trace elements in the mississippi river delta outflow region:...

Gmchrmca er Cosmochvmca Acfa Vol. 55. pp. 3241-3251 Copyright 6 1991 Pergamon Press pk. Printed in U.S.A.

0016-7037/91/$3.00 + .oO

Trace elements in the Mississippi River Delta outflow region: Behavior at high discharge

ALAN M. SHILLER’ and EDWARD A. BOYLE’

‘Center for Marine Science, University of Southern Mississippi, Stennis Space Center. MS 39529. USA *Department of Earth. Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02 I39

(Received July 3 1, 1990; accepted in revi.red form August 21, I99 1)

Abstract-Samples for dissolved trace element analysis were collected in surface waters of the plume of the Mississippi River during a period of high river discharge. These field data are compared with results of laboratory mixing experiments. The studies show that Cu. Ni, and MO are largely unreactive in the plume. Surprisingly, Fe also appears to show little reactivity: the pronounced flocculation removal of Fe frequently observed in other estuaries is not seen in this system. This difference may be a consequence of the alkaline nature of the Mississippi which results in low dissolved Fe concentrations in the river ( ~50 nmol/ kg). Zinc, another particle-reactive element, also shows little reactivity. This lack of reactivity for Zn, as well as Cu and Ni, is partly a result of the short residence time of plume waters in shallow areas affected by sedimentary interactions. The chromium distribution shows apparent non-conservative behavior indicative of estuarine removal: however, temporal variation in river concentrations is a more likely explanation for this behavior. For some other elements, complex distributions occur as a consequence of the interplay of physical-chemical and/or biological processes with the dynamic mixing regime. For Cd, desorption from the suspended load plays a major role in determining the distribution. However, sedimentary input may also play a role in the spatial variability of Cd. For V, biological uptake in the plume exerts a strong influence on its distribution. At the time of this study, uptake was large enough to consume both the river flux of V as well as a substantial amount of vanadium supplied by the ocean.

INTRODUCTION

ESTUARIES ARE THE interface between rivers and the ocean. Chemical, biological, geological, and physical processes all combine to make estuaries dynamic geochemical reactors wherein the flux of dissolved materials from the continent to the ocean can be substantially modified. Thus, understanding the geochemical dynamics of estuaries is an important part of constructing elemental mass balances for the coastal zone as well as for the oceans as a whole. An additional reason for studying the chemistry of estuaries relates to the fact that some oceanic geochemical processes are greatly magnified in estuaries due to the high suspended loads and primary productivities of estuarine waters.

Trace elements are a particularly interesting aspect of estuarine chemistry because their differing physical chemistries lead to a variety of geochemical behaviors. For example, previous studies have indicated conservative mixing behavior for dissolved Cu and Ni (BOYLE et al., 1982), biological uptake for V and Ge (SHILLER and BOYLE, 1987a; FROELICH et al.. 1985), desorption for Cd and Ba (EDMOND et al., 1985; HANOR and CHAN, 1977 ), sedimentary input for Mn (EVANS et al. 1977), and removal through flocculation for Fe ( SHOL- KOVITZ, 1976; BOYLE et al., 1977). Understanding these various behaviors is important beyond simply quantifying fluxes of dissolved materials to the ocean. Potentially, trace element distributions can be used to indicate the magnitudes or time scales of estuarine geochemical processes relative to mixing. Additionally, since some trace elements can limit phytoplankton growth at low concentrations (BRAND et al., 1983; MARTIN and FITZWATER, 1988) or can be toxic (e.g., FITZ- WATER et al., 1982), estuarine trace element chemistry may be a relevant factor in determining the distribution of estuarine primary productivity.

The area of the northern Gulf of Mexico, where Mississippi River waters mix with seawater, is an attractive area for the study of estuarine trace element chemistry (HANOR and CHAN, 1977; MOORE and SCOTT, 1986). The Mississippi River is one of the ten largest rivers in the world. During high-flow periods, discharge through the Mississippi Delta is great enough that the river water forms a plume or lens of water on top of the more dense Gulf of Mexico waters with which it mixes (e.g., WRIGHT and COLEMAN, 197 1). This type of estuary without the usual land boundaries is char- acteristic of large rivers. During low-discharge periods, however, much estuarine mixing of the Mississippi is confined to the distributary channels which then form a classic salt- wedge estuary (e.g., PRITCHARD, 1989). Additionally, ap- proximately 30% of the discharge of the Mississippi is chan- neled through the Atchafalaya River which enters the Gulf through Grand Lake and Atchafalaya Bay some 175 km west of the delta. (This distribution of outflow is regulated by the US Army Corps of Engineers.) Other river waters enter the Gulf through various marshes and lakes along the Louisiana coast. Thus, the Mississippi River estuarine system, both spatially and temporally, provides a variety of physical mixing systems with roughly the same end-member composition. Since the basin of the Mississippi River is heavily developed for both industry and agriculture, there is, of course, some concern that there may be an anthropogenic stamp on the chemistry occurring in the estuarine mixing zone. Recent work shows that most trace elements in the river itself have dissolved concentrations similar to those of less disturbed systems ( SHILLER and BOYLE, 1987b). This suggests that the common perception of extreme metal contamination of the Mississippi River is not correct. Additionally, many processes controlling trace element distributions are not likely to have changed.

3241

3242 A. M. Shiller and E. A. Boyle

In this study, we have confined our observations of trace element concentrations to surface waters of the delta outflow region (Fig. 1) during a period of high discharge (April 15- 22, 1982). At that time, discharge of the river through the delta was -2.0 X lo4 m3/sec, a flow rate which is exceeded only about 20% of the time (WELLS, 1980). Most of this discharge reaches the Gulf through three major distributaries: Pass a Loutre, Southwest Pass, and South Pass. About 20% ofthe flow from the delta exits through various minor passes, bayous and crevasses (WRIGHT, 1970).

METHODS

Near-surface plume waters were obtained aboard the R/V Gyre by pumping water into the ship laboratory using acid-cleaned, Teflon- lined tubing attached to and extending in front of a “fish” towed several meters off the side of the ship (BOYLE et al., 1982). The tubing was connected to an acid-cleaned polycarbonate receiving vessel. The system pump was downstream from the Teflon-lined tubing and receiving vessel. To collect a sample, Teflon/polypropylene stopcocks on the receiving vessel were turned so as to isolate the vessel from the pumping system. The receiving vessel was then drained into an acid-cleaned polyethylene bottle for immediate filtration.

Efficient retention of particles during filtration is essential for the accurate determination ofdissolved metal concentrations in particle- rich natural waters. For example, the results of TREFRY et al. ( 1986) indicate that the Mississippi River typically carries 1000 times more suspended iron than dissolved. In our work, water samples were vac- uum-filtered through acid-rinsed 0.4 pm Nucleopore filters. The filters were held in 47 mm diameter Millipore Sterifil holders. The silicone o-ring gasket of these holders was replaced with a gasket fashioned from thin Teflon sheet. Each filter holder was attached to a cylindrical Plexiglas evacuation jar which contained an acid-cleaned polyethylene bottle for receiving the filtrate. In this way, samples can be filtered directly into storage bottles. Filtered samples were acidified to pH < 2 with double vycor-distilled HCL.

Because of occasional particle leakage around the gaskets used in our filter holders, some samples were inadequately filtered. Six samples

were discarded due to visible suspended matter in the acidified f&rate. Several other samples were also suspected to be inadequately filtered due to very high Fe concentrations (> 100 nmol/kg) and were also eliminated.

Mixing experiments were performed using water samples that were returned to the lab unfiltered and unacidified. Besides waters collected during the field survey in April 1982, river waters for additional mixing experiments were collected during high-discharge periods in March and May of 1983. These samples were collected at Baton Rouge, LA, using a bottle attached to a non-metallic pole as described by SHILLER and BOYLE ( 1987b). Discharge, suspended load, pH, and specific conductivity for the river during these sampling periods are given elsewhere ( SHILLER and BOYLE, 1987b). Transport and short-term storage (several days) of these unfiltered samples appeared to have a minimal effect on trace element concentrations. This was evidenced by a comparison of field-filtered and transported samples for April 1982 (e.g., compare river concentrations in Figs. 3a,b).

In the mixing experiments, known aliquots of filtered or unfiltered river and seawater were mixed in varying proportions in acid-cleaned polyethylene bottles. Two types of experiments were performed. In one series, unfiltered river and seawater were mixed in order to ex- amine trace element sorption behavior during mixing. To distinguish colloid flocculation from other reactions, a second series of mixtures were made using pre-filtered (0.4 pm Nuclepore) river and seawater. All experimental mixtures were placed on a shaker table to equilibrate for a number of hours before filtration and acidification for storage. Unfiltered mixtures were equilibrated for about four hours during the April 1982 experiments, six hours during March 1983, and over- night for the May 1983 experiments. Pre-filtered mixtures (April 1982 only) were equilibrated for an hour.

Dissolved trace elements were determined by graphite furnace atomic absorption spectrophotometry using a Perkin-Elmer 5000/ HGA400 combination. Samples were preconcentrated prior to analysis using the cobalt-pyrollidine dithiocarbamate method (BOYLE et al., 198 I ). Trace element recovery by the method is pH dependent. The co-precipitation was therefore carried out at pH 1.8 for Cu, Ni, and Cd, and at pH 4.5 for V, Zn, Fe, MO. and Cr. Recovery by this separation procedure was determined by the addition of stable element spikes. Recoveries ranged from -87% for Cr to - 100% for V. Similar tests demonstrated that Cr( III) and Cr( VI) were recovered with equal

29.4

Pass A Loutre

28.8

28.6 90.0 89.5 89.0 88.5

Longitude

FIG. 1. Sampling locations in the plume of the Mississippi River. Different symbols correspond to transects through different areas of the plume; the same symbols are used in plots of the field data (Figs. 2- 10). (Open Gulf of Mexico sample locations not shown.) Results from experiments performed at different times are shown as follows: April 1982, triangles and circles; March 1983, diamonds; May 1983, squares.

Geochemistry of trace metals in the Mississippi River Delta 3243

efficiencies as were V(IV) and V(V). Precision is estimated to be + 10% for Zn and Fe, and +5% for the other elements. Not all elements were determined in all samples.

As reported previously ( SHILLER and BOYLE, 1987b), a filtered river water sample was refiltered through a 100,000 MW Nucleopore ultrafilter. There were no analytical differences between 0.4 pm filtered and ultrafiltered samples, which indicates that for this river the 0.4 pm data are representative of the truly dissolved concentration with no significant colloidal fraction for the elements reported here.

Nutrients were determined on acidified samples using standard techniques (STRICKLAND and PARSONS, 1972). Salinity was calcu- lated by measuring dissolved magnesium with flame atomic absorption and using a Mg/S ratio of I .5 1 mM / %o.

RESULTS

The field results are listed in the Appendix. Figure 2 shows the dissolved phosphate, silicate, and suspended matter concentrations in the field samples. The top panel of Figs. 3-10 shows trace element concentrations in the field samples plot- ted as a function of salinity. Differing symbols in the plots of the field data represent different locations in the delta outflow region as shown in Fig. 1. Open circles in these graphs denote samples collected in a transect extending seaward from

120

Gi < QO a 1 60

N

gj 30

i

00

A4lOo

A 0

0 O 0

0 0 1

20 Salinity

40

FIG. 2. Field results of(a) silicate, (b) phosphate, and (c)suspended load for surface samples in the plume of the Mississippi River. Symbols correspond to different areas of the plume as shown in Fig. 1. See Fig. I for explanation of symbols.

Southwest Pass as well as open Gulf of Mexico samples and very low salinity samples; squares denote samples collected on an approach towards Pass B Loutre; triangles denote samples collected on a transect south of the delta between South and Southwest Passes. The trace element field data show sys- tematic variations with salinity and/or correspondences with nutrient concentrations, as would be expected in a mixing regime with high primary production.

The second panel in all of the trace element figures shows the results of the unfiltered mixing experiments. As with the field data, particle leakage during filtration was a problem in some of the mixing experiments. Half of the mixtures from April 1982 showed evidence of particle leakage as indicated by dissolved iron concentrations above 100 nM. In general, this level of particulate contamination is much more of a problem for particle-reactive elements such as Fe, Zn, and Cr than for the other elements. Thus, these high-Fe samples were eliminated from the graphs for Fe, Zn, and Cr but are shown as circles on the graphs for Cu, Ni, Cd, V, and MO.

The third panel in all of the trace element figures (except MO) shows the results of the pre-filtered mixing experiments. In all cases the behavior seems to be that of simple mixing with little or no apparent removal by flocculation.

DISCUSSION

Phosphate, Silicate and Suspended Load

Nutrients and suspended load, as shown in Fig. 2, clearly demonstrate the heterogeneous and dynamic nature of the plume. The dissolved silicate data are most simply interpreted as representing three distinct mixing trends with high salinity end-members of -20%0, -25L, and -3Ok. For the transect from Southwest Pass offshore (circles), conservative behavior is indicated by the straight mixing line from the river to -300/w. Silica removal only appears to be significant at salinities higher than this. For the other mixing trends there are fewer data, making interpretation problematical.

This simple interpretation of three separate mixing trends in three different areas of the delta outflow is reasonable within the context of what little is known or can be inferred from the dynamics of this region. As mentioned above, the river exits the delta through several main distributaries as well as numerous minor passes and crevasses. The delta itself provides various inter-distributary bays which can serve to con- fine waters of intermediate salinities. Limited surveys of salinity in the plume indicate more than one high salinity end- member ( WISEMAN et al., 1982). Landsat images of this area also indicate the spatially heterogeneous nature of the plume (ROUSE and COLEMAN, 1976). Visually, we observed the outflow region to be characterized by a variety of fronts, across which the color of the water would abruptly change from brown to green or green to blue.

The phosphate data add another level of complexity to the picture of dynamics in the outflow region. There are two important differences between the distributions of dissolved phosphate and silicate. First, for stations between South and Southwest Passes (triangles), silicate decreases with increasing salinity whereas phosphate shows a maximum at approx. 13%0. This may be indicative of an additional intermediate salinity mixing endmember. More likely though, this water


has received a phosphate input due to organic matter regeneration in the shallow near-delta sediments.

A second difference between the distributions of the two nutrients can be seen in the transect from Southwest Pass (circles). Silicate goes to zero concentration (i.e., below the detection limit) at about 30%0 salinity, whereas the phosphate in this mixing trend drops below the detection limit at about 25% salinity. That is, each nutrient shows a different high salinity endmember. Since the biota cannot use silica without phosphate, either silica uptake above 25%0 salinity is main- tained by rapid turnover of phosphate or the zero silica water is a remnant of an earlier bloom. Whatever the actual explanation, it is clear that the nutrient distributions reflect the complexities of mixing within the outflow region.

The distribution of suspended matter also shows spatial variability akin to that of the nutrients. These particulate concentration differences probably relate to differences in lo- cal currents (i.e., resuspension) and to the age of the water parcel (i.e., settling time). Zooplankton may also act to ag- gregate and remove particles and thus suspended loads may be affected by grazing rates.

Copper, Nickel, and Molybdenum

Despite the complexities of the mixing regime as revealed by the nutrient distributions, Cu, Ni, and MO all mix ap- proximately conservatively out to high salinities and with no differences shown between the three spatial sample groupings (Figs. 3a, 4a, Sa). Mixing experiments, as shown in Fig. 3b,c for Cu, confirm the lack of reactivity and emphasize that not even removal via flocculation is observed.

Conservative mixing of Cu has also been observed in the plumes ofthe Amazon (BOYLE et al., 1982) and Chang Jiang (Yangtze) Rivers (EDMOND et al., 1985). This unreactive behavior could result from the strong complexation of Cu by organic ligands and conservative dilution of the complex ( APTE et al., 1990). Nonconservative estuarine mixing of copper only seems to be observed in some smaller estuaries (e.g., WINDOM et al., 1983). A key difference between small estuaries and the plume is bathymetry. Since the delta extends nearly to the shelf break, there is only a short residence time of plume waters in shallow areas affected by sedimentary interactions. In low pH rivers, mineral reactions have also been suggested to account for low salinity input of Cu (WIN- DOM et al., 1991).

For nickel, two of the three river end-member samples have concentrations significantly lower than anticipated by straight-line (i.e., conservative) extrapolation of the estuarine data (Fig. 4a). Since the mixing experiments (Fig. 4b,c) show only conservative behavior for Ni, the low river values may reflect temporal variability in the river concentration ( SHILLER and BOYLE, 1987b). Conservative behavior for Ni was also observed in the plume of the Amazon (BOYLE et al., 1982); however, it has been suggested that the particle- rich plume of the Chang Jiang River shows evidence for nickel desorption at low salinities ( EDMOND et al., 1985). Addi- tionally, non-conservative behavior for nickel has been observed in a number of instances in small estuaries (e.g., WIN- DOM et al., 1991 ).

Pluma

0 10 20 30 40

Salinity

FIG. 3. Field and mixing experiment results for copper: (a) field results, symbols as in Fig. 1; (b) unfiltered mixing experiment-see Fig. 1 for explanation of symbols; (c) prefiltered mixing experiment.

The conservative MO distribution is not unexpected in view of the conservative behavior of this element in the ocean (MORRIS, 1975; COLLIER, 1985 ) as well as previous reports of its lack of estuarine reactivity (HEAD and BURTON, 1970; PRANCE and KREMLING, 1985; VAN DER SLOOT et al., 1985; 1989). The biological requirement for this element is low relative to its high concentration in plume waters, and as an oxyanion it is not likely to be appreciably scavenged by particulate matter at the pHs encountered.

Cadmium

A nonconservative distribution is observed for cadmium, with peak concentrations found between 5 and 15%0 salinity (Fig. 6a). Other workers have suggested that cadmium is desorbed from suspended particles during estuarine mixing (BOYLE et al., 1982; EDMOND et al., 1985) and our mixing experiments confirm this (Fig. 6b). On the basis of laboratory experiments and equilibrium calculations, COMANS and VAN DIJK ( 1988) concluded that this desorption results from the


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Salinity

FIG. 4. Results for nickel; see Fig. 3 for explanation.

reduction of free Cd2+ activity in solution due to chloride complexation and ionic strength effects.

Although some spatial variability is observed in cadmium concentrations at equivalent salinities, this variability does not appear to be related to differences in suspended load (compare Fig. 6a with Fig. 2~). Biological activity is probably of little importance to the estuarine Cd distribution since the most nutrient-depleted samples (Pass a Loutre transect, open squares) have Cd concentrations only slightly lower than other samples at equivalent salinities. The interesting aspect of the Cd distribution is that samples collected from South Pass to Southwest Pass (triangles), show a broad maximum in Cd concentrations at intermediate salinity. This maximum comes at a higher salinity than the desorption peak shown by both the main Southwest Pass transect (circles) and the mixing experiment. This is not likely to be a kinetic effect since mixing experiments indicated that Cd desorption occurs very rapidly, on the order of minutes. As with the phosphate data, another intermediate end-member or sedimentary input ( TREFRY et al., 1986) is indicated. This is reasonable since samples in the main transect from Southwest Pass (circles)

come from waters where the bottom depth is generally greater than 80 m, whereas the samples taken between South and Southwest Passes (triangles) come from shallower waters and are in close proximity to the shallows of East Bay. It is thus more likely that these inshore samples would see the influence of mixing with waters affected by sedimentary inputs. TREFRY et al. ( 1986) found Cd to be far more strongly mobilized from Mississippi delta sediments than other metals they ex- amined.

Iron and Zinc

Iron and zinc field data show the greatest scatter. This is due to the particle-reactive nature of these two elements. As mentioned above, there is typically 1000 times more particulate iron in the river than dissolved ( TREFRY et al., 1986). For Zn, experiments involving the acidification of aliquots of unfiltered Mississippi River water indicated more than an order of magnitude greater acid leachable Zn than dissolved Zn (SHILLER and BOYLE, 1987b). Thus, slight leakage of particles during filtration can result in a measurable increase in the dissolved concentration when the filtrate is acidified. Nonetheless, some important conclusions can be drawn from the data in Figs. 7 and 8.

In rivers less alkaline than the Mississippi, dissolved iron is typically micromolar in concentration (e.g., BOYLE et al., 1977). Much of the Fe in these rivers is apparently in colloidal form and flocculates when the river water mixes with seawater. This results in removal of dissolved Fe from estuarine waters ( SHOLKOVITZ, 1976; BOYLE et al., 1977). However, in the alkaline Mississippi River, Fe is at a low concentration

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FIG. 5. Results for molybdenum: see Fig. 3 for explanation.


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FIG. 6. Results for cadmium; see Fig. 3 for explanation.

( - 30 nmol / kg) to begin with and the dramatic estuarine Fe removal observed elsewhere is not seen here. This observation is also in accord with an ultrafiltration experiment ( SHILLER and BOYLE, 1987b) which revealed no significant colloidal fraction for any of the trace elements. The mixing experiments (Fig. 7b,c) confirm the lack of significant Fe flocculation.

For Zn, there is also no strong evidence of nonconservative behavior in the main field mixing trend (circles). Other workers have suggested, on the basis of radiotracer experiments, that Zn will be desorbed from particles in estuarine waters (EVANS and CUTSHALL, 1973; LI et al., 1984). How- ever, only in the May 1983 mixing experiment (Fig. 8b, boxes) do we see any possibility of this. We also note that for Zn, and possibly Fe, there appear to be lower concentrations in the most nutrient-depleted samples (squares) than in the main Southwest Pass mixing trend (circles). The largely unreactive behavior of Zn in the plume can be contrasted with evident non-conservative behavior of Zn in some small estuaries ( WINJJOM et al., 199 1). Again, differences in bathymetry and sedimentary interactions are likely keys to this contrasting behavior.

Chromium

Dissolved Cr appears to show nonconservative behavior (i.e., removal) in the field data (Fig. 9a). This behavior could be due to biological uptake, flocculation, or adsorption onto particle surfaces; however, there are reasons to suspect that none of these processes accounts for the apparent Cr removal we observe. Biological uptake is probably not the cause since there is no correspondence between the Cr and nutrient distributions. Because no colloidal form was observed and because flocculation was not observed for any other element, the apparent removal is not likely due to a flocculation of river-borne material. Mixing experiments (Fig. 9b) indicate little or no removal of chromium onto particles. Additionally, since adsorption would mainly involve the removal of dissolved Cr( III) (CRANSTON and MURRAY, 1980; MAYER et al., 1984), it is unlikely that the proportion of dissolved Cr( III) in either the river or seawater is great enough to account for the apparent removal we observe in our field data (CRANSTON and MURRAY, 1978; 1980).

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FIG. 7. Results for iron; see Fig. 3 for explanation.

Geochemistry of trace metals in the Mississippi River delta 3247

0

A A

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55

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2- 2%. ‘&.. =-4..

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FIG. 8. Results for zinc; see Fig. 3 for explanation.

Cr distribution is that it results from temporal changes in the river Cr concentration. LODER and REICHARD ( 1981) and OFFICER and LYNCH ( 198 1) have demonstrated that under certain conditions temporal variations in river concentrations can lead to non-linear mixing curves. In the present case, we note that river discharge (as recorded at Tarbert Landing, MS) decreased by about 20% in the week prior to our sampling. Chromium in the Mississippi River has the largest variation in concentration with discharge of the metals con- sidered here ( SHILLER and BOYLE, 1987b). The decreased river discharge could have been accompanied by a 0.5 nM increase in rivet-me dissolved chromium. An increase in the river end-member would lead to a downward bend in the Cr distribution, as we observe. A critical parameter in determining the shape and extent of curvature in the estuarine distribution is the ratio of the residence time of freshwater in the mixing zone to the period of the source variation (OF- FICER and LYNCH, 198 1). Ratios on the order of one can produce a broad shallow curvature. Based on the discharge of the river and estimates of the amount of fresh water on the shelf (S. P. DINNEL, pers. comm.), we estimate a freshwater residence time in the delta outflow region of between

3 and 30 days. Thus, the time scale and magnitude of the likely change in the chromium concentration of the river could account for the observed curvature in the estuarine chromium distribution.

Vanadium

The estuarine geochemistry of V has been discussed in detail elsewhere ( SHILLER and BOYLE, 1987a). In the present case, the V distribution (Fig. 10) is seen to bear a strong resemblance to the nutrient distributions (Fig. 2), suggesting that biological uptake is primarily responsible for the nonconservative behavior of V. ( SHILLER and BOYLE, 1987a).

Despite the similarities, there are some significant differences in the information provided by the V and nutrient distributions. Vanadium has a high seawater concentration whereas the nutrients are depleted in surface seawater, and this results in the V distribution revealing certain mixing trends within the plume more clearly than do the nutrients. For example, in the low salinity samples of the Southwest Pass transect (circles), V concentrations increase with increasing salinity on a trend which can be extrapolated to the

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FIG. 9. Results for chromium; see Fig. 3 for explanation.


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FIG. 10. Results for vanadium; see Fig. 3 for explanation.

open Gulf seawater end-member. Thus, the V data indicate that the Southwest Pass transect can be subdivided into mixing segments from 0 to - 12%0 salinity and - 12 to -30%0 salinity. Close inspection of the phosphate data indicates that this distribution can also be interpreted as showing two distinct trends within the Southwest Pass transect. However, without the V data, such an interpretation of the phosphate distribution would be more speculative. This is even more clearly the case for the silicate distribution: the low salinity silicate data could easily be extrapolated to zero concentration at either 30 or 36%0 salinity. Thus, V, being biologically active and with a high seawater concentration, can reveal mixing regimes not readily apparent in the nutrient distributions.

There is another important difference between vanadium and nutrient distributions: their fluxes from the river to the ocean are very different. These fluxes can be estimated by extrapolating the high salinity data points back to zero salinity, thereby yielding an “effective river concentration” (BOYLE et al., 1974; OFFICER, 1979). For the nutrients, uptake at intermediate salinities results in the flux of dissolved phosphate or silicate from the river being entirely consumed in the estuary. That is, the flux to the ocean is negligible compared to the flux from the river, at least for this time period.

For V, extrapolation of the high salinity data yields a negative effective river concentration. That is, the biological removal of V in the plume is great enough to consume both the river flux as well as a substantial amount of V supplied by the ocean. This consumption is large enough ( 1 to 2 times the river flux) that if it is sustained through the high-discharge months and if there is little sedimentary regeneration of V, then the plume is a net sink for oceanic V. Probably, though, a significant fraction of the removed V (as well as the phosphate) is regenerated (COLLIER, 1984 ).

Biological Removal of Trace Elements

Our chemical distributions indicate that biological activity was an important process affecting certain chemical distributions in the plume during April 1982. Since phosphate from the river was entirely consumed within the plume, we can use the fluvial phosphate flux to make a minimum estimate of plume primary production. This calculation ignores recycling of phosphate within the plume and the supply of phosphate from oceanic waters. Based on our river phosphate concentration and the flow of the Mississippi, the fluvial phosphate flux was 44 mol P/set at the time of our sampling. Using a Redfield Ratio of 106 C/P yields an overall plume primary productivity of 4.8 X lo9 gC/day. This estimate agrees in magnitude with the results of LOHRENZ et al. ( 1990). who used 14C primary productivity distributions to estimate a primary production of 1.6 X IO9 gC/day in the southwestern third of the outflow region during April 1988. During their study period, river discharge was similar to the April 1982 discharge, though river phosphate concentrations were about 50% higher ( 3 pmol/ kg).

In an oceanic regime, primary productivity as high as that estimated for the plume would be expected to have a large effect on trace element distributions. It is therefore instructive to compare our field data with an a priori prediction of trace element uptake in the plume ( WINDOM et al., 199 1). This prediction can be made by using oceanic trace element and phosphate removal rates. Taking the ratio of oceanic trace element removal rates to the oceanic phosphate removal rate and then multiplying by the observed estuarine phosphate removal yields the predicted estuarine trace element removal. In Table 1, the first column lists the oceanic removal ratios as taken from COLLIER and EDMOND ( 1984) and COLLIER ( 1984). In the second column of the table, these ratios are multiplied by the 44 mol/sec river phosphate flux to yield the predicted trace element removal. This calculation requires the same assumptions as the primary productivity calculation above: recycling within the plume is ignored as is the nutrient / metal supply from oceanic waters. These assumptions, as well as the uncertainty of the oceanic removal ratios, mean that the predicted uptakes are only crude estimates.

For the purpose of comparison, columns 3 and 4 of Table 1 show the river dissolved flux and the predicted removal as a percentage of the river flux. For Cu and Ni, the small predicted removal is consistent with the observed lack of removal in the field data. given the uncertainty of the calculation. For V, a more significant removal is predicted; in this case, a greater removal is observed (i.e., > 100%) than predicted. This may simply reflect the uncertainty of the estimate or

Geochemistry of trace metals in the Mississippi River delta 3249

Table 1. Reported oceanic trace element to phosphorus removal ratios and predicted plume trace element uptake based on these ratios and the uptake of the river phosphate flux within the plume. (Oceanic ratios from Collier & Edmond, 1984 and Collier, 1984)

Oceanic Element/PO4 Predicted Plume River Predicted Uptake as a Removal Ratios Untake Flux Percent of River Flux

Cd Ni cu Zn Fe V

0.6 - 1.1 1.4 - 2.6 1.7 - 2.0 12 - 30 30 - 90

4

(mmol/mol)

26 - 48 3 >lOO% 62 - 110 530 12 - 22% 75 - 88 500 15 - 18%

530 - 1300 84 >lOO% 1300 - 4000 660 >lOO%

180 440 40%

(mmol/sec) (mmol/sec)

may indicate that the removal of V (reldive to phosphate) is more efficient in the plume than in oceanic waters.

For Fe, Zn, and Cd, the predicted removal rates are substantially greater than the river fluxes. ‘;‘hus, complete removal of these elements at intermediate salinities might be expected. However, though the field data are suggestive of a possible small biological effect for these elements, such extreme removal is not observed. This lack of an easily ob- servable depletion for Fe, Zn, and Cd likely results from the fact that these elements are particle-reactive. That is, their dissolved concentrations in the plume are to some extent buffered by their much greater adsorbed particulate concentrations.

CONCLUSIONS

1) Mixing in the Mississippi River Delta outflow region is a dynamic process influenced by various large and small river outflows which enter the mixing zone in areas of differing energy and bottom depth. These factors create heterogeneities in the distributions of suspended matter and primary productivity (as reflected in the nutrient distributions). Complex distributions for certain trace elements can also result. For example, the cadmium distribution appears to be strongly influenced by desorption from the suspended load and probable input from the sediments. Vanadium, on the other hand, is strongly influenced by biological processes. The d;stributions ofboth of these trace elements can be useful in distinguishing waters in the outflow region which have similar salinities but different histories.

2) The residence time of waters within the delta outflow region may be as long as a month. Based on observations of the variability of trace element concentrations in the Mississippi River ( SHILLER and BOYLE, 1987b), nonlinear conservative mixing ( LODER and REICHARD, 198 1; OF- FICER and LYNCH, 198 1) may be observed for some elements during periods of change in river discharge. In par- ticular, the Cr distribution appears to be susceptible to this effect due to the large range of its Mississippi River concentration (SHILLER and BOYLE, 1987b). If uptake and removal factors are small for this element, then the Cr distribution could provide information about the time scale of mixing.

3) Previous reports have shown that the flocculation of colloidal iron along with the co-removal of other trace ele-

ments is an important process in estuaries of soft water, organic-rich rivers (BOYLE et al., 1977; SHOLKOVITZ, 1978). BOYLE et al. (1977) noted, however, that hard water rivers might behave differently. For the Mississippi River plume that would appear to be the case. The dissolved iron concentration in this river is more than an order of magnitude lower than is typical of soft water rivers and flocculation is unimportant in the plume.

4) Copper, nickel, and zinc are largely unreactive in the plume. However, in small estuaries nonconservative behavior of these elements has been observed (e.g., WINDOM et al., 199 1). Besides the lack of flocculation in the plume, a key factor in this contrast appears to be bathymetry. Since the delta extends nearly to the shelf break, there is only a short residence time of delta outflow waters in shallow areas affected by sedimentary interactions.

5) Biological activity can affect both oceanic and estuarine trace element distributions. However, one cannot use oceanic relative elemental removal rates to predict estuarine behavior. This appears to be due both to the buffering of certain dissolved trace element concentrations by their adsorbed concentrations and to differences in estuarine versus oceanic uptake. Biological uptake can be substantial enough to result in a negative flux of dissolved V through the plume, though regeneration could reintroduce this V back into the ocean.

Acknowledgments-We thank M. R. Scott (Texas A & M) and the captain and crew of the R/V Gyre for assistance at sea and L.-H. Chan (LSU) for aid in collecting additional river water. This manu- script was improved by the comments of E. R. Sholkovitz; we also thank J. D. Burton and A. van Geen for their constructive reviews. Support for this work was provided by the NSF through a Postdoctoral Fellowship ( AMS) and research grant OCE-8 I 17929 (EAB). The sampling and analysis were performed while AMS was in residence at MIT.

Editorial handling: E. R. Sholkovitz

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APPENDIX; Field results for April 1982. (Latitude in degrees North; Longitude in degrees West; Salinity in ppt; Nutrients in umol/kg; Trace elements in nmol/kg except Cd in pmol/kg; Suspended load is mg/l.)

Stn# Lat. Low S PO4 Si02 V CU Ni Cd Cr MO Fe Zn me/l 1 28.50 86.00 36.3 0.00 -- 38.4 1.30 1.64 8 2.42 5.0 0.11 __ 8 28.70 9 28.80

10 28.89 11 28.84 12 28.88 14 28.96 15 29.01 16 29.06 18 29.09 19 29.07 23 28.99 26 28.85 27 28.85 28 28.84 29 28.84 30 28.84 31 28.84 32 28.83 33 28.83 34 28.87 35 28.90 36 28.61 37 28.65 38 28.67 39 28.69 40 28.72 41 28.74 42 28.76 43 28.78 44 28.80 45 28.82 48 28.88 49 28.96 50 28.98 51 28.99 52 29.00 53 29.01 55 29.00 56 29.03 57 29.04

88.01 88.20 88.37 88.79 88.83 88.89 88.93 88.98 89.02 89.03 89.11 89.27 89.30 89.33 89.36 89.39 89.42 89.45 89.48 89.58 89.63 89.54 89.52 89.52 89.51 89.50 89.50 89.49 89.48 89.47 89.47 89.44 89.15 89.13 89.12 89.10 89.09 89.14 89.17 89.18

0.00 0.00

__ -_ -_

36.9 36.5 35.0 34.2 31.7 31.1 31.2 31.2 24.1 14.7 17.4 27.8 26.2 22.7 18.6 16.0 12.8 11.8 9.7

14.9 18.2 36.3 36.1 31.3 27.7 25.9 23.4 19.7 16.9 12.6 11.0 5.8 6.6 4.5 1.0 2.1 1.5 0.4 0.4 0.4

1 35.7 1 37.4

-" 2016 1 -_ 2 __ 2 -_ 0 __

1: 0.0 13 6 1:o 3 -_

1.19 1.41 3.57 4.12 5.63 6.06 6.50 6.39 11.9 16.6 14.5 a.23

10.7 11.7 13.5 15.5 16.6 17.5 19.1 17.3 14.7 1.62 2.49 6.61 a.23 9.75 11.5 14.7

1.42 1.64 2.95 4.04 5.35 5.90 7.10 6.01 11.0 16.4 14.8 a.84 ii.8 14.4 14.0 16.0 17.5 19.9 19.7 18.1 14.4 8.63

10.7 6.66 a.74

10.0 11.6 14.0

8 2.61 9 2.55 39 2.21 52 2.20 98 1.71 110 1.91 109 -- 95 1.86 133 1.30 219 1.15 171 1.18 188 1.51 226 1.51 252 1.51 173 1.47 260 1.96 252 1.76 299 1.04 233 0.95 232 199 1162 ii 2.48 29 -- 93

145 1166 151 -- 161 1.49 200 --

__ __ __ _- __ __ __ __ __ __ 48 49

4.0 0.20 3.0 0.36 6.4 -- 6.1 5.78 5.2 -- __ __

__ _- __ __ _- __

0.00 0.03 0.05 -_

0.03 0.09 0.07 0.37 0.66 0.54 0.30 1.11 0.34 0.00 0.03 0.28 0.05 0.09 0.07 0.30 0.75 0.94 1.47 1.93 1.70

5.2 -- 11.6 1.38 a.4 2.41 9.6 1.28

-- 13.8 -- -- 24.7 2.16 69 __ __

__ 0.7 a.7 15 11 3.2 3.3 3.7 5.3 4.5 5.7 6.8 7.6 4.3 4.9 _- _-

;:2 3.2 5.4 5.7

11 14 15 28 35 42 54 47 36

159 159 128

1' 2.9 7.8

2 2.2 42 12.4 57 17.4 58 15.2 64 14.5 57 28.2 29 12.2 1 31.6 1 27.8 -- 27.4 10 11.3 17 13.0 24 14.8 41 19.7

60 75.6 1.80 51 86.0 5.74 44 38 2;:5 2181 31 11.4 2.25 47 _- __ 56 73.2 2.89 LO4 18.6 -- __ __ __ __ __ __

74 38.7 2.26 -- __ 2.83 72 17.0 1.64 __ __ __

50 16.1 3.08 __

36 1819 3198 22 23.8 3.59 -- __ __ __

10 2;:9 4:;O 12 29.8 4.76 9 20.8 4.35 __

9 36:4 6:;l -- __ 4.90

51 21.7 15.6 16.2 235 1.21 70 26.3 19.9 22.8 297 -- 73 21.6 18.8 16.7 302 1.12 97 23.9 294 0.98 97 27.0

21.4 23.9

_- -_ 25.8 114 25.5 23.7 111 22.6 22.4 -- 21.6 23.1

123 -- 24.6 116 21.8 24.3 121 23.3 26.8

24.7 24.8 28.6 28.8 27.0 26.8 21.5 20.6 25.2

271 -- 274 -- 198 1.06 226 0.96 194 1.05 162 -- 161 1.77 178 --

1.70 2.12 1.66

2.31 2.14

trace elements in the mississippi river delta outflow region:...

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