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 (0.2–0.4mm 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 concen- tration 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 3.5also 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 molecu- lar size but also in chemical composition.671,1314,1315In 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 adjectivegilvusmeaning 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 suffi- cient 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 acronymCDOM (pronounced ‘seedom’), which is taken to stand for eithercoloured dissolved organic matter, orchromophoric dissolved organic matter. In recognition of 3.3 The major light-absorbing components of the aquatic system 71
this common usage, but without prejudice as to which interpretation is implied, I shall, in the remainder of this book, use ‘CDOM’ interchange- ably 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
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 truein situabsorption coefficient due to gilvin.
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 forg440to 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
aðlÞ ¼aðl0ÞeSðll0Þ ð3:5Þ wherea(l) anda(l0) are the absorption coefficients at wavelengthsland
l0nm, respectively, andSis 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 0.01760.0020 nm1. For 12 New Zealand lakes,Svaried from 0.015 to 0.020 nm1, with a mean of 0.0187 nm1;285 and for 22 Australian inland waters, S varied from 0.012 to 0.018 nm1, with a mean of 0.016 nm1.720 When partial photobleaching of CDOM takes place, the surviving dissolved colour has a spectrum with a some- what higherSvalue.1417In agreement with this observation, Twardowski and Donaghay (2002) found surface waters to have higherSvalues 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 ~355 nm or ~370 nm, CDOM fluoresces with a very broad peak in the blue region. The quan- tum yield is about 0.8%.1418The 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 (seeChapter 7).1418,94
Table 3.2contains a selection of published data in the form of values of g440 for marine as well as fresh waters in various geographical regions. 3.3 The major light-absorbing components of the aquatic system 73
Table 3.2Value of absorption coefficient at 440 nm due to dissolved colour (g440)and particulate (phytoplanktonþtripton) 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 (gþp)was available.
Water body g440(m 1 ) p440(m 1 ) Reference I. Oceanic waters Atlantic Ocean Sargasso Sea ~0 0.01 760 Off Bermuda 0.01a – 616 Caribbean Sea 0.03 (gþp) – 636
Gulf of Mexico – Gulf loop intrusion 0.005 – 204 Romanche Deep 0.05 (gþp) – 636 Mauritanian upwelling 0.034–0.075a – 161 Gulf of Guinea 0.024–0.113 – 161 Pacific Ocean Galapogos Islands 0.02a – 189 Galapogos Islands 0.16 (gþp) – 636 Central Pacific 0.04 (gþp) – 636 Off Peru 0.05a – 189 Indian Ocean Oligotrophic water 0.02 – 748 Mesotrophic water 0.03 – 748 Eutrophic water 0.09 – 748 Mediterranean Western 0.0–0.03a – 574 Arctic Ocean
Beaufort & Chukchi Seas 0.054 0.13 1439
Southern Ocean North–South transect,
42–55S, along 142E
0.019–0.099b – 242
II. Shelf, coastal & estuarine waters
Gulf of Mexico
Yucatan Shelf 0.006 – 204
Bay of Campeche 0.022 – 204
Cape San Blas 0.054 – 204
Mississippi plume 0.028 – 204
North America, Atlantic coast
New England shelf, summer 0.060 0.044 1269
Southeast USA, Georgia, 60 km offshore, spring 1993 (high river discharge)
0.200b 0.026b 980
Summer 1992 0.021b 0.031b 980
Table 3.2 (cont.) Water body g440(m 1 ) p440(m 1 ) Reference Chesapeake Bay (Rhode
R. Mouth)
0.27 0.80 428
Georgia salt marsh 1.52 – 1460
Arabian Sea Cochin, India, 3 km offshore 0.24c(gþp) 1113 30 km offshore 0.10c(gþp) 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 Zealand), av. of 19 stations
0.044 0.026 1218
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
A˚rhus Bay, Denmark (Kattegat)
0.232 834
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,~18S 0.0500.028 0.010 126 Mossman-Daintree estuarine,
16S. Dry season
0.0820.087 0.085 126
Wet season 0.2460.254 0.531 126
Fitzroy R./Keppel Bay system, 23S. Dry season. Estuary station 2
0.471 – 1025
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, mouth sites, low water
0.1–0.6 – 1401
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 ~2.00.4 – 314 (8 months) 1.4–2.8 –
Blaxter L. (bog lake), England 9.65 – 891
Ireland:
Carmean Quarry 0.23 – 643
Table 3.2 (cont.) Water body g440(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 catchments 0.074 av. (0.00–0.28) – 776 Alpine meadow catchments 0.154 av. (0.09–0.21) – 776
Forested catchments 0.232 av. (0.02–0.53) – 776 Africa L. George, Uganda 3.7 – 1336 L. Victoria (Uganda), Murchison Bay 0.45 1013 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, USA
0.90 0.65 1445
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 0.480.24 – 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 1.540.78 0.16–0.58 696, 697, 701, 720 (3-year range) 0.67–2.81 – L. George 1.801.06 3.73–4.21 696, 697, 701, 720 (5-year range) 0.69–3.04
Table 3.2 (cont.) Water body g440(m 1 ) p440(m 1 ) Reference Burrinjuck Dam 2.211.13 0.63–1.44 696, 697, 701, 720 L. Burley Griffin 2.951.70 2.91–2.96 (5-year range) 0.99–7.00 – 696, 697, 701, 720 Googong Dam 3.42 0.83 701 Queanbeyan R. 2.42 – 720 Molonglo R. 0.44 – 720
Molonglo R. Below confluence with Queanbeyan R.
1.84 – 720
Creek from boggy catchment 11.61 – 720 (b) Murray-Darling system Murrumbidgee R., Gogeldrie Weir (10 months) 0.4–3.2 – 1014 L. Wyangan 1.13 0.38 720 Griffith Reservoir 1.34 3.73 720
Barren Box Swamp 1.59 2.55 720
Main canal, MIA 1.11 5.35 720
Main drain, MIA 2.12 10.34 720
Murray R., upstream of Darling confluence
0.81–0.85 – 1014
Darling R., above confluence with Murray
0.7–2.5 – 1014
(c) Northern Territory (Magela Creek billabongs) Mudginberri 1.11 1.13 725 Gulungul 2.28 1.68 725 Georgetown 1.99 18.00 725 (d) Tasmania Huon R. 7–14 – 241 Lakes: Perry 0.06 – 152 Ladies Tarn 0.40 – 152 Risdon Brook 0.98 – 152 Barrington 3.05 – 152 Gordon 8.29 – 152
(e) Southeast Queensland, coastal dune lakes
L. Wabby 0.06 – 151 Basin L. 0.46 – 151 L. Boomanjin 2.59 – 151 L. Cooloomera 14.22 – 151 (f) South Australia Mount Bold Reservoir 5.40 2.25 433
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 inTable 3.2show that marine waters generally have much less dissolved colour than inland waters, and the greater the dis- tance 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 Table 3.2 (cont.) Water body g440(m 1 ) p440(m 1 ) Reference New Zealand Waikato R. (330 km, L. Taupo to the sea): L. Taupo (0 km) 0.070 0.033 282 Ohakuri (77 km) 0.22 0.32 282 Karapiro (178 km) 0.82 0.71 282 Hamilton (213 km) 0.97 0.98 282 Tuakau (295 km) 1.37 1.67 282
Lakes (mean of monthly values over 11 months): Rotokakahi 0.09 – 285 Rotorua 0.23 – 285 Opouri 0.86 – 285 Hakanoa 1.84 – 285 D 4.87 – 285
aValues measured at a wavelength less than 440 nm and converted to
g440on the basis of an appropriate CDOM absorption spectrum.
bMeasurements carried out at 443 nm, to conform with the corresponding
SeaWiFS band.
cPublished values for
cat 440 nm corrected for scattering.
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 inTable 3.2give an indication of the extent of this variability. For example, in Lake Burley Griffin (Canberra, Australia) the value ofg440varied 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 deter- mined 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 quantita- tively 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,805 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.496The 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.685,751The breakdown products have been found to include a range of low molecular weight carbonyl compounds, such as pyruvate, formal- dehyde and acetaldehyde, and lower molecular weight carboxylic acids (oxalic, malonic, formic and acetic)685,105all 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 sub- stances of riverine origin in the oceanic mixed layer is 5 to 15 years. Mopperet 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,685it 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, Va¨ha¨taloet al. (2000) calculated that UVB con- tributed 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 Milleret 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), Osburnet 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, Stedmonet al. (2000) found values ranging from 0.0727 m2g1C in the Skagerrak to 0.630 m2g1C in the Kattegat, with a data set mean of 0.290.11 m2g1C, at 375 nm. For a large number of mountain