Estuarhze, Coastal and Shelf Science (I982) x4, xox-x x6

Sediment Nutrient Regeneration in Three North Carolina Estuaries

T. R. Fisher" Duke University ~]/Iarine Laboratory, Beaufort, North Carolina 285z6, U.S.A.

P. R. C a r l s o n ~ University of North Carolbza, Institate of 3Iarhze Sciences, l~orehead City, North Carolhla 28577, U.S.A.

a n d R. T. B a r b e r Duke University ]~larbte Laboratory, Beaufort, North Carolhta 285z6, U.S.A.

Received x6.Tuly ~98o

Keywords: Nutrients; phytoplankton; sediments; estuaries; primary production

Sediment-water column exchanges of oxygen, ammonium, nitrate and phos- phase were measured in three North Carolina estuaries by means of diver- installed chambers placed in the sediments. Significant fluxes wcrc observed in two of the estuaries characterized as organic-rich, dcpositional environ- ments. The third was a highly flushed system dominated by sandy to shelly muds, and no measurable fluxes wcrc found. In the former two estuaries, fluxes were weakly influenced by temperature, and ammonium and phos- phate fluxes were highly correlated. Nitrate fluxes were very small, and phosphate sorption was frequently observed at temperatures less than x5 ~ Data from this research and the literature show a general correlation of sediment inorganic N and P fluxes and the computed water column N and P uptake, demonstrating that sediments supply, as an annual average, 28-35% of the N and P required for the primary production of shallow marine systems.

Introduction

The inorganic forms of nitrogen and phosphorus imported into estuaries and onto continental shelves via rain, river runoff and nitrogen fixation (new N and P) have been shown to supply less than the amounts required by primary producers (Dugdale & Goering, x967; Haines, x976; Windom et al., i975~ Stanley & Hobble, x977; Kuenzler et al., x979; Harrison & Hobbie, x974; Nixon, i98o ). In situ regeneration and recycling (old N and P) must, therefore, supply the remainder of the photosynthetic nutrient demand for sustained productivity.

Two primary mechanisms of nutrient regeneration have been described: heterotrophie excretion in the water column and sediment-water column exchange. Heterotrophic excre- tion in the water column is conveniently divided into r, vo categories based on size differences

* Supported by NSF OCE 76-82o84. bPresent address: University of Maryland, Itorn Point Environmental Labora- tories, Cambridge, MD 2x6x3, U.S.A.

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ioa T. R. Fisher, P. R . Carlson ~ R. T. Barber

and experimental techniques. Using the terminology of Sieburth et al. (x978), metazoo- plankton (>zoo p) have been shown to supply less than lO~o of the phytoplankton nutrient demands in shallow systems (Williams et al., x968; Smith, I978 ). Microheterotrophs ( < zoo ~t: protozoo-, myco- and bacterioplankton) appear to supply much more recycled N and P than the metazooplankton. So few measurements (Caperon et al., 1979; Harrison, i978; Johannes, i964) are available, however, that quantitative assessments are still pre- mature. The isotope dilution method used by Caperon et al. (1979) and Harrison (I978) for obtaining microheterotroph community excretion rates promises to be a useful tool in this area, and more measurements are needed to quantify this contribution of N and P to photo- synthetic nutrient demand.

The other major mechanism for nutrient cycling is the release of inorganic N and P from sediments, and considerable research has been conducted in this area in the last 5 years. In this paper we report our measurements of sediment N and P release from several estuaries in North Carolina, and compare then to N and P demand in the water column. We also attempt to summarize existing data on sediment nutrient release and set it within the above perspec- tive of old and new sources of N and P required for phytoplanktonie photosynthesis.

R e s e a r c h a r e a s

This research was conducted in three North Carolina estuaries of contrasting hydrologic character ('/'able x). The South River (Figure I) is a coastal plain estuary connecting with the Neuse River estuary in the Pamlico Sound complex. The outer banks of North Carolina damp out lunar tidal oscillations in this area, but wind-driven tides occur in Pamlico Sound which influence water levels in connecting areas such as the Neuse and South River estuaries. The major contrasts between the latter two systems are the size and freshwater input rates. The Newport River estuary is somewhat similar to the South River estuary in size and freshwater flow but the Newport is directly on the coast and constantly flushes via the high- salinity shelf waters carried by lunar tides. On a gradient of water residence times, South> Neuse> > > Newport. The flushing behavior and volume of the freshwater flow have a great effect not only on the sediment nutrient release, but also on levels of primary productivity. There were four stations in the South River estuary, one in the Neuse River estuary and two in the Newport River estuary.

TABLE I. Comparison of some physical characteristics of the thrco estuaries investigated

South River Neuse River Newport River estuary estuary estuary

Surface area ('kin l) Mean depth (m) Tidal range (m) Mean tidal volume (xo 6 m ~) Tidal frequency

Area of drainage basin (xooo km') Tidal range/mean depth

2:5 2 ' 0

0"2:

5 " 0 "-, 2 w e e k - I

(wifid-driven) 0 " 2

0 " I

Longitudinal extent of estuary (kin) 14 Mean freshwater input (m s s- 1) x'o "Ib'picai peak currents (m s -1) o.x Character of drainage basin coastal plain

400 z7 3"0 x'o o.z 0.8

80.0 '~2"o ,"-2 week -a ,-,2 day -a

(wind-driven) (lunar) I I ' O I ' I

o'o7 o'8 67 18 55"0 5"0 0"2 I ' o

piedmont coastal plain

Nutrient regeneration Io 3

Area enlarged

below

...o.

" Newporl R i v e r

estuor~ ~or~h~o~

Pomiico

" estt

At lan t ic Ocean

Cope Lookout

Figure I. Research areas. The locations of the Neuse River, South River and New- port River estuarie's are shown on this map of eastern North Carolina.

Methods

The fluxes of oxygen, ammonium, phosphate and nitrate across the sediment-water interface were measured by trapping a volume of water above the sediments in clear, one-quarter inch thick Plexiglas cylindrical chambers (diameter = o.6 3 m, height = o.zo m, area = o.3I m 2, total volume = 6z 1). Concentration changes (lamol I - I h - l ) in the water were measured over time. Flux rates (pmol m 2 h -x) were calculated from concentration changes using the known surface area and computed water volume (estimated from the height of the water column trapped above the sediments after insertion of the chamber).

T h e chambers were stirred by means of a xz-V submersible bilge pump ( ' rule 5oo') with an oil-sealed universal motor. Pump speed was controlled by reducing the voltage applied to the pump by means of a variable resistor in parallel with the motor. Tile pump sat on top of the chamber (Figure z) withdrawing water from the top center of the cylinder and distributing it to three locations at the top periphery via L-shaped glass tubes. The result was a gentle rotation of the water with linear velocities near the periphery of o . I - IO cm s -1, depending on pump speed. All measurements were made at low velocities (x- 3 cm s -1) to prevent resuspension and bulk flow of interstitial pore water.

Io 4 T. It. Fisher, P. R. Carlson 6~ 12. T. Barber

Marker )

Sample Oxygen tube IEV D.C. electrode

J Marker float line

pump L-shaped tube

. Sample volume replacement

:'.:.-'c a be .:~'.?:;: ~:.-.-)....--..-......... :......-.-..

Figure 2. ' Benth ic c h a m b e r s ' u sed to obta in ne t s e d i m e n t - w a t e r co lumn exchanges o f oxygen, a m m o n i u m , ni t rate and phospha te . C h a m b e r dimea'zsions: d iamete r = 0"63 m , he igh t = o.2o m , total vo lume = 62 1. T h e s e chamber s operate well at dep ths u p to 5 m , and can easily be modif ied for deeper work.

Continuous records of oxygen concentration were obtained with a YSI model 57 oxygen meter connected to a Linear model I42 portable chart recorder. The electrode was inserted into the chamber near one of the pump outlets to eliminate the need for stirring.

Between four and eight discrete samples for nutrient analyses were withdrawn from the chambers over a 2-4-h period. Less than ioo ml were withdrawn for each sample, which was then filtered and subdivided for ammonium, nitrate and phosphate analyses. Ammonium and phosphate reagents were added immediately in the field, and later read on a spectrophoto- meter in the laboratory. Nitrate (+nitri te) samples were refrigerated in the dark and analysed within 48 h. All methods were miniaturized versions of standard techniques requiring 5-20 ml (Fisher & Parsley, I979). During chamber operation less than 0. 5 1 was withdrawn from the chamber (< 2~o of typical chamber operating volume), and no correction for withdrawal of sample volumes was made. However, a I5-cm piece of tubing (I mm internal diameter) protruded from the chamber to allow bottom water to flow into the chamber during sample removal.

In order to obtain measurements in a given area, a small boat was anchored securely at three points to prevent drift. Two chambers were then inserted by a diver into the sediments

Nutrient regeneration 1o 5

within several meters of each other. Great care was used to prevent disturbance of the sediments under the chambers. The pump cords, sampling tubes and oxygen electrode cables were led inboard and connected, and the measurements begun. Chamber runs typically began around noon, but the amount of light at the bottom was frequently negligible (see Discussion following). The measurements were made during the period September 1977 to December 1978.

Results

Typical results from the Neuse River and South River cstuaries are shown in Figure 3 at different temperatures. At temperatures > x 5 ~ oxygen was consumed and both phosphate and ammonium were released; at temperatures < 15 ~ the oxygen and ammonium fluxes were reduced, and phosphate sorption, rather than release, was observed (see also Figure 4). Significant nitrate release to the water (P< o'os) was observed only occasionally during the course of the study, and nitrate uptake by the sediments was never found.

The consumption of oxygen by the sediments was continuous, consistent and usually linear over time. However, in several experiments performed under a strongly stratified

s I ~ I ; I = 250 L. 'A

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"~'et 200

4 5 0 I - ~ 0 1 " 150

(o) 00"0 1

425 I I t I t I t I 00

0.4. -1.74./.,reel P04 m-Zh -I 1.5

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0.5

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i i i i i I i

2 3 4 0 / t m o t 0:, Mz h "j 0 . 0 p m o l NO s m "2 h " l

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I 219/.Jmol NH 4 m'Zh -I !

t I t I t I t

I.o

o.s .z.

0

0 I 2 3 4 0 I 2 3 4 Time (h) Time (h )

Figure 3- Typical results from the use of benthic chambers at low (x r and higher (22 ~ temperatures. (a) South River estuary Station 5 on x February 1978 at x ~ (b) South River estuary Station 2 on 26 iXiay 1978 at 22 ~ All concentrations within the chamber showed significant changes with time, with the exception of ammonium on I February ]978 and nitrate on 26 ]Xlay 1978.

lO6 T. R. Fisher, P. R. Carlson &, R. T. Barber

-~" 6 ~ ~ I ~ I

'" t �9 o E �9 �9

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i I

T I t I l I

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I T I i I

�9 I L

I I . ee l I I I0 20 30

T e m p e r a t u r e (~

Figure 4. Relationship between sediment-water column fluxes and water tempera- ture. For selected seasons there aro significant correlations (P<o'os) , but overall the correlations are marginal (r 2 = o.2-o.4, o'z >.P>o.ox). Note phosphate sorp- tion at lower temperatures.

water column and low in situ oxygen conditions, oxygen consumption was dependent on concentration (partial pressure) when oxygen was less than ,-, 60 pM, or ,.~ 25% of saturation. This resulted in decreasing oxygen uptake rates over time; i.e. logarithm of oxygen concen- tration vs. time was linear. Except in experiments >6--xo h, ammonium, phosphate and nitrate release rates from sediments were unaffected and remained linear. For comparison with the other fluxes, oxygen consumption in these experiments was computed from the initial concentration independent portion of the curve.

The concentration at which oxygen uptake becomes dependent on its concentration is probably a function of the horizontal shear within the chambers. We attcmpted to match the chambers' internal velocities to estimated average velocity conditions outside the chamber, and the 6o/aM value reflects average velocity conditions in the Neuse and South River estuaries.

In the Newport River estuary, the chambers did not function well because of tidal current erosion of the sediments (sand to sandy, shelly mud) around the chamber. However, even during measurements at slack tide, there were no concentration changes in the chamber,

Nutrient regeneration xo7

suggesting that the oxygen and nutrient flux rates were very low and comparable to the production or uptake rates by plankton trapped within the chamber. In contrast, the cham- bers were very successful in the other, less energetic environments and most of the subsequent discussion concerns these latter estuaries.

Statistics and seasonal ranges of the data are given in Table 2. There are no significant differences between the data for the Neuse River and South River estuaries ( P > o q ) with the exception of ammonium release, which was significantly greater in tile Neuse River estuary ( o . o 5 > P > o ' o t ) . Comparing the one Ncuse River station with the closest South River station, there were no significant differences ( P > o . 0 including ammonium release. This suggests that the sample size was too small to estimate statistically any differences in nutrient fluxes between the taro basically similar systems.

TABLE 2. Statistics of the sediment-water column fluxes. All values are in ltmol m 2 h -x. Positive values are release to the water column, and negative values repre- sent loss from the water column

South River Neuse River Newport River estuary estuary estuary

Oj mean --x77o --15xo o ' o

s.E. 205 260 - - range ( - - 8 2 5 ) - ( - - 3 x 5 o ) ( - - 815 ) - ( - -216o ) - - n x 4 5 3

NItt mean xx3 224 O'O

S.E. X8"9 5X'4 - - range 0"0-267 70"6--454 - -

n 2 I 7 3

mean 6.2 5 x4"o o ' o

s.c. 2.x6 6"07 - - range (--8"34)-22"8 (--2"28)-46'0 - -

n 2 x 7 3

mean x .42 3.2I o ' o

s.E. 0"932 3"2t - - range 0"0-5"78 o'o-6"42 - - n 8 z 3

PO~

NO,

The effect of temperature on sediment-water column exchange processes is shown in Figure 4. For these relationships r 2 varies between o.2 and o. 4 and is only marginally signifi- cant ( o . t o > P > o . o t ) , indicating that temperature is influencing sediment metabolism and exchange, but that other processes arc probably more important. A log transform does not improve the correlation (o.t < r 2 < o.4). The data shown in Figure 4 arc similar in magnitude to data obtained by other investigators (e.g. Rowe et al., I977; Hale, x975; Pamatmat, x968 ) in other shallow marine systems.

There are no significant relationships between oxygen consumption and either ammonium or phosphate release (P>o.x) ; however, there is a significant correlation ( o . o s > P > o . o i ) between tile ammonium and phosphate release rates, as shown in Figure 5- The slope of the line through these data is an indication of tile relative mobility of N and P in sediment nutrient fluxes. However, as can be seen in this data set, such an approach yields an under- estimate of the N : P ratio because of the significant intercept (N value) at P = o, probably caused by phosphate sorption (uptake of phosphate by ferric hydroxides and other complexes) and heterotrophic utilization of phosphates. Using the slope alone (N : P = 7.x) ignores the contribution of these other important processes represented by the positive value of the intercept (at P = o). Computing N : P painvise using only those data whose values of

Io8 T. R. Fisher, P. R. Carlson ~ R. T. Barber

k: 7 E

5 0 0

4 0 0

B 3 0 0 E

~ 2 o o

i

I00

t I i I 0 I i I I I o

0

�9 8 �9 g

. o

0 v % l v I t I T I I I I - I 0 0 I0 2 0 30 4 0 50

PO 4 re lecse ( / . tmol m'Zh "1)

Figure 5. Ammonium and phosphate exchange betnveen sediments and water column. These exchanges are significantly correlated (r 2 = 0-06, P<o'o5). For discussion ofN : P, see text.

phosphate exchange were greater than zero yields an average N : P ratio of i6 (range 5.2- 28), representative of the warmer portions of the year (April-November). This value is in agreement with the average N : P composition of phytoplankton (Redfield et aL, x963). However, because several processes may be co-occurring, the release of ammonium and phosphate in accordance with the average composition of phytoplankton must be interpreted cautiously, especially since these estuaries are subject to inputs of allocthonous materials of varying N : P.

Small-scale horizontal heterogeneities in sediment exchange rates were assessed from the paired chambers. Differences between nutrient exchanges within the two chambers are expressed as a coefficient of variation (IOO% S.D./mean). For ammonium and phosphate exchange, the coefficients of variation ranged from o to 38~ (average = I5) and o to 14o % (average = 48), respectively. Phosphate exchange was more variable than ammonium exchange, especially in late fall and early spring when the balance of exchange was shifting from net uptake to net release. For nitrate release, the coefficients of variation ranged from 2"I-Z4o~ (average = 72~). Nitrate release was always small relative to ammonium release. The sediments appear to be too reduced to allow significant nitrification to occur, although visual inspection usually suggested an oxidized (brown) layer of surficial sediments.

Discussion

In this paper we have reported rates of exchange of oxygen, ammonium, nitrate and phos- phate betnvecn the water column and sediments of three shallow estuaries in North Carolina. In one estuary strongly flushed by lunar tides, no significant fluxes were observed between the sandy, shelly sediments and water column. In the other two, the organlc-rich sediments exhibited exchange with the overlying water at rates similar to those reported for other environments. Oxygcn consumption was not closely coupled with ammonium or phosphate exchange, but the latter two processes were highly correlated. Nitrate exchange was very small relative to ammonium exchange.

Nutrient re~,eneration t o 9

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F i g u r e 6 . S u m m a r y o f p r o c e s s e s r e s p o n s i b l e f o r n u t r i e n t e x c h a n g e a t t h e s e d i m e n t - w a t e r c o l u m n i n t e r f a c e .

The sediments of these shallow aquatic systems represent storage of former water column material, some fraction of which is eventually returned. There are probably three potential fates of newly sedimented materials (Figure 6): (i) rapid, at least partly aerobic degradation in the surficial sediments (o-io cm) by macro- and microbenthos and return to the water column in less than I year through the combined effects of bioturbation and diffusion; (2) temporary burial in the top meter of the sediments, anaerobic decomposition by micro- organisms and return to the water column over decades or centuries via diffusion and bulk flow of interstitial waters (Berner, x97I); and (3) effective removal from the aquatic system by deeper ( > I m) burial on time scales equivalent to the filling and draining of shallow marine systems by sedimentation and glaciatlon-modulated sea-level changes (Riggs & O'Connor, 1975; Schubel & Meade, 1977). The partitioning between these possibilities will depend on the composition of the material, sedimentation rate, oxygen availability, resuspen- sion etc. Nixon (i98o) has shown that one-quarter to one-half of the organic matter input to shallow marine sediments is oxidized, suggesting that one-quarter to one-half of the material is returned to the water column (fates i and 2), and one-half to three-quarters is buried (fate 3) or is oxidized in the water column.

Evidence for the first two parts of these schemes has been presented by Davies (I975) who showed rapid degradation and return of newly sedimented material (zooplankton fecal pellets) folloMng a spring-phytoplankton bloom. This caused a seasonal asymmetry in his data which appeared to be a short-term pulse imposed upon a strong seasonal signal. The data from the South River and Newport River estuaries are of a similar nature, except that blooms are more frequent and of lesser magnitude, and tend to alternate with allocthonous inputs of terrestrial materials associated Mth runoff events (Hobble & Smith, I975; Barber et al., I98o ). Accordingly, we observed a poor correlation between the fluxes and temperature over the entire seasonal cycle, although shorter time periods could be found with good correlation. Hale (x975) reports similar results. We suggest here that the non-seasonal short- term pulses of sediment nutrient regeneration are due to rapid (probably aerobic) degradation

r Io 7'. R. Fisher, P. R. Carlson ~ R. 7". Barber

of newly sedimentcd materials at the sediment surface, whereas the seasonal temperature- related regeneration represents diffusion and bulk flow of interstitial waters from deeper ( < x m) sediments to the water column. There are no data on the relative magnitudes of these two types of nutrient regeneration processes within sediments, but it is clear that they arc of the same order of magnitude, at least on a daily basis, but not necessarily on an annual basis. The relative independence of oxygen uptake and nutrient release in our data could be due to tile differential dependence of these fluxes on short-term or longer-term degradation of organic matter.

The longer-term nutrient fluxes may be seasonal in nature because of temperature effects on diffusion and benthic faunal/floral metabolic activity and because of bulk flow of inter- stitial water due to seasonal groundwater hydraulic gradients. In two cores from the South River estuary, we observed a continuous decrease in salinity in interstitial water to less than 3~oat 3 ~ cm depth, well below the mean annual salinities for these stations. This suggests a hydraulically driven bulk flow of groundwater through these sediments similar to that reported for lakes (Winter, i979). Lee (i977, x98o ) and Smctaeek et al. (i976) have reported pumping of interstitial waters by lunar tides, and a longer-term groundwater-driven flushing mechanism may exist as well, contributing to the io-iooo-year nutrient return postulated in Figure 6.

The diffusion of materials such as ammonium and phosphate, normally a very slow proccss, is the rcsult of the large concentration gradients between interstitial pore waters, typically in the mmol 1 -I range (Berner, x971 ), and the overlying water column, typically in the Itmol 1-1 range. A ioo--Iooo-fold concentration gradient thus drives fluxes of phosphate and ammonium into the overlying water. The concentrations found within interstitial waters are the result of long water residence times within the sediment, heterotrophic degradation of organic matter and in situ formation and dissolution of minerals. The formation of ferric phosphate complexes is well known (e.g. Hutchinson, 1957); however, Martens et al. (1978) have also reported in situ formation of struvite and vivianite. Both of these minerals involve phosphate, and struvite also involves ammonium equilibrium within sediments. Other processes such as sorption stabilize interstitial concentrations (Nelson, x96z; Rosenfcld, i979). These ideas are summarized in Figure 6.

The nutrient store in the sediments acts to smooth the supply of new nutrients, enabling shallow marine ccosystems to maintain productivity between nutrient pulses from new sources such as river runoff, or shelf onwelling/upwelling events. The relative magnitudes of the short-term and longer-term regeneration will determine the rapidity of the nutrient return to the water column.

This research was initiated in order to assess the relative importance of sediment-water exchange as a proccss in the nitrogen and phosphorus cycles of shallow marine systems. The data obtained are of the same order of magnitude rcportcd by other investigators using differ- ing techniques in other marine systcms. Table 3 is a summary of available data. With the exception of the sandy sediments of La Jolla Bight and the Newport River estuary, average values of oxygen uptake and inorganic nitrogen release from sediments of the other eight environments were similar, ranging from --xx to --4 z and from x to io mmol m --~ day -x, respectively. Phosphate exchange was more variable; ranging from o.o 3 to x.z mmol m -z day -I, probably due to the greater importance of sorption and mineral equilibria. The eight areas were characterized in the references as depositional environments with soft organic sediments, whereas the La Jolla Bight and the Newport River estuary are both sandy, relative- ly non-depositional environments. The sediment surface of La Jolla Bight receives sufficient light to sustain a benthic microflora, which influences the nutrient flux to the water column

Nutr ien t regeneration x x x

TABLE 3. Values of sediment-water column exchange rates in mmol m -z day-L Positive values are net release to the water column, and negative values represent loss from the water column. YDIN (dissolved inorganic nitrogen) = NO~+NO~+ NtI~

Environment (region) NOs NOz NHa EDIN PO~ Oz Reference

South River estuary 0"03 - - 2"7 2"7 o'x5 --4z m (NC, U.S.A.)

Neurse River estuary 0"07 - - 5"4 5"5 0"34 --36 (NC, U.S.A.)

Newport River estuary o'oo - - o.o o'o o.oo o (NC, U.S.A.)

La Jolla Bight (CA, U.S.A.) o'o8 0"03 0"87 0.86 0"077 - - i i Hartwig, I974 Narragansett Bay --0"03 o-o 4 z.x z.i o's3 --z8 Ilale, x975

(RI, U.S.A.) Buzzards Bay (MA, U.S.A.) o'o 5 o'oz 1"3 1.4 --0"03 --23 Rowe et al., 1975 Coastal North Sea z'3 - - 1"8 4"x - - - - Billen, x978

(Belgium) Offshore North Sea x-z - - 0"90 3"x - - - - Billen, x978

(Belgium) Loch Thumaig (Scotland) o'o o.o 0"96 0"96 - - -- x x Davies, x975 Cap Blanc (Africa) 3"8 0"34 5'6 9"7 x.z - - zo Rowee ta l . , x977

Average 0"73 0'09 2-2 3-0 0.28 - -z t -t-S.E. o"4z 0"06 o'6o o'9o o'I6 4"9

(Hartwig, 1978). Likewise the shallow Newport with its sandy, shelly bottom, is too energetic an environment for large-scale sediment accumulation. There is a black reducing zone 5 - i o cm down in the sand, and undoubtedly there are fluxes of materials across this interface. However, the rates are too low to be accurately measured with the method used here. Tlmycr (i974) has reported that bacteria and phytoplankton compete for the small quantities of nutrients available in the water column of this estuary, and the uptake of ammonium and phosphate by plankters t rapped within the chamber might potentially keep pace with the flux across the sediment interface in this nutr ient- impoverished estuary.

T h e relative impact of planktonic uptake of ammonium and phosphate within the chambers is an important technical question concerning the use of tile benthic chambers. T h e concen- tration changes observed within the chambers are the result of several processes described by the equation:

dc .4 = Rs ~ + r p - u p ,

where c is the nutrient concentration 0tM), t is t ime (h), Rs is tile sediment nutrient regenera- tion (pmol m -"~ h - l ) , .4 is tile surface area of sediment within the chamber (me), V is the volume of water within the chamber (1), % is planktonic nutrient regeneration (pmol 1-1 h -1) and ttp is planktonic nutr ieat uptake (pmol 1-1 h-X). In most studies of benthic regeneration, the first term is assumed to be quite large relative to the latter two, or a control bottle is used for monitoring net concentration changes driven by planktonic processes ( rp- - t%, usually found to be small or zero). However, a bottle is not a true control because the major concen- tration changes occur within the chamber, not the bottle. Because there is an hyperbolic relationship between nutrient uptake and nutrient concentration, the bottle ' con t ro l ' does not mimic conditions within tile chamber and may underest imate the uptake term.

~SN-labelled ammonium uptake was measured in the water column during chamber runs (Fisher et al., in preparation), from which it is possible to evaluate the magni tude of ttp

x x2 T. I~. Fisher, P. R. Carlson ~ R. 2". Barber

relative to dc]dt for ammonium. For each chamber, ammonium uptake rates were calculated for the average light and nutrient conditions prevailing during a chamber measurement period. Uptake rates were, in each ease, for the same station and day as the chamber measure- ments. In the ease of ammonium, uptake within the chambers (up) was equivalent to an average of x ,% (s.n. = 4"7, range = 0.5-220%) of the observed concentration changes (de/dr), ignoring those experiments where no concentration change was observed within the chamber. Because of the turbidity of these estuaries, dark uptake rates corrected for concen- tration were appropriate for the above computations; therefore, the use of darkened chambers would not have reduced the magnitude of the planktonic uptake within ttle chamber.

These calculations suggest that the data presented here on sediment nutrient fluxes are slightly underestimated because of uptake by plankton within the chamber. However, regeneration by heterotrophie plankton within the chamber (rp) will reduce the magnitude of the underestimation (and could possibly cause overestimation). Harrison (x978) and Caperon et al. (, 979) have shown a good correlation of water column uptake and regeneration, which, if generally true, will cause the rp--% term to be zero. Nevertheless, until three of the four variables in the above equation are measured, a quantitative assessment is impossible. The preliminary data presented here suggest that up is, in fact, small relative to Rs(A/V) , and that deldt largely reflects Rs(A/V) .

In order to estimate the quantitative importance of sediment-water column exchange as a process in the nutrient cycles of shallow marine systems, we have assembled phytoplankton productivity data from the literature on the environments listed in Table 3 for which sedi- ment nutrient release data were available. From the carbon productivity data we have estima- ted N and P phytoplankton demand using Redfield atomic ratios (io6 : x6 : i), and then computed, on an average daily basis, the proportion of that demand supplied by sediment release (Table 4). The values range from o to 79% (average = 35%) for nitrogen and from o to 75% (average = a8%) for phosphorus. For the North Carolina estuaries, we also have data on directly measured ammonium uptake rates, which are about one-half of the com- puted DIN demand. For simplicity in discussing the results from other investigations, we have used only the computed total nitrogen demand. When considering only ammonium turnover in the water column, the sediment nitrogen fluxes are more significant than Table 4 suggests because the ammonium fltux dominates the ZDIN release.

The data of Table 4 are illustrated in Figure 7, which demonstrates a correlation between water column N demand and benthic EDIN release. T h e ' • ' on tile graph is the data from La Jolla Bight. Omitting this point improves the r 2 from o-59 to o.77 , in either case demon- strating a strong relationship between sediment DIN release and computed water column DIN demand. Sediment phosphate release is also correlated with calculated water column P demand (r 2 = o'65; o'74 without La Jolla data). These data demonstrate that tile absolute rates of benthic nutrient recycling increase with increasing system productivity, suggesting that sediment accumulation and nutrient regeneration are ecosystem mechanisms which scale recycling to inputs of new N and P. Eppley et al. (i979) have suggested that ammonium regeneration acts as a 'multiplier of the nitrate input', and sediment accumulation is one mechanism which could act in this manner. However, considering the sediment nitrate fluxes in Table 3, it is probably best not to characterize nitrate as strictly a 'new' N source, as originally proposed by Dugdale & Goering (x967).

The data represented in Tables 3 and 4 and Figure 7 are consistent with a recent review of the literature on shallow marine systems by NLxon (x98o), who showed that bet~veen one- quarter and one-half of the allocthonous carbon inputs plus autocthonous carbon primary

�9 productivity are oxidized by tile sediments. The data in Table 4 do not include allocthonous

Nutrient regeneration x 13

~176 o

o~ vC.

0

~.~_

~ "g z

0

"6

2

9 . ~ '~, ~ I I I

() ~ 0 0 0 0 0 0 0 ~,

9.r-

~ m.......~r~,~o 0 ~ 0 ~ 0 0 0 0 0

"-:z .<

~ z ~ ' ' ~ ~ 1 7 6 2~,-I Z m o O ~ o

0 0

, ~ o 0

0 el

0 o9..~ 0 0

I x4 T . R . Fisher, P. R. Carlson ~ R. T. Barber

I0

8

8 - - 6

Z

2

1 | �9

�9 X

J I I I0 20

Water column N demand (mmol N m -z day -j)

0 30

Figure 7. Sediment inorganic nitrogen ( E D I N = N H 4 + N O : + N O 3 ) release and calculated water column nitrogen demand for the io shallow marine systems listed in Tables 3 and 4. These two processes are significantly correlated (r ~ = 0-59 , P<o-os ) and demonstrate the importance of sediment recycling in supplying inorganic nitrogen for primary production in the water column. Phosphate release is similarly coupled with calculated water column P demand (r ~- = o'65, P<o 'o5) . • in this figure is La Jolla Bright data, influenced by benthic algae and upwelling.

inputs and, unlike Nixon's (198o) data, there is no significant corrclation of sediment oxygen consumption and water column carbon inputs. However, assuming an R/Q of I-o, tile oxygen demand of the sediments represented about one-half of the water column primary production. Including estimates of allocthonous inputs would have made these data even more similar to those of Nixon (x98o).

It is clear from Table 4 and Figure 7 that sediment nutrient fluxes play an important recycling role in tile nutrient cycles of most shallow marine systems. Furthermore, there seems to be a loose coupling between nitrogen and phosphorus fluxes from the sediment and nitrogen and phosphorus water column demand. It can be argued either way which is cause and which is effect, since both processes are linked on the one hand by sedimentation rate and on the other by nutrient availability in the water column. We feel that future research should conccntrate on mechanisms which couple new and old nutrient supplies (input and recycled N and P, sensu Dugdale & Goering, x967) and their impact on ecosystem produc- tivity. Further quantification of planktonic nutrient regeneration will provide valuable information on a poorly known recycling mechanism. In addition, investigation of sediment nutrient release as a function of net sedimentation or sediment type is also suggested as a useful direction for future research.

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