Downward irradiance PAR

As a broad indication of the availability of light for photosynthesis in an aquatic ecosystem, information on the penetration of the whole

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Pig. 6.4 Spectral distribution of downward irradiance in marine and inland waters, (a) The Gulf Stream (Atlantic Ocean) off the Bahama Islands (plotted from data of Tyler and Smith, 1970). (b) Batemans Bay, NSW, Australia (after Kirk, 1979). (c) Lake Burley Griffin, ACT, Australia, 29 September 1977; water comparatively clear (turbidity = 3.7 NTU) (after Kirk, 1979). (d) Lake Burley Griffin, 6 April 1978; water turbid (turbidity = 69 NTU) (after Kirk, 1979).

photosynthetic waveband is of great value. As solar radiation penetrates a water body, it becomes progressively impoverished in those wavelengths which the aquatic medium absorbs strongly and relatively enriched in those wavelengths which are absorbed weakly. We would therefore expect the attenuation coefficient for total PAR to be higher in the upper few metres and to fall to a lower value with increasing depth. This change in the rate of attenuation of PAR with depth can readily be observed in most marine waters and the clearer inland systems: two of the curves in Fig. 6.5 - for the Tasman Sea, and for a relatively clear 1ake - show the increase in slope of the log Ed curve with increasing depth. The curve eventually becomes approximately linear, indicating that the downward flux is now confined to wavebands all with about the same, relatively low, attenuation coefficient. In oceanic waters the light in this region is predominantly blue-green (Fig. 6.2a), whereas in inland waters the penetrating waveband is likely either to be green (Fig. 6.2b), to extend from the green to the red (Fig. 6.2c), or to be predominantly red (Fig. 6.2d).

A countervailing tendency, which exists at all wavelengths, is for attenuation to increase with depth as a result of the downward flux becoming more diffuse, due to scattering. By counteracting the effect of changes in spectral composition, it may partly explain why graphs of log Ed against depth for turbid waters are so surprisingly linear (Fig. 6.5, L. Burley Griffin), and lack the biphasic character seen in the clearer waters. However, since high turbidity is commonly associated with increased absorption at the blue end of the spectrum (see §3.3) it is also true that in such waters the blue waveband is removed at even shallower depths than usual and so the change in slope of the curve occurs quite near the surface and is not readily detectable.

Even when, as in the clearer waters, the graph of log Ed against depth is noticeably biphasic, the change of slope is usually not very great. Thus, the attenuation of total PAR with depth is nearly always approximately, and often accurately, exponential in agreement with eqns 6.1 and 6.2. Attenuation of PAR in a given water body can therefore generally be characterized by a single value of Kd, or, at worst, by two values, one above and one below the change in slope. The vertical attenuation coefficient for downward irradiance of PAR provides a convenient and informative parameter in terms of which to compare the light-attenuating properties of different water bodies. Table 6.2 presents a selection of values, including some obtained by summation of spectral distribution data across the photosynthetic range. Oceanic waters have the lowest values of Kd (PAR) as might be expected from their low absorption and

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Irradiance Attenuation

Fig. 6.5 Attenuation of downward quantum irradiance of PAR with depth in a coastal water (Tasman Sea, off Batemans Bay, NSW) and two inland waters (Lake Burley Griffin, ACT; Burrinjuck Dam, NSW) in Australia (Kirk, 1977a, and unpublished). The marked decrease in rate of attenuation in Burrinjuck Dam below about 7 m is particularly noteworthy: spectro-radiometric measurements showed that most of the light below this depth was confined to the 540 to 620 nm (green-yellow) waveband.

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Fig. 6.5 Attenuation of downward quantum irradiance of PAR with depth in a coastal water (Tasman Sea, off Batemans Bay, NSW) and two inland waters (Lake Burley Griffin, ACT; Burrinjuck Dam, NSW) in Australia (Kirk, 1977a, and unpublished). The marked decrease in rate of attenuation in Burrinjuck Dam below about 7 m is particularly noteworthy: spectro-radiometric measurements showed that most of the light below this depth was confined to the 540 to 620 nm (green-yellow) waveband.

scattering. Inland waters, with rare exceptions such as Crater Lake in Oregon, USA, have much higher values, with coastal and estuarine waters coming in between. The highest values are found in very turbid waters (e.g. L. George and Georgetown billabong in Australia) in which the suspended tripton strongly absorbs, as well as scatters, the light. High

Table 6.2 Vertical attenuation coefficients for downward quantum irradiance of PAR in some marine and fresh 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.

Water body

I. Oceanic waters

Atlantic Ocean

Sargasso Sea

Sargasso Sea

Gulf Stream, off Bahamas Tropical East Atlantic Guinea Dome

Mauritanian upwelling, offshore Mauritanian upwelling, coastal Pacific Ocean Off Oahu, Hawaii 100 km off Mexico

2-week period South Pacific, East of New Zealand Subtropical convergence zone (41-42° S), av. 5 stns

West Chatham Rise, av. 4 stns

East Chatham Rise

East of shelf, ~ 100 km off Dunedin

II. Coastal and estuarine waters

Europe North Sea

Offshore Netherlands Dogger Bank

Ems-Dollard Estuary (Netherl./Germany) Inner region Outer region

Arhus Bay, Kattegat, Denmark Bjornafjord, Norway

Kongsfjord, Spitsbergen (spring, clearest water conditions) Shannon Estuary, Ireland Upper Middle Lower

Coastal Clyde Sea, Scotland Clyde R. Estuary

0.03

0.08-0.096

0.032 0.11

0.112-0.187

0.087

0.100 0.104

0.41

1382 1250

1386

942 942 942

1386

1224

721 721 721 721

1129 1129

254 254 834

638 521

892 892 892 148

Water body

Kd (PAR) (m-1)

Referen

Gare Lough entrance

0.429

148

Upstream (Dumbarton)

1.158

148

Lough Etive, Scotland

0.3-0.4

887

North America

Gulf of California

0.17

1386

Chesapeake Bay, Rhode R. mouth

1.10-2.05

428

Delaware R. Estuary

0.6-5.0

1317

Hudson R. Estuary, N.Y., 10 stns av., July

2.02

1312

San Francisco Bay

Shallows, inner estuary

10-13

246

Outer estuary

246

Georgia Embayment

St Catherine's Sound

2.9

1005

8 km offshore

1.8

1005

30 km offshore

0.27

1005

60 km offshore

0.09

1005

Fraser R., Strait of Georgia (Canada)

River mouth

0.8

539

Porlier Pass

0.27

539

Australia

Tasman Sea, coastal New South Wales

0.18

697

Port Hacking Estuary, NSW

0.37

1199

Clyde R. Estuary, NSW

0.71

697

Coastal sea lakes, NSW

Lake Macquarie

0.55 ± 0.09

1200

Tuggerah Lakes

1.25 ± 0.18

1200

Coastal turbid-zone coral reef, Cleveland Bay

0.147-0.439

26

(Great Barrier Reef system), Queensland Swan R. Estuary, Western Australia 7 km upstream from mouth 39 km upstream from mouth New Zealand

9 estuaries, North Island, mouth sites, low water

III. Inland waters

North America Great Lakes L. Superior L. Huron L. Erie L. Ontario

Irondequoit Bay, L. Ontario

Finger Lakes, N.Y.

Otisco

Seneca

Skanateales

Crater L., Oregon

San Vicente reservoir, California

0.564 ± 0.111 0.468 ± 0.075 0.238 ± 0.029 0.06 0.64

749 749

1401

639 639

639, 1285

1445

1386

Water body

Kd (PAR) (m-1)

Reference

L. Minnetonka, Minnisota

0.7-2.8

896

McConaughy reservoir, Nebraska

1.6 (av.)

1144

Yankee Hill reservoir, Nebraska

2.5 (av.)

1144

Pawnee Hill reservoir, Nebraska

2.9 (av.)

1144

Alaskan lakes

44 clear lakes, little colour

0.31 ± 0.12

736

21 clear lakes, yellow

0.70 ± 0.07

736

23 turbid lakes, little colour

1.63 ± 1.51

736

Europe

L. Zurich, 10-month period

0.25-0.65

1182

Esthwaite Water, England

0.8-1.6

536

Loch Croispol, Scotland

0.59

1274

Loch Uanagan, Scotland

2.35

1274

Forest lakes, Finland

Nimeton

3.45

658

Karkhujarvi

2.49

658

Tavilampi

1.75

658

Mountain lakes (Alps, Pyrenees)

Predominantly rock catchments (10 lakes)

0.16 av.

776

Alpine meadow catchments (5 lakes)

0.35 av.

776

Forested catchments (10 lakes)

0.40 av.

776

Las Madres L., Spain

0.42-0.88

21

Middle East

Sea of Galilee (L. Kinneret), Israel

0.5

331

during Peridinium bloom

3.3

331

Africa

L. Simbi, Kenya

3.0-12.3

897

Saline-alkaline lakes, Kenya

Bogoria

12.7 ± 0.2

1003

Nakuru

13.8 ± 0.3

1003

Elmentaita

12.5 ± 0.3

1003

L. Tanganyika

0.16 ± 0.02

550

Volcanic lakes, Cameroon

Barombi Mbo

0.148

733

Oku

0.178

733

Wum

0.305

733

Beme

0.353

733

South African impoundments

Hartbeespoort

0.67

1431

Rust de Winter

1.70

1431

Bronkhorstspruit

4.23

1431

Hendrik Verwoerd

13.1

1431

Australia

(a) Southern Tablelands

Corin Dam, ACT

0.87

720

L. Ginninderra, ACT

1.46 ± 0.68

697, 720

3-year range

0.84-2.74

Burrinjuck Dam, NSW 6-year range L. Burley Griffin 6-year range L. George 5-year range

(b) Murray-Darling system Murrumbidgee R., Gogeldrie Weir, 10 months Murray R. upstream of Darling R. confluence Darling R. upstream of confluence with Murray

(c) Snowy Mountains impoundments Blowering

Eucumbene

Jindabyne

Talbingo

(d) Southeast Queensland coastal dune lakes Wabby

Boomanjin Cooloomera

(e) Northern Territory (Magela Creek billabongs) Mudginberri

Gulungul Georgetown

(f) Tasmania (lakes) Perry

Ladies Tarn Barrington Gordon Pedder

New Zealand (lakes)

Taupo

Rotokakahi

Ohakuri

Rotorua

Hakanoa Japan L. Biwa

North Basin, clear station EW6 South Basin, turbid site NS9

1.65 ± 0.81 0.71-3.71 2.81 ± 1.45 0.86-6.93 15.1 ± 9.3 5.7-24.9

1014 1014 1014

1200 1200 1200 1200

151 151

725 725 725

152 152 152 152 152

287 286 286 286 286 286

95 95

values (>2.0 m"1) may also be associated with dense algal blooms (Sea of Galilee - Peridinium), with intense soluble yellow colour but low scattering (L. Pedder, Tasmania), or with a combination of high soluble colour and scattering (L. Burley Griffin, Australia). In shallow lakes, resuspension of bottom sediments by wind-induced wave action can increase the attenuation coefficient severalfold, and if the sediments contain a substantial proportion of clay particles then the increased attenuation can last for a week or so after the initial storm event.551 In shallow coastal waters, such as those in and adjoining coral reefs, resuspension of sediments by wave action can greatly reduce light availability for benthic plant life. On the basis of a two-year study in a turbid coastal-zone reef within the Great Barrier Reef Lagoon (Australia), Anthony et al. (2004) concluded that the main factor (74 to 79%) limiting availability of PAR for the coral was high turbidity caused by wave-induced resuspension: clouds accounted for only 14 to 17% and tides for 7 to 10% of the variations in benthic irradiance.

In the enormous (68 800 km2) tropical African lake, L. Victoria, the factors controlling penetration of PAR vary from place to place. Loiselle et al. (2008) observed that in nearshore areas where extensive wetlands are present, dissolved yellow colour plays the dominant role. Attenuation due to tripton was important around a river outflow, and biomass-related attenuation increased in significance towards the open lake. In the Swan River estuary, Western Australia, which has a highly coloured freshwater inflow from coastal wetlands, Kostoglidis et al. (2005) found, using multiple regression, that 66% of the variation in Kd (PAR) was explained by CDOM and an additional 8% by total suspended solids. In the rather turbid waters (average b —1.9m"1) of Arhus Bay, Kattegat (North Sea-Baltic estuarine transition) Lund-Hansen (2004) estimated that water contributes 9%, CDOM 17%, phytoplankton 32% and suspended par-ticulate matter (inorganic) 42% to total Kd (PAR).

At high latitudes for much of the year, PAR has to pass through a layer of sea ice before it can pass into the water column. Ehn et al. (2004) measured spectral transmittance through a 28 cm thick layer of landfast sea ice near the entrance to the Gulf of Finland (Baltic Sea). The Baltic Sea has higher levels of yellow substances than most other marine ecosystems, and the sea ice contained higher levels of dissolved and particulate colour than are typical for the Arctic. Spectral albedo values integrated over 400 to 700 nm were commonly between 0.33 and 0.42 and Kd(PAR) values were in the range 3.2 to 4.7 m"1.

A useful, if approximate, rule-of-thumb in aquatic biology is that significant phytoplankton photosynthesis takes place only down to that depth, zeu, at which the downwelling irradiance of PAR falls to 1% of that just below the surface. That layer within which Ed (PAR) falls to 1% of the subsurface value is known as the euphotic zone. Making the assumption that Kd (PAR) is approximately constant with depth, the value of zeu is given by 4.6/Kd. This, as we have seen, is a reasonable assumption for the more turbid waters and so will give useful estimates of the depth of the euphotic zone in many inland, and some coastal, systems. In the case of those clear marine waters in which there is a significant increase in slope of the log Ed (PAR) versus depth curve, a value of Kd (PAR) determined in the upper layer could give rise to a substantial underestimate of the euphotic depth.

Another useful reference depth is zm, the mid-point of the euphotic zone. This, by definition, is equal to % zeu: given the approximately exponential nature of the attenuation of PAR with depth, it follows that zm ~ 2.3/Kd, and corresponds to that depth at which downward irradiance of PAR is reduced to 10% of the value just below the surface.

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