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Enrichment of estuarine phytoplankton by the addition of dissolved manganese
ENRICHMENT OF ESTUARINE PHYTOPLANKTON BY THE ADDITION OF DISSOLVED MANGANESE
JAMESG. SANDERS'~"
Marine Science Program, Unirersity of North Carolina, Chapel Hill, North Carolina 27514, USA
ABSTRACT
The response of natural phytoplankton to additions of excess Mn in an estuary receiving sewage effluent varied with tidal amplitude. During periods of low tidal amplitude, when DOC concentrations were high, carbon uptake by phytoplankton was stimulated. When tidal amplitudes were relatively high, carbon uptake was not affected by Mn addition. The link between high DOC concentrations and stimulation suggests that the Mn addition either relieves a deficiency in available Mn caused by organic complexation or complexes organics from the sewage effluent which are otherwise harmful to phytoplankton productivity. Sewage effluent entering estuaries can be both beneficial and detrimental to the phytoplankton population. Productivity is increased by the addition of inorganic nutrients but may be depressed by the organics contained in the effluent.
INTRODUCTION
Estuarine phytoplankton populations, among the most productive marine plankton systems, are subject to stresses not encountered elsewhere. Due to tidal flux and the relative shallowness of estuaries, wide daily ranges in salinity, temperature, dissolved oxygen, pH and nutrient concentrations require that the populations adapt to constant change. Estuaries are also susceptible to the impacts of man such as commercial fishing, recreational use and the addition of sewage and industrial effluents. f Present address: Skidaway Institute of Oceanography, PO Box 13687, Savannah, Georgia 31406, USA.
59
Marine Environ. Res. (1) (1978)--© Applied Science Publishers Ltd, England, 1978 Printed in Great Britain
60
JAMES G. SANDERS
Sewage effluent often benefits an estuary's productivity since it can provide abundant essential nutrients such as nitrogen and phosphorus. Negative effects are also exerted in that the addition of organics often increases the biochemical oxygen demand (gOD), leading to large fluctuations in the concentration of dissolved oxygen. The addition of effluent also increases the possibility of toxic materials entering the estuary, especially if part of the effluent has an industrial origin. Trace metals are often found in sewage effluent. Many metals are essential in small quantities but are inhibitory in larger concentrations. Due to this dual role, the response of an estuarine system to metal addition is complex. This paper describes the response of natural phytoplankton to one such metal, manganese, in an estuary that receives sewage effluent, Calico Creek, North Carolina, USA.
DESCRIPTION OF THE STUDY AREA
Calico Creek is a small tidal tributary of the Newport River estuary, located approximately 3 km northwest of the port of Morehead City (Fig. 1). It is narrow and shallow, fringed for most of its length by Spartina and Juncus marsh. The "79°W ,
BR
.'.t~'~ :'
"
75 ° ~'37°N
..', STP
""
:C::~: .,:, ~~',-..,,
13 "'::'~ STREET
0
~
3
,NEWPORT R E
!
""
METRES I 0 0 0
Fig. 1. Calico Creek, North Carolina, showing the sample station used during the investigation (20th Street), The inset shows the location of Calico Creek in coastal North Carolina.
hydrography and chemical characteristics of the creek water have been previously reported (Kuenzler et al., 1971 ; Sanders, 1978; Sanders & Kuenzler, details to be published later). Of the approximately 2 x 108 litres of water which flow daily in and out of the creek due to tidal action, about 2 ~'/o,4.7 x 10 6 litres/day, is effluent from Morehead City's sewage treatment plant. Calico Creek has an average yearly concentration of dissolved Mn of 21 #g/litre,
ENRICHMENT OF ESTUARINE PHYTOPLANKTON BY MANGANESE
61
considerably higher than surrounding systems. This is due mainly to resuspension of dissolved Mn trapped in pore waters (Sanders, 1978)and input of sewage effluent. The concentration of Mn in the effluent is somewhat higher--27/~g/litre--and contributes 15 % of the Mn excess. The productivity of the creek is high. The annual production of the Spartina marsh averages around 560gC/m 2 (E. J. Kuenzler, pers. comm.). The phytoplankton productivity is also quite high, 230 g C/m 2 . y, approximately three times higher than the yearly production rate in the nearby Newport River estuary (details to be published later). The seasonal productivity and population composition of the phytoplankton Are somewhat different from those of surrounding populations. High population densities during the summer months, caused by the high nutrient concentrations in the effluent, are dominated by three species. I have found, together with E. J. Kuenzler, that Naricula arrensis, a small pennate diatom, Nannochloris sp., a small coccoid green, and a prasinophyte, tentatively identified as Nephroselmis gilva, comprise more than 80 ~/o of the population from May to September. During the winter, the species composition, population density and productivity of the Calico Creek population are similar to populations in surrounding estuaries.
SAMPLING AND ANALYTICAL PROCEDURES
In the field, aliquots of Mn 2+ as MnSO4 were added to 125ml glass bottles containing natural water collected between the mid- and high-water levels at 20th Street (Fig. 1). The added Mn concentration of the water in the bottles ranged from zero (control group) to 1.6mg/litre. There were five replicates of each added concentration. The bottles were inoculated with approximately 1/~Ci of 14C as HCO~, placed in the creek and incubated just under the surface of the water for 4 h. A set of dark controls was incubated with each group of bottles. After the incubation, the bottles were removed from the creek, placed in a lighttight box and returned to the laboratory. They were then filtered through HA type (0.45 pm) Millipore filters, rinsed with filtered seawater and placed in a refrigerated desiccator overnight. The filters were then placed on planchets, loaded into a sample changer and counted with a Nuclear Chicago Model 1043 gas flow counting system. A sample of the water used for the above Mn addition was analysed for ambient Mn concentration (Strickland & Parsons, 1972, with modifications as in Sanders, 1975). Another aliquot was used to determine the phytoplankton population density and species composition. Several separate water samples were taken at high water under periods of different tidal amplitude. These samples were analysed for dissolved organic carbon (DOC) by the method of Menzel & Vaccaro (1964) and for copper complexation capacity using an Orion Cu electrode (R. G. Smith, pers. comm.).
JAMES G. S A N D E R S
62
7
1
r~
z Z < Z <
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L2"s • i
6 A6
A~-., A - - A
0,,.., 0'~
"~ Z "
z
L
r,
i lao
~"nz -"a.O
..; Z
~.~
- ~
,,, o ~s
Z
~ I
a, Z
Z <
..~ Z
e
<
0>"
Z -- Z
0
Z
ENRICHMENT OF ESTUARINE PHYTOPLANKTON BY MANGANESE
63
RESULTS
The incubation of natural phytoplankton populations that had been enriched with varying concentrations of Mn was performed eight times during the spring and summer of 1975. Although all the incubations were performed in a similar fashion, the phytoplankton reacted differently. On three occasions, carbon uptake was enhanced by the addition of Mn. Only once was uptake depressed. Uptake was essentially unchanged during the remaining four incubations (Table 1). On any given incubation all the different concentrations of Mn added gave the same response. The different responses were not a function of the species composition. Variations in class and species dominance and population size throughout the study period did not appear to be linked to any of the three responses. The response of the phytoplankton (with one exception) was linked to tidal amplitude. Stimulation occurred when tidal amplitude was low, during periods of neap tides. Similarly, during spring tides when tidal amplitudes were relatively high, the addition of Mn had no significant effect on carbon uptake. A physical factor affected by changes in tidal amplitude is the dilution of sewage effluent. The amount of effluent entering Calico Creek remains fairly constant (Table TABLE 2 FLOW RATE ( X 106 LITRES/DAY)AND BOD (IN MG/LITRE)OF THE MOREHEADCITY SEWAGE EFFLUENT, DURING THE FOUR-HOURMANGANESEINCUBATIONS
Date 8 28 15 28 16 24 16 30
February 1975 February March March May May July July
( x 106 litre/day)
Effluent flow
BOD (mg/litre)
5-26 4.73 4.01 3.75 3-48 4.69 7.12 4.20
>62 15 25 34 20 26 14 23
TABLE 3 DOC CONCENTRATIONS(IN MG/LITRE)AND COPPERCOMPLEXATIONCAPACITY(IN/.ZG/LITRE)OF CALICOCREEK SURFACE WATER, DURING PERIODSOF VARYINGTIDAL AMPLITUDE
Date
2 August 1976 25 October 15- November 16 November 17 November 22 November 23 November
Tidal amplitude (m)
DOC (mg/litre)
0.92 1.22 0.64 0.67 0.73 1.22 1.20
90 86 94 100 90 80 82
Cu complexation capacity (Izg/litre) 315 415
(94
JAMES G. SANDERS
1.2.
i = - (.034)X -~4 . 0 2
1.0 a D Io,.. < .J
.8"
t-
.6 70
80
90
100
DOC, m g . l ' l The variation of D O C (in mg/litre) in Calico Creek surface water with tidal amplitude (in metres). The calculated linear regression is significant at the 1 ~o level of significance (r = 0.89).
Fig. 2.
2); therefore, the larger volum,e of water flushed through the creek during a spring tide causes greater dilution of the effluent. The dilution can easily be seen by examining variations in DOC concentrations (Table 3, Fig. 2, the calculated regression is significant at the 1 ~ level of significance) and the copper complexation capacity (Table 3) with tidal amplitude.
DISCUSSION
Additions of Mn resulted in stimulation of phytoplankton productivity only during periods of reduced tidal amplitude when DOC concentrations were relatively high. It is therefore likely that interactions between added Mn and dissolved organics in the creek were responsible for the stimulation. There are two possible mechanisms by which manganese-organic interactions could cause stimulation and these are presented below. The ambient concentration of dissolved Mn in the surface water ranged between 7 and 34 #g/litre (Sanders, 1978), values higher by two orders of magnitude than the Mn requirement of phytoplankton determined by Harvey (1947) and Walker (1954). The formation of Mn complexes with the dissolved organics present in the water,
ENRICHMENT OF ESTUARINE PHYTOPLANKTON BY MANGANESE
65
however, will alter Mn speciation and mobility, affecting its availability to phytoplankton. The experiments performed indicate that under periods of high tidal amplitude (lower DOC concentrations) ample Mn is available to the phytoplankton; no stimulation occurs when Mn is added. It is possible, however, that Mn becomes limiting during periods of higher DOC concentration. Manganese additions during this time would lead to stimulation of productivity. The added Mn may also complex organics which are otherwise detrimental to phytoplankton productivity. Many organic compounds inhibit the growth of phytoplankton (Korzep, 1962; Malina, 1964; Gross, 1968; Hellebust, 1970; Ukeles & Rose, 1976), and the addition of sewage effluent to the creek is a good mechanism for their introduction. Hellebust & Lewin (1972) found that only completely dissociated organic acids are taken up by Cylindrotheca sp., a marine diatom. Complexation by free Mn ions would therefore reduce the effect an organic could have on phytoplankton by forming a largely non-reactive complex. Under the above assumption, populations sampled on the spring tide (with lower DOC concentrations and presumably lower concentrations of harmful organics) should be more productive than populations sampled during periods of higher DOC concentrations. This is indeed true. Under similar environmental conditions (light intensities, water temperatures, nutrient concentrations, population size and composition) the phytoplankton productivity measured on 28 February during a period of spring tides and lower DOC concentration was higher than productivity measured on 15 March, sampled during a period of neap tides (Table 1). It is interesting to note that stimulation caused by Mn addition on 15 March allowed the population to reach a level of productivity which equalled that of 28 February (Table 1). Since both the above mechanisms have the same cause and effect, it is difficult to separate the two without further characterisation of the organic species present in the surface water. At present, the sewage efffuent entering Calico Creek is both beneficial and detrimental to the phytoplankton population. Productivity is increased by the addition of inorganic nutrients but can be depressed by the organic load entering in the effluent. Trace metals entering the creek in the effluent are able to exert a positive effect on phytoplankton productivity by either relieving metal deficiencies or complexing harmful organic compounds. However, since the organic compounds in the sewage effluent are largely undescribed, the possibility of detrimental metal-organic compounds exists. This may" have caused the single occurrence of inhibition seen during the experiments. The sewage characteristics (Table 2) show that the effluent on this occasion contained an unusually large BOD, and the phytoplankton population density was quite small. The inhibition seen could have been caused by either a toxic organic present in concentrations larger than the Mn could complex or a manganese-organic complex that was harmful to the phytoplankton.
66
JAMES G. SANDERS
Before conclusions can be reached c o n c e r n i n g the effects that metal ions a n d sewage effluents have o n estuarine productivity, further work is necessary to determine the organics present in the effluent a n d their interactions with metal ions. However, it is obvious that effluents can be detrimental to a n estuarine system. The benefits caused by the i n p u t of inorganic n u t r i e n t s m u s t be carefully balanced by the possible detrimental effects for each estuarine system.
ACKNOWLEDGEMENTS 1 a m grateful to Dr W. W o o d s for his guidance d u r i n g the course of the study, to P. R. C a r l s o n for his help in sample collection, a n d to H. M. Bacon for his analysis o f copper c o m p l e x a t i o n capacity. M r J. C l a y t o n of the M o r e h e a d City sewage t r e a t m e n t plant kindly supplied data o n the B O D a n d flow rates of the effluent. I t h a n k Drs H. L. W i n d o m , W. M. D u n s t a n a n d D. G. W a s l e n c h u k for their criticism of the manuscript. This project was s u p p o r t e d by the M a r i n e Science P r o g r a m of the University of N o r t h Carolina.
REFERENCES GROSS, R. E. (1968). Mechanism of toxicity and resistance to D-mannose and certain derivatives in species of the genus Chlorella Beij. J. Phycol., 4, 140-51. HARVEY,H. W. (1947). Manganese and the growth of phytoplankton. J. mar. biol. Ass. UK, 26, 562-79. HELLEBUST,J. A. (1970). The uptake and utilization of organic substances by marine phytoplankton. In Organic matter in natural waters, ed. by D. W. Hood, 225-56. University of Alaska, Institute of Marine Science Occasional Publication No. t. HELLIEaUST,J. A. & LEWIN,J. (19723.Transport systems for organic acids induced in the marine pennate diatom, Cylindrotheca JusiJormis. Can. J. Microbial., 18, 225-33. KORZEP, D. A. (19623. Toxicity of organic compounds. MS thesis, University of Texas, Austin. KUENZLER,E. J., WYMAN,S. D. & MCKELLAR,H. N. ( 1971). Notes on hydrography and phytoplankton of Calico Creek. In Structure and Jmwtioning of estuarine ecosystems exposed to treated sewage wastes 11. Supplement to the annual reportJor 1970-1971,ed. by E. J. Kuenzler and A. F. Chestnut, North Carolina Sea Grant Publication, 9-22. MALINA,J. F. (1964). Toxicity of petrochemicals in the aquatic environment. Water Sewage Works, 3, 456-9. MENZEL,D. W. & VACCAaO,R. F. (19643.The measurement of dissolvedorganic and particulate carbon in seawater. Limnol. Oeeanogr., 9, 138-42. SANDERS,J. G. (1975). Variations and interactions oJ manganese and phytoplankton in Calico Creek, North Carolina. MS thesis, University of North Carolina, Chapel Hill. SANDERS,J. G. (1978). The sources of dissolved manganese to Calico Creek, North Carolina. Estuar. coastal mar, Sei., 6, 231-8. SIRIeKLANKg,J, D. H. & PARSONS,T. R. (1972). A practical handbook ql seawater analysis. Ottowa, Fisheries Research Board of Canada. UKELES,R. & RosE, W. E. (1976). Observations on organic carbon utilization by photosynthetic marine microalgae. Mar. Biol., 37, 11-28. WALKER,J. B. (1954). Inorganic micronutrient requirements of Chlorella. 1I. Quantitative requirements for iron, manganese, and zinc. Arch. Bioehem. Biophys, 53, 1-8.
JAMESG. SANDERS'~"
Marine Science Program, Unirersity of North Carolina, Chapel Hill, North Carolina 27514, USA
ABSTRACT
The response of natural phytoplankton to additions of excess Mn in an estuary receiving sewage effluent varied with tidal amplitude. During periods of low tidal amplitude, when DOC concentrations were high, carbon uptake by phytoplankton was stimulated. When tidal amplitudes were relatively high, carbon uptake was not affected by Mn addition. The link between high DOC concentrations and stimulation suggests that the Mn addition either relieves a deficiency in available Mn caused by organic complexation or complexes organics from the sewage effluent which are otherwise harmful to phytoplankton productivity. Sewage effluent entering estuaries can be both beneficial and detrimental to the phytoplankton population. Productivity is increased by the addition of inorganic nutrients but may be depressed by the organics contained in the effluent.
INTRODUCTION
Estuarine phytoplankton populations, among the most productive marine plankton systems, are subject to stresses not encountered elsewhere. Due to tidal flux and the relative shallowness of estuaries, wide daily ranges in salinity, temperature, dissolved oxygen, pH and nutrient concentrations require that the populations adapt to constant change. Estuaries are also susceptible to the impacts of man such as commercial fishing, recreational use and the addition of sewage and industrial effluents. f Present address: Skidaway Institute of Oceanography, PO Box 13687, Savannah, Georgia 31406, USA.
59
Marine Environ. Res. (1) (1978)--© Applied Science Publishers Ltd, England, 1978 Printed in Great Britain
60
JAMES G. SANDERS
Sewage effluent often benefits an estuary's productivity since it can provide abundant essential nutrients such as nitrogen and phosphorus. Negative effects are also exerted in that the addition of organics often increases the biochemical oxygen demand (gOD), leading to large fluctuations in the concentration of dissolved oxygen. The addition of effluent also increases the possibility of toxic materials entering the estuary, especially if part of the effluent has an industrial origin. Trace metals are often found in sewage effluent. Many metals are essential in small quantities but are inhibitory in larger concentrations. Due to this dual role, the response of an estuarine system to metal addition is complex. This paper describes the response of natural phytoplankton to one such metal, manganese, in an estuary that receives sewage effluent, Calico Creek, North Carolina, USA.
DESCRIPTION OF THE STUDY AREA
Calico Creek is a small tidal tributary of the Newport River estuary, located approximately 3 km northwest of the port of Morehead City (Fig. 1). It is narrow and shallow, fringed for most of its length by Spartina and Juncus marsh. The "79°W ,
BR
.'.t~'~ :'
"
75 ° ~'37°N
..', STP
""
:C::~: .,:, ~~',-..,,
13 "'::'~ STREET
0
~
3
,NEWPORT R E
!
""
METRES I 0 0 0
Fig. 1. Calico Creek, North Carolina, showing the sample station used during the investigation (20th Street), The inset shows the location of Calico Creek in coastal North Carolina.
hydrography and chemical characteristics of the creek water have been previously reported (Kuenzler et al., 1971 ; Sanders, 1978; Sanders & Kuenzler, details to be published later). Of the approximately 2 x 108 litres of water which flow daily in and out of the creek due to tidal action, about 2 ~'/o,4.7 x 10 6 litres/day, is effluent from Morehead City's sewage treatment plant. Calico Creek has an average yearly concentration of dissolved Mn of 21 #g/litre,
ENRICHMENT OF ESTUARINE PHYTOPLANKTON BY MANGANESE
61
considerably higher than surrounding systems. This is due mainly to resuspension of dissolved Mn trapped in pore waters (Sanders, 1978)and input of sewage effluent. The concentration of Mn in the effluent is somewhat higher--27/~g/litre--and contributes 15 % of the Mn excess. The productivity of the creek is high. The annual production of the Spartina marsh averages around 560gC/m 2 (E. J. Kuenzler, pers. comm.). The phytoplankton productivity is also quite high, 230 g C/m 2 . y, approximately three times higher than the yearly production rate in the nearby Newport River estuary (details to be published later). The seasonal productivity and population composition of the phytoplankton Are somewhat different from those of surrounding populations. High population densities during the summer months, caused by the high nutrient concentrations in the effluent, are dominated by three species. I have found, together with E. J. Kuenzler, that Naricula arrensis, a small pennate diatom, Nannochloris sp., a small coccoid green, and a prasinophyte, tentatively identified as Nephroselmis gilva, comprise more than 80 ~/o of the population from May to September. During the winter, the species composition, population density and productivity of the Calico Creek population are similar to populations in surrounding estuaries.
SAMPLING AND ANALYTICAL PROCEDURES
In the field, aliquots of Mn 2+ as MnSO4 were added to 125ml glass bottles containing natural water collected between the mid- and high-water levels at 20th Street (Fig. 1). The added Mn concentration of the water in the bottles ranged from zero (control group) to 1.6mg/litre. There were five replicates of each added concentration. The bottles were inoculated with approximately 1/~Ci of 14C as HCO~, placed in the creek and incubated just under the surface of the water for 4 h. A set of dark controls was incubated with each group of bottles. After the incubation, the bottles were removed from the creek, placed in a lighttight box and returned to the laboratory. They were then filtered through HA type (0.45 pm) Millipore filters, rinsed with filtered seawater and placed in a refrigerated desiccator overnight. The filters were then placed on planchets, loaded into a sample changer and counted with a Nuclear Chicago Model 1043 gas flow counting system. A sample of the water used for the above Mn addition was analysed for ambient Mn concentration (Strickland & Parsons, 1972, with modifications as in Sanders, 1975). Another aliquot was used to determine the phytoplankton population density and species composition. Several separate water samples were taken at high water under periods of different tidal amplitude. These samples were analysed for dissolved organic carbon (DOC) by the method of Menzel & Vaccaro (1964) and for copper complexation capacity using an Orion Cu electrode (R. G. Smith, pers. comm.).
JAMES G. S A N D E R S
62
7
1
r~
z Z < Z <
© -r.
L2"s • i
6 A6
A~-., A - - A
0,,.., 0'~
"~ Z "
z
L
r,
i lao
~"nz -"a.O
..; Z
~.~
- ~
,,, o ~s
Z
~ I
a, Z
Z <
..~ Z
e
<
0>"
Z -- Z
0
Z
ENRICHMENT OF ESTUARINE PHYTOPLANKTON BY MANGANESE
63
RESULTS
The incubation of natural phytoplankton populations that had been enriched with varying concentrations of Mn was performed eight times during the spring and summer of 1975. Although all the incubations were performed in a similar fashion, the phytoplankton reacted differently. On three occasions, carbon uptake was enhanced by the addition of Mn. Only once was uptake depressed. Uptake was essentially unchanged during the remaining four incubations (Table 1). On any given incubation all the different concentrations of Mn added gave the same response. The different responses were not a function of the species composition. Variations in class and species dominance and population size throughout the study period did not appear to be linked to any of the three responses. The response of the phytoplankton (with one exception) was linked to tidal amplitude. Stimulation occurred when tidal amplitude was low, during periods of neap tides. Similarly, during spring tides when tidal amplitudes were relatively high, the addition of Mn had no significant effect on carbon uptake. A physical factor affected by changes in tidal amplitude is the dilution of sewage effluent. The amount of effluent entering Calico Creek remains fairly constant (Table TABLE 2 FLOW RATE ( X 106 LITRES/DAY)AND BOD (IN MG/LITRE)OF THE MOREHEADCITY SEWAGE EFFLUENT, DURING THE FOUR-HOURMANGANESEINCUBATIONS
Date 8 28 15 28 16 24 16 30
February 1975 February March March May May July July
( x 106 litre/day)
Effluent flow
BOD (mg/litre)
5-26 4.73 4.01 3.75 3-48 4.69 7.12 4.20
>62 15 25 34 20 26 14 23
TABLE 3 DOC CONCENTRATIONS(IN MG/LITRE)AND COPPERCOMPLEXATIONCAPACITY(IN/.ZG/LITRE)OF CALICOCREEK SURFACE WATER, DURING PERIODSOF VARYINGTIDAL AMPLITUDE
Date
2 August 1976 25 October 15- November 16 November 17 November 22 November 23 November
Tidal amplitude (m)
DOC (mg/litre)
0.92 1.22 0.64 0.67 0.73 1.22 1.20
90 86 94 100 90 80 82
Cu complexation capacity (Izg/litre) 315 415
(94
JAMES G. SANDERS
1.2.
i = - (.034)X -~4 . 0 2
1.0 a D Io,.. < .J
.8"
t-
.6 70
80
90
100
DOC, m g . l ' l The variation of D O C (in mg/litre) in Calico Creek surface water with tidal amplitude (in metres). The calculated linear regression is significant at the 1 ~o level of significance (r = 0.89).
Fig. 2.
2); therefore, the larger volum,e of water flushed through the creek during a spring tide causes greater dilution of the effluent. The dilution can easily be seen by examining variations in DOC concentrations (Table 3, Fig. 2, the calculated regression is significant at the 1 ~ level of significance) and the copper complexation capacity (Table 3) with tidal amplitude.
DISCUSSION
Additions of Mn resulted in stimulation of phytoplankton productivity only during periods of reduced tidal amplitude when DOC concentrations were relatively high. It is therefore likely that interactions between added Mn and dissolved organics in the creek were responsible for the stimulation. There are two possible mechanisms by which manganese-organic interactions could cause stimulation and these are presented below. The ambient concentration of dissolved Mn in the surface water ranged between 7 and 34 #g/litre (Sanders, 1978), values higher by two orders of magnitude than the Mn requirement of phytoplankton determined by Harvey (1947) and Walker (1954). The formation of Mn complexes with the dissolved organics present in the water,
ENRICHMENT OF ESTUARINE PHYTOPLANKTON BY MANGANESE
65
however, will alter Mn speciation and mobility, affecting its availability to phytoplankton. The experiments performed indicate that under periods of high tidal amplitude (lower DOC concentrations) ample Mn is available to the phytoplankton; no stimulation occurs when Mn is added. It is possible, however, that Mn becomes limiting during periods of higher DOC concentration. Manganese additions during this time would lead to stimulation of productivity. The added Mn may also complex organics which are otherwise detrimental to phytoplankton productivity. Many organic compounds inhibit the growth of phytoplankton (Korzep, 1962; Malina, 1964; Gross, 1968; Hellebust, 1970; Ukeles & Rose, 1976), and the addition of sewage effluent to the creek is a good mechanism for their introduction. Hellebust & Lewin (1972) found that only completely dissociated organic acids are taken up by Cylindrotheca sp., a marine diatom. Complexation by free Mn ions would therefore reduce the effect an organic could have on phytoplankton by forming a largely non-reactive complex. Under the above assumption, populations sampled on the spring tide (with lower DOC concentrations and presumably lower concentrations of harmful organics) should be more productive than populations sampled during periods of higher DOC concentrations. This is indeed true. Under similar environmental conditions (light intensities, water temperatures, nutrient concentrations, population size and composition) the phytoplankton productivity measured on 28 February during a period of spring tides and lower DOC concentration was higher than productivity measured on 15 March, sampled during a period of neap tides (Table 1). It is interesting to note that stimulation caused by Mn addition on 15 March allowed the population to reach a level of productivity which equalled that of 28 February (Table 1). Since both the above mechanisms have the same cause and effect, it is difficult to separate the two without further characterisation of the organic species present in the surface water. At present, the sewage efffuent entering Calico Creek is both beneficial and detrimental to the phytoplankton population. Productivity is increased by the addition of inorganic nutrients but can be depressed by the organic load entering in the effluent. Trace metals entering the creek in the effluent are able to exert a positive effect on phytoplankton productivity by either relieving metal deficiencies or complexing harmful organic compounds. However, since the organic compounds in the sewage effluent are largely undescribed, the possibility of detrimental metal-organic compounds exists. This may" have caused the single occurrence of inhibition seen during the experiments. The sewage characteristics (Table 2) show that the effluent on this occasion contained an unusually large BOD, and the phytoplankton population density was quite small. The inhibition seen could have been caused by either a toxic organic present in concentrations larger than the Mn could complex or a manganese-organic complex that was harmful to the phytoplankton.
66
JAMES G. SANDERS
Before conclusions can be reached c o n c e r n i n g the effects that metal ions a n d sewage effluents have o n estuarine productivity, further work is necessary to determine the organics present in the effluent a n d their interactions with metal ions. However, it is obvious that effluents can be detrimental to a n estuarine system. The benefits caused by the i n p u t of inorganic n u t r i e n t s m u s t be carefully balanced by the possible detrimental effects for each estuarine system.
ACKNOWLEDGEMENTS 1 a m grateful to Dr W. W o o d s for his guidance d u r i n g the course of the study, to P. R. C a r l s o n for his help in sample collection, a n d to H. M. Bacon for his analysis o f copper c o m p l e x a t i o n capacity. M r J. C l a y t o n of the M o r e h e a d City sewage t r e a t m e n t plant kindly supplied data o n the B O D a n d flow rates of the effluent. I t h a n k Drs H. L. W i n d o m , W. M. D u n s t a n a n d D. G. W a s l e n c h u k for their criticism of the manuscript. This project was s u p p o r t e d by the M a r i n e Science P r o g r a m of the University of N o r t h Carolina.
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