Seasonal Investigations of Dissolved Organic Carbon Dynamics in the Tamar Estuary, U.K.

Estuarine, Coastal and Shelf Science (1999) 49, 891–908 Article No. ecss.1999.0552, available online at http://www.idealibrary.com on

Seasonal Investigations of Dissolved Organic Carbon Dynamics in the Tamar Estuary, U.K. A. E. J. Miller NERC Centre for Coastal and Marine Studies, Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 3DH, U.K. Oceanography Laboratories, Department of Earth Sciences, The University of Liverpool, Bedford Street North, Liverpool L69 3BX, U.K. Present address: University of Highlands and Islands project, The Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, P.O. Box 3, Oban, Argyll PA34 4AD, U.K. Received 19 February 1999 and accepted in revised form 16 August 1999 A high temperature catalytic oxidation (HTCO) technique was used to measure dissolved organic carbon (DOC) during seasonal surveys of the Tamar Estuary, U.K. At the time of the programme, the field of DOC analysis had been plagued by numerous analytical difficulties. However, using thorough calibration of the analytical systems and the systematic analysis of an internal reference material, a valuable estuarine DOC data set was produced. The range of DOC concentrations observed (478–110
M C) is consistent with the published data for riverine and coastal sea waters respectively. The Tamar Estuary is a freshwater DOC-dominated system, with strong correlation between lateral DOC distribution and salinity. However, mixing behaviour was not strictly conservative. During tidal cycle studies at a fixed station, DOC concentrations appeared to be uncoupled from salinity, and were inversely related to turbidity. It is concluded that tidally-induced resuspension of bottom sediments provided the dominant control mechanism for DOC concentration. The Tamar Estuary shows contrasting behaviour to the larger, more heavily impacted, Severn Estuary. Hence it is likely that the behaviour of DOC in estuaries cannot be classified as typical per se, but is a function of the natural and anthropogenic characteristics of the catchment and hydrology.  1999 Academic Press Keywords: dissolved organic carbon; seasonal variations; macrotidal; north European

Introduction Dissolved organic matter provides the largest oceanic pool of organic carbon. This reservoir is of global importance, as it carries a carbon load (dissolved organic carbon, DOC) equivalent to that held as atmospheric CO2(c. 660 giga tonnes; Hedges, 1992). Consequently, DOC is intimately involved in many facets of the global carbon cycle and is inextricably linked to the aquatic biogeochemical cycling of numerous chemical species, including nitrogen and phosphorus (Mantoura et al., 1991). In rivers and estuaries DOC is also intimately involved in a range of locally important biogeochemical processes. Biological removal of DOC through heterotrophic remineralization exerts a controlling influence on the oxygen mass balance of aquatic systems (Tipping et al., 1997). Potential toxicity of industrial and urban effluents is ameliorated through complexation with trace metals (Suffet & MacCarthy, 1989) and partitioning of organic micro pollutants such as pesticides, PAHs and PCBs (Chiou et al., 1986). Adsorption of DOM 0272–7714/99/120891+18 $30.00/0

onto particles causes alteration of their physicochemical properties, and makes the material more readily available to the biota (Keil et al., 1994). The biogeochemistry of carbon in marine and estuarine environments has most recently been reviewed by Sharp (1991) and Lee and Wakeham (1992). Estuaries are highly dynamic systems, undergoing sharp geochemical changes in salinity, pH and turbidity during the transition from freshwater to marine environments. It is known that concentration of DOC through an estuary may vary over an order of magnitude (e.g. Mantoura & Woodward, 1983), although the nature of delivery remains open to question (Suzuki & Tanoue, 1991; Sharp et al., 1993). Precise estimates of DOC fluxes are confounded through difficulties in defining the average river water concentration and quantifying physicochemical removal processes (i.e. aggregation, coagulation, flocculation and precipitation) in the mixing zone (Sholkovitz et al., 1978; Alber & Valiela, 1994). Desorption of OM from estuarine particulates has also been reported (Eisma et al., 1982). Depending on the relative magnitude of  1999 Academic Press

892 A. E. J. Miller

the contributing mixing and biogeochemical processes, the reported nature of estuarine DOC transport has been described as both conservative (Sieburth & Jensen, 1968; Mantoura & Woodward, 1983; Sharp et al., 1988, 1993; Aminot et al., 1990) and non-conservative (Susuki & Tanoue, 1991; Sharp et al., 1993; Alberts & Griffin, 1996; Argyrou et al., 1997). Since the mid 1970s it has been accepted that only a small pool of information exists in relation to the nature and reactivity of estuarine DOM (Head, 1976); and that statement still holds true today. In the Tamar Estuary, non-conservative transport of DOC has been observed in the 0·10–1·00 salinity region of the freshwater-seawater interface (FSI), using ultraviolet/persulphate (UV/PS) techniques (Morris et al., 1978; Mantoura & Mann, 1979). The objective of this work was to provide analytically validated seasonal DOC data for the estuary, using a high temperature catalytic oxidation (HTCO) technique (Miller et al., 1993b), to improve our understanding of the biogeochemical reactivity of DOC in estuarine environments.

inputs are largely focused on the naval dockyard and centres of population concentrated towards the mouth of the estuary. Hydrography The Tamar system forms a macrotidal estuary, with tidal ranges at Devonport of 6·5 m at springs and 1·5 m at neaps, decreasing to 1–2 m in the upper reaches. The River Tamar provides the dominant freshwater input with typical monthly averaged flow decreasing from a maximum in January of 38 m3 s
1 to a minimum of 5 m3 s
1 in June (Uncles et al., 1983). The annual average is 30 m3 s
1 (Akroyd, 1983), but instantaneous flows can exceed 100 m3 s
1 (Bale, 1987). The rivers Tavy and Lynher contribute a further 30% and 20% of average flow respectively (Uncles et al., 1983), but cause deviations in the overall salinity of less than 2 units (Morris et al., 1982b). At mid-tide, the estuary holds approximately 1·6108 m3 water (Law, 1989). The flushing time is around one week, although under summer conditions, where freshwater flow is low, this may be reduced to one day in the low salinity region.

Location and methods Sampling and analytical methods Site description The Tamar Estuary is probably the most studied system of its kind in the U.K. Most notably in the present context, the work of Morris and co-workers in the late 1970s and early 1980s (Morris et al., 1978; Morris et al., 1981; Morris et al., 1982a, b, c; Morris, 1983; Morris et al., 1985) provides a broad description of local estuarine biogeochemical processes. The Tamar is also close to the average length of estuaries in the U.K. (Gameson, 1973) and therefore represents a suitable system in which to study environmentally important processes. The River Tamar (Figure 1) rises in north Cornwall, and flows southward for approximately 100 km, collecting the major tributaries of the River Tavy and the River Lynher along the way. The estuary is defined from the limit of tidal incursion at Gunnislake Weir to the seaward boundary at the entrance to Plymouth Sound: a topographical distance of 31 km. Water depths through the estuary are typically less than 5 m below chart datum, although a deep water channel (up to 40 m) extends downstream from Torpoint. The catchment covers an area of approximately 1700 km2 (Evans et al., 1995, where land use is generally rural. The Tamar is a relatively clean river with little visible evidence of pollution or eutrophication in the upper reaches. Anthropogenic

A comparative study of techniques for the determination of DOC was conducted on waters from the Tamar Estuary during March 1990 (Miller et al., 1993b). High temperature catalytic oxidation (HTCO) was performed alongside wet chemical oxidation, using the UV/PS system described by Mantoura and Woodward (1983). Axial DOC distributions varied somewhat between techniques, but these were subsequently related to methodological problems at the time of the analyses (Hedges & Lee, 1993). However, at the time of the seasonal study reported here, internal measures to address these problems had already been implemented, as discussed below. A survey strategy was composed to include at least monthly surveys of the Tamar Estuary from April through to December, 1991. The majority of surveys were scheduled for sampling during neap tide conditions, both for logistical reasons and for ease of comparability. Axial transects were sampled in mid channel, along the salinity gradient, from the mouth of the estuary to as near the tidal limit as possible. Occasionally, additional surveys were performed to investigate differing states of the tide. An axial survey was conducted during spring tide conditions during June (hereafter referred to as ‘ Junes ’). Variability over a tidal cycle was examined using a fixed-position,

Estuarine DOC dynamics 893 River Tamar Location

N 0 Gunnislake Weir

5 Calstock

10

Key Distance from tidal limit Gunnislake Weir (km)

10

River Tamar 15

River Tavy

City boundary

0

4

Kilometres 0

Miles

20

2

River Tamar

Saltash

Tamar Bridges

River Lynher Camels Head

25

Torpoint

Plymouth

Devonport Dockyard

St. John’s Lake 30

The Sound English Channel

F 1. Map of the Tamar Estuary and its catchment.

894 A. E. J. Miller

platform-based survey at Calstock (c. 8 km from the weir) during September. Altogether, 10 estuarine investigations were completed, including nine transects through the mixing region. Surveys were performed aboard RV Tamaris, a 12 m flat-hulled survey vessel (Rotork Sea Truck). A traverse of the entire estuarine region took approximately 3·5–4 h, and environmental data (temperature, salinity and turbidity) were recorded continuously throughout this time. Discrete samples were collected for DOC, chl a and particulate organic carbon (POC), for analysis in the laboratory. Salinity and suspended particulate matter (SPM) samples were also collected, for subsequent calibration of continuous parameters. Continuous measurements Aboard RV Tamaris, water was pumped from a depth of 0·5 m using a submersible (Flyght B2040) centrifugal pump through a constant-levelled reservoir that held detecting sensors. The system flushing time was <20 s. A multi-parameter DMP data logging system (Type 1478 Water Logger; DMP Electronics, Camelford, Cornwall U.K.), was used to collect a standard suite of measurements: temperature, salinity and turbidity. The logger was interfaced with a portable computer (Amstrad Alt 287) and the time at which readily identifiable landmarks were passed was electronically noted on the computer. In the absence of a global positioning system, this procedure fixed the data in a framework of time and geographical position (Figure 1). Calibration of sensors Calibrations for the logger sensors were performed prior to, during or after each survey, as a matter of routine, according to the following procedures. Salinity. Discrete salinity samples were analysed against ‘ IAPSO Standard Sea Water ’ (Ocean Scientific International Ltd.), using an inductively-coupled salinometer. (Model 601 Mk. III; Autolab). Continuously monitored data were corrected as a factor of the regression slope obtained by plotting discrete against logger-derived salinity values. (It is acknowledged that the use of a Guideline salinometer is not appropriate for very low salinities (<5 units), but the method provides an approximation within the scope of this paper.) Temperature. The DMP temperature probe was calibrated in water against a standard mercury thermometer, to an accuracy of 0·1 C. Continuously

monitored data were corrected as a factor of the regression slope obtained by plotting discrete against logger temperature values. Turbidity. Output from the DMP turbidity probe was calibrated in the field using a 1000 mg l
1 standard, and by gravimetric analysis of discrete samples in the laboratory. Continuously monitored data were calibrated using the regression slope between gravimetric data and logger turbidity values. Discrete chemical sampling and analysis Sampling. Prior to each survey, all glassware was cleaned by soaking overnight in Decon-90 detergent (Decon Laboratories) followed by copious rinsing with Mill-R/Q water (MQ, Millipore). Between 20–25 samples were taken over the salinity range during each survey. Samples were collected in a 2 l glass bottle, held in a stainless steel frame. A remotely operated Teflon stopper allowed samples to be obtained at a depth of 0·5 m. Aliquots were immediately filtered through muffled (450 C, 4 h) 47 mm GF/F filters (Whatman). For the main body of monthly surveys, duplicate all-glass filtration systems were used simultaneously: one containing a pre-weighed GF/F filter, for gravimetric determination of SPM and subsequent POC analysis; the other housing an unweighed GF/F, for chl a analysis. Used filters were placed into icepacked cool box immediately after filtration, and were stored frozen at
20 C, upon return to the laboratory. Aliquots of each filtrate were transferred to cleaned, 160 ml glass-stoppered bottles and stored in the dark whilst on board. Immediately upon return to the laboratory the samples were refrigerated and analysis of the replicate samples commenced. Even though work continued through the night there was an inherent time lag of up to 30 hours between collection and analysis of the final sample. The effects of heterotrophic DOC turnover were not investigated. But, whilst oceanic heterotrophs may turn over organic matter on time-scales relevant to this work (Kirchman et al., 1991), estuarine bacterial populations have been shown to be inefficient consumers of DOC (Uncles et al., 1997). Chlorophyll a analysis. Chl a was determined by the HLPC method of Mantoura and Llewellyn (1985), using a Shimadzu system and following an experimental procedure described elsewhere (Barlow et al., 1993). Instead of the gradient elution used for highresolution pigment analysis, the system was set up to the isocratic optimum for chl a, using 90% methanol (CH3OH) and 10% acetone ((CH3)2CO) at a flow

Estuarine DOC dynamics 895 T 1. Calibration data for curves determined during Tamar Estuary surveys

April May Junes June July September October November December

Start slopeSD Area/
M C spike

R2

n

End slopeSD Area/
M C spike

R2

n

73·140·82 71·033·72 69·612·07 69·341·94 72·922·41 74·844·00 77·092·04 92·173·03 78·192·47

0·9976 0·9430 0·9869 0·9854 0·9797 0·9669 0·9902 0·9841 0·9890

6 6 5 6 6 5 5 5 4

74·171·48 62·432·39 73·841·36 65·412·11 57·281·65 73·104·44 83·942·51 67·453·28 76·882·67

0·9925 0·9730 0·9902 0·9797 0·9819 0·9442 0·9894 0·9724 0·9811

6 6 6 6 6 5 4 4 5

rate of 1 ml min
1, for 8 min (to give rapid throughput of samples). Absorbance, using a Shimadzu UVvisible spectrophotometer (SPD-6AV), was used to determine chl a at 440 nm. Standardization, against chl a (Sigma Chemical Co.), was performed with a Unicam SP8000 Ultraviolet Recording Spectrophotometer (Pye Unicam Ltd.). HPLC analyses were all performed between 13–20 January 1992. Particulate organic carbon analysis. Determination of POC was performed using the method of Verado et al. (1990). Filters were freeze-dried overnight (Lyolab A; LSL Secfroid). Meanwhile, a laboratory-built punch was used to press 21 mm ø Al foil discs (Elemental Microanalysis Ltd.) into cups (‘ dull ’ side forming the inner surface), which were then ashed (450 C; 4 h). Three punches of 6 mm ø were taken for each sample, where filtered material covered 35 mm ø (i.e. areas of 328·27=84·81 mm2 and 962·11 mm2, respectively). A high loading on the filter was about 24 mg SPM, leaving c. 2·1 mg SPM for elemental analysis. As the analyser provides a measure of total carbon, any inorganic carbon must be removed prior to determination of the organic carbon. To perform this decarbonation samples were rinsed with 50
l MQ water, dried in an oven at 110 C, treated by addition of 100
l of 8% sulphurous acid (SOc(aq); BDH Ltd.), and dried once more. Samples were analysed using a Carlo Erba NA1500 Elemental Analyser, driven by EAGER 200 software (Fisons Scientific Equipment). The reaction column (straight bore silica) was prepared with 80 mm copper wires (fine, reduced; 50 g); overlain by 100 mm tungstic oxide on alumina (WO3
Al2O3) oxidizing agent (0·85–1·77 mmø; 25 g); separated by silica wool (very fine), which was also used to plug either end of the column. All components and chemicals were supplied by Elemental Microanalysis Ltd.

The calibration standard was acentanilide (C6H5NHCOCH3) (OAS grade; BDH Ltd.), as its C:N ratio (C=67·01%, N=7·82%) is similar to that of Tamar Estuary sediments (Law, 1989). System blanks were obtained by running several empty, ashed Al cups. Whilst three replicate analyses were performed on all samples from Survey A, logistical constraints necessitated that only duplicate samples were analysed subsequently. All analyses were completed between 16–29 May 1992. DOC analysis. DOC analyses were carried out in accordance with the protocols described in Miller et al. (1993b), with the following amendments to the routine. Analytical columns were packed using a fresh batch of the ‘ standard ’ Shimadzu 0·5% Pt/AL2O3 catalyst for each survey. Prior to each analytical run the fresh column was installed in the TOC-500 furnace and multiple injections of MQ water were made, for cleaning/conditioning purposes. This procedure was of an empirical nature: varying volumes of water were injected until the infrared gas analyser (IRGA) signal reached an equilibrium response. Between 20 and 40 injections of 80
l over 1–2 h were normally sufficient for this purpose. Calibration of the instrument was performed immediately prior to and after sample analysis. Freshly prepared caffeine stock and working standards (covering the range 50–200
M added-C) were used, to avoid the problems of rapid degradation encountered previously (Miller et al., 1993a). Typically, the calibration slopes were internally consistent, with differences between ‘ Start ’ and ‘ End ’ of less than 5% around the mean slope in most cases (Table 1). This drift may be the result of gradually enhanced conditioning of the analytical column, or low frequency change in IRGA response, and is accounted for by application of an incremental slope, averaged over the number of

896 A. E. J. Miller T 2. ‘ Total blank ’ and ‘ reference sample ’ data determined during Tamar Estuary surveys

April May Junes June July September October November December a

MQ
M C

SD
M C

CVa, %

n

NADW
M C

SD
M C

CVa, %

n

52 50 27 56 60 48 18 57 36

1 2 4 5 2 3 4 3 1

2·2 3·7 13·7 8·5 3·7 6·2 24·5 6·0 3·5

3 3 4 5 3 3 11 3 3

92 98 89 88 93 99 87 72 85

3 3 4 3 1 4 4 3 3

3·7 2·8 4·0 3·8 0·7 3·7 4·5 4·3 4·1

3 3 3 3 3 3 3 3 3

CV: coefficient of variation.

samples between calibrations, for determination of individual sample concentrations. In all cases the correlation coefficients of the calibration slopes are high (R2 >0·94). Where discrepancies were >5%, the slope that fell within the mean SD range of slopes for calibrations from all surveys has been used. As a result, the relative accuracy of the data for these surveys may be compromised (in the order of a 10–15% offset) but still provides a useful record of pronounced features of the distribution through the estuary. Correction of the DOC measurements. Analysis by HTCO carries a blank associated with the hardware and chemical components of the analyser — the system blank. During the Tamar surveys, true system blanks were not determined. However, pragmatic attempts to compensate for their absence are described. Measurements of the combined ‘ system blank plus water blank ’ were made, through the analysis of MQ water (Table 2). This provides an estimate of the combined blank resulting from components of the analytical system and residual carbon in the MQ water. The MQ can typically contribute 10–30
M C, such that the system blank is likely to be less than 50
M C, and possibly below 20
M C. However, the remaining uncertainty precluded useful derivation and application of this system blank blank value. Subsequent studies using the same type of catalyst have quantified the system blank at <15
M C (Alvarez-Salgado & Miller, 1998). Hence, as the estuarine DOC concentration range is of the order 102
M C, the unaccounted for blank would not have a significant impact on the data interpretation. As a consequence, it was felt that blank correction of this magnitude is too small to be significant in the context of this estuarine study, and blank correction in retrospect has not been applied. To provide an internal reference, against which to examine the consistency of individual analytical runs,

aliquots of aged seawater were repeatedly analysed. This North Atlantic Deep Water (NADW, 4173 m) had been collected on 12/05/90, filtered through an ashed GF/F (450 C, 4 h and stored in a dark glass bottle at 4 C). Sub-samples were taken immediately prior to analysis for each survey. With the exception of November, there is a strong coherence (mean=91
M C; SD=5
M C; n=8) between measured NADW-DOC values over eight months (Table 2). (Note, the NADW value is high relative to the approximately 40–50
M C expected for deep Atlantic Water (Hansell & Carlson, 1998). This is likely the result of contamination during the sample collection stage, rather than during storage, and does not preclude its use as a reference material in the context of this work. The reasoning here is that, in light of the water’s age (in the order of 102 years), DOC concentration changes due to heterotrophic respiration during storage over the time-scale of this investigation were negligible. Further, potential contamination from the volatile organic compounds during storage is also assumed to be negligible, as any such material would be subsequently removed during the purging stage of the decarbonation process.) Adjustment of the measured DOC concentrations was made by normalization to the mean NADW value for each survey. This involved a simple calculation of the difference between the NADW value determined for a particular survey and the mean NADW value (91
M C), followed by subtraction of this difference from the sample DOC concentrations for that particular survey. Although the resulting data remain without blank correction, this method provides internal consistency for this seasonal study. Theoretical dilution lines. To provide an interpretative framework for the investigation of estuarine systems, a chemical variable can be plotted against a conservative

1.20 1.00 0.80 0.60 0.40 0.20 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00

10.0 8.0 6.0 4.0 2.0

POC (µM C)

Chlorophyll a (µg l–1)

10.20 35.00 30.00 9.80 25.00 9.40 20.00 15.00 9.00 10.00 8.60 5.00 8.20 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00

Temperature (°C)

mixing parameter (i.e. chlorinity or salinity) to produce a theoretical dilution line (Liss, 1976; Mantoura & Morris, 1983). This provides a mathematical representation of the conservative mixing of constituents in freshwater and coastal water end-members, where their estuarine distribution is outwith the influence of any input or removal processes. However, the theoretically valid construction of such indices is often precluded by the typically complex nature of estuarine systems, and the lines constructed often deviate from the absolute definition. For this reason the term ‘ applied dilution line ’ (ADL) will be used here to describe the conservative mixing between the most geographically extreme river water end-member and the lowest-DOC-concentration sample from the seaward end of the sampling transect.

Salinity

Estuarine DOC dynamics 897

0

Measured estuarine variables Measurements of estuarine variables made during each survey are presented chronologically in Figures 2–11, as a function of distance measured from the weir at Gunnislake (0 km, Figure 1). All data logger parameters have been calibrated. Spiking on some temperature and turbidity profiles resulted from occasional interference with the logger probes and does not reflect high frequency environmental variability. Where data series are illustrated by lines only, the trace represents output from the data logging system. Where symbols are included, the data points represent observations from discrete samples. In August logger data was unavailable. Thus discrete values for all parameters are plotted and distances along the estuary have been extrapolated from discrete fixed geographical reference points against time, with 10 recorded sightings spread across the full range of survey times/ positions. Data for September, representing a partial tidal cycle, are presented against time, which is more appropriate for the semi-diurnal format. Salinity The calibrated salinity output occasionally appears to deviate from the expected distribution. The most pronounced example of this was during November, where riverine input was maximal following heavy rain, and the FSI was consequently displaced down estuary (c. 12 km). As most surveys were conducted under neap tide conditions, the saline influence was not maximized. Surveys also typically commenced approximately one hour after low tide, with seaward samples being collected first. Hence, fairly low coastal

SPM (mg l–1)

Results 40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Distance from Gunnislake Weir (km)

F 2. Axial transects of physical, physicochemical and biogeochemical parameters along the Tamar Estuary. Samples collected on 22 April 1991. (Series in grey correspond to the right hand axis.)

salinities were sometimes recorded (e.g. November), when the freshwater influence was greatest. A localized minimum is observed at around 20 km during October and, to a lesser extent, December; representing the more significant influence of the River Tavy on the flood tide under autumn conditions of enhanced freshwater influx. Pronounced maxima were observed around 21·5 and 23·5 km during October and November. The former is due to localised run off from the extensive tidal mudflats and salt marsh, and the latter represents an effect of the strong lateral salinity gradient induced when the flood tide is constricted through the narrows under the Tamar Bridge. Localized decreases in salinity around 30 km and 26 km are the result of inputs from St. John’s Lake and the River Lynher respectively. Temperature Temperature profiles reflect seasonality, with maximum temperatures recorded in August (16·8– 18·4 C), and minimum in December (5·9–9·4 C).

Salinity Chlorophyll a (µg l–1)

Chlorophyll a (µg l–1)

0

80.0 60.0 40.0 20.0 0.0 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Distance from Gunnislake Weir (km)

SPM (mg l–1)

SPM (mg l–1)

100.0

18.00 16.00 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00

120.0 100.0 80.0 60.0 40.0 20.0

POC (µM C)

40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0

40.00 16.00 35.00 15.50 15.00 30.00 25.00 14.50 14.00 20.00 15.00 13.50 13.00 10.00 5.00 12.50 0.00 12.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00

Temperature (°C)

60.00 50.00 40.00 30.00 20.00 10.00 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00

POC (µM C)

18.00 35.00 30.00 16.00 25.00 14.00 20.00 15.00 12.00 10.00 10.00 5.00 8.00 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00

Temperature (°C)

Salinity

898 A. E. J. Miller

0

300.0 250.0 200.0 150.0 100.0 50.0 0.0 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Distance from Gunnislake Weir (km)

F 3. Axial transects of physical, physicochemical and biogeochemical parameters along the Tamar Estuary. Samples collected on 21 May 1991. (Series in grey correspond to the right hand axis.)

F 4. Axial transects of physical, physicochemical and biogeochemical parameters along the Tamar Estuary. Samples collected on 13 June 1991 (spring tide). (Series in grey correspond to the right hand axis.)

Estuarine temperature is predominantly controlled by conservative mixing of coastal and river water. In the spring and early summer (April–July), freshwater had temperatures over 3 C higher than the coastal influx. By August, there had been a switch, with highest temperatures noted at the seaward end, decreasing to the FSI, after which there was an increase into the freshwater which did not plateau within the geographical extent of this study. This trend continued throughout the autumn, with a maximum difference in excess of 2 C. By late December, the minimum estuarine temperature had dropped below 5 C, whilst coastal water was still above 9 C. A strong semi-diurnal conservative behaviour was apparent during the partial tidal cycle study in September, where temperature displayed a conservative relationship to salinity.

mum was observed, ranging from c. 35 mg l
1 SPM in April, to almost 400 mg l
1 SPM in July. Regardless of the geographic penetration of saline waters, these maxima were generally encountered in the low salinity (<1 unit) region. However, August and October had most definitely pronounced maxima around mid-estuary (c. 16·1–17·4 km), where salinity was around 25 units. Following a period of heavy rainfall in November, the turbidity maximum was partially masked by the influx of highly turbid freshwater. This influence is illustrated by the presence of a broad secondary turbidity maximum well into the low salinity zone. Whilst the typical pattern below the maximum is of decreasing turbidity with increasing salinity, October shows a very different distribution. The characteristic signal is masked by a relatively high seaward SPM concentration in the region of 80 mg l
1. The development of a broad turbidity minimum in mid-estuary, from c. 16·5–21·5 km, corresponds to complex salinity and temperature distributions. (Note, although there was an element of logger signal interference, resulting

Turbidity Turbidity (reported as SPM) was the most variable parameter. In most surveys a distinct turbidity maxi-

Salinity Chlorophyll a (µg l–1)

60.0 40.0 20.0 0

180.0 160.0 140.0 120.0 100.0 80.0 60.0 40.0 20.0 0.0 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Distance from Gunnislake Weir (km)

F 5. Axial transects of physical, physicochemical and biogeochemical parameters along the Tamar Estuary. Samples collected on 20 June 1991. (Series in grey correspond to the right hand axis.)

in localized artefactual ‘ plateauing ’ of the observed distribution, the data are included as they clearly show the major trends, as reflected by profiles for complementary logger parameters.) Over the partial tidal cycle, in September, turbidity shows semi-diel behaviour in relation to salinity. On the flood tide, salinity increases from c. 1–12, between 09:00–14:00h, after which there is a steady, but slightly shallower decrease with the ebb tide. During the flood, turbidity increases in a steady fashion from must below 16 mg l
1, reaching an apparent plateau (c. 27·0 mg l
1) by 12:30h, where salinity was c. 10 units. On the ebb, turbidity drops off just after 17:00h, with a more pronounced rate of decrease than on the flood. SPM falls from 26·0–15·0 mg l
1, over a salinity range c. 7–2 units, in around 2·5 h. This tidally related pattern illustrates the effect of reinforcement of the turbidity maximum at times of relatively high current velocity, by scouring of bottom sediments. Concentrations plateau at slack water, decreasing 1–2 h later, as sedimentation processes become increasingly significant during periods of decreased tidal current velocities.

SPM (mg l–1)

Chlorophyll a (µg l–1)

80.0

9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00

250.0 200.0 150.0 100.0 50.0

POC (µM C)

SPM (mg l–1)

100.0

18.00 35.00 30.00 17.50 25.00 17.00 20.00 15.00 16.50 10.00 16.00 5.00 15.50 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00

Temperature (°C)

60.00 50.00 40.00 30.00 20.00 10.00 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00

POC (µM C)

16.00 35.00 30.00 15.00 25.00 14.00 20.00 15.00 13.00 10.00 12.00 5.00 11.00 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00

Temperature (°C)

Salinity

Estuarine DOC dynamics 899

0

350.0 300.0 250.0 200.0 150.0 100.0 50.0 0.0 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Distance from Gunnislake Weir (km)

F 6. Axial transects of physical, physicochemical and biogeochemical parameters along the Tamar Estuary. Samples collected on 10 July 1991. (Series in grey correspond to the right hand axis.)

Chlorophyll a Chl a data show broad seasonality, with concentrations over two orders of magnitude higher in May and June compared to July–December and April. The highest overall values were observed during May, where a broad band (c. 7·5–17·5 km) peaked at >50
g l
1. In April, August, November and December the chl a data clearly track turbidity profiles. During October, the picture of a very broad enhanced chl a concentration band stretches from around 15–31 km, centred on the region of significant salinity and temperature perturbation described above. Over the partial tidal cycle there was a major chl a maximum band around 09:30–11:00h, corresponding to a salinity increase of c. 1–4·5 units. Subsequently, a band of relatively minor perturbations are observed up to high tide at 14:00h. Throughout the ebb, chl a concentrations are stable, forming a plateau between c. 2·5–5
g l
1. Hence, on the ebb tide, the level of chl a is relatively constant due to the absence of intact phytoplankton cells.

0

45.0 40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Distance from Gunnislake Weir (km)

F 7. Axial transects of physical, physicochemical and biogeochemical parameters along the Tamar Estuary. Samples collected on 20 August 1991. (Series in grey correspond to the right hand axis.)

Particulate organic material POC has been used as an indicator of the total particulate organic matter (POM). A number of common features occur between surveys. During April– July and December POC increases in tandem with turbidity, producing pronounced maxima at the FSI. Regression of POC against SPM (data not shown) produced significant linear correlation, R2 >0·9 for May–July, and >0·6 for April. The magnitude of the POC maxima rises from c. 8–180
M C, between April and July respectively, then gradually decreases to <12
M C by October. During November, POC showed a pronounced peak at 20 km. Downstream POC values typically decrease to between zero and c. 6
M C with increasing salinity. There was a dip in POC concentration, seawards from around 24 km, during April. This was matched by a minor turbidity peak and dips in salinity and chl a, reflecting the influence of enhanced particulate/low-phytoplankton River Lynher input. Over the partial tidal cycle the POC profile shows a very significant trough over 30 min either side of high

Salinity Chlorophyll a (µg l–1)

5.0

6.00 4.00 2.00

18.00 17.60 17.20

35.00 25.0 30.00 20.0 25.00 15.0 20.00 15.00 10.0 10.00 5.0 5.00 0.00 0.0 07:00 09:00 11:00 13:00 15:00 17:00 19:00 21:00

30.0 SPM (mg l–1)

Chlorophyll a (µg l–1)

10.0

18.40

8.00

POC (µM C)

SPM (mg l–1)

20.0 15.0

18.80

10.00

0.00 16.80 07:00 09:00 11:00 13:00 15:00 17:00 19:00 21:00

POC (µM C)

14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00

12.00

Temperature (°C)

18.80 35.00 30.00 18.40 25.00 18.00 20.00 17.60 15.00 17.20 10.00 16.80 5.00 16.40 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00

Temperature (°C)

Salinity

900 A. E. J. Miller

26.0 22.0 18.0 14.0 10.0 07:00 09:00 11:00 13:00 15:00 17:00 19:00 21:00 Time (hr:min GMT)

F 8. Axial transects of physical, physicochemical and biogeochemical parameters along the Tamar Estuary. Samples collected on 18 September 1991 (partial tidal cycle, from Calstock Barge). (Series in grey correspond to the right hand axis.)

water. This is a coincident with the dip in turbidity, as resuspended detrital bottom sediments settle out during slack water. Axial DOC profiles Discrete DOC concentrations, normalized to the NADW reference water, are presented as the means of duplicate measurements carried out on filtrates from the pigments filtration system (‘ P ’) and the SPM filtration system (‘ S ’) during each survey (Figure 12). The difference between the two measurements is included as the error bar around the mean. Data for August were subject to a number of unresolved analytical problems. These included relatively low (15–20% below the mean) calibration curve slopes, exceptionally high concentrations for the NADW (after analyses of two replicate aliquots), significantly higher MQ water concentrations relative to other surveys, and the over-riding magnitude of the error between replicate (‘ S ’ and ‘ P ’) profiles. Hence, these data are not discussed further in the text.

Salinity Chlorophyll a (µg l–1)

Chlorophyll a (µg l–1)

0

80.0 60.0 40.0 20.0 0.0 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Distance from Gunnislake Weir (km)

SPM (mg l–1)

SPM (mg l–1)

100.0

2.50 2.00 1.50 1.00 0.50 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00

40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0

POC (µM C)

12.0 10.0 8.0 6.0 4.0 2.0

10.00 35.00 30.00 9.50 25.00 9.00 20.00 15.00 8.50 10.00 8.00 5.00 7.50 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00

Temperature (°C)

1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00

POC (µM C)

14.50 35.00 30.00 14.00 13.50 25.00 13.00 20.00 15.00 12.50 10.00 12.00 11.50 5.00 11.00 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00

Temperature (°C)

Salinity

Estuarine DOC dynamics 901

0

60.0 50.0 40.0 30.0 20.0 10.0 0.0 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Distance from Gunnislake Weir (km)

F 9. Axial transects of physical, physicochemical and biogeochemical parameters along the Tamar Estuary. Samples collected on 17 October 1991. (Series in grey correspond to the right hand axis.)

F 10. Axial transects of physical, physicochemical and biogeochemical parameters along the Tamar Estuary. Samples collected on 18 November 1991. (Series in grey correspond to the right hand axis.)

For all surveys the normalized estuarine DOC concentrations fall within the range presented in the literature (Table 3). From the eight analytically verified axial transects (April–July, October–December), the magnitude of DOC concentrations and their relationship with salinity, are highly variable across both spatial and temporal scales. DOC is generally highest in June, July and November, the latter in coincidence with highest freshwater input. Most transects (April– Junes and October–December) show a pronounced DOC spike in the low salinity region. Distributions for individual profiles, and their biogeochemical interpretation will be discussed in detail below. For the partial tidal cycle (Figure 13) direct comparison of DOC concentration with complementary variables (Figure 8) is made straightforward by the use of ‘ Time ’ as the reference axis in both cases. A number of symmetric and asymmetric patterns were observed around high tide (c. 14:00h). The DOC plot, however, shows pronounced internal variability, and appears to bear no simple relationship to the observed behaviour in any other variable (Table 4)

including that expected with salinity. Nevertheless, it is possible to discern a general trend in DOC concentration over the duration of this survey. There is a gradual decrease in DOC from the first samples (c. 9:30h) to around 16:00h, after which concentrations appear to steadily increase to the final sample. The only coincident feature at 16:00h is the peak of the ebb tide temperature deviation, indicating the influx of relatively warm freshwater from upstream. Discussion Applied dilution lines Deviations from the dilution line indicate nonconservative behaviour only if the composition of the riverine and coastal components have remained constant over estuarine water replacement times (Tamar: 2–10 day; Morris et al., 1985), and if deviation within the salinity-variable relationship can be unequivocally confirmed. But, the chemical composition of the riverine end-member typically varies over

Chlorophyll a (µg l–1)

10.00 35.00 30.00 9.00 25.00 8.00 20.00 7.00 15.00 6.00 10.00 5.00 5.00 4.00 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00

Temperature (°C)

Salinity

902 A. E. J. Miller

1.00 0.80 0.60 0.40 0.20 0.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00

SPM (mg l–1)

25.0

dramatic or prolonged rainfall events had occurred. As a consequence, the riverine and coastal endmembers should have similar DOC concentrations. Extrapolation of the respective ADLs to zero salinity produces a spring tide (Junes) DOC concentration of 346
M C, and a neap tide (June) value of 320
M C. The companion values for a salinity of 35 units are 166
M C and 163
C respectively. For November, where samples were only collected at salinities up to 25, extrapolation of the dilution line to 35 units produces a DOC concentration of 58
M C. Whilst this value is within reported range for DOC in coastal waters (e.g. Cauwet et al., 1990; Cauwet, 1990; Cauwet, 1992; Guo et al., 1994), it is significantly below 50% of the value calculated for other surveys in this study. It would thus appear that the high freshwater input and/or absence of higher salinity values are biasing the construction of the dilution line in this case.

20.0 15.0 10.0 5.0 0.0 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Distance from Gunnislake Weir (km)

F 11. Axial transects of physical, physicochemical and biogeochemical parameters along the Tamar Estuary. Samples collected on 16 December 1991. (Series in grey correspond to the right hand axis.)

time scales that are short relative to the freshwater residence time. Advection of these riverine fluctuations into the estuary will disrupt the mixing control of estuarine distributions (Loder & Reichard, 1981; Morris et al., 1985); producing deviation within salinity-parameter relationships, regardless of in situ reactivity (Morris et al., 1981). In addition, significant subsidiary freshwater inputs, such as river tributaries, serve to increase the system’s overall complexity. Consequently, it is generally not possible to unambiguously interpret sporadic deviations from the dilution/mixing curves, often evident around the FSI. In the longer term, only persistent non-conservative features of the estuarine distributions may be considered to be fully reliable indicators of estuarine reactivity (Morris et al., 1985). Confidence in the value of the DOC data series and the ADL approach can be gained using the data for the both surveys in June. These represent different states of the tide, but are relatively closely spaced in time, just seven days apart. Between surveys there was no indication of enhanced biological activity and no

Spatial distribution In the majority of cases (April–July and October) DOC concentrations show complex and significant deviations from the ADL. Notwithstanding the limits of precision, April shows a major input between salinities of 3–6. Whilst this occurs downstream of the turbidity, chl a and POC peaks, on an incoming tide, it coincides with the position of a ‘ primary treatment ’ sewage effluent outfall near Bere Alston (c. 10 km from the weir, South West Water, unpubl. data). May and June also show broad positive deviation corresponding with this geographical region. Mantoura and Mann (1979) suggested that positive anomalies of up to 4000
M C (sic) in DOC signal in the Bristol Channel, U.K. were probably due to major localized sewage inputs. The influence of other large-scale sewage inputs is most clearly demonstrated during April–Junes and October. The very distinct peak at salinities around 25, during April, corresponds directly with the outfall at Saltash (c. 24 km). The secondary rise at salinity 32 coincides with Camels Head (c. 26 km) and is present during May and October around salinities 34 and 30 units respectively. Peaks are also observed during October and December in the region of the outfall at Ernesettle (c. 21·5 km, salinities c. 27 and c. 30 units respectively). It has previously been concluded that such localized anthropogenic inputs are quantitatively insignificant compared to the estuarine DOC flux (Mantoura & Woodward, 1983), and results from this study concur with that conclusion. The natural DOC signal may be strongly influenced by a number of in situ environmental variables. In an

Estuarine DOC dynamics 903

F 12. Axial transects of DOC concentrations and the applied theoretical dilution lines along the Tamar Estuary. (Concentrations are represented as the means of duplicate samples from separate filtration systems. The difference between the two measurements is represented by the error bar around the mean.)

attempt to define simple relationships, linear regression analyses were performed between DOC and the other parameters measured (Table 4). DOC in this study shows a consistently strong relationship with salinity over estuarine transects, although this can not be categorized as truly conservative. The single exception to this may be during November, where the enhanced, high-DOC river flow clearly dominated the estuarine gradient. Even here there is a definite sag, induced by removal processes acting around the FSI. In this study is appears that DOC is generally uncoupled from sedimentary and biological activity within the estuary. Sharp DOC (and POC) peaks often occur in the region of the turbidity maximum and these appear to be a function of the extreme physicochemical conditions. It has been proposed (Morris et al., 1978) that there is an intimate link between biological activity and DOC through release of osmolytes at the FSI, during mass mortality of stenohaline phytoplankton. Whilst chl a peaks do generally occur in the region of the FSI, there are no overall trends between DOC and chl a concentration in this region. Turbidity also consistently peaks in this region, with generally concomitant peaks in POC. It is thus proposed that the observed peak in DOC may result from mixing of organic rich porewaters (c. 3000–6500
M C; Mantoura & Woodward, 1983),

904 A. E. J. Miller T 3. Estuarine DOC concentrations determined using a range of analytical methodologies Reference

Technique DOC,
M C

Estuary

Morris et al., 1978 Mantoura and Mann, 1979 Laane, 1980 Degens, 1982

Tamar, U.K. Bristol Channel, U.K. Ems–Dollart, NL Elbe, Germany Weser, Germany Ems–Dollart, NL Mantoura and Woodward, 1983 Severn Estuary and Bristol Channel, U.K. Sharp et al., 1988 Delaware Estuary, U.S.A. Suzuki and Tanoue, 1991 Sharp et al., 1993 Aminot et al., 1990 Elorn Estuary, France Albert and Griffin, 1996 Saltmarsh Estuaries, south-eastern U.S.A. Cauwet and Siderov, 1996 Lena River, Siberia Argyrou et al., 1997 Lake Pontchartrain Estuary, Louisiana, U.S.A. Bianchi et al., 1997 Sabine–Neches Estuary, Texas/Louisiana, U.S.A.

Salinity

UV/PS UV/PS PS PS PS PS UV/PS HTCO

310–465 60–315 35–1300 165–175 150–400 130–480 60–580 400–480 280

0–10 17–35 <1–31 <1–32 <2–31 14–30 0–>34·5 River Saline

HTCO HTCO HTCO HTCO HTCO

65–380 100–2870 203–661 441–707 366–1740

1–31 0–31 0·06–33·21 0–8 0·2–9·4

450

12.00

400

10.00

350 300 250

8.00 6.00 4.00

Salinity

DOC (µM C)

UV/PS, persulphate-promoted ultraviolet photo-oxidation (wet photochemical DOC analysis); PS, persulphate oxidation (wet chemical DOC analysis technique); HTCO, high temperature catalytic oxidation).

2.00

450

30.0

400

25.0

350 300 250

20.0 15.0

SPM (mg l–1)

DOC (µM C)

0.00 200 07:00 09:00 11:00 13:00 15:00 17:00 19:00 21:00

200 10.0 07:00 09:00 11:00 13:00 15:00 17:00 19:00 21:00 Time (hr:min)

F 13. Discrete DOC concentrations against continuous salinity and discrete SPM concentrations during a partial tidal cycle of the Tamar Estuary, sampled at Calstock Barge on 18 September 1991. (Series in grey correspond to the right hand axis.)

desorption from particles, and/or disaggregation of macroaggregates (Eisma et al., 1982) during the tidally induced scouring of bottom sediments. More recently, a similar phenomenon has been observed around the turbidity maximum during surveys of the Humber Estuary, but the mechanisms responsible are not clearly defined (Alvarez-Salgado & Miller, 1999). So, with the exception of salinity, there are no intimately linked linear relationships along the extent of

the estuary. Hence, an internally complex system is presented, showing considerable lateral variability along the salinity gradient. Temporal variability There is a general seasonality to the amplitude of the DOC signal throughout the estuary. Lowest overall concentrations occur during December and April, with highest values observed from early June clearly through to July. After periods of prolonged rainfall (November), river water exerts a strong influence on concentration, as this is characteristically richer in DOC than seawater and has shown the greatest overall variability in signal. Regardless of the magnitude of riverine input or the end-member DOC concentrations (Table 5), the consistently dominant influence on DOC distribution is clearly the relationship with salinity (Table 4). Whilst the discussions immediately above have demonstrated, at least qualitatively, that there are significant deviations from ADLs, it appears that these are the result of allochthonous inputs, and not indicative of in situ production. Extrapolation of the ADLs enabled determination of theoretical concentrations for the freshwater and saline end-members of the estuary (Table 5). Subsequently, these values have been used to calculate a first-order estimate of the monthly-averaged DOC input to the estuary; using mean daily flow over Gunnislake Weir (Evans et al., 1993) and the ‘ Salinity-0 ’ DOC concentration (Figure 14). Contrary to the findings of Mantoura and Woodward (1983). DOC does not show linear covariation with river flow.

Estuarine DOC dynamics 905 T 4. Squared correlation coefficient matrix for DOC vs complementary parameters from Tamar Estuary surveys

April May Junes June July September October November December

Temperature

Salinity

Chl a

POC

Turbidity

0·5539 0·8141 0·6546 0·4208 0·4128 0·3027 0·4447 0·7346 0·5730

0·6119 0·6882 0·8610 0·7503 0·8491 0·1211 0·7252 0·9717 0·4096

0·0903 0·0244 0·2847 0·0020 0·2084 0·2606 0·2761 0·5644 0·1034

0·2434 0·1308 0·2360 0·2800 0·0555 0·0375 0·3330 0·4309 n/a

0·1997 0·0969 0·0909 0·2102 0·0738 0·2268 0·2577 0·0311 0·0812

T 5. End-members of the applied dilution lines and resultant riverine DOC fluxes calculated for the Tamar Estuary

April May Junes June July October November December a

Salinity ‘ 0 ’ DOC,
M C

Salinity ‘ 35 ’ DOC,
M C

River flowa 1000 m3 month
1

DOC influx tonnes C month
1

193 294 346 320 442 284 433 193

153 152 166 169 201 152 58 138

52 496 17 961 14 964 14 964 24 706 22 356 94 878 47 969

122 63 62 58 131 76 509 111

Data taken from Evans et al. (1993): monthly average derived from the mean daily flow over the weir (m3 s
1)

From the values in Table 5, apparent seasonality in overall DOC concentrations is not duplicated by the carbon fluxes, which are controlled by the combination of river flow and freshwater DOC concentration. In this context, November provides the greatest C-flux (c. 500 tonnes) into the estuary; up to c. 300% higher than the second largest (c. 130 tonnes; July). As discussed above, November is clearly representative of an extreme situation, brought about by significantly enhanced freshwater input. At a different level of intercomparison, it may be possible to define consistent small-scale features, or significant differences between profiles. There appear to be no replicated deviations in DOC concentration at points that are common to the majority of surveys, and no consistent trends between DOC and complementary estuarine variables. Whilst significant sewage inputs have been proposed to account for features in a spatial context, there must be some mechanisms(s) of control leading to variation in behaviour between surveys. One possibility is that the magnitude of the sewage influence is a function of the relation-

ship between discharge and sampling times. A further mechanism for the masking of a sewage-related DOC signal, might be the occurrence of localized large-scale microbiological respiration of DOC in this region. (Sags in DO distribution have been related to enhanced microbial respiration at sites of sewage input (Morris et al., 1978; Mantoura & Mann, 1979)). Another possibility is the displacement of high-DOC river water down estuary during periods of enhanced flow (e.g. July and November). Although a sewage outfall exists at Calstock, there were no noticeable effects on DOC concentrations during the partial tidal cycle survey (Figure 13). There is only circumstantial evidence in support, but it is possible to propose a tentative mechanism for the observed DOC distribution. Initially, concentrations decrease on the flood tide, reflecting the increasing proportion of relatively low-DOC saline water. Although salinity maximises around 14:00h, DOC continues to decrease. There is no apparent relationship with chl a. Although there is little coherence on the flood tide, POC and DOC track quite closely,

906 A. E. J. Miller 600

Riverine [DOC] (µM C)

450 500

400 350

400

300 250

300

200 200

150 100

Conc. Flux

50 0

5

10

15 20 25 30 River flow (m3 s–1)

35

100 0 40

Riverine DOC flux (tons C month–1)

500

F 14. Riverine DOC concentrations and monthly averaged riverine DOC efflux to the Tamar Estuary for each of the axial transects. Monthly river flows are derived from the mean daily flow over Gunnislake Weir (Evans et al., 1993).

onwards from c. 15:00h on the ebb. It appears that turbidity is a major controlling influence on the distribution of DOC during this survey (Figure 13). As turbidity increases DOC concentrations generally go down. This suggests that there is considerable adsorption onto particles (Keil et al., 1994) and/or flocculation (Alber & Valiela, 1994) within this limited salinity zone, and that rapid reactions involving the reversible adsorption of DOC onto SPM may be important mechanisms of estuarine DOC control (Mantoura & Woodward, 1983). At c. 15:00h the influence of the freshwater is seen as a peak in the temperature signal and dip in salinity for approximately 2·5 h (Figure 8). Outwith the between-DOC-sample variability over this period, this freshwater input is not manifest on the DOC profile. Whilst a distinct increase in DOC concentration would have been expected, its absence suggests that the capacity for adsorption and/or flocculation is great enough to effectively remove the additional supply of DOC flowing into the region on the ebb tide. It is not possible to categorically confirm this process, as these ‘ fixed station ’ measurements were not complemented by lateral coverage. In a similar study, Mantoura and Woodward (1983) showed strong inverse correlation between DOC and salinity, but poor agreement with SPM concentration. The authors concluded that advection was providing the dominant control, and rejected the reversible adsorption hypothesis. Conclusions DOC measurements from nine estuarine surveys have produced a valuable seasonally comparable data set.

Although absolute blank correction was not possible, the repeated analysis of a ‘ stable ’ reference material has enabled an adjustment of concentrations against a narrowly defined mean value. The resulting series of DOC profiles are internally consistent throughout the survey period. As a result, the data provided here constitute a comprehensive study of HTCO-DOC in the Tamar Estuary. Over seasonal and tidal timescales a picture of great variability in DOC distribution has emerged. It has been possible to identify specific geographically/hydrodynamical zones of perturbation and propose mechanisms of control for the observations made in those regions. A number of major temporal and spatial features have been elucidated: (1) Concentrations reported here are within the wide overall range (c. 35–2870
M C) reported across a number of other estuarine studies (Table 3), even though measurements were performed with a variety of analytical methodologies. (2) The dominant control on lateral DOC transport is salinity distribution, indicating tidal mixing between freshwater and saline waters. This is consistent with published data over the last 40 years (e.g. Duursma, 1961; Sieburth & Jensen, 1968; Loder & Hood, 1972; Laane, 1980; Mantoura & Woodward, 1983; Sharp et al., 1988). The mixing gradient was clearly a function of the highly variable organic-rich riverine input, compared to relatively low frequency fluctuation in the seawater signal. Surveys at spring and neap tide conditions (spaced closely in time) show coherence in DOC concentrations and distributions, largely reflecting the environmentally stable conditions in the catchment between sampling periods. During the partial tidal cycle, the DOC signal was apparently uncoupled from salinity, but inversely related to the turbidity. Under this circumstance, it is proposed that the control mechanism is related to potentially reversible adsoprtion processes (acting between the DOC and SPM) exerting an over-riding influence on the mixing process. (3) Construction of ADLs has facilitated observation of significant deviations from conservative mixing over the estuary. There is distinct reactivity around the FSI, and major sewage-related inputs from mid-to-outer estuary. There appear to be no clearly defined relationships between lateral DOC distribution and pigments POM or turbidity. This contrasts dramatically with the classical work of Mantoura and Woodward (1983), on waters from the Severn Estuary and Bristol Channel. They found strong correlation between Riverine DOC concentration and river flow and inverse correlation with salinity over a tidal cycle, but no apparent relationship with turbidity, and positive seasonal seawater DOC variation with chl a (indicating a significant coastal phytoplankton source). It is probable that

Estuarine DOC dynamics 907

the greatly contrasting DOC-related behaviour between estuaries stems from the systems’ widely differing characteristics. The River Severn/Bristol channel system has a large catchment area (c. 104 km2), producing highly turbid waters with long residence time (c. 200 day). In comparison, the Tamar Estuary has a much smaller catchment (c. 1700 km2), a short water residence (c. 2–10 day) and exhibits generally low turbidity, apart from its characteristic tidally-induced maxima. Consequently, the comparison of chemical reactivity between such environments should not necessarily be expected to demonstrate consistency at any level. (4) The clearly defined deviations from the linear mixing process illustrate that DOC is a reactive component within the estuary. However, there were no investigations of the nature of DOM through the estuary, and the absence of DO and nutrient data has necessarily left the proposal of input/removal mechanisms in a tentative state. Ideally, to fully interpret DOM-related processes in such complex biogeochemical environments, future studies must aim to incorporate high quality environmental data and resolution of component species/groups of the bulk DOC. Under these circumstances it may be possible to identify which components are most reactive, and under what circumstances. Acknowledgements The author offers thanks to the many staff from the Plymouth Marine Laboratory who collaborated during the fieldwork programme and in the laboratory. This work contributed to a Ph.D. thesis (Miller, 1996) that was conducted under the supervision of Dr M. R. Preston (University of Liverpool) and Professor R. F. C. Mantoura (Plymouth Marine Laboratory). Studies were supported by the U.K. Natural Environment Research Council through BOFS Special Topic award GST/02/372. Comments supplied by two anonymous reviewers have proved invaluable in the completion of this manuscript—thank you. References Ackroyd, D. R. 1983 The removal and remobilisation of heavy metals during estuarine mixing. Ph.D. thesis, C.N.A.A., Plymouth Polytechnic. Alber, M. & Valiela, I. 1994 Biochemical composition of organic aggregates produced from marine macrophyte-derived dissolved organic matter. Limnology and Oceanography 39, 717–723. Alberts, J. J. & Griffin, C. 1996 Formation of particulate organic carbon (DOC) in salt marsh estuaries of the southeastern United States. Archive of Hydrobiology Special Issues, Advances in Limnology 47, 401–409. Alvarez-Salgado, X. A. & Miller, A. E. J. 1998 Simultaneous determination of dissolved organic carbon and total dissolved

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