Time-series observations during the low sub-surface oxygen events ...

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Marine Chemistry 97

Time-series observations during the low sub-surface oxygen

events in Narragansett Bay during summer 2001

Deanna L. Bergondo *, Dana R. Kester, Heather E. Stoffel, Wendy L. Woods

Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA

Received 8 December 2003; received in revised form 7 January 2005; accepted 22 January 2005

Available online 6 September 2005

Abstract

A series of automated water column time-series measurement systems has been established in Narragansett Bay, Rhode

Island. These systems measure near surface and near bottom temperature, salinity, oxygen, pH, chlorophyll and pressure at 15-

min intervals. The data obtained from two buoy sites during the period of July through September 2001 reveal the occurrence of

episodic surface phytoplankton blooms followed by subsurface hypoxic events, particularly in the upper portions of the estuary

including the area known as the Providence River. Three hypoxic events occurred at monthly intervals in July, August and

September. Their timing, and that of the phytoplankton blooms that preceded them, is linked to the periodic weak neap tidal

cycles that occur alternately with somewhat stronger neap tidal cycles. The connection between surface blooms/subsurface

hypoxia and tidal range is attributed to water column stratification and the ability of moderate changes in tidal amplitude to

reduce stratification through vertical mixing. During the summers of 2002 and 2003 we predicted and subsequently observed

hypoxic events in the Upper Bay. Based on these findings we can project into future years the times when summer blooms and

hypoxia are most likely to occur in the upper portions of Narragansett Bay. The observations from summer 2001 suggest that

the oxygen consumption and renewal in the subsurface waters is delicately balanced. Further increases in inputs of nutrients,

organic matter, or oxygen-consuming substances could shift this balance from hypoxic to anoxic with substantial impacts on

fish and other marine organisms.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Estuaries; Oxygen; Phytoplankton; Stratification; Tides; Time-series analysis

1. Introduction

Eutrophication and its associated hypoxic condi-

tions are increasing concerns in coastal waters. Nutri-

ents and organic matter enter coastal waters from

0304-4203/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.marchem.2005.01.006

* Corresponding author.

E-mail address: [email protected] (D.L. Bergondo).

sewage and wastewater treatment facility effluents.

Fertilizers used for agriculture and landscaping in

urban and suburban coastal watersheds, as well as

animal organic wastes, contribute non-point source

pollution, aiding in the eutrophication process. In-

creases in organic material loading of coastal waters

can lead to oxygen depletion, especially beneath the

pycnocline of a stratified water column. Nutrients

(2005) 90–103

D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–103 91

entering coastal waters stimulate algal photosynthesis

and the production of organic matter. Upon reaching

waters beneath the pycnocline this organic matter

contributes to oxygen depletion.

Hypoxia refers to conditions where the dissolved

oxygen concentration decreases to the point where

organisms are adversely affected. Various investiga-

tors have chosen different oxygen concentrations,

usually either V0.06 mmol/L (2 mg/L) or V0.09mmol/L (3 mg/L), as the criterion for hypoxia. Miller

et al. (2002) reported the results of a number of

experiments on the lethality of low oxygen to various

marine organisms. While some species and life-stages

are more tolerant than others to low oxygen levels,

concentrations at 50% lethality (LC50) as high as 0.09

mmol/L were found in a number of the experiments.

In this paper we consider oxygen concentrations

V0.09 mmol/L (38.3% saturation when t=19 8C and

S =29) to be hypoxic and a cause for concern regard-

ing the quality of waters for marine organisms in

Narragansett Bay.

Diaz (2001) provided a worldwide summary of

coastal water hypoxia during the last half of the

20th century based on observations from more than

20 countries and about 10 states. In the USA, hypoxic

conditions have been studied extensively in the Che-

sapeake Bay, the Gulf of Mexico, western Long Island

Sound, and several other coastal systems. Taft et al.

(1980) summarized the occurrence of hypoxic and

anoxic bottom waters in a northern section of Chesa-

peake Bay from near Baltimore to the Patuxent River

from 1964 to 1977. Oxygen-depleted waters were

associated with thermohaline stratification created by

river input and summer seasonal warming. In the

northern Gulf of Mexico, hypoxia is related to exces-

sive nutrients discharged from the Mississippi River,

and water column stratification caused by the river’s

plume along the shelf (Rabalais and Turner, 2001;

Rabalais et al., 2001). In Long Island Sound (LIS),

Parker and O’Reilly (1991) summarized data from

1950 to 1990 showing east-to-west depletion of bot-

tom O2 and progressively more severe hypoxia from

1975 to 1989. Welsh and Eller (1991) further exam-

ined the processes associated with LIS hypoxia. They

found a continual decline in bottom oxygen from May

to September when the water column was thermally

stratified. Anderson and Taylor (2001) reported results

of weekly bottom water oxygen measurements during

the summers of 1992 and 1993 in which episodic

hypoxia, as opposed to persistent hypoxia, was

observed throughout the summer in LIS.

The uppermost reaches of Narragansett Bay,

including the Seekonk and Providence Rivers, located

in the major urban center of Rhode Island (Fig. 1),

become seasonally hypoxic. While geographically

referred to as briversQ, the Seekonk and Providence

are actually tidal estuaries. Poor water quality condi-

tions have been known to exist for quite some time in

the Seekonk and Providence Rivers (Doering et al.,

1988a,b; Granger, 1994; Kester et al., 1996). Below

the Providence River, conditions are considered par-

tially to well mixed (Kremer and Nixon, 1975; Pilson,

1985). This has led to a general expectation that most

other portions of Narragansett Bay are in good con-

dition for public utilization (e.g., swimming and fish-

ing) and habitat utilization by organisms vital to the

diverse ecology of the Bay. However, during a July

1998 training exercise, investigators with the Envir-

onmental Protection Agency (EPA) Environmental

Monitoring and Assessment Program (EMAP)

observed lower than expected subsurface oxygen con-

centrations (b0.06 mmol/L) in the region south of

Greenwich Bay and north of Wickford Harbor (Fig.

1). Fish kills and sporadic measurements of oxygen

produced additional evidence of low oxygen condi-

tions beyond the tidal river section of the estuary

(Deacutis, 1999). In 1999 the north and south Pru-

dence Island buoys were established by the Rhode

Island Department of Environmental Management

(DEM) and the Graduate School of Oceanography

(GSO) to conduct more comprehensive monitoring

of oxygen in the upper and mid Bay regions. In

2001 additional monitoring sites in the Providence

River were established. These new measurement tech-

nologies have been used to investigate conditions and

processes occurring in Narragansett Bay.

This paper presents observations and analyses from

the summer of 2001 showing hypoxic conditions not

only in the Providence River, but also in the main

parts of Narragansett Bay. Low oxygen events are

documented in July, August, and September 2001 at

two buoy sites located in the Providence River (Bul-

lock Reach, BR) and the Upper Bay (North Prudence,

NP) (Fig. 1). This paper examines the mechanisms

that cause low oxygen concentrations, and the factors

that restore oxygen concentrations to more moderate

Fig. 1. Map of Narragansett Bay showing the locations of the two moored time-series buoys: BR=Bullock Reach Buoy, NP=North Prudence

Buoy, X=meteorological data from T.F. Green Airport. The Blackstone River, not shown, flows into the Seekonk River.

D.L. Bergondo et al. / Marine Chemistry 97 (2005) 90–10392

values. We also show an ability to predict when sur-

face water phytoplankton blooms and sub-surface

hypoxia are most likely to occur in future years.

2. Study site

Narragansett Bay is a partially mixed estuary, run-

ning north–south from Rhode Island Sound (Fig. 1). It

was formed by the drowning of three river valleys,

which today form the three main regions of the bay:

West Passage, East Passage, and Sakonnet River. The

mean depth of Narragansett Bay is approximately 8.3

m includingMt. Hope Bay (Pilson, 1985). Narragansett

Bay is a relatively saline estuary (i.e., mean salinity 29–

31), with freshwater inputs mainly localized in the

uppermost areas of the bay (Ely, 2002). Seventy-eight

percent of the total surface freshwater input to the bay

comes from five major rivers: Tauton, Blackstone,

Pawtuxet, Woonasquatucket, and Moshassuck (Des-


bonnet and Lee, 1991). The Tauton River empties into

Mount Hope Bay, while the remaining rivers empty

into the Providence River. Additional fresh water input

comes from smaller rivers and land runoff (5%), sew-

age treatment effluent carried by rivers (4%), and pre-

cipitation (13%) (Ries, 1990, as cited in Desbonnet and

Lee, 1991). Flushing time of the bay is primarily con-

trolled by fresh water input rates, and varies from 10 to

40 days with a mean time of 26 days (Pilson, 1985).

Circulation in the bay is primarily tidally controlled,

though non-tidal circulation induced by winds and

freshwater inputs can exert considerable influence on

shorter temporal and spatial scales (Hicks, 1959; Weis-

berg and Sturges, 1976).

3. Methods

The measurements in this study were made with

Yellow Springs Incorporated (YSI) sensors and

sondes. Instrumented buoys were moored at two

sites: Bullock Reach (BR) and North Prudence (NP)

(Fig. 1). Each buoy consisted of a 1.2 m diameter

foam discus with a water-tight chamber that housed

the data logger, controlling electronics, and 12-V

batteries. A tripod structure on the buoy supported

three solar panels that recharged the batteries. Two

sondes (YSI model 6820 at Bullock Reach and YSI

model 600XL at North Prudence) were suspended

from each buoy at depths 0.5 m below the sea surface

and 1.0 m above the seafloor. The average depths of

the bottom sondes were 8.8 m and 10.3 m for Bullock

Reach and North Prudence respectively. Every 15 min

the sondes measured temperature, conductivity (from

which salinity was computed), dissolved oxygen

(using a rapid pulsed oxygen sensor), pH, and pres-

sure (from which the depth of the sensor was deter-

mined). Chlorophyll fluorescence was measured at the

near-surface depth at both sites. Chlorophyll was

measured at the Bullock Reach site with a YSI chlor-

ophyll fluorescence probe and at the North Prudence

site with a Seapoint Systems fluorometer. Data were

transmitted from the Bullock Reach buoy by radio

every 15 min and from the North Prudence buoy by

cellular telephone modem every 8 h.

During summer months, when biofouling rates were

relatively rapid in Narragansett Bay, the buoys were

serviced at 2-week intervals by swapping the surface

and bottom sondes with clean and recently calibrated

sondes. The sonde temperature sensors were factory

calibrated and did not require further calibration prior

to deployment. The conductivity sensors were cali-

brated using a secondary standard 0.2 Am filtered

coastal seawater with a salinity range typical of Narra-

gansett Bay. The salinity of this secondary standard

was determined using a Guildline Autosal based on

IAPSO standard seawater. The dissolved oxygen sen-

sor was calibrated at the atmospheric partial pressure of

oxygen as recommended by YSI, Inc. In earlier works

with these oxygen sensors, they showed that calibra-

tions were within F0.5% of Winkler titration values

(Kester and Magnuson, 1994; Magnuson and Kester,

1995). The pressure sensors were calibrated at atmo-

spheric pressure with a barometer.

Each 2-week deployment began with recently cali-

brated sensors. At the end of each deployment, the

data were examined to determine if post-deployment

corrections were needed to account for sensor drift or

biofouling. Several independent measures were used

to determine whether corrections were needed. The

sondes removed from the field were re-calibrated in

the laboratory prior to removal of any biofouling.

These post-deployment calibrations provided one

measure of the combined effects of sensor drift and

biofouling. As a second measure the last reading of

the sonde removed from the field and the first reading

of the newly deployed (recently calibrated) sonde

were compared. During each servicing of a buoy, a

recently calibrated sonde was used to measure the

vertical profile of properties at the buoy site. These

profiles were used to obtain information on vertical

gradients at the times when sondes were swapped.

To remove high frequency noise from the data

analysis, the 15-min measurements were digitally fil-

tered to 6-h and hourly values using a 24- and 4-point

filters. Hourly and 6-h filters were applied to the 15-

min data in a manner that eliminated potential phase

shifts between the original data and the filtered data.

4. Results

Hourly values of surface to bottom density differ-

ences and surface and bottom dissolved oxygen at the

two buoy locations (July 1, 2001 to September 30,

2001) are shown in Fig. 2. The variations in surface to

0

2

4

6

8

10

7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30

Bullock Reach North Prudence

0.0

0.2

0.4

0.6

0.8

7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30

Oxy

gen

(mm

ol/L

) .

Surface Bottom Bullock Reach

0.0

0.2

0.4

0.6

0.8

7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30

Oxy

gen

(mm

ol/L

) . Surface BottomNorth Prudence

0.5

0.8

1.1

1.4

1.7

7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30

Tid

al R

ange

(m) .

-10

-5

0

5

10

7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30

N-S

Win

d (m

/s) From the south

From the north

-10

-5

0

5

10

7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30

E-W

Win

d (m

/s) From the west

From the east

0

10

20

30

40

7/1 7/8 7/15 7/22 7/29 8/5 8/12 8/19 8/26 9/2 9/9 9/16 9/23 9/30

a.

b.

c.

d.

e.

f.

g.

Riv

er F

low

(m3 /s

) (k

g/m

3 )

∇ σ T


Table 1

Summary of 2001 oxygen depletion rates at Bullock Reach and North Prudence Buoy Sites

Start date End date Initial dissolved

oxygen (mmol/L)

Final dissolved

oxygen (mmol/L)

Oxygen depletion

rate (mmol/L/day)

Bullock Reach

7/13/01 5:17 7/19/01 12:47 0.126 0.061 �0.008

8/13/01 8:32 8/17/01 13:32 0.101 0.033 �0.018

8/27/01 21:02 8/29/01 16:02 0.071 0.047 �0.026

9/2/01 4:47 9/4/01 2:32 0.112 0.061 �0.025

9/9/01 9:47 9/10/01 21:32 0.151 0.040 �0.033

9/12/01 5:02 9/14/01 0:02 0.115 0.052 �0.029

Average �0.02

North Prudence

7/13/01 11:47 7/17/01 5:17 0.100 0.037 �0.012

8/13/01 3:17 8/16/01 5:47 0.106 0.042 �0.013

8/28/01 12:17 8/31/01 3:02 0.148 0.045 �0.026

9/1/01 22:32 9/3/01 15:32 0.203 0.160 �0.025

9/6/01 0:32 9/7/01 18:47 0.180 0.115 �0.031

9/11/01 11:02 9/13/01 6:17 0.154 0.093 �0.039

Average �0.024


bottom density difference at the buoy sites reveal

episodic stratification events. The density differences

were primarily driven by changes in salinity. The

surface to bottom density difference at the Bullock

Reach site was greater than at the North Prudence site.

While the amplitudes of events varied with location,

they were nearly always in phase (Fig. 2).

During the summer of 2001 three major episodes

of hypoxia were observed in subsurface waters of

Narragansett Bay (Fig. 2). The first of the low oxygen

events occurred between July 13 to 23 at both the

North Prudence and the Bullock Reach sites (Fig. 2).

During this period, the tidal range was less than 0.9 m,

the winds were below 7 m/s and variable, and the

Blackstone River peaked (11 July, Fig. 2). At the

North Prudence site the dissolved oxygen dropped

below 0.09 mmol/L on 13 July and, based on the

15-min data, reached a low of 0.036 mmol/L on 17

July. The dissolved oxygen at the Bullock Reach site

declined from 13 July to 19 July at a rate of �0.008

mmol/L/day (Table 1), becoming hypoxic on 17 July.

The dissolved oxygen reached a minimum value of

0.043 mmol/L on 22 July. At about this time, fish kills

Fig. 2. The hourly time-series and metrological data: a) surface to bottom

(black), b) dissolved oxygen concentration at Bullock Reach, c) dissolved

RI, e) north–south component of wind from T.F. Green Airport, f) east–we

flow. The diamonds indicate dates of RIDEM dissolved oxygen surveys.

were reported and low oxygen was observed in shal-

low water (~3–4 m) along the western shore of Green-

wich Bay (Deacutis, personal communication, 2001).

At the North Prudence site, the event ended with

bottom oxygen values increasing steadily (19 July–

23 July) from 0.063 mmol/L to 0.19 mmol/L with

salinity and temperature indicating full mixing of the

water column. At the Bullock Reach site, the event

ended with a series of very large tidally driven varia-

tions in oxygen between 21 July and 23 July. The end

of the July event coincided with increased tidal range

during the new moon of July 20. Surface oxygen

values were more variable at the Bullock Reach site,

ranging from 0.22 to 0.53 mmol/L, with the North

Prudence site ranging from 0.19 to 0.35 mmol/L.

The second low oxygen event occurred between 13

August and 1 September (Fig. 2). At the Bullock Reach

site, dissolved oxygen concentrations decreased from

0.13 mmol/L on 13 August to 0.033 mmol/L on 17

August (rate=�0.018 mmol/L/day (Table 1)). The

tidal range at this time was less than 0.9 m, winds

were variable, and the river flow peaked on 14 July.

Based on 15-min readings, concentrations reached a

density difference for North Prudence (grey) and Bullock Reach

oxygen concentration at North Prudence, d) tidal range at Newport,

st component of wind from T.F. Green Airport, g) Blackstone River


minimum of 0.021 mmol/L on 18 August. Between 18

and 22 August, tidal range was above 0.9 m, and

dissolved oxygen concentrations ranged tidally

between 0.03 and 0.09 mmol/L. The tidal range

began to fall on 21 August. Dissolved oxygen concen-

tration decreased from 0.103 mmol/L on 22 August to

0.041 mmol/L on 27 August, recovered briefly on 27–

28 August, and decreased to less than 0.06 mmol/L late

on August 28. The dissolved oxygen remained low

until 1 September. At the North Prudence site, hypoxia

occurred between 13 and 17 August, reaching a mini-

mum value of 0.042mmol/L on 16 August. The bottom

oxygen recovered to 0.16 mmol/L by the new moon on

18 August and remained high until 25 August. A short

Fig. 3. The 6-h surface (solid grey) and bottom (solid black) oxygen values

the relationships between bottom water hypoxia and surface phytoplankto

2001. Shaded regions are periods of low tidal range.

hypoxic event at North Prudence was observed

between 29 and 31 August. Surface oxygen values

ranged from 0.14 to 0.44 mmol/L at Bullock Reach

and 0.19 to 0.47 mmol/L at North Prudence.

In September, hypoxia occurred intermittently

from 2 to 14 September at the Bullock Reach site

(Fig. 2). Concentrations reached lows of 0.061 mmol/

L on 4 September, 0.040 mmol/L on 10 September,

and 0.052 mmol/L on 14 September. Surface oxygen

values ranged from 0.16 to 0.42 mmol/L at Bullock

Reach. Bottom water oxygen also declined at the

North Prudence site during this time period, but did

not decline to levels below 0.09 mmol/L for more than

a tidal cycle.

from (a) Bullock Reach and (b) North Prudence buoy data showing

n blooms and tidal range (grey with circles) during the summer of


5. Discussion

5.1. Tidal range and hypoxia

There are several important factors that lead to the

monthly subsurface hypoxic conditions observed

during the summer of 2001. The most important

factors were availability of nutrients in the euphotic

zone to support phytoplankton blooms, meteorologi-

cal conditions that enhanced water column stratifica-

tion (freshwater input and surface heating), and

variations in tidal amplitude. Our time-series data

show that tidal range has a strong influence on the

oxygen content of waters in Narragansett Bay. Fig. 3

shows the tidal range (low–high height difference)

during the summer of 2001, based on tidal predic-

0.0

0.1

0.2

0.3

0.4

0.5

0.6

8/7 8/8 8/9 8/10 8/11 8/12 8/13 8

0.00

0.05

0.10

0.15

8/7 8/8 8/9 8/10 8/11 8/12 8/13 8

0

200

400

600

800

1000

8/7 8/8 8/9 8/10 8/11 8/12 8/13 8

.

a.

b.

c.

Time

Sola

r Rad

iatio

n (W

/m2 )

Chl

orop

hyll

a ( µ

mol

/L)

Oxy

gen

(mm

ol/L

)

Fig. 4. The 15-min time-series data from the Bullock Reach buoy for A

dissolved oxygen values (b) chlorophyll a (Amol/L, molecular weight=89

Laboratory in Newport, RI.

tions at Newport, Rhode Island (which is character-

istic of nearly all portions of Narragansett Bay). The

values shown are the absolute fall (or rise) which

occurs about every 6 h due to the semi-diurnal

nature of tides in the Bay. In 2001, the tidal range

was less than 0.9 m on 11–17 July, 28 July–2

August, 10–16 August, 26–31 August, 7–13 Septem-

ber, and 23–29 September.

The relationship between bottom water hypoxia

and tidal range during the summer of 2001 is illu-

strated in the 6-h oxygen values from the Bullock

Reach and North Prudence buoy data (Fig. 3). Dur-

ing periods of low tidal range (around the neap

tides), there is oxygen depletion in subsurface

waters. A decrease in bottom oxygen concentration

was observed during each of the six neap tidal

/14 8/15 8/16 8/17 8/18 8/19 8/20 8/21

Surface Bottom

/14 8/15 8/16 8/17 8/18 8/19 8/20 8/21

Chlorophyll a

/14 8/15 8/16 8/17 8/18 8/19 8/20 8/21

Solar Radiation

-2001

ugust 7–21, 2001 (a) surface (solid grey) and bottom (solid black)

3.49 g/mol) (c) incoming solar radiation data measured by Eppley


events. However, the occurrence of hypoxia in only

the four lower tidal range events indicates that

hypoxia does not occur during all neap tides. During

periods of high tidal range (around spring tides),

bottom oxygen concentrations are restored. Subsur-

face oxygen values were restored during the 20

August spring tide period at the North Prudence

site but not at the Bullock Reach site. This is con-

sistent with our observations that stratification tends

to be stronger at Bullock Reach than at North Pru-

dence, thereby requiring more tidal mixing to restore

oxygen to the bottom water. Dissolved oxygen con-

centrations decreased at rates of �0.008 to �0.033

mmol/L/day at Bullock Reach and at rates of

�0.012 to �0.39 mmol/L/day at North Prudence

during the summer of 2001 (Table 1).

5.2. Tidal range and phytoplankton blooms

There is a strong relationship between light and

photosynthetically produced oxygen. The large in-

creases in chlorophyll a and photosynthetically pro-

duced oxygen observed during clear sunny days are

absent during cloudy conditions (Fig. 4). The major

controlling variable for surface oxygen values is solar

irradiation. Important secondary variables causing

lower surface oxygen concentrations at high tide and

higher levels at low tide, are the horizontal oxygen

gradient within the Providence River and the tidal

excursion. The influence of these factors is much

smaller that the influence of solar irradiation. A

power spectral density plot of the Bullock Reach sur-

face water oxygen concentration shows that the semi-

0

1000

2000

3000

4000

5000

6000

7000

8000

0.0 0.5 1.0Frequency (c

Pow

er S

pect

ral D

ensi

ty

Fig. 5. Power spectral density of surface di

diurnal tidal frequency (1.932 cycles/day) is of very

small consequence in the oxygen variations, com-

pared to the day–night events (1 cycle/day) and the

lower frequency events such as the components at 8–9

days and longer (Fig. 5). These lower frequency

changes in oxygen correspond to bloom conditions

during which surface oxygen concentrations reach 0.3

mmol/L and greater (Fig. 2).

The periods of high surface oxygen values,

which are indicative of phytoplankton blooms, are

sensitive to water column stratification (Fig. 2).

Blooms occur when stratification is enhanced

(such as during neap tides or increases in freshwater

input). Blooms terminate during periods of increased

vertical mixing caused by spring tides, surface cool-

ing, or storm-induced wind mixing. Fig. 3 shows

evidence that changes in tidal range also affect the

surface oxygen concentration. At the North Pru-

dence buoy, high levels of photosynthetically pro-

duced oxygen were observed when the tidal range

was less than 0.9 m on 11–17 July, 27 July–2

August, 10–16 August, 27 August–2 September,

11–16 September and 23–26 September. Photosyn-

thetic blooms tend to occur during periods of low

tidal range (i.e., neap tides) while, conversely,

blooms are disrupted or cannot be sustained during

high tidal ranges (spring tides).

5.3. Restoration of bottom oxygen

Restoration of oxygen to bottom waters can occur

by two processes: downward mixing of euphotic

water or lateral advection of oxygen-rich bottom

1.5 2.0 2.5ycles per day)

BR Surface DO

ssolved oxygen concentrations at BR.


water from the southern portion of the Bay. From 22

to 25 July, bottom oxygen concentrations increased

with each tidal cycle, in phase with tidal height (Fig.

2). The large tidal pulses imply a large lateral oxygen

gradient over the tidal excursion distance (about 3

km). The change in surface to bottom density shows

(Fig. 2) that large spikes in dissolved oxygen concen-

tration occur during times when the surface to bottom

density differences are smallest (i.e., when the water

column is vertically mixed). The bottom dissolved

oxygen concentrations during the well-mixed periods

reached values as high as 0.27 mmol/L (Fig. 2). Sur-

face dissolved oxygen concentration ranged from 0.16

to 0.43 mmol/L at the BR site, whereas surface dis-

solved oxygen concentrations further south at the NP

site barely reached over 0.22 mmol/L in the same

period (Fig. 2). Therefore, it appears that both lateral

advection and vertical mixing restore oxygen to the

bottom waters at BR.

5.4. Predictions for hypoxia

The observed sensitivity of surface phytoplankton

blooms and subsurface hypoxia to tidal range in the

upper and mid-portions of Narragansett Bay is a sig-

nificant finding. While a number of factors influence

the occurrence of blooms and bottom water oxygen

depletion, tidal mixing is especially important during

the summer months. Other factors required to support

a phytoplankton bloom include availability of nutri-

ents and sunlight. Factors important for subsurface

oxygen depletion include surface salinity decreases

due to rainfall and river flows, and surface warming

due to solar irradiance. Hypoxia is, conversely, alle-

viated or eliminated by rapid surface cooling and wind

mixing events.

Despite a variety of influences on oxygen levels

in Narragansett Bay, observation in the present study

show that bloom and hypoxic events can be pre-

dicted. Fig. 6 shows the predicted tidal ranges at

Newport for the months of June through September

for 2002, 2003, and 2004. Based on the results for

2001, the critical events occurred when the tidal

range was less than 0.9 m over a period of 5–7

days. In 2002 the first of such periods occurred

from 30 June to 7 July and the second such period

was from 29 July to 6 August. During both of these

periods hypoxic conditions were observed at the

Bullock Reach and North Prudence buoy sites (Ber-

gondo, 2004). During the early August event the

subsurface oxygen concentration at the North Pru-

dence site reached 0.021 mmol/L, lower than any

value seen in 2001. The hypoxia was eliminated by

6 August when north winds completely mixed most

of the Bay. The critical periods for low oxygen in

2003 were 20–26 June, 20–27 July and 18–25

August. Hypoxia was observed at the NP site during

these periods (Narragansett Bay Window Collabora-

tive, unpublished data). On 20 August, a very large

fish kill was reported in Greenwich Bay (RIDEM,

2003). Projecting ahead to the summer of 2004,

exceptionally weak tidal ranges during the periods

of 7–14 August and 5–11 September were expected

to enhance water column stratification to an even

greater extent than was the case in 2001, 2002 and

2003 (compare Figs. 3 and 6). The 2004 dissolved

oxygen data from the North Prudence site showed

hypoxic events occurring around 11 August and 9

September (Narragansett Bay Window Collaborative,

unpublished data).

5.5. Past dissolved oxygen surveys

Are summer hypoxic events a recent occurrence in

upper Narragansett Bay? Low bottom water dissolved

oxygen concentrations were reported in the Seekonk

and Providence Rivers during surveys in 1923, 1947,

1955, 1959, 1983, and 1987 (Desbonnet and Lee,

1991). Summertime surveys of the Seekonk and Pro-

vidence in 1947, 1955 and 1987 showed increases in

dissolved oxygen over time, and also with distance

from the head of the Seekonk River (Desbonnet and

Lee, 1991). Assuming that the onset of hypoxia in

Narragansett Bay is as closely linked to tidal amplitude

as was observed in the 2001 time-series data, it is

interesting to relate tidal amplitude to past surveys

south of Conimicut Point (Table 2). During the summer

of 1959, the weak neap tide occurred during 27 June–3

July and 27 July–1 August. Low oxygen conditions

were observed on the 28, 29, 30, and 31 July surveys. In

the summer of 1980, four surveys occurred during the

weak tidal cycle, 20 June, 18 July, 15 August and 12

September; however, low oxygen concentrations were

not observed. In the summer of 1983, Granger (1994)

found 0.031–0.02 mmol/L oxygen concentrations in

the northern half of the Providence River; 0.062–0.16

Fig. 6. Relative tidal range predictions at Newport for (a) 2002, (b) 2003, and (c) 2004.


mmol/L in the southern portion of the Providence

River; and 0.13–0.19 mmol/L in the upper Bay near

the present NP buoy site. These observations were

based on oxygen profiles taken from a small boat

about twice per month. Of the four surveys between

mid-June and mid-August 1983, three occurred near

times of high tidal range (N1.6 m), and one occurred

just prior to weak neap tide period (b0.8 m). The four

SINBADD cruises took place 21–24 October 1985,

18–21 November 1985, 7–10 April 1986 and 19–22

May 1986, times when low oxygen conditions were not

likely to occur. Another relevant historical data were

reported by Doering et al. (1988a,b) for the six SPRAY

surveys over an annual cycle in 1987–88. They also did

not observe hypoxia to the extent seen in 2001. The

automated time-series measurements reported in this

study obviously provide a more comprehensive record

of hypoxic events than can be obtained with occasional

boat surveys. However, properly timed surveys, such

as those reported by Deacutis et al. (in press), are of

great value in determining the spatial pattern and extent

of hypoxic conditions. In the eleven surveys conducted

prior to 1999 during periods when hypoxia was likely

to occur, low oxygen concentrations were only

Table 2

Previous dissolved oxygen studies in Narragansett Bay

Year Date Dates of low

tidal range

Likelya Observed

1959 29 Jun–3 Julb 27 Jun–4 Jul Y N

1959 13 Jul–17 Julb N N

1959 27 Jul–31 Julb 25 Jul–2 Aug Y Y

1959 11 Aug–13 Augb N N

1972 20–21 Junc 17 Jun–26 Jun Y N

1972 26–27 Julc 16 Jul–24 Jul Y Y

1980 6–Jund N N

1980 20–Jund 17 Jun–26 Jun Y N

1980 7–Juld N N

1980 18–Juld 17 Jul–25 Jul Y N

1980 15–Augd 15 Aug–23 Aug N N

1980 12–Sepd 14 Sep–21 Sep N N

1982 16–Sepe N N

1982 30–Sepe 21 Sep–30 Sep Y N

1982 15–Octe N N

1982 23–Nove N N

1982 10–Dece N N

1983 10–Dece N N

1983 24–Maye N N

1983 16–June N N

1983 1–Jule 28 Jun–7 Jul Y N

1983 14–Jule N N

1983 11–Auge 16 Aug–22 Aug N N

1983 28 Sep–3 Octe 26 Sep–2 Oct Y N

1985 21–24 Octf N N

1985 18–21 Novf N N

1986 7–10 Aprf N N

1986 19–22 Mayf N N

1986 11–Octg N N

1986 15–Decg N N

1987 11–Marg N N

1987 22–Aprg N N

1987 27–Jung 28 Jun–7 Jul N N

1987 12–Augg N N

1987 19–20 Augg 15 Aug–19 Aug Y N

1989 7–Seph 4 Sep–12 Sep Y N

1999 27 Jul–30 Juli 19 Jul–27 July Y Y

a The likelihood of hypoxia occurring is determined based on a

survey being conducted during a summer month (June–September)

and when tidal range is less than 0.9 m (Y=Yes, N=No).b US Army Corp of Engineers (1960).c Olsen and Lee (1979).d Oviatt (1980).e Granger (1994).f Pilson and Hunt (1989)Hunt et al. (1987).g Doering et al. (1988ab).h Doering et al. (1990).i North Prudence Buoy, unpublished data.


observed twice. It is likely that if hypoxia in the upper

Bay had been as extensive and extreme as was

observed in 2001–2003, it would have been detected

and reported in studies of the Bay prior to the mid-

1990s.

6. Summary

The occurrence of neap tidal subsurface hypoxic

events only in summer is likely caused by several

factors. One factor is stronger stratification of the

water column during summer and weaker sustained

winds, typically out of the southwest, versus strong

north to northeast winds typical of other seasons

(Magnuson, 1997). These seasonal changes in wind

direction may also affect the estuarine circulation and

flushing rate of the bay (Kincaid, personal commu-

nication). Another factor is accelerated rates of

organic matter decomposition by bacteria under

warm conditions. Summer decomposition rates are

sufficient to cause hypoxia during weeklong periods

of reduced vertical mixing (Nixon et al., 1976). At the

North Prudence buoy site, the spring tide mixing

events in 2001 were sufficient to restore the subsur-

face oxygen to values of 0.13 mmol/L and greater.

During the July and September events at the Bullock

Reach buoy site, this was also the case; however,

during the August hypoxic event, the strong spring

tide did not restore the subsurface oxygen. This was

due to stronger density-driven water column stratifi-

cation in this area of the Bay. These results indicate

that there is a delicate balance between oxygen con-

sumption and renewal in the upper portions of Narra-

gansett Bay, and particularly in the Providence River.

Increases in water column nutrients, organic matter, or

other oxygen-consuming substances could shift this

delicate balance from its current state of intermittent

hypoxic to a state of prevalent anoxia, with large

effects on water quality and survivability of fish and

other marine organisms.

Acknowledgements

This paper is dedicated in memory of Dana R.

Kester. We would like to thank Donald Pryor for

compiling the information on past dissolved oxygen

surveys in Table 2. We are grateful to Donald Pryor,

Christopher Deacutis, Warren Prell, Mimi Fox, and

Candace Oviatt for their guidance and support in


preparing this paper, for their discussions on hypoxia

and their helpful suggestions for improving the manu-

script. The thoughtful comments of Bob Byrne and

two anonymous reviews substantially improved the

manuscript. Support for this work came from the

NOAA CMER and EPA EMPACT programs.

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