Coloured dissolved organic matter light absorption

From the point of view of aquatic ecology, the significance of the soil humic material is that as water, originating as rainfall, drains through soil and into rivers and lakes, and ultimately into estuaries and the sea, it extracts from the soil some of the water-soluble humic substances and these impart a yellow colour to the water, with major consequences for the absorption of light, particularly at the blue end of the spectrum. James and Birge (1938) in the USA and Sauberer (1945) in Austria carried out extensive quantitative studies on the absorption spectra of lake waters with varying degrees of yellowness. The colour of humic substances is due to the presence of multiple double bonds, many of them conjugated, some in aromatic nuclei. In any sample of humic material there are numerous different chromophores, and consequently a multitude of electronic excitation levels, which, because they overlap, give rise to a rather undif-ferentiated UV/visible absorption spectrum. Visser (1984) found that in surface waters derived from coniferous forest catchments in Quebec

(Canada), the colour intensity per unit mass of the humic acid was nearly four times that of the fulvic acid: its concentration, however, was only about one seventh that of the fulvic acid, implying that in these waters, humic acid contributed about one third, and fulvic acid two thirds, of the total colour. As well as absorbing light, dissolved humic material in natural waters has a broad fluorescence emission band in the blue region.

The light absorption properties of these dissolved humic materials in natural inland waters can be determined relatively easily by measuring the absorption spectrum of a water sample that has been filtered (0.2-0.4 mm pore size) in 5 cm or 10 cm pathlength cells.696 Figure 3.5 shows the absorption spectrum of this material from some Australian inland waters. These are typical humic substance absorption spectra, with absorption being very low or absent at the red end of the visible spectrum, and rising steadily with decreasing wavelength towards the blue: absorption in the ultraviolet region is higher still.

The presence of dissolved yellow material in inland waters is often easily apparent to the eye. Its presence in marine waters (where its concentration is much lower) is not so readily apparent, and the fact that it is important in these ecosystems too was pointed out by Kalle (1937, 1966). Figure 3.5 also shows the absorption spectra of soluble yellow material in estuarine and coastal waters.

The dissolved yellow material in natural waters has been variously referred to as 'yellow substance', 'gelbstoff (the same name, in German), 'yellow organic acids', 'humolimnic acid', 'fulvic acid', 'humic acid' etc. Dissolved yellow materials from different waters vary not only in molecular size but also in chemical composition.671,1314,1315 In the context of light attenuation it would be useful to have a general name, applicable to all or any of these compounds regardless of their chemical nature, which simply indicates that they preferentially absorb light at the blue end of the spectrum. 'Yellow substance' or 'gelbstoff are too non-specific and could apply to anything from butter to ferric chloride. I have suggested the word 'gilvin', a noun derived from the Latin adjective gilvus meaning pale yellow.696 'Gilvin' would thus be defined as a general term to be applied to those soluble yellow substances, whatever their chemical structure, which occur in natural waters, fresh or marine, at concentrations sufficient to contribute significantly to the attenuation of PAR. While 'gilvin' is certainly to be found in the aquatic literature, it is nevertheless true that the term that is most commonly used in recent papers is the acronym CDOM (pronounced 'seedom'), which is taken to stand for either coloured dissolved organic matter, or chromophoric dissolved organic matter. In recognition of

Dissolved Organic Matter

Fig. 3.5 Absorption spectra of soluble yellow material (gilvin, CDOM) in various Australian natural waters (from Kirk, 1976b). The lowest curve (Batemans Bay, NSW) is for coastal sea water near the mouth of a river; the next curve (Clyde River, NSW) is for an estuary; the remainder are for inland water bodies in the southern tablelands of New South Wales/Australian Capital Territory. The ordinate scale corresponds to the true in situ absorption coefficient due to gilvin.

Fig. 3.5 Absorption spectra of soluble yellow material (gilvin, CDOM) in various Australian natural waters (from Kirk, 1976b). The lowest curve (Batemans Bay, NSW) is for coastal sea water near the mouth of a river; the next curve (Clyde River, NSW) is for an estuary; the remainder are for inland water bodies in the southern tablelands of New South Wales/Australian Capital Territory. The ordinate scale corresponds to the true in situ absorption coefficient due to gilvin.

this common usage, but without prejudice as to which interpretation is implied, I shall, in the remainder of this book, use 'CDOM' interchangeably with 'gilvin' for the dissolved yellow substances in natural waters.

As the absorption spectra in Fig. 3.5 show, the concentration of CDOM varies markedly, not only between marine and fresh waters, but also among different inland waters. A convenient parameter by means of which the concentration of CDOM may be indicated is the absorption coefficient at 440 nm due to this material within the water: this we shall indicate by g440. This wavelength is chosen because it corresponds approximately to the mid-point of the blue waveband peak that most classes of algae have in their photosynthetic action spectrum.

In many waters, certainly most inland waters, the CDOM absorption is sufficiently high for g440 to be measurable with reasonable accuracy, using 50 or 100 mm pathlength cells. When CDOM absorption is low, as in most marine waters, the absorption coefficient can be measured on a filtrate with an absorption meter of the ICAM, or PSICAM type (§3.2 above) with very long equivalent pathlength, or with a reflective tube absorption meter. Alternatively, measurement can be carried out in the near-ultraviolet (350-400 nm) where absorption is higher, and g440 determined by proportion from a typical CDOM absorption spectrum, or using the approximate relationship.161

where a(l) and a(10) are the absorption coefficients at wavelengths l and 10 nm, respectively, and S is a coefficient describing the exponential slope of the absorption curve. For ~350 stations in various coastal waters around Europe (English Channel, Adriatic, Baltic, Mediterranean and North Seas) Babin et al. (2003b) found that S varied within a narrow range around 0.0176 ± 0.0020 nm-1. For 12 New Zealand lakes, S varied from 0.015 to 0.020 nm-1, with a mean of 0.0187 nm-1;285 and for 22 Australian inland waters, S varied from 0.012 to 0.018 nm-1, with a mean of 0.016 nm-1.720 When partial photobleaching of CDOM takes place, the surviving dissolved colour has a spectrum with a somewhat higher S value.1417 In agreement with this observation, Twardowski and Donaghay (2002) found surface waters to have higher S values than bottom waters in a coastal fjord: they proposed that the higher spectral slopes commonly associated with oceanic CDOM relative to coastal CDOM may be due to increased cumulative photobleaching.

When excited with near-UV light, typically ~355nm or ~370nm, CDOM fluoresces with a very broad peak in the blue region. The quantum yield is about 0.8%.1418 The intensity of this fluorescence in natural waters is found to be highly correlated with CDOM absorption, and it can be used as an alternative way of measuring CDOM, either in the water, using a submersible fluorometer, or remotely by excitation of the fluorescence by an airborne laser (see Chapter 7).1418,94

Table 3.2 contains a selection of published data in the form of values of g440 for marine as well as fresh waters in various geographical regions.

Table 3.2 Value of absorption coefficient at 440 nm due to dissolved colour (g44o) and particulate (phytoplankton + tripton) colour (p440) in various natural waters. Where several measurements have been taken, the mean value, the standard deviation, the range and the time period covered are in some cases indicated. For some waters only the absorption coefficient for CDOM plus particles (g + p) was available.

Water body g440 (m 1)

p440 (m 1) Reference

I. Oceanic waters

Atlantic Ocean

Sargasso Sea

Off Bermuda

Caribbean Sea

Gulf of Mexico - Gulf loop intrusion Romanche Deep Mauritanian upwelling Gulf of Guinea Pacific Ocean Galapogos Islands Galapogos Islands Central Pacific Off Peru Indian Ocean Oligotrophic water Mesotrophic water Eutrophic water Mediterranean Western Arctic Ocean Beaufort & Chukchi Seas Southern Ocean North-South transect, 42-55° S, along 142° E

II. Shelf, coastal & estuarine waters

Gulf of Mexico Yucatan Shelf Bay of Campeche Cape San Blas Mississippi plume North America, Atlantic coast

New England shelf, summer Southeast USA, Georgia, 60 km offshore, spring 1993 (high river discharge) Summer 1992 Rhode R. Estuary

0.034-0.075a

0.024-0.113

0.02a

0.05a

0.019-0.099b

0.01

0.13

0.060 0.200b

0.021b 0.72

0.044 0.026b

0.031b 8.4

760 616 636 204

636 161 161

189 636 636 189

748 748 748

1439

204 204 204 204

1269 980

980 428

Table 3.2 (cont.)

Water body

g44o(m 1)

p440 (m *)

Reference

Chesapeake Bay (Rhode

0.27

0.80

428

R. Mouth)

Georgia salt marsh

1.52

-

1460

Arabian Sea

Cochin, India, 3 km

0.24c (g + p)

1113

offshore

30 km offshore

0.10c(g + p)

1113

Bay of Bengal

Near Ganges mouths

0.37a

-

745

Yellow Sea

0.20-0.23a

-

574

Eastern Pacific

Peruvian coast

0.29a

745

South Pacific

Chatham Rise (east of New

0.044

0.026

1218

Zealand), av. of 19 stations

North Atlantic/North Sea/Baltic

system

W. Greenland

0.004a

-

574

North Atlantic

0.02a

-

670

Iceland

0.016a

-

574

Orkney-Shetland

0.016a

-

574

North Sea (Fladen Ground)

0.03-0.06a

-

574

Wadden Sea

~0.64 (0.0-3.0)

-

583

Skagerrak

0.05-0.12a

-

574

Kattegat

0.12-0.27a

-

574

Arhus Bay, Denmark

0.232

834

(Kattegat)

Baltic Sea

0.36-0.42a

-

574

South Baltic Sea

0.26

-

636

Bothnian Gulf

0.41a

-

636

Loch Etive, Scotland

0.7-1.0

-

887

Mediterranean

Villefranche Bay

0.060-0.161a

-

161

Marseilles drainage outfall

0.074-0.646a

-

161

R. Var mouth

0.136a

-

161

R. Rhone mouth

0.086-0.572a

-

161

Tyrrhenian Sea

Gulf of Naples

0.02-0.20a

-

391

Northern Adriatic Sea

Sacca di Goro (R. Po mouth)

0.32-3.43a

-

391

Venice Lagoon

0.44-0.73a

-

391

Black Sea

Crimean Peninsula (coastal)

0.081-0.197b (av. 0.13)

-

311

Southeast Australia

(a) Jervis Bay

3 stations

0.09-0.14a

0.03-0.04

720

Table 3.2 (cont.)

Water body

g440 (m 1)

P440 (m 1)

Reference

(b) Tasman Sea/Clyde R. system

Tasman Sea

0.02a

-

697

Batemans Bay

0.18

-

696

Clyde R. Estuary

0.64

-

697

(c) Gippsland (estuarine) lakes

system

L. King

0.58

0.25

720

L. Victoria

0.65

0.22

720

L. Wellington

1.14

2.27

720

Latrobe R.

1.89

2.78

720

(d) Tasmania

Huon R. Estuary mouth

0.16-0.30

-

241

Northeast Australia/Great

Barrier Reef

Great Barrier Reef, -18° S

0.050 ± 0.028

0.010

126

Mossman-Daintree estuarine,

0.082 ± 0.087

0.085

126

16° S. Dry season

Wet season

0.246 ± 0.254

0.531

126

Fitzroy R./Keppel Bay system,

0.471

-

1025

23 °S. Dry season. Estuary

station 2

Offshore, 28 km

0.006

-

1025

Western Australia

Swan R. Estuary:

7 km upstream from mouth

0.66 (0.09-2.95)

749

39 km upstream

3.82 (2.21-10.6)

749

New Zealand

South Island, 11 shelf stations

0.04-0.10 (av. 0.07)

-

283

North Island, 9 estuaries,

0.1-0.6

-

1401

mouth sites, low water

Japan, Pacific coast

17 km off Shimoda

0.024

0.133

727

5 km off Shimoda

0.011

0.095

727

Nabeta Bay

0.054

0.140

727

Funka Bay, Hokkaido

0.065

-

1171

III. Inland waters

Europe

Rhine R.

0.48-0.73a

-

574

Donau R., Austria

0.85-2.02

-

574

Ybbs R., Austria

0.16a

-

574

Neusiedlersee, Austria

~2.0 ± 0.4

-

314

(8 months)

1.4-2.8

-

Blaxter L. (bog lake), England

9.65

-

891

Ireland:

Carmean Quarry

0.23

-

643

Killea Reservoir

0.5

-

643

Table 3.2 (cont.)

Water body

g44o(m 1)

P440 (m 1)

Reference

Lough Neagh

1.9

-

643

Lough Fea

4.7

-

643

Lough Erne

5.3

-

643

Loughnagay

6.4

-

643

Lough Bradan

17.4

-

643

Lough Napeast

19.1

-

643

Lough Leven, Scotland

1.2

-

1336

Mountain lakes (Alps,

Pyrenees):

Predominantly rock

0.074 av.

-

776

catchments

(0.00-0.28)

Alpine meadow

0.154 av.

-

776

catchments

(0.09-0.21)

Forested catchments

0.232 av. (0.02-0.53)

-

776

Africa

L. George, Uganda

3.7

-

1336

L. Victoria (Uganda),

0.45

1013

Murchison Bay

North America

Crystal L., Wisc., USA

0.16

-

620

Adelaide L., Wisc., USA

1.85

-

620

Otisco L., NY, USA

0.27

0.27

1446

Irondequoit Bay, L. Ontario,

0.90

0.65

1445

USA

Lake Erie

0.23 (0.08-0.75)

-

116

Bluff L., N.S., Canada

0.94

-

495

Punch Bowl, N.S., Canada

6.22

-

495

South America

Guri Reservoir, Venezuela

4.84

-

805

Carrao R., Venezuela

12.44

-

805

China

L. Tianmuhu

0.48 ± 0.24

-

1501

Japan

L. Kizaki

0.30

0.71

727

L. Fukami-ike

0.85

3.11

727

Australia

(a) Southern Tablelands

Cotter Dam

1.28-1.46

0.77

701, 720

Corin Dam

1.19-1.61

0.11

701, 720

L. Ginninderra

1.54 ± 0.78

0.16-

0.58

696, 697, 701, 720

(3-year range)

0.67-2.81

-

L. George

1.80 ± 1.06

3.73-

4.21

696, 697, 701, 720

(5-year range)

0.69-3.04

Water body g440 (m 1)

p440 (m 1) Reference

Googong Dam 3.42 0.83

Queanbeyan R. 2.42 -

Molonglo R. Below confluence 1.84 -

with Queanbeyan R.

Creek from boggy 11.61 -catchment

(b) Murray-Darling system

Murrumbidgee R., Gogeldrie 0.4-3.2 -

Weir (10 months)

L. Wyangan 1.13 0.38

Griffith Reservoir 1.34 3.73

Barren Box Swamp 1.59 2.55

Main canal, MIA 1.11 5.35

Main drain, MIA 2.12 10.34

Darling confluence

Darling R., above confluence 0.7-2.5 -with Murray

(c) Northern Territory (Magela Creek billabongs)

Mudginberri 1.11 1.13

Gulungul 2.28 1.68

Georgetown 1.99 18.00

(d) Tasmania

Ladies Tarn 0.40 -

Risdon Brook 0.98 -

Barrington 3.05 -

(e) Southeast Queensland, coastal dune lakes

L. Cooloomera 14.22 -

(f) South Australia

Mount Bold 5.40 2.25 Reservoir

696, 697, 701, 720 701 720 720 720

1014

720 720 720 720 720 1014

1014

725 725 725

152 152 152 152 152

151 151 151 151

p440 (m *) Reference

New Zealand

Waikato R. (330 km, L. Taupo to the sea): L. Taupo (0 km) Ohakuri (77 km) Karapiro (178 km) Hamilton (213 km) Tuakau (295 km) Lakes (mean of monthly values over 11 months): Rotokakahi Rotorua Opouri Hakanoa D

0.070

0.22

0.82

0.97

1.37

0.033

0.32

0.71

0.98

1.67

282 282 282 282 282

285 285 285 285 285

a Values measured at a wavelength less than 440 nm and converted to g440 on the basis of an appropriate CDOM absorption spectrum. b Measurements carried out at 443 nm, to conform with the corresponding cSeaWiFS band.

c Published values for c at 440 nm corrected for scattering.

This compilation gives some idea of typical gilvin concentrations that may be expected in natural waters, but the lack of measurements in what are otherwise limnologically well-characterized parts of the world is apparent. The data in Table 3.2 show that marine waters generally have much less dissolved colour than inland waters, and the greater the distance from land, the lower the concentration. The high concentration (for a marine water) within the Baltic Sea is noteworthy: it decreases from the Bothnian Gulf southwards, as the proportion of river water in the sea diminishes. The increase in concentration with distance from the sea upstream in estuarine systems can be seen in the data for Clyde River-Batemans Bay and Latrobe River-Gippsland Lakes (Australia). The Amazon River, with its massive outflow of coloured water, contributes very large quantities of CDOM to the western tropical North Atlantic Ocean, its influence on the optical properties being detectable at distances greater than 1000 km from the river mouth.301

Although gilvin is chemically rather stable - its concentration in a refrigerated stored sample usually shows only small changes over a few weeks - its concentration within any inland water body changes, in either direction, with time, in accordance with rainfall events in different parts of the catchment and consequent changes in the concentration of gilvin in the inflowing waters. Some of the data in Table 3.2 give an indication of the extent of this variability. For example, in Lake Burley Griffin (Canberra, Australia) the value of g440 varied seven-fold over a five-year period. Nevertheless, for any given water body, variation does tend to be around a certain mean value and the water body can usefully be regarded as typically high, low or intermediate in gilvin concentration. The category in which a particular lake, impoundment or river falls is determined by the drainage pattern, vegetation, soils and climate in the catchment.

The factors governing the concentration of gilvin in surface waters are not well understood and in view of the great influence of this material on aquatic primary production it would be desirable to know more. One generalization that can be made is that gilvin concentration is high in drainage water from bogs or swamps: this can readily be seen, for example, in the peat bogs of northern Europe, and is shown quantitatively in the g440 values for an English bog lake and a creek draining boggy ground in the Australian southern tablelands (Table 3.2). Gilvin concentration is also high in the waters draining from humid tropical forests, as the g440 values for two Venezuelan waters in Table 3.2 show. The lack of oxygen in the permanently, or frequently, waterlogged soil of such areas leads to a build-up of partially decomposed organic matter, and gilvin is derived from the soluble component of this. The other side of the coin is that water draining limestone-rich catchments tends to be low in gilvin. An inverse relationship between water colour and lake depth has been observed for North America.496 The effects of vegetation type, soil mineralogy, agricultural practices, climate and other environmental parameters on gilvin concentration are not well understood.

Although, as we have noted, gilvin is chemically quite stable, it does undergo photochemical degradation by intense sunlight in the surface layer.685,751 The breakdown products have been found to include a range of low molecular weight carbonyl compounds, such as pyruvate, formaldehyde and acetaldehyde, and lower molecular weight carboxylic acids (oxalic, malonic, formic and acetic)685,105 all of which would be readily utilized by aquatic microbes. On the basis of their rate measurements, Kieber et al. (1990) estimate that the half-life of dissolved humic substances of riverine origin in the oceanic mixed layer is 5 to 15 years. Mopper et al. (1991) present evidence suggesting that the photochemical pathway is in fact the main route for the degradation of the long-lived biologically refractory, dissolved organic carbon of the ocean. Microbial decomposition of CDOM is, however, also significant: in water from the Mississippi River plume it occurred at about 30% of the photo-oxidation rate.552

Although action spectra indicate that the UVB region is the part of the solar spectrum (280-320 nm) that is the most damaging for CDOM,685 it does not follow that it is UVB that carries out most of the photo-oxidation through the water column in natural water bodies. The very high absorption of UVB by CDOM ensures that this spectral waveband is rapidly attenuated with depth, so that photo-oxidation occurring further down the water column must be due to light of longer wavelengths. For a humic lake in Finland, Vahatalo et al. (2000) calculated that UVB contributed 9%, UVA (320-400 nm) 68% and visible light 23% to the photochemical mineralization. For lakes in Argentina and Pennsylvania, USA, Osburn et al. (2001) calculated that the contribution of UVB radiation to photobleaching of CDOM was small (<20% of total decrease in the absorption coefficient) compared to that of UVA and blue light. For continental shelf waters of the South Atlantic Bight (southeastern USA), the calculations of Miller et al. (2002) indicate that it is the UVA region of the solar spectrum that is primarily responsible for photo-oxidation of dissolved organic matter. For the estuarine waters of Chesapeake Bay (USA), Osburn et al. (2009) found that the long-wave photobleaching of CDOM increased with increasing salinity.

In the case of freshwater reservoirs, the colour of the water can be a function of how long it is exposed to sunlight before use. In tropical northern Australia, Townsend et al. (1996) found that in two closely adjacent reservoirs with similar catchments, the average concentration of gilvin over eight years was inversely proportional to retention time. The Manton River Reservoir colour was typically two to three times that in the Darwin River Reservoir, which they attributed to the shorter retention time, and therefore shorter time for photobleaching, in the former water body.

Since the total organic matter in a water body may contain some undetermined proportion of colourless organic material, it is difficult to know what meaning to give measurements of the specific absorption coefficient (absorption per unit weight) of CDOM. For Danish coastal waters, Stedmon et al. (2000) found values ranging from 0.0727 m2g_1 C in the Skagerrak to 0.630 m2g_1 C in the Kattegat, with a data set mean of 0.29 ± 0.11m2g_1C, at 375 nm. For a large number of mountain lakes in the Alps and the Pyrenees, Laurion et al. (2000) found specific absorption coefficients of total dissolved organic carbon at 320 nm over the range 0.43 to 3.51m2g~1C. Specific absorption coefficients were generally lower in surface waters than in deeper layers, an effect possibly attributable to photobleaching.

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