Article pubs.acs.org/est
Simulation of Observed PCBs and Pesticides in the Water Column during the North Atlantic Bloom Experiment Lin Zhang,†,‡ Louis Thibodeaux,§ Lee Jones,∥ and Rainer Lohmann*,† †
Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882-1197, United States Biological Science and ∥Mathematical Science, University of Massachusetts Lowell, Lowell, Massachusetts 01854, United States § Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70802, United States ‡
S Supporting Information *
ABSTRACT: The dynamics of persistent organic pollutants in the oceans are not well constrained, in particular during a bloom formation and collapse. Polychlorinated biphenyls (PCBs) and some pesticides were measured in air, water, and zooplankton tracking the North Atlantic Bloom in May 2008. Lower weight PCBs were entering the water column from the atmosphere during the main bloom period but reached equilibrium after the bloom collapsed. The PCBs in the lipids of zooplankton Calanus were in equilibrium with those in the dissolved phase. A Lagrangian box model was developed to simulate the dissolved phase PCBs and pesticides by including the following processes: air−water exchange, reversible sorption to POC, changes in mixed layer depth, removal by sinking particles, and degradation. Results suggest that sorption to (sinking) POC was the dominant removal process for hydrophobic pollutants from seawater. Statistical test suggested simulated results were not significantly different from observed values for hydrophobic pollutants (p,p’-DDE).
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directions were not always depositional.9 This study utilizes the measured gaseous and dissolved phase POPs concentrations as well as numerical models to study the fate of dissolved POPs in the surface water, which include exchange with the gas phase in the overlying atmosphere, sorption to and release from (settling) particulate organic carbon (POC), exchange between upper and lower water column resulting from stratification and deepening of the mixed layer depth (MLD), and chemical degradation. Due to logistical constraints (cost of ship-time for pumping hundreds of liters seawater at sites), concentrations of dissolved POPs below the mixed layer had to be modeled. Air−water gas exchange is considered one of the most important processes that affects the fate of POPs in the surface ocean. However, there are large uncertainties in estimating the air−water gas exchange fluxes.10 Estimation of fluxes of PCBs between air and water are made using empirical models relying on air−water exchange velocities (Vgas/seawater). Over the past several decades, several empirical wind parametrization of Vgas/seawater have been made, such as the piecewise linear relationship by Liss and Merlivat,11 the quadratic relationship,12−14 and the cubic relationship.15 These different wind
INTRODUCTION Polychlorinated biphenyls (PCBs) and several organochlorine pesticides (OCPs) are considered persistent organic pollutants (POPs) ubiquitously present in the remote oceans.1−4 Similar to greenhouse gas CO2, POPs can be removed by both the biological pump and physical pump. Lipophilic POPs bind to sinking POC generated during phytoplankton blooms and subsequent vertical export into depth (biological pump for POPs). The physical pump refers to the air-sea deposition of POPs and deep water formation that transport water and dissolved POPs to depth. Although the biological pump may dominate the fluxes on a global ocean scale, the removal by physical pump may occur on a longer time scale.5 POPs associated with settling organic particles may get released to water column when the organic particles get remineralized. At certain locations, remineralization occurs in relative shallow waters (60−100 m).6 The released CO2, nutrients, and POPs are mixed back into the water column and returned to the surface ocean by winter storms. In addition to the biological and physical pumps, POPs can also be removed by degradation. A recent study reported that the magnitude of the degradation can be as large as the biological pump7 and emphasized the importance of biological pump in the scenario that the air−water−plankton−deep water system was close to steady state.8 The purpose of this study was to investigate different removal processes of POPs during a dynamic phytoplankton bloom in the North Atlantic when the air−water exchange © XXXX American Chemical Society
Special Issue: Ron Hites Tribute Received: January 14, 2015 Revised: May 1, 2015 Accepted: June 8, 2015
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DOI: 10.1021/acs.est.5b00223 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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were decanted, and samples were extracted again with the same amount of solvent mixture. The two extracts were mixed together, rotoevaporated down, and solvent exchanged to hexane, and the lipid fraction was quantified gravimetrically. Concentrated sulfuric acids were then added to remove lipids and other interferences.27 Extracts were then passed through alumina and silica cartridges (Agilent) to further remove polar interferences, which were modified from previous studies21−23,28 and concentrated to about 100 μL. Tribromobiphenyl was added as an internal standard to the concentrates prior to GC-MS-MS (Waters Quattro MicroGC) analysis. Detailed GC temperature profile can be found elsewhere.1 The amount of PCBs in the laboratory blanks and recoveries for different 13C-PCBs are in Table SI-1. Ten different PCB congener (#8, 18, 28, 52, 44, 66, 101, 118, 153, and 138) concentrations were measured and reported in Tables SI-2 and 3. Concentrations of measure OCPs were reported elsewhere.9 Lagrangian Box Model Formulation. A Lagrangian float mimicked the movement of a water parcel and stayed in the same phytoplankton bloom patch from year-day (YD) 95145.6,25 If the measurements of POPs were conducted on the float, that would represent the time rate of change in the Lagrangian reference frame which does not include the advective effects. Our measurements were conducted on the R/V Knorr which was following the float and conducted geographically fixed patterns (bowtie) in a relatively small area (60.6°N, 25.4°W, to 62.1°N, 27.6°W). In essence, the sampling campaign was close to being “Lagrangian” but was limited by sampling constraints for POPs. Initially, we assume that these transport processes (lateral advection and eddy diffusion driven transport in and out of the box) did not cause significant changes in dissolved POPs budgets with time. We also examined the residuals defined as the difference between the simulated dissolved POPs concentrations with our observations to verify if any processes were missed from our model. A mass balance tracking the concentration of dissolved POPs in the upper 100 m water column, both above and below the mixed layer depth (MLD, m), was set up to study how different biogeochemical processes affected the environmental behavior and fate of POPs in the dissolved phase. During the NABE the MLD varied greatly in the top 100 m water column. The different biogeochemical processes (Figure 1) include air− water gaseous exchange fluxes (Fa‑w), chemical degradation reactions in the water (Fr), partitioning into and release from particles (Fpartitioning), removal from the surface ocean mixed layer by sinking particles and release into lower water columns (Fsinking), and exchanges caused by variations in MLD with time (FU/L). These individual processes in the mass balance were described below (eqs 2-6) and then combined into the mass balance equation (eqs 7 and 8) All fluxes are in pg m−2 d−1. Air−Water Gas Exchange Fluxes (Fa‑w). Air−water fugacity ratios (FRs) were calculated to assess the air−water gas exchange directions of PCBs following previously established methods1,3,9
speed parametrizations lead to large discrepancies in calculated global and regional CO2 fluxes13 and resulted in as much as 40% differences in air-sea exchange of O 2 during a phytoplankton bloom.16 How these differences in fluxes affect the fate of dissolved POPs in the phytoplankton bloom has not been investigated. Partitioning onto and settling with POC that sinks out of the upper water column represent two removal processes of POPs from the seawater dissolved phase.10 The plankton mass in surface waters is considered to be the largest fractions of POC and contributes greatly to the food web,17 especially during a phytoplankton bloom. There is disagreement on whether POPs reach equilibrium with plankton - some studies imply not,17−20 while several field and laboratory studies showed that bioaccumulation of PCBs was at steady-state.21−23 The equilibrium state of POPs between the lipid of the biota and the dissolved phase in the water column can be assessed by correlating the lipid−water partitioning coefficient (Klipid) against octanol−water partitioning coefficients (Kow) for a series of compound with the same type of interactions with the biological lipids (i.e., PCBs).22−24 Klipid can be derived using field measured POPs concentration in the lipids of biota and in the dissolved phase. Zooplankton samples were collected in this study to assess if the POPs between the lipid of the biota and the dissolved phase were at equilibrium during the dynamic evolution of phytoplankton blooms. To assess the fate of dissolved POPs in the mixed layer, the concentrations of POPs in the air, water, and POC phase need to be measured, but also supplementary biogeochemical, oceanographic, and meteorological information are necessary. The extensive multidisciplinary study during the North Atlantic Bloom Experiment (NABE) in 2008 provided such a unique opportunity to study the fate of hydrophobic POPs. The North Atlantic spring bloom is characterized by its rapid production of POC in the mixed layer and high vertical export rate of POC out of upper water column. POC and sinking POC concentration, mixed layer depth, and wind speed were measured during NABE in 2008. During NABE, the R/V Knorr followed a patch of phytoplankton bloom south of Iceland in the North Atlantic.6,9,25,26 Simultaneous air, water, and zooplankton samples were collected to determine the air− water gas exchange directions and the partitioning process of PCBs and OCPs between zooplankton and seawater. Different removal processes were compared to assess their contributions in determining the fate of PCBs and OCPs during NABE.
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MATERIALS AND METHODS Sampling Location, Sample Collection, Extraction, and Analysis. Sampling locations, collection approaches, extraction, and analysis of air and water samples have been described elsewhere.9The plankton collected using bongo net tows were of two size fractions, 200−500 μm and >500 μm. Most of the zooplankton biomass was in the fraction >500 μm. In total, there were 11 zooplankton samples collected during the three-week cruise (Table SI-6). The majority of the zooplankton collected was Calanus f inmarchicus, the most abundant copepods in the North Atlantic waters. The extraction of zooplankton samples was modified from a previously established method.23 Briefly, the samples were extracted using liquid−liquid extraction with 200 mL of hexane and acetone (1:1, v/v) adopted from Sobek et al.23 Samples were shaken for 12 h. Seven different 13C-labled PCBs were added prior to extraction as surrogate standards.1,9 The solvents
fg fw
=
Cg Cw ·K aw
(1)
where fg and f w are the POPs’ fugacities in air and seawater, Cg and Cw are gaseous (pg m−3) and dissolved (pg L−1) concentrations of PCBs, and Kaw is the air−water partitioning coefficient.29 Considering the uncertainties introduced in analytical measurements, air−water partitioning coefficients, B
DOI: 10.1021/acs.est.5b00223 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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where Psinking is the flux of POC exported out of the surface ocean (aka sinking) mixed layer (kg C m−2 d−1). The POC export fluxes were estimated as the difference between the net community production (NCP) minus time rate of changes in POC concentrations and advection assuming the accumulation of DOC is insignificant. DOC was not measured during this cruise. But based on C:N ratios of DOM in previous studies, DOC accumulation was only observed in oligotrophic areas subtropical gyres.16 It was suggested that DOC accumulation was only a small fraction of NCP, and POC export accounted for 60−70% of the estimated NCP. The exported POC sank very fast out of the mixed layer at a relative constant rate partially due to the ballast effect provided by siliceous shells in diatoms and calcite shells in coccolithophore,16 which is a removal process for hydrophobic POPs. Depth profiles of O2:NO3− uptake ratios, comparisons between 234thorium derived fluxes and those measured by the Lagrangian float suggested very high remineralization rates between 60 m and 100 m.16,25,35 Thus, the POPs removed from upper water column by sinking POC would be released back to the lower water column below the MLD once the POC were remineralized, which is a source term for POPs budget in the lower water layer. Chemical Degradation Reaction in the Surface Ocean Mixed Layer (Fr). The pseudo-first-order rate coefficient for chemical degradation reaction in the surface ocean mixed layer was calculated as kr = ((In 2)/λ1/2) (day−1). λ1/2 is the half-life of POPs in the surface seawater, and the degradation loss Fr is
Figure 1. Biogeochemical processes and physical exchanges of PCBs and pesticides during the North Atlantic bloom 2008 in the upper 100 m water column. C1(i) is the concentration in the mixed layer at time step i, whereas C2(i) is the concentration in the water column below the mixed layer depth (MLD) but above 100 m. See more detailed descriptions in the main and SI text.
and error propagation,9,30,31 only FRs > 3.1 were considered as net dry deposition and FRs < 0.32 as net volatilization with 95% certainty. Fugacity ratios in between 0.32−3.1 are deemed not significantly different from air−water equilibrium.31 Air−water gas exchange flux (Fa‑w, pg m−2 d−1) was calculated according to a modified version of a two-film model32 as detailed elsewhere.15,33,34 A positive Fa‑w flux value indicates a net volatilization flux from water to air ⎛ Cg ⎞ Fa‐w = Vgas/seawater·⎜Cw − ⎟ K aw ⎠ ⎝
Fr = k r*Cw*h
Exchange between Upper and Lower Water Layer across Mixed Layer Depth (FU/L). The vertical change of the MLD with time can also redistribute the dissolved POPs in the water column. There was a storm before the main bloom period6,16 which deepened the mixed layer depth to close to 100 m. Temperature, salinity, O2, and nitrate depth profiles showed uniform distribution through the mixed layer6,16 which indicates the dissolved POPs should have be well mixed initially. At a later time the thermal stratification of the water column resulted in a shoaling of MLD to less that 30 m until the year-day 139 (Figure 2). A reduced (shallower) MLD
(2)
where Vgas/seawater is the air−water gas exchange mass transfer velocity (m d−1) Partitioning into and Release from Particles (Fpartitioning). Fpartitioning = h*P*COC = h*P*K OC*Cw
(3)
where hu (m) is the height of the upper layer (sea surface to mixed layer depth), hL is the height of the bottom layer (mixed layer depth to 100 m below sea surface), and U and L denote upper and lower layer. P is the concentration of particulate organic carbon (kg C L−1) obtained from beam attenuation and backscatter during NABE which were calibrated with shipboard measurement of POCs6 in the upper layer (Pu) and lower layer (PL). COC (pg g−1 POC) is the concentration of POPs in POC, which equals Cw * KOC if the partitioning exchange of POPs is at equilibrium between the dissolved phase and sorbed to POC. KOC is the equilibrium partitioning coefficient (L/kg C) between fresh organic carbon and seawater. Temporal variation of this flux determines whether it is a source or sink term. If there was net accumulation of POC within δt, it would be a removal process for dissolved POPs as they absorb into POC. If there was net loss of POC due to remineralization, sorbed POPs would be released into water column as a source to the dissolved phase. Fluxes Caused by Sinking Particles (Fsinking). Fsinking = Psinking*COC = K OC*PsinkingCw
(5)
Figure 2. Mixed layer depth and particulate organic carbon concentrations in the water column above and below the mixed layer depth as a function of year-day of 2008.
(4) C
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with the concentrations (average of 9.6 ± 9.0 pg m−3) after the bloom started to evolve in this study from YD 123.5 as well as the findings from cruise ARKXX in the Greenland Sea and Arctic,4 which suggest there was no significant concentration gradients in background gas phase concentrations of PCBs between Arctic and North Atlantic. The few high spikes in the concentration detected in this study (YD 121.5 to 123, average of 60.5 pg m−3) and the ones found in the southern tip of Norwegian Sea (100 pg m−3)4 probably reflected local sources of PCBs. The dissolved phase concentrations of PCBs ([ΣICESPCBs]dissolved) ranged from 0.05 to 5.64 (1.82 ± 1.80) pg L−1 (Table SI-3). During the first half of the cruise YD 123 to 128, the [ΣICESPCBs]dissolved was significantly greater (3.58 pg L−1 p < 0.05) than the during the remainder of the cruise (0.79 pg L−1). The main bloom period was from YD127-134, and the percentage of relatively more lipophilic PCB congeners (hexachlorinated ones PCB 138 and 153) dropped from 31% to 13% on average from prebloom (before YD 128) to after bloom. This suggested that partitioning to particulate organic matter (POC) produced in the bloom from seawater may at least be partially responsible for the decrease in the observed [ΣICESPCBs]dissolved. Previous studies conducted in the Arctic and North Atlantic reported similar concentrations. The ARKXVI cruise in 1999 in the Arctic (62°−75°N) found an average [ΣICESPCBs]dissolved of 1.20 ± 0.83 (0.53−2.27) pg L−1.4 The Swedish Arctic Ocean expedition (SWEDARCTIC 2001) reported [ΣICESPCBs]dissolved from 0.13 to 3.92 (1.38 ± 1.13) pg L−1.36 A few samples from the SWEDARCTIC 2001 were collected from the Norwegian Sea Current (Sample No. 1−3) with a concentration of 2.94 ± 0.97 pg L−1. The samples collected in this study represent the Irminger Current,9 which is one of the two branches of the North Atlantic Current (the other one is the Norwegian Sea Current). Comparison between the two currents suggested that there was no significant difference between them (p > 0.05). Previous studies also reported concentrations in the North Atlantic close to the continents. The [ΣICESPCBs]dissolved in the English Channel was 1.35 pg L−1(49°N)3 and 2.27 pg L−1(40.5°N, 71.2°W) on the US east coast.37 The differences in concentration levels within the Atlantic were small compared to other ocean basins such as Mediterranean Sea (6.63 ± 7.26 pg L−1) and Black Sea and Marmara Sea (12.91 ± 12.12 pg L−1),28 which suggests there was no significant concentration gradient from the Northern North American and Northern European coasts to the remote Atlantic Ocean and the distribution of PCBs in the North Atlantic was relatively uniform. Air−Water Gas Exchange of PCBs. Calculated air−water fugacity ratios (FRs) suggested that most of the PCBs congeners analyzed were depositing from the atmosphere to the underlying water at the beginning of the cruise until YD 125.5 (Table SI-4). After YD 125.5 only lightweight PCB congeners were detected in both gas and dissolved phases rendering the assessment of air−water exchange directions only available for PCB 8, 18, and 28. These lightweight congeners were found to be net depositing during the main bloom period which was from YD 127 to YD 134. The net air−water exchange was at equilibrium after the bloom collapsed after YD 135. There have been only a few studies conducted in the North Atlantic and Arctic in which PCBs were detected simultaneously in both gas and dissolved phase to determine air−water exchange directions. It was found that PCBs were
would lose volume (and POPs) from the upper layer to the lower layer, whereas deepening of MLD would gain water volume and POPs from the lower layer into the upper layer. If MLD deepens, the upper layer mass change is approximated by the average concentration in the lower layer multiplied by the time change of the MLD position and vice versa for the lower layer.
FU/L = Cwδh/δt
(6)
The mass balance equations for upper and lower layer are hu
⎛ Cg ⎞ δCu δ(hu*Pu*Cu) = Vg/w ·⎜Cu − − kr ⎟ − K OC* δt K δt ⎝ aw ⎠ *Cu*hu − K OC*Cu*Psinking − Cu*δh/δt
hL
(7)
δC L δ(hL*PL*C L) = −K OC* − k r*C L*hL + K OC*Cu δt δt *Psinking − C L*δh/δt
(8)
These two differential equations for the Lagrangian box model were solved using the finite difference method in Matlab (Mathworks, Matlab 2013b) to simulate the changes of PCBs and OCPs dissolved phase concentrations with time. The dissolved POPs concentrations on year-day 123 were used for initial conditions for both layers because the water column was initially well mixed down to ∼100 m. We let the initial dissolved water concentration, KOC, KAW, and kr vary 10 times each following normal distribution. The maximum likelihood fitting was applied to the 10,000 Monte Carlo simulation curves to find the best fit. Chi-square test was conducted to assess the relationship between the best fit and observations (see SI for more details). The residuals were also calculated (Figure SI-4).
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RESULTS AND DISCUSSION PCBs Concentrations in Air and Water. The PCB concentration of seven ICES (International Council for Exploration of the Sea) congeners in the gas phase ([ΣICESPCBs]gas) ranged from 2.4 to 70.9 (20.3 ± 23.0) pg m−3 (Table SI-2). The sample with highest [ΣICESPCBs]gas was collected on year-day (YD) 122 when the ship was still in the Reykjavik harbor and back trajectories indicated that the air masses were from the Arctic.9 The concentrations were greater at the beginning of the cruise (average [ΣICESPCBs]gas 60.5 pg m−3) until YD 123.5. During the rest of the cruise, the [ΣICESPCBs]gas were relatively low with an average of 6.5 pg m−3 except at YD 125.5 and 135 with two relatively higher concentrations detected. The lowest [ΣICESPCBs]gas was found on YD 139 when the main bloom had collapsed and the air masses were originated from the East Greenland Sea according to the back trajectories.9 The concentration range detected in this study was comparable to published concentrations across the North Atlantic and Arctic Ocean. Two cruises conducted on a similar track in 20011 and 20043 reported [ΣICESPCBs]gas of 31 ± 28 and 53 ± 55 pg m−3, respectively. On another cruise (ARKXX) covering the Norwegian Sea (60−72° N), Greenland Sea (72−78° N), and the Arctic Ocean (78−85° N) average [ΣICESPCBs]gas of 30 (6−100), 9 (3.5−22), and 7 (0.76−43) pg m−34 were reported. The Arctic Monitoring and Assessment Program (AMAP) also reported [ΣAMAPPCBs]gas concentrations at long-term land-based monitoring stations in the Arctic and North Atlantic area. The AMAP measurements were in line D
DOI: 10.1021/acs.est.5b00223 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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bloom evolved after year-day 131 of 2008, the biomass collected increased, and the major zooplankton in the >500 μm fraction were determined to be copepod Calanus f inmarchicus. The lipid percentage in the samples were 4.09 ± 1.56% (1.72−6.45). The total ICES PCB concentrations in the Calanus normalized to lipid content ([ΣICESPCBs]lipid) ranged from 0.45 to 6.28 (2.72 ± 1.80) ng g−1 lipid (SI-6). TriCl and Hepta-Cl PCBs were the most abundant congeners in the ΣICESPCBs accounting for 32 ± 15% and 30 ± 10%, respectively. Sobek et al.23 also found various Calanus species (C. finmarchicus, C. glacialis, C. hyperboreus) dominate the >500 μm fraction of their zooplankton samples with PCB 28, 52, and 153 concentrations of 4.67 ± 3.40, 6.82 ± 3.81, and 7.52 ± 4.11 ng g−1, respectively. At one specific station where C. f inmarchicus was the major component, the [PCB28, PCB 52, PCB 153]lipid were 3.1, 4.6, and 8.3 ng g−1, respectively. The same group also reported concentrations of other smaller copepods collected from Gullmar Fjord on the Swedish west coast (56°N, 11°E), which were 3.3−14 (median 5.2) ng g−1 lipid for PCB 52 and 10−26 (median 19) ng g−1 lipid for PCB 153.22 These previously reported concentrations were greater than the ones found in this study (Table SI-6). The differences likely stemmed from different locations and Calanus species. Another possible reason would be the biomass dilution effect28 due to the great amount of organic matter produced in the bloom in this study. During NABE, diatoms bloomed followed by coccolithophores upon silica depletion, both of which were heavily grazed by zooplanktons and produced a constant “POC rain” in the water column.6,16 In a previous study in the Mediterranean Sea, the dry weight concentrations of PCB 52, 101, 138, and 180 were negatively correlated with the biomass abundance of planktons.28 Lipid to water partition coefficients (Klipid L/kg) of zooplankton samples were calculated according to eq 9. Klipid is the equilibrium partitioning coefficients of POPs:
depositing in the Norwegian Sea, Greenland Sea, and Arctic during the ARKXX cruise on R/V Polarstern between June and August 2004.4 During a later cruise on the same ship from Germany to South Africa from October to December 2005, PCBs were found to be net depositing from air to water in the North Atlantic but were at air−water exchange equilibrium in the South Atlantic.3 Three different wind-speed dependency relationships from Liss and Merlivat,11 Nightingale et al.,14 and Wanninkhof and McGillis15 were chosen to compare different air−water exchange velocities and resulting PCBs fluxes. The Nightingale 2000 relationship was chosen because it was derived from an experiment conducted during an open ocean algal bloom. Results suggested that the Wanninkhof and McGillis relationship always produced the highest transfer velocity, whereas the linear one gave the lowest velocity with Nightingale 2000 in between (Table SI-5). The Wanninkhof and McGillis was also the only relationship among the three which considered Weibull probability distribution of wind speeds which describes the nonlinear effects of wind speeds.15 The transfer velocity decreased from lightweight congeners (i.e., PCB 28) to heavier ones (i.e., PCB 153) regardless of which relationship was used. For PCB 28, the velocities calculated based on Wanninkhof and McGillis were on average 82% higher than the Liss and Merlivat, whereas it was 52% for PCB 153. To assess how different relationships would affect the air− water exchange fluxes of PCBs and their fate in the water column, transfer velocities derived from three different relationships were tested in the model (see below). Correspondingly, these different wind-speed dependency relationships produced different air−water exchange fluxes. Figure 3 showed the air−water gas exchange flux variation for
Klipid =
C lipid Cw
(9)
where Clipid is the concentrations of POPs in the lipids of biota (ng g−1 lipid), and Cw is the concentrations of POPs in the water (pg L−1). Log Klipid was plotted against log KOW to assess if the bioaccumulation of PCBs by copepods can be treated as an equilibrium partitioning process. KOW describes the partitioning coefficient of chemicals of interest between dissolved phase and n-octanol.34 When a series of compounds with different KOW values such as different PCB congeners (Table SI-7) have the same kind of molecular interactions with the lipids in the organisms, the log Klipid vs log KOW will have a linear relationship which is consistent with equilibrium partitioning.34,38 The slopes of the linear relationship between log Klipid and log KOW reflect the differences in qualities as solvents for various PCBs between lipids and n-octanol. The intercepts indicate additional sorbent−sorbate interactions such as H-donor and acceptor properties.38 The average slope for the linear regressions between the log Klipid of various PCBs in zooplanktons and their corresponding log KOW was 0.95 ± 0.41 (0.38−1.78), and the intercept was 0.03 ± 2.55 (−4.54−4.04) (Table SI-8). These values imply, on average, equilibrium partitioning in the field and agreed with those from several previous studies in both lab and field investigations. The slope and intercept were reported to be 0.47 and 2.58 (r2 = 0.69) for plankton samples collected in the Mediterranean Sea.28 For
Figure 3. Air−water exchange fluxes of PCB 8 calculated using three different relationships (ng m−2 d−1), average wind speed (m s−1) as a function of year-day 2008.
PCB 8 as a function of year-day 2008 at different sampling locations. The flux ranged from −19.97 to 33.63 ng m−2 d−1 based on the Wanninkhof and McGillis relationship, which were 38−95% higher than those according to the Liss and Merlivat relationship. At the same location on year-day 123, PCB 8 had the largest deposition flux (−19.97 ng m−2 d−1) among all PCB congeners, whereas PCB 138 (−0.98 ng m−2 d−1) had the smallest one according to the Wanninkhof and McGillis relationship which again were 30−61% higher compared to the Liss and Merlivat one (Figure SI-2). PCBs in Zooplankton. At the beginning of the cruise, only phytoplankton (
