Oversimpiifications and Confusion Page 19

Appendix
Chronology of major events that potentially influence hypoxia on the
inner continental shelf of the northern Gulf of Mexico

Period Major Events

* Log jam between Old River and Atchafalaya breached (1863)

* Extensive land clearing and cultivation in upper basin: suspended sediment
load in 803 and 90s twice that of 1930s

= Eads begins training of delta passes (79)

* First organized tile drainage in Illinois (79)

* Increase in MR Bight primary production (biogenic silica) first evidenced in
mid 1800s

Pre-1900

Atchafalaya flow ca. 13% of combined flow
Flow down Bayou Lafourche (15%) terminated (04)

Changes in foraminiferan communities in MR Bight indicate hypoxic stress
Manufactured fertilizers begin to be used

Land drainage increases 6 fold over two decades

Floods of 1912-13 prompt Congress to authorize federal funding of levees

i910

Great 1927 Flood, ultimately leads to decision to regulate Atchafalaya flow

1929 and levee and straighten river

Jadwin Plan levees and channel straightening essentially end overbank flow

1930 and shorten river's course by 240 km (30s-40s)

Growth in US consumption of manufactured fertilizers begins (ca 40)
Increase of glauconite indicates greater anoxia in MR Bight (42)

More land is drained, area more than doubles from 50-70
Extensive dam building in Missouri and Arkansas reduces suspended
sediment flux by 50% in a decade ($3-62)

Use of nitrogen fertilizer in Mississippi Basin begins rapid growth (60)
Decline in diversity of ostracods and further changes in forams in MR Bight
Si:N ratios in lower river begin rapid decline from 4 to | (60-80)

Period of higher average river flows begins (72), new record flood in 73
First actual measurements of hypoxia off SE Louisiana and Texas coasts
(72-74)

Atchafalaya flow peaks at 35% stabilized at 30% (75-76)

Doubling in deposition of marine organic carbon in MR Bight begins (ca 77)
High flow in 79 transports twice as much nitrate as 73

N fertilizer use in basin begins fluctuating plateau ca. 7 x 1960 level (81)
Systematic surveys and concerted research on hypoxia begin (85)

NECOP project started (90)
1993 flood results in dramatic expansion of areal extent of hypoxic waters
Greatest N fertilizer use in basin (94-98)

Areal extent of hypoxia greatest since surveys began in 85

Federal assessment of Gulf hypoxia undertaken (98-00)



Oversimplifications and Confusion Page 13

flow by 1950 (Topic ! report, Fig. 32). No substantial over-bank flow across the deltaic
plain has occurred since the 1927 flood. While reconstructing wetlands in the watershed,
reopening flood plains, management of Atchafalaya flow, and diversion of lower river
flows through deltaic wetlands may all be important in reducing nutrient delivery to the
Gulf in the future, none of these geomophic/hyrologic changes appears to be primarily
responsible for the late 20 century intensification of hypoxia.

18

1.6 283

14

12 ag 083-—S79
#4]

rs as ha oe eed nitrate
eos 75 74 70 BS a organic N
2 02
zo

20 25 30 35

Annual discharge (10°12 cu ft/yr)
Discharge from UA report; N flux from CENR Topic 3 report

False Paradigm Conflicts

Currently, the data are consistent with the eutrophication paradigm but are insufficient to allow wus
to distinguish berween the quantitative roles of inorganic nutrients and organic carbon on the
fertilization processes compared to the effects of stratification on the fertilization processes versus
the effects of stratification on the eutrophication process. UA report p. 55

The UA report outlines three “paradigms” for oxygen depletion in Gulf of Mexico
bottom waters (p. 8-9, Figure 3): (1) eutrophication, involving the increased deposition
of organic matter produced in situ by phytoplankton stimulated by an increase in
nutrients, especially nitrogen; (2) terrestrial organic matter loading in which the
microbial degradation of organic matter transported in rivers induces oxygen depletion;
and (3) stratification, wherein strong density differences between fresh surface and salty
bottom waters inhibits mixing and prevents the exchange of oxygen to bottom waters.

The UA report introduces unnecessary confusion in understanding and polarity in
perspectives in its confection of three “paradigms,” There is but one paradigm here, the
eutrophication paradigm. It encompasses multiple sources of organic matter and the
influences of density stratification of water masses,

Eutrophication has been defined as the increase in organic loading to an aquatic system,
be it from allochthonous organic matter or autochthonous biological production



Oversimpiifications and Confusion Page 4

the Gulf over similar time periods during residence on the shelf, particularly in oxygen-
limited sediments.

Several lines of evidence indicate that the fraction of the organic carbon loading that is
labile or oxidizable is substantially less than 35%. Average concentrations of total
organic carbon (dissolved and particulate) in the lower river at St. Francisville averaged
6.38 mg L” during 1996-98°, while the mean 5-day biochemical oxygen demand (BOD)
over the same period was 1.58 mg L. It would require complete oxidation of 9% of that
organic carbon pool to meet that oxygen demand if the only substance oxidized was
reduced carbon (nitrite, ammonia and other oxygen demanding materials also contribute
to BOD).

In addition, the longitudinal distribution of dissolved organic carbon in the Mississippi
River system strongly suggests that the organic carbon contained in the lower river is a
very refractory residual from the microbial degradation that takes place along the long
ete se mca . + : course to the sea. DOC
concentrations averaged about
10 mg L* at Minneapolis/St.
Paul (nearly 3,000 km upriver
from the Gulf), but declined to
4.2 mg L" at a point 1,500 km
upriver from the Gulf (just
below the entry of the Ohio
River), DOC concentrations
remained relatively constant

tewoveesssara

1,500 km. After the confluence of the Arkansas, the last major tributary to join the
Mississippi, the decline in DOC concentrations suggested that less than 7% of the DOC
was degradable over the last 800 km of transit, It is reasonable to assume that the
processes that degrade the DOC also act on the POC during transit down the river.

All of this indicates that the organic carbon discharged into the Gulf is predominantly
refractory—the recalcitrant remains from hundreds of years of soil processing and a
3,000 km long secondary treatment system—and that the proportion of the residual that
could be further degraded over time scales of weeks to months is less than 10%.
Although there are other suggestions” that a greater percentage (up to 50%) of the
particulate organic carbon discharged by large rivers such as the Mississippi cannot be
accounted for in the sediments in depositional areas, this does not necessarily mean that
that “missing carbon” contributes to shelf metabolism. It could be lost due to desorption
and transported away as dissolved organic carbon or other mechanisms.

* USGS NASQAN data.

* Keil, R.G. et al. 1997. Loss of organic matter from riverine particle in deltas. Geochemica
Cosmochemica Acta 61: 1507-1511.



Oversimplifications and Confusion Page 17

annual flows approximated the long-term average (¢.g., 1992 and 1995) experienced
extensive hypoxia.

Piecing together the evidence in a chronological timeline (Appendix) provides a mosaic
that makes the sequence of multiple changes clearer. Although the Louisiana inner shelf
may have experienced hypoxia, especially after floods, in the historic past, the first clear
evidence of eutrophication (increasing supply of organic matter) was evidenced in the
Mississippi River Bight (the eastern most part of the present hypoxic region) during the
19" century, probably as a result of large soil losses associated with land clearing.
Microfossil evidence suggests that biological effects of hypoxia in this most proximal (to
the river discharge) part of the region began to be witnessed in the early 20" century. It
is quite possible that this hypoxia was driven by the massive terrestrial particulate organic
and nutrient inputs that were experienced at this time, and consequently, limited to the
Mississippi River Bight region. During the first half of this century the proportion of
flow discharged through the Atchafalaya progressively increased.

Toward the middle of the century, while suspended sediments (including POC and PON)
decreased as a result of post-dust bow! soil conservation and, later, dam construction,
inorganic nitrogen supplies increased, first gradually, then in 60s and 70s dramatically.
This infusion of biostimulatory material, the effects of which are magnified by recycling,
resulted in increased intensity of hypoxia in the Mississippi River Bight and,
hypothetically, significant expansion of hypoxia to the west. However, because there are
few sites on the shelf outside of the Mississippi Delta Bight that experience the high
sediment deposition rates needed for the types of paleo-biogeochemical studies that have
been used in the Bight, no clear evidence of this post-60s expansion yet exists, The lack
of highly depositional areas elsewhere in the present hypoxic region itself is a strong
reason why terrestrial organic carbon deposition cannot be important in oxygen
consumption processes over the broad region.

Closing

The UA report does acknowledge that terrigenous nutrient loading plays a role in shelf
hypoxia. However, in its emphases on the role of terrigenous organic carbon, particulate
nitrogen and river hydrology, it seeks to cast doubt on the importance of secular increases
of nitrate loading and the efficacy of land-based nutrient controls. This critique has
revealed that the UA report substantially misunderstands of the scale, geography and
history of Gulf hypoxia; lacks balance in the treatment of biostimulatory effects of
organic carbon versus nutrients; and selectively cites literature sources without careful
attention to the nature of their evidence and assumptions. Furthermore, it misrepresents
the extent to which physical forces, such as freshwater inputs, stratification and mixing,
have already been addressed in the many scientific papers on the factors responsible for
hypoxia.

In addition, other sections of the UA report betray either by lack of rigor or selective
emphasis a certain bias toward the interests of sponsors. There is, for example, a very
shallow and dismissive analysis of the effects of hypoxia on the ecosystem and on living



Oversimpiifications and Confusion Page 9

management regarding the importance of nutrients in coastal eutrophication, not to
mention the even longer experience with managing nutrient enrichment in lakes.

Raising of the organic carbon argument by consultants for the fertilizer industry is
reminiscent of arguments of the detergent industry that it was organic carbon and not
phosphorus that was causing lake eutrophication. Once we moved past that red herring,
thanks to classic studies in the Canadian lakes and the Great Lakes, society was able to
adapt to low phosphorus detergents and routinely apply phosphorus removal in waste
treatment. Our laundry remains clean, but many lakes are much cleaner as a result!

Meanwhile, the total nitrogen flux of the Mississippi River system has more than doubled
since the publication of the Ryther and Dunstan classic (comparison of 1969, a year of
about average flow, with the 1986-95 mean, Topic 3 report), while the terrigenous
organic carbon loading has declined (UA report)!

Sediment Leftovers

Carbon isotopic studies af Gulf af Mexico continental shelf sediments, up until 1997, had concluded
that $0% or more of surficial organic matter derived from marine or phytodetrital sources.
However this came into dowbt when Goi et ai, (1997) pointed out that the ''C “ages” of surface (D-
2 cm) sediments are very ald (~2600 to 6800 years),

Using Gulf of Mexico shelf and slope sediment cores collected in 1987, Goi et al. (1998) have
recently shown the importance of land-derived C, plant (corn and grasses) organic matier in these
sediments. They also conclude that this material is old, highly degraded and has been cycled
through the soil several times, UA report p. 47

Based on these recent published reports, the UA report the interpretation of
several studies of the ratio of stable isotopes of carbon ( and °C) that concluded that
away from the vicinity of the river a php Rt in Louisiana shelf
sediments is predominantly phytoplankton-derived. However, none of the sediment
samples reported on by Gofii and colleagues’? was collected from the inner-mid shelf
hypoxic zones, rather their samples were from the continental slope at water depths of 74
or more, particularly down the prodelta slope, where most of the river-derived POC is
deposited. Furthermore, Goiii et al. suggest that little of the C4 terrestrial plant carbon
that confuses the interpretation of the ' C deposits on the shelf as opposed to the slope.
Nonetheless, these new interpretations of the '"C do raise some important questions that
merit further study and caution care in interpretation of these ratios. Nonetheless, the
changing ratios found with distance from riverine sources, the differences in isotope
ratios in contemporary river-borne organic carbon and shelf sediments, and the changes
in isotope ratios in sediments laid down during the late 20" century reported by previous
and summarized in Report 1—all of which are consistent with a marine

investigators
origin of the carbon—still require explanation.

" Gofi, M.A. et al. 1998. A reassessment of the sources and importance of land-derived organic matter in
surface sediments from the Gulf of Mexico. Geochemica et Cosmochemica Acta 62:3055-3075.



Oversimpiifications and Confusion Page 6

hypoxic zone. Of course, this pertains to the average conditions for the hypoxic zone;
nearer the rivers a greater percentage could be accounted for by terrigenous carbon, while
regions remote from the rivers receive insubstantial labile terrigenous carbon. Curiously,
the UA report pays no attention to the losses of terrigenous organic carbon to
sedimentation, dispersion and metabolism as it is transported over distances of 30-380
km during the several weeks’, yet devotes an appendix to calculations that seek to
demonstrate that less than 25% of the marine organic carbon produced in waters
overlying hypoxic zones is deposited to the bottom—over a vertical distance of 10 m or
less. Actual published observations from sediment traps show that there is sufficient
seston deposition from surface production, especially during the spring, to supply the
organic carbon required (Topic | report).

The UA report also suggests that large injections of terrigenous organic carbon during
floods may have consequences for oxygen depletion in subsequent years. Considered in
the context of the previous discussion, this would have to assume that the small labile
portion of this organic matter that is deposited on the shelf would remain largely
undegraded over six months in relatively warm Gulf of Mexico bottom waters until
stratification sets up the next year and then be actively mineralized. Also, while the
multi-year consequences of flood events deserve more attention, it is important to keep in
mind that these floods also inject elevated masses of nutrients, which by stimulating plant
production greatly magnify the pool of labile organics on the shelf (see below). Organic
matter produced in situ as a result of such pulses could also remain stored in the shelf
system until hypoxia sets up in the following spring. At least in the 1993 flood, a much
larger than normal portion of the flow through the Mississippi delta proper was lost from
the shelf environment as a result of the
strong discharge jet that propelled the
plume onto the slope and wind forcing of
currents toward the east. So, a smaller
than normal proportion of the organic
carbon spike could be “credited” to the
carbon metabolism of the Louisiana shelf.
Again, this underscores the need to
consider the importance of interannual
inputs of both carbon and nitrogen from a
dynamic and geographically realistic
perspective rather than as a simplistic
black box.

Concomitant with the greatly diminished suspended particulate concentrations
experienced in the lower Mississippi River during the last half of the 20" century (50 to
70 % reductions'®), the concentrations and loadings of organic carbon have also declined.

° Mean currents available to transport DOC or POC are quite slow, <25 cm sec-| in the coastal boundary
jets and slower in most of the region experiencing hypoxia (Wiseman, WJ. and F. J. Kelly. Seasonal
variability within the Louisiana Coastal Current during the 1982 flood season. Estuaries 17:732-739. )

"* Kesel, RH. 1988. The decline in the suspended load of the Lower Mississippi River and its influence on
adjacent wetlands. Environmental Geology and Water Sciences 11-271-281.



Oversimpiifications and Confusion Page 5

Moreover, the use of benthic respiration as a basis of comparison with terrigenous
organic inputs ignores the role of respiration in the bottom layers of the water column in
depleting oxygen. Water-column respiration in the isolated bottom waters is at least
equal to benthic respiration and may be 2 to 3 times the rate of benthic respiration’.

The UA report did not consider other published reports* of total respiration below the
pycnocline in hypoxic areas, which estimate the annually integrated subpycnocline
respiration to be 197 g Q, mTM yr. Using the same C:0 and areal assumptions as the UA
report, this translates to an annual requirement of 8.0 x 10'° moles of C to satisfy the
observed metabolism. This is within the range, but above the mean, of the two bounding
estimates of the UA report.

Given that not more than about 11% of the terrigenous labile organic matter could
contribute to hypoxia and less than 35% of this organic matter may actually be oxidizable
within the time frames in which hypoxia develops, the contribution of terrigenous organic
sources to hypoxia has to be a very small part of the carbon respired in hypoxic waters.
The UA report states that: “ even if only 10% of the total organic carbon flux could be
metabolized within the bottom
waters or at the sediment-water
ad interface, this could account for a
Led Hypoxic Zone significant portion of 2 loss.”
Carbon Economy Because of the losses en route, it is
likely, based on the calculations
developed here, that not more than
4% of the total terrestrial organic
flux is metabolized within hypoxic
layers and this proportion may be
less than 1% if only 10% of the
3% organic carbon is labile.
Furthermore, in either case, this
Subpyenoclime Riverine Labile Net Primary would account for only a small part
Respiration © CSupplied Production of the subpycnocline respiration.

On the other hand, the production of plankton carbon in the surface waters overlying
hypoxic waters (summarized in CENR report 1) is 9 to 32 times greater than the
maximum amount of riverine labile carbon reaching hypoxic zones, depending on
assumptions about the lability of the terrestrial carbon. If, as argued above, the
proportion of the degradable organic carbon is closer to 10%, this would suggest that
terrestrial organic carbon sources could be responsible no more than about 7% of the
oxygen respiration below the pycnocline, with the remaining carbon coming from the
surface production (44% of which would be required to support subpycnocline
respiration) or the productive coastal boundary layer along the inshore border of the

10 motes C
S5=z

coneaaw

* CENR Report | and Bierman, et al. (1994). A preliminary mass balance model of primary productivity
and dissolved oxygen in the Mississippi River phunve/inner Gulf shelf region. Estuaries 17: 886-899.

* Justic, D., N.N. Rabalais, and RE, Tumer. 1996. Effects of climate change on hypoxia in coastal waters:
a doubled CO, scenario for the northern Gulf of Mexico. Limnology and Oceanography 41:992-1003.


Oversimpiifications and Confusion Page 18

resources in which it is stated: “,.,Dagg (1995) recently proposed that increased primary
production in Louisiana-Texas Continental Shelf coastal waters may be increasing
secondary production of commercially important fishery species. Dagg suggests that any
proposed decrease in riverine nutrient loading to alleviate hypoxia be weighed against the
possible effects this decrease may have on species at higher trophic levels.” Yet the
Paper cited'* is limited to an analysis of copepod grazing rates and does not discuss
increases in primary and secondary productivity, much less present evidence or reasoning
to suggest that decreases in riverine nutrient loadings would affect higher trophic levels.
At best, this is a serious misrepresentation.

Whether intended or not the effect of the UA report is to confuse and cloud the reader’s
understanding of the causes of Gulf hypoxia, rather than to clarify and illuminate.
Nonetheless, it is hoped that in the long run, the report will have a positive effect by
revealing uninformed viewpoints that can be corrected and legitimate issues that require
further research and analysis,

TM Dugg, M.J, 1995, Copepod grazing and the fate of phytoplankton in the northern Gulf of Mexico,
Continental Shelf Research 15: 1303-1317.



Oversimplifications and Confusion Page 8

This actually underestimates the relative mass of utilizable N because it excludes all
organic N, even though some of the organic N is either directly utilizable by
phytoplankton or would be released as useable ammonium following heterotrophic
mineralization. Even if fully 35% of the riverine organic carbon were labile, just the
readily bioavailable, inorganic nitrogen discharged by the rivers would support the
production of almost three times as much readily labile organic carbon by the marine
plankton. However, the real impact of the nutrient additions is that they are recycled,
used again and again to produce more marine carbon. As discussed in the UA report (p.
41) the phytoplankton production beyond the immediate dilution plume close to the river
passes is based on ammonium and other recycled compounds. Evidence reviewed in
CENR Report | suggests that an atom of nitrogen discharged by the river is used to
produce organic carbon an average of four times before it is lost due to burial,
denitrification of flushing off of the shelf. This is plausible, even conservative, based on
known recycling rates in other coastal ecosystems''. This means that at least 11 times
more labile organic carbon is produced by phytoplankton supported by terrigenous
nutrients than directly provided in the form of terrigenous carbon, and this might be 39
times as much if only 10% of the riverine organic carbon is effectively labile.

This difference of an order of magnitude or more is consistent with the previous
estimation that terrigenous organic carbon could contribute less than 10% of the oxygen-
consuming carbon in hypoxic waters. It further suggests that if nitrogen is subject to the
proportionally similar advective, depositional and metabolic (in this case denitrification)
losses as river-supplied organic carbon, the nitrogen inputs are sufficient to match the
respiratory demands resulting in hypoxia.

The magnifying effects of the stoichiometry and recycling of nutrient additions in
comparison to organic carbon inputs to coastal ecosystems have been widely appreciated
over the thirty years since the landmark paper of Ryther and Dunstan'?, As a result, the
science and management of coastal water quality shifted from a preoccupation with
biochemical oxygen demand (BOD) of organic wastes to an emphasis on nutrient
loadings and dynamics. Organic loadings from point sources produced intense, but
relatively localized impacts, mainly on the benthic environment as influenced by
deposition of highly labile particulate organic carbon. These impacts have been largely
ameliorated as a result of waste treatment (solids removal and secondary treatment). On
the other hand, the more widely distributed and often distant consequences of
anthropogenic nutrient inputs, including those from the harder-to-control nonpoint
sources, have been documented, analyzed and modeled in many areas around the world.
Nutrient enrichment has been accepted as a major cause of environmental degradation in
many regions of the US, Europe and Japan, where major commitments to reduce nutrient
loadings have been made by governmental, industrial and agricultural sectors. The UA
assessment does not consider this 30-year development in science and environmental

" Howarth, R.W. 1991. Comparative responses of aquatic ecosystems to toxic chemical stress, p. 169-195.
In J. Cole, et al, (eds.). Comparative Analysis of Ecosystems. Springer-Verlag, N.Y.

" Ryther, J.H., and W.M. Dunstan, 1969. Nitrogen, phosphorus and eutrophication in the coastal marine
environment. Science 171:375-380,



(Ormce oF THe ParsipenT

Pers Orrick Bene 775
Covance, SO) 21613-0775
——— (410) 228-9250 wxx. 602

—_——_ University of Maryland Fax (410) al
boeschieca unces
CENTER FOR ENVIRONMENTAL SCIENCE et nee ures ot

December 16, 1999

Gulf of Mexico Hypoxia Working Group

National Oceanic and AtmosphericAdministration
National Centers for Coastal Ocean Science, Room 9127
1305 East West Highway

Silver Spring, MD 20910

To whom it may concern,

Please accept the attached document as a contribution to the Gulf of Mexico
Hypoxia Integrated Assessment.

This white paper addresses issues raised by the report of the University of
Alabama entitled The Role of the Mississippi River in the Gulf of Mexico Hypoxia, which
was referenced in various public comments on the technical reports on which the
assessment is based.

An earlier draft of my paper was distributed prior to and discussed at the
December 3 workshop held in St. Louis. The current version reflects changes and
additions resulting from those discussions,

1 am submitting this evaluation as an individual scientist knowledgeable about
scientific issues related to eutrophication, including hypoxia in the Gulf of Mexico.

Sincerely yours,
Donald F. Boesch
Professor
Attachment
APPALACHIAN LABORATORY CHESAPEAKE BIOLOGICAL LABORATORY HOkN Point LABORATORY

An InSTIFUFKON OF THE LiewniewrTy SrSrEM OF MARYLAND



Oversimpiifications and Confusion Page it

(based on just two samples) to the concentrations, adjusted for particle surface area
effects, in 15 surface sediment samples in the depocenter offshore. Not only is this
highly dependent on scant but much higher estimates of particulate nitrogen
concentrations that the voluminous USGS data, but also on numerous assumptions
about the contemporary age of the surface sediments and unknown gains and losses
of N in the sediments (¢.g. denitrification) as discussed by Mayer et al. If the
concentrations of the PN in these two SPM samples are indeed too high an estimate
of average concentrations by a factor of 2, much less more than 4, there would be no
evidence that any of the riverine PN is labile, as is the case for several other rivers
Mayer et al. studied.

Using the more data-rich USGS estimates (1980-98) of PN (27 4M reported as PON) and
DIN flux (108 .M) in recent years, assuming that the PN/SPM ratio is constant and the
same SPM concentrations, DIN + PN concentrations would have increased by 12% since
50-52, despite a 47% decline in PN. Because only a fraction (<<S0%) of the PN and
DON is bioavailable, the increase in labile N must be substantially greater than that.

Long-Term Hydrologic Changes

[There is evidence of a significant increase in the discharge of the Mississippi since 1967. This
amounts to appraximately an 11% increase in river discharge and is most likely the result of climate
and precipitation changes and also im reservoir operation by the Corps of Engineers since that time,
Discharge to Atchafalaya Bay has also increased during this century as a result of artificial control of
Row, UA report p. 32
{4}s a result of the channelization and levecing of the river. loodwaters spend significanty less time in
historical floodplain during flooding events. This results bt more rapid transit of river water fo the
Gob ant tcc agoramiy fe reams nih neiiends the hectpiat chee amt rnen ved
occur. Both of these factors reduce the opportunity for nitrate removal via denitrification. p. 34

Climate and, consequently, river discharge vary dramatically, We have known that for a
long time. Furthermore, we are becoming increasingly aware that there may be long-
term, secular trends superimposed on this variation. It is also clear that humankind’s
plumbing of this great river system has affected how much freshwater flow and its
constituents are delivered to the Gulf, and when and where. But, the appropriate question
is: Could these climatic variations and changes and hydrological modifications alone,
without any increases in anthropogenic loadings of nutrients in the watershed, have
caused observed changes in the delivery of nutrients and hypoxia in Gulf?

Much attention is paid in the UA report about whether the hypoxic zones have increased
since systemic mapping began in 1985 and following the 1993 flood and the relationship
of these trends to river flow and nutrient delivery. I do not believe that one can conclude
that nutrient loading or hypoxia have increased during the 1980s and 1990s, The
interannual variations in estimates of the extent of hypoxic bottom waters are functions of
interannual variations and seasonal timing of the delivery of fresh water and nutrients,
meterological and oceanographic forces that affect stratification (e.g. formation of a
secondary thermocline causing an oxycline), mixing and currents, and the imprecision of
the survey as a means to characterize the extent of hypoxia for the year (a still frame in an



Oversimpiifications and Confusion Page 10

Furthermore, if most of the organic carbon in shelf sediments is thousands of years old
(from the time it was originally incorporated in plant tissue), then the carbon isotope
ratios tell us nothing regarding the contemporary sources of the new, labile organic
carbon that is responsible for shelf hypoxia. If the carbon were indeed thousands of years
old, it would suggest that the majority of the organic carbon pool in the sediments is very
refractory material. Phytoplankton-derived carbon could be almost totally labile, fuel
respiration, and contribute little to the sediment carbon trash heap. To deduce the relative
importance of contemporary supplies of organic carbon from analysis of remnants
thousands of years old might then be analogous to describing the diet of modern
Americans solely from studies of Indian shell middens.

Total Available Nitrogen Loadings Have Decreased

Recent work by Mayer et al. (1998) has indicated that although the dissalved inorganic nitrogen (DIN)
concentrations of the Mississippi River have increased over time from 1950-52 until 1973-82, the amount
of “labile” or bioavailable particulate nitrogen (LPN) has decreased.

Ione argues, as Mayer and colleagwes have, that this LPN can be rapidly solubilized and wtilized by
coastal phytoplankton, the actual bioavailable M introduced to the shelf from the Mississippi River may
have actually decreased from 1950-52 ti 1973-82 (Figure 31) UA Report, p. 48.

Do the results of Mayer et al. (1998) truly demonstrate that total labile nitrogen loading
has actually decreased since 1950s as a result of declines in PN as suggested in the UA

Report?

Mayer et al. made only two measurements of total nitrogen in suspended particulates
the Mississippi River in 1994 and 1997, yielding estimates of 2.5 to 3.5 mg g”. All
estimates of PN concentrations for various time periods were made by multiplying these
concentrations by Meade and Parker's (1985) estimates of average suspended particulate
material concentrations. There are several problems with this approach:

1. All extrapolations were based only on these two estimates. Using the average of 3
mgs " and Meade and Parker's average SPM for 1973-1982, the volumetric PN
concentration would be 1.7 mg L”', which is 4.5 times higher than the average PON
concentrations (1080-98) measured by the USGS NASQAN based on a large number
of samples over many months and years,

2. No actual measurements were made for any of the time periods considered (50-52,
53-72, and 73-82). The estimates for these periods assume the same concentrations
of PN per mass of suspended particulates, despite the fact that the factors acting to
reduce SPM concentrations by half could have also affected these concentrations.
Moreover, the factors causing the observed increase in DIN during these time periods
and between 1982 and the time of collection of Mayer et al.’s samples could have
also affected the PN.

3. The assumption regarding how much of the PN delivered by the river is bioavailable
in the Gulf is based on differences in the concentrations of PN per mass of SPM



Oversimplifications and Confusion Page 7

Thus, if it were primarily injections of terrigenous organic carbon that were fueling
hypoxia on the shelf, the intensity of hypoxia should have decreased since mid-century.
Biological and chemical indicators in Mississippi Delta Bight sediments, as summarized
in the CENR Topic 1 report, suggest the opposite is true.

Stoichiometry and Cycling of Carbon and Nitrogen

If one were to make the case that terrigenous organic carbon is an important contributor
to the metabolism depleting oxygen on the shelf it would seem that a requisite part of that
argument should be comparison with other sources of organic carbon, particularly
plankton production. However, the UA report makes no attempt to compare directly the
contributions to shelf hypoxia of terrigenous carbon versus in situ marine production
stimulated by terrigenous nutrient inputs. How then could one conclude which is more
important? A first-order approach to determination of relative importance is the
comparison of the flux of labile organic carbon from the rivers to the primary production
that could be supported by the flux of bioavailable nitrogen or phosphorus.

First, let us summarize the various estimates of contemporary carbon and nitrogen flux
from the Mississippi-Atchafalaya River system:

Component Average annual flux (10" g) Source
Total organic carbon, 1973-95 2 Carey et al. (1999)
Total organic carbon 5.76 Trefrey et al. (1994)
Total organic carbon, 1996-98 5.80 USGS NASQAN data
Total nitrogen, 1980-96 1.57 Goolsby et al. (1999)
Total nitrogen, 1996-98 133 USGS NASQAN data
Inorganic nitrogen 1980-96 0.99 Goolsby et al. (1999)
Inorganic nitrogen 1996-98 0.99 USGS NASQAN data

Even using the higher values for total organic carbon flux, a few straightforward
calculations quickly lead one to the conclusion that terrigenous nitrogen has to be far
more important than terrigenous organic carbon without having to address the losses of
carbon due to advection, deposition and metabolism considered above.

Average annual flux 10" moles Marine C/
Riverine C
Riverine inorganic nitrogen 0.71
Riverine total organic carbon 4.83
Riverine labile organic carbon (@35% of total) 1.69
Riverine labile organic carbon (@ 10% of total) 0.48
Marine organic carbon directly supported by new riverine 4.68 2.8-10.2
nitrogen (Redfield atomic ratio of 106/16 = 6.625)
Marine organic carbon supported by new and recycled 18.74 11.1-39.0
nitrogen (average recycling = 4)



Oversimpilifications and Confusion Page3

to be metabolized in the bottom layers of hypoxic zones. These estimates of losses en
Toute are in all cases conservative and, except for respiration in the surface waters above
the hypoxic zone, do not include metabolic losses in transit. In reality, far less than 11%

of the river's organic carbon reaches hypoxic bottom waters on the shelf. This
hypothetical budget is based on the following assumptions and estimates:

Loss of C Estimated rate Rationale

Deposition amd 15% of Atchafalaya | Huge swamp basin and emerging delta and

mineralization in flow surrounding tidal wetlands trap POC and provide

Atchafalaya Basin and opportunity for mineralization of labile organic

delta matter.

Near-field deposition of 60% of the POC A large portion of the suspended load is deposited
{POC flux is 48% of | when river currents enter Gulf and lose momemtum
the TOC’) and as a result of flocculation. Most of the primary

depositional zones around the Mississipi dela
proper* and along the Chenier Ptain coast (primary
shelf depocenser for the Atchafalaya discharge) are
not subject to hypoxia.

Advection of flow through | 50% of Mississippi Only half of flow through the Mississippi delta

Mississippi delta into deep | delta flux Proper stays on shelf and flows to the west; much

water small proportion of this flow may remain on shelf

under very high di and west wind conditions.

Transport to of 40% of remaining ‘The area on the shelf over which the river effluents

the shelf outside the labile TOC spread (as indicated by reduced surface salinity) is

hypoxic zone far larger than the hypoxic zone, extending both
farther inshore and offshore.

Respiratory consumption | 80% of the remaining Hypoxic bottom waters originate from the deeper

in surface waters over labile DOC shelf and mix little with surface waters. Although

hypoxic waters terrigenous POC may sediment directly into this
layer from river-influenced surface waters, DOC
must be assimilated by microorganisms in the surface
mixed layer, which consume most of this carbon
through their respiration. The assumption that 20%
is biodeposited to the bottom layer is very liberal
given the known efficiency of retention within the
microbial loop.

The assumption that 35% of the TOC flux of the Mississippi-Atchafalaya rivers is readily
oxidizable or labile is very probably too high. Although this value was taken from a

Paper on organic fluxes in world rivers, the Mississippi is a long, nutrient-rich and well-
oxygenated river that provides ample opportunity for mineralization of organic matter
during transit. It is not clear how a significant portion of the labile carbon pool would
travel down the river undegraded over weeks to months only to be rapidly degraded in

* Although the UA report cites Trefrey et al. that 66% of the TOC is POC and 34% is DOC, more recent
and extensive sampling by the USGS NASQAN for the lower Mississippi and Atachafalaya (1996-98)
indicates that the ratio of TOC-DOC in terms of average concentration is 37:63, but the ration in terms of
average annual flux is 48:52. This assumes POC and DOC are equally labile.

*As depicted in the UA report (Fig. 30), Trefry et al, (1994) showed that approximately half of the near
bottom POC in the nearfield plume within 30 km of Southwest Pass was terrigenous in origin, reflecting
the rapid deposition of suspended sediments upon leaving the turbulent flow regime of the river.



Oversimpiifications and Confusion Page 14

stimulated by nutrient additions'*, Clearly, the metabolism of all coastal ecosystems is
based on allochthonous and autochthonous organic carbon; it is a question of proportion,
not of one state or another. Based on the rationale developed above, allochthonous
organic carbon, while contributing to the pool of organic carbon, can support but a minor
portion of the metabolism over the more than 10,000 km? of continental shelf influenced
by seasonal hypoxia. Besides, by all accounts, human activities have resulted in
declining terrigenous organic loadings and diminished lability of this organic matter
during the last half of the 20 century, a period during which nitrate flux tripled (Topic 3)
and the sediment record reveals organic deposition and hypoxia increased. Taken on its
own, the evidence from the northern Gulf of Mexico strongly supports increased nutrient
loadings rather than terrigenous organic carbon inputs as the reason for this
eutrophication. Considered in the context of the global scientific understanding of
eutrophication causes and trends, uncertainties diminish even more.

Similarly, the eutrophication paradigm (a paradigm in the sense originally used by
Thomas Kuhn'* is not simply a hypothesis but a set of causal explanations widely
accepted within and used in practice by the scientific community) includes the notion that
density stratification is necessary for the development of hypoxia in bottom waters'®,
Nutrients and stratification are not competing hypotheses, but interacting factors. In fact,
variations in river flow affect both the delivery of nutrients but also the buoyancy of
surface waters, and thus the strength of stratification, If the Louisiana shelf waters were
not stratified, there would be no hypoxia. Seasonal destratification and mixing from
storms demonstrate that every year, The driving force of river flow means that nutrient
delivery and stratification strongly covary. By the same token, if there were no readily
oxidizable organic carbon in the ecosystem, by what process would the oxygen in the
isolated bottom layers be depleted?

‘The causes of stratification and its role in inducing hypoxia are described in the Topic 1
report, which makes it clear that stratification is a requisite condition. Rather than
arguing whether changes in the delivery of fresh water are responsible for increased
hypoxia as opposed to increased nutrient loading when both factors may have, in fact,
changed, it would be more appropriate and useful to pose the following question: Given
the changes in and variability of flow, what would be the extent and severity of hypoxia
had nutrient concentrations of the effluents not increased? This question is partially
approached by the modeling in the CENR Topic 4 report. The model estimated that, for
three years modeled, if nutrient loading were 20 to 30% less (river flows and water mass
structure held constant), bottom water dissolved oxygen would have been 15 to 50%
higher. These at-least-internally-consistent predictions together with the historical
reconstructions support the conclusion that reductions in nutrient loadings would reduce

"* Nixon, S.W. 1995, Coastal marine eutrophication: a definition, social causes, and future concerns.
Ophetia 41:199-219,

* Kuhn, T.S. 1962. The Structure of Scientific Revolutions. University of Chicago Press, Chicago. 212 p.
TM Hypoxia occurs in unstratified coastal waters only in poorly flushed backwaters where intense organic
loading or over-night respiration is sufficient to deplete the oxygen inventories faster than they can be
resupplied by diffusion or photosynthesis.



Oversimpiifications and Confusion Page 16

Factor Short-term (days-a few years) | rm (decades)
Riverine organic carbon Relatively unimportant over the ‘Unimportant, loadings have
hypoxic zone, may be more important decreased substantially during the
near the river sources 20° century
Riverine nutrients Important factor; stimulates the Dominant factor affecting long-

Production of the vast majority of the | term changes in hypoxia: flux of
‘Organic matter that, on decomposition, | nitrate increased 3-fold between
depletes the dissolved oxygen 1970 and 1983, but increased flux
of utilizable N probably began
during late 19" century with land-

clearing and cultivation. |
‘Water column structure Important factor: extent of hypoxia Increased flows down the
and circulation depends on current patterns, wind Atchafalaya during early 20"
mixing. and intrusions of salty bottom | century influenced stratification
water (secondary pycnocline). and circulation, setting the stage
for hypoxia om southwest
Louisiana shelf during the latter
half of the century.
Freshwater flows Imporant factor affecting both nutrient | Much less important than
flux and shelf stratification, thus increases in nutrient fluxes.
playing a major role in the extent and | Increased 30% during 1970-83,
severity of hypoxia. but equivalent flows during the
carly part of this period
fransported half as mach nitrate.

As explained above, stratification is a key factor that determines the extent and severity
of hypoxia. Factors that influence it, including freshwater inputs, circulation, mixing and
the intrusion of denser water masses from offshore, greatly influence the extent of
hypoxia during a given period (month or year), For that reason, there is far too much
emphasis on the relationship of the spatial extent of hypoxia during one annual survey to
trends in nutrient enrichment. For the most part, those factors that affect water column
structure and circulation on the shelf have not changed over time (except for changes in
discharges, discussed next). ‘The one exception is the changes in the apportionment of
flow between the Atchafalaya and Mississippi deltas. However, it should be noted that at
the beginning of the 20® century, the Atchafalaya was already carrying about 13% of the
combined flow and this proportion reached present levels by 1950. This historical
adjustment may not have “caused” hypoxia off the southwestern Louisiana shelf, but
certainly set the stage for the hypoxia that has occurred as a result of nutrient enrichment
of the rivers,

Finally, there is no question that the volume of freshwater discharges of the Mississippi
and Atchafalaya rivers has a major effect on the extent and severity on hypoxia on the
shelf. River discharges bring not only fresh water that Strengthens density stratification
and affects circulation on the shelf, but also nutrients that fuel the production of organic
carbon. But, the timing of the discharges and meteorological and oceanographic factors
also influence the extent and severity of hypoxia, causing considerable variation around
any expected linear relationships between discharge and extent of hypoxia. Over the long
term, however, changes in freshwater discharge have been much less important than
changes in nutrient delivery in increasing hypoxia on the shelf. Recent years in which the



Oversimptifications and Confusion Page 2

Land-Based Carbon Sources are Relatively Unimportant

The UA report's premise that terrigenous organic carbon is likely to be an important
component of the total organic matter flux, and thus oxygen consumption in bottom
waters that become hypoxic, is based on estimates of the flux of readily degradable
organic carbon discharged by the Mississippi and Atchafalaya rivers; the relationship of
this flux to the metabolic demands required to deplete the oxygen in bottom hypoxic
waters (quoted in the following box); and new interpretations of the ratios of stable
carbon isotopes in shelf sediments that may indicate a dominant terrigenous source of
organic carbon to the shelf.

How important could this allochthonous carbon flux be for the benthic metabolism and O; consumption in
the nearshore and shelf region of the Gulf of Mexico? To answer this question, we have performed a
rather simple-minded calculation We have used Dortch et al.'s (1984) total benthic respiration rate from
the hypaxic zone obtained in 1991 (range of 160 to 500 mg O; mi” d'). We have taken the estimated area
of the hypoxic zane for that year and multiplied the equivalent amount of carbon respired by the measured
benthic respiration rate (Dortch et al, 1994) cited above, to produce an estimate of total benthic
respiration in the kypaxic zone. This yields a carbon respiration or organic carbon consumption af 2.4 fo
11.8210" moles of carbon annwally, This value is quite similar both to that of Trefrey et al. (1994) and the
mean USGS fluc discussed above, even if we assume that 35% (Ettekott, 1988) of the total organic
carbon is labile or axidizable (17x10 moles C and 9.2x10" moles C, respectively). UA report, p. 48.

This comparison is far too simplistic in assuming that the organic carbon in the river
discharges is completely conveyed, as if by pipeline, to hypoxic layers lying 30 to 380
km from the points of the river discharges. In fact, the hypoxic layers constitute only a
small portion (approximately 20% or less) of the water mass on that part of the shelf
influenced in one way or another by river discharges. The comparison completely
neglects the losses of the allochthonous organic carbon due to advection, deposition, and
metabolism en route to the hypoxic zones (schematically depicted below). Furthermore,
it very probably overestimates the proportion of the organic matter that is labile and

underestimates the oxygen
SIMPUSTIC REAUSTIC consumption required to
100% of labite 100% of labile create and maintain
Carton in ver Coon in ter | bottom-water hypoxia.
Deposition Respiration
Atchafalaya Basinidelta ®% Simply put, the “simple-
minded calculation” in the
Laetoli UA report does not take
into account the
eee eaters ane geography, scale and
es a, physical and biochemical
wet — Lael processes associated with
see! shelf hypoxia. The
Respiration in surface 45 4, schematic diagram shows
ba ear aro ¢ why only a fraction of
Al can conttoute to Maxima depostion | river-borne labile organic
poner Sr eeeenze | carbon could reasonably

find its way into position



The Role of the Mississippi River in Gulf of Mexico Hypoxia
Oversimplifications and Confusion

Donald F, Boesch
University of Maryland Center for Environmental Science
November 29, 1999

A report produced for The Fertilizer Institute by the University of Alabama’ entitled The
Role of the Mississippi River in the Gulf of Mexico Hypoxia raises several questions about
whether the large-scale oxygen depletion (hypoxia) in bottom waters of the Louisiana
continental shelf is the result of anthropogenic nutrient loading from the Mississippi
River, as has been reported in the scientific literature and concluded by the Gulf of
Mexico Hypoxia Assessment carried out under the Committee on Environment and
Natural Resources’, The University of Alabama (UA) report calculates that “terrigenous
(land-derived) organic carbon is likely to be an important component of the total organic
matter flux” driving oxygen depletion in hypoxic zones. It also suggests that discharges
of biologically available actually decreased since the 1950s. In addition, it draws
inferences that hydrological changes in the river basin coupled with increased river flow
may be responsible for increased hypoxia rather than just increased loadings of nutrients
from fertilizers and other human sources,

These assertions casting doubt on the importance of increased land-derived nutrient
(particularly nitrogen) loading in increasing the extent and intensity of hypoxia have been
widely cited by agricultural and industrial interests, such as the American Farm Bureau
Federation and The Fertilizer Institute, and some upstream states in their comments on
CENR Assessment.

This white paper is not a comprehensive critique of the entire UA report, but an
examination of the evidence and arguments in the report supporting its contentions that
terrestrial organic loading is important relative to nitrogen, total bioavailable nitrogen
loadings have actually decreased, and hydrological changes contribute to more pervasive
hypoxia. It is intended to inform the CENR Assessment and address the concerns by
upriver and agricultural interests—apparently bolstered by the UA report—concerning
the role of anthropogenic nutrient loadings in hypoxia in the Gulf of Mexico.

Carey, A.E., J.R. Pennock, J.C. Lehwter, W.B. Lyons, W.W. Schroeder and J.-C. Bonzongo. 1999, The
Role of the Mississippi River in Gulf of Mexico Hypoxia. Environmental Institute Publication Number 70.
Prepared for The Fertilizer Institute by The University of Alabama, Tuscaloosa, Alubama 35487-0207.

? Pour reports produced for the CENR assessment summarize relevant literature, data and analyses: Topic
1. Characterization of Hypoxia, Topic 3. Flux and Sources of Nutrients in the Mississippi-Atchafalaya
River Basin, Topic 4. Effects of Reducing Nusrient Loads to Surface Waters within the Mississippi River
Basin and the Gulf of Mexico, and Topic $. Reducing Nutrient Loads, Especially Nitrate-Nitrogen, to
‘Surface Water, Goundwater, and the Gulf of Mexico.



Oversimpiifications and Confusion Page 12

action-packed movie). The big changes in the trophic regime on the inner shelf that led
to more frequent, persistent, severe and extensive hypoxia by all evidence (from the
sediments and river flux) and reasoning occurred earlier, particularly during the 1950s,
1960s and 1970s.

Much is also made in the UA report of a “jump” in river flow after 1967, but careful
examination of the graphs of annual mean flow reveals that flows during the period 1967
to 1972 were at or actually below the 1817-1966 mean. The major features of the flow
record after 1972 are the two highest flow years on record, 1973 and 1993, coming
twenty years apart. Without those two years the 1967-1995 mean flow is very close to
the 1817-1966 mean. Should one conclude that there has a secular jump in river flow just
because of the coincidence of two 100+ year floods within a twenty year period?
Nonetheless, the Topic 3 report also concludes that there has been an increase in flow, so
long-term changes in river flow must be considered, both retrospectively and
prospectively.

How then has increased flow affected nitrogen flux during a period in which we know the
loadings of soluble and highly reactive nitrogen in the watershed have also increased?
Fertilizer consumption also grew rapidly from the late 1950s through the 1970s (UA
report Fig. 13). Atmospheric deposition of nitrate in the watershed also grew during this
same period as it did throughout eastern North America. Nitrate concentrations in the
river were rising during the 1960s (UA report Fig. 12), but fluxes did not grow greatly
until the 1970s when river flow increased dramatically with the record breaking 1973
flood. Flow during 1973 topped the legendary 1927 flood year and brought down more
fresh water, and probably more organic carbon, to the Gulf than the better studied 1993
flood. But something seems to have happened with regard to nutrient delivery of the
river between 1973 and 1993.

Comparison of nitrogen flux to river flow for the eight highest flow years between 1973
and 1993 reveals some interesting patterns (see following graph). First, organic nitrogen
flux seems not to be influenced by flow, at least at these high flow rates. There is no
obvious temporal trend. High river flows afler 1975 have delivered far more nitrate than
previously, Coincidentally, this was a time near the end of the rapid growth in the
consumption of nitrogen fertilizers (UA report Figure 13). To determine if there is a
linkage would obviously require better understanding of the saturation, time lags, and
small scale hydrology, but the point here is that the changes in nitrate delivery were not
due to changes in hydrology alone.

The UA report correctly observes that exclusion of flood plains, channelization of flow
within levees that prevents dispersion of flow through the wetland-estuarine deltaic plain,
and increased flow down Atchafalaya all could work to increase the amount of nutients
the river system delivers to the Louisiana shelf. However, all of these changes occurred
before the rapid increase in nitrogen flux and the apparent increase in hypoxia evident in
the sediment record, both post-1950s (see Appendix). The levees cutting off flood plains
were essentially completed well before then, the Lafourche distributary had been closed
(1904) and the Atchafalaya had already captured approximately 30% of combined river



Oversimpiifications and Confusion Page 15

the extent and severity of hypoxia in the future. This conclusion is bolstered by
comparative observations of the dramatic contraction of the area of hypoxia on the
northwestem shelf of the Black Sea during the 1990s that coincided the substantial
reductions in nitrogen concentrations in the Danube River, which in tum followed the
large reductions in fertilizer use as a result of perestrotka in eastern Europe'”

The question of how much hypoxia would be alleviated in the future if nutrient fluxes
from the rivers were reduced is a legitimate one, This should be the subject of future
research and modeling to inform the policy process, but the answer will remain somewhat
uncertain because of the influence of meteorologic and oceanographic variability.
However, it is abundantly clear that, short of building a conduit for direct discharge into
the deep Gulf, reduction of upstream sources of nutrients or restoration of aquatic
environments that trap them en route is the only thing that society can do to reduce the
scale and intensity of hypoxia on the Louisiana-Texas shelf.

Historical Perspective

It is important to consider all the claims regarding causes of hypoxia on the inner
Louisiana-Texas shelf in historical perspective (see Appendix for chronology). Some
factors may be important on a transient (real time or interannual) basis, but are not the
cause of secular (long-term) changes in the spatial and temporal extent or severity of
hypoxia. The following is an attempt to provide such a perspective, the intent of which is
to organize and clarify the confusion created in the UA report. It considers the four
explanations offered as competing in the UA report (the three “paradigms” plus the factor
of increased freshwater discharges, which is related to the issue of stratification).

As demonstrated above, riverine organic carbon sources cannot be a major factor today
and could not be related to the increased organic enrichment during the second half of the
20" century (as evidenced in the sediment record) because particulate organic carbon
loadings have decreased. Nutrient loading (particularly bioavailable nitrogen), on the
other hand, is the driver of the organic carbon budget of the hypoxic zones of the shelf,
The fact that nitrate loadings have increased dramatically during the last half of the
century—and were very likely elevated above pristine conditions before then—coupled
with evidence and strong arguments for increased organic production on the shelf support
the conclusion that nutrient-eutrophication is the primary cause of secular changes in
hypoxia.

" Mee, L.D. and G, Topping (eds.). 1999. Black Sea Pollution Assessment. Black Sea Environmental
Series Vol. 10. UN Publications, New York.