Marrowstone Sodium Hypochlorite Mesocosm September 2004 

The first test of the sodium hypochlorite generator/filtration treatment system was started 

on September 3, 2004. The experiment included a chlorine/no filtration treatment and 

chlorine with filtration treatment. The target chlorination level was 4 mg/L. Initial 

chlorine levels for the filtered treatment were 3.5 mg/L and 2.95 mg/L without filtration 

(Table 1). There were 4 mesocosms (tanks) per treatment and 4 control mesocosms. The 

mesocosms were analyzed for Total Residual Oxidant (Cl2 mg/L), culturable 

heterotrophic bacteria, chlorophyll a, and zooplankton at 5, 24, 48, 120, and 240 hours 

following treatment. 

Methods 

Total Residual Oxidant (TRO) was measured as Cl2 using a Hach Spectrophotometer and 

Hach Colorimeter. DPD powder pillows were used for the analysis. TRO measurements 

were after the initial fill and at all of the time points listed above. 

The 1500-gallon tank of raw seawater was tested for bacteria levels before the start of the 

experiment. Water samples taken at the 5 time points were also tested for bacteria levels. 

Bacteria colonies were cultured from known volumes of water to calculate colony 

forming units (CFU) per liter of water. Colonies were cultured on petri dishes containing 

a growth medium suited for marine heterotrophic bacteria. The inoculated medium was 

analyzed for colony formation after a few days at room temperature incubation. 

Water samples were also taken for chlorophyll a analysis. Chlorophyll a is an indicator 

for the presence of live phytoplankton. A known volume of sample water was filtered 

through glass fiber filters with a pore size small enough to retain phytoplankton cells. 

The filters were frozen for later analysis in our Seattle laboratory. Chlorophyll a was 

extracted from the filters using acetone and then analyzed for fluorescence to determine 

the concentration in μg/L. 

Mesozooplankton were collected in Mystery Bay, using a 110μm mesh net 1 meter 

diameter the morning of the test. Enough were collected to achieve approximately 150 

mesozooplankton per liter. A Stempel pipette was used three times to collect a 

randomized 5 ml sample of the mesozooplankton “soup”. The densities counted using a 

dissecting scope were used to calculate how much of the “soup” was needed for the 

1500-gallon tanks to achieve 150 mesozooplankton per liter. The calculated amount of 

soup was poured into the tank and allowed to settle for at least an hour to let zooplankton 

acclimate. Three preliminary samples were collected out of the 1500-gallon tank after 

mixing. These were used to check calculation and to see effects of pumps. Sampling 

periods for mesozooplankton were 5,24,48,120, and 240 hours. Samples were collected 

out of the 72-gallon mesocosms using a 1-liter Nalgene bottle after thoroughly mixing the 

contents of the mesocosm. Liter sample was filtered through a 73μm sieve and placed in 

counting tray. Mesozooplankton counted were placed into one of eight generic 
categories and then by state; live, dead (absolutely no response to poking) or moribund 

(internal movement and no flight response to needle poke). 

Results 

Total Residual Oxidant (TRO) levels declined steadily for the duration of the experiment 

(Figure 1). TRO in the nonfiltered test tanks dropped more than the filtered test tanks in 

the first 5 hours and then the TRO dissipated equally for the remaining time points. 

Bacteria were greatly reduced in both treatments and showed minimal rebound over the 

10 days of the experiment (Figure 2). There was a slight rebound of bacteria in the 

treatment without filtration. 

Chlorophyll a is an indicator of phytoplankton. In the treated seawater, Chlorophyll a 

levels were at or below the detection limit starting with the 5-hour time point and 

continued to drop for both of the treatments (Figure 3.B, Figure 3.C). In the control 

tanks, chlorophyll a levels declined over the duration of the experiment, likely due to the 

absence of light since all of the mesocosms were covered (Figure 3.A). 

After the first treatment trial it was concluded that no statistical difference could be found 

between the two treatments, at all time points and states (Figure 4A-4C). The chlorine 

plus filter 5 hour time point had one live polychaeta larvae in two of the replicates Figure 

4.E. This was the only time that live organisms were found in any of the treatments and 

time points. The difference between the control and treatments was highly significant. 

Some mesozooplankton were still able to get through the 50μm filter, all were dead 

except for the 2 mentioned above. 
Heterotrophic Bacteria 

Marrowstone Sodium Hypochlorite Mesocosom 

September 2004 

Marrowstone Sodium Hypochlorite Mesocosm 

Total Residual Oxidant Cl2 9-3-04

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0 50 100 150 200 250

Time (hrs)

TRO Cl2 (ppm)

Control

Cl w/ filter

Cl w/o filter

Marrowstone Sodium Hypochlorite Mesocosm

Heterotrophic Bacteria 9-3-04

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

0 50 100 150 200 250

Time (hrs)

Colony Forming Units / liter

Control

Cl w/ filter

Cl w/o filter

Figure 1 

Figure 2 
Chlorophyll a (μg/L) 

Marrowstone Sodium Hypochlorite Mesocosom 

September 2004

Control

0

2

4

6

8

10

12

14

020406080100120140160180200220240

Time (hours)

chl a (ug/L)

Control

Pretreatment

Chlorine with Filter

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

020406080100120140160180200220240

Time (hours)

chl a (ug/L)

Chlorine with no Filter

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

020406080100120140160180200220240

Time (hours)

chl a (ug/L)

Figure 3.A 

Figure 3.B 

Figure 3.C 
Zooplankton analysis 

Marrowstone Sodium Hypochlorite Mesocosm 

September 2004 

Live Mesozooplankton 

0

50

100

150

5 24 48 120 240

Time(hours)

Zooplankton/L

Pre-treatment Control

Chlorine Filter Chlorine no-filter

Total Mesozooplankton

0

50

100

150

200

5 24 48 120 240

Time(hours)

Zooplankton/L

Pre-treatment Control

Chlorine Filter Chlorine no-filter

Percent Live Mesozooplankton

0%

20%

40%

60%

80%

100%

5 24 48 120 240

time(hours)

percent live

Control Chlorine Filter

Chlorine no-filter
Control Treatment

0%

20%

40%

60%

80%

100%

5 24 48 120 240

Time(hours)

Percent 

Mesozooplankton

Dead

Moriund

Live

Chlorine Filter Treatment

0%

20%

40%

60%

80%

100%

5 24 48 120 240

Time(hours)

Percent 

Mesozooplankton

Dead

Moriund

Live

Chlorine Non-filter Treatment

0%

20%

40%

60%

80%

100%

5 24 48 120 240

Time(hours)

Percent 

Mesozooplankton

Dead

Moriund

Live