large flow volumes or large sediment loads. Boron has a rather fortunate geochemistry tbr this kind of investigation. Except tbr a few rivers draining volcai~ic or desert terrain, most of the world's rivers have a low boron concentration compared to the ocean-- about 1.5 ~zM compared to 450 ~M (LIVINGSTON~, 1963). Dissolved boron is considered a conservatiw- elenlent in seawater. Thus, unlike other compounds in river plumes, the're is little difficulty in selecting the end members of a conservative mixing line. The fresh water end member is zero tbr most rivers, and the saline end member is determined by the salinity of the surrounding ocean. For most river plumes, departure t?om linearity in a boronsalinity plot is much more likely to be produced by a chemical process affecting boron than by the presence of several types of runoff or diluting sea water. Boron is examined along with silicon to help decide it an uptake of silicon is biological or abiological. Dissolved silicon could conceivably interact with both aluminosilicate minerals and silica-utilizing organisms such as diatoms. The simultaneous uptake of boron a~d silicon in a river plume makes a stronger case ibr the abiological remowd of silicon---i.e, surface reactions are at least occurring during the removal of silicon. Boron is also of interest by itself because of its possible usethlness as an indicator of paleosalinity in sedimentary rocks (see above referenees).

Cruises to two major rivers provided us an opportmlity to consider boron along with silicon and other nutrients under conditions reasonably typical of much of the freshwater-scawater boundary of the ocean. The two rivers arc the Zaire, which enters the equatorial Atlantic, and the Magdalena, which enters the Caribbean Sea at Barranquilla, Colombia. Like most of the world's river runoff, both enter warm ' ~:>20° C) or tropical ocean waters. According to HOL~MAN (1968), the Zaire is the second largest river on earth, with a flow volume of about 1 x 1015 1.y --t. The Magdalena River is comparatively

DISSOLVED BORON

347

unknown. HOLEMANdoes not include it in his compilation, .although PULIDO (1967) gives data showing that the Magdalena discharges 2 × 1014 l ' y -1, roughly as much as the Columbia River in the U.S.A. or the Indus River. It is a turbid river, having 200 mg. 1-1 suspended load (BETZER, 1971). Thus, it delivers a large amount of sediment to the ocean--4 × 1013 g . y - l - - w h i c h is about one half of the sediment delivered annually by the Orinoco according to HOLEMAN (1968). By contrast, the Zaire River has a very low suspended load--52 mg-1-1 according to data in HOLEMAN and 32 mg'1-1 according to EISMA, KALF & VAN DER GAAST (1978). Thus, our study spans a range of conditions for sediment-water interactions in river plumes at low latitudes. Acknowledgements.--We appreciate the assistance of Aleido J. van Bennekom, Doeke Eisma, and others from the Netherlands Institute for Sea Research for providing us with a suite of Zaire plume samples covering the full salinity range. This research was supported by National Science Foundation Grants 74-01480-A02 and OCE76-82416 and by Office of Naval Research Contract N00014-75-C-0539. II. METHODS The plume of the Zaire River was sampled between November 8 and 22, 1976 by workers from the Netherlands Institute for Sea Research. A general outline of the hydrography of the Zaire estuary is given by EISMA & VAN BENN~KOM (1978). Samples of water from the plume were membrane-filtered, stored in polyethylene bottles, and shipped to our laboratory in the United States. The sampling sites in the Zaire River were from various parts of the plume--in the main channel, offshore toward the outer fringe, from surface water, and from deeper and more saline water. However, we did not process any samples from the "older" part of the plume in which nutrient concentrations were definitely below the linear mixing line (VAN BENNEKOM, BERGER, HELDER • DE VRIES, 1978). The Magdalena River was sampled on August 12 and August 18, 1977 aboard the R/V GYRE, which belongs to Texas A & M University. These samples were taken by bucket thrown from the bow as the ship steamed down the river from Barranquilla. Thus, they represent waters that had probably been at their present salinity for a moderate time. However, the riverine sediment had not settled out as much as in more "aged" parcels. The water samples were filtered immediately through 0.4 ~m Nuclepore ® membrane filters and were stored in clean polyethylene bottles until analyzed.

348

K. A. F A N N I N G

& V. I. M A Y N A R D

All filtered samples were analyzed for salinity and dissolved boron after storage. Salinity was determined with a Guildline Autosal ® conductimetric salinometer. Dissolved boron was determined spectrophotometrically as a complex with curcumin using the method of UPPSTROM (1968). In this method, interference by water is eliminated by addition of propionic anhydride with oxalyl chloride as a catalyst. The Magdalena samples were analyzed for dissolved nutrients using a Technicon Autoanalyzer II ®. Ascorbic acid was the reducing agent tbr the silica determination (CARDER et al., 1977). The technique of ARMST RONG, STEARNS • STRICKLAND (1967) was used for nitrate, and the method of STRICKLAND ~L PARSONS (1968) was used for phosphate. The extremes of ionic strength in a river plume can vary the intensity of color in spectrophotometric methods. However, ours were not seriously effected. The only consistent effect of salinity was for the silica technique in which the slope of the least-squares linear regression of concentration against peak height increased about 3% as the salinity of the standards went from 0 to 36~o. There was no consistent effect of salinity on the other methods beyond the normal scatter. The silica calculations were appropriately corrected for salt error. Estimates of the reliabilities of the colorimetric analytical methods are as follows. The reported silica concentrations are accurate to within 3 [xmoles.kg 1, as an average, with a range of 1 ~moles-kg 1 above to 5 #moles'kg 1 below tile true value. The average difference in silica concentration for 58 pairs of duplicate analyses was 0.8 ~amoles.kg 1. Nitrate concentrations are accurate to within 0.6 ~ m o l e s . k g 1 on the average with a range of 0.8 ~moles-kg -I above to 2 ~xmoles•kg- 1 below the true value. The average difference in nitrate concentration for 18 pairs of duplicate analyses was 0.2 ~moles'kg '1. Phosphate concentrations were accurate to within 0.1 ~moles.kg 1 on the average with a range of 0.14 ~xmoles.kg -1 above to (}.34 ~xmoles. kg -1 below the true value. The average difference in phosphate concentration for 43 pairs of duplicate analyses was 0.2 ~zmoles.kg-L Boron concentrations were accurate to within 0.002 m g . k g 1 on the average with a range of 0.20 mg- kg- 1 above to 0.16 mg. kg 1 below the true value. The average difference in boron concentrations tbr 74 pairs of duplicate analyses was 0.05 mg. kg-L Before the boron analyses were begun, the samples had been stored-the Zaire samples for 11 months and the Magdalena samples for 3 months. However, tests to discover potential effects of storage were negative. The samples were in polyethylene bottles, so there was no possibility of contamination from borosilicate glass containers. Samples of river plumes and sea water sealed in polyethylene containers for 3 months maintained their original boron-chlorinity ratio. Samples of

DISSOLVED

BORON

349

sea water left open to the air for 2 weeks also kept a constant boronchlorinity ratio. III. RESULTS

AND DISCUSSION

Within our ability to tell, the boron-salinity diagrams for the Zaire and the Magdalena rivers were identical, and a combined diagram was therefore prepared (Fig. 1, upper diagram). This similarity in the behaviour of boron appeared despite some obvious dissimilarities between the two major rivers: different drainage basins on two continents, different years and seasons of sampling, diffeient suspended loads, and different distributions of the sampling sites within the two plumes. Also, Fig. 1 presents combined data from two or more transects of each river plume. During the survey of each plume, these transects were several days apart, time enough for mixing and chemical processes to have altered the properties of most parcels of water. A least-squares regression to a straight line was determined for all the data points in Fig. 1 (upper diagram). The slope of that regression is 0.1278 (mg B) (kg sw) -1 (~o S) -1, and the coefficient of determination of the regression is 0.995, signifying a good fit of the data points to a straight line. When multiplied by the salinity-chlorinity ratio of sea water (1.80655), the slope of the regression becomes: 0.2309 (mg B) (kg sw)-I (~o chlor.) -1. This value compares quite favourably with the accepted values of the boron-chlorinity ratio of sea water. CULKIN (1965) gives an average boron-chlorinity ratio of 0.240 (mg B) (kg sw) -1 (~oo chlor.) -1 with a range of 0.222-0.264 (mg B) (kg sw) -1 (~oo chlor.) -1- RILEY & Chester (1971) quote data from GREENHALOH and RILEY that show a boron-chlorinity ratio of 0.230 (mg B) (kg sw) -1 (~oo chlor.) -1. UPPSTROM (1974) found 0.232 (mg B) (kg sw)-I (~o chl°r.) -1 in the Pacific Ocean. The distribution in Fig. 1 and the value of the slope of the regression are exactly what would be expected for a major inert component in the oceans that is nearly non-existent in rivers. Because the distribution of boron in the two plumes was predictable from the salinity, we conclude that boron behaves conservatively during the mixing of large lowlatitude rivers with the ocean. It does not seem to matter whether the rivers carry m u c h sediment or little sediment. The mineralogy of the riverine sediment appears not to be important either. Quartz (18 to 36% of the inorganic material) and kaolinite (44 to 62%) are the main constituents of the suspended sediment in the Zaire river plume, along with 6 to 8% illite and mica (EIsMA, KALF & VAN DER GAAST, 1978).

350

K. A. F A N N I N G & V. I. MAYNARD

GRIFFIN, WINDOM & GOLDBERG (1968) also claimed that the Zaire sediment was primarily kaolinite, but the Magdalena sediment has a 5xx

4x

x

el

2-

25--250

•

0

"o

"--...

{Co

•

°

"~'-I.%

5

o

5

I0

15

2O

25

30

55

8alinit y (%0)

Fig. l. Upper diagram: The variations of dissolved boron with salinity during the mixing of the Zaire ( x ) and Magdalena Rivers (O, O) with the surrounding ocean. Slope of least-squares line: 0.1278 (mg B) (%0 S) -1 or 0.2309 (mg B) (%0 chlor.) -1. b. Lower diagram: The variations of dissolved silicon (O, O) and nitrate (,~, A) with salinity during the mixing of the Magdalena River with the surrounding ocean. Sampling on 12 August 1977 (closed symbols) and on 18 August 1977 (open symbols).

DISSOLVED BORON

351

large quantity of chlorite (JOHN HATHAWAY and PETER BETZER, personal communication). There is a chance that the available samples did not come from the oldest parts of the two plumes where the suspended sediment might have had m a x i m u m contact with higher boron concentrations. As mentioned, we did not receive any samples of water from a zone to the southwest of the Zaire mouth showing biological uptake of phosphate and nitrate. However, the samples analyzed came from as far as 120 miles away from the mouth of the Zaire, implying that the riverine suspended sediment at those far stations could have been exposed to sea water for m a n y days. Surely some reaction should have occurred in that time if it were ever going to occur. The Magdalena samples were obtained from surface buckets within 5 miles of the mouth; so there is a possibility, albeit small, that we somehow missed some boron uptake in a remote part of the Magdalena plume. However, the survey of Llss & POINTON (1973) consisted entirely of bucket samples quite probably within 5 miles of the River Alde in England. Why then did boron seem to behave differently near the River Alde than near the Zaire or the Magdalena? It is only possible to speculate or to eliminate some unacceptable explanations. Based on the laboratory studies discussed in the Introduction, temperature differences between waters off the English coast and low-latitude ocean waters are unlikely answers. Higher temperatures in warmer oceans and rivers should increase the likelihood of boron adsorption, not lessen it. Suspended load is not a factor either because the Magdalena River has 2 to 20 times more suspended sediment per litre than the Alde, comparing BETZER'S (1971) data to that ofLxss & POINTON (1973). There are at least two possible explanations for the uptake of boron in one plume and not in the other two. The first is coastal circulation. I f the load of suspended sediment in the ocean adjacent to the Alde were high due to re-suspension by storms or tides, then a longer time of contact between sediment and saline water would result. Longer contact time increases boron uptake in the laboratory. The second explanation is that the clay minerals in suspension off the Alde are more likely to contain iUite, which has a large capacity to adsorb boron from solution. Illite tends to be important in temperate marine sediments and rivers (GRIFFIN, WINDOM & GOLDBERG, 1968) and was present in the Alde, although its relative abundance was not reported by Liss & POINTON. The distributions of dissolved silicon did not show any evidence of abiological adsorption in the mixing zones of either the Zaire or the Magdalena. The silica distribution of VAN BENNE~O~, BERGER, HELDER & DE VRIES (1978) for the mixing zone of the Zaire does not drop

352

K. A. F A N N I N G

& V. I. M A Y N A R D

below a straight line between riverine and oceanic end members except for the area to the southwest already mentioned. In fact, their plots of nutrients against salinity had small to large upward bulges at intermediate salinities which indicate production by some means or injection from richer sources around the Zaire. The silica distribution in the Magdalena plume showed only conservative mixing (Fig. 1, lower diagram). The slope of the least-squares regression line was --6.229 (~mole SiO2) (kg sw) -~ (~'/oo S) -1, and the coefficient of determination was 0.998. Thus, in August of 1977, there were neither biological nor abiological processes affecting silicon in the Magdalena's mixing zone. In contrast to the nitrate distribution in the Zaire estuary (VAN BENNEKOM, BERGER, HELDER & DE VRIES, 1978), the nitrate distribution in the Magdalena plume resembled a straight line (Fig. 1, lower diagram). The slope of its least-squares regression was - 0 . 4 7 7 (~mole NOs) (kg sw) -1 (~o S) -1, and the coefficient of determination was (i).988, suggesting a lack of biological activity affecting nitrate. However, the points for nitrate in Fig. 1, lower diagram, do seem to show a larger relative scatter than the silica points. Some of this nitrate scatter may have resulted from analytical uncertainty; however, during August of 1977, the Magdatena plume may have contained patchy biological activity that was utilizing nitrate. The phosphate data from the Magdalcna showed :L tremendous scatter compared to either silica or nitrate. Riverine phosphate conccntrations ranged from 1 to 3 ~ m o l e . k g t. While there was a general tendency for phosphate concentrations to decrease as salinity increased in the plume, for the region of the plume out to 25~),, S, some surface samples having salinity differences of 1~o or less had phosphate concentrations that differed by 2 to 3 ~mole.kg -1. These variant samples were only a few hundred metres apart. Eventually, the surface phosphate levels dropped to a few"tenths ofa ~mole per kg when the salinity reached 36~oo. This spotty pattern could have been caused by patchy biological activity because some of the lower phosphate concentrations corresponded to slightly lower nitrate concentrations. Howew:r, some ot" the lowered nitrate concentrations corresponded to increased phosphate concentrations. This fact suggests that there may be several sources of phosphate around the Magdalena. One of these is the city of Barranquilla. The river has been channelized near its mouth to aid navigation, and there are industries located along this portion of the river as well as upstream. Another explanation may be an injection [i'om the mangrove thickets and swamps that are abundant near the mouth. VAN BENNEKOM, BERGEN, HELDER & DE Ved~s (1978) suggest a similar source for phosphate enrichment in the plume of the Zaire.

DISSOLVED BORON

353

IV. S U M M A R Y W a t e r samples f r o m the plumes o f the Zaire R i v e r in Africa a n d the M a g d a l e n a R i v e r in S o u t h A m e r i c a were analyzed for dissolved b o r o n in o r d e r to search for interactions between b o r o n and suspended sediments. These interactions have been r e p o r t e d only for smaller rivers. T o help characterize a n y sediment-water interactions, dissolved nutrients were m e a s u r e d as well. O n a single graph, b o r o n concentrations were plotted against c o r r e s p o n d i n g salinities for samples f r o m b o t h river plumes. T h e d a t a points fell o n a straight line, and, w h e n multiplied b y the salinitychlorinity ratio o f sea w a t e r (1.80655), the slope o f t h a t line gave the a c c e p t e d b o r o n - c h l o r i n i t y ratio o f sea w a t e r 0.231 m g ' k g -1 (%0 chlor.) -1. T h e excellent fit o f all the boron-salinity d a t a to a single straight line with t h a t p a r t i c u l a r slope indicated t h a t b o r o n acted conservatively in b o t h plumes. Serious d o u b t is therefore cast on the idea t h a t inorganic adsorption o f b o r o n onto suspended sediment is a significant geochemical process at the freshwater-seawater interface o f the ocean. Silica was completely conservative in the M a g d a l e n a p l u m e a n d n e a r l y so in the p l u m e o f the Zaire, according to VAN BENNEKOM, B~RGER, HELD~R & DE VRIES (1978). Such b e h a v i o u r for silica also indicates a lack o f m a j o r sediment-water interactions, t h o u g h m i n o r or trace elements m a y be effected. Few chemical processes affected nitrate in the M a g d a l e n a plume, b u t the large scatter ill the M a g d a l e n a p h o s p h a t e distribution i n d i c a t e d i m p o r t a n t biological reactions, additional p h o s p h a t e sources, or both.

V. R E F E R E N C E S

ARMSTRONG,F. A.J., C. R. STEARNS• J. D. H. STRICKLAND,1967. The measurement of upwelling and subsequent biological processes by means of the Technicon Autoanalyzer and associated equipment.--Deep Sea Res. 14: 381-389. BENNEKOM, A.J. VAN, G. W. BERGER, W. HELDER & R. T. P. DE VRIES, 1978. Nutrient distribution in the Zaire estuary and river plume.--Neth. J. Sea Res. 12 (3/4) : 296-323. BETZER, P.R., 1971. The concentration and distribution of particulate iron in waters of the Northwest Atlantic Ocean and Caribbean Sea. University of Rhode Island (Fh. D. Thesis). BOYLE, E., R. COLLIER, A. T. DENGLER, J. M. EDMOND, A. C. No & R. F. STALLARD, 1974. On the chemical mass-balance in estuaries.--Geochim, cosmochim. Acta 38: 1719-1728. CARDER, K. L., K. A. FANNING, P. R. BETZER & V. I. MAYNARD, 1977. Dissolved silica and the circulation in the Yucatan Strait and deep eastern Gulf of Mexico.--Deep Sea Res. 24:1149-I 160.

354

K. A. F A N N I N G & V. Io M A Y N A R D

COUCH, E. L. & R. E. GRIM, 1968. Boron fixation by illites.--Clays Clay Miner. 16* 249-256. CULKIN, F., 1965. The major constituents of sea water, h i : J. P. RILLY & G. SKmROW. Chemical oceanography. Academic Press, London, 1 : 121-161. EISMA, D. & A. J. VANBENNEKOM,1978. T h e Zaire river and estuary and the Zaire outflow in the Atlantic O c e a n . - - N e t h . J. Sea Res. 12 (3/4) : 255-272. EISMA, D., J. KALE & S . J . VAN DER GAAST, 1978. Suspended matter in the ZMre estuary and the adjacent Atlantic O c e a n . - - N e t h . J. Sea Res. 12 (3/4) : 382-406. FANNINO, K. A. & M. E. O . PILSON, 1973. T h e lack of inorganic removal of dissolved silica during river-ocean m i x i n g . - - G e o c h i m , cosmochim. Acta 37: 2405-2415. FLEET, M. E. L., 1965. Preliminary investigations into the sorption of boron by clay m i n e r a l s . - - C l a y Miner. 6: 3-16. GRIFFIN, J . J . , H. WmDOM & E. D. GOLDBERG, 1968. The distribution of clay minerals in the world o c e a n . - - D e e p Sea Res. 15: 433-459. HARDER, H., 1961. Einbau yon Bor in detritische T o n m i n e r a l e - E x p e r i n ~ e n t e zur Erkl/irung des Borgehaltes toniger S e d i m e n t e . G e o c h i m . cosmochim. Acta 21: 284-294. HOLEMAN, J. N., 1968. The sediment yield of major rivers of the world. Wat. Resour. Res. 4: 737-747. HOSOKAWA, I., F. OHSHIMA & N. KONI)O, 1970. O n the concentrations of the dissolved chemical elements in the estuary water of the Chikugogawa R i v e r . - J. oceanogr. Soc. J a p a n 26: 1-5. LANDERGREN, S. & M. C. CAP,VAJAL, 1969. Contribution to the geochemistry of boron I I I . T h e relationship between boron concentration in marine clay sediments and the salinity of the depositional environments expressed as an adsorption isotherm.--Ark. Miner. Geol. 5 (2) • 11-22. LERMAN, A., 1966. Boron in clays and estimation of paleosalinities.---Sedimentology 6" 267-286. LEVlNSON, A. A. & J. C. LUDWmK, 1966. Speculation on the incorporation of boron into argillaceous sediments.--Geochim, cosmoehim. Acta 30: 855-861. Llss, P. S. & M . J . POINTON, 1973. Removal of dissolved boron and silicon during estuarine mixing of sea and river waters.--Geochim, cosmochim, Acta 37: 1493-1498. Ltss, P. S. & C. P. SPENCER, 1970. Abiological processes in the removal of silicate from sea w a t e r . - - G e o c h i m , cosmochim. Acta 3"1: 1073-1088. LIVlNOSTONE, D . A . , 1963. Chemical composition of rivers and lakes. The data of geochemistry. U.S. Geol. Survey Prof. Paper 440-G: 1-64. PULIOO, E. R., 1967. Las obres de bocas de ceniza Puerto de Colombia. Technical Report, Government of Colombia: 1 -100. RII,EY, J. P. & R. CttESTER, 1971. Introduction to marine ch~:mistr}. Academic Press, London: 1-465. STRICKLAND, J. D. H. & T. R. PARSONS, 1968. A practical handbook of seawater analysis. -Bull. Fish. Res. Bd Can. 167: I -311. UPPSTR6M, L. R., 1968. A modified method for deternfination of boron with curcumin and a simplified water-elimination procedure.--Analytica ehim. Aeta 4 3 : 4 7 5 486. , 1974. The boron/chlorinity ratio of seawater fl'om the Pacific O c e a n . - - D e e p Sea Res. 21: 161-162.