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Estuarine, Coastal and Shelf Science 79 (2008) 707–717

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Estuarine, Coastal and Shelf Science

journal homepage: www.elsevier .com/locate/ecss

Spatial and temporal distribution of chromophoric dissolved organic matter(CDOM) fluorescence and its contribution to light attenuation in UK waterbodies

J. Foden*, D.B. Sivyer, D.K. Mills, M.J. Devlin 1

Centre for Environment, Fisheries and Aquaculture Science (Cefas), Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK

a r t i c l e i n f o

Article history:Received 12 September 2007Accepted 24 June 2008Available online 2 July 2008

Keywords:chromophoric dissolved organic matter(CDOM) fluorescenceeuphotic depthlight attenuationsalinitysuspended particulate matter (SPM)UK waters

* Corresponding author: School of EnvironmentalAnglia, Norwich, Norfolk NR4 7TJ, UK.

E-mail address: jo.foden@uea.ac.uk (J. Foden).1 Present address: Catchment to Reef Research Gro

versity, Townsville, QLD 4811, Australia.

0272-7714/$ – see front matter Crown Copyright � 2doi:10.1016/j.ecss.2008.06.015

a b s t r a c t

Vertical attenuation of light through the water column (Kd) is attributable to the optically active com-ponents of phytoplankton, suspended particulate material (SPM) and chromophoric dissolved organicmatter (CDOM). Of these, CDOM is not routinely monitored and was the main focus of this study.Concentrations and spatio-temporal patterns of CDOM fluorescence were investigated between August2004 and March 2006, to quantify the correlation coefficient between CDOM and salinity and to bettercharacterise the contribution of CDOM to Kd. Sampling was conducted at a broad range of UK and Re-public of Ireland locations; these included more than 15 estuaries, 30 coastal and 70 offshore sites in thesouthern North Sea, Irish Sea, Liverpool Bay, Western Approaches and the English Channel.An instrument package was used; a logger with multi-sensor array was deployed vertically through thewater column and concurrent water samples were taken to determine salinity, CDOM fluorescence andSPM. Surface CDOM fluorescence values ranged between 0.05 and 16.80 S.Fl.U. (standardised fluores-cence units). A strong, negative correlation coefficient of CDOM to salinity (r2 ¼ 0.81) was found. CDOMabsorption (aCDOMl) was derived from fluorescence measurements and was in the range 0.02–2.2 m�1

with mean 0.15 m�1. These results were comparable with direct measurements of aCDOMl in the samegeographic regions, as published by other workers.Spatial differences in CDOM fluorescence were generally explicable by variation in salinity, in localconditions or catchment areas; e.g. CDOM at the freshwater end was 3.54–11.30 S.Fl.U., reflecting thevariety of rivers sampled and their different catchments. Temporal changes in CDOM fluorescence wererelated to salinity. A significant and positive correlation was found between CDOM and Kd, and althoughCDOM was found to be less influential than SPM on Kd, it was still of significance particularly in coastaland offshore waters of lower turbidity.

Crown Copyright � 2008 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Formal assessments of marine eutrophication are required underEuropean legislation, such as the Water Framework Directive (WFD)and The Convention for the Protection of the Marine Environment ofthe North–East Atlantic, known as the ‘‘OSPAR Convention’’ (OSPAR,1997). A simple dose-response model of nutrient enrichment to riskof eutrophication does not consider the important role light plays inmarine waters. Cloern (2001) recognises system attributes that‘filter’ responses to changes in nutrient loading, including: the un-derwater light climate, horizontal exchange, tidal mixing, grazingand biogeochemical processes. Tides, turbulence, salinity gradients,

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bed stress and turbidity are some of the component parts of thefilter; e.g. Monbet (1992) found responses in mean annual chloro-phyll concentrations to mean dissolved inorganic nitrogen(DIN ¼ NO2 þ NO3 þ NH4) concentrations differed in estuariesdepending on mean tidal range. This complex response determinessusceptibility, which influences the assessment of eutrophicationstatus. The light climate is highly variable in UK waters and thereforeof particular significance with regard to the risk of eutrophication.

The vertical attenuation of downwelling light through the watercolumn (Kd) is attributable to four optically active compounds(OAC): chromophoric dissolved organic matter (CDOM), phyto-plankton, suspended particulate matter (SPM) and seawater itself(Roesler et al., 1989; Babin et al., 2003). It is recognised that gen-erally there is an inverse gradient in Kd with increased distancefrom shore; highest coefficients occurring in transitional watersand the lowest in the offshore environment (e.g. Pfannkuche, 2002;Retamal et al., 2008). The depth of the euphotic zone can be cal-culated from measurements of Kd (e.g. Kirk, 1994; Behrenfeld andFalkowski, 1997; Goosen et al., 1999). It is the depth at which

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J. Foden et al. / Estuarine, Coastal and Shelf Science 79 (2008) 707–717708

photosynthetically active radiation (PAR) is reduced to 1% of thelevel just below the sea surface. This is the maximum depth of thewater column in which net primary production by phytoplanktontakes place. Quantifying euphotic depth and the extent to which Kd

is determined by one or more OACs, will provide better informationon the light climate for phytoplankton and for submerged aquaticvegetation (SAV), and susceptibility to eutrophication.

Of the four main OACs that determine Kd and therefore euphoticdepth, CDOM is not routinely monitored in UK waters. CDOM isa light-absorbing constituent of the dissolved organic matter pool.In estuaries and coastal waters CDOM is affected by land–oceaninteractions (Nelson et al., 2004; Kostoglidis et al., 2005). Terrestrialorigin CDOM largely consists of a variety of aliphatic and aromaticpolymers derived from the degradation of terrestrial and aquaticplant material, excretion and grazing (Kirk, 1994; Steinberg et al.,2000), which enter the coastal environment via rivers and runoff(Harvey et al., 1984). In coastal environments, light absorption byCDOM may protect aquatic organisms from potentially harmfulradiation, but can also reduce the amount of PAR available forgrowth (Blough and Del Vecchio, 2002).

Studies have shown that the mixing processes between riverand marine waters mainly control the distributions of CDOM incoastal and estuarine waters (Blough et al., 1993; Højerslev et al.,1996; Chen et al., 2004, 2007). In Tampa Bay, Florida, Chen et al.(2007) found a conservative relationship between CDOM fluores-cence and salinity along a salinity gradient and CDOM distributionwas dominated by mixing. Variability in CDOM concentrationsbetween rivers entering the bay appeared to be related to differ-ences in watersheds; e.g. agriculture-intensive or industrial landuse. Deviation from conservative mixing in the Pearl River has alsobeen explained by differences in water mass source (Chen et al.,2004).

CDOM can be measured by absorption or fluorescence methods.CDOM fluorescence was measured during this study and these re-sults are reported in detail, but to facilitate further comparison withalternative CDOM measurements by other workers, absorption hasalso been derived. Absorption spectra are broad and unstructured,typically decreasing with increasing wavelength in an exponentialfashion (Blough and Del Vecchio, 2002). Measurements of fluo-rescence can be used to derive CDOM absorption coefficients wherea linear relationship is established (Hoge et al., 1993; Green andBlough, 1994; Vodacek et al., 1995). The fluorescence emission hasbeen found to be well correlated with absorption coefficient andlinear relationships have been observed in numerous studies, invaried types of water bodies (e.g. Ferrari and Tassan, 1991; Hogeet al., 1993; Ferrari et al., 1996; Nieke et al.,1997; Ferrari and Dowell,1998). Within a geographical area the variation in the ratio isgenerally no more than w15–30% (Blough and Del Vecchio, 2002)and such relationships have been used to derive CDOM absorptioncoefficients from in situ measurements of fluorescence (e.g. Hogeet al., 1995; Vodacek et al., 1995).

A comparative study of the spatio-temporal variability in Kd andthe main light-attenuating OACs of estuaries, coastal and offshorewaters of the UK was conducted over an 18-month period, betweenAugust 2004 and January 2006, to improve the description and theunderstanding of OACs therein. CDOM fluorescence, suspendedparticulate load and chlorophyll concentration were measured indiscrete water samples. Turbidity, chlorophyll fluorescence, CDOMfluorescence, conductivity, temperature, pressure and downwellingPAR irradiance were recorded in vertical profiles using a loggerwith an instrument array. This paper focuses in particular on CDOMfluorescence concentration: its influence on the vertical light at-tenuation coefficient (Kd); its correlation with salinity; its spatialvariation and variability by water body type; its temporal vari-ability; and derived CDOM absorption values. The influence of SPMon Kd is discussed briefly herein, but is presented in greater detail

elsewhere (Devlin et al., 2008). It is intended that the data pre-sented provide a useful baseline study characterising CDOM con-centrations in a variety of UK waters.

2. Materials and methods

2.1. Survey design

The fieldwork programme investigated the spatial and temporalvariation of the OACs within and between water bodies and ty-pologies, and also along gradients that may traverse boundariesacross water bodies. Sampling was either conducted opportunis-tically on research cruises in the North Sea, English Channel, IrishSea and Western Approaches, or during dedicated near-shore sur-veys in Portland Harbour, Solent, Milford Haven, Isles of Scilly, LochLinnhe, the firths of Lorne and Forth and the southern North Sea.The locations of sampling sites are plotted in Fig. 1. The UK gov-ernment agency, Cefas (Centre for the Environment, Fisheries andAquaculture Science), deploys and maintains SmartBuoys at foursites in UK waters. Each buoy houses a suite of autonomous in-struments measuring a range of variables at high frequency (dailyto hourly) at fixed-point locations (http://map.cefasdirect.co.uk/smartbuoyweb/StaticMapPage.asp). Two buoys are located on theeast coast at the Warp anchorage in the Thames estuary and at theWest Gabbard in the southern North Sea. Two buoys are also lo-cated off the west coast in the Liverpool Bay region and are knownas Liverpool Bay 1 and Liverpool Bay 2 (Fig. 1). Sampling was alsoconducted opportunistically during maintenance and researchcruises focussed on these sites.

The water body classification scheme adopted is the typologydesigned for the European WFD (UKTAG, 2003). The WFD requiresEU Member States to type water bodies according to specific fac-tors. Transitional waters (TW) are characterised by tidal range,mixing, salinity and substratum. Coastal waters (CW) are defined asthose waters within 1 nautical mile of the coast and are charac-terised by tidal range, salinity and exposure (defined by windstrength) (Vincent et al., 2002; Rogers et al., 2003). Table 1 detailsthe specific characteristics of these types. This was felt to be anappropriate scheme because the factors on which it is based, suchas horizontal exchange and tidal mixing, are broadly in line withthose abiotic components of the ‘filter’ described by Cloern (2001)(see Section 1). Sites falling outside WFD water bodies have beenclassified as either offshore, if further offshore than the seawardlimits of coastal water bodies (CW), or as riverine if upstream ofa TW.

2.2. Sample collection and analysis and water column profiles

For CDOM and SPM, bucket or niskin samples were collected atthe surface (w0.5 m depth). Following the methods of Chen andBada (1992), CDOM sample water was filtered immediately aftercollection through pre-combusted (500 �C for 5 h) Whatman GF/F47 mm glass fibre filters (Whatman plc, USA). Water was filtered asa precaution to remove particulate matter and eliminate any pos-sible contribution from phytoplankton degradation during storage,even though filtered and unfiltered samples have been found toyield identical results (Chen and Bada, 1992). Filtered samples werestored in amber glass bottles with aluminium foil-lined caps.Initially duplicate samples were stored in pre-combusted and non-combusted bottles, but as the fluorescence results were correlatedto a high degree of significance (r2 > 0.98 and p < 0.05) furthersamples were stored in clean, non-combusted amber glass bottles.

Samples were stored at w12 �C and analysed within 4 days.Natural CDOM fluorescence is highly dependent upon the tem-perature at which analyses are made; maximum sensitivity occursbetween 21 and 25 �C, but shows no or minimal change after

Fleet &Portland

0°0'0"

5°0'0"W

5°0'0"W

10°0'0"W

10°0'0"W

55°0'0"N

55°0'0"N

50°0'0"N50°0'0"N

0 100 200 Kilometers50

MilfordHaven

Isles ofScilly

Western Approaches

LiverpoolBay

SolwayFirth

Firth ofClyde

Firth of Forth

Orwell & Stour

Deben

Crouch

Thames

Fig. 1. Location of CDOM sampling sites (þ) around the UK and Republic of Ireland coasts and offshore. Greyed areas are WFD water bodies (TWs and CWs). Cefas SmartBuoylocations are indicated; in Liverpool Bay SmartBuoy 1 (:) and Liverpool Bay SmartBuoy 2 (B) (main map) and at West Gabbard (6) and Warp (:) (insert map). The insert mapshows the East Anglia coast and the boxed area within is the West Gabbard sampling grid.

J. Foden et al. / Estuarine, Coastal and Shelf Science 79 (2008) 707–717 709

storage at room temperature (Willey and Atkinson, 1982; Chen andBada, 1992). To minimise such effects samples and standards wereanalysed in a constant temperature room. Filtered near-shore waterhas been shown to increase in fluorescence over several monthswhen left at room temperature in a dark bottle (Chen and Bada,1992), so samples not analysed within 4 days were frozen.

For SPM, a known volume was filtered immediately after col-lection through pre-weighed Cyclopore� Track Etched Membrane0.4 mm polycarbonate (hydrophilic) 47 mm diameter filter papers(Whatman plc). Filters were then rinsed with demineralised waterto remove salt residues. The papers were dried under vacuum ina desiccator until a constant weight was obtained and the SPM thencalculated.

To measure chlorophyll concentrations 250 cm3 of seawaterwere passed through Whatman GF/F glass fibre filters. These filterswere extracted overnight in 90% acetone neutralised with NaHCO3

in darkness in a refrigerator as described by Tett (1987). Aftercentrifugation of the extract fluorescence was measured usinga Turner Designs Model 10 fluorometer before and after

acidification with 8% HCl and used to calculate concentrations ofchlorophyll. The fluorometer had been calibrated using pure solu-tions of chlorophyll-a (Sigma Chemical Co, ORIGIN) with concen-tration determined spectrophotometrically. This method throughthe acidification step will correct for degradation products, i.e.phaopigments, however, it will not correct for presence of chlor-ophyllide-a, a photosynthetically inactive precursor to chlorophyll-a. Only through chromatography will it be possible to separatechlorophyllide-a. Therefore all results from this method arereported as measurements of extracted chlorophyll and notchlorophyll-a.

Concurrent with these water samples, data for a variety of pa-rameters were recorded vertically through the water column usinga profiler that could be deployed by hand or boat-winch. Profileswere taken using an instrument package controlled by a logger,ESM2 (Ecosystem Monitor 2); a Cefas in-house design, logging at2 Hz. Pressure was measured with a Druck 5 bar PDCR (GE Sensing,UK) and converted into depth (m). CTS-C-1E OEM-type FSI (Fal-mouth Scientific Instruments, USA) sensors measured temperature

Table 1Predominant typology characteristics of main transitional (TW) and coastal water (CW) types and the number of CDOM samples taken

Type Exposure Tidalrange

Mixingcharacteristics

Salinity Depth No. of samples Example water bodiessampled

Offshore Exposed – – Marine – 204 Western approaches,southern North Sea

CW1 Exposed Macro – Marine – 7 South PembrokeshireCW2 Exposed Meso – Marine – 40 Isles of ScillyCW4 Moderately exposed Macro – Marine – 37 Milford Haven (coastal),

North Norfolk, Solway FirthCW5 Moderately exposed Meso – Marine – 78 Essex, North KentCW8 Sheltered Meso – Marine – 63 Portland Harbour,

Chichester HarbourCW10 Sheltered Micro – Marine – 19 Fleet LagoonCW11 Sheltered Meso – Marine – 2 Loch FeochanCW12 Sheltered Meso – Marine – 2 Loch LinnheTW1 Sheltered Macro Partly mixed/stratified Mesohaline/polyhaline (5–30) Intertidal/shallow sub-tidal 37 Bure, Waveney, Yare & LothingTW2 Sheltered Meso Partly mixed/stratified Mesohaline/polyhaline (5–30) Intertidal/shallow sub-tidal 45 Orwell & Stour, Beaulieu RiverTW3 Sheltered Macro Fully mixed Predominantly polyhaline (18–30) Extensive intertidal areas 22 Milford Haven (estuarine),

ThamesTW4 Sheltered Meso Fully mixed polyhaline

or euhalinePolyhaline/euhaline (>18) Extensive intertidal areas 4 Southampton Water

Riverine Sheltered – – Freshwater – 25 Norfolk Broads

J. Foden et al. / Estuarine, Coastal and Shelf Science 79 (2008) 707–717710

and conductivity, from which salinity was calculated. DownwellingPAR (E) irradiance was recorded with a cosine 192 SA quantum ir-radiance sensor (LI-COR Biosciences, USA) at wavelengths 400–700 nm, modified by Cefas with a logarithmic amplifier to improveresolution at low light levels. Chlorophyll was measured witha Seapoint Chlorophyll Fluorometer (Seapoint Sensors Inc, USA)and CDOM with a TriOS MicroFlu CDOM fluorometer (TrioOSGmbH, Germany). Turbidity (as optical backscatter) was recordedwith an OBS STM-BH Seapoint instrument. Chlorophyll, CDOM andturbidity records were all calibrated with the concurrent watersamples described above.

Profiles were measured on the sunny side of the vessels, fromjust below the water surface (E0) to the bottom, or a maximumdepth of w49 m. The profiler’s rate of descent was�0.5 m s�1 (datalogged at <0.25 m depth intervals). The light attenuationcoefficient Kd (m�1) was determined as the slope of the linear re-gression equation of the natural logarithm of PAR against depth.

Ln PAR ðrangeÞ ¼ Kd*depth ðrangeÞ þ aþ error (1)

Where ‘range’ is generally the euphotic depth or water depth ifshallower than euphotic depth and the error is assumed to havea normal distribution with mean zero and constant variance. Eu-photic zone depth (Zeu) was assumed to be equal to the 1% lightdepth and calculated as:

Zeu ¼ 4:6=Kd (2)

(e.g. Kirk,1994; Behrenfeld and Falkowski,1997; Goosen et al.,1999).

2.3. CDOM analysis

Fluorescence of CDOM samples was measured at controlledroom temperature (18–19 �C) using a Turner Fluorometer Model 10Series (Turner Designs, USA) fitted with a 310–390 nm excitationfilter and 410–600 nm emission filter, with 10-327R attenuatorplate. Fluorescence measurements were based on the method ofHoge et al. (1993) as modified by Ferrari and Tassan (1991) andFerrari and Dowell (1998). The modifications were made to allowfor uniformity and comparison with findings of other workers (e.g.Hoge et al., 1993; Green and Blough, 1994; Ferrari and Dowell, 1998;Ferrari, 2000). A solution of 0.01 mg l�1 quinine sulphate in 0.5 MH2SO4 identically analysed is defined as 10 normalised fluorescenceunits and sample fluorescence intensity, Fs, is reported in

standardised fluorescence units (S.Fl.U.) (Ferrari and Dowell,1998) –also reported as St.F.U. (Ferrari, 2000):

�Fs=Fqs

�� 10 ¼ FsðlÞ ðS:Fl:U:Þ (3)

Where Fs is the standardised fluorescence, Fs is the fluorescence ofthe sample, Fqs is the fluorescence of the quinine sulphate and l isthe excitation wavelength. As the Turner fluorometer’s sensitivity ismaximised at 8 the fluorescence standard was a solution of0.008 mg l�1 quinine sulphate in 0.5 M H2SO4 and the instrumentwas set to read 8 using this standard, relative to the blank of ultra-pure water (UPW). To account for any possible instrument drift thequinine sulphate standard and UPW blanks were measured at thebeginning and end of each sample run.

The correlation between fluorescence emission and absorptioncoefficient of CDOM at the wavelength used for fluorescence exci-tation has been recognised (Hoge et al., 1993; Green and Blough,1994; Ferrari and Dowell, 1998; Ferrari, 2000; Blough and DelVecchio, 2002). The improved correlation of absorption withstandardised fluorescence measurements as opposed to absorptionwith normalised fluorescence measurements, in which the CDOMsignal is normalised to the Raman signal, has previously beendescribed by Ferrari and Dowell (1998). CDOM absorption can bederived from standardised fluorescence measurements using Eq.(4):

aCDOMl ¼ aþ b Fs (4)

where aCDOMl is the wavelength-dependent CDOM absorption, Fsis the standardised fluorescence and a and b are the intercept andslope, respectively (Ferrari and Dowell, 1998). The relationshipbetween CDOM fluorescence and aCDOM must be established forspecific geographic regions (Blough and Del Vecchio, 2002). A lit-erature search for CDOM fluorescence to absorption regressionequations identified Ferrari’s (2000) sampling area to most closelyapproximate the types of waters sampled for our study. For theNorth Sea and Atlantic area aCDOM355 Ferrari (2000) establishedthe regression equation:

a CDOM355 ¼ 0:015ð � 0:0066Þ þ 0:138ð � 0:0031Þ S:Fl:U:

(5)

With a strongly positive correlation coefficient of r ¼ 0.98. Conse-quently Eq. (5) was applied to derive calculated aCDOMl

(aCDOMcalc) from the measured fluorescence values reportedherein. Blough and Del Vecchio (2002) compiled a summary table

0

10

20

30

40

50

60

70

Calculated photic depth (m)

No.

sam

ples

0.1 -

2.02.1

- 4.0

4.1 -

6.06.1

- 8.0

8.1 -

10.0

10.1

- 12.0

12.1

- 14.0

14.1

- 16.0

16.1

- 18.0

18.1

- 20.0

20.1

- 22.0

22.1

- 24.0

24.1

- 26.0

26.1

- 28.0

28.1

- 30.0

30.1

- 32.0

32.1

- 34.0

34.1

- 36.0

36.1

- 38.0

38.1

- 40.0

>40.0

Fig. 2. Histogram of calculated photic depths (Zeu ¼ 4.6/Kd) from all sampling stations.

J. Foden et al. / Estuarine, Coastal and Shelf Science 79 (2008) 707–717 711

of CDOM optical properties from datasets published by numerousworkers, converting data to a common set of parameters so thatcomparisons could be made. Utilising this table the aCDOMcalc ofour study have been compared with reported aCDOMl of otherstudies in Section 4.

3. Results

3.1. Kd variability in UK waters

Kd coefficients were recorded between 0.06 and 18.0 m�1 withnotable contrasts in measurements from the east and west coasts ofthe UK. Light attenuation was greatest in the transitional andcoastal waters of East Anglia (Fig. 1), where Kd coefficients of>2.0 m�1 were measured. The highest coefficients were measuredin TW types 1 and 2 (Table 1); respective maxima Kd of 18.0 m�1

(mean 5.61 m�1) and 9.0 m�1 (mean 3.37 m�1). These water bodiesare either macrotidal or strongly meso-tidal with sand and mudsubstrates, e.g. the estuaries of the Bure, Waveney, Yare & Lothing,Orwell & Stour. In all other UK waters Kd measurements were<2.0 m�1. On the UK’s west coast Kd coefficients recorded in theIrish Sea, Liverpool Bay, Celtic Sea and Bristol Channel were lower,in the range 0.06–0.91 m�1, with mean Kd of 0.36 m�1. Euphoticdepths calculated from Kd ranged from 0.3 m in the Blyth estuary,Suffolk (TW1) to 76.7 m in the Western Approaches, the meandepth being 12.3 m. The frequencies of euphotic depths are pre-sented in Fig. 2, in 2 m ranges. In 40% of samples euphotic depthwas 0.2–6.0 m, and in a quarter of samples it was <4 m. The ma-jority of these were shallow estuaries with large tidal ranges and allof the sampled estuaries of TW types 1 and 2 (Table 1) had euphoticdepths of <6 m.

The relative contributions of all the OACs to Kd were measuredduring this study (unpublished data). Two simple Poisson modelswere first fitted to investigate the association of particulate (SPM)and dissolved (CDOM) light absorbers with Kd, separately. Theunadjusted slope coefficient of SPM when regressed against Kd was0.013 m�1 (p < 0.01, s.e. ¼ 0.001), and for CDOM it was 0.17 m�1

(p < 0.01, s.e. ¼ 0.028).When considered together in a multiple regression model, the

particulate matter and dissolved matter accounted for 94% of thevariance in Kd. Although CDOM and SPM were found to be signif-icantly correlated with each other (r ¼ 0.1, p < 0.01), the inclusionof each variable in the model did not substantially affect the slope

coefficient or statistical significance of the other, and CDOM andSPM were both shown to be highly significant predictors of Kd

(p < 0.01). The regression equation was:

Kd ¼ 0:18* CDOMðS:Fl:U:Þ þ 0:062*SPM�

mg�1�þ 0:116 (6)

where 0.166 m�1 is the intercept. Standardised beta coefficients(the predicted change in Kd associated with 1 standard deviationchange in the independent variables) were 0.95 for SPM and 0.086for CDOM.

CDOM and Kd values for each water body type are shown inFig. 3a; it was not possible to derive Kd values for the rivers sam-pled because they were <3 m deep, which was too shallow foreffective profiler deployment. There is a positive correspondencebetween the two variables for all water types. In moderately ex-posed, meso-tidal coastal waters (CW5) and in sheltered, partiallymixed, meso-tidal transitional waters (TW2) Kd values were highrelative to CDOM concentrations, compared with all other watertypes. CW5 is typified by the coastal waters of Essex and TW2 bythe Alde, Orwell and Deben estuaries in East Anglia. In all transi-tional waters and in one coastal water type (CW5), high concen-trations of SPM (mean 10.1–81.4 mg l�1) were reflected in highvalues of Kd (mean 0.82–5.61 m�1) (Fig. 3b). When multiple re-gression of SPM and CDOM was performed on CW5 data sepa-rately, SPM was found to explain the same proportional variance inKd as for all water bodies – slope of 0.062 and standardised betacoefficient of 0.90. The slope of Kd to CDOM in CW5 was 1.22(p < 0.05), which is higher than for the whole data set. In transi-tional water bodies of type 2 CDOM was only found to be a statis-tically significant contributor to Kd (p < 0.05) in three of the sevenestuaries of this type. The five estuaries in which CDOM was notstatistically significant (p > 0.05) were all in East Anglia – thesewere Blackwater & Colne, Crouch, Alde & Ore, Orwell & Stour andDeben. With the exception of these five estuaries, there was anotherwise significant (p < 0.05) correlation of CDOM to Kd in allsampled waters.

3.2. Spatial distribution of CDOM concentrations

The pattern of decreasing CDOM with increased distance fromland is evident in both the ranges and means. Table 2 summarisesthe range and mean of CDOM found in offshore waters, CWs, TWsand rivers. CDOM concentrations during the sampling period

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

TW1

TW2

TW3

TW4

CW10

CW11

CW12

CW8

CW4

CW5

CW1

CW2

Offsho

re

TW1

TW2

TW3

TW4

CW10

CW11

CW12

CW8

CW4

CW5

CW1

CW2

Offsho

re

CD

OM

(S.

Fl.U

.) Mean K

d (m-1)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

Mean K

d (m-1)

CDOMKd

0.0

20.0

40.0

60.0

80.0

100.0

120.0

Water body type

Mea

n SP

M (

mg

l-1)

SPMKd

b

a

Fig. 3. Mean Kd (m�1) for water bodies between August 2004 and December 2005, with standard error bars: (a) mean surface CDOM (S.Fl.U.); and (b) mean surface suspendedparticulate material (SPM) (mg l�1). Water body types on x-axis represent an increasing gradient of exposure, left to right.

Table 2Summary of CDOM ranges by water body type

Distance fromterrestrial influence

Type (nm ¼nautical miles)

CDOM range(S.Fl.U.)

CDOM mean(S.Fl.U.)

No. of surfacesamples

Offshore (>1 nm) <0.1–1.0 0.3 204CW (<1 nm) 0.1–7.2 0.3 248TW (estuaries) 0.2–13.7 2.4 108Riverine 0.9–16.8 6.6 25

J. Foden et al. / Estuarine, Coastal and Shelf Science 79 (2008) 707–717712

varied over more than two orders of magnitude from 0.05 to 16.80S.Fl.U.. CDOM means are presented for each typology and offshoresites (Fig. 3a). CW1, CW2 water bodies and offshore sites are at fullmarine salinity and are the most exposed types. These waters havethe lowest CDOM values of <0.15 S.Fl.U.. In contrast, TW1 waterbodies are the most sheltered and strongly freshwater influencedwith salinity of 5–30. Lagoons and lochs (CWs 10, 11 and 12) aresimilarly sheltered and are therefore directly affected by CDOM ofterrestrial origin. The wide range in CDOM values in freshwatershown in Fig. 4a (3.54–11.30 S.Fl.U. at 0 salinity) reflects the varietyof rivers and their different catchments sampled during the study,from small tributaries of Fleet Lagoon, to large rivers in Norfolk andSuffolk. The r2 of 0.81 (p < 0.05) indicates a strong negative cor-relation with salinity. Ninety percent of CDOM samples are in therange 0.10–3.00 S.Fl.U., and were sampled in the salinity range23.3–35.5.

The east and west coasts of the UK had contrasting CDOMmeasurements. To the east of England in the southern North Sea(Fig. 1 inset) CDOM values in coastal and offshore waters (salinity33.4–35.2) were recorded at the lower end of the range shown inFig. 4a; varying between 0.15 and 0.59 S.Fl.U. (Fig. 4b). The corre-lation coefficient between CDOM and salinity in these marine sa-linities is r2 ¼ 0.33 (p < 0.05). CDOM sample locations to the west ofEngland and Wales included the Solway Firth, the Irish Sea, Liver-pool Bay, the Isles of Scilly and the Western Approaches (Fig. 1).Compared with the data from east England and the southern NorthSea, there is a greater salinity range (31.2–35.6) in the coastal andoffshore waters of this area and CDOM values vary between 0.05and 0.89 S.Fl.U. (Fig. 4c). The r2 coefficient of CDOM and salinity is

0.53 (p < 0.05). Within Fig. 4c data from four particular regionshave been identified; Liverpool Bay, the Solway Firth, the Isles ofScilly and the Western Approaches. Variability in local conditionsmay account for the characteristic salinity-CDOM relationships ineach region. Liverpool Bay had reduced salinity (�32.0) and a highCDOM signal (�0.59 S.Fl.U.). Waters of the highest salinity (>35.3)were furthest offshore in the Western Approaches and were char-acterised by very low CDOM values (�0.12 S.Fl.U.). Salinity in theIsles of Scilly was similar to that of the Western Approaches (w35);however, CDOM concentrations were elevated in comparison(0.21–0.36 S.Fl.U.). At three sites in the Solway Firth of <33 salinity,CDOM was relatively low (�0.08 S.Fl.U.) in comparison with sitesimmediately outside the Firth (�0.16 S.Fl.U.), in the northeast IrishSea.

Correlation between CDOM fluorescence and salinity was alsofound to vary between estuaries. Table 3 presents the CDOM andsalinity ranges of eight exemplar estuaries sampled during thestudy, the r2 correlation and the number of samples on which thelatter statistic is based (locations shown in Fig. 1).

y = -0.174x + 6.288

r 2 = 0.809N = 585

0.0

2.0

4.0

6.0

8.0

10.0

12.0

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0

CD

OM

(S.

FL

.U.)

All CDOM samples

y = -0.121x + 4.475

r 2 = 0.330N = 151

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

33.0 33.5 34.0 34.5 35.0 35.5

CD

OM

(S.

Fl.U

.)

North Sea and east coast

y = -0.118x + 4.280

r 2 = 0.530N = 114

0.0

0.2

0.4

0.6

0.8

1.0

30.0 31.0 32.0 33.0 34.0 35.0 36.0

Salinity

CD

OM

(S.

Fl.U

.)

Irish Sea and west coast

a

b

c

Fig. 4. CDOM correlation with salinity: (a) all sample data; (b) east coast of Englandand southern North Sea; and (c) CDOM from the west coast of England and Wales, theIrish Sea and the Western Approaches. In (c) clusters of CDOM are identified in fourlocations; Liverpool Bay (6), the Solway Firth (>), the Isles of Scilly (B) and theWestern Approaches (,). Note, different x and y-axes scales.

J. Foden et al. / Estuarine, Coastal and Shelf Science 79 (2008) 707–717 713

3.3. Temporal distribution of CDOM concentrations

Three water bodies were subject to repeat sampling on three ormore occasions during the course of the study; Fleet Lagoon &Portland Harbour, the West Gabbard and Milford Haven (Fig. 1).

Table 3CDOM and salinity ranges and their correlation in nine estuaries (TWs & CWs) of the UK

Water body Type Salinity range CDOM range

Thames (inner and outer) TW3 & CW4 20.6–35.1 0.2–3.1Milford Haven (inner & outer) TW3 & CW4 23.3–34.7 0.2–1.6Deben TW2 26.8–34.0 0.3–1.4Orwell & Stour TW2 28.1–34.2 0.6–1.9Crouch TW2 32.5–33.5 0.5–0.7Firth of Forth (inner & outer) TW2 & CW8 29.1–33.4 0.2–1.5Fleet & Portland CW10 & CW8 30.9–35.2 0.1–0.9Clyde (inner & outer) CW5 & CW8 32.7–33.6 0.3–0.6

These data were analysed for temporal changes in the CDOM signal.The data from the Fleet lagoon & Portland Harbour water bodyshowed CDOM ranges varied inversely with salinity and that therewere temporal variations between sampling occasions (Fig. 5a). Theranges were greatest in January 2005; CDOM was 0.14–0.85 S.Fl.U.and salinity 30.9–35.1. In March and April 2005 the ranges of bothvariables were smaller, with salinity of 34.7–35.2 and 34.2–35.1,respectively, and CDOM of 0.19–0.24 and 0.14–0.21 S.Fl.U.,respectively.

The Milford Haven estuary in South Wales was sampled on threeoccasions; December 2004, March and May 2005. Fig. 5b presentsthese temporal data and there is a strongly linear, negative corre-lation between CDOM and salinity in Milford Haven across allsampling occasions; r2 ¼ 0.85 (p < 0.05). A conservative dilutioncurve is evident on all days and there is no significant differencebetween the three datasets as evidenced in the CDOM ranges andmeans, suggesting no significant temporal differences in the CDOMsignal over these three sampling occasions.

The West Gabbard site is w55 km offshore in the southern Bightof the North Sea and samples were taken in a grid around the CefasSmartBuoy on several occasions (see Fig. 1 inset map). CDOMsampling was conducted on five occasions between February 2005and March 2006 at the SmartBuoy site and in the sampling grid;data are presented in Fig. 5c. The salinity range on all samplingoccasions was small; 34.3–35.2. The CDOM range was corre-spondingly narrow, from 0.15 to 0.35 S.Fl.U. It is not possible toidentify a correlation coefficient of salinity to CDOM over suchnarrow ranges. There is no significant temporal difference betweenthe CDOM signals on the five sampling occasions with mean CDOM0.20–0.27 S.Fl.U..

3.4. CDOM absorption

Ferrari (2000) established the equation of directly measuredfluorescence to absorption at l 355 nm (Eq. (5)). This was used toderive aCDOMcalc from the measured fluorescence values reportedherein, because 355 nm is within the measured range 310–390 nm.The mean aCDOMcalc for all fluorescence samples was 0.15 m�1 andwas in the range 0.02–2.3 m�1. The highest aCDOMcalc of >1.0 m�1

were found in rivers on the south and east coast of UK, and in theEast Anglia (Fig. 1) transitional water known as ‘Bure, Waveney,Yare & Lothing’ (UKTAG, 2003). For North Sea transitional, coastaland offshore waters only, the mean was 0.2 m�1 with a range of0.04–1.6 m�1. In the Atlantic waters aCDOMcalc was sampled in therange 0.04–0.1 m�1, with mean 0.1 m�1. However, aCDOMcalc

values are only presented for comparison with other workers’measured aCDOMl in very general terms. Fluorescence in this studywas measured across the range 310–390 nm rather than at pre-cisely 355 nm. The absorbance of CDOM is known to increase sig-nificantly at lower wavelengths (Hoge et al., 1993; Green andBlough, 1994; Ferrari and Dowell, 1998); for example in the BalticaCDOM355 was up to four times higher than aCDOM420, with sea-sonal variation (Ferrari and Dowell, 1998). Therefore, aCDOMcalc

values are only estimates because of the instrumental range used to

(S.Fl.U.) r2 correlation CDOM mean (S.Fl.U.) No. of CDOM samples

1.0 0.6 290.9 0.6 351.0 0.8 110.8 1.4 120.2 0.6 60.8 0.5 161.0 0.3 430.9 0.5 11

Fleet Lagoon & Portland Harbour

Milford Haven

West Gabbard SmartBuoy site

CDOM

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

CD

OM

(S.

Fl.U

.)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

CD

OM

(S.

Fl.U

.)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

CD

OM

(S.

Fl.U

.)

b

c

a

30.0 31.0 32.0 33.0 34.0 35.0 36.0

23.0 25.0 27.0 29.0 31.0 33.0 35.0

32.0 33.0 34.0 35.0 36.0

Salinity

SamplingOccasion N Range Mean

Feb '05 7 0.15 - 0.24 0.20Dec'05 12 0.16 - 0.26 0.21Jan '06 27 0.21 - 0.35 0.27

0.17 - 0.31Feb '06 15 0.23Mar '06 21 0.19 - 0.33 0.23

SamplingOccasion N Range MeanDec '04 12 0.17 - 1.58 0.66Mar '05 14 0.26 - 1.11 0.53May '05 12 0.19 - 0.92 0.43

CDOM

Samplingoccasion N

CDOMrange Mean

Jan '05 13 0.14 - 0.85 0.39Mar '05 20 0.19 - 0.24 0.21Apr '05 8 0.14 - 0.21 0.17

y = -0.171x + 6.157r2 = 0.899N = 41

y = -0.113x + 4.121 r2 = 0.820N = 39

Fig. 5. CDOM concentrations: (a) in Fleet lagoon & Portland Harbour in > January, , March and : April 2005; (b) in Milford Haven in > December 2004, , March 2005 and :

May 2005; and (c) from a sampling grid around the West Gabbard SmartBuoy site in > February 2005, , December 2005, : January 2006, C February 2006 and þMarch 2006.Note different scales.

J. Foden et al. / Estuarine, Coastal and Shelf Science 79 (2008) 707–717714

measure fluorescence, and it was outside the scope of this study toprecisely quantify absorbance at one wavelength.

4. Discussion

It is recognised that growth of phytoplankton and submergedaquatic vegetation is reduced in conditions of limited light avail-ability (e.g. Vant and Davies-Colley, 1986; Ruiz et al., 2001; Kempet al., 2004; Koch et al., 2004; Smith et al., 2006; Sfriso and Facca,2007), because the light climate as recognised by Cloern (2001) isone of the filters modulating the response of a system to nutrientloading. For photosynthesis to occur light plays a major role, withOACs having a controlling effect on the underwater PAR irradianceand therefore on photosynthesis. A wide variety of underwaterlight climates were measured in UK water bodies during this study,which were comparable to Kd coefficients reported by otherworkers in the same geographic regions or similar water types (e.g.Bowers and Mitchelson-Jacob, 1995; Bowers et al., 2000; Kocumet al., 2002; Lund-Hansen, 2004; McKee et al., 2007). Comparablewith Kd measured by Pfannkuche (2002) in the shelf waters of New

Zealand, 65% of measurements were between 0.06 and 0.8 m�1. Theremaining 35% were >0.8 m�1 and were predominantly from theturbid estuaries and coastal waters of East Anglia (Fig. 1). Indeed,the highest coefficients (>3.1 m�1) were found in transitional wa-ters and near-shore coastal waters of the east coast of the UK;comparable with previously recorded Kd levels in an East Anglianestuary (e.g. Kocum et al., 2002). OACs restrict the light climate inUK waters, as evidenced in the euphotic depths calculated from Kd.Euphotic depth was <6 m in all of the shallow estuaries with largetidal ranges, TW types 1 and 2. At the Warp SmartBuoy in theThames estuary (Fig. 1), euphotic depth was <6 m in 70% of sam-ples. Even as far as 35 km offshore at the West Gabbard SmartBuoysite (Fig. 1) euphotic depth was <6 m on 40% of occasions. Highconcentrations of OACs effectively restrict primary production toa thin euphotic zone, even in nutrient enriched waters (Murrellet al., 2007; Naik and Arthur Chen, in press; Retamal et al., 2008)and will severely limit or even prevent productivity of benthic floracommunities (e.g. Aumack et al., 2007; Sfriso and Facca, 2007),thereby reducing susceptibility to eutrophication and undesirabledisturbance in these systems.

J. Foden et al. / Estuarine, Coastal and Shelf Science 79 (2008) 707–717 715

The contributions of CDOM, SPM and chlorophyll to Kd weremeasured during this study and empirical analyses found SPMconcentration to be the most important explanatory variable ofattenuation (unpublished data). The high levels of suspendedmatter measured in UK waters during this study (Fig. 3b) have alsobeen recorded by other workers (e.g. Kocum et al., 2002; Martinoet al., 2002; Kirby and Shaw, 2005; Turner and Mawji, 2005; Uncleset al., 2006; Devlin et al., 2008). In particular, CDOM data frommoderately exposed meso-tidal coastal waters (type 5) weredominated by observations taken at the Warp SmartBuoy site(Fig. 1) in the Thames estuary (61 of 109 samples, with a mean SPMof 29.1 mg l�1), which is an area recognised as highly turbid(Sanders et al., 2001). Nonetheless, CDOM was still a statisticallysignificant influence on Kd in all observations from this and othersimilar types of water bodies. Only in five highly turbid TWs of type2 in East Anglia was CDOM not statistically significant. AlthoughSPM makes the greatest contribution to light attenuation of theOACs, it has been shown that CDOM still has a measurable andsignificant, positive correlation with Kd in UK waters. The pro-portional contributions of SPM and CDOM to light attenuation inthe coastal and offshore waters of the North Sea are comparable tothose reported by Lund-Hansen (2004) in the transitional NorthSea-Baltic. However, they are in contrast to other studies in thewider Baltic Sea where the underwater light regime is largelygoverned by CDOM, with the largest source being riverine input(Schwarz, 2005; Kowalczuk et al., 2005, 2006).

The range in CDOM fluorescence found in this study was similarto that found by Hoge et al. (1993) who recorded fluorescence from0.13 to >10.0 S.Fl.U. at sites in the western North Atlantic, Gulf ofMexico and Monterey Bay. In the transitional and coastal waters ofthe North Sea the range in CDOM was within that reported byObernosterer and Herndl (2000), who measured CDOM fluores-cence in the Dutch coastal part of the North Sea. The inverse cor-relation of decreasing CDOM with increased exposure was alsofound (exposure characterised by water body type in Table 1). Themore sheltered water body types, heavily influenced by freshwater,e.g. rivers, all TWs and lochs and lagoons, had mean CDOM con-centrations >5.0 S.Fl.U., where runoff from adjacent land was thelikely cause of elevated concentrations. The more exposed and fullymarine types (e.g. CWs 1 and 2) and offshore waters, had the lowestCDOM values (�0.15 S.Fl.U.) CDOM in UK waters appears to be ofterrestrial origin and its distribution dominated by mixing; a strongnegative correlation with salinity, indicative of a conservative re-lationship. This supports previous findings by other authors (e.g.Green and Blough, 1994; Højerslev et al., 1996; Nieke et al., 1997;Ferrari and Dowell, 1998; Blough and Del Vecchio, 2002; Chen et al.,2007), indicating freshwater to be the main source of CDOM in thecoastal water.

The spatial differences are generally explicable by salinity vari-ation. For example, where the salinity range was narrow, as in thedata from the east coast of England and southern North Sea, theCDOM signal’s range was narrow and correlation between thesevariables was weak, but still statistically significant. Over largersalinity ranges, as in the data from the west coast from the Solwayin the north to the Western Approaches in the southwest, theCDOM range was correspondingly large and correlation stronger.However, in common with other studies (e.g. Chen et al., 2004,2007), CDOM fluorescence did show some regional distinctivenessand analysis of the data revealed groups of data points related tolocation.

Differences between locations may be explained by local char-acteristics. For example, the large riverine inputs from the Mersey,Dee, Ribble and Conwy, estuaries, into Liverpool Bay deliver ele-vated CDOM likely to be of terrestrial origin derived from theircatchments (0.63–0.96 S.Fl.U.). Comparative studies of the humicsubstances (fulvic and humic acids) signal in this area show it to be

positively related to the freshwater signal, with higher concentra-tions recorded in Colwyn Bay and at the mouths of the Ribble andMersey estuary during December 2005 sampling (L. Laglera, 2007,pers comm).

The three samples from the Solway Firth in waters of salinity<33, had low CDOM concentrations (�0.08 S.Fl.U.), comparedwith samples taken immediately outside the Firth (0.17–0.26S.Fl.U.). These low CDOM concentrations may have been causedby the combination of the characteristics of the catchment’s landuse, the weather conditions during the days preceding samplingand the more large scale circulation of the eastern Irish Sea. TheFirth drains hilly areas where pastoral farming and forestry arecommon (B. Miller, SEPA 2005, pers. comm). The weather wasfine and settled for the 10 days prior to sampling, which wouldlikely result in lower runoff into the rivers of the Solway catch-ment and reduced CDOM inputs of terrestrial origin. The circu-lation of the eastern Irish Sea could be influencing CDOMconcentrations outside the mouth of the Solway Firth, as thegeneral surface circulation shows a net northward movementalong the west coast of England (Davies and Xing, 2003; Howarthet al., 2007). The circulation draws northwards waters from Liv-erpool Bay along the English coast towards Scotland, passing themouth of the Solway estuary and exiting through the NorthChannel (Brown and Gmitrowicz, 1995). This would account forthe difference in low CDOM concentrations recorded in the Sol-way Firth estuary and higher concentrations found in the moresaline waters immediately outside the mouth of the estuary,which are influenced by the relatively high CDOM concentrationsoriginating from Liverpool Bay.

The Isles of Scilly are fully marine waters with no significantfreshwater inputs from the islands, but CDOM concentrations of0.21–0.36 S.Fl.U. (Fig. 4c) were above the levels recorded elsewhereto the west of England and Wales, in waters of the same 35.1–35.2salinity range (�0.21 S.Fl.U.). This difference might have beencaused by water masses carrying different characteristic CDOMsignals. Local agricultural practices, such as fertilising bulb andflower crops, and diffuse surface runoff may also have had a localinfluence, elevating the CDOM signal.

In order to further investigate patterns and causes of regionalvariability in CDOM in UK waters, sampling was carried outopportunistically in 20 estuaries (TW and CW types) and, in eightcases, conducted along salinity gradients (Table 3). The differencesin the CDOM signal between these eight estuaries were largelyexplicable by the range in salinity, with r2 values of�0.78 (p < 0.05)in all estuaries apart from the Crouch. It is evident that water bodieswith the largest salinity ranges have correspondingly large CDOMranges, e.g. Thames and Milford Haven. The more exposed estuaries(as characterised in Table 1), such as the Clyde, have marine sa-linities in a narrow range and a correspondingly low CDOM range.There were further differences between specific estuaries; for ex-ample the range of salinity sampled in Milford Haven (23.3–34.7)was much greater than that sampled in the Orwell & Stour waterbody (28.6–34.5), but the differences in CDOM within each salinityrange were similar (1.31 and 1.35 S.Fl.U., respectively). The mixingcharacteristics may account for this dissimilarity. Milford Havenextends across types TW3 and TW4, which are characterised asfully vertically mixed, whereas the Orwell & Stour water body istype TW2, characterised as partially mixed or stratified (Table 1).Physical barriers preventing sampling water of salinity <28 in theOrwell & Stour where CDOM concentrations may have been higher.The poor correlation between the CDOM signal and salinity in theCrouch (r2 ¼ 0.14) may be because of the influence of the Roachestuary which empties into the Crouch w4 km from its mouth.Furthermore, sampling was carried out during a flooding tide,further complicating the possibility of separating these differentwater masses.

J. Foden et al. / Estuarine, Coastal and Shelf Science 79 (2008) 707–717716

Temporal differences in the CDOM signal were found in waterssampled at the mouth of Fleet Lagoon and Portland Harbour, be-tween the three sampling occasions during winter and spring 2005.Overall CDOM fluorescence was highly correlated with salinity(r2 ¼ 0.90). The salinity and CDOM ranges were similar on 12thMarch and 28th April, but the ranges in both variables were greateron 20th January 2005. This variability is most likely to have beencaused by the difference in tidal states. In March and April samplingbegan during the high water slack and continued for the first 3 h ofthe ebb tide. However, in January sampling began 2 h after the lowwater slack and continued for 3 h during the flooding tide. The lowersalinity water recorded in January was probably as a result of sam-pling beginning after low water when salinity would be mostinfluenced by the fresher waters flowing from Fleet Lagoon. The sixstreams draining into Fleet Lagoon sampled during this study hada CDOM mean of 9.7 S.Fl.U., which would explain the elevated CDOMconcentrations sampled at the point where Fleet Lagoonwaters flowinto Portland Harbour. Salinity in March and April was >34.2 andwhen the samples from these days are compared with those fromwaters of the same salinity in January, the CDOM ranges all fall be-tween 0.14 and 0.22 and CDOM means for each sampling date are all0.20 S.Fl.U.. No other temporal signal was identified in these waters.This study showed that in Milford Haven any temporal differences inthe CDOM signal were similarly related to salinity and a dilutiongradient dominated the inverse correlation of salinity and CDOM onall sampling occasions. The ranges in CDOM concentration and sa-linity at the West Gabbard site were very small; 0.15–0.35 S.Fl.U. and34.3–35.3, respectively. No temporal difference was detectable inCDOM in the fully marine waters at this site.

Although CDOM absorption was not directly measured duringthis study, absorption was derived from standardised fluorescenceunits (aCDOMcalc), using Eq. (5). This was carried out in order tocompare the data more widely with aCDOMl reported by otherworkers as compiled by Blough and Del Vecchio (2002). The derivedaCDOMcalc are only compared in general terms with results of otherworkers for reasons already given (Section 3.4). The range and meanaCDOMcalc were found to be comparable with those recorded in thesame geographic regions by other workers (e.g. Ferrari, 2000; Sted-mon et al., 2000; Kitidis et al., 2006). The mean aCDOMcalc in thecoastal and offshore waters of the North Seawas similar to aCDOM380

measured by Højerslev et al. (1996). However, it was lower than thatmeasured by Obernosterer and Herndl (2000), who recorded meanaCDOM365 in the Marsdiep Tidal Inlet, a region strongly influencedby the river Rhine’s outflow and Waddenzee, with high concentra-tions of OACs. In open Atlantic waters Kitidis et al. (2006) measuredan aCDOM300 range comparable with the aCDOMcalc derived herein.These findings support the results of other studies in suggestingreasonable CDOM absorption values can be derived from measure-ments of fluorescence (Hoge et al., 1993; Green and Blough, 1994;Vodacek et al., 1997; Ferrari and Dowell, 1998; Ferrari, 2000; Bloughand Del Vecchio, 2002), using an equation from another worker(Ferrari, 2000) and that aCDOMcalc is broadly comparable with di-rectly measured aCDOMl from a variety of other studies.

5. Conclusions

In conclusion, the methods used to profile the water column,and to collect, store and analyse CDOM fluorescence were suc-cessful. A large range of light attenuation coefficients was measuredin UK waters, with highest Kd values recorded in transitional wa-ters. Although SPM is the best single explanatory variable of Kd,CDOM fluorescence was still found to be influential, particularly incoastal and in low turbidity environments of offshore waters.CDOM absorption was derived using a fluorescence-absorptionequation published from work in similar water types and producedcomparable absorption values. There was a strong, negative

correlation of CDOM to salinity; r2 of 0.81 and estuarine dilutiongradients of CDOM were indicative of conservative mixing. Incommon with other studies, spatial differences were evident andgrouping of data by region reflected local conditions or catchmentareas. Temporal differences in the CDOM signal were correlatedwith salinity variations. The results show that CDOM makes a con-tribution to Kd in UK waters, especially near to shore where highestanthropogenic nutrient concentrations are encountered. ThereforeCDOM acts to reduce the amount of light available for plant growthand reduce the subsequent risk of eutrophication.

Acknowledgements

This work was undertaken by Cefas, funded by Defra contracts;research supporting assessment of marine eutrophication andmonitoring trends in nutrient concentrations in marine waters ofEngland and Wales (ME2202 and AE004). The work was supportedby Agri-Food and Biosciences Institute in Northern Ireland, byproviding the opportunity for collaborative fieldwork on board theRV Corystes. The authors would like to thank Environment Agencystaff on the Vigilance, Water Guardian and Sea Vigil who assisted infieldwork, and the captains, officers and crews of the RV CefasEndeavour, RV Corystes and THV Alert for their assistance duringsurveys. The constructive comments of the reviewers of this paperwere extremely useful in making substantial improvements, andour thanks are also offered to them. The views expressed in thispaper are those of the authors and do not necessarily reflect thoseof Defra, Cefas, AFBI or EA.

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