Gas Exchange, Dispersion, and Biological Productivity on the West Florida Shelf: Results from a Lagrangian Tracer Study

Rik Wanninkhof, Ocean Chemistry Division NOAA/AOML, 4301 Rickenbacker Causeway, Miami FL 33149
Gary Hitchcock, Marine Biology Department RSMAS/U. Miami, 4600 Rickenbacker Causeway, Miami FL 33149
Bill Wiseman, Coastal Studies Institute, Louisiana State University, Baton Rouge. LA 70803
Gabe Vargo, Department of Marine Science, University of South Florida, 140  7 th Avenue, St. Petersburg FL 33701.
Peter Ortner, Ocean Chemistry Division NOAA/AOML, 4301 Rickenbacker Causeway, Miami FL 33149
Bill Asher, JISAU, University of Washington, Box 354235, Seattle WA 98105
David Ho, Department of Geochemistry, LDEO, Palisades NY. 10964
Peter Schlosser, Department of Geochemistry, LDEO, Palisades NY. 10964
Mary-Lynn Dickson, Graduate school of Oceanography, URI, Narragansett RI 02882
Meredith Anderson, Department of Marine Science, University of South Florida, 140  7 th Avenue, St. Petersburg FL 33701.
Rob Masserini, Department of Marine Science, University of South Florida, 140  7 th Avenue, St. Petersburg FL 33701.
Kent Fanning, Department of Marine Science, University of South Florida, 140  7 th Avenue, St. Petersburg FL 33701.
Jai-Zhong Zhang, Ocean Chemistry Division NOAA/AOML, 4301 Rickenbacker Causeway, Miami FL 33149
(who else?)

Abstract
A Langrangian study was performed on the West Florida shelf in April, 1996 using deliberate chemical tracers.  The horizontal dispersion and the gas transfer velocity were determined over a period of two weeks from the change in concentration and concentration ratio of the two injected trace gases, sulfur hexafluoride (SF6), and helium-3 (3He).  The tracer patch initially moved southward due to strong wind forcing of the shelf water and then drifted slowly northward. The horizontal diffusion estimates from surveys of surface concentration of SF6 were about an order of magnitude greater than empirical equations.  The difference is attributed to vertical shear. The second moment of the patch grew to 1.6 x 103 km2 in eleven days. The gas transfer velocity, normalized to CO2 exchange at 20 oC, was 8.4 cm hr-1 at an average wind speed of 4.7 m s-1 for the duration of the experiment in good agreement with empirical estimates.  Dissolved inorganic carbon measurements show that remineralization rates exceeded productivity causing an increase in dissolved inorganic carbon of about  1 (µmol kg-1) day-1 in the water column. This net remineralization in accordance with in situ and on-deck incubation experiments.

Introduction

Coastal seas are a dynamic interface between continents and open ocean. Significant riverine and diffuse inputs of nutrients, carbon, and pollutants find their way onto the continental shelves where they undergo biogeochemical transformations, sink to the sediments, and/or are transported to the open ocean.  Because of the continental inputs and shallow depths, coastal oceans are thought to contribute 14 % of the global biological production, while comprising 8 % of the ocean surface area. Many studies have focused on chemical and biological processes on the continental shelves but quantitative assessment of fluxes remains difficult, in large part because of the heterogeneous and dynamic nature of the environment.  
During the Florida Shelf Lagrangian Experiment (FSLE), a water mass was tagged with the deliberate tracers sulfur hexafluoride (SF6) and the isotope 3He.  The tracer patch was followed from April 1 to April 18, 1996 to study water mass movement, air-sea gas exchange, and biological productivity on the West Florida shelf.  The shelf is gently sloping with isobaths parallel to the coast and a width of 2 x 102 km (to the 100 m isobath).  By following a water mass on the shelf, rather than studying a fixed location, spatial variability issues are minimized.  Intensive surveys of the tracer patch, including vertical profiles, made it possible to investigate dispersion processes on 102 km length scales.  This effort complemented several other ongoing studies in the region, notably investigation the large scale flow field on the West Florida shelf by biweekly drifter deployments during 1996, and modeling efforts to study water movement on the shelf in response to meteorological forcing (Johnson and Weisberg, personal communication).  
In the spring, the West Florida shelf is horizontally and vertically stratified.   Horizontal and vertical stratification varied during the experiment due to diurnal heating, and changing meteorological conditions. There is a gradual shoreward decrease in temperature (T), salinity (S) and density (D).  To the south there are strong T, S, and D gradients associated with the warm loop current.  A general indication of the magnitude of the stratification in the region is obtained from a series of five CTD profiles taken over a distance of 25 km perpendicular to the bottom bathymetry immediately prior to injection of the tracer. The profiles show that the top 20-m are well mixed while below this there is a near linear T decrease of 0.4 oC, a S increase of 0.1, and a D increase of 0.15 kg m-3 to the bottom at 25 m.  Horizontal stratification in T, S, and D was 0.04 oC km-1, 0.03 o/oo km-1, and 0.01 (kg m-3) km-1, respectively.  
Water movement on the shelf is strongly influenced by meteorological events and tides.  The tidal ellipse is roughly aligned perpendicular to the shelf [Weisberg et al., 1996].  On average, net movement is southwestward along shore, with cold air outbreaks and associated north/northeasterly winds causing rapid transport. 

Injection and Methodology

A mixture of (99.7 %) SF6 and (0.3 %) of 3He was bubbled into the water column through 0.5 m of porous tube at approximately 19-21 m depth from a small boat.  The injection occurred in a 3.5-km streak perpendicular to the depth contours at 29 oN , 84 oW.  Based on injection flow rates, approximately 23 mol of SF6 and 0.07 mol of 3He was injected with an unknown fraction escaping in gaseous form from the water column into the atmosphere. The ratio of 3He and SF6 in the initial samples was 0.33 % .  Laboratory studies [Wanninkhof et al., 1993] show a significant fractionation between SF6 and He during partial dissolution.  The constancy in the ratio between the injection mixture and water column thus suggests that most of the tracer dissolved.  Inventory estimates from the surveys of the patch corrected for loss by gas exchange yielded inventories accounting for 70 to 110 % of the estimated injection volume.  Total water depth was 25 m with a pycnocline at 20 m and a decrease in density shoreward (see above). 
"Surface" water SF6, and CO2 levels, temperature, and salinity were mapped by automated instruments sampling water pumped at a rate of 40 L min-1 from the bow at 6-m depth on the NOAA ship MALCOLM BALDRIGE.  When SF6 levels were above background, samples were drawn for SF6 at 3 to 10-minute intervals and measured quantitatively with a precision of 3 %. Sulfur hexafluoride and 3He analyses were performed as in [Wanninkhof et al., 1993; Wanninkhof et al., 1991]. Partial pressure of CO2 (pCO2) and total dissolved inorganic carbon (DIC) were measured to estimate biological productivity from water column inventory changes. These analyses were done using the techniques described in [Johnson et al., 1993; Wanninkhof and Thoning, 1993]. 

Advection and dispersion

Surface realizations of the patch are shown for 3 days, 7 days, and 11 days after injection in Figure 1 based on surveys.  The patch grows rapidly in size as a result of tidal and wind mixing.  The variance of the patch is determined by assuming an elliptical shape and measuring the major and minor axes.  The effective diffusion coefficients and scale of diffusion is given in Table 1. As predicted for two-dimensional Fickian diffusion in the ocean, the effective diffusivity increases with time and diffusion length scale. In the diffusion estimate no correction for loss due to gas exchange is applied as this is a minor term compared to concentration decrease by diffusion and dispersion.  The effective horizontal diffusivity, Ka is about an order of magnitude greater than the values described in [Okubo, 1971] which are primarily from dye dispersion studies. The higher values are attributed to vertical shearing of the patch.  [GET REFERENCE]
During the two hour injection, the winds averaged 9 m s-1 from the NW (or parallel to the coast).  This caused a rapid southward movement of the surface part of the patch while the bottom part moved towards the coast.  This behavior is in agreement with observations by [Marmorino, 1983] who described counterclockwise veering with depth, as in a bottom Ekman layer, from moored current meter observations in this area .  The first vertical SF6 profiles taken 1 day after the injection showed vertically uniform distributions in the water column but subsequent profiles throughout the first week had little SF6 below 15 m.  The profiles clearly show the complex water movement and vertical shear. Profiles showed SF6 maxima at the surface or at mid-depth, while at other locations double maxima were found, with high levels at the surface and mid-depth.  This suggests  that on short time scales the spread of tracer is not a simple eddy diffusive process, but an interplay of advection and diffusion at different length and time scales. Since only the surface levels of SF6 were measured continuously, the sub-pycnocline maximum was not resampled till a week after injection.  3He/SF6 ratios in this bottom maximum were the same as the highest ratios observed immediately after injection. Since 3He escapes the water column by gas exchange about three times faster than SF6 this unequivocally shows that the bottom water was isolated from surface gas exchange for the entire experiment.  The bottom layer was completely separated from the surface layer during much of the first week since no elevated SF6 concentrations were encountered near the bottom during extensive profiling of the surface patch.  Towards the end of the experiment the top and bottom layers of the patch again coalesced.  Surface CODE drifters released simultaneous with the tracer showed an initial strong southward movement, and subsequent movement back to the initial location of the deployment during the experiment. This is in agreement with the inferred movement of the surface tracer patch.

Gas Exchange  

The dual tracer release of SF6 and 3He makes it possible to determine the air-sea gas exchange of these compounds.  As detailed in [Wanninkhof et al., 1993; Watson et al., 1991] the absolute exchange rates of either gas can be determined with knowledge of the relative rates of escape of SF6 and 3He.  For 3He, the gas transfer velocity is expressed as:

	k3He	= h d(ln(3He/SF6)/(1-(ScSF6 / Sc3He)-1/2))/dt		(1)

where k3He is the gas transfer velocity of 3He; ScSF6 and Sc3He are the Schmidt numbers for SF6 and 3He (1251 and 163, respectively at the average SST of 16.8 oC); -1/2 is the Schmidt number exponent; h is the average mixed depth; and SF6 and 3He are the excess SF6 and 3He concentrations in the mixed layer.  The implicit assumptions in the analysis are that on the 2-week time scale of the experiment, (1) gases in the top 20-m of the water column could exchange with the atmosphere, (2) that a negligible amount of the deep tracer maximum diffused into the mixed layer, (3) and that bubble mediated gas exchange was negligible, justifying use of the -1/2 Schmidt number dependence [Asher and Wanninkhof, 1995].  Using these criteria, a subset of the samples were used to determine the gas transfer velocities.  The seven stations used in this analysis had a vertically uniform 3He/SF6 profile down to 20-m and they were removed from the location of the deep SF6 maximum.  Samples drawn near the deep maximum had anomalously high 3He/SF6 ratios.  The gas transfer velocity for 3He, k3He determined from the slope in ln(3He/SF6) (Fig. 2) is 
17 ±  2 cm hr-1.  For comparison with previous observations [Wanninkhof et al., 1993; Watson et al., 1991] this value is normalized to CO2 exchange at 20 oC using a square root Schmidt number dependence, kCO2 = k3He (Sc3He/ScCO2)0.5, where ScCO2 is the Schmidt number of CO2 in seawater at 20 oC of 660.  The kCO2 of 8.4 ± 1 cm hr-1 at a wind speed of 4.7 ± 2.4 m s-1 as measured at 20 m on the ship agrees well with previous work on Georges Bank and the empirical equation described in [Wanninkhof, 1992].  The error bar is the uncertainty in the slope of a linear fit between ln(3He/SF6) and time.  An added uncertainty is the depth of the column exchanging with the atmosphere above the pycnocline. This varied from day to day due to variable heating and mixing during the experiment. We estimate conservatively that this increases the uncertainty by 20 %.  In this initial analysis only a single gas exchange velocity is obtained because of the periodic vertical inhomogeneity in tracer distribution and absence of prolonged unique wind events. 

Biological production

Changes in the water column inorganic carbon budget can be used to estimate biological production if advection and dispersion are properly accounted for [Chipman et al., 1993].  An estimate of net production/remineralization was made from the continuous surface water pCO2 measurements and select measurements of total inorganic carbon (DIC).  For an isolated water parcel, the rate of biological production is equal to the change in total inorganic carbon in the water column, dDIC/dt.  For a system open to the atmosphere, a correction must be applied for gas flux which can be done quantitatively from the gas transfer velocity derived above.  For this experiment productivity was estimated from the change in inorganic carbon near the center of the patch over time.  An implicit assumption in this exercise is that there is negligible mixing with external waters of different DIC near the center of the patch. Since only a limited amount of DIC samples were taken while pCO2  is measured eight times per hour, and because pCO2 has a greater sensitivity to changes in biological production than DIC, DIC changes were calculated from changes in pCO2.  For instances when pCO2 and DIC were measured simultaneously at the surface, the alkalinity and pCO2 at average temperature and salinity (16.9 oC and S=35.17) were calculated using the dissociation constants of [Mehrbach et al., 1973] and a computer program developed by [Lewis and Wallace, 1995].  The temperature correction takes into account the change in pCO2 due to changes in temperature.  The salinity normalization for DIC and pCO2 corrects for dilution.  The temperature normalized pCO2 was linearly correlated to DIC, 35 (= DIC * 35/Sal) such that a DIC,35 could be inferred for all pCO2 points.  The resulting purely empirical linear relationship was, (Fig. 3a):

 pCO2(16.9 C)[±2] = 0.80 [±0.05] DIC,35 -1315 [±113], r2 = 0.89, n = 27	(2)

 where the numbers in brackets are the uncertainties in the linear regression.  pCO2(16.9 C) is in µatm, while DIC(35) is in µmol kg-1.  This regression is used in combination with the trend of pCO2 (16.9 C) with time to infer the change of DIC.  For the period from April 5 through April 16, 1996, the DIC,35 of the water column increased by 0.92 [±0.29] (µmol kg-1) day-1 (Fig. 3b).  Using the average gas transfer velocity calculated from the 3He/SF6 over the period, an average mixed layer depth of 20m, and an average ∆pCO2 of -48 ± 6 µatm for the period, the increase of DIC,35 due to gas exchange is 0.17 (µmol kg-1) day-1. The difference between the increase of DIC in the water column and the increase due to gas exchange of 0.75 (µmol kg-1) day-1 is attributed to net remineralization.  The calculated TAlk,35 shows a weak increase with DIC,35 with a slope of 0.34 [±0.22] (Fig. 3c).  If the DIC,35 increase is corrected for the gas exchange contribution the ratio observed during the experiment increases to 0.4.  This ratio gives the amount of calcium carbonate dissolution relative to remineralization of soft tissue.  The ratio is in line with the global average of 0.25 [Broecker and Peng, 1982].  
This simple calculation incorporated several approximations and assumptions. The relationship between pCO2 and DIC, used to infer changes in DIC, is  influenced by the very process we want to determine such that the relationship has an implicit time dependence. Secondly, the pCO2 will increase over time due to gas exchange.  The theoretical change of pCO2 with respect DIC to  due to gas exchange, dpCO2/dDICgas≈ 1.5, and the theoretical change due to biological productivity is  dpCO2/dDICgas≈ 1.2 which differs from the slope in Eq. 2.  However,  the trend in Figure 3A is primarily spatial rather than temporal and effects will increase the scatter in Figure 3A but  it does not change the slope of the relationship.
The remineralization over the two week period corresponds to a 10 µmol kg-1 increase in water column DIC.  This increase is similar to the spatial variations observed around the patch illustrating the importance of performing the calculations in a Langrangian reference frame.  The remineralization of DIC should correspond to increases in nutrients.  Select samples for nitrate, nitrite, ammonia, phosphate and silicate were taken but not systematically in the center of the patch.  Levels were low with average values for ammonia of  0.33 ± 0.4 µmol kg-1 (n=182) ; for nitrate of  0.23 ± 0.1 µmol kg-1 (n=156); for nitrite of  0.17 ± 0.08 µmol kg-1 (n=171); for phosphate of  0.35 ± 0.2 µmol kg-1 (n=76); and for silicate 1.57 ± 0.3 µmol kg-1 (n=76).  Ammonia and nitrate did not  increase systematic with time, while phosphate showed increase of 0.006 ± 0.005 µmol kg-1 day-1  which corresponds with the change in DIC but is statistically not significant.
Independent estimates of productivity using oxygen incubation and phytoplankton growth experiments confirm that the remineralization exceeded productivity during most of the cruise.  The large undersaturation of pCO2 and relatively high chlorophyll values indicate that the area was likely sampled following a spring bloom.  Diatoms blooms are a common occurrence in this area of the shelf during the spring (GABE IS THIS TRUE).

Conclusion

The tracer measurements during the Langrangian study on the West Florida shelf showed complex mixing and dispersion of a shallow coastal region. A weak stratification below 20 m was sufficient to isolate the bottom part of the tracer patch from the influence of gas exchange throughout the entire experiment.  The inhomogeneity of the tracer patch complicated interpretation of gas exchange and water column productivity estimates on short time scales. Estimates over the duration of the experiment did yield robust values.  Horizontal surface dispersion estimates from the second moment of the tracer patch are greater than the conical values of [Okubo, 1971].  We hypothesize that this is caused by vertical shearing of the tracer patch.  The air-sea gas transfer velocity obtained for the study agrees well with the empirical equation of [Wanninkhof, 1992].  From the change in water column inorganic carbon inventory, a net remineralization of organic matter was deduced in accordance with incubation experiments.  The study illustrates the merit of deliberate tracers to augment "natural tracers" such as salinity and temperature to study biological and physical processes in complex coastal circulation systems such as encountered on the West Florida Shelf. 

Figure captions:

Figure 1.  Surface SF6 concentrations contours at 3 days (Fig. 1a), 7 days (Fig. 1b), and 11 days (Fig. 1c) after injection.  The concentration scale is parts per trillion by volume (pptv). The plus symbols denote the cruise track and the solid thick line shows the location of injection.  The solid contour depicts the variance, from which the effective horizontal diffusion, Ka was calculated. The first two surveys were performed in 24 hours while the third survey took 40 hours.  Contours were generated using Surfer for Windows using a Kriging interpolation/extrapolation scheme. All three plots are on the same scale with a aspect ratio (latitude/longitude) of 1:2.  Unrealistic concentrations far removed from the cruise tracks are blanked. 

Figure 2. Decrease in ln (3He/SF6) versus time.  The gas transfer velocity can be determined from the slope of the least squares linear fit according to Eqn. (1). The error bars are the standard deviation from the mean of depth profiles comprised of three to five samples.

Figure 3.  Determination of net productivity/remineralization by changes in carbon inventory.  Fig. 3a shows the linear relationship between DIC,35 and pCO2(16.9 C) which is used to deduce the DIC,35 in the patch.  The solid line in Fig. 3b is the trend of calculated DIC,35 versus time caused by net remineralization and air-sea gas exchange, and the dashed line is the calculated increase in DIC due to air-sea gas exchange.  Fig. 3c. shows the relationship between DIC,35 and TAlk,35 from which the partitioning between calcium carbonate dissolution and soft tissue remineralization is determined.

Table 1. 
Horizontal diffusion coefficients

aTime 		bMax. SF6	cs 2		dKa		el		fpredict. Ka
(Days) 	(ppt)		(10 12cm2)	(10 5cm2 s-1)	(106 cm)	(10 5cm2 s-1)
						
3		300		0.18		1.70		1.25		0.49
7		52		5.82		24.08		7.24		1.51		
11		32		10.12		26.63		9.55		2.77

a: time since injection.  The mid point of the respective surveys were used for time
b: Maximum SF6 levels encountered during the survey
c: variance determined from the variance in the major (sx) and minor (sy) axes (s 2 = 2 sx sy )
d :Effective horizontal diffusion coefficient ( Ka = s2/4 t)
e: diffusion length scale (3 * s)
f: predicted Ka= 0.0108 t2.34 [Okubo, 1971]

References
Asher, W., and R. Wanninkhof, The effects of breaking waves on dual-tracer gas exchange experiments, in Air-water gas transfer: Selected papers from the third international symposium on air-water gas transfer, edited by B. Jähne, and E.C. Monahan, pp. 517-528, Aeon Verlag & studio, Hanau, Germany, 1995.
Broecker, W.S., and T.-H. Peng, Tracers in the Sea, Eldigio Press, Palisades, 1982.
Chipman, D.W., J. Marra, and T. Takahashi, Primary production at 47oN and 20oW in the North Atlantic Ocean: A comparison between the 14C incubation method and mixed layer carbon budget observations, Deep-Sea Research II, 40, 151-169, 1993.
Johnson, K.M., K.D. Wills, D.B. Butler, W.K. Johnson, and C.S. Wong, Coulometric total carbon dioxide analysis for marine studies: maximizing the performance of an automated continuous gas extraction system and coulometric detector, Mar. Chem., 44, 167-189, 1993.
Lewis, E.R., and D.W.R. Wallace, Basic programs for the CO2 system in seawater, Brookhaven National Laboratory, 1995.
Marmorino, G.O., Variability of current, temperature, and bottom pressure across the west Florida continental shelf, winter 1981-1982, J. Geophys. Res., 88, 4439-4457, 1983.
Mehrbach, C., C.H. Culberson, J.E. Hawley, and R.M. Pytkowicz, Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure, Limnology and Oceanography, 18, 897-907, 1973.
Okubo, A., Oceanic diffusion diagrams, Deep-Sea Res., 18, 789-802, 1971.
Wanninkhof, R., Relationship between gas exchange and wind speed over the ocean., J. Geophys. Res., 97, 7373-7381, 1992.
Wanninkhof, R., W. Asher, R. Weppernig, H. Chen, P. Schlosser, C. Langdon, and R. Sambrotto, Gas transfer experiment on Georges Bank using two volatile deliberate tracers, J. Geophys. Res., 98 (C11), 20237-20248, 1993.
Wanninkhof, R., J.R. Ledwell, and A.J. Watson, Analysis of sulfur hexafluoride in seawater, J. Geophys. Res., 96, 8733-8740, 1991.
Wanninkhof, R., and K. Thoning, Measurement of fugacity of CO2 in surface water using continuous and discrete sampling methods, Mar. Chem., 44 (2-4), 189-205, 1993.
Watson, A.J., R.C. Upstill-Goddard, and P.S. Liss, Air-sea exchange in rough and stormy seas, measured by a dual tracer technique, Nature, 349, 145-147, 1991.
Weisberg, R.H., B.D. Black, and H. Yang, Seasonal modulation of the West Florida continental shelf circulation, Geophys. Res. Let., 23, 2247-2250, 1996.

Figure 1A

Figure 1B

Figure 1C  

Fig. 2