eutrophication and the rate of denitrification and $n_2o$ production in coastal marine sediments

9
Eutrophication and the Rate of Denitrification and $N_2O$ Production in Coastal Marine Sediments Author(s): Sybil P. Seitzinger and Scott W. Nixon Source: Limnology and Oceanography, Vol. 30, No. 6 (Nov., 1985), pp. 1332-1339 Published by: American Society of Limnology and Oceanography Stable URL: http://www.jstor.org/stable/2836489 . Accessed: 18/06/2014 14:55 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . American Society of Limnology and Oceanography is collaborating with JSTOR to digitize, preserve and extend access to Limnology and Oceanography. http://www.jstor.org This content downloaded from 185.44.77.89 on Wed, 18 Jun 2014 14:55:16 PM All use subject to JSTOR Terms and Conditions

Upload: sybil-p-seitzinger-and-scott-w-nixon

Post on 20-Jan-2017

212 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Eutrophication and the Rate of Denitrification and $N_2O$ Production in Coastal Marine Sediments

Eutrophication and the Rate of Denitrification and $N_2O$ Production in Coastal MarineSedimentsAuthor(s): Sybil P. Seitzinger and Scott W. NixonSource: Limnology and Oceanography, Vol. 30, No. 6 (Nov., 1985), pp. 1332-1339Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2836489 .

Accessed: 18/06/2014 14:55

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

American Society of Limnology and Oceanography is collaborating with JSTOR to digitize, preserve andextend access to Limnology and Oceanography.

http://www.jstor.org

This content downloaded from 185.44.77.89 on Wed, 18 Jun 2014 14:55:16 PMAll use subject to JSTOR Terms and Conditions

Page 2: Eutrophication and the Rate of Denitrification and $N_2O$ Production in Coastal Marine Sediments

1332 Notes

populations of marine phytoplankton. Limnol. Oceanogr. 17: 738-748.

, W. R. TAYLOR, AND J. L. TAFT. 1977. Ni- trogenous nutrition of the plankton in the Ches- apeake Bay. 1. Nutrient availability and phyto- plankton preferences. Limnol. Oceanogr. 22: 996- 1011.

, D. WYNNE, AND T. BERMAN. 1982. The up- take of dissolved nitrogenous nutrients by Lake Kinneret (Israel) microplankton. Limnol. Ocean- ogr. 27: 673-680.

MALONE, T. C. 1980. Size-fractionated primary pro- ductivity of marine phytoplankton. Brookhaven Symp. Biol. 31, p. 301-319. Plenum.

NYDAHL, F. 1976. On the optimum conditions for

the reduction of nitrate to nitrite by cadmium. Talanta 23: 349-357.

OILSON, R. J. 1980. Nitrate and ammonium uptake in Antarctic waters. Limnol. Oceanogr. 25: 1064- 1074.

PROBYN, T. A. 1985. Nitrogen uptake by size-frac- tionated phytoplankton populations in the south- ern Benguela upwelling system. Mar. Ecol. Prog. Ser.: In press.

RONNER, U., F. SORENSSON, AND 0. HOLM-HANSEN. 1983. Nitrogen assimilation by phytoplankton in the Scotia Sea. Polar Biol. 2: 137-147.

Submitted: 27 February 1985 Accepted: 10 June 1985

Limnol. Oceanogr., 30(6), 1985, 1332-1339 ? 1985, by the American Society of Limnology and Oceanography, Inc.

Eutrophication and the rate of denitrification and N20 production in coastal marine sediments

Abstract-Large (13 m3, 5 m deep) microcosms with coupled pelagic and benthic components were used to measure the effect of nutrient load- ing and eutrophication in coastal marine ecosys- tems on the rates of benthic denitrification (N2) and N20 production. After 3 months of daily nutrient addition, average denitrification rates ranged from about 300 ,gmol N m-2 h- in the sediments of the control microcosm to 880 in the most enriched microcosm, which received 65 times the nutrient input of the control. Increases in the production of N20 were more dramatic and increased by a factor of about 100, from 0.56 ,gmol N m-2 h-I in the control to 51 in the most enriched microcosm. Although there was a clear increase in the denitrification rate in the more eutrophic systems, the amount of fixed nitrogen removed was a constant or progressively smaller fraction of the nitrogen input. Even in the most enriched microcosm, at least 16% of the N input was removed by denitrification.

Many estuaries and other coastal marine ecosystems are receiving increased nitrogen from sewage, industrial wastes, and agri- cultural runoff (Walsh et al. 198 1; Meybeck 1982; U.S. EPA 1982). In some cases these anthropogenic sources have come to dom- inate natural inputs from the watershed, the atmosphere, and coastal seawater. For ex- ample, sewage inputs alone account for 50% or more of the inorganic nitrogen loading

in Long Island Sound, New York Bay, Rar- itan Bay, Delaware Bay, and San Francisco Bay (Nixon and Pilson 1983). Similar sit- uations no doubt exist elsewhere.

The ecological consequences of increas- ing nitrogen inputs in coastal marine eco- systems are not as well known as the impact of phosphorus loading on lakes (Schindler 1981; Nixon and Pilson 1983). The fate of the increased nitrogen input to coastal sys- tems is even more poorly known. Increasing amounts of nitrogen may be buried in sed- iments, transported in various forms to nearshore ocean waters, or lost to the at- mosphere as a result of denitrification (N2) and N20 production. In Narragansett Bay, R.I., denitrification is an important sink for fixed nitrogen and is removing, as N2, an amount of N equal to half of the inorganic nitrogen loading to that estuary each year from urban sewage (Seitzinger et al. 1980, 1984; Seitzinger 1982). N20 is also pro- duced in coastal sediments, although its production does not appear to be a major mechanism of removal of nitrogen loading (Seitzinger et al. 1983). The production of N20 is of global geochemical interest, how- ever, because of the role of this trace gas in the destruction of stratospheric ozone

1332

This content downloaded from 185.44.77.89 on Wed, 18 Jun 2014 14:55:16 PMAll use subject to JSTOR Terms and Conditions

Page 3: Eutrophication and the Rate of Denitrification and $N_2O$ Production in Coastal Marine Sediments

Notes 1333

(Crutzen 1970; Hahn and Crutzen 1982) and in the radiative heat budget of the atmo- sphere (Wang et al. 1976).

Recognition of the magnitude and prob- able widespread occurrence of denitrifica- tion in nearshore sediments (Nixon 1981; Billen 1982; Seitzinger 1982; Kemp et al. 1982) leads to the question of how the rate of this process varies in response to in- creased nitrogen loading and eutrophica- tion. To what extent does denitrification "buffer" the nitrogen levels of coastal ma- rine systems in the face of increasing an- thropogenic inputs? We report here the first results of a series of experiments designed to address this question. Our work was part of a cooperative study of the eutrophication of a coastal marine ecosystem using the large microcosms at the Marine Ecosystems Re- search Laboratory (MERL) at the Univer- sity of Rhode Island.

The MERL facility, started in 1976, con- sists of 14 large cylindrical fiber-glass tanks on the shore of the lower West Passage of Narragansett Bay. They contain coupled benthic and pelagic components with a 5-m- deep, 13-m3 mixed water column overlying a 40-cm-deep layer of sediment. Sediments are collected from Narragansett Bay with a large (0.25 im2) box corer, placed in a. tray with care to preserve the orientation of the sediment, and lowered to the bottom of the tanks. Relatively unpolluted coastal sea- water from lower Narragansett Bay is pumped by diaphragm pump into each tank at a rate sufficient to replace the water about every 23 days, approximating the flushing time of the bay.

Nutrients, chlorophyll, zooplankton numbers, dissolved oxygen, pH, and total system metabolism are measured at least once a week. The abundance of benthic an- imals, porewater and sediment analyses and exchange of materials across the sediment- water interface are determined less fre- quently. The microcosms exhibit behavior similar to that of Narragansett Bay in terms of species composition and abundance, bi- ological rate processes, and the concentra- tions of various nutrients, metals, and or- ganic compounds. A detailed description of the systems, data comparing microcosm processes with bay processes, and discus-

sion of variability among tanks are given elsewhere (e.g. Pilson et al. 1979, 1980; Nix- on et al. 1980; Hunt and Smith 1982; Elm- gren and Frithsen 1982).

The eutrophication experiment began in April 1981 with sediments from a relatively unpolluted site in mid-Narragansett Bay and water flowing into the tanks from off our dock. Nutrients were added daily beginning on 1 June to six tanks: NH4+ = (NH4CI), P043- = (KH2PO4), and Si(OH)4 =

(Na2SiO3- 7 H20) in a ratio of 12.8: 1.0: 0.9 by atoms. Three tanks served as controls. Six levels of nitrogen loadings above those in the control tanks were chosen to range in a geometric series from a level comparable to the inorganic nitrogen loading to Nar- rangansett Bay (- 100 ,umol N m-2 h-1: Nixon 1981) up to a level comparable to the extreme loading of 3,645 ,umol N m-2 h-I estimated for New York Bay (Nixon and Pilson 1983). Each tank also received nutrients from the Narragansett Bay input water, so that the estimated total dissolved inorganic nitrogen (DIN) loading (umol m-2 h-1) during the first year of the experiment averaged as follows: controls = 60; 1 x = 170; 2x = 300; 4x = 520; 8x = 1,000; 16 x = 2,000; 32 x = 3,900. Because of slightly different flow rates entering each tank, the 32 x tank actually received about 65 times the DIN input of the controls and 23 times that of the 1 x treatment. The treatment names are retained here for con- sistency with various other papers dealing with this experiment. Results of the first 6- 12 months of the experiment are given by Nixon et al. (1984).

Sediment cores (47 cm2) were collected in early September by a SCUBA diver from each of five microcosms (a control, 4 x, 8 x, 16 x, and 32 x treatments) and transferred to gastight glass incubation chambers for measurements of N2 and N20 production. Details of the method have been given pre- viously (Seitzinger et al. 1980, 1984; Seit- zinger 1982). Only the general outline and recent modifications of the methods are de- scribed here.

The cores (7 cm deep) were incubated in the dark with unfiltered seawater (- 300 ml) which was stirred continuously with a float- ing magnetic stirring bar to facilitate the

This content downloaded from 185.44.77.89 on Wed, 18 Jun 2014 14:55:16 PMAll use subject to JSTOR Terms and Conditions

Page 4: Eutrophication and the Rate of Denitrification and $N_2O$ Production in Coastal Marine Sediments

1334 Notes

equilibration of dissolved gases with the overlying gas phase (-70 ml). Water over the cores was changed daily with freshly pre- pared low-N2 seawater obtained by flushing water with a gas mixture of 21% 02, 312 ppm C02, and the balance helium. The water used was collected daily from the same mi- crocosms as the respective sediment cores. The incubation temperature was 18?C, which approximated the microcosm tem- peratures at the time of sediment collection.

Duplicate samples (100 ,l) of the gas phase were taken for N2 analysis from each of four chambers (control, 4 x, 16 x, and 32 x) about 2 h after the water was changed and between 9 and 24 h later. There was no correlation between incubation time be- tween samples and N2 production rate. The differences in concentration between se- quential samples within an incubation were used to calculate the net N2 flux across the sediment-water interface. Gas samples were injected directly into a gas chromatograph (Tracor, model MT-1 50G) equipped with dual ultrasonic phase shift detectors (2-m x 0.318-cm-o.d. stainless steel columns packed with 45/60 mesh Molecular Sieve 5A; He carrier gas flow rate, 25 cm3 min- 1).

Previous experiments (Seitzinger 1982) showed that the N2 initially dissolved in the porewaters was equilibrated with the low- N2 overlying water in about 1 week. For safety, we report only those measurements made after 10 days preincubation. Four N2 flux measurements on cores from the con- trol, 4 x, and 32 x treatments and two from the 16 x treatment were complete 2 weeks after the cores were taken from the micro- cosms.

Duplicate samples (1.5 ml) ofthe gas phase were taken for N20 analysis from each of the five chambers (control, 4 x, 8 x, 16 x, and 32 x) about 2 h after the water was changed and at about 4-h intervals there- after. There was no correlation between N20 production from a core and time after the water was changed (up to 12 h). N20 was determined on days when N2 was not.

Production of N20 in the water was checked in early October (ambient water temp 12?C) by incubating water samples from the microcosms in serum bottles fitted

with rubber serum stoppers. Two water samples (80 ml each) were siphoned into serum bottles (125 ml) from each of the microcosms in which N20 sediment fluxes had been measured. One bottle from each microcosm was injected initially with 1 ml of saturated HgCl2 solution. All bottles were incubated in the dark at 18?C. After 9 h the second sample from each microcosm was injected with 1 ml of HgCl2 solution. The bottles were hand-shaken for 2 min to equil- ibrate the headspace and water. Duplicate samples from each bottle were analyzed for N20 and the water temperature in the bot- tles measured within 5 min of sampling.

Samples from the chambers and from the serum bottles were analyzed for N20 by in- jecting duplicate gas samples directly into a gas chromatograph (Hewlett-Packard, model 57 10A) equipped with a 63Ni elec- tron capture detector (350?C) and a 2.44-m x 0.318-cm-o.d. stainless steel col- umn packed with Poropak Q, 80/100 mesh (CH4/Ar carrier gas flow 25 cm3 min- 1). The total amount of N20 in the water and gas phases of the chambers or bottles was cal- culated with the N20 solubility equation of Weiss and Price (1980) for the appropriate temperature and salinity.

N2 fluxes ranged from about 160 ,mol N m-2 h-I in the 4x treatment to over 1,000 in the 32 x treatment (Fig. 1). The average denitrification rates (Table 1) from the con- trol and 4 x treatment sediments were not statistically different (a = 0.05). However, N2 production in the 16 x treatment was greater than in the control and 4 x treat- ments and increased still further in the 32 x treatment.

The above data clearly show that deni- trification was enhanced at the higher treat- ment levels, but the exact relationship be- tween N input to the microcosms and denitrification is not as clear. If we simply calculate the percentage of DIN input re- moved by denitrification from the sediment of each microcosm, it appears that denitri- fication removes a smaller percentage of the N loading as the input increases (Table 1). However, even in the most enriched system, denitrification removed as N2 an amount of N equivalent to -23% of the DIN input.

This content downloaded from 185.44.77.89 on Wed, 18 Jun 2014 14:55:16 PMAll use subject to JSTOR Terms and Conditions

Page 5: Eutrophication and the Rate of Denitrification and $N_2O$ Production in Coastal Marine Sediments

Notes 1335

60 -

32 x 1000 50

800 E 20

E E 40 - +1

E E 0

-~600 --6 0

N 400 T 50 0

disle 4nrai niroe (DN)

4x,~~~~~~~~~~~~~~ z ~~~~~~~~~16x 20 0 1 0 4x 8

0 0 ..t lETTTT 100 200 500 1000 2000 5000 100 200 500 1000 2000 4000

DIN Loading , ~g -atoms m2 h-1 DIN Loading / Lg-atoms m2 hI

Fig. 1. Rate of N2 and N20 flUX out of sediments from MERL microcosms receiving various levels of dissolved inorganic nitrogen (DIN).

Alternatively, the four rate measurements fit a linear regression model (N2 flux = 0.16 DIN input + 257) very well (r2 = 0.98), sug- gesting that a relatively constant 16% of the nitrogen input was denitrified at all treat- ment levels once a correction is made for the background nitrogen loss. Neither in- terpretation is very secure because of the small data set and the fact that the rate of denitrification measured in the control tank was three times higher than rates measured in mid-Narragansett Bay at a similar tem- perature (Seitzinger et al. 1984). Even if rates from the three enriched tanks are used with a linear regression to extrapolate back to the control, the calculated flux of 220 ,umol N m-2 h-I is still twice that in the bay. We do not know the reason for this, but it must be transitory since this rate of nitrogen loss ex- ceeds the DIN input to the control system.

Denitrification requires a supply of or- ganic matter and of nitrate or nitrite. The response of the denitrification rate in the microcosms to the increased nutrient load- ing likely reflects a change in the availability in the sediments of these two components. Although we do not have direct measure- ments of organic matter sedimentation, diel changes in water column oxygen in the mi- crocosms show that the production of or-

ganic matter increased with increased nu- trient loading (Nixon et al. 1984). In the more enriched tanks, apparent daytime pro- duction increased by a factor of 2-3, similar to the increase in the denitrification rate.

The supply of NO3- and N02- in the mi- crocosms also increased with increased nu- trient loading. Concentration of N02- and N03- in the water ranged from 1 ,M (con- trol) to 58 ,uM (16 x treatment) (Nixon et al. 1984). However, it appears that the N03- and N02- for denitrification in the sedi- ments arose not by diffusion from the over- lying water, but rather from nitrification oc- curring in the porewaters themselves. We base this conclusion on the results of sedi- ment-water N02- and N03- flux measure- ments which showed a net flux out of the sediments in all but the 32 x treatment (Be- rounsky and Nixon 1985) (Table 1) in- dicating not only that nitrification was an active process in the sediments, but that nitrification rates exceeded denitrification rates in all but the 32 x treatment.

The sediment-water fluxes of N03- and N02- were measured with a benthic cham- ber which covered the entire surface of the microcosm sediments (Kelly 1982). One potential error in this approach arises from the assumption that nitrification in the water

This content downloaded from 185.44.77.89 on Wed, 18 Jun 2014 14:55:16 PMAll use subject to JSTOR Terms and Conditions

Page 6: Eutrophication and the Rate of Denitrification and $N_2O$ Production in Coastal Marine Sediments

1336 Notes

(U U

r- oo en (U -.4 tn 4-b0 tn o (U

.00 C14 r- v tn 06

0 0 z C 0 cl cl

tn

00

0 z + +1 +1 +1 +1 +1 +1

"o "o 00 -.4 N tn 't 00

0 z 0

(U 0 0

u 0 - 0 0 C,4 'O

z U. + 00 E! :3 0 -.;, - t I N c) 5 '21 < (A 0 tn V.C - + cd 0 0 (U 0

'nE u Z 1-0 0 0 0 0 - 0

0 0 o 0 N N I'D 00 C) 0 C'i 'i C'i 4

Z Z 0 U.0 C)

R,

0

aN oo C) 0

N. 0 in

oo t-,

(ON C) C) 10, 0 + +I >1 oo r-- en oo

0 t= Z 0 0 on z z + Ow C- C-) z 00 0

0 10 0 t-, X o on

-4 Z.2

ZS 0 0 5 en CN 'S zz

C5 -4 eri N z o + 0 C. V.- , r.0 0 0 +1 +1 +1 +1 +1 +1 ow ,o N oo C) - 0 z 0 In Z :4 N a) C) ci 0 4 0 10

z Cd + 0 - , 0 +o U

0 0 I-) Z .U +

c.. z E 0 0 0 cq C) 0 0 , 0

M O.- ,, x .- 0 t= Z x il "O' 0

Z (.4)

o t= 4-1 +1 +I +I +1 In 00 00 N 7 Z C) C) in 00 o en en tn 00

0 cd 0

cd C-)

0 x x C-) x x cq W)

't 00 ow 2 4) z .0 u C) C) C) C) C) cn o E

= 0'a C's9 N C) C) C) o ZA > C) 0 ON Z

over the sediments during a benthic flux incubation was similar to that in water iso- lated from sediments in a "control" cham- ber. Changes in NO3- + NO2- in the "con- trol" are subtracted from those in the combined sediment-water system to arrive at a value for the sediments alone. We might overestimate the flux from the sediments if nitrification in the benthic chamber water is significantly higher than in the "control" water, either because nitrification rates in the benthic chamber waters were markedly enhanced by ammonium coming out of the sediments or because nitrification rates in the "control" bottles were reduced due to significant decreases in ammonium as a re- sult of nitrification.

The following calculations show that nei- ther of the above caused significant changes in nitrification rates during our measure- ments of benthic flux. Berounsky and Nixon (1985) give an equation for ammonium concentration vs. nitrification rates in the MERL microcosms during the eutrophica- tion experiment,

0.70 x log[NH4+ M]) - 1.23 = log N03- + NO2- production

(,umol liter-' d-1). With this equation we calculate that the maximum increase in ammonium concen- tration inside the benthic chambers during the 2-h benthic flux incubations (sum of the measured sediment efflux of NH4+, NO3-, N02-, and N2) would have increased the nitrification rates in the chamber water by an amount <2 ,umol m-2 h-I in any treat- ment. The ammonium concentration at time zero was 0.6 AM in the control treatment and ranged from 42 to 310 ,uM in the other treatments; nitrification in the "control" bottles would have decreased the ammo- nium concentrations by 0.003 ,uM in the control treatment bottle and by <0.3 ,uM in the other "control" bottles during the in- cubation. Such small decreases in NH4+ could not possibly have reduced nitrifica- tion rates significantly. These calculations thus support our conclusion that nitrifica- tion was neither significantly enhanced in the benthic chambers nor reduced in the control bottles, that the increases in NO3-

This content downloaded from 185.44.77.89 on Wed, 18 Jun 2014 14:55:16 PMAll use subject to JSTOR Terms and Conditions

Page 7: Eutrophication and the Rate of Denitrification and $N_2O$ Production in Coastal Marine Sediments

Notes 1337

and NO2- during the benthic flux incuba- tions were indeed due to excess production in the sediments over that used by denitri- fication, and that the source of NO3- and NO2- for denitrification was from nitrifi- cation in the sediments.

We therefore calculated total nitrification rates within the sediments as the sum of the nitrate and nitrite flux out of the sediments plus the nitrate and nitrite necessary to sup- ply the measured N2 flux (Table 1); this may be an underestimate, as the possible reduc- tion of nitrite to ammonium is not consid- ered (Koike and Hattori 1978; Sorenson 1978). Sediment nitrification rates in- creased with increased nutrient input and were twice as high in the 32 x treatment as in the control. This may be related to the increased concentration of NH4+ in the porewaters in the more eutrophied micro- cosms. For example, NH4+ concentrations in the top 5 cm of sediment in the 32 x treatment were 5-10 times those in the con- trol sediments (Pilson unpubl. data). The absolute numbers as well as abundance of polychaetes relative to bivalves also in- creased with increased nutrient loading (Nixon et al. 1984); these burrowing infauna irrigate the sediment and may have en- hanced the oxygen supply in the porewaters, and, in conjunction with the porewater NH4+ concentrations, increased nitrification there. Chatarpaul et al. (1980) noted such an effect when tubificid worms were added to stream sediment and Henriksen et al. (1983) showed that potential nitrification rates in coastal sediments of Denmark increased by at least a factor of two when infauna was present.

Only in the 32 x treatment was the in- creased nitrification rate in our sediments insufficient to supply the increased denitri- fication demand: there was a net flux of NO3 and NO2- from the water into the sediments (Table 1) which supplied about 32% (281/ 888) of the denitrification demand. Rates of nitrification in the water column in the 32 x treatment during this time (2.8 Amol liter-' d- 1) were sufficient to supply the measured flux into the sediments as well as maintain the high concentration of NO3- (Table 1) in the water column (Berounsky and Nixon 1985).

Nitrification in the sediments accounts for a significant portion of the sediment oxygen consumption, from about 30 to 51% (Table 1). A smaller percentage of the benthic oxy- gen consumption seemed to be due to ni- trification as the nutrient input to the mi- crocosms increased. This trend was similar to the Narragansett Bay results (Seitzinger et al. 1984) where the nitrification-related sediment oxygen demand was lowest at the station with the highest nutrient loading.

Net N20 flux was lowest in the control microcosm sediments and increased by about a factor of 100 in sediments from the 32x treatment (Fig. 1, Table 1). No pro- duction or consumption of N20 was mea- sured in water samples incubated without sediment. The amount of N lost from the microcosms as N20 accounted for between 0.2% (control) and 5.8% (32 x) of the gas- eous N (N20 + N2) losses from the micro- cosms.

Although we have no evidence to identify the pathway responsible for the N20 pro- duction measured here, it is interesting that the ratios of N20: N2 production and N20 production: nitrification increased with in- creasing nutrient loading (Table 1). (Nitri- fication was calculated as described above.) The highest ratios were found in the 32 x treatment which showed a net uptake of N03- + NO2- by the sediments (Table 1).

Our experimental results that benthic de- nitrification and N20 production increase in response to increased nutrient inputs sup- port conclusions drawn from measurements made in Narragansett Bay with a more lim- ited range of nutrient input rates (Seitzinger et al. 1983, 1984). The Narragansett Bay data are mainly from two areas with nu- trient input rates approximating the MERL 8 x and control treatments. The bay sedi- ments receive various pollutants including heavy metals and petroleum hydrocarbons and have received their present rate of nu- trient input for a number of years, while the MERL measurements were made after 3 months of nutrient addition. As such, the two data sets may not be directly compa- rable. Therefore, we will compare only gen- eral trends. Fluxes of N2 and N20 were high- est in both systems from sediments under

This content downloaded from 185.44.77.89 on Wed, 18 Jun 2014 14:55:16 PMAll use subject to JSTOR Terms and Conditions

Page 8: Eutrophication and the Rate of Denitrification and $N_2O$ Production in Coastal Marine Sediments

1338 Notes

areas receiving the highest rate of nutrient input. The increase relative to nutrient in- puts was more pronounced for N20 than for N2 fluxes. Nitrate from the overlying water was an important source for denitri- fication in the most eutrophied microcosm and area in the bay but probably not in the other sediments. The N20: N2 production and N20: nitrification ratios were highest in both MERL and bay sediments when there was a net uptake of nitrate and nitrite by the sediments from the water. One differ- ence between the MERL data and the Nar- ragansett Bay data was that the absolute rates of N2 and N20 fluxes were about two-three times greater in the MERL microcosms than in the bay with similar rates of N input; the reason for this is not known.

Although the loss of fixed nitrogen as N2 and N20 from the bottom community in- creased with increased inorganic nitrogen loading in the overlying water, denitrifica- tion removed a constant or progressively smaller fraction of the input. If this rela- tionship is general, it seems clear that the capacity of estuaries and other nearshore systems to remove anthropogenic nitrogen inputs through denitrification is limited. Nevertheless, even at the extreme range of our 32 x treatment, the "tertiary treatment" function is impressive, with over 15% of the added nitrogen removed from the system. Of course in nature there may be additional complexities. In areas with large nitrogen inputs that are less well mixed than the MERL tanks, prolonged periods of low oxy- gen or anoxic bottom water may occur. Un- der these conditions the availability and production of nitrate may be much reduced and thereby result in a decrease in denitri- fication. The few recent direct measure- ments of denitrification cited earlier, those reported here, the common observation of low N: P ratios in coastal waters, and mea- surements of sediment-water nutrient flux- es (Nixon 1981; Kemp et al. 1982; Hopkin- son and Wetzel 1982) all support the belief that nearshore environments are important sinks in the marine nitrogen cycle. How- ever, our experimental results, and the lim- ited historical and comparative studies from heavily impacted natural systems, suggest

that the homeostatic capacity of coastal eco- systems can be exceeded by nitrogen inputs of a magnitude experienced by an increasing number of estuaries, bays, and lagoons.

This research was supported by the Office of Sea Grant, NOAA, U.S. Department of Commerce. We thank E. Laws for his re- views of the manuscript.

Sybil P. Seitzingerl Scott W. Nixon

Graduate School of Oceanography University of Rhode Island Narragansett 02882-1197

References BEROUNSKY, V., AND S. W. NIXON. 1985. Eutrophi-

cation and the rate of net nitrification in a coastal marine ecosystem. Estuarine Coastal Shelf Sci. 20: 773-781.

BILLEN, G. 1982. An idealized model of nitrogen re- cycling in marine sediments. Am. J. Sci. 282: 512- 541.

CHATARPAUL, L., J. B. ROBINSON, AND N. K. KAUSHIK. 1980. Effects of tubificid worms on denitrification and nitrification in stream sediment. Can. J. Fish. Aquat. Sci. 37: 656-663.

CRUTZEN, P. J. 1970. The influence of nitrogen oxides on the atmospheric ozone content. Q. J. R. Me- teorol. Soc. 96: 320-325.

ELMGREN, R., AND J. B. FRITHSEN. 1982. The use of experimental ecosystems for evaluating the envi- ronmental impact of pollutants: A comparison of an oil spill in the Baltic Sea and two long-term, low-level oil addition experiments in mesocosms, p. 153-165. In G. D. Grice and M. R. Reeve [eds.], Marine mesocosms. Springer.

HAHN, J., AND P. J. CRUTZEN. 1982. The role of fixed nitrogen in atmospheric photochemistry. Phil. Trans. R. Soc. Lond. Ser. B 296: 521-541.

HENRIKSEN, K., M. B. RASMUSSEN, AND A. JENSEN. 1983. Effect of bioturbation on microbial nitro- gen transformations in the sediment and fluxes of ammonium and nitrate to the overlying water. Ecol. Bull. 35: 193-205.

HOPKINSON, C. S., AND R. L. WETZEL. 1982. In situ measurements of nutrient and oxygen fluxes in a coastal marine benthic community. Mar. Ecol. Prog. Ser. 10: 29-35.

HUNT, C. D., AND D. L. SMITH. 1982. Controlled marine ecosystems-a tool for studying stable trace metal cycles: Long-term response and variability, p. 1 11-122. In G. D. Grice and M. R. Reeve [eds.], Marine mesocosms. Springer.

1 Present address: Academy of Natural Sciences, Di- vision of Environmental Research, Philadelphia, Pennsylvania 19103.

This content downloaded from 185.44.77.89 on Wed, 18 Jun 2014 14:55:16 PMAll use subject to JSTOR Terms and Conditions

Page 9: Eutrophication and the Rate of Denitrification and $N_2O$ Production in Coastal Marine Sediments

Notes 1339

KELLY, J. R. 1982. Benthic-pelagic coupling in Nar- ragansett Bay. Ph.D. thesis, Univ. Rhode Island. 195 p.

KEMP, W. M., R. L. WETZEL, W. R. BOYNTON, C. F. D'ELIA, AND J. C. STEVENSON. 1982. Nitrogen cycling and estuarine interfaces: Some current con- cepts and research directions, p. 209-230. In V. S. Kennedy [ed.], Estuarine comparisons. Aca- demic.

KoiKE, I., AND A. HATTORI. 1978. Simultaneous de- terminations of nitrification and nitrate reduction in coastal sediments by a 15N dilution technique. Appl. Environ. Microbiol. 35: 853-857.

MEYBECK, M. 1982. Carbon, nitrogen and phos- phorus transport by world rivers. Am. J. Sci. 282: 401-450.

NIXON, S. W. 1981. Remineralization and nutrient cycling in coastal marine ecosystems, p. 111-138. In B. J. Neilson and L. E. Cronin [eds.], Estuaries and nutrients. Humana.

,,D. ALONSO, M. E. PILSON, AND B. A. BUCKLEY. 1980. Turbulent mixing in aquatic microcosms, p. 818-849. In Microcosms in ecological research. DOE Symp. Ser. CONF 781101. NTIS.

, AND M. E. PILSON. 1983. Nitrogen in estu- arine and coastal marine ecosystems, p. 565-648. In E. J. Carpenter and D. G. Capone [eds.], Ni- trogen in the marine environment. Academic.

, AND OTHERS. 1984. Eutrophication of a coast- al marine ecosystem-an experimental study using the MERL microcosms, p. 105-135. In Flows of energy and materials in marine ecosystems: The- ory and practice. NATO Adv. Res. Inst. Proc. Ple- num.

PILSON, M. E., C. A. OVIATT, AND S. W. NIXON. 1980. Annual nutrient cycles in a marine microcosm, p. 753-778. In Microcosms in ecological research. DOE Symp. Ser. CONF 781101. NTIS.

, G. A. VARGO, AND S. L. VARGO. 1979. Replicability of MERL microcosms: Initial ob- servations, p. 359-381. In Advances in marine environmental research. U.S. EPA-600/9-79-035. Narragansett, R.I.

SCHINDLER, D. W. 1981. Studies of eutrophication in lakes and their relevance to the estuarine environ- ment, p. 71-82. In B. J. Neilson and L. E. Cronin [eds.], Estuaries and nutrients. Humana.

SEITZINGER, S. P. 1982. The importance of denitri- fication and nitrous oxide production in the ni- trogen dynamics and ecology of Narragansett Bay, Rhode Island. Ph.D. thesis, Univ. Rhode Island, 145 p.

, S. W. NIXON, AND M. E. PILSON. 1984. Deni- trification and nitrous oxide production in coastal marine ecosystems. Limnol. Oceanogr. 29: 73-83.

, AND S. BURKE. 1980. Deni- trification and N2O production in near-shore ma- rine sediments. Geochim. Cosmochim. Acta 44: 1853-1860.

, M. E. PILSON, AND S. W. NIXON. 1983. N20 production in near-shore sediments. Science 222: 1244-1246.

SORENSON, J. 1978. Capacity for denitrification and reduction of nitrate to ammonia in a coastal ma- rine sediment. Appl. Environ. Microbiol. 35: 301- 305.

U.S. EPA. 1982. Chesapeake Bay Program technical studies: A synthesis. U.S. EPA, Chesapeake Bay Program. Annapolis, Maryland.

WALSH, J. J., G. T. ROWE, R. L. IVERSON, AND C. P. McRoy. 1981. Biological export of shelf carbon is a sink of the global CO2 cycle. Nature 291: 196- 201.

WANG, W. C., Y. L. YUNG, A. A. LACIs, J. Mo, AND J. E. HANSEN. 1976. Greenhouse effects due to man-made perturbations of trace gases. Science 194: 685-690.

WEISS, R. F., AND B. A. PRICE. 1980. Nitrous oxide solubility in water and seawater. Mar. Chem. 8: 347-359.

Submitted: 3 May 1983 Accepted: 4 June 1985

This content downloaded from 185.44.77.89 on Wed, 18 Jun 2014 14:55:16 PMAll use subject to JSTOR Terms and Conditions