Estuaries and Coasts
DOI 10.1007/s12237-010-9351-9
Sediment and Lower Water Column Oxygen Consumption
in the Seasonally Hypoxic Region of the Louisiana
Continental Shelf
Michael C. Murrell & John C. Lehrter
Received: 30 April 2010 / Revised: 6 September 2010 / Accepted: 14 September 2010
# Coastal and Estuarine Research Federation (outside the USA) 2010
Abstract We report integrated measurements of sediment
oxygen consumption (SOC) and bottom water plankton
community respiration rates (WR) during eight cruises from
2003 to 2007 on the Louisiana continental shelf (LCS)
where hypoxia develops annually. Averaged by cruise, SOC
ranged from 3.9 to 25.8 mmol O2 m−2 day−1, whereas WR
ranged from 4.1 to 10.8 mmol O2 m−3 day−1. Total belowpycnocline respiration rates ranged from 46.4 to
104.5 mmol O2 m−2 day−1. In general, below-pycnocline
respiration showed low variability over a large geographic
and temporal range, and exhibited no clear spatial or interannual patterns. SOC was strongly limited by dissolved
oxygen (DO) in the overlying water; whereas, WR was
insensitive to low DO, a relationship that may be useful for
parameterizing future models. The component measures,
WR and SOC, were similar to most prior measurements,
both from the LCS and from other shallow estuarine and
coastal environments. The contribution of SOC to total
below-pycnocline respiration averaged 20 ± 4%, a finding
that differs from several prior LCS studies, but one that was
well supported from the broader estuarine and oceanic
literature. The data reported here add substantially to those
available for the LCS, thus helping to better understand
oxygen dynamics on the LCS.
Keywords Gulf of Mexico . Sediment oxygen
consumption . Plankton community respiration . Hypoxia
M. C. Murrell (*) : J. C. Lehrter
US Environmental Protection Agency, Gulf Ecology Division,
1 Sabine Island Dr.,
Gulf Breeze, FL 32561, USA
e-mail: [email protected]
Introduction
Anthropogenic nutrient loading (principally nitrogen and
phosphorus) to coastal and estuarine waters is a global
environmental concern, chiefly because of its role in
eutrophication of many coastal ecosystems (NRC 2000).
One well-documented consequence of eutrophication is the
increased incidence of hypoxia (dissolved oxygen < 2 mg
O2 L−1) in coastal and estuarine systems worldwide (Diaz
and Rosenberg 2008). One of the largest hypoxic zones in
the world forms every summer in the northern Gulf of
Mexico on the Louisiana continental shelf (LCS). Based on
July surveys from 1985 to 2007, the areal extent of hypoxia
averaged 13,500 km2 (Rabalais et al. 2007).
Over the past 25 years, there has been active research on
the LCS exploring linkages between riverine delivery of
fresh water and nutrients, the timing and extent of primary
productivity, and the dynamics of hypoxia formation in
bottom waters (reviews, Rabalais et al. 2007; Dagg et al.
2007; Bianchi et al. 2010). Various models developed for
the LCS predict spatial (e.g., Bierman et al. 1994; Scavia et
al. 2003; Hetland and DiMarco 2008), seasonal (e.g., Justić
et al. 1996), and inter-annual oxygen dynamics (e.g., Turner
et al. 2005; Greene et al. 2009).
Prior studies of oxygen metabolism on the LCS include
sediment studies (Miller-Way et al. 1994; Morse and Rowe
1999; Rowe et al. 2002) or water column studies (e.g.,
Turner et al. 1998), but only Dortch et al. (1994) reported
paired water column and sediment measurements. Here, we
report paired measurements of sediment respiration and
lower water column respiration on the LCS in the region
subject to summer hypoxia. Specific objectives were: (1) to
explore environmental factors that contributed to variability
in measured rates, (2) to estimate the relative magnitude of
water column and sediment respiration in below-pycnocline
Estuaries and Coasts
waters, (3) to examine how below-pycnocline respiration
varied spatially and temporally, and (4) to compare results
with model predictions to test our conceptual understanding
of the mechanisms regulating hypoxia.
Materials and Methods
Data reported herein were collected as part of a 5 year field
research program conducted by the US Environmental
Protection Agency, which included a total of 12 shelf-wide
hydrographic surveys from Dec. 2002 to Aug. 2007.
Station locations largely followed the standard LUMCON
sampling design (Rabalais et al. 1999). We present data
from a subset of eight spring and summer cruises (Mar.–
Sep.) coinciding with the time of year when hypoxia tends
to develop (Fig. 1). During 2003–2005 sediment measurements were made as part of shelf-wide hydrographic
surveys, so stations were staggered across the shelf, with
numbers and locations limited by the time required to
process sediments. During 2006–2007, sediment measurements were made at sites chosen to represent contrasting
trophic zones as conceptualized by Rowe and Chapman
(2002) and Breed et al. (2004). The 2006–2007 stations
were part of a more detailed study of sediment biogeochemistry to be reported elsewhere. We included the
integrated respiration measurements from both sets of
cruises to expand the temporal coverage.
Integrated respiration measurements were made at a total
of 31 sites: 19 in spring (Mar.–Apr.) and 12 in summer
(Jun.–Sep.). The water depth at the sites ranged from 8.8 to
31 m. All sites were located within the region where
hypoxia has been frequently observed, based on long-term
monitoring (e.g., Rabalais et al. 2007).
Sediment Measurements
Different sediment coring devices were used over the study,
requiring different processing methods. From 2003 to 2005,
we used a 0.25 m2 box corer that collected a square sediment
plug. It was often necessary to repeatedly deploy the box
corer to retrieve a suitably undisturbed set of cores. Subsamples were collected in cylindrical plexiglas cores (10 cm
ID × 40 cm), which were gently pushed ~25 cm into the
sediment surface, being careful to minimally disturb the
fragile surface flocculent layer, and excavated using rubber
stoppers to seal the ends. The water overlying the sediment
was carefully siphoned off, and refilled with site-collected
bottom water. The overlying water height averaged 16.0 cm
(range, 11.5–19.0 cm). This handling method introduced
some oxygen to the overlying water, especially at stations
where ambient DO was strongly under-saturated. Therefore
initial conditions in cores at the beginning of incubation were
typically higher than ambient DO concentrations as measured by the CTD; the average difference (DOinitial–DOCTD)
was 44 mmol O2 m−3 (range, −46 to 151 mmol O2 m−3).
From 2006 to 2007, sediment cores were collected with a
hydraulically dampened multi-corer (Ocean Instruments MC400). The multi-corer deployed four cylindrical plexiglas core
tubes (10 cm ID × 40 cm) that were mounted directly on the
frame. The overlying water height averaged 20.0 cm (range,
11.0–25.0 cm). Visual inspection of these cores revealed an
undisturbed flocculent layer and the overlying water was
clear, indicating minimal resuspension. Thus, the multi-corer
was consistently more reliable than the box corer, and had the
added advantage of collecting the true bottom water overlying
the sediments. Hence, water overlying these cores was not
replaced. Compared with box core collections, the initial DO
concentrations more closely matched CTD measurements of
bottom water DO with an average DOinitial–DOCTD of
19 mmol O2 m−3 (range,−18 to 61 mmol O2 m−3).
Regardless of coring apparatus, the cores were prepared
similarly for SOC measurements. The core tubes were
sealed with top and bottom plates and held together with
elastic cords, as described by Cowan and Boynton (1996).
The top plate included a sampling port and a suspended
magnetic stir bar assembly at the core top. Upon sealing the
cores, any remaining air bubbles were displaced with
additional bottom water. The cores were immersed in a
water bath that maintained bottom water temperatures
(±1°C) using a thermostatically controlled heat pump
system. The cores were incubated in darkness to prevent
photosynthetic oxygen production. The stir bar was slowly
rotated (~60 rpm) by external rotating magnets situated at
the core top to mix the overlying water. After allowing the
cores to acclimate to incubation conditions (1–2 h), DO
measurements were made hourly for 4 to 6 h using either an
Orion® 862A (2003–2005) or a YSI® 5000 (2006–2007)
probe. Each probe had a stirrer mechanism and a beveled
fitting that matched the opening in the top plate. The DO
probes were calibrated and stored at room temperature in
water-saturated air. Sensor drift was evaluated by monitoring
variation in water-saturated air readings, which was minimal
during experiments, typically varying <0.5% of full scale.
SOC rates were calculated for each core as the slope of the
least squares regression of DO concentration versus time,
normalized to the overlying water height (mean, 20 cm; range,
9–27 cm), and expressed in units of mmol O2 m−2 day−1. Most
commonly the reported SOC rates were the average of three
replicate cores; at five sites, only two replicate cores were
incubated (three in Mar. 2005, one in Aug. 2006, and one in
Apr. 2007). The minimum detection limit was ~4 mmol O2
m−2 day−1, and coefficient of variation among replicate cores
averaged 20%. SOC rates were below detection at three sites
(either zero or slightly positive flux rates), so one half of the
detection limit was assumed in those cases.
Estuaries and Coasts
Fig. 1 Maps of sampling locations: a Jun 2003; b Apr 2004 (open circles) and Mar 2005 (closed circles); c Apr (circles), Jun (triangles), and Sep
2006 (squares); and d Apr (circles) and Aug 2007 (triangles)
During June 2003, we conducted several experiments to
explore how sediments responded to oxygenation after a
period of hypoxia. At five sites where bottom waters were
strongly under-saturated (<35%), we re-oxygenated the
bottom water sample before using it to replace the
overlying water in sediment cores. Re-oxygenation was
achieved by placing water into a polyethylene cubitainer
with air space and vigorously shaking for ca. 30 s.
Bulk sediment properties were measured on samples
collected from the top 1–2 cm, which were frozen (−20°C)
and returned to the laboratory for analysis. Sediment
samples were collected from unused portions of the box
core samples and from an additional replicate core from the
multi-corer. Analyses included water content, grain size,
chlorophyll-a (chl-a), and solid phase carbon and nitrogen.
Sediment water content was measured by sequential
gravimetric analysis of sediment aliquots (~5–20 g) before
and after drying (60°C, 24 h). Grain size analysis (percent
sand, silt, and clay) was determined using a standard
sieving and settling velocity method (ASTM D422-63
1998). Samples for particulate carbon and nitrogen were
dried, milled to a fine powder, and analyzed on an
Elementar® Vario EL elemental analyzer. Samples were
not treated to remove inorganic carbon. For chl-a, sediment
samples (~0.5 g) were placed in 90% buffered methanol,
extracted, and measured using the same fluorometric
method as water column filter samples.
Water Column Measurements
At each site, water column profiles of temperature, salinity,
depth, and DO were measured with a Seabird 911 CTD
system. The pycnocline was determined as the depth of
maximum Brunt–Väisälä Frequency (Pond and Pickard
1983) calculated from CTD profiles. At most sites, there
was a single, clearly definable pycnocline, but sometimes a
distinct second pycnocline was evident. In those cases, the
lower pycnocline was chosen to represent the demarcation
between the surface and bottom layers. The CTD was
outfitted with a rosette of 10 L Go-Flo® bottles, which were
used to collect surface-layer and bottom-layer water
samples for standard water quality constituents. Bottom
water samples were collected 1–2 m above the sediment
surface, which was always below the pycnocline. Samples
for chl-a were filtered (100–500 ml) onto GF/F filters and
frozen at −70°C until analysis. In the laboratory, chl-a was
extracted with buffered methanol and sonicated with a
micro-probe sonicator. Fluorescence was measured with a
Turner Designs TD 700 fluorometer with optical filters
specific for chl-a (Welschmeyer 1994), and calibrated using
commercially available standards (Sigma Chemicals).
Samples for ambient DO concentrations and WR rates
were drawn first from the Go-Flo® bottles, before sampling
for other water quality constituents, using a length of
Tygon® tubing inserted into the bottom of 300 ml
biological oxygen demand (BOD) bottles. Water was
allowed to overflow the BOD bottle by 1–2 volumes before
capping, taking care to expel all bubbles from the sample.
DO was measured using an Orion® 862A probe, designed
to fit tightly into the tapered opening of the BOD bottles.
When inserting the probe, care was taken to avoid
introducing bubbles. After reading, the probe was gently
removed and the stopper was immediately replaced, adding
deionized water (~5 ml) to fill the flared top of the BOD
Estuaries and Coasts
bottle. A secondary plastic cap was used to insure a tight
seal. Prior experience with this method indicated that this
handling practice did not significantly alter ambient
dissolved oxygen concentrations in the bottles (Murrell et
al. 2009). After initial readings, the BOD bottles were
placed into a dark running seawater incubator that maintained near in situ temperature. After 24 h, the bottles were
removed and a second DO measurement was made.
At a subset of stations, an additional set of duplicate
BOD samples were collected for DO determination via
Winkler chemistry (Parsons et al. 1984) for comparison to
the initial DO probe measurements. A total of 398 paired
measurements was made (30–74 per cruise); the two
independent measures typically agreed within ±5%, yielding
the linear regression equation: DOWinkler = 0.969 × DOmeter −
0.036, R2 = 0.947 (data not shown).
WR rates were calculated as the change in DO over
time, expressed as mmol O2 m−3 day−1. To account for
slight discrepancies between in situ water temperature and
the water bath temperature (usually <2°C), raw respiration
rates were adjusted (WRraw) assuming Q10 = 2, as: WR =
WRraw × 2(Tb-Ti)/10, where Tb was the ambient bottom water
temperature, and Ti was the incubator temperature. Although
the correction typically was small (mean, 0.99; range, 0.80–
1.27), it helped minimize a known bias. The detection limit
for this method, based on our ability to resolve changes in
DO concentration, was ~1–2 mmol O2 m−3 day−1. The WR
values reported are the average of duplicate BOD bottle
incubations. The median coefficient of variation among
duplicates was 19% (data not shown).
Below-pycnocline integrated water respiration rates
(IWR; mmol O2 m−2 day−1) were calculated by multiplying
WR rates by the bottom-layer depth, as defined by the
depth between the pycnocline and the bottom. Total belowpycnocline volumetric respiration rates (TRV) were calculated by summing IWR and SOC and dividing by the
bottom-layer depth.
During 2003–2005, IWR rates were typically calculated from a single near-bottom WR measurement.
However beginning in 2006, WR measurements were
made from two or more depths below the pycnocline or
from bottom water collected several times over 36 h
while the ship remained anchored on site. In these cases,
IWR was calculated using all available data by trapezoidal mid-point integration.
Statistical Analysis
Correlation analysis was used to examine relationships
among environmental variables and metabolic measurements. We used analysis of variance (ANOVA) to examine
spatial and temporal patterns in integrated respiration rates,
binned by year (spring only), month, longitude, and water
depth. Post-hoc tests were performed using the Bonferroni
adjustment for multiple comparisons.
Results
Hydrographic Setting and Site Characteristics
Hydrographic conditions over this study were typical for
the LCS with widespread water column stratification and
under-saturated bottom water DO concentrations during
spring–summer (Table 1). Salinity stratification was strongest
in March and April and weaker during late August and
September, consistent with the prevailing freshwater flow
regime. As illustrated for 2006 in Fig. 2, the depth of the
pycnocline was shallowest during spring near the Mississippi
River, and deepened both temporally from April to September and spatially with increasing distance from the Mississippi River plume. Bottom water DO ranged from 2.6 to
193 mmol m−3, but was depleted (<100 mmol O2 m−3
or ~40% of saturation) at 11 of the 31 sites, consistent with
expectations from long-term monitoring. Bottom water chl-a
(Table 1) ranged from 0.3 to 11.1 mg m−3 across all sites, and
correlated negatively with bottom water salinity (Pearson’s
r = −0.67, p < 0.01, n = 31).
The analysis of sediment bulk properties is briefly
summarized here. Generally, sediments had high silt + clay
content, averaging 75% ± 3.1% (range, 19–98%) and
moderate water content, averaging 43 ± 2.0% (range, 22–
72%). Sediment chl-a averaged 2.2 ± 0.2 μg gww−1, and
was about 2-fold higher during 2003–2004 (2.6–4.2 μg
gww−1) than during 2005–2007 (0.9–1.7 μg gww−1), a
difference that was statistically significant (ANOVA, p <
0.001). Sediment carbon and nitrogen averaged 13.0 ±
0.7 mg gdw−1 and 0.98 ± 0.07 mg gdw−1, respectively.
SOC rates ranged from 1.3 to 23.3 mmol O2 m−2 day−1
with a month–year average (i.e., averaged by cruise) of
11.6 ± 2.2 mmol O2 m−2 day−1 (Table 2). WR rates ranged
from 1.4 to 14.0 mmol O2 m−3 day−1 with a month-year
average of 6.8 ± 0.7 mmol O2 m−3 day−1. Belowpycnocline IWR column rates ranged from 11.7 to
109.3 mmol O2 m−2 day−1with a month–year average of
53.4 ± 6.3 mmol O2 m−2 day−1. IWR rates were typically
about 3-fold higher than corresponding SOC rates. Total
below-pycnocline respiration (total respiration (TR) = SOC +
IWR) ranged from 21.3 to 122.6 mmol O2 m−2 day−1with a
month–year average of 65.0 ± 6.6. The sediment contribution to total below-pycnocline respiration (SOC:TR) ranged
from 0.02 to 0.56 across all sites, with a month–year average
of 0.20 ± 0.04.
The relationships among respiration and environmental
variables were examined using simple correlation analysis
(Table 3). Notably, SOC rates strongly correlated with in
1.7–2.3
0.5–2.2
0.5–4.4
1.7–8.4
3.6–4.8
±0.4
1.9
1.5
2.6
4.2
4.1
2.5
90–171
2.6–168
20–149
145–193
3.8–156
±11.5
125
105
76
164
56
107
21.3–21.7
24.0–26.7
29.1–30.4
21.7–22.9
28.3–30.6
±1.3
21.5
25.5
29.7
22.2
29.2
24.0
1.8–21.0
6.3–11.7
4.9–9.3
2.0–5.5
1.7–4.0
±1.0
8.7
8.6
6.7
4.2
3.1
7.6
7.5–18.0
7.0–9.8
9.4–15.9
2.0–16.0
4.5–16.4
±0.8
12.0
8.4
12.4
7.7
11.9
9.6
20.3
20.0
19.7
16.1
17.1
18.2
5–10 Apr. 2006
13–17 Jun. 2006
13–17 Sep. 2006
25–29 Apr. 2007
25–29 Aug. 2007
Average ± SE
3
3
3
3
3
18.2–23.1
18.8–21.4
19.1–20.7
7.7–22.0
8.8–22.8
±0.6
0.6–11.1
3.1
74–178
110
18.7–21.0
20.2
5.6–14.6
9.7
3.0–12.0
7.8
17.2
22–30 Mar. 2005
6
7.6–23.4
M04, F05, H04
A04, C04, D01,
D02, E03, M03,
M05, M21
M04, A04, B08,
C03, D06, F03, H04
A04, C06, H04
A'04, C06, H04
A'04, C06, H04
Z05, C02, C06, H04
C02, C06, H04
0.3–3.4
0.9–1.9
1.5
1.5
78–165
72.5–120
100
122
20.6–21.3
21.7–23.7
22.5
21.0
4.4–16.3
5.7–20.6
11.1
9.2
3.0–11.0
5.0–13.0
10.0
6.4
8.0–31.0
16.8
7
15.0–22.5
18.8
3
2–6 Apr. 2004
12–19 Jun. 2003
Mean
Range
Mean
Range
Mean
Range
Mean
Range
Mean
Range
Mean
Range
Station names
Bottom chlorophyll-a
(mg m−3)
Bottom DO
(mmol m−3)
Bottom
temperature
(°C)
Delta salinity
Pycnocline
depth (m)
Bottom
depth (m)
# sites
Dates
Table 1 Summary of water column conditions at sampling sites. Included are cruise averages and ranges of bottom depth, pycnocline depth, delta salinity (bottom salinity minus surface salinity),
temperature, DO, and chl-a. Station names are also included
Estuaries and Coasts
situ bottom water DO and with initial DO at the beginning
of the SOC incubations. WR rates positively correlated with
temperature and chl-a, and correlated negatively with
several sediment variables. Unlike SOC, WR rates did not
covary with either DO variable. Total respiration, when
expressed volumetrically, correlated negatively with bottom
depth and salinity, and correlated positively with bottom
water chl-a. TRV also negatively correlated with most
sediment variables. However, the variation in sediment bulk
properties was not significantly correlated with sediment
metabolic rates.
Spatial and temporal patterns in respiration rates were
explored by binning the respiration rates by year (spring
only), month, longitude, and water depth (Figs. 3, 4, 5, and
6). From 2004 to 2007, spring respiration rates showed
relatively little inter-annual variation. For example, TR
ranged less than 2-fold, from 50 to 78 mmol O2 m−2 day−1
(Fig. 3). When viewed by month (Fig. 4), respiration rates
were similar from March to September. Similarly, respiration rates showed little variation with longitude (Fig. 5),
though TR appeared higher at 89°W and 90°W than farther
west. SOC:TR was significantly higher at 91°W than 90°W
(p = 0.027). Finally, when binned by depth, both TRV and
SOC:TR were significantly higher in the 5–15 m bin than
the deeper depth bins (p < 0.03, Fig. 6).
The re-oxygenation experiments conducted during June
2003 (Fig. 7) showed that adding oxygen to overlying water
sharply increased SOC rates. The first two experiments
directly showed that SOC increased from ~3 mmol O2 m−2
day−1when DO was under-saturated to ~26 mmol O2 m−2
day−1when waters were re-oxygenated. The other three
experiments had no under-saturated treatment, but the reoxygenated SOC rates were similarly high, exceeding all other
SOC measurements made at near in situ conditions (Table 2).
During 2003–2005, IWR rates were calculated from singlepoint measurements of bottom water WR at 12 of 16 sites. For
the remaining sites, IWR rates were calculated from WR
measurements made at multiple depths below the pycnocline
(range, 2–6) or from WR measurements made at multiple
times over ~36 h (range, 5–8). We used the more complete
dataset to compare single-point IWR estimates with IWR
calculated from all available data. A scatter plot comparing the
methods (Fig. 8) showed strong coherence (R2 = 0.82) with
regression slope and intercept values that were statistically
indistinguishable from one and zero, respectively.
Discussion
Below-pycnocline respiration measurements fell within
fairly narrow ranges with few clear spatial or temporal
patterns. This result was somewhat surprising considering
the large variability in physical and biogeochemical
Estuaries and Coasts
Temperature (˚C)
20
22
24
26
28
30
20
Temperature (˚C)
22 24 26 28
30
20
Temperature (˚C)
22 24 26 28
30
Salinity
Salinity
Salinity
10 15 20 25 30 35 40 10 15 20 25 30 35 40 10 15 20 25 30 35 40
Sigma t
15
20
5
10
25
0
DO (mmol O2 m-3)
100
200
300
Sigma t
15
20
5
10
25
0
DO (mmol O2 m-3)
100
200
300
10
0
DO (mmol O2 m-3)
100
200
300
C
5
5
10
10
15
15
20
20
0
Depth (m)
B
0
E
F
5
5
10
10
15
15
20
20
0
Depth (m)
D
0
G
H
I
5
5
10
10
15
15
20
20
characteristics across the LCS. However, because this study
spanned several years, and sampled over a large geographic
range, it seems likely that we captured a reasonable range in
spring–summer oxygen metabolic rates for the region.
Sediment Respiration
This study significantly expands the number of SOC rate
measurements available for the LCS, which were
Depth (m)
Depth (m)
25
0
A
Depth (m)
Sigma t
15
20
5
0
Depth (m)
Fig. 2 Representative
hydrographic profiles of
temperature (red crosses),
salinity (green squares), sigma t
(blue triangles), and DO (black
circles) from three stations
sampled during 2006 in Apr
(top row: a–c), Jun (middle row:
d–f), and Sep (bottom row: g–i).
Sites were distributed
longitudinally along the ~20 m
isobath at 89.5°W (left-hand
column: a, d, g), 90.5°W
(middle column: b, e, h), and
92.5°W (right-hand column: c,
f, i). The horizontal line depicts
the depth of the pycnocline;
whereas, the vertical line
indicates the DO concentration
considered to be hypoxic,
defined as DO < 62.5 mmol O2
m−3. A notable pattern is the
deepening and weakening of the
pycnocline spatially from east to
west and temporally from Apr to
Sep
generally similar those reported in prior literature
(Table 4). SOC rates showed clear evidence of DO
limitation (Fig. 9), similar to findings in other continental
shelf studies (e.g., Archer and Devol 1992; Rowe et al.
2002). While SOC rates were almost certainly sensitive to
other environmental variables (e.g., temperature, organic
matter quality and quantity, sediment porosity, etc.), DO
availability appeared to be of primary importance. This
DO–SOC functional relationship, if robust, maybe useful
0.04–0.55
0.14–0.56
0.02–0.34
0.27–0.34
0.02–0.17
0.05–0.12
0.18–0.38
0.03–0.22
±0.16
±0.07
±0.05
±0.02
±0.04
±0.02
±0.05
±0.06
±0.04
0.23
0.36
0.16
0.31
0.10
0.08
0.28
0.10
0.20
3.5–7.4
2.3–13.2
2.2–9.4
6.9–11.2
7.6–10.4
4.7–9.3
6.4–14.9
7.3–17.2
±1.2
±1.4
±1.1
±1.5
±0.8
±1.4
±2.3
±2.9
±0.8
21.3–78.2
26.3–116.3
21.5–71.5
57.4–73.6
86.0–122.6
22.7–90.0
38.5–108.8
35.2–74.4
±16.8
±11.3
±7.3
±4.8
±10.6
±19.6
±17.9
±12.6
±6.6
Jun. 2003
Apr. 2004
Mar. 2005
Apr. 2006
Jun. 2006
Sep. 2006
Apr. 2007
Aug. 2007
Average ± SE
9.1
16.4
6.0
19.9
11.1
3.9
19.1
6.8
11.6
±6.4
±1.5
±1.6
±0.9
±4.8
±0.6
±2.2
±4.9
±2.2
2.0–21.9
11.3–22.1
1.3–12.8
18.4–21.4
2.0–18.0
2.8–4.6
14.5–23.3
1.9–16.6
4.1
5.1
5.6
5.9
8.2
7.2
7.3
10.8
6.8
±1.6
±0.9
±1.1
±1.0
±0.6
±1.5
±1.8
±2.0
±0.7
1.8–7.2
1.4–8.8
1.4–8.8
4.6–8.0
7.5–9.4
4.3–9.0
4.1–11.2
7.3–14.0
37.3
41.5
43.7
44.5
93.4
54.9
58.3
53.6
53.4
±18.8
±11.5
±8.1
±4.6
±8.0
±19.1
±16.4
±10.8
±6.3
17.7–74.8
11.7–97.3
14.1–70.2
38.9–53.6
84.0–109.3
19.9–85.4
23.9–89.2
33.2–69.7
46.4
57.8
49.8
64.5
104.5
58.9
77.5
60.4
65.0
5.0
8.1
6.2
8.3
9.0
7.6
9.8
11.9
8.2
Range
SE
Mean
Range
SE
Mean
Range
SE
Mean
Range
SE
Mean
SE
Range
SE
Mean
Mean
Range
SOC:TR
TRV
(mmol O2 m−3 day−1)
TR
(mmol O2 m−2 day−1)
IWR
(mmol O2 m−2 day−1)
WR
(mmol O2 m−3 day−1)
SOC
(mmol O2 m−2 day−1)
Cruise
Table 2 Summary of sediment respiration (SOC), bottom water volumetric respiration (WR), integrated below-pycnocline water column respiration (IWR), total below-pycnocline respiration
(TR), and total below-pycnocline respiration expressed volumetrically (TRV). SOC:TR is also included. Data are summarized by cruise with mean, standard error, and range
Estuaries and Coasts
for parameterizing models of oxygen dynamics. For
example, the hydrodynamic model of Hetland and
DiMarco (2008) used a temperature-adjusted version of
the DO–SOC model of Rowe et al. (2002) to parameterize
SOC rates. Our data indicated much lower SOC rates at
lower DO concentrations than predicted by the Rowe et al.
(2002) model. If true, then the Hetland and DiMarco
(2008) may have overestimated the role of SOC in their
model of below-pycnocline respiration.
We found that SOC rates increased when overlying
water was re-oxygenated. Miller-Way et al. (1994) did a
similar experiment and found that SOC rates increased from
6.3 mmol O2 m−2 day−1when under-saturated to 31.7 mmol
O2 m−2 day−1after re-oxygenation. This response is consistent with the expected accumulation of anaerobic metabolites (e.g., Fe2+, HS−) during hypoxia, which then became
chemically re-oxidized as oxygen availability was increased. In this sense, sediments acted to buffer the system
against re-oxygenation, thus likely helping to maintain
hypoxia during summer.
Bottom Water Respiration
WR rates provide a useful and relatively simple
measure of aquatic metabolism, thus have seen wide
use (review: del Giorgio and Williams 2005). The
average WR for this study (6.8 ± 0.7 mmol O2 m−3
day−1), was lower than the mean of 19.6 ± 3.9 mmol C
m−3 day−1 from 24 estuarine systems worldwide (Hopkinson and Smith 2005). A similar global compilation of
marine surface water literature reported a mean WR of
3.3 ± 0.15 mmol O2 m−3 day−1 (Robinson and Williams
2005). Our average WR rates fell between these global
averages, which is perhaps not surprising given that the
LCS is a coastal margin with both estuarine and marine
character.
The WR literature for LCS bottom waters varies
widely, with study means ranging from 0.43 mmol O2
m−3 day−1 (Dortch et al. 1994) to 15.9 mmol O2 m−3
day−1 (Turner et al. 1998). The low end of this range is
>10-fold lower than what we observed in this study, and
similarly much lower than expected from the literature
(del Giorgio and Williams 2005). This discrepancy may
have been methodological given that Dortch et al. (1994)
used an electron transport activity assay to estimate WR
rates.
In contrast to SOC, WR rates appeared insensitive to DO
concentrations, such that WR rates at sites where DO was
severely depleted (<35 mmol O2 m−3) were indistinguishable from those measured at normoxic sites. Similarly,
Sampou and Kemp (1994) found that plankton community
respiration rates only showed evidence of limitation at DO
< 0.8 mg O2 L−1 (<25 mmol O2 m−3).
Estuaries and Coasts
IWR
TR
TRV
SOC:TR
−0.24
−0.16
0.61**
0.76**
−0.07
−0.31
−0.04
−0.02
−0.03
0.16
0.16
0.20
−0.32
0.04
−0.10
−0.18
0.38*
0.50**
−0.29
−0.38*
−0.27
−0.45*
−0.40
−0.57**
0.37*
−0.26
0.00
−0.08
−0.19
0.27
0.26
−0.06
0.07
−0.07
−0.58**
−0.40
0.30
−0.29
0.16
0.12
−0.20
0.19
0.24
−0.06
0.07
−0.03
−0.51*
−0.32
−0.58**
0.05
0.08
0.13
0.39*
0.31
−0.44*
−0.47**
−0.47**
−0.49**
−0.31
−0.52**
−0.42*
−0.02
0.43*
0.65**
−0.06
−0.41*
−0.18
0.00
−0.18
0.05
0.38
0.35
−0.02
−0.03
0.31
−0.03
Methodological Considerations
It is prudent to consider potential methodological artifacts
that could have materially affected the results and interpretation. Firstly, the practice of retrieving sediment cores and
conducting laboratory incubations may have caused artifacts when compared with less disruptive in situ methods.
While both methods have been used extensively, core
incubation methods can be problematic in deep ocean
sediments, permeable sandy sediments, and in sediments
dominated by deep-dwelling macrofauna (review, Glud
2008). However, such artifacts should be relatively small
on the LCS, which has shallow depths, has predominately
silty sediments, and has a macro-faunal community
comprised largely of epi-benthic polychaetes (Baustian
and Rabalais 2009). A prior LCS study (Miller-Way et al.
1994) compared in situ benthic chambers, diver-collected
cores, and box cores, finding that all methods produced
similar results. Similarly, other comparison studies from
shallow environments found insignificant differences
amongst core incubation and in situ methods (Belanger
1981; Kemp et al. 1992).
A second potential methodological concern was our use
of two different sediment collection apparatus over the
study (box corer vs. multi-corer). While we made no direct
comparisons, the averages and ranges of SOC rates
estimated from each method were almost identical (Table 2).
Also, when viewed in relation to initial DO concentrations
(Fig. 9) the two methods appeared to fall along the same
distribution. In sum, both methods produced similar SOC
rate estimates that were also similar to prior estimates from
the LCS (Table 4).
A third concern was our use of single bottom water
WR rate measurements to calculate below-pycnocline
plankton IWR rates. This method assumed that WR rates
0.28
−0.01
were uniform from the pycnocline to the bottom. We
tested this assumption by comparing IWR calculated
from a single bottom water WR measurement with IWR
calculated from multiple below-pycnocline measure-
100
TR (mmol O2 m-2 d-1)
*p < 0.05; **p < 0.01,
significant
WR
TRV (mmol O2 m-3 d-1)
The number of observations
ranged from 23 to 31
Bottom depth
Longitude
DO in situ
DO initial
Chl-a
Temp.
Salinity
Sed. chl-a
Silt + clay
Sed. % water
Sed. C
Sed. N
Sed. C/N
SOC
IWR
A
SOC
80
60
40
20
0
15
B
10
5
0
0.6
SOC:TR
Table 3 Pearson correlations
between respiration and
environmental variables
C
0.4
0.2
0.0
2004
2005
2006
2007
(3)
(3)
3)
Year
(7)
(6)
Fig. 3 Coupled water column and sediment below-pycnocline
respiration from 2004 to 2007 during Mar–Apr. a Total respiration
(TR), showing contributions of SOC (solid bars) and IWR (open bars)
components. b TRV, and c SOC:TR. Error bars are standard errors.
Sample sizes are included in parentheses at bottom
Estuaries and Coasts
100
IWR
A
SOC
80
60
40
20
TR (mmol O2 m-2 d-1)
TR (mmol O2 m-2 d-1)
100
TRV (mmol O2m-3d-1)
15
TRV (mmol O2 m-3 d-1)
0
B
10
5
0
C
SOC:TR
SOC:TR
0.6
0.4
IWR
A
SOC
80
60
40
20
0
15
B
10
5
0
0.6
C
0.4
0.2
0.2
0
89
0.0
Mar
(6)
Apr
(13)
May
Jun
July
Month
Aug
Sep
(6)
(3)
(3)
(3)
Fig. 4 Coupled water column and sediment respiration by month
from Mar. to Sept.: a TR with stacked bars showing SOC (solid bars)
and IWR (open bars) components, b TRV, and c SOC:TR. Error bars
are standard errors. Sample sizes are included in parentheses at bottom
ments (Fig. 8). The strong agreement between the two
IWR estimates suggested that the single-point method
produced accurate IWR estimates, and further indicated
that below-pycnocline waters had uniform metabolic
characteristics.
A fourth concern was our method of measuring WR
with a DO probe, which was relatively imprecise
compared with Winkler titration methods. While most
of our WR measurements were above the 1–2 mmol O2
m−3 detection limit, the resolution was rather coarse. This
lack of precision should not cause a major bias in an
overall mean, but it could have obscured patterns that
would otherwise be evident using more sensitive methods.
This may, in part, explain the lack of clear spatial or
temporal gradients in this dataset.
Relative Importance of Water Column and Sediment
Respiration Rates
Based on paired measurements of IWR and SOC,
sediments accounted for 22 ± 5% of the total belowpycnocline respiration. This proportion agrees with that
90
91
Longitude (˚W)
92
(14)
(9)
(5)
Fig. 5 Coupled water column and sediment respiration by longitude.
a Total respiration (TR), showing contributions of SOC (solid bars)
and IWR (open bars) components, b TRV, and c SOC:TR. Error bars
are standard errors. Sample sizes are included in parentheses at bottom
expected for coastal and estuarine systems worldwide
(Kemp et al. 1992) (Fig. 10), but differs from prior LCS
studies. For example, Dortch et al. (1994) estimated that
sediments accounted for ~75% of below-pycnocline
respiration based on paired measurements of WR and
SOC. However, compared with the wider literature (del
Giorgio and Williams 2005), it seems clear that the WR
rates in Dortch et al. (1994) were anomalously low. More
recently, Quiñones-Rivera et al. (2007, 2010) estimated
that SOC accounted for ~75% of the below-pycnocline
respiration based on δ18O measurements and an isotope
fractionation model. The reason for this very different
result is unclear, but likely resulted from differences in
assumptions, and in the spatial and temporal domains
studied. One limitation of this stable isotope approach is
that only relative fractions of benthic and water column
respiration can be calculated, thus cannot be compared
with direct measurements of sediment and water column
metabolic rates. Further work is needed to test the
assumptions of both methodologies to reconcile the
different conclusions about the relative importance of
sediment and water column respiration rates on the LCS.
100
SOC
80
60
40
20
0
15
B
10
150
100
2 point
3 point
4 point
6 point
1:1 Line
50
0
0
50
100
150
200
IWR multi-point (mmol O2m-2d-1)
5
Fig. 8 Comparison of different methods of estimating integrated, belowpycnocline respiration (IWR). Single-point IWR was calculated by
multiplying WR rates from a single near-bottom water sample by the
depth of the below-pycnocline portion of the water column. For multipoint IWR, WR rates from 2, 3, 4, or 6 depths were used to calculate
IWR via trapezoidal mid-point integration. The linear regression
describing the relationship: IWRsingle point = 0.99 × IWRmulti-point −
2.52, R2 = 0.82, n = 43
0
0.6
C
SOC:TR
200
IWR
A
IWR single-point (mmol O2m-2d-1)
TRV (mmol O2 m-3 d-1)
TR (mmol O2 m-2 d-1)
Estuaries and Coasts
0.4
0.2
0.0
5-15
15-20
Depth (m)
(8)
(14)
20-31
Integrated Below-Pycnocline Respiration and Hypoxia
(9)
Fig. 6 Coupled water column and sediment respiration by water
depth. a TR with stacked bars showing SOC (solid bars) and IWR
(open bars) components, b TRV, and c SOC:TR. Error bars are
standard errors. Sample sizes are included in parentheses at bottom
50
SOC (mmol O2 m-2 d-1)
Ambient
Re-oxygenated
40
30
20
10
0
A
B
C
D
E
Experiment
81 ⏐ 165
26
⏐ 147
39 ⏐ 209
4 ⏐ 145
26 ⏐ 144
Fig. 7 Results from re-oxygenation experiments conducted in June
2003 at five sites where bottom waters were oxygen-depleted at the
time of sampling. The open bars (two experiments only) represent
SOC rates measured at ambient DO concentrations and the shaded
bars represent SOC rates measured after re-oxygenation. The paired
numbers below the x-axis denote DO concentrations before and after
re-oxygenation (mmol O2 m−3). Error bars represent the standard
errors
In this study, below-pycnocline TRV averaged 8.2 mmol
O2 m−3 day−1. Assuming no mixing, this rate implies that
below-pycnocline waters would go from saturation
(~250 mmol O2 m−3) to hypoxia (~62.5 mmol O2 m−3) in
22 days. Continuous oxygen data collected from bottom
waters at a 20 m station (C6) on the eastern LCS suggest a
similar DO turnover rate. Based on spring–summer 1993
time series, Rabalais et al. (2007) noted that DO concentrations decreased from 6 mg L−1 to less than 2 mg L−1 in
9–18 days. Assuming an average below-pycnocline depth
of 10 m (Justić et al. 1996), this DO decline rate would be
equivalent to a TRV rate ranging from 6.9 to 13.9 mmol
O2 m−3 day−1, consistent with those estimated herein.
Several models have been developed that predict oxygen
dynamics on the LCS, but two in particular generated
below-pycnocline respiration estimates, thus inviting comparison with our direct empirical measurements. Firstly,
Bierman et al. (1994) used a mass balance box model to
estimate shelf-wide patterns in oxygen metabolism. Most
notably, Bierman et al. (1994) calculated a TRV varying
from 0.15 mg O L−1 day−1 near the Mississippi River
increasing westward to 0.69 mg O L−1 day−1 on the western
Louisiana shelf (4.7 to 22 mmol O2 m−3 day−1, respectively). We observed similar TRV rates (Table 2) and a similar
trend, though statistically insignificant, with longitude
(Fig. 5). Secondly, Justić et al. (1996, 1997, 2002, 2003)
developed a vertical two-box coupled biological-physical
model for a 20 m station (C6) on the eastern LCS. Based on
this model, monthly average below-pycnocline total respiration (TR) rates ranged from 0.05 to 33.4 g O2 m−2
Estuaries and Coasts
Table 4 Summary of sediment (SOC)
(SOC) and
and bottom
bottom water
water (WR)
(WR)
respiration rates reported for the Louisiana
Louisiana continental
continental shelf
shelf region,
region,
including mean, standard error (SE), range, and
and number
number of
of observaobserva−2
−1
tions (n).
were
converted
to mmol
O2 mO
m−2
(n).When
Whennecessary,
necessary,units
units
were
converted
to mmol
2day
SOC (mmol O2 m−2 day−1)
Month
July, Nov.
Oct.
Mar., Apr., July, Aug.
July
May, June, July, Aug., Sep., Oct.
Apr.
Apr., Aug.
Apr., July, Aug.
Mar., Apr., Jun., Aug., Sep.
a
−1 −3
−1
for
mmol
m−3 day
for WR
to facilitate
daySOC
forandSOC
andO2mmol
O2 m
day−1
for WR comparisons.
to facilitate
Also,
when necessary,
standard
errors standard
were calculated
from calculated
respective
comparisons.
Also, when
necessary,
errors were
standard
deviations
from respective
standard deviations
WR (mmol O2 m−3 day−1)
Mean
SE
Range
n
Mean
SE
Range
n
Source
–
–
–
–
16.0
6.3
24.8
19.2
11.6
–
–
–
–
±4.1
–
±8.8
±4.7
±1.4
–
–
–
–
5.0–25
–
1.9–56.0
0.8–56.4
1.3–23.3
–
–
–
–
4
1
5
12
31
1.87
3.28
15.9
9.1
0.43
–
–
–
6.45
±0.22
–
±2.82
±0.70
±0.06
–
–
–
±0.54
0.08–6.00
3.09–3.46
0.60–75.0
–
0.06–1.72
–
–
–
1.4–14.0
61
2
36
81
55
–
–
–
31
Turner and Allen 1982a
Biddanda et al. 1994b
Turner et al. 1998 (suppl.)
Fry and Boyd (2010)
Dortch et al. 1994c
Miller-Way et al. 1994
Morse and Rowe 1999
Rowe et al. 2002
This Studyd
Weighted mean and standard deviation among sites distributed east and west of the Mississippi River delta
b
Estimated from their Fig. 3
c
Weighted mean and pooled standard deviation calculated from their Table 2; range estimated from their Fig. 3. SOC only from summer
d
Statistics were calculated weighting all observations equally, thus do not perfectly match Table 2
month−1 (0.1 to 34.3 mmol O2 m−2 day−1), with highest
rates in January and lowest rates September. In contrast, our
estimates of TR were about 4-fold higher than those of the
Justić model. The reason for the difference is unclear, but
warrants further study.
In conclusion, we reported 31 measurements of coupled
below-pycnocline WR and SOC rates on the LCS. The
25
60%
Rowe 2001
Roweet al. 2002
Eldridge & Morse 2008
Box Core
Multicorer
This study
20
Kemp et al. 1992
This study below pycnocline
This study whole water column
50%
40%
S0C:TR
SOC (mmol O2 m-2 d-1)
component measures, WR and SOC were similar to most
prior measurements, both from the LCS and from other
shallow estuarine and coastal environments. We consistently found that WR was the major sink for dissolved
oxygen below the pycnocline, a finding that differed
from earlier LCS studies, but one that is well supported
from the broader estuarine and oceanic literature. We also
15
30%
20%
10
10%
5
0%
0
20
40
60
Bottom Depth (m)
0
0
100
200
Initial DO (mmol O2 m-3)
Fig. 9 Relationship between SOC and initial DO concentration of the
overlying water at the beginning of the incubation, including box core
(open circles) and multi-corer (closed circles) collections Superimposed are curves showing model relationships from Rowe (2001),
Rowe et al. (2002), and Eldridge and Morse (2008). For this study:
SOC = 0.094 × DO − 1.35, R2 = 0.57
Fig. 10 Ratio of SOC:TR plotted in relation to water depth. The open
circles represent study means reported by Kemp et al. (1992) from a
variety of coastal and estuarine systems during spring and summer.
The curve, also from Kemp et al. (1992), is described by the equation
y = 55.5–27.8 log(x). The mean from the present study for the belowpycnocline portion of the water column is shown as a closed triangle.
The comparable mean for the entire water column, assuming WR was
vertically uniform, is shown as a closed circle. Error bars are the
standard error of the 31 observations
Estuaries and Coasts
observed that SOC (but not WR) was strongly limited by
DO concentrations, a relationship that may be useful for
parameterizing future models. The incidence of hypoxia
on the LCS and its linkage to anthropogenic nutrients
remain topics of interest to scientists and resource
managers. Clearly, additional modeling and empirical
studies are needed to further our understanding of
oxygen dynamics on the LCS.
Acknowledgments This study was made possible by the Gulf
Ecology Division’s Nutrients Team, including R. Greene, J. Hagy, J.
Kurtz, A. Almario, J. Aukamp, D. Beddick, G. Craven, and D. Yates.
Thanks to J. Campbell, B. Quarles, and R. Stanley for assistance with
respiration measurements. The US EPA’s Office of Water provided
ship time. J. Caffrey, R. Devereux, and B. Roberts, and two
anonymous reviewers provided helpful suggestions. This study was
approved for publication by the US EPA Office of Research and
Development; however the contents are solely the views of the
authors. Use of trade names or commercial products does not
constitute endorsement by the US EPA. Contribution number 1365
from the US EPA, Gulf Ecology Division. We dedicate this paper to
the memory of our friend and colleague, Pete Eldridge, who inspired
us all.
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