WHY IS THE RHODE RIVER SO MUDDY?

BY KWADWO OMARI

(INTERN: PHYTOPLANKTON ECOLOGY LAB)

INTRODUCTION

• Background

• The Rhode river which was once vegetated experienced less under water grasses in the late 1960’s. Since then there has been a sporadic occurrence of these under water grasses in the Rhode River.

• Sea grasses are important as food sources, habitats (protection from predators, breeding ground) for some aquatic organisms.

• Their presence at any point in a water body much depends on the clarity of the water body which also depends on Chlorophyll and suspended particulate matter in the water.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Mea

n S

ecch

i D

epth

(m

)

Month

error bars represent +/-1 standard error

Rhode River Water ClarityCBP Station WT8.2

1984-2006

Factors Affecting Sediment Concentration

0 30 60 90 120 150 180 210 240 270 300 330 3600

2

4

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ISS

(g m

-3);

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(km

h-1)

Bio

ma

ss (

kg t

raw

l-1)

Day of Year Inorganic soids Windspeed Epibenthic feeders

0

20

40

60

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100

Sed

iment

Flu

x (k

g h

a-1 w

k-1)

Sediment Flux

J F M A M J J A S O N D

Wind

Runoff Inorganic Solids

• Wind peaks in March

• Runoff peaks in June-July

• Sediments peak in August

• Peak coincides with peak in mobile benthic animals

INTRODUCTION CONT’D.

• Water clarity in the Rhode River is shown to be the best during winter. In summer when there’s lighter wind activity and no inflows into the river, water clarity is shown to be the worst.

• These observations make these two questions very important:

(i) Why the Rhode river, in the first place, is so muddy?

(ii) Why is the water clarity so much worse in summer than winter?

OBJECTIVES

• To determine the settling rate of sediments in the Rhode River

• To determine the susceptibility of the Rhode River (muddy up estuary versus sandy down estuary) to resuspension by a standard disturbance in the early and late summer.

• Experimental Procedure

• The whole approach of this experimental set up was to enclose a volume of water that will reduce turbulence.

• Short cylinder used at Fox Point and long cylinder was used at the Canning House bay.

• Cylinders placed in water and held firmly in place by the iron rods.

• YSI probe was programmed to take data (turbidity and chlorophyll readings) every 30 seconds.

METHODOLOGY

METHODOLOGY CONT’D

• Each experiment was in two parts;

(i) Before anchor/weight was dropped

(ii) After anchor/weight was dropped

• The ambient turbidity measurement (profile) was taking before the start of the experiment and the turbidity profile of the water trapped in the cylinder was again taken after the experiment.

Canning House Bay

Fox Point

METHODOLOGY

• Study Area

• Fox Point (Muddy bottom)

• Canning House Bay (Sandy bottom)

METHODOLOGY CONT’D

• Materials

(i) YSI probe

(ii) Cylinders

(iii) Weight (anchor)

Water samples were brought from the site and analyzed for the Total Suspended Solids (TSS) and Fixed Suspended Solids (FSS). The water samples were usually taken from the water trapped in the cylinder.

RESULTS

EXPT RESULTS

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-0.3 0.2 0.7 1.2 1.7 2.2

TIME (HOURS)

TU

RB

IDIT

Y (

NT

U)

OBSERVED

FIT

t

TTTT bb exp0

Tb

Before disturbance After disturbance

Turbidity Signal is Dominated by Inorganic Suspended Solids

EXPT RESULTS-CHLOROPHYLL

0

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40

-0.3 0.2 0.7 1.2 1.7 2.2

TIME (HOURS)

CH

LO

RO

PH

YL

L (

mg

/m3)

•After anchor drop, chlorophyll slowly increases while turbidity is decreasing.

•TSS were 75% inorganic and 25% organic.

Date Site TSSmg/l

FSS mg/l

% Inorg.

30 Jul

Canning House Bay

25 15 60

08 Aug

Fox Point 62.5 45 72

09 Aug

Fox Point 78 58 74.4

10 Aug

Fox Point 52.5 40 76.2

COMPARISON OF SITES

TIME CONSTANT

0

0.05

0.1

0.15

0.2

0.25

0.3

CHB FP

HO

UR

S

Series1

BASELINE TURBIDITY

0

5

10

15

20

25

30

CHB FP

NT

U

No significant difference between sites for Time Constant or Baseline Turbidity

BEFORE DISTURBANCE

COMPARISON OF SITES

AFTER DISTURBANCE

BASELINE TURBIDITY

0

5

10

15

20

25

30

35

40

45

50

CHB FP

NT

U

TIME CONSTANT

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

CHB FP

HO

UR

S

TIME CONSTANT WAS SIGNIFICANTLY SHORTER AND BASELINE TURBIDITY WAS SIGNIFICANTLY HIGHER AT FOX POINT

COMPARISON OF SITESCHANGE IN BASELINE TURBIDITY

AFTER DISTURBANCEChange in baseline turbidty

-5

0

5

10

15

20

25

30

CHB FP

NT

U

CHANGES THROUGH TIMEBASELINE TURBIDITY BEFORE

DISTURBANCE

FOX POINT

0

5

10

15

20

25

5/28/07 6/7/07 6/17/07 6/27/07 7/7/07 7/17/07 7/27/07 8/6/07 8/16/07DATE

NTU

CANNING HOUSE BAY

0

5

10

15

20

25

30

35

40

45

50

6/17/07 6/27/07 7/7/07 7/17/07 7/27/07 8/6/07DATE

NTU

Decreasing baseline turbidity at Canning House Bay and constant baseline turbidity at Fox Point.

CHANGES THROUGH TIMEBASELINE TURBIDITY AFTER

DISTURBANCE

CANNING HOUSE BAY

0

5

10

15

20

25

30

35

40

6/17/07 6/27/07 7/7/07 7/17/07 7/27/07 8/6/07DATE

NTU

FOX POINT

0

10

20

30

40

50

60

5/28/07 6/7/07 6/17/07 6/27/07 7/7/07 7/17/07 7/27/07 8/6/07 8/16/07DATE

NTU

Decreasing baseline turbidity at Canning House Bay and slightly increasing (ns) baseline turbidity at Fox Point.

SUMMARY

• TSS were 75% inorganic and 25% organic.

• Before the disturbance, there was no significant difference between sites for time constant or baseline turbidity.

• After the disturbance, time constant was significantly shorter and baseline turbidity was also significantly higher at Fox Point.

SUMMARY

• Before disturbance there was a decreasing baseline turbidity at Canning House Bay and a constant baseline turbidity at Fox Point through time.

• After disturbance there was a decreasing baseline turbidity at Canning House Bay and an increasing baseline turbidity at Fox Point through time.

CONCLUSIONS

• The turbidity consists of rapidly and slowly settling components.

• The water column clearance time for rapidly settling component was about 0.6 hour (3*time constant)

• The difference between sites was consistent with expectations for the sandy and muddy bottoms.

• The comparisons between sites indicates that shallow muddy sites may be the source of turbidity for most of the river.

• Also the susceptibility of the bottom to resuspension seemed to progress through the summer.

FUTURE WORK

• Work should be repeated in winter.