The nature of the underwater light field
6.3 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
6.3 Downward irradiance – PAR 159
Fig.6.4Spectraldistributionofdownwardirradianceinmarineandinlandwaters.(a)TheGulfStream(AtlanticOcean) BahamaIslands(plottedfromdataofTylerandSmith,1970).(b)BatemansBay,NSW,Australia(afterKirk,1979).(c)Lake Griffin,ACT,Australia,29September1977;watercomparativelyclear(turbidity¼3.7NTU)(afterKirk,1979).(d)Lake Griffin,6April1978;waterturbid(turbidity¼69NTU)(afterKirk,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 pene-trating 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 logEdagainst 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 coeffi-cient 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 ofKd(PAR) as might be expected from their low absorption and
6.3 Downward irradiance – PAR 161
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
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.
Table 6.2Vertical 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 Kd(PAR) (m1) Reference
Gulf Stream, off Bahamas 0.08 1386
Tropical East Atlantic
100 km off Mexico 0.11 1386
Eastern North Pacific (33N, 142W) 2-week period
0.112–0.187 1224 South Pacific, East of New Zealand
Subtropical convergence zone (41–42S), av. 5 stns
0.087 721
Sub-antarctic water (46–47S), av. 4 stns 0.100 721
West Chatham Rise, av. 4 stns 0.104 721
East Chatham Rise 0.126, 0.134 721
East of shelf,100 km off Dunedin 0.059, 0.080 721 II. Coastal and estuarine waters
A˚rhus Bay, Kattegat, Denmark 0.152–0.557 av.
0.293
Coastal Clyde Sea, Scotland 0.163 148
Clyde R. Estuary
6.3 Downward irradiance – PAR 163
Table 6.2 (cont.)
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
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 0.550.09 1200
Tuggerah Lakes 1.250.18 1200
Coastal turbid-zone coral reef, Cleveland Bay (Great Barrier Reef system), Queensland
0.147–0.439 26 Swan R. Estuary, Western Australia
7 km upstream from mouth 0.40 (0.25–0.69) 749 39 km upstream from mouth 2.19 (0.72–3.69) 749 New Zealand
9 estuaries, North Island, mouth sites, low water 0.3–1.1 1401 III. Inland waters
Irondequoit Bay, L. Ontario 1.030.11 1445
Finger Lakes, N.Y.
Otisco 0.5640.111 346
Seneca 0.4680.075 346
Skanateales 0.2380.029 346
Crater L., Oregon 0.06 1386
San Vicente reservoir, California 0.64 1386
Table 6.2 (cont.)
Water body Kd(PAR) (m1) 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 0.310.12 736
21 clear lakes, yellow 0.700.07 736
23 turbid lakes, little colour 1.631.51 736 Europe
L. Zurich, 10-month period 0.25–0.65 1182
Esthwaite Water, England 0.8–1.6 536
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
duringPeridiniumbloom 3.3 331
L. Tanganyika 0.160.02 550
Volcanic lakes, Cameroon
L. Ginninderra, ACT 1.460.68 697,720
3-year range 0.84–2.74
6.3 Downward irradiance – PAR 165
values (>2.0 m1) 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 Table 6.2 (cont.)
Water body Kd(PAR) (m1) Reference
Burrinjuck Dam, NSW 1.650.81 697,720
6-year range 0.71–3.71
L. Burley Griffin 2.811.45 697,720
6-year range 0.86–6.93
L. George 15.19.3 697, 720
5-year range 5.7–24.9
(b) Murray-Darling system
Murrumbidgee R., Gogeldrie Weir, 10 months 1.4–8.0 1014 Murray R. upstream of Darling R. confluence 1.85–2.16 1014 Darling R. upstream of confluence with Murray 2.78–8.6 1014 (c) Snowy Mountains impoundments
Blowering 0.48 1200
Eucumbene 0.38 1200
Jindabyne 0.49 1200
Talbingo 0.46 1200
(d) Southeast Queensland coastal dune lakes
Wabby 0.48 151
Boomanjin 1.13 151
Cooloomera 3.15 151
(e) Northern Territory (Magela Creek billabongs)
Mudginberri 1.24 725
North Basin, clear station EW6 0.3 95
South Basin, turbid site NS9 1.71 95
bottom sediments by wind-induced wave action can increase the attenu-ation 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.551In 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), Anthonyet 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, Kostoglidiset al. (2005) found, using mul-tiple 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 (averageb1.9 m1) of A˚rhus 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 totalKd(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 ecosys-tems, 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 andKd(PAR) values were in the range 3.2 to 4.7 m1.
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 whichEd(PAR) falls to 1%
of the subsurface value is known as the euphotic zone. Making the
6.3 Downward irradiance – PAR 167
assumption thatKd(PAR) is approximately constant with depth, the value ofzeuis 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 logEd(PAR)versusdepth curve, a value ofKd(PAR) deter-mined 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 ½ 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.