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The impact of sewage discharges at ocean outfalls on phytoplankton populations in waters surrounding the Hawaiian Islands
Marine Em'ironmental Research 8 (1983) I 01- I 17
The Impact of Sewage Discharges at Ocean Outfalls on Phytoplankton Populations in Waters Surrounding the Hawaiian Islands Edward A. Laws & Kenneth L. Terry University of Hawaii, Oceanography Department, Honolulu, HI 96822, USA (Received: 23 August, 1982)
ABSTRACT Primary production rates, chlorophyll a and nutrient concentrations, particulate carbon, nitrogen and phosphorus concentrations, Secchi depths and submarine light levels were measured at stations within, and near, the zone of mixing of the Sand Island and Mokapu Point ocean sewage outfalls near the island of Oahu in the Hawaiian Islands. Multiyear averages of several parameters indicated that the Sand Island outfall had no adverse impact on phytoplankton communities or water quality. The major effect of the Mokapu Point outfall was a reduction in photosynthetic rates, probably due to the rapid uptake of nutrients introduced into the mixed layer with the secondarily treated sewage. The Sand Island outfall, which discharged coarsely screened raw sewage within the thermocline, had less of an impact because the inorganic nutrient concentrations in the sewage were lower and because the sewage rarely rose into the mixed layer. The results indicate that, under appropriate conditions, ocean outfalls may be preferable to estuarine or freshwater outfalls and that secondary treatment is not always necessary to reduce the impacts of ocean outfalls to an acceptable level.
INTRODUCTION The discharge of sewage that has received primary treatment has been proposed as an alternative to secondary sewage treatment to prevent 101 Marine Environ. Res. 0141-1136/83/0008-0101/$03-00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
Edward A. Laws, Kenneth L. Terry
102
eutrophication problems in coastal areas (Officer & Ryther, 1977). Assuming nitrogen to be the principal nutrient limiting phytoplankton biomass in marine systems (Ryther & Dunstan, 1971), Officer & Ryther (1977) have pointed out that the biochemical oxygen demand (BOD) associated with the mineralisation of the phytoplankton biomass produced from the inorganic N in secondarily treated sewage may be as much as twice the BOD of raw sewage. Since the effective dilution volume and flushing rate for sewage is likely to be much larger in the ocean than in an estuary, it is logical to expect that the environmental impact of the ocean outfall will be much less than that of an estuarine outfall. The impact of the ocean outfall on the phytoplankton community may be further reduced if primarily treated, rather than secondarily treated, sewage is discharged, because the latter typically contains a higher concentration of inorganic N (Officer & Ryther, 1977). In cases where ocean outfalls are not likely to cause oxygen depletion, primary treatment may therefore be preferable to secondary treatment. During 1977 the County of Honolulu, Hawaii, began discharging 3.9 x 10+ m 3 d a y - t of largely domestic secondarily treated sewage at a depth of 30 m at an ocean outfall I km offshore of Mokapu Point on the windward side of the island of Oahu (Fig. 1). In December, 1976 the County of Honolulu began discharging 2.5 x 105 m 3 d a y - 1 of essentially
d
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u
13<:
~
L.._--~Mz
¢*' ~=, 41,~
.,IrM,okapu Ocea 13
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O=fall ~,~o~ ~
I j
Fig. !.
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Outlall
~
.
.. L ~
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/
Mokotea
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+~ ~
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Location of Mokapu Point sewage ouffall and samplint stations.
Sewage discharge effects on phytoplankton
103
21"20"
N 16~5~B°
Fig. 2.
Ikm i i 56'
i 54'
W 157"52'
Location of new and old Sand Island sewage outfails and sampling stations.
raw (coarsely screened) and largely domestic sewage at a depth of 73 m at an ocean outfall approximately 3 km offshore of Sand Island on the leeward side of the island of Oahu (Fig. 2). Characteristics of the sewage discharged at these two outfalls are given in Table 1. Prior to the use of these two outfalls, about 60 % and 40 % of the Mokapu Point sewage had been discharged into Kaneohe Bay and Kailua Bay, respectively (Fig. 1), and the Sand Island sewage had been discharged at a depth of approximately 15 m about 1 km offshore (Fig. 2). The Kaneohe Bay discharge had created serious water quality problems in Kaneohe Bay (Laws & Redalje, 1979; Smith et al., 1981) and pathogens had repeatedly been isolated in water samples taken near the old Sand Island outfall (Loh & Fujioka, 1980). The purpose of our study was to determine the impact of the two new sewage outfalls on water quality and the phytoplankton communities in the vicinity of the outfalls. In the case of the Mokapu TABLE 1 Characteristics of Sewage Discharges at Sand Island and Mokapu Outfali Sites
Flow rate (m 3 day -t) Total N (mg litre-t) Total P (mg litre- t)
Sand Island
Mokapu Point
2.5 × l0 s 18-29 5-4-5-8
3-9 x 104 22 5.8
Edward A. Laws, Kenneth L. Terry
104
Point outfall we were able to compare pre-discharge and discharge conditions, because the old outfalls were far ( > 4 kin) from the new outfall site. This comparison was impossible in the case of the new Sand Island outfall because of the proximity of the old outfall, but the Sand Island raw sewage discharge results do provide a revealing comparison with the Mokapu Point secondarily treated sewage discharge study.
MATERIALS AND METHODS Sampling sites at Sand Island and Mokapu Point are indicated in Figs 1 and 2. The annual average current directions at Mokapu Point are in a northerly and westward direction around the point, and at Sand Island in a westward and southerly direction (Laevastu et al., 1964; Armstrong, 1973). On the basis of these mean current directions, stations were established in an upcurrent (UC) and downcurrent (DC) direction from the zone of mixing station (MZ). However, current directions near the outfall sites may vary seasonally and tend to reverse direction on the ebb and flood tides (Laevastu et al., 1964; Bathen, 1978). Thus, the UC and DC stations were distinguished from the MZ station primarily by their distance and only secondarily by their direction from the zone of mixing. The stations were chosen to lie along the isobaths of the outfall diffusers, and were separated by distances of 1.25 km at Mokapu Point and 2.5 km at Sand Island. Monthly sampling was done for a 3-year period (December, 1976-November, 1979) at Sand Island and during two separate 1-year periods (February, 1976-January, 1977 and January, 1979-December, 1979) at Mokapu Point. Water samples were taken from depths of 5, 25 and 50m at Sand Island and 5, 15 and 25 m at Mokapu Point using an ordinary garden hose connected to a bilge pump. Preliminary studies on surface water samples collected in a plastic bucket and subsequently drawn through the hose and bilge pump system revealed no significant effect of the hose-pump system on either chlorophyll a (Chl a) concentrations or primary production rates (t-test, p > 0.10). The samples were prescreened through a 102#m mesh nylon gauze to remove the larger zooplankton.
Primary production Primary production rates were measured using the 14C method
Sewage discharge effects on phytoplankton
105
(Strickland & Parsons, 1972). Sixty-millilitre light and dark glass bottles were filled with sample water and then stored in the dark at ambient temperature prior to inoculation with t4C-bicarbonate. Maximum storage time was I h. The bottles were incubated in Honolulu Harbor in the case of the Sand Island samples or in Kaneohe Bay in the case of the Mokapu Point samples at water depths corresponding to the light levels ( ~ of surface light) from which the water samples were taken. Light levels were estimated from Secchi depths or from submarine light measurements made using a LI-COR model 185-A submarine quantum sensor. The samples were incubated from approximately 10a.m. to noon and then filtered under low vacuum (<0-35atm) onto Sartorius 0.45tan membrane filters. The filters were rinsed with 10 ml of filtered seawater, placed in liquid scintillation vials containing 0.5 ml of 1N HCI to drive off any t4CO2, and dried at room temperature. The activity on the filters was then determined using a Tracor Delta 300 liquid scintillation counter with Aquasol as a fluor. The activity of 14C spikes was determined following the recommendations of Iverson et al. (1976).
Chlorophyll a Chlorophyll a concentrations were determined fluorometrically (Strickland & Parsons, 1972), but using the acidification procedure for phaeopigment corrections recommended by Holm-Hansen & Riemann (1978). About 350 ml of sample water were filtered onto glass fibre GF/C filters and placed in 100~ acetone for the extraction procedure. The fluorometer was calibrated using pigment extracts from a mixed population of phytoplankton, as recommended by Strickland & Parsons (1972). The pigment concentrations in the mixed population were determined on a Beckman Acta II spectrophotometer, and the ratio of the Chl a concentrations to the fluorometer (Turner model III) reading used to determine the fluorometric calibration constant.
Particulate carbon, nitrogen and phosphorus Samples for particulate carbon (PC), particulate nitrogen (PN) and particulate phosphorus (PP) were collected only during 1979. The samples were collected in 2 litre dark polypropylene bottles, stored at ambient temperature and filtered within 4 h. For PC and PN analyses, 2 litres of sample were filtered onto precombusted GF/C filters. The loaded
106
Edward A. Laws, Kenneth L. Terry
filters were stored frozen prior to analysis on a Hewlett-Packard model 185B CHN analyser. Procedures recommended by Sharp (1974) were followed in analysing the samples. For PP analyses, 2 litres of sample water were again filtered onto a precombusted GF/C filter. The organic material on the filter was oxidised first by heating at 100°C for I h in concentrated H2SO 4, and then heating at 100°C for 30min in dilute H2SO 4. The acid solution was then neutralised with NaOH and NaHCO 3 and analysed for molybdate reactive phosphorus (MRP) as indicated below. Filter blanks were subjected to identical acid treatment and the results used to correct sample values for MRP contamination.
Nutrients All nutrient analyses were performed on a Technicon AutoAnalyzer. Filtrates from either the Chl a or PP analyses were used for the dissolved nutrient determinations. The samples were stored frozen prior to analysis. MRP, nitrate plus nitrite, ammonium and silicate were measured following the procedures of Murphy & Riley (1962), Wood et al. (1967), Solorzano (1969) and Strickland & Parsons (1972), respectively.
Light intensity and extinction coefficients Submarine light intensities were measured only during 1979. A LI-COR model 185A submarine quantum sensor (400-700 nm radiation) attached to a 30 m conducting cable was used to make the light measurements. Light extinction coefficients were calculated from the slope of a plot of In (I) versus depth, where I is the measured light intensity. Secchi depth measurements were made from the sun side of the boat (a Boston Whaler) using a 30-cm diameter white disk. Extinction coefficients (K) were calculated from Secchi depths (D) using the relationship K = 1.7/D (Sverdrup et al., 1942).
RESULTS Data from the Sand Island and Mokapu Point stations are listed in Tables 2 and 3. Because sampling extended over an integral number of years, these values are not biased by seasonal cycles. In order to test for differences between stations and for pre-discharge versus discharge
Particulate carbon (juu) Particulate nitrogen ~M) Particulatephosphorus (riM) Chl a (mgm =s) Pbotmynthesis (rag C m - 3 h - i) Productivity Index (gC g- t Chl a h - l) Molybdate reactive P NO~ + NO~ (pM) Nh~ (jtN) Silicate (/JM) Temperature (°C) Light ( ~ Surface) Secchi (m)
4.9 + I.I 0"51±0"10 35 ± 35 0.18±0.06 0.91 ±0.32 8.7 + 4.3 0.33±0.19 0.17±0-14 0.38+0.15 5.0 ± 2-4 25.1 +0-4 17 _+5 27 ± 4
10.0 + 4.7
0.29+0.11 0.22+0.15 0-48-1-0.17 8.1 ± 5.2 25.2±0-5 45 ± 9
~m
Upcurrent
0.38±0.24 0.18±0-11 0.53+0.21 5.5 ± 3-4 24-9±0.4
4.3 ± 2.4 0.49+0.49 0.21±0.13 0.67±0-56 6.1 ± 3.7 25.2±0.5 44 ± 7
8.8 _+3.5
4-0 :t: 1.0 0.43±0-11 23 ± 3 0.15±0.04 1.15+0.47
5m
0.59±0.53
0-18-1-0.08 0.70±0-40 5.5 ± 2.4 24.9±9.4
0.16±0.09 0.61±0-27 7.0 ± 4.9 25-0+0-4 18 ± 4 27 ± 5
4-7 + 2-0
5.6 ± 3.5 0.70_+0"45 28 + 9 0.20±0.05 0.64±0.19
~m
3.9±1.1 0.~ ±0.II
14±9 0'16±0-~ 1'22±0"55 9-8±4.9 0"28 + 0"11 0"12+0-08 0"50:120"21 5"9 +_3.7 25" I ± 0-4 16±4
3.9±1-3 0.~ ±0.II
20±15 0'13±0-03 0"87±0-31 9'8±5"3 0"63 + 0"68 0.16+0-09 0"48+0"22 6"7 + 4"6 25"2 ± 0"4 41±5 27±5
Downcurrent
1976-Novem~r,
5m
D~ember,
0.50±0.46
8.8 + 4-1
4.6 ± 1.4 0"53+0"13 59 + 59 0.16±0-04 1.13±0-48
25m
50m
Sandlsland,
Mixing zone
TABLE2 MonthlySamplesfrom
6.3 ± 1.5 0.74±0.22 24 ± 7 0.21 ± 0-07 0.49±0-20
50m
ConfidenceLimits)~r
4.7 ± 1.0 0-50+0-10 13 5:12 0-15±0.03 !.12±0-46
5m
MeansofVariables(±95%
0-54 + 0"49 0"20:1:0"11 0.62+0-40 8'9 ± 5-5 25-0 ± 0'4
5.2±2.4
29±26 0-19±0-~ 0"~±0"19
4.8±I-7 0.57±0.17
JOm
1979
Sccchi (m)
Light ( ~ Surface)
Temperature (°C)
Silicate (~M)
NH~ (,uM)
NO_; + N O z (aM)
Molybdate reactive P
(gCg-IChlah -l)
Photosynthesis (rag C m-~ h -j ) Productivity Index
Chi a (rag m -3)
Particulate phosphorus (riM)
Particulate nitrogen (t~M)
Particulate carbon (t~M)
46+-6
18 0.11+-0-03 0-20±0.16 1.5 ± 0,5 0-6±0-3** 17±7 6±5** 0.15±0.04 0.14±0.03 0.31 4-0.16 0.20±0.22 0-42±0-20 0.71±0.46 3"3 ± 1"5 12±10 24.4+0.5 25.4+-0-9
15_.+
0.38_+0.06
3.2±0-7
5m
29+9 24 __.3 24+_3
5:t: 10 0.12±0-03 0.16±0.07 1.3 ± 0.4 0.4±0.1"* 13±4 2"5±0'5"* 0.15±0-1)4 0-11±0-02 0.59+0.48 0.20±0-25** 0-44±0.37 0-86±0.98 3"0 ± 1'2 13+- 13 24.3±0.4 25.3+-0.9
0.40-t-0.09
3-3+--I.I
15m
Upcurrent
17 +- 7
17+ 12 0-12±0-04 0.16±0-08 0.95:0.4 0.4±0.2** 8±3 4±2** 0-18±0.03 0.12+0.03"* 0-29±0-13 0.14+0-13 0.44+-0-17 0.41±0.10 2-0 ± 0-9 7_+6* 24-2±0.4 25.2+0.9
0.45+0.08
4.0+_.1.1
25m
26+ 14 0.10±0-03 0.17±0-08 1.4 ± 0.3 0.6±0.3** 145:2 5_+2** 0.14 ± 0.04 0.12+_.0.03 0-58+0-48 0.21±0.19" 0.44±0.30 0.60+0-45 3"3 ± 1'6 10±g* 24.3±0-5 25.4±0.9 . . . . 42+-8
0.44+0.06
3'3+0-7
5m
14+ II 0.12±0.03 0.16±0.08 I. I ± 0.4 0-7 ± 0.3" 12±5 6±5** 0-15 ±0-03 0.14±0.02 0.35±0.24 0-18±0.18" 0.33±0.18 0-70±0.59 3"0 ± I' 1 13+_7" 24.2+-0-4 25-2 +- 0.9 . . . . . 25 +- 3 23 5:3 24+-3
0.44+0.08
3"6__+0"6
15m
Mixing zone
16+ II 0.12_+0-04 0.225:0-09 I. 3 ± 1.6 0-5_+0.2 10±8 3±3** 0.16+-0.03 0.14±0.03 0.46+0,32 0.08±0.04** 0-36±0.29 0-56±0.24 2"9 ± 0"9 8+-7 24.2+-0.4 25.2+_0.8 . . . . 14+- 5
0"47520"06
4"1+--0,6
25m
22+20 0-10 ± 0.03 0.25+0.12" 1.2 +- 0.3 0.5 + 0.2"* 12+-2 3-t-I** 0.14+-0-04 0.13±0.01 0-70±0.81 0.12+-0-06 0.44±0-32 0.47+_0-23 3"I +_ I-4 5.3+- 3'5 24.3+-0.5 25.3+_0.9 . . . . 40+- 7
0.39+0-04
3'3 +- if4
5m
36+ 55 0-09_+0.03 0.18+-0.007" 0-9 _+0-3 0.5+-0.2** 10+_2 3+1"* 0.15+-0-04 0.16+-0.04 0.95+-0-72 0-33+0.30" 0.44+-0.18 0-49_+0.15 2"9 +- I-2 14+- 11" 24.2+-0-4 25.2+_0.8 . . . . . 22 + 5 23 +- 3 24_+3
0-46+-0.08
_~.7~ 0 . 9
15m
Downeurrent
_
_
~4+ 2
38___54 0.11+0.04 0.24+0.11 O.5 + O.I 0.4+_03 4+- I 3+-3* 0-14_+0.04 0.13+-0-02 0.41 +0.32 0.13_+0.11"* 0.45 +0.28 0-50+_0-14 2-8 ___0.9 II _+8* 24.2+-0.4 25-1 +_0.8
_
0-54+_0.14
42+1.0
25m
TABLE 3 M e a n s o f V a r i a b l e s ( + 9 5 j'~,, C o n f i d e n c e L i m i t s ) for M o n t h l y S a m p l i n g at t h e M o k a p u P o i n t S t a t i o n s , D e c e m b e r , 1976 N o v e m b e r , 1 9 7 9 U p p e r a n d L o w e r N u m b e r s a r e P r e - d i s c h a r g e a n d D i s c h a r g e V a l u e s . Single a n d D o u b l e A s t e r i s k s I n d i c a t e D i f f e r e n c e s S i g n i f i c a n t at 95 ?,', a n d 99 % C o n f i d e n c e Levels, R e s p e c t i v e l y
Sewage discharge effects on phytoplankton
109
differences at Mokapu Point, a non-parametric Kruskal Wallis test was performed (Sokal & Rohlf, 1969). The test revealed no significant interstation differences at Sand Island (p > 0.15). At Mokapu Point, there was a significant inter-station difference in productivity indices (PI's) at 25 m during pre-discharge conditions (p < 0.05) and a significant inter-station difference in MRP concentrations at 15 m (p < 0-04) during discharge conditions. Otherwise there were no significant (p > 0.05) inter-station differences for any parameter during either pre-discharge or discharge conditions at Mokapu Point. However, a large number of significant differences were detected in comparing pre-discharge and discharge conditions at Mokapu Point. PI's were significantly different and lower at all stations and depths during discharge conditions. This change in PI's was due in part to a reduction in photosynthetic rates, which were significantly different and lower at seven of the nine station-depths during discharge conditions. Chl a concentrations were consistently higher during discharge conditions, but the differences were statistically significant only at 5 m and 15 m at the DC station. Of the nutrients studied, silicate was consistently higher and nitrate plus nitrite was consistently lower during discharge conditions, the difference being significant at five of nine station-depths for silicate and six of nine station-depths for nitrate plus nitrite. There was no significant change in NH~ concentrations due to sewage discharges, and MRP concentrations changed significantly only at the UC station 25 m depth.
DISCUSSION The fact that almost no significant inter-station differences were found during discharge conditions indicates that (a) the effects of the sewage discharges were so small as to be statistically undeteetable in the zone of mixing or (b) the effects of the sewage discharges were evenly distributed at the UC, MZ and DC stations. At Sand Island the ouffall diffuser is located at a depth within the thermocline during much of the year, and submergence of the effluent field was expected about 90 % of the time (C. L. Lau, pers. comm.). We suspect, therefore, that the uniformity in mean parameter values at all three Sand Island stations reflects a lack of any significant impact on the system from the sewage ouffall rather than a significant but uniform perturbation. The one piece of information which argues against such an interpretation is the fact that the Sand Island MRP
110
Edward A. Laws, Kenneth L. Terry
concentrations are uniformly high (0-3-0.6~M) compared with those measured at Mokapu Point. However, we suspect that the high MRP concentrations at our Sand Island stations reflect the proximity of Keehi Lagoon and Honolulu Harbor rather than the sewer outfall. Streams draining into Keehi Lagoon and Honolulu Harbor contain total P concentrations as high as 10/~M and total P concentrations in Keehi Lagoon are 3-4 times higher than those measured at nearshore stations to the east and west of Honolulu (City and County of Honolulu, 1971). Tidal flushing injects these nutrients directly into the nearshore mixed layer, a process which is undoubtedly much more efficient than the slow diffusion of sewage-introduced nutrients through the thermocline. At Mokapu Point, on the other hand, the shallowness of the outfall diffuser resulted in the discharge occurring within the mixed layer throughout the year. The uniformity of the post-discharge parameter values, as well as the significant changes in PI's, productivity, silicate and nitrate plus nitrite-concentrations, must therefore be interpreted as suggesting that the time-averaged effects of the sewage discharge on the water column were distributed rather uniformly over a wide area. This conclusion is consistent with the known variability in current patterns and turbulence of the water column near the Mokapu Point outfall site. The reduction in photosynthetic rates during sewage discharges at Mokapu Point seems at first surprising if one assumes that the populations are naturally nutrient limited. The effect could be due to the presence of toxins in the sewage, but this explanation seems unlikely, since the sewage is largely of domestic origin. A more likely explanation follows from the recent work of Healey (1979) and Lean & Pick (1981). These authors have observed that photosynthetic rates may be markedly suppressed when nutrient-limited populations are suddenly allowed to rapidly assimilate nutrients. The effect on photosynthetic rates is transient and is caused by the fact that energy is diverted from carbon assimilation to nutrient assimilation until ~ the populations become nutrient saturated. The fact that water column NH4 "~ and MRP concentrations were not significantly elevated over pre-discharge values, and that NO 3 plus NO 2 concentrations were actually reduced, suggests that the phytoplankton stripped virtually all of the added NH~- from the water, and then took up additional nitrate and nitrite to compensate for the low N: P ratio in the sewage. The low nutrient concentrations during discharge conditions (Table 3) are by no means inconsistent with a suppression of photosynthesis by rapid nutrient uptake, since the
Sewage discharge effects on phytoplankton
111
observed suppression has been found to persist for at least several hours after stimulated uptake is complete (Healey, 1979). A current speed of only about I km h-1 would be adequate to transport water from the outfall site to either the MZ or DC station in only a little more than 2 h, and, based on information given by Bathen (1978), current speeds in the outfall area frequently exceed 2-4 km h- 1. Thus, the time scales involved are consistent with our explanation for the suppression of photosynthetic rates during discharge conditions. The fact that silicate concentrations were higher during discharge conditions is probably due to the very high concentrations of silicate (> 500/aM) in freshwater on Oahu (Stearns & Vaksvik, 1935), and the fact that relatively little of that silicate is required to support the additional biomass produced from the N and P in the sewage. Although we conclude that sewage discharges did significantly affect the phytoplankton community and certain water quality parameters at Mokapu Point, none of the means of variables measured at either the Mokapu Point or Sand Island stations is objectionable from a water quality standpoint. A comparison with other oceanographic regions is revealing. Chl a concentrations of about 10-40mgm -3 are typical of phytoplankton blooms in fertile coastal waters (Riley & Chester, 1971), whereas Chl a concentrations of 0-05 mgm-a are commonly found in barren tropical seas (Bienfang & Gundersen, 1977). Average Chl a values in temperate seas are about 0.5 mg m-3 (Riley & Chester, 1971). Eppley et al. (1977) reported surface water Chl a values in near-shore surface waters off Southern California as high as 9 mg m - 3, with values 100 km offshore approaching 0.2mgm -3. Eppley et al. (1973) and Sharp et al. (1980) reported surface water Chl a concentrations in the central gyre of the North Pacific to be 0.05-0.15 mg m- a. Based on these comparisons, the Chl a concentrations at the Sand Island and Mokapu Point stations would make them intermediate between barren tropical seas and temperate seas. Examination of PC, PN and PP concentrations provides another means of comparing the water quality near the sewage outfalls with other oceanographic areas. Eppley et al. (1972) reported PC concentrations in unpolluted coastal waters off southern California of about 17/aMwhereas, in areas affected by sewage discharges, the PC concentrations were 30-80 #M. PN concentrations measured by Eppley et al. (1972) in control areas averaged about 1.5-3.0/zM whereas, in sewage discharge areas, the
t12
Edward A. Laws, Kenneth L. Terry
concentrations were 3"5-10#M. These PC and PN concentrations are 3-24 times larger than the PC and PN values at Sand Island and Mokapu Point reported here. In the central gyre of the North Pacific, Eppley et al. (1973) and Sharpet al. (1980) have reported PC concentrations ranging from 1.2 #M to 5"6 gM and PN concentrations ranging from 0.17/~M to 0"84 ~M. These concentration values span the range of mean concentrations found at the Sand Island and Mokapu Point stations. Thus, from the standpoint of PC and PN concentrations, the Sand Island and Mokapu Point stations were distinctly oligotrophic. It has often been observed that the C: N: P ratio of particulate material in the ocean is approximately equal to the Redfield ratio of 106:16:1 by atoms (Goldman et al., 1979), although Sharp et al. (1980) have reported a significant correlation between particulate N:C ratios and growth rates in the North Pacific. Given the mean PC and PN values in Tables 2 and 3, we calculate median C: N ratios at Sand Island and Mokapu Point of 8-5 and 8.3 by atoms, respectively, compared with the Redfield ratio of 6.6 by atoms. Based on the correlations noted by Sharp et al. (1980), these C:N ratios would imply growth rates equal to about 75 ~/0of maximum rates. Median N: P ratios at the Sand Island and Mokapu Point stations are 22 and 25 by atoms, respectively--values which are high compared with the range of I0-15 by atoms which Ryther & Dunstan (1971)consider typical of marine phytoplankton, and values which are certainly high compared with the typical N: P ratio in sewage of 7-8 or less by atoms (Table 1). In fact, the particulate N: P ratios are more typical of the range of 23 or higher reported for zooplankton (Corner & Davies, 1971) than of phytoplankton, although Goldman et al. (1979) have noted that the N:P ratio in phytoplankton is often identical to the N:P inorganic nutrient supply ratio over a wide range of growth rates and N:P ratios. Examination of PC:Chl a ratios shows median values of 336 and 233 by weight at the Sand Island and Mokapu Point stations, respectively-values which are certainly high compared with the range of 50-100 expected in marine phytoplankton, unless the populations are growing at a very low nutrient-limited growth rate (Laws & Bannister, 1980). Thus, much of the particulate material sampled by us probably represented zooplankton and detritus. Given this conclusion, the only inference we can make from the particulate N:P ratios is that these ratios have probably not been greatly influenced by the low N: P ratio in the sewage. Photosynthetic rate comparisons are made most easily if values are
Sewage discharge effects on phytoplankton
113
integrated over 24 h and over the depth of the euphotic zone. Laws & Redalje (1979) have found that for samples incubated from approximately 10 a.m. to noon in Kaneohe Bay, multiplication of the hourly rate by 7. I gives a good estimate of 24 h production rates corrected for dark respiration losses and diel periodicity in photosynthetic rates. We applied this factor to our hourly production values in Tables 2 and 3 to estimate daily production. Integration of these values over the depth of the euphotic zone is impossible at Mokapu Point, since the light level at 30 m was about 19 % of surface light. Ifwe assume the euphotic zone to extend to three times the Secchi depth, then the euphotic zone at Sand Island is about 81 m deep. Since our incubations extended to a depth of only 50 m, considerable extrapolation of our results would be required to estimate production to the bottom of the euphotic zone. We have therefore calculated daily production rates over the entire 30 m water column at Mokapu Point and over the upper 50 m of the water column at Sand Island, using the general theory of Gauss Quadrature (Hornbeck, 1975) to choose appropriate weighting factors for the numerical integrations. The results of these calculations are shown in Table 4. For comparative purposes, we have listed primary production rates reported by other investigators in the North Pacific gyre, nearshore areas around the Hawaiian Islands and California coastal areas. From these comparisons, primary production rates at the outfall stations appear to be 2-30 times higher than open ocean values and 3-9 times higher than values reported 10-20 km from the Hawaiian Islands. However, the values reported by Eppley et al. (1972) for unpolluted California coastal waters are 2-5 times higher than the Mokapu Point and Sand Island values, and their values from sewage polluted coastal waters are even higher. It thus seems reasonable to interpret the Sand Island and Mokapu Point production rates as characteristic of a rather oligotrophic coastal water column. The assimilation numbers (maximum productivity indices) calculated at the Sand Island and Mokapu Point stations nevertheless are almost all within, or above, the range of values (5-10) which Curl & Small (1965) consider indicative of eutrophy. However, Curl & Small (1965) were concerned with comparisons of mainly open ocean systems, and there is no doubt that the Sand Island and Mokapu Point stations are more productive than waters further offshore. The low assimilation numbers (0-3) often reported in oligotrophic open ocean waters have usually been interpreted as being indicative of nutrient limitation (Laws & Bannister,
0-50 0-50 0-50 0-30 0-3O 0-30 0-30, 0-50 0-30, 0-60 0-30, 0-50 0-30, 0-50 0 30, 0- 50
Mokapu Point--UC MZ I)(2 North Pacific gyr¢ south of Hawaiian Islands
North Pacific gyre north of Hawaiian Islands Keahol¢ Point, Island of Hawaii, 22 km from shore
Kahe Point, Island of Oahu, 9-3 km from shore
California coastal---control sewage polluted
Depth range (m)
Sand lsland--UC MZ DC
Station
19 32 44- 73
0-9
1.9
1.3 -3-9
This study
Pre-discharge Di~harge 3.4 8.7 4-2 9. I 6.1 3.3 0.3- 1-5
Reference
Estimated from Bienfang (1977) by multiplying 1000- 1640 production values by 1.16 Estimated from Noda & Bienfang (1981) by setting equal to 9-h production, beginning at dawn Eppley et al. (1972)
Estimated from Bienfang & Gundersen (1977) by multiplying 0900-1500 production values by 1.28 Eppley et al. (I 973)
This study
6.3 7.4 7.3
Daily production rate (rag C m - 3 day- l )
TABLE 4 Daily Primary Production Rates at Sand Island, Mokapu Point, and at Other Areas in the North Pacific
r-,
Sewage discharge effects on phytoplankton
115
1980), but, as indicated, we feel the decline in PI's at Mokapu Point reflects a transient response of a nutrient-limited population to nutrient additions and therefore cannot be interpreted straightforwardly from steady-state theory. With respect to water clarity, Jerlov (1948) indicates that typical light extinction coefficients in subtropical oceanic waters are about 0.08 m- 1. Light extinction coefficients estimated from Secchi depths or calculated from light intensity measurements during 1979 at Sand Island and Mokapu Point are in good agreement, the estimated values ranging between 0.05 and 0.07 m-~. Thus, water clarity near the outfalls was as good as, or better than, that in many subtropical ocean areas. Inorganic nitrogen concentrations are in general not particularly sensitive indicators of sewage enrichment, since phytoplankton are capable of rapidly stripping the water of any added nitrogen (McCarthy & Goldman, 1979). Phosphate concentrations, on the other hand, may be sensitive indicators of sewage enrichment, because the N:P ratio in sewage is low relative to the needs of most phytoplankton (Rytber & Dunstan, 1971 ; Smith et al., 1981). However, Eppley et al. (1972) found depth profiles of phosphate, silicate, nitrate and ammonium to be little different at unpolluted and sewage polluted stations off the southern California coast. Their surface water concentrations of MR P, nitrate plus nitrate, NH~ and silicate averaged about 0.25, 0.25, 1.0 and 3"0/~M, respectively. These values are comparable with the nutrient concentrations shown in Tables 2 and 3. In the offshore ocean around Hawaii, typical surface water values of MRP, nitrate plus nitrite, NH~" and silicate are about 0.12, 0.22, 0.15 and 2 #M, respectively (Bienfang, 1981), values which are somewhat lower than the mean concentrations at Sand Island and Mokapu Point. Judging from the findings of Eppley et al. (1972) and considering the fact that neither inorganic N or MRP concentrations increased significantly during discharge conditions at Mokapu Point, we conclude that sewage discharges did not adversely affect water quality with respect to nutrient concentrations. We conclude that, in areas such as Hawaii, where the surrounding waters are naturally unproductive and where the bottom shoals rapidly, ocean outfall sites are greatly preferable to estuarine or freshwater sites, especially if the latter involve secondarily treated sewage. In such unproductive areas the impact of ocean outfall sewage discharges on the phytoplankton community can apparently be reduced to an insignificant level if the outfall is properly designed.
116
Edward A. Laws, Kenneth L. Terry REFERENCES
Armstrong, R. W. (Ed.) (1973). Atlas of Hawaii. University of Hawaii Press, Honolulu, 222 pp. Bathen, K. H. (1978). Circulation atlasJbr Oahu. University of Hawaii Sea Grant Report MR-78-05, Honolulu, 94 pp. Bienfang, P. J. (1981). Phytoplankton dynamics in the oligotrophic waters off Kahe Point, Oahu, Hawaii. Oceanic Institute. Waimanalo, Hawaii, 43 pp. Bienfang, P. J. & Gundersen, K. (1977). Light effects on nutrient-limited, oceanic primary production. Mar. Biol., 43, 187-91. City and County of Honolulu (1971). Water Quality Program for Oahu with Special Emphasis on Waste Disposal--Final Report Work Areas 6 and 7: Analysis of Water Quality Oceanographic Studies, Parts I and II. Dept. of Public Works. Corner, E. D. S. & Davies, A. G. (1971). Plankton as a factor in the nitrogen and phosphorus cycles in the sea. Adv. Mar. Biol., 9, 101-204. Curl, H. & Small, L. F. (1965). Variations in photosynthetic assimilation ratios in natural, marine phytoplankton communities. Limnol. Oceanogr., 10, R67R73. Eppley, R. W., Carlucci, A. F., Holm-Hansen, O., Kiefer, D., McCarthy, J. J. & Williams, P. M. (1972). Evidence for eutrophication in the sea near southern California coastal sewage outfalls--July 1970. Calif. Mar. Res. Comm., Cal. COFI Rept, 16, 74-83. Eppley, R. W., Renger, E. H., Venrick, E. L. & Mullin, M. M. (1973). A study of plankton dynamics and nutrient cycling in the central gyre of the North Pacific Ocean. Limnol. Oceanogr., 18, 534-51. Eppley, R. W., Harrison, W. G., Chisholm, S. W. & Stewart, E. (I977). Particulate organic matter in surface waters off Southern California and its relationship to phytoplankton. J. Mar. Res., 35, 671-95. Goldman, J. C., McCarthy, J. J. & Peavey, D. G. (1979). Growth rate influence on the chemical composition of phytoplankton in oceanic waters. Nature, 279, 210-15. Healey, F. P. (1979). Short-term responses of nutrient-deficient algae to nutrient addition. J. Phycol., 15, 289-99. Holm-Hansen, O. & Riemann, B. (1978). Chlorophyll a determination: Improvements in methodology. Oikos, 30, 438-47. Hornbeck, R. W. (1975). Numerical methods. Quantum Publishers, New York, 310pp. Iverson, R. L., Bittaker, H. F. & Myers, V. B. (1976). Loss of radiocarbon in direct use of Aquasol for liquid scintillation counting of solutions containing ~4C-NaHCO3. Limnol. Oceanogr., 21,756-8. Jerlov, N. G. (1948). Optical studies of ocean waters. Reports of the Swedish Deepsea Expedition. Volume IlL Physics and chemistry, No. 1, pp. 1-59. Laevastu, T., Avery, D. E. & Cox, D. C. (1964). Coastal currents and sewage disposal in the Hawaiian Islands. Hawaii Inst. of Geophysics Tech. Report 64-1, 101 pp. Laws, E. A. & Bannister, T. T. (1980). Nutrient- and light-limited growth of
Sewage discharge effects on phytoplankton
117,
Thalassiosirafluriatilis in continuous culture, with implications for phytoplankton growth in the ocean. Limnol. Oceanogr., 25, 457-73. Laws, E. A. & Redalje, D. (1979). Effects of sewage enrichment on the phytoplankton population of a subtropical estuary. Pac. Sci., 33, 129-44. Lean, D. R. S. & Pick, F. R. (1981). Photosynthetic response of lake plankton to nutrient enrichment: A test for nutrient limitation. Limnol. Oceanogr., 26, 1001-19. Loh, P. C. & Fujioka, R. S. (1980). Viruses in water: Their detection, survival and disease potential. In: Environmental Survey Techniques for Coastal Water Assessment. University of Hawaii Sea Grant Report CR-80-01, Honolulu, pp.41-70. McCarthy, J. J. & Goldman, J. C. (1979). Nitrogenous nutrient of marine phytoplankton in nutrient-depleted waters. Science, 203, 670-2. Murphy, J. & Riley, J. P. (1962). A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta, 27, 31-6. Noda, E. K. & Bienfang, P. K. (1981). OTEC environmental benchmark survey offKeahole Point, Hawaii. Dept. of Ocean Engineering and Hawaii Natural Energy Institute, Univ. of Hawaii, HNEI Tech. Report 81-05, 522 pp. Officer, C. B. & Ryther, J. H. (1977). Secondary sewage treatment versus ocean outfalls: An assessment, Science, 197, 1056-60. Riley, J. P. & Chester, R. (1971). Introduction to marine chemistry. Academic Press, New York, 465 pp. Ryther, J. H. & Dunstan, W. M. (1971). Nitrogen, phosphorus, and eutrophication in the coastal marine environment, Science, 171, 1008-13. Sharp, J. H. (1974). Improved analysis for 'particulate' organic carbon and nitrogen from seawater. Limnol. Oceanogr., 19, 984-9. Sharp, J. H., Perry, M. J., Renger, E. H. & Eppley, R. W. (1980). Phytoplankton rate processes in the oligotrophic waters of the central North Pacific Ocean. J. Plankton Res., 2, 335-53. Smith, S. V., Kimmerer, W. J., Laws, E. A., Brock, R. E. & Walsh, T. W. (1981). Kaneohe Bay sewage diversion experiment: Perspectives on ecosystem responses to nutritional perturbation. Pac. Sci., 35, 279-395. Sokal, R. R. & Rohlf, F. J. (1969). Biometry. W. H. Freeman and Co., San Francisco, 776 pp. Solarzano, L. (1969). Determination of ammonia in natural waters by the phenolhypochlorite method. Limnol. Oceanogr., 14, 799-801. Steams, H. T. & Vaksvik, K. N. (1935). Geology and ground-water resources of Oahu, Hawaii. US Geology Survey, Maui Publ. Co. Ltd, Wailuku, 479 pp. Strickland, J. D. H. & Parsons, T. R. (1972). A practical handbook of seawater analysis (2nd edn). Bulletin of the Fisheries Research Board of Canada No. 167. Sverdrup, H. U., Johnson, M. W. & Fleming, R. H. (1942). The oceans: Their physics, chemistry and general biology. Prentice-Hail, Englewood Cliffs, NJ, 1087 pp. Wood, E. D. F., Armstrong, A. J. & Richards, F. A. (1967). Determination of nitrate in sea water by cadmium-copper reduction in nitrite. J. Mar. BioL Assoc. U.K., 47, 23-31.
The Impact of Sewage Discharges at Ocean Outfalls on Phytoplankton Populations in Waters Surrounding the Hawaiian Islands Edward A. Laws & Kenneth L. Terry University of Hawaii, Oceanography Department, Honolulu, HI 96822, USA (Received: 23 August, 1982)
ABSTRACT Primary production rates, chlorophyll a and nutrient concentrations, particulate carbon, nitrogen and phosphorus concentrations, Secchi depths and submarine light levels were measured at stations within, and near, the zone of mixing of the Sand Island and Mokapu Point ocean sewage outfalls near the island of Oahu in the Hawaiian Islands. Multiyear averages of several parameters indicated that the Sand Island outfall had no adverse impact on phytoplankton communities or water quality. The major effect of the Mokapu Point outfall was a reduction in photosynthetic rates, probably due to the rapid uptake of nutrients introduced into the mixed layer with the secondarily treated sewage. The Sand Island outfall, which discharged coarsely screened raw sewage within the thermocline, had less of an impact because the inorganic nutrient concentrations in the sewage were lower and because the sewage rarely rose into the mixed layer. The results indicate that, under appropriate conditions, ocean outfalls may be preferable to estuarine or freshwater outfalls and that secondary treatment is not always necessary to reduce the impacts of ocean outfalls to an acceptable level.
INTRODUCTION The discharge of sewage that has received primary treatment has been proposed as an alternative to secondary sewage treatment to prevent 101 Marine Environ. Res. 0141-1136/83/0008-0101/$03-00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
Edward A. Laws, Kenneth L. Terry
102
eutrophication problems in coastal areas (Officer & Ryther, 1977). Assuming nitrogen to be the principal nutrient limiting phytoplankton biomass in marine systems (Ryther & Dunstan, 1971), Officer & Ryther (1977) have pointed out that the biochemical oxygen demand (BOD) associated with the mineralisation of the phytoplankton biomass produced from the inorganic N in secondarily treated sewage may be as much as twice the BOD of raw sewage. Since the effective dilution volume and flushing rate for sewage is likely to be much larger in the ocean than in an estuary, it is logical to expect that the environmental impact of the ocean outfall will be much less than that of an estuarine outfall. The impact of the ocean outfall on the phytoplankton community may be further reduced if primarily treated, rather than secondarily treated, sewage is discharged, because the latter typically contains a higher concentration of inorganic N (Officer & Ryther, 1977). In cases where ocean outfalls are not likely to cause oxygen depletion, primary treatment may therefore be preferable to secondary treatment. During 1977 the County of Honolulu, Hawaii, began discharging 3.9 x 10+ m 3 d a y - t of largely domestic secondarily treated sewage at a depth of 30 m at an ocean outfall I km offshore of Mokapu Point on the windward side of the island of Oahu (Fig. 1). In December, 1976 the County of Honolulu began discharging 2.5 x 105 m 3 d a y - 1 of essentially
d
~
u
13<:
~
L.._--~Mz
¢*' ~=, 41,~
.,IrM,okapu Ocea 13
("
MarineCocpsA i r .St~ion
O=fall ~,~o~ ~
I j
Fig. !.
J
Outlall
~
.
.. L ~
",,
/
Mokotea
" .o~
+~ ~
it
o,.,tmu
1
?
Location of Mokapu Point sewage ouffall and samplint stations.
Sewage discharge effects on phytoplankton
103
21"20"
N 16~5~B°
Fig. 2.
Ikm i i 56'
i 54'
W 157"52'
Location of new and old Sand Island sewage outfails and sampling stations.
raw (coarsely screened) and largely domestic sewage at a depth of 73 m at an ocean outfall approximately 3 km offshore of Sand Island on the leeward side of the island of Oahu (Fig. 2). Characteristics of the sewage discharged at these two outfalls are given in Table 1. Prior to the use of these two outfalls, about 60 % and 40 % of the Mokapu Point sewage had been discharged into Kaneohe Bay and Kailua Bay, respectively (Fig. 1), and the Sand Island sewage had been discharged at a depth of approximately 15 m about 1 km offshore (Fig. 2). The Kaneohe Bay discharge had created serious water quality problems in Kaneohe Bay (Laws & Redalje, 1979; Smith et al., 1981) and pathogens had repeatedly been isolated in water samples taken near the old Sand Island outfall (Loh & Fujioka, 1980). The purpose of our study was to determine the impact of the two new sewage outfalls on water quality and the phytoplankton communities in the vicinity of the outfalls. In the case of the Mokapu TABLE 1 Characteristics of Sewage Discharges at Sand Island and Mokapu Outfali Sites
Flow rate (m 3 day -t) Total N (mg litre-t) Total P (mg litre- t)
Sand Island
Mokapu Point
2.5 × l0 s 18-29 5-4-5-8
3-9 x 104 22 5.8
Edward A. Laws, Kenneth L. Terry
104
Point outfall we were able to compare pre-discharge and discharge conditions, because the old outfalls were far ( > 4 kin) from the new outfall site. This comparison was impossible in the case of the new Sand Island outfall because of the proximity of the old outfall, but the Sand Island raw sewage discharge results do provide a revealing comparison with the Mokapu Point secondarily treated sewage discharge study.
MATERIALS AND METHODS Sampling sites at Sand Island and Mokapu Point are indicated in Figs 1 and 2. The annual average current directions at Mokapu Point are in a northerly and westward direction around the point, and at Sand Island in a westward and southerly direction (Laevastu et al., 1964; Armstrong, 1973). On the basis of these mean current directions, stations were established in an upcurrent (UC) and downcurrent (DC) direction from the zone of mixing station (MZ). However, current directions near the outfall sites may vary seasonally and tend to reverse direction on the ebb and flood tides (Laevastu et al., 1964; Bathen, 1978). Thus, the UC and DC stations were distinguished from the MZ station primarily by their distance and only secondarily by their direction from the zone of mixing. The stations were chosen to lie along the isobaths of the outfall diffusers, and were separated by distances of 1.25 km at Mokapu Point and 2.5 km at Sand Island. Monthly sampling was done for a 3-year period (December, 1976-November, 1979) at Sand Island and during two separate 1-year periods (February, 1976-January, 1977 and January, 1979-December, 1979) at Mokapu Point. Water samples were taken from depths of 5, 25 and 50m at Sand Island and 5, 15 and 25 m at Mokapu Point using an ordinary garden hose connected to a bilge pump. Preliminary studies on surface water samples collected in a plastic bucket and subsequently drawn through the hose and bilge pump system revealed no significant effect of the hose-pump system on either chlorophyll a (Chl a) concentrations or primary production rates (t-test, p > 0.10). The samples were prescreened through a 102#m mesh nylon gauze to remove the larger zooplankton.
Primary production Primary production rates were measured using the 14C method
Sewage discharge effects on phytoplankton
105
(Strickland & Parsons, 1972). Sixty-millilitre light and dark glass bottles were filled with sample water and then stored in the dark at ambient temperature prior to inoculation with t4C-bicarbonate. Maximum storage time was I h. The bottles were incubated in Honolulu Harbor in the case of the Sand Island samples or in Kaneohe Bay in the case of the Mokapu Point samples at water depths corresponding to the light levels ( ~ of surface light) from which the water samples were taken. Light levels were estimated from Secchi depths or from submarine light measurements made using a LI-COR model 185-A submarine quantum sensor. The samples were incubated from approximately 10a.m. to noon and then filtered under low vacuum (<0-35atm) onto Sartorius 0.45tan membrane filters. The filters were rinsed with 10 ml of filtered seawater, placed in liquid scintillation vials containing 0.5 ml of 1N HCI to drive off any t4CO2, and dried at room temperature. The activity on the filters was then determined using a Tracor Delta 300 liquid scintillation counter with Aquasol as a fluor. The activity of 14C spikes was determined following the recommendations of Iverson et al. (1976).
Chlorophyll a Chlorophyll a concentrations were determined fluorometrically (Strickland & Parsons, 1972), but using the acidification procedure for phaeopigment corrections recommended by Holm-Hansen & Riemann (1978). About 350 ml of sample water were filtered onto glass fibre GF/C filters and placed in 100~ acetone for the extraction procedure. The fluorometer was calibrated using pigment extracts from a mixed population of phytoplankton, as recommended by Strickland & Parsons (1972). The pigment concentrations in the mixed population were determined on a Beckman Acta II spectrophotometer, and the ratio of the Chl a concentrations to the fluorometer (Turner model III) reading used to determine the fluorometric calibration constant.
Particulate carbon, nitrogen and phosphorus Samples for particulate carbon (PC), particulate nitrogen (PN) and particulate phosphorus (PP) were collected only during 1979. The samples were collected in 2 litre dark polypropylene bottles, stored at ambient temperature and filtered within 4 h. For PC and PN analyses, 2 litres of sample were filtered onto precombusted GF/C filters. The loaded
106
Edward A. Laws, Kenneth L. Terry
filters were stored frozen prior to analysis on a Hewlett-Packard model 185B CHN analyser. Procedures recommended by Sharp (1974) were followed in analysing the samples. For PP analyses, 2 litres of sample water were again filtered onto a precombusted GF/C filter. The organic material on the filter was oxidised first by heating at 100°C for I h in concentrated H2SO 4, and then heating at 100°C for 30min in dilute H2SO 4. The acid solution was then neutralised with NaOH and NaHCO 3 and analysed for molybdate reactive phosphorus (MRP) as indicated below. Filter blanks were subjected to identical acid treatment and the results used to correct sample values for MRP contamination.
Nutrients All nutrient analyses were performed on a Technicon AutoAnalyzer. Filtrates from either the Chl a or PP analyses were used for the dissolved nutrient determinations. The samples were stored frozen prior to analysis. MRP, nitrate plus nitrite, ammonium and silicate were measured following the procedures of Murphy & Riley (1962), Wood et al. (1967), Solorzano (1969) and Strickland & Parsons (1972), respectively.
Light intensity and extinction coefficients Submarine light intensities were measured only during 1979. A LI-COR model 185A submarine quantum sensor (400-700 nm radiation) attached to a 30 m conducting cable was used to make the light measurements. Light extinction coefficients were calculated from the slope of a plot of In (I) versus depth, where I is the measured light intensity. Secchi depth measurements were made from the sun side of the boat (a Boston Whaler) using a 30-cm diameter white disk. Extinction coefficients (K) were calculated from Secchi depths (D) using the relationship K = 1.7/D (Sverdrup et al., 1942).
RESULTS Data from the Sand Island and Mokapu Point stations are listed in Tables 2 and 3. Because sampling extended over an integral number of years, these values are not biased by seasonal cycles. In order to test for differences between stations and for pre-discharge versus discharge
Particulate carbon (juu) Particulate nitrogen ~M) Particulatephosphorus (riM) Chl a (mgm =s) Pbotmynthesis (rag C m - 3 h - i) Productivity Index (gC g- t Chl a h - l) Molybdate reactive P NO~ + NO~ (pM) Nh~ (jtN) Silicate (/JM) Temperature (°C) Light ( ~ Surface) Secchi (m)
4.9 + I.I 0"51±0"10 35 ± 35 0.18±0.06 0.91 ±0.32 8.7 + 4.3 0.33±0.19 0.17±0-14 0.38+0.15 5.0 ± 2-4 25.1 +0-4 17 _+5 27 ± 4
10.0 + 4.7
0.29+0.11 0.22+0.15 0-48-1-0.17 8.1 ± 5.2 25.2±0-5 45 ± 9
~m
Upcurrent
0.38±0.24 0.18±0-11 0.53+0.21 5.5 ± 3-4 24-9±0.4
4.3 ± 2.4 0.49+0.49 0.21±0.13 0.67±0-56 6.1 ± 3.7 25.2±0.5 44 ± 7
8.8 _+3.5
4-0 :t: 1.0 0.43±0-11 23 ± 3 0.15±0.04 1.15+0.47
5m
0.59±0.53
0-18-1-0.08 0.70±0-40 5.5 ± 2.4 24.9±9.4
0.16±0.09 0.61±0-27 7.0 ± 4.9 25-0+0-4 18 ± 4 27 ± 5
4-7 + 2-0
5.6 ± 3.5 0.70_+0"45 28 + 9 0.20±0.05 0.64±0.19
~m
3.9±1.1 0.~ ±0.II
14±9 0'16±0-~ 1'22±0"55 9-8±4.9 0"28 + 0"11 0"12+0-08 0"50:120"21 5"9 +_3.7 25" I ± 0-4 16±4
3.9±1-3 0.~ ±0.II
20±15 0'13±0-03 0"87±0-31 9'8±5"3 0"63 + 0"68 0.16+0-09 0"48+0"22 6"7 + 4"6 25"2 ± 0"4 41±5 27±5
Downcurrent
1976-Novem~r,
5m
D~ember,
0.50±0.46
8.8 + 4-1
4.6 ± 1.4 0"53+0"13 59 + 59 0.16±0-04 1.13±0-48
25m
50m
Sandlsland,
Mixing zone
TABLE2 MonthlySamplesfrom
6.3 ± 1.5 0.74±0.22 24 ± 7 0.21 ± 0-07 0.49±0-20
50m
ConfidenceLimits)~r
4.7 ± 1.0 0-50+0-10 13 5:12 0-15±0.03 !.12±0-46
5m
MeansofVariables(±95%
0-54 + 0"49 0"20:1:0"11 0.62+0-40 8'9 ± 5-5 25-0 ± 0'4
5.2±2.4
29±26 0-19±0-~ 0"~±0"19
4.8±I-7 0.57±0.17
JOm
1979
Sccchi (m)
Light ( ~ Surface)
Temperature (°C)
Silicate (~M)
NH~ (,uM)
NO_; + N O z (aM)
Molybdate reactive P
(gCg-IChlah -l)
Photosynthesis (rag C m-~ h -j ) Productivity Index
Chi a (rag m -3)
Particulate phosphorus (riM)
Particulate nitrogen (t~M)
Particulate carbon (t~M)
46+-6
18 0.11+-0-03 0-20±0.16 1.5 ± 0,5 0-6±0-3** 17±7 6±5** 0.15±0.04 0.14±0.03 0.31 4-0.16 0.20±0.22 0-42±0-20 0.71±0.46 3"3 ± 1"5 12±10 24.4+0.5 25.4+-0-9
15_.+
0.38_+0.06
3.2±0-7
5m
29+9 24 __.3 24+_3
5:t: 10 0.12±0-03 0.16±0.07 1.3 ± 0.4 0.4±0.1"* 13±4 2"5±0'5"* 0.15±0-1)4 0-11±0-02 0.59+0.48 0.20±0-25** 0-44±0.37 0-86±0.98 3"0 ± 1'2 13+- 13 24.3±0.4 25.3+-0.9
0.40-t-0.09
3-3+--I.I
15m
Upcurrent
17 +- 7
17+ 12 0-12±0-04 0.16±0-08 0.95:0.4 0.4±0.2** 8±3 4±2** 0-18±0.03 0.12+0.03"* 0-29±0-13 0.14+0-13 0.44+-0-17 0.41±0.10 2-0 ± 0-9 7_+6* 24-2±0.4 25.2+0.9
0.45+0.08
4.0+_.1.1
25m
26+ 14 0.10±0-03 0.17±0-08 1.4 ± 0.3 0.6±0.3** 145:2 5_+2** 0.14 ± 0.04 0.12+_.0.03 0-58+0-48 0.21±0.19" 0.44±0.30 0.60+0-45 3"3 ± 1'6 10±g* 24.3±0-5 25.4±0.9 . . . . 42+-8
0.44+0.06
3'3+0-7
5m
14+ II 0.12±0.03 0.16±0.08 I. I ± 0.4 0-7 ± 0.3" 12±5 6±5** 0-15 ±0-03 0.14±0.02 0.35±0.24 0-18±0.18" 0.33±0.18 0-70±0.59 3"0 ± I' 1 13+_7" 24.2+-0-4 25-2 +- 0.9 . . . . . 25 +- 3 23 5:3 24+-3
0.44+0.08
3"6__+0"6
15m
Mixing zone
16+ II 0.12_+0-04 0.225:0-09 I. 3 ± 1.6 0-5_+0.2 10±8 3±3** 0.16+-0.03 0.14±0.03 0.46+0,32 0.08±0.04** 0-36±0.29 0-56±0.24 2"9 ± 0"9 8+-7 24.2+-0.4 25.2+_0.8 . . . . 14+- 5
0"47520"06
4"1+--0,6
25m
22+20 0-10 ± 0.03 0.25+0.12" 1.2 +- 0.3 0.5 + 0.2"* 12+-2 3-t-I** 0.14+-0-04 0.13±0.01 0-70±0.81 0.12+-0-06 0.44±0-32 0.47+_0-23 3"I +_ I-4 5.3+- 3'5 24.3+-0.5 25.3+_0.9 . . . . 40+- 7
0.39+0-04
3'3 +- if4
5m
36+ 55 0-09_+0.03 0.18+-0.007" 0-9 _+0-3 0.5+-0.2** 10+_2 3+1"* 0.15+-0-04 0.16+-0.04 0.95+-0-72 0-33+0.30" 0.44+-0.18 0-49_+0.15 2"9 +- I-2 14+- 11" 24.2+-0-4 25.2+_0.8 . . . . . 22 + 5 23 +- 3 24_+3
0-46+-0.08
_~.7~ 0 . 9
15m
Downeurrent
_
_
~4+ 2
38___54 0.11+0.04 0.24+0.11 O.5 + O.I 0.4+_03 4+- I 3+-3* 0-14_+0.04 0.13+-0-02 0.41 +0.32 0.13_+0.11"* 0.45 +0.28 0-50+_0-14 2-8 ___0.9 II _+8* 24.2+-0.4 25-1 +_0.8
_
0-54+_0.14
42+1.0
25m
TABLE 3 M e a n s o f V a r i a b l e s ( + 9 5 j'~,, C o n f i d e n c e L i m i t s ) for M o n t h l y S a m p l i n g at t h e M o k a p u P o i n t S t a t i o n s , D e c e m b e r , 1976 N o v e m b e r , 1 9 7 9 U p p e r a n d L o w e r N u m b e r s a r e P r e - d i s c h a r g e a n d D i s c h a r g e V a l u e s . Single a n d D o u b l e A s t e r i s k s I n d i c a t e D i f f e r e n c e s S i g n i f i c a n t at 95 ?,', a n d 99 % C o n f i d e n c e Levels, R e s p e c t i v e l y
Sewage discharge effects on phytoplankton
109
differences at Mokapu Point, a non-parametric Kruskal Wallis test was performed (Sokal & Rohlf, 1969). The test revealed no significant interstation differences at Sand Island (p > 0.15). At Mokapu Point, there was a significant inter-station difference in productivity indices (PI's) at 25 m during pre-discharge conditions (p < 0.05) and a significant inter-station difference in MRP concentrations at 15 m (p < 0-04) during discharge conditions. Otherwise there were no significant (p > 0.05) inter-station differences for any parameter during either pre-discharge or discharge conditions at Mokapu Point. However, a large number of significant differences were detected in comparing pre-discharge and discharge conditions at Mokapu Point. PI's were significantly different and lower at all stations and depths during discharge conditions. This change in PI's was due in part to a reduction in photosynthetic rates, which were significantly different and lower at seven of the nine station-depths during discharge conditions. Chl a concentrations were consistently higher during discharge conditions, but the differences were statistically significant only at 5 m and 15 m at the DC station. Of the nutrients studied, silicate was consistently higher and nitrate plus nitrite was consistently lower during discharge conditions, the difference being significant at five of nine station-depths for silicate and six of nine station-depths for nitrate plus nitrite. There was no significant change in NH~ concentrations due to sewage discharges, and MRP concentrations changed significantly only at the UC station 25 m depth.
DISCUSSION The fact that almost no significant inter-station differences were found during discharge conditions indicates that (a) the effects of the sewage discharges were so small as to be statistically undeteetable in the zone of mixing or (b) the effects of the sewage discharges were evenly distributed at the UC, MZ and DC stations. At Sand Island the ouffall diffuser is located at a depth within the thermocline during much of the year, and submergence of the effluent field was expected about 90 % of the time (C. L. Lau, pers. comm.). We suspect, therefore, that the uniformity in mean parameter values at all three Sand Island stations reflects a lack of any significant impact on the system from the sewage ouffall rather than a significant but uniform perturbation. The one piece of information which argues against such an interpretation is the fact that the Sand Island MRP
110
Edward A. Laws, Kenneth L. Terry
concentrations are uniformly high (0-3-0.6~M) compared with those measured at Mokapu Point. However, we suspect that the high MRP concentrations at our Sand Island stations reflect the proximity of Keehi Lagoon and Honolulu Harbor rather than the sewer outfall. Streams draining into Keehi Lagoon and Honolulu Harbor contain total P concentrations as high as 10/~M and total P concentrations in Keehi Lagoon are 3-4 times higher than those measured at nearshore stations to the east and west of Honolulu (City and County of Honolulu, 1971). Tidal flushing injects these nutrients directly into the nearshore mixed layer, a process which is undoubtedly much more efficient than the slow diffusion of sewage-introduced nutrients through the thermocline. At Mokapu Point, on the other hand, the shallowness of the outfall diffuser resulted in the discharge occurring within the mixed layer throughout the year. The uniformity of the post-discharge parameter values, as well as the significant changes in PI's, productivity, silicate and nitrate plus nitrite-concentrations, must therefore be interpreted as suggesting that the time-averaged effects of the sewage discharge on the water column were distributed rather uniformly over a wide area. This conclusion is consistent with the known variability in current patterns and turbulence of the water column near the Mokapu Point outfall site. The reduction in photosynthetic rates during sewage discharges at Mokapu Point seems at first surprising if one assumes that the populations are naturally nutrient limited. The effect could be due to the presence of toxins in the sewage, but this explanation seems unlikely, since the sewage is largely of domestic origin. A more likely explanation follows from the recent work of Healey (1979) and Lean & Pick (1981). These authors have observed that photosynthetic rates may be markedly suppressed when nutrient-limited populations are suddenly allowed to rapidly assimilate nutrients. The effect on photosynthetic rates is transient and is caused by the fact that energy is diverted from carbon assimilation to nutrient assimilation until ~ the populations become nutrient saturated. The fact that water column NH4 "~ and MRP concentrations were not significantly elevated over pre-discharge values, and that NO 3 plus NO 2 concentrations were actually reduced, suggests that the phytoplankton stripped virtually all of the added NH~- from the water, and then took up additional nitrate and nitrite to compensate for the low N: P ratio in the sewage. The low nutrient concentrations during discharge conditions (Table 3) are by no means inconsistent with a suppression of photosynthesis by rapid nutrient uptake, since the
Sewage discharge effects on phytoplankton
111
observed suppression has been found to persist for at least several hours after stimulated uptake is complete (Healey, 1979). A current speed of only about I km h-1 would be adequate to transport water from the outfall site to either the MZ or DC station in only a little more than 2 h, and, based on information given by Bathen (1978), current speeds in the outfall area frequently exceed 2-4 km h- 1. Thus, the time scales involved are consistent with our explanation for the suppression of photosynthetic rates during discharge conditions. The fact that silicate concentrations were higher during discharge conditions is probably due to the very high concentrations of silicate (> 500/aM) in freshwater on Oahu (Stearns & Vaksvik, 1935), and the fact that relatively little of that silicate is required to support the additional biomass produced from the N and P in the sewage. Although we conclude that sewage discharges did significantly affect the phytoplankton community and certain water quality parameters at Mokapu Point, none of the means of variables measured at either the Mokapu Point or Sand Island stations is objectionable from a water quality standpoint. A comparison with other oceanographic regions is revealing. Chl a concentrations of about 10-40mgm -3 are typical of phytoplankton blooms in fertile coastal waters (Riley & Chester, 1971), whereas Chl a concentrations of 0-05 mgm-a are commonly found in barren tropical seas (Bienfang & Gundersen, 1977). Average Chl a values in temperate seas are about 0.5 mg m-3 (Riley & Chester, 1971). Eppley et al. (1977) reported surface water Chl a values in near-shore surface waters off Southern California as high as 9 mg m - 3, with values 100 km offshore approaching 0.2mgm -3. Eppley et al. (1973) and Sharp et al. (1980) reported surface water Chl a concentrations in the central gyre of the North Pacific to be 0.05-0.15 mg m- a. Based on these comparisons, the Chl a concentrations at the Sand Island and Mokapu Point stations would make them intermediate between barren tropical seas and temperate seas. Examination of PC, PN and PP concentrations provides another means of comparing the water quality near the sewage outfalls with other oceanographic areas. Eppley et al. (1972) reported PC concentrations in unpolluted coastal waters off southern California of about 17/aMwhereas, in areas affected by sewage discharges, the PC concentrations were 30-80 #M. PN concentrations measured by Eppley et al. (1972) in control areas averaged about 1.5-3.0/zM whereas, in sewage discharge areas, the
t12
Edward A. Laws, Kenneth L. Terry
concentrations were 3"5-10#M. These PC and PN concentrations are 3-24 times larger than the PC and PN values at Sand Island and Mokapu Point reported here. In the central gyre of the North Pacific, Eppley et al. (1973) and Sharpet al. (1980) have reported PC concentrations ranging from 1.2 #M to 5"6 gM and PN concentrations ranging from 0.17/~M to 0"84 ~M. These concentration values span the range of mean concentrations found at the Sand Island and Mokapu Point stations. Thus, from the standpoint of PC and PN concentrations, the Sand Island and Mokapu Point stations were distinctly oligotrophic. It has often been observed that the C: N: P ratio of particulate material in the ocean is approximately equal to the Redfield ratio of 106:16:1 by atoms (Goldman et al., 1979), although Sharp et al. (1980) have reported a significant correlation between particulate N:C ratios and growth rates in the North Pacific. Given the mean PC and PN values in Tables 2 and 3, we calculate median C: N ratios at Sand Island and Mokapu Point of 8-5 and 8.3 by atoms, respectively, compared with the Redfield ratio of 6.6 by atoms. Based on the correlations noted by Sharp et al. (1980), these C:N ratios would imply growth rates equal to about 75 ~/0of maximum rates. Median N: P ratios at the Sand Island and Mokapu Point stations are 22 and 25 by atoms, respectively--values which are high compared with the range of I0-15 by atoms which Ryther & Dunstan (1971)consider typical of marine phytoplankton, and values which are certainly high compared with the typical N: P ratio in sewage of 7-8 or less by atoms (Table 1). In fact, the particulate N: P ratios are more typical of the range of 23 or higher reported for zooplankton (Corner & Davies, 1971) than of phytoplankton, although Goldman et al. (1979) have noted that the N:P ratio in phytoplankton is often identical to the N:P inorganic nutrient supply ratio over a wide range of growth rates and N:P ratios. Examination of PC:Chl a ratios shows median values of 336 and 233 by weight at the Sand Island and Mokapu Point stations, respectively-values which are certainly high compared with the range of 50-100 expected in marine phytoplankton, unless the populations are growing at a very low nutrient-limited growth rate (Laws & Bannister, 1980). Thus, much of the particulate material sampled by us probably represented zooplankton and detritus. Given this conclusion, the only inference we can make from the particulate N:P ratios is that these ratios have probably not been greatly influenced by the low N: P ratio in the sewage. Photosynthetic rate comparisons are made most easily if values are
Sewage discharge effects on phytoplankton
113
integrated over 24 h and over the depth of the euphotic zone. Laws & Redalje (1979) have found that for samples incubated from approximately 10 a.m. to noon in Kaneohe Bay, multiplication of the hourly rate by 7. I gives a good estimate of 24 h production rates corrected for dark respiration losses and diel periodicity in photosynthetic rates. We applied this factor to our hourly production values in Tables 2 and 3 to estimate daily production. Integration of these values over the depth of the euphotic zone is impossible at Mokapu Point, since the light level at 30 m was about 19 % of surface light. Ifwe assume the euphotic zone to extend to three times the Secchi depth, then the euphotic zone at Sand Island is about 81 m deep. Since our incubations extended to a depth of only 50 m, considerable extrapolation of our results would be required to estimate production to the bottom of the euphotic zone. We have therefore calculated daily production rates over the entire 30 m water column at Mokapu Point and over the upper 50 m of the water column at Sand Island, using the general theory of Gauss Quadrature (Hornbeck, 1975) to choose appropriate weighting factors for the numerical integrations. The results of these calculations are shown in Table 4. For comparative purposes, we have listed primary production rates reported by other investigators in the North Pacific gyre, nearshore areas around the Hawaiian Islands and California coastal areas. From these comparisons, primary production rates at the outfall stations appear to be 2-30 times higher than open ocean values and 3-9 times higher than values reported 10-20 km from the Hawaiian Islands. However, the values reported by Eppley et al. (1972) for unpolluted California coastal waters are 2-5 times higher than the Mokapu Point and Sand Island values, and their values from sewage polluted coastal waters are even higher. It thus seems reasonable to interpret the Sand Island and Mokapu Point production rates as characteristic of a rather oligotrophic coastal water column. The assimilation numbers (maximum productivity indices) calculated at the Sand Island and Mokapu Point stations nevertheless are almost all within, or above, the range of values (5-10) which Curl & Small (1965) consider indicative of eutrophy. However, Curl & Small (1965) were concerned with comparisons of mainly open ocean systems, and there is no doubt that the Sand Island and Mokapu Point stations are more productive than waters further offshore. The low assimilation numbers (0-3) often reported in oligotrophic open ocean waters have usually been interpreted as being indicative of nutrient limitation (Laws & Bannister,
0-50 0-50 0-50 0-30 0-3O 0-30 0-30, 0-50 0-30, 0-60 0-30, 0-50 0-30, 0-50 0 30, 0- 50
Mokapu Point--UC MZ I)(2 North Pacific gyr¢ south of Hawaiian Islands
North Pacific gyre north of Hawaiian Islands Keahol¢ Point, Island of Hawaii, 22 km from shore
Kahe Point, Island of Oahu, 9-3 km from shore
California coastal---control sewage polluted
Depth range (m)
Sand lsland--UC MZ DC
Station
19 32 44- 73
0-9
1.9
1.3 -3-9
This study
Pre-discharge Di~harge 3.4 8.7 4-2 9. I 6.1 3.3 0.3- 1-5
Reference
Estimated from Bienfang (1977) by multiplying 1000- 1640 production values by 1.16 Estimated from Noda & Bienfang (1981) by setting equal to 9-h production, beginning at dawn Eppley et al. (1972)
Estimated from Bienfang & Gundersen (1977) by multiplying 0900-1500 production values by 1.28 Eppley et al. (I 973)
This study
6.3 7.4 7.3
Daily production rate (rag C m - 3 day- l )
TABLE 4 Daily Primary Production Rates at Sand Island, Mokapu Point, and at Other Areas in the North Pacific
r-,
Sewage discharge effects on phytoplankton
115
1980), but, as indicated, we feel the decline in PI's at Mokapu Point reflects a transient response of a nutrient-limited population to nutrient additions and therefore cannot be interpreted straightforwardly from steady-state theory. With respect to water clarity, Jerlov (1948) indicates that typical light extinction coefficients in subtropical oceanic waters are about 0.08 m- 1. Light extinction coefficients estimated from Secchi depths or calculated from light intensity measurements during 1979 at Sand Island and Mokapu Point are in good agreement, the estimated values ranging between 0.05 and 0.07 m-~. Thus, water clarity near the outfalls was as good as, or better than, that in many subtropical ocean areas. Inorganic nitrogen concentrations are in general not particularly sensitive indicators of sewage enrichment, since phytoplankton are capable of rapidly stripping the water of any added nitrogen (McCarthy & Goldman, 1979). Phosphate concentrations, on the other hand, may be sensitive indicators of sewage enrichment, because the N:P ratio in sewage is low relative to the needs of most phytoplankton (Rytber & Dunstan, 1971 ; Smith et al., 1981). However, Eppley et al. (1972) found depth profiles of phosphate, silicate, nitrate and ammonium to be little different at unpolluted and sewage polluted stations off the southern California coast. Their surface water concentrations of MR P, nitrate plus nitrate, NH~ and silicate averaged about 0.25, 0.25, 1.0 and 3"0/~M, respectively. These values are comparable with the nutrient concentrations shown in Tables 2 and 3. In the offshore ocean around Hawaii, typical surface water values of MRP, nitrate plus nitrite, NH~" and silicate are about 0.12, 0.22, 0.15 and 2 #M, respectively (Bienfang, 1981), values which are somewhat lower than the mean concentrations at Sand Island and Mokapu Point. Judging from the findings of Eppley et al. (1972) and considering the fact that neither inorganic N or MRP concentrations increased significantly during discharge conditions at Mokapu Point, we conclude that sewage discharges did not adversely affect water quality with respect to nutrient concentrations. We conclude that, in areas such as Hawaii, where the surrounding waters are naturally unproductive and where the bottom shoals rapidly, ocean outfall sites are greatly preferable to estuarine or freshwater sites, especially if the latter involve secondarily treated sewage. In such unproductive areas the impact of ocean outfall sewage discharges on the phytoplankton community can apparently be reduced to an insignificant level if the outfall is properly designed.
116
Edward A. Laws, Kenneth L. Terry REFERENCES
Armstrong, R. W. (Ed.) (1973). Atlas of Hawaii. University of Hawaii Press, Honolulu, 222 pp. Bathen, K. H. (1978). Circulation atlasJbr Oahu. University of Hawaii Sea Grant Report MR-78-05, Honolulu, 94 pp. Bienfang, P. J. (1981). Phytoplankton dynamics in the oligotrophic waters off Kahe Point, Oahu, Hawaii. Oceanic Institute. Waimanalo, Hawaii, 43 pp. Bienfang, P. J. & Gundersen, K. (1977). Light effects on nutrient-limited, oceanic primary production. Mar. Biol., 43, 187-91. City and County of Honolulu (1971). Water Quality Program for Oahu with Special Emphasis on Waste Disposal--Final Report Work Areas 6 and 7: Analysis of Water Quality Oceanographic Studies, Parts I and II. Dept. of Public Works. Corner, E. D. S. & Davies, A. G. (1971). Plankton as a factor in the nitrogen and phosphorus cycles in the sea. Adv. Mar. Biol., 9, 101-204. Curl, H. & Small, L. F. (1965). Variations in photosynthetic assimilation ratios in natural, marine phytoplankton communities. Limnol. Oceanogr., 10, R67R73. Eppley, R. W., Carlucci, A. F., Holm-Hansen, O., Kiefer, D., McCarthy, J. J. & Williams, P. M. (1972). Evidence for eutrophication in the sea near southern California coastal sewage outfalls--July 1970. Calif. Mar. Res. Comm., Cal. COFI Rept, 16, 74-83. Eppley, R. W., Renger, E. H., Venrick, E. L. & Mullin, M. M. (1973). A study of plankton dynamics and nutrient cycling in the central gyre of the North Pacific Ocean. Limnol. Oceanogr., 18, 534-51. Eppley, R. W., Harrison, W. G., Chisholm, S. W. & Stewart, E. (I977). Particulate organic matter in surface waters off Southern California and its relationship to phytoplankton. J. Mar. Res., 35, 671-95. Goldman, J. C., McCarthy, J. J. & Peavey, D. G. (1979). Growth rate influence on the chemical composition of phytoplankton in oceanic waters. Nature, 279, 210-15. Healey, F. P. (1979). Short-term responses of nutrient-deficient algae to nutrient addition. J. Phycol., 15, 289-99. Holm-Hansen, O. & Riemann, B. (1978). Chlorophyll a determination: Improvements in methodology. Oikos, 30, 438-47. Hornbeck, R. W. (1975). Numerical methods. Quantum Publishers, New York, 310pp. Iverson, R. L., Bittaker, H. F. & Myers, V. B. (1976). Loss of radiocarbon in direct use of Aquasol for liquid scintillation counting of solutions containing ~4C-NaHCO3. Limnol. Oceanogr., 21,756-8. Jerlov, N. G. (1948). Optical studies of ocean waters. Reports of the Swedish Deepsea Expedition. Volume IlL Physics and chemistry, No. 1, pp. 1-59. Laevastu, T., Avery, D. E. & Cox, D. C. (1964). Coastal currents and sewage disposal in the Hawaiian Islands. Hawaii Inst. of Geophysics Tech. Report 64-1, 101 pp. Laws, E. A. & Bannister, T. T. (1980). Nutrient- and light-limited growth of
Sewage discharge effects on phytoplankton
117,
Thalassiosirafluriatilis in continuous culture, with implications for phytoplankton growth in the ocean. Limnol. Oceanogr., 25, 457-73. Laws, E. A. & Redalje, D. (1979). Effects of sewage enrichment on the phytoplankton population of a subtropical estuary. Pac. Sci., 33, 129-44. Lean, D. R. S. & Pick, F. R. (1981). Photosynthetic response of lake plankton to nutrient enrichment: A test for nutrient limitation. Limnol. Oceanogr., 26, 1001-19. Loh, P. C. & Fujioka, R. S. (1980). Viruses in water: Their detection, survival and disease potential. In: Environmental Survey Techniques for Coastal Water Assessment. University of Hawaii Sea Grant Report CR-80-01, Honolulu, pp.41-70. McCarthy, J. J. & Goldman, J. C. (1979). Nitrogenous nutrient of marine phytoplankton in nutrient-depleted waters. Science, 203, 670-2. Murphy, J. & Riley, J. P. (1962). A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta, 27, 31-6. Noda, E. K. & Bienfang, P. K. (1981). OTEC environmental benchmark survey offKeahole Point, Hawaii. Dept. of Ocean Engineering and Hawaii Natural Energy Institute, Univ. of Hawaii, HNEI Tech. Report 81-05, 522 pp. Officer, C. B. & Ryther, J. H. (1977). Secondary sewage treatment versus ocean outfalls: An assessment, Science, 197, 1056-60. Riley, J. P. & Chester, R. (1971). Introduction to marine chemistry. Academic Press, New York, 465 pp. Ryther, J. H. & Dunstan, W. M. (1971). Nitrogen, phosphorus, and eutrophication in the coastal marine environment, Science, 171, 1008-13. Sharp, J. H. (1974). Improved analysis for 'particulate' organic carbon and nitrogen from seawater. Limnol. Oceanogr., 19, 984-9. Sharp, J. H., Perry, M. J., Renger, E. H. & Eppley, R. W. (1980). Phytoplankton rate processes in the oligotrophic waters of the central North Pacific Ocean. J. Plankton Res., 2, 335-53. Smith, S. V., Kimmerer, W. J., Laws, E. A., Brock, R. E. & Walsh, T. W. (1981). Kaneohe Bay sewage diversion experiment: Perspectives on ecosystem responses to nutritional perturbation. Pac. Sci., 35, 279-395. Sokal, R. R. & Rohlf, F. J. (1969). Biometry. W. H. Freeman and Co., San Francisco, 776 pp. Solarzano, L. (1969). Determination of ammonia in natural waters by the phenolhypochlorite method. Limnol. Oceanogr., 14, 799-801. Steams, H. T. & Vaksvik, K. N. (1935). Geology and ground-water resources of Oahu, Hawaii. US Geology Survey, Maui Publ. Co. Ltd, Wailuku, 479 pp. Strickland, J. D. H. & Parsons, T. R. (1972). A practical handbook of seawater analysis (2nd edn). Bulletin of the Fisheries Research Board of Canada No. 167. Sverdrup, H. U., Johnson, M. W. & Fleming, R. H. (1942). The oceans: Their physics, chemistry and general biology. Prentice-Hail, Englewood Cliffs, NJ, 1087 pp. Wood, E. D. F., Armstrong, A. J. & Richards, F. A. (1967). Determination of nitrate in sea water by cadmium-copper reduction in nitrite. J. Mar. BioL Assoc. U.K., 47, 23-31.