MARINE ECOLOGY PROGRESS SERIES Vol. 197: 103-119.2000 Published May 12 Mar Ecol Prog Ser l

Seasonal benthic organic matter mineralisation measured by oxygen uptake and denitrification along a transect of the inner and outer River Thames estuary, UK

'Department of Biological Sciences. University of Essex. Colchester C04 3SQ. United Kingdom 'Centre for Environment Fisheries and Aquaculture Sciences, Lowestoft NR33 OM, United Kingdom

ABSTRACT: Seasonal measurements of organic matter mineralisation by oxygen uptake and denitrifi- cation were carried out from July 1996 to March 1998 along a -200 km transect of the River Thames estuary, UK. There was a distinct gradient of decreasing rates of organic matter mineralisation sea- ward, which was related to the concentration of suspended solids and sedimentary organic carbon (C) at each site. There was clear seasonality and highest rates of oxygen uptake (10056 pm01 O2m-2 h-l) at the muddy sites, but lower rates and non-temperature-dependent oxygen uptake at the sandier sites. Denitnfication, both that driven by nitrate from the overlying water (D,) and that coupled to nitrifica- tion in the sediment (D,), followed a sirmlar trend to oxygen uptake, from negliqble rates of approxi- mately 1 1.lmol N m-' h-' for both D, and D, at the furthest offshore site, Site 12, to l1 407 and 8209 pm01 N m-' h-', respectively, at the inner muddy Site 1. The Thames estuary is heterotrophic and a very effi- cient organic C filter, trapping and remineralising 77% of its organic C input. Attenuation of fluvial nitrate loads was regulated by freshwater flow. Minimal attenuation (3 %) occurred during peak flows (i.e. during periods of shortest freshwater flushing time) and >loo% attenuation during periods of low- est freshwater flow (longest flushing times). Including the sewage treatment works (STWs) nitrate load in this calculation reduced the degree of attenuation of the nitrate load to, on average, 11 %. Annual rates of D, and D, for an inner area of 125 kmz were 112 and 85 Mm01 N yr-l, respectively, with a total rate of 196 Mm01 N yr-' (2744 t), which was equivalent to 9% of the total dssolved inorganic nitrogen (DIN) load for 1995-96. A mean denitrification rate (D,) of 0.64 m01 N m-' yr-', based on measurements in 4 east coast estuaries, was used to estimate a total rate of denitrification for the entire area of UK east coast estuaries. The total rate of 0.81 Gm01 N yr-' represented 16% attenuation of the total fluvial dis- charge of nitrate (-6 Gm01 N yr-') to the UK's east coast estuaries (1995-96) and hence a 16% reduction in the UK nitrate load to the North Sea.

KEY WORDS: Organic mineralisation . Oxygen uptake - Denitrification - Nutrient attenuation

INTRODUCTION may have anthropogenic components. Estuaries act as traps for both particulate and dissolved organic loads, Estuaries are the major conduits between land and as well as mineral particles. Settlement of organic mat- sea, through which flow the loads of soluble and par- ter in turn stimulates benthic respiration and a rem- ticulate materials derived from the catchment area of ineralisation of organic material. Oxygen is rapidly each estuary. These loads may be derived from both depleted with depth in surface sediments and anoxic leaching and run-off from the land and from atmos- mineralisation proceeds via alternative electron accep- pheric deposition within the catchment, each of which tors. In nutrient-enriched estuaries nitrate is often abundant and may be respired and in turn lost as N2 through denitrification (Nedwell 1975, Seitzinger et al. 'Corresponding author. E-mail: [email protected] 1980, Billen et al. 1985, Law et al. 1991, Nielsen et al.

0 Inter-Research 2000 Resale of fullarticle not permitted Mar Ecol ProgSer 197: 103-119, 2000

1995, Nixon et al. 1996, Ogilvie et al. 1997, Trimmer et the Thames estuary as a biological filter for dissolved al. 1998). inorganic nitrogen (DIN) and organic C. Studies on benthic mineralisation in estuarine and coastal sediments have tended to be either spatially or temporally restricted (see Heip et al. 1995). Although MATERIALS AND METHODS both Kelley et al. (1990) and Middelburg et al. (1996) demonstrated gradients of decreasing mineralisation Sample sites. The tidal Thames estuary covers an rates along estuaries, others have shown little variation area of approximately 131 km2 (Fig. 1) and is the between sediment sites (Hargrave & Philips 1981, Jer- 100 km stretch of the River Thames between Tedding- gensen & Ssrensen 1985, Carnrnen 1991, Trimmer et ton Weir (landward), where it is 100 to 200 m wide, and al. 1998). Nowicki et al. (1997) reported denitrification Southend Pier (seaward), where it is 7 km wide. It data along an extensive gradient from Boston Harbour drains a catchment of 14 000 km2 with a population of and Massachusetts Bay and did, at least for denitrifica- 12 million people. Spring and neap tides at Southend tion, show a clear relationship between sedimentary are 6 and 3 m respectively (Environment Agency un- organic content and rates of mineralisation. However, publ. public access data, Environment Agency Data their study did not include measurements of oxygen Centre, Twirton, UK). uptake the sediiient. ??-m aim cf L!e presezt study This sb~dyof the Thtxes estuary (Fig. 1) was divided was to provide a detailed assessment (spatially and into an inshore and an offshore transect visited be- temporally) of mineralisation rates via oxygen uptake tween July 1996 and March 1998. Initially, from July and denitrification along an estuarine and coastal gra- 1996 to February 1997, in the inner Thames, 6 sites dient and to then use this data to assess the capacity of were visited between Mucking Flats (Site 4) and

OOE2" 4' 6" 8" 10'

Sample sites o R.Lee STWs

R Ravensbournc . :...

Fig.1. Locations of the sample sites in both the inner and outer Thamestransects. STWs: sewage treatment works Trimmer et al.: Mineralisation rates via oxygen uptake and denitniication 105

Southend (Sites 8 and 9). This was followed by a 3 mo collected 30 to 50 cm of sediment and 15 to 25 1 of site crossover period of visiting 9 sites, when the transect water; any cores that had drained were discarded and was extended upstream to Beckton (Site l), where the the box corer was redeployed. Three large sediment major sewage treatment works (STWs) serving London cores (-30 cm deep) and overlying water (1 1) were col- discharge. Finally, from August 1997 to March 1998, lected to measure benthic oxygen uptake at each loca- 6 sites were visited between Beckton and Southend. tion using Perspex core tubes (65 cm long X 8 cm i.d.), The inshore transect was visited with a small (15 man) each sealed at the bottom with a silicon rubber bung. hovercraft 2 h after high-tide (water depth over sedi- Smaller sediment cores (-10 cm deep) and overlying ments at high-tide = 2 to 3 m). Sample site positions water (-100 ml) were collected to measure benfkc were determined using a global positioning system denitrification at the same time using small perspex (GPS) to within -100 m2. The offshore transect ex- core tubes (20 cm long X 3.4 cm id.),each sealed with tended from 16 km from Southend (Site 10, water a rubber bung. Cores from the inner sites were trans- depth at high-tide = 15 m) to the furthest extent of the ported back to the laboratory usually within 3 h. All Thames plume, 122 km offshore (Site 12, water depth cores were then stored open and submerged in aerated at high-tide = 45 m) and was visited on board the RV site water at the in situ temperature until required (<2 'Cirolina'. See Table 1 for locations and Table 2 for to 12 h). The upper part of the inner Thames estuary frequency and dates of visits. suffers from a dissolved oxygen sag (Fig. 2). Therefore, Water column characteristics. Water samples (50 1) storing the cores in aerated site water would have for core storage and analysis were collected by hand in affected the in situ conditions and this would have plastic containers from each of the inner Thames sites. been most dramatic at Sites 1 to 4, where the mean in Bottom water samples (50 1) for storage of offshore sed- situ dissolved oxygen levels were between 55 and 70 % iment cores (see below) were collected using Niskin of air saturation. bottles mounted on a CTD rosette sampler. Subsam- Sediment characteristics. Sediment cores (30 cm X ples of water (20 ml) were taken from each inner estu- 6 cm i.d.) were taken at each site, to a depth of at least ary site for nutrient analysis, filtered (0.2 pm Minisart 15 cm and sealed at both ends with rubber bungs. PlusTM,Sartorius UK Ltd) and fixed with mercuric chlo- Sediment porosity, particle size distribution, organic C, ride solution (200 1-11, 0.2 % w/v; lrkwood 1992). Addi- total nitrogen (N) and chlorophyll concentrations were tional water samples were taken for salinity (200 ml) measured at 1 cm depths on extruded samples down to and chlorophyll (500 ml) analyses. The water and air 10 cm. Porosity was determined from the dry weights temperatures at each site were measured with a mer- and wet weights of known volumes of sediment. Par- cury thermometer and the air pressure for the survey ticle size distribution was measured by re-suspending day recorded. Similar samples were taken at the off- each 1 cm slice in deionised water using an ultrasonic shore sites directly from the Niskin bottles mounted on water bath and wet sieving through a range of stain- the CTD and analysed immediately. All nutrient analy- less steel meshes (2000, 1000, 500, 250, 125 and 63 pm ses (NO3-, NO2-, NH4+,SiOB and Kirkwood 1996) pore sizes). Each fraction was then collected, re-dried were carried out using a continuous flow autoanalyser and re-weighed. Particle size was expressed as a per- (Skalar, SA4000, De-Breada, The Netherlands). Salin- centage of the total dry weight. Each sample for C and ity was measured using a salinometer (Autosal, Guild- N analysis was homogenised with hydrochloric acid line, USA). Suspended loads were measured by filter- (5 ml, 1 M) to remove carbonate, and after re-drying ing known volumes of water through preweighed glass subsamples (20 mg) were analysed for their organic C fibre filters (5 cm diameter, GF/F, Whatman Ltd, UK), and N contents with a CHN analyser (model 2400, drying and reweighing. Chlorophyll was measured by Perkin-Elmer Ltd, Norwalk, Connecticut, USA) cali- filtering between 0.1 and 0.25 1 through GF/F glass brated with standard acetanilide (71.09% C, 10.36% fibre filters and extracting in neutralised 90 % acetone N). Sediment chlorophyll pigments were extracted (2:l for 24 h at 4°C in the dark. Measurements of extracted sediment volume to acetone) and measured as de- chlorophyll fluorescence (Tett 1987) were made scribed above. Sediment temperatures were measured (before and after acidification with 8% hydrochloric with a mercury thermometer inserted into the surface acid to distinguish phaeopigments) using a fluorome- sediment layer. ter (Turner Designs Model 10). Sediment biological rate measurements. Oxygen up- Sediment collection and storage. At each site in the take: The 3 large core tubes were capped and com- inner Thames, sediment samples were collected by pletely water-filled to exclude air bubbles. A mid-col- hand in approximately 50 cm of water. At the offshore umn water sample (20 ml) was then drawn off into a sites, sediment samples were first collected using a gas-tight syringe, gently transferred to an Exetainer NIOZ cylindrical box corer (31 cm inner diameter Ii.d.1, (12 m1 Labco Ltd, High Wycombe, UK) and fixed for Netherlands Institute for Sea Research). The box corer Winkler analysis of dissolved oxygen. The water col- 106 Mar Ecol Prog Ser 197: 103-1 19,2000

umn in each tube was stirred with an induction motor centration of oxygen in the overlying water never fell driving a magnetic follower (Rank Brothers Ltd, Cam- below 80% of air saturation (see above). Following bridge, UK) at 300 rpm (Nedwell & Trimmer 1996). The incubation, the I5NO3- enriched cores were processed rates of benthic O2 uptake were measured using dis- as the reference cores. solved O2 electrodes (model 1302, Strathkelvin Ltd, All the slurry samples were sent to the National Glasgow, UK) coupled to a 6 channel O2 meter (Essex Environmental Research Institute, Silkeborg, Den- Electronics Centre, Colchester, UK). The multi-chan- mark, to be analysed for abundance and concentration nel 0, meter was connected to a computer (V.1.P com- of 28N2, and 30N2on a gas chromatograph coupled puters, UK) with data analysis software (Notelog@, to a dual inlet isotope ratio mass spectrometer (Europa Garrat Consultants, Colchester, UK) which logged, dis- Instruments, Crewe, UK). Denitnfication rates were played and analysed the data. The on-screen computer calculated according to Nielsen (1992). logging enabled the oxygen concentration in the water Calculation of freshwater flushing times, nutrient column to be continuously monitored and statistically loads and attenuation. The Thames estuary was subdi- analysed for linearity with respect to time. Significant vided into 2 main sections according to the type of rates of oxygen uptake (p < 0.05) could usually be bathymetric data available (H. R. Wallingford Ltd, determined within 1 to 4 h. Triplicate incubation charn- UK)-Section 1: Teddington Weir to Westminster beis contakig only site water (500 TJ),with nc sedi- Eridge and Section 2: Westmhster Eridge to Southend ment present, were treated and incubated as above, to Pier. Section 1 was subdivided into 25 sectors, each act as controls for water column remineralisation pro- 1000 m long. The cross-sectional area of each sector cesses in the absence of sedimenWwater exchange. (given at 0.5 m depths) was multiplied by its length Denitrification measured by isotope pairing: The 9 (1000 m) to give the volume of water within each sec- small sediment cores were used to measure denitrifica- tor at mid-tide. Section 2 was subdivided into 75 sec- tion rates using the 15N03 isotope pairing technique of tors, each 1000 m long. The width of the channel at Nielsen (1992). Six of the nine cores were spiked by 1.5 m intervals above or below chart datum (at rnid- addition of ''NO3 (10 mM Na15N03- (99.3 atom%, tide) was then used to construct a series of trapezia. Europa, Crewe, UK]) into the overlying water and left The volume of water within each sector was calculated to equilibrate for 0.5 h (Rysgaard et al. 1995). A pre- by multiplying the area of each trapezium (i.e. cross- requisite of the isotope pairing technique is a uniform sectional areas) by the length of each box (i.e. 1000 m) mixing of the 14N03- and 15N03- isotopes throughout and summing the volume of all sectors. the denitrification zone (Nielsen 1992), which could Monthly (1992 to 1997) axial surveys of chlorinity (Cl) have been a problem at the outer sites with a deeper (Environment Agency unpubl. public access data) at oxic layer. In order to test for this we added 15N03- at a discreet sampling sites along the estuary were used to range of concentrations (5 to 40 FM), and if the calcu- assign a salinity (S) value to each sector, using Knud- lated rates of coupled denitrification proved to be sub- sen's formula: sequently independent of the concentration of I5NO3- added, thls was taken as good evidence for uniform mixing (Nielsen 1992, Nielsen & Glud 1996). Once this The volume of freshwater within each sector was had been proven cores were amended at the higher then calculated using: concentration of 40 pM 15N03- to minimise the under- freshwater (m3) = total volume of sector (m3) estimation of denitrification (Nielsen & Glud 1996). At X [l-(SO/S')] the inner, higher nitrate sites enrichment of the NO3- pool was usually about 30% (Rysgaard et al. 1995). Where So is the salinity value assigned to each sector Water samples (2 ml), collected from all core tubes to and S' is the salinity of the offshore coastal water out- determine the initial 14N03-:15N03- ratio in the water side the estuary, in this case 35%0 for the southern overlying the treatment cores, were filtered and pre- North Sea. served prior to analyses (see above). The sediment in Seasonal variations in freshwater flushing times for the 3 reference cores was then gently mixed into a the Thames estuary were calculated by dividing the slurry with the overlying water and a sample (20 rill) seasonal freshwater volumes by seasonal freshwater carefully drawn off into an Exetainer (12 m1 Labco Ltd, flows; taken as the sum of flows from the River Lee, High Wycombe, UK) containing ZnC12 solution (500 1-11, River Roding and the River Thames flowing over Ted- 25 % w/v) and sealed. The ''NO3- enriched cores were dington Weir (Environment Agency unpubl. public sealed and incubated in the dark at the in situtemper- access data). ature, with gentle stirring (-60 rpm) of the water Nutrient loads (monthly averages for 1995-96) for the column for 2 to 6 h. The on-screen data logging in the Thames and all the UK east coast estuaries (The Wick parallel oxygen uptake cores ensured that the con- in Scotland to The Medway in England) were calcu- Trimmer et al.: Mineralisation rates via oxygen uptake and denitrification 107

lated by multiplying fluvial freshwater flows by l10 1 nutrient concentrations (Harmonised Moni- toring data, Department of the Environment, Transport and the Regions). Axial suspended l00 - solid and dissolved oxygen profiles (expressed as distance from London Bridge) along the Thames estuary (1993 to 1998) were provided by the Environment Agency (unpubl. public access data); sampling was usually within 2 to 3 d of our own sampling dates. The fluvial and STWs organic C loads to the Thames estuary are based on the most recent available data (Environment Agency unpubl. public access data) and are made up of average concentration data and average flow data for all data available to date from 11 rivers (81 % being the Thames flowing into the estuary at Teddington), 16 STWs and 9 trade discharges (only 1 of which contributes g more than 1 % of the total organic C budget). 50 -

RESULTS

Water column characteristics

Dissolved oxygen increased down the inner estuary (Fig. 2), with minimum saturation (with respect to air) either side of Site 1 (mean 55 +4 % wwwPr-*Fr.PPmm SE, n = 14) and maximum saturation at Site 9 mmmmmmmmmmmm -5CO>nL cm- (92 & 2% SE, n = 12). Dissolved oxygen in- /322~$~22~~~ creased at all sites during late spring. - (May 1997) and mid.rnnter (January 1998). Maximum Fig. 2. Water column dissolved oxygen (% of air saturation). Point nitrate concentrations were measured at the samples collected by the Environment Agency (unpubl. public ac- cess data) w~thin2 to 3 d of this study's sampling date head of the transect at Site 1 (range 511 to 797 pM, mean = 611 PM), decreasing down- stream to 0.6-9 pM (mean = 4 PM) at Site 12. Average chlorophyll concentrations did not vary sig- mum was reached, between 35 and 53 km down- nificantly along the inner estuary; increasing from only stream around the Tilbury Basin mudflats, where sus- 4 pg 1-l (range 1.3 to 7.3 pg 1-') at Site 1, to a peak of pended loads peaked at 90 + 5.6 mg I-' (SE, n = 219, 8 pg I-' (range 3.5 to 23 pg 1-l) at Site 7 and then 5 yr mean). Suspended loads decreased again further decreasing to 1.5 pg 1-' (range 0.6 to 2.9 pg I-') at Site 9. seaward to their lowest values for the inner estuary at Maximum off-shore spring chlorophyll concentrations Southend of 28 t 2.9 mg 1-' (SE, n = 58, 5 yr mean). of 18.6 and 12.8 pg 1-' were measured at Sites 10 and There was then a third turbidity peak at Site 10 of 12, respectively. Salinity increased from 2.1 t 0.2%0 34 t 2.7 mg 1-' (SE, n = 127) followed by a steady (SE, n = 45, 5 yr mean) at London Bridge to 31.7 * 0.?%0 decrease seaward out to Site 12 (3.8 mg 1-' + 0.3 SE, (SE, n = 57, 5 yr mean) at Southend. The outer transect n = 96). sites (Sites 10 to 12) were essentially fully marine, with salinity varying (by a few %o) from around 34.4%0 at

Site 10 to around 35 %O at Site 12. Sediment characteristics Suspended loads initially increased down the estu- ary to 81 * 3.8 mg 1-I (SE, n = 218, 5 yr mean) 18 km From Site 1 seaward along the transect there was a downstream from London Bridge, where both the general increase in particle size until, offshore, the major Beckton and Crossness STWs discharge sediments were dominated by fine and medium sands (Fig. 3). Suspended loads then decreased further (Table 1). Porosity and organic C were greatest at the downstream until the broad high-tide turbidity maxi- muddy sites (Sites 1, 4 and 5), decreasing in proportion Mar Ecol Prog Ser 197: 103-1 19, 2000

to each other (p c 0.05) seaward along the transect recorded at the offshore Site 12, the percent total N (Fig. 3). The lowest C:N ratios were also recorded at was probably below the limit of detection (0.02%) and the 3 muddy sites. Although a C:N ratio of 12:l was the value of the ratio therefore suspect.

InnerThames (salinity 2-32 pp) Southern Nonh Sea (salin~ty34-35 ppl) k 4

1.0 1 1 100

A

- 0.4 - Mainarea of deposition (Tilbury Basin)

Distance down stream from London Bridge (km)

Fig. 3. Sediment porosity (0) and organic carbon (D); mean of all measurements for all sites from July 1996 to March 1998. Sus- pended load (A); 18 to 80 km: 5 yr mean (Environment Agency unpubl, public access data), 80 to 200 km: mean of 6 surveys from July 1996 to April 1997

Table 1. Site locations and sediment characteristics for the 12 sites investigated on 6 occasions. Particle size measured once in May 1996 for inner Sites 4 to 10, July 1996 for outer Sites 1 to 3 and Apnl 1997 for inner Sites 11 to 13

Site" Location Particle sizeb PorosityC Organic Cd Total Ne C:N ratio N E 0-10 cm 0-2 cm 0-2 cm 0-2 cm 0-2 cm (250% w/w) (cm3 H,O cm-3 sed.) (% dry wt) (% dry wt) (% dry wt) Inner 1 (TH 11) 51°31'07 0°07'47 Siltdclays 2 (TH 12) 51°27'76 0" 16'47 Very fine sands 3 (TH 13) 51°27'09 0°22' 12 Very fine sands 4 (TH 9) 51°28'80 0°26'46 SilWclays 5(TH5) 51°29'20 0°31'50 Silt/clays 6(TH10) 51°29'49 0°35'10 Very fine sands 7 (TH 4) 51°29'47 0°38'50 Fine sands 8 (TH 7) 51°31'28 0°40'83 Very fine-fine sands 9 (TH 8) 51°31'56 0°44'44 Very fine sands Outer l0 (TH 1) 51°30'80 0°58'00 Very fine sands-silffclays l1 (TH 2) 51°48'00 1°35'00 Fine sands 12 (TH 3) 52" 00' 00 2" 20' 00 Medium sands

aSites have been renumbered sequenhally for clarity but the orig~nalTH number is given for cross reference to other JoNuS project papers bWentworth classification (Tait 1981): medlurn sands, 500 to 250 pm; fine sands, 250 to 125 PI; very fine sands, 125 to 63 pm; silvclays. <63 pm =CV 4 % dCV 14 % eCV 18% Trimmer et al.: Mineralisation rates via oxygen uptake and denitrification

Chlorophyll concentrations in the surface 0 to 1 cm significant (p 5 0.05) inter-site differences from Sites 6 sediment layer showed no significant variation along to 12 but a consistent peak in oxygen uptake at the the inner estuary (mean 27 mg chl a m-') and showed inner muddy sites. ANOVA indicated significant sea- no significant seasonal variation, apart from a small sonal differences in oxygen uptake rates at each site peak in May (mean 62 mg chl a m-'). (p 10.003).However, the clearest seasonal trends (i.e. significant correlation with seasonal temperature) and highest rates of oxygen uptake were at the muddy Sediment rate measurements sites, whilst lower and non-temperature-dependent oxygen uptake rates were recorded at the sandier sites Oxygen uptake (Fig. 5). There was a significant positive correlation (p < 0.001) between overall average rates of oxygen There was a broad range of sedimentary oxygen uptake and sedimentary organic C content along the uptake rates along the transect (Table 2), from transect. 507 pm01 O2 m-' h-' at the furthest offshore Site 12 to 10 056 p01 0' m-' h-' at the inner muddy Site 4. The overall trend of mean rates is illustrated in Fig. 4A. Denitrification ANOVA indicated significant inter-site variation for rates of oxygen uptake on each occasion (p 1 0.05). The calculated rates of coupled denitrification from However, post hoc Tukey analysis showed very few the initial trial at the outer Sites 10 to 12 proved to be

Total

80 100 120 140 160 180 200

Distancedown stream from LondonBridge (km)

Fig. 4. Mean rates of (A) oxygen uptake and (B) denitnfication at each site for all samples collected along the Thames transect 110 Mar Ecol Prog Ser 197: 103-1 19, 2000

Table 2. Sedimentary rates of oxygen uptake and denitrification (D, and D,) for each site in the Thames estuary, numbers in brackets are standard errors, n = 3 and 6 respectively. Note standard errors of

Site Ju196 Aug 96 Oct 96 Nov 96 Feb 97 Mar 97 Apr 97 May 97 Jun 97 Ju197 Aug 97 Oct 97 Jan 98 Mar 98

Inner 1 02 2870 3562 7485 3113 1740 2427 4721 (256) (142) (1037) (184) (578) (184) (449) l Dw 11407 2311 398 4389 2047 6626 5757 (1259) (586) (67) (296) (280) (855) (337) 1 Dn 8209 2428 558 437 138 301 2991 (486) (293) (89) (98) (57) (144) (290) 2 02 2610 2495 6304 2884 1391 3220 2792 (383) (318) (1205) (538) (405) (434) (20) 2 D, 418 43 235 98 36 2240 58 (129) (11) (66) (21) (4) (1035) (7) 2 D" 385 15 0 38 16 271 63 (114) (4) (7) !.5! (108) (S) 3 02 1671 2924 2957 5384 1632 1527 1647 (360) (237) (830) (186) (389) (140) (82) 3 Dw 156 30 80 93 74 39 85 (46) (7) (10) (24) (17) (5) (10) 3 D, 157 21 145 47 19 100 116 (32) (6) (57) (9) (6) (38) (14) 4 O2 4313 6022 2387 1415 1546 2574 3714 6443 5486 2736 4301 4789 (283) (1439) (101) (104) (221) (186) (511) (1307) (504) (121) (509) (1008) 4 D, 310 243 183 333 1646 741 123 109 124 165 96 58 (26) (30) (63) (20) (423) (163) (25) (29) (22) (30) (19) (12) 4Dn 213 334 173 227 1816 962 81 158 151 53 215 90 (23) (39) (39) (37) (557) (198) (11) 35 (33) (11) (44) 19 5 D,, 4328 10056 1772 1489 1412 5358 2625 4379 (714) (1326) (326) (273) (78) (727) (140) (914) 502 217 99 48 193 569 3261 986 178 (28) (17) (6) (119) (285) (519) (172) (18) 5 D, 238 264 71 324 275 1513 187 284 (12) (41) (4) 137 (166) (440) (36) 50 6 02 1798 836 1006 502 500 955 2407 2220 3680 539 1502 1843 (255) (74) (127) (182) (1) (197) (451) (428) (413) (45) (70) (243) 6 D, 35 7 74 59 18 0 0 11 103 11 19 8 (5) (1) (22) (20) (8) (4) (9 (1) (9) (1) 6D, 45 15 110 67 0 0 0 19 0 14 17 8 (6) (4) (34) (19) (6) (1) (5) 7 O2 1277 1636 932 620 490 705 2147 823 (95) (393) (47) (48) (122) (117) (485) (453) 7 D, 35 54 27 4 27 14 17 17 (1) (11) (7) (1) (2) (9) (7) (6) 70, 25 16 22 0 0 0 0 0 (2) (1) (3) 8 02 1358 1306 566 558 535 697 672 1004 2500 1576 1136 1248 (266) (63) (80) (207) (23) (175) (101) (214) (161) (540) (85) (142) 80, 35 11 0 2 0 3 7 15 14 5 0 4 (9) (2) (2) (1) (2) (3) (2) (1) 80, 35 14 0 5 0 0 11 0 10 9 0 4 (3) (2) (1) (1) (3) (1' (1) 9 O2 2242 3956 553 898 410 689 1137 2264 (552) (449) (59) (125) (32) (250) (308) (1393) 9D, 17 18 1 2 0 0 1 6 (2) (4) (1) 90, l6 12 1 1 0 0 0 11 (4) (3) (3) Trimmer et al.: mineralisation rates via oxygen uptake and denitrification Ill

Table 2. (continued)

Site Ju196 Aug 96 Oct 96 Nov 96 Feb 97 Mar 97 Apr 97 May 97 Jun 97 Jul97 Aug 9? Oct 97 Jan 98 Mar 98

Outer l0 Oz 2325 1383 1058 2278 (425) (252) (58) (233) l0 D, 5 5 3 4 (1) lOD, 27 9 5 7 (2) (1) (1) (1) 11 0, 1481 1480 473 1458 (129) (27) (77) (368) l l D 2 1 2 0

11 D, 1.3 4 0 2 (1) 120, 541 609 507 1427 (136) (35) (59) (127) 12 D, 0 0 1 0

12D, 0 0 0 0 independent of the concentration of 15N03- added, for all sites along the transect. However, this relation- which was taken as good evidence for a uniform mix- ship was strongest (r = 0.92 compared with r = 0.53) at ing of the I4NO3- and 15N03- isotope pools. Sites 6 to 12, with a sediment organic C content of Denitrification, both that driven by nitrate from the <1.5 % (dry wt). The D, fraction dominated denitrifica- overlying water (D,) and that coupled to nitrification'in tion at the majority of the inner sites, with a maximum the sediment (D,) (Table 2), followed a trend along the transect similar to that 10000 , 26 of oxygen uptake, from negligible I l rates of approximately 0 to 1 pm01 N l 1 " 24 m-' h-' for both D, and D, at the fur- I j #, thest offshore site, Site 12, to maxi- ,! \ - 22 8000 c mum values of 11 407 and 8209 pm01 - 1 N m-' h-', respectively, at the inner muddy Site 1. The overall trend of 'E mean values is illustrated in Fig. 4B. ANOVA indicated significant inter- 0 site variation for rates of D, and D, on each occasion (p I0.038), with signifi- cantly higher rates consistently being measured at the muddy sites. ANOVA indicated significant seasonal varia- tion (p I 0.004) in the rates of D, (except Site 3, p = 0.142) and D, (except Sites 3 and 7, p = 0.090 and 0.105, respectively) measured at each of the sites. The average rates of both D, and D, were significantly (p < 0.05) positively correlated with the WWIDPr-r.F-I'.Pr. sediment organic C content along the -mmmmQ10\mmmm-,Q>D &2sU)- transect. There was a significant posi- - Q222Qr-283 tive 0'001) (p between Fig. 5.Oxygen uptake rates at a muddy s~te(Site 4, open bars), and a sandy site the overall average rates of Dxvand (Site 6, hatched bars) together with the sediment temperature (a).Bars indlcate overall average NO3- concentrations standard error, n = 3 Mar Ecol Prog Ser 197: 103-119, 2000

C delivered to the Thames estuary from either STW or of organic loading on nitrification and denitrification in a fluvial discharge was deposited and remineralised marine sediment microcosm. FEMS Microbiol Ecol 12: before the mouth of the estuary at Southend. Zwols- 159-167 Cammen LM (1991) Annual bacterial production in relation to mann (1994) also estimated that 60 % of the fluvial sed- benthic microalgal production and sediment oxygen up- iment discharge to the Thames estuary was retained take in an intertidal sandflat and an intertidal mudflat. within the estuary. Mar Ecol Prog Ser 71:13-25 The average area-normalised rate of organic C min- Cooper AB, Cooke JG (1984) Nitrate loss and transformation in 2 vegetated headwater streams. NZ J Mar Freshw Res eralisation in the Thames estuary (21 m01 C m-2 yr-l) 18:441-450 was similar to that measured in the Great Ouse (29 m01 Davidson NC, Laffoley Dd'A, Doody JP, Way LS, Gordon, J C m-2 yr-l, Trimmer et al. 1998).Extrapolating the com- Key R, Drake CM, Pienkowsh MW, Mitchell RM, Duff KL bined average area-normalised rate (25 m01 C m-2 yr-l) (1991) Nature conservation and estuaries in Great Britain. Nature Conservancy Council, Peterborough to the entire estuarine area of the UK (6 X log m2) sug- de Jonge DJ, de Jonge VN (1995) Dynamics of rnicrophyto- gests that 150 Gm01 C (1.8X 106 t) is recycled back to benthos chlorophyll-a in the Scheldt estuary (SW Nether- the atmosphere each year. The annual estuarine pro- lands). Hydrobiologia 31 1:21-30 duction of CO, in the UK is, therefore, comparable to Digest of Environmental Statistics (1997) Department of the that produced from agriculture, shipping, civil aircraft Environment, Transport and the Regions, Vol 19. The Sta- tionery Office, London and railways, i.e. 1 X 1c6 each, respectively (Eigest of : Fielding PJ, Damstra KSJ, Branch GM (1988) Benthic diatom Environmental Statistics 1997). The recycling of fixed biomass, production and sediment chlorophyll in Lange- C back to atmospheric CO2 by estuarine sediments fur- baan Lagoon, South Africa. Estuar Coast Shelf Sci 27: ther emphasises the significance of estuaries as biolog- 413-426 ical filters, not only of DIN loads but also of particulate Hargrave BT, Philips GA (1981) Annual in situ carbon dioxide and oxygen flux across a subtidal marine sediment. Estuar and dissolved organic C. Coast Shelf Sci 12:725-737 Heip CHR, Goosen NK, Herman PMJ, Krornkamp J, Middel- burg JJ, Soetaert K (1995) Production and consumption of Acknowledgements. This work was part of the JoNuS (Joint biological particles in temperate tidal estuaries. Oceanogr Nutrient Study) programme funded by the Ministry of Agri- Mar Biol Annu Rev 33:l-149 culture, Fisheries and Food and the Department of the Envi- Jsrgensen BB (1982) Mineralisation of organic matter in the ronment, Transport and the Regions (both UK). The views sea bed-the role of sulphate reduction. Nature 296: expressed are those of the authors and do not reflect the poli- 643-645 cies of the funding departments. The authors wish to thank Dr Jsrgensen BB, Ssrensen J (1985) Seasonal cycles of 02, NOs- Andrew Sage for compiling the nutrient load data and the and SOA2-reduction in estuarine sedirnents: the signifi- Captain, Officers and crew of the RV 'Cirolina'. cance of an No3- reduction maximum in spring. Mar Ecol Prog Ser 24:65-74 Jsrgensen KS (1989) Annual pattern of denitrification and LITERATURE CITED nitrate ammonification in estuarine sediment. Appl Envi- ron Microbiol55: 1841-1847 Aller RC (1994) The sedmentary Mn cycle in Long Island Kelley CA, Martens CS, Chanton JP (1990) Variations in sed- Sound: its role as intermediate oxidant and the influence imentary carbon mineralisation rates in the White Oak of bioturbation, O,, and COrgflux on diagenetic reaction River estuary. North Carolina. Limnol Oceanogr 35: balances. J Mar Res 52:259-295 372-383 Aller RC, Blair NE, Xla Q, Rude PD (1996) Remineralisation Kukwood DS (1992) Stability of solutions of nutrient salts dur- rates, recycling, and storage of carbon in the Amazon shelf ing storage. Mar Chem 38: 151-164 sediments. Cont Shelf Res 16:753-786 Kirkwood DS (1996) Nutrients: practical notes on their deter- Anderson FO. Helder W (1987) Comparison of oxygen micro- mination in seawater. Techniques in marine environmen- electrodes, oxygen flux rate and electron transport system tal science, No. 17. ICES, Copenhagen activity in coastal manne sedments. Mar Ecol Prog Ser 37: Law CS, Rees AP, Owens NJP (1991) Temporal variability of 259-264 denitrification in estuarine sediments. Estuar Coast Shelf Billen G, Somville M, de Becker E, Servais P (1985) A nitrogen Sci 33:37-56 budget of the Scheldt hydrographical basin. Neth J Sea Lohse L. Kloosterhuis R, van Raaphorst W, Helder W (1996) Res 19:223-230 Denitnfication rates in continental shelf sediments of the Binnerup SJ, Jensen K, Revsbech NP, Jensen MH, Ssrensen J North Sea: acetylene block technique versus isotope pair- (1992) Denitrification, dissimilatory reduction of nitrate to ing. Mar Ecol Prog Ser 132:169-179 ammonium, and nitrification in a bioturbated estuarine Luther GW, Shellenbarger PA, Brendel PJ (1996) Dissolved sediment as measured with l5N and micro sensor tech- organic Fe (111) and Fe (11) complexes in salt marsh pore- niques. Appl Environ Microbiol 58:303-313 waters. Geochirn Cosmochim Acta 60:951-960 Blackburn TH, Lund BA, Krom MD (1988) C- and N-mineral- Mackin JE, Swider KT (1989) Organic matter decomposition isatlon in the sedirnents of earthen marine fishponds. Mar pathways and oxygen consun~ptionin coastal marine sed- Ecol Prog Ser 44:221-227 ments. J Mar Res 47:681-716 Cadbe GC, Hedgman J (1974) Primary production of the ben- Middelburg JJ, Klaver G, Nieuwenhuize J, Vlug T (1995) Car- thic microflora living on tidal flats in the Dutch Wadden bon and nitrogen cycling in intertidal sedirnents near Sea. Neth J Sea Res 8:260-291 Doel. Scheldt Estuary. Hydrobiologia 311:57-69 Caffrey JM, Sloth NP, Kaspar HF, Blackburn TH (1993) Effect Middelburg JJ, Klaver G, Nieuwenhuize J. Wielemaker A, de Trimmer et al.: Mineralisation rates via oxygen uptake and denitnfication 119

Haas W, Vlug T, van der Nat JFWA (1996) Organic miner- ation in nitrification and denitrification in estuarine sedi- alisation in intertidal sediments along an estuarine gradi- ment colonised by benthic microalgae and bioturbating ent. Mar Ecol Prog Ser 132:157-168 infauna. Mar Ecol Prog Ser 126:lll-121 Mulder A, van de Graaf AA, Robertson LA, Kuenen JG (1995) Sampou P, Oviatt CA (1991) Seasonal patterns of sedimentary Anaerobic ammonium oxidation dscovered in a denitnfy- carbon and anaerobic respiration along a simulated ing fluidized bed reactor. FEMS Microbiol Ecol 16: eutrophication gradient. Mar Ecol Prog Ser 72:2?1-282 177-183 Schlegel HG (1975) Mechanisms of chemoautotrophy. In: Nedwell DB (1975) Inorganic nitrogen metabolism in a Kinne 0 (ed) Marine ecology, Vol 2. John Wiley, London, eutrophicated tropical mangrove estuary. Water Res 9: p 9-60 221-231 Seitzinger SP (1988) Denitnfication in freshwater and coastal Nedwell DB, Trimmer M (1996) Nitrogen fluxes through the marine ecosystems: ecological and geochemical signifi- upper estuary of the Great Ouse, England: the role of the cance. Lirnnol Oceanogr 35:702-724 bottom sediments. Mar Ecol Prog Ser 142:273-286 Seitzinger SP (1990) Denitrification in aquatic sediments. In: Nedwell DB, Blackburn TH, Wiebe WJ (1994) Dynamic Revsbech NP, Serrensen J (eds) Denitrification in soil and turnover of organic carbon, nitrogen and sulphur in the sediment. Plenum Press, New York, p 301-322 sediments of Jamaican mangrove forest. Mar Ecol Prog Seitzlnger S, Nlxon S, Pilson MEQ, Burke S (1980) Denltrifi- Ser 110:223-231 cation and N20 production in near-shore marine sedi- Nielsen K, Nielsen LP, Rasmussen P (1995) Estuarine nitrogen ment~.Geochim Cosmochim Acta 44:1853-1860 retention independently estimated by the denitrification Sloth NP, Blackburn TH, Hansen LS, Risgarrd-Petersen N, rate and mass balance methods: a study of Norsminde Lomstein BA (1995) Nitrogen cycling in sediments with Fjord. Denmark. Mar Ecol Prog Ser 119:275-283 different organic loading. Mar Ecol Prog Ser 116:163-170 Nielsen LP (1992) Denitnfication in sediment determined Serrensen J, Jerrgensen BB, Revsbech NP (1979) A comparison from nitrogen isotope pairing. FEMS Microbio Ecol 86: of oxygen, nitrate and sulphate respiration in coastal 357-362 marine sedlments. Microbial Ecol5: 105-1 15 Nielsen LP, Glud RN (1996) Denitrification in a coastal sedi- Serrensen J, Rasmussen LK, Koike I (1987) Micromolar sulfide ment measured in situ by the nitrogen isotope pairing concentrations alleviate acetylene blockage of nitrous technique applied to a benthic flux chamber. Mar Ecol oxide reduction by denitrifying Pseudornonas fluorescens. Prog Ser 13?:181-186 Can J Microbiol33: 1001-1005 Nixon SW, Arnmerman JW, Atkinson LP, Berounsky VM, Tait RV (1981) Elements of marine ecology, 3rd edn. Butter- Bden G, Bolcourt WC, Boynton WR, Church W, DiToro worths, London DM. Elmgren R, Garber JH. Giblin AE, Jahnke RA, Owens Tett P (1987) Plankton. In: Baker JM, Wolff WM (eds) Biologi- NJP, Pilson MEQ, Seitzinger SP (1996) The fate of nitro- cal surveys of estuaries and coasts. Cambridge University gen and phosphorus at the land-sea margm of the North Press, Cambridge, p 280-341 Atlantic Ocean. Biogeochemistry 35:141-180 Trimmer M, Nedwell DB, Sivyer D, Malcolm SJ (1998) Nitro- Nowicki BL, Requintina E, van Keuren D. Kelly JR (1997) gen fluxes through the lower estuary of the Great Ouse, Nitrogen losses through sediment denitrification in Boston England: the role of the bottom sediments. Mar Ecol Prog Harbor and Massachusetts Bay. Estuaries 20:626-639 Ser 163:109-124 Owvie B, Nedwell DB, Harrison RM, Robinson A, Sage A Upton AC, Nedwell DB, Parkes RJ, Harvey SM (1993) Sea- (1997) High nitrate, muddy estuaries as nitrogen sinks: the sonal benthic microbial activity in the southern North Sea; nitrogen budget of the hver Colne estuary (United King- oxygen uptake and sulphate reduction. Mar Ecol Prog Ser dom). Mar Ecol Prog Ser 150:217-228 101:273-281 Peierls B, Caraco N, Pace M, Cole J (1991) Human influence Yoon WB, Benner R (1992) Denitrification and oxygen con- on river nitrogen. Nature 350:386-387 sumption in sediments of two south Texas estuaries. Mar Reise K (1985) Tidal flat ecology: an experimental approach to Ecol Prog Ser 90:157-167 species interactions. Springer Verlag, Berlin Zirnrnerman AR, Benner R (1994) Denitnfication, nutrient Richards FA, Cline JD, Broenkow W, Atkinson LP (1965) regeneration and carbon mineralisation in sediments of Some consequences of the decomposition of organic Galveston Bay. Texas. USA. Mar Ecol Prog Ser 114: matter in Lake Nitinat, an anoxic fjord. Limnol Oceanogr 275-288 (Suppl 10):R185-201 Zwolsmann JJG (1994) North Sea estuaries as filters for con- Rysgaard S, Christensen PB, Nielsen LP (1995) Seasonal vari- taminants. Report T1233. Delft Hydraulics, Delft

Editorial responsibility: Otto Kinne (Editor), Submitted: April 19, 1999; Accepted: November 23, 1999 Oldendorf/Luhe, Germany Proofs receivedfrom author(s): April 14, 2000