OPTICAL PROPERTIES OF LAKB COLERIDGE: THE IMPACT OF' IIYDROELECTRIC PO\ryER DEVELOPMENT I

OPTICAL PROPERTIES OF LAKE COLERIDGE:

POI/üER DEVELOPMENT for Electricorp)

ROBERT J DAVTES-COLLEY !ìIATER QUAI,ITY CENTRE D.s .r.R. P O BOX 11-115 HA}IILTON, NZ

JOHN K FEN}IICK WATER RESOURCES SURVEY D.S.I.R. P O BOX 13-695 CHRISTCHURCH, NZ

Running Head,Iine: Lake Coleridge, Optical Properties

TABLE OF CONTENTS

Page

EXECUTIVE SUMMARY

TECHNICAL SUMMARY

INTRODUCTION

STUDY AREA

METHODS 9

RESULTS 11

Lake Water Colour 11 Visual CIaritY 12 r,ight Attenuatj-on and. Reflectance 13 Water Composition and Optical Character 15 Absorption and Scattering of r,ight 16 Inflow of Suspended Sediments 18

DISCUSSION 19 Effects of the Diversion Waters on Visual Clarity 19 Regression Models 20 optical Model 22 Relation of In-Lake Suspended Sediments to Sediment Inflow 24

CONCLUSIONS 25

27 RECO¡4MENDATIONS FOR FUTURE WORK EXECUTIVE SUMMARY

Lake Coleridge is generally a visually attractive, clear, water body the optical character of which largely reflects the i-nfluence of fresh water rather than that of water constituents. The greenish-blue hue is essentially that of pure water, but the colour is brighter and greyer than optically pure water because of the presence of fine suspended inorganic sediments. These sediments appear to be derived mainly from diverted \,raters of the Harper and V{ilberforce Rivers although there is only a weak correlation of in-lake concentrations with diversion sediment loads. a significant decrease in visual clarity of the lake as measured by Secchi disk depth (which fell from 13.4 to 8.6 m) has occurred since the oakden Canal was comnissioned. Relationships developed in this study, between Secchi depth and other optical measures¡ suggest that this decrease in visual clarity has been acconpanied by a drop of lesser magnitude in light penetration for plant photosynthesis and an increase in brightness of the water colour.

A relationship between inlake suspended matter and inflowing suspended Loads in the diversion \,¡ater \¡¡as not established because of incompleÈe sediment inflow records, no data on sediment grain sizes involved, and the complicated hydro and sedirnent dynamics of the lake. A programme of monitoring for the life of the water right, based on Secchi disk observations at a central Iake site, is recommended in order to track any long term trend in clarity and to provide a database against which to measure the impact of any future changes in lake management.

If a change in lake management policy is being contemplated then a more detailed study to enable the optical properÈies to be modelled, from the quantity and type of inflow sedimentsr mâY be required. TECHNICAL SUMMARY

A one year study of the optical- properties of Lake Coleridge has been carried out to determine the effects of inflows of turbid waters in the Wil-berforce and Harper Rivers which augment hydro-electric po\¡rer generation at the Coleridge Power Station. The objectives of the study were (a) to determine the factors controlling the optical character of the Iake, including visual- clarity, Iight climate and colour, (b) to assess the ecological and aesthetic impacts (if any) of the dj-version waters via changed optj-cal properties and (c) to recommend a programme of long term monitoring f or the l-ake.

The study has shown that the blue colour of the water in Lake Coleridge largely reflects the influence of v/ater itself, rather than that of additionaf constituents, on the optical properties. However, at certain tirnes the lake water colour appears somewhat greyer and appreciably brighter than pure water. This depends on the concentration of inorganic suspended sediments from the d.iversion waters. These inorganic sediments dominate the attenuation of image-forming light in Lake Coleridge, mainly by the process of scattering, and thus contribute strongly to reduction in visual clarity as measured by Secchi depth. Phytoplankton biomass, in this oligotrophic lake, is low and has littl-e influence on the overall optical character.

Comparison of mid-lake Secchi depth measurements made in this study with historical- data suggests that visual clarity has been significantly reduced from about 13.4 m to 8.6 m since Èhe Oakden canal was commissioned. The visual clarity is still "high" r but the reduction may represent an impact on aesthetic quality of the lake and, to a lesser extent, on the ecology, by reducing the sighting range of fish for example. However, it is difficult to assess the significance of this reduction in clarity in terms of recreational use of the Iake in the absence of guidelines or criteria for aesthetic quality. It is thought likeJ-y that this reduction in clarity has probably stabilised at a ner¡r steady state, but this is not known with certainty.

The penetration of diffuse ambient light into lake Coleridge is "high" (cornparable to that in Lake Taupo) with 1Z of light reaching about 30 m 5 depth on average. This euphotic depth may be less compared with pre-diversion times, although to a lesser extent than the visual clarity. This is because the diversion \¡tater sediments scatter light. strongly but are only weakly lighÈ absorbing. OnIy s1ight impacÈs on lake ecology are expected from this change in light climate, namely a small reduction in benthic plant cover and thus in available habitat for aquatic fauna.

Simple models have been developed, based on linear regression and on established optical theorY. These models relate the oPtical, characteristics of Lake Coleridge to in-lake total suspended sediment (TSS) concentrations. However, it was not possible to predict in-lake TSS from mass flow of sediment in the diversion waters (and other inflows) because of incomplete mass infl-ow data, and the complexity of inflow/Iake hydraulics, and the wide dispersion of grain sizes (and hence of faII velocities and of scattering per unit mass of solids). Thus we cannot, at present, model the effects on the optical character of Lake Coleridge of changed diversion water Ioadings.

Long term monitoring, at a low level of activity, is desirable to detected any long term trends in clarity (e.g. from continued shore erosion) and Èo provide data for assessment of the impact of any future changes in management of the lake. It is recommended that Secchi depths be measured at monthly intervals at a mid-take site, and thaÈ water samples be obtained for analysis of suspended sediments and turbidity (the latter as a quick and simple check on data consistency). The Secchi, turbidity and TSS data should be plotted immediately they are available, to provide a time series which will facilitate examination of trends in optical quality of Lake Coleridge. If changes in Lake management are being conÈemplated Èhen a more detailed study to develop a model of Iake optical properties, from Èhe quantity and quality of inflow sedimentsr f,âY be worthwhile. INTRODUCTION

Lake Coleridge, in the Canterbury high country (Fig 1), is a rnajor fishery and a tourist asset of regional, and perhaps, national importance. The lake also provides water storage for hydro-electric power generation at the Coleridge Power Station, situated near the southern shore. The station began operating in 1914 using water flowing naturall-y into the lake. Ho\^/ever, increasing electricity demand has resulted in the need to augment the inflows (to increase ttre generating potential) with waters from the glaciated Harper and !{i}berforce catchments in 1921 and 1977, respectively. An environmental impact report was prepared (in 1975) to cover the final-, Wilberforce, part of the development (Jo\^/ett 1984). one of three major concerns identified in submissi-ons on this report was that the discharge of Wilberforce waters, including glacial neltvrater laden with high suspended matter concentrations, \^roul-d reduce the general clarity of Lake Coleridge (Jowett 1984). A further concern has been with the potential for increased phytoplankton production in response to the increased nutrient load in the diverted water (t',titcfrett 1984) .

Suspended sofids mainly affect receiving waters by attenuating light (principally through scaÈtering - Kirk 1985). This results in reduction of clarity and. changes in colour. The change in colour is mainly attributable to an increase in the ratio of Iight scattering to tight absorption. This causes an increase in brightness of the water, but a d,ecrease in colour saturation (ie, increased "gtreyness" of the colour) also occurs. Hue (eg, blue, green, yellow) may also change if the mineral sediment is associated v/ith light-absorbing organic materj-al.

Reduction in waÈer clarity could irnpact the lakets ecology and recreational value in two main ways. Firstly, reduced visual clarity (traditionally measured by Secchi disk depth) would reduce the visual range of aquatic organisms and of recreational users of the lake. Secondly, the penetration of diffuse photosynthetically available radiation (PAR) which determines the depth to which plants can grohT in the water (euphotj-c depth) rnight be reduced with consequent impact on primary production and on the depth range of plants. These two aspects of v/ater clarity reflect rather different optical processes. Thus, there is no analyticat relationship between 7 visual range in water and penetration of diffuse light in water, although a broad correlation is often noted.

To date it has not been demonstrated that any significant reduction in water clarity (or change in colour) has occurred following diversion of llilberforce vrater to the lake. Hovrever, Jowett (1984) using a regression model he developed for Secchi depth (based on residence time, mean depÈh and percentage glaciation of 24 large New Zealand lakes , t2 = O.77) predicted that the Secchi disk clarity in the centre of Lake Coleridge would stabilise at about 1 metre less than pre-diversion levels. AIso, in a thorough review of past studies on the lake, Graynoth (1987) was not able to find results which demonstrated that any lake-wide ecological damage has resulted from the diversj-ons. Hor¡Iever it may be that such damage, if it has occurred, could be subtle and difficult to detect (Mitchell, 1984).

A one year study of the optical character of Lake Coleridge was therefore proposed with the objecÈives of determining (a) Èhe factors conÈrolling the water clarity and colour and light climate for aquaÈic planÈ growth, and (b) whether there has been a significant change in the optical character' particularly clarrty, in the lake \,rater since diversion. The results of this study are presented below together with reconmendations for future monitoring. Fig.1

N À I Oakden\ Harper Diversion I

-rNilr¡=' Canal \ I gge¡ o 1 2 3 4 Skm

e.9 ðf

ñtake)

Power

Fig.1 Map showing tTre location of Lake Coleridge and of the samPling stations in the lake.

STUDY AREA

Lake Coleridge, at an altitude of 507 m, occupies a 200 m deep trough which hras formed by the Wilberforce Glacier (fig.1). The lake is 17.8 km long and, relatively narrow (maximum widÈh, 3.4 km) with a surface area of 32.g km2 (Irwin 1975). The littoral is very steep except at both ends and on the delta of the Ryton River (fig. 1 ). Thus much of the lake is very dee,o with over 602", by area, being over 100 m deep (Bowden 1983).

The natural catchment area is 21O km2 which increases to g87 km2 if the diversion catchments are included (Bowden 1983). The vegetation cover of the natural catchment is 7OZ tussock t 48.1v" of the catchment has a steep (26-35.) slope, and 39.8% is affected by sheet erosion (Livingston et. al.

1 986). prior to hydroelectric porÀrer development only small flows (about + n3¡s¡ of largely sediment-free water entered Lake Coleridge from four tributaries. This has nohT increased to 24 *3/" of water, with an estirnated 34r0OO tonnes of sediment per year entering from the Harper diversion and 1O0r00O tonnes per year from the Wilberforce (Bowden et. aI.

1 983, Jowett 1 984) .

Lake Coleridge has had a range of limnological studies carried out on it' but few have been in detail. The lake is oligotrophic (Livingston et. al. 1986) and as a resuft of the frequent strong winds funnelled down the glacial basin, it is deeply mixed, even in mid sumrner. METHODS

Eight transects were established down the Iake in a logarithmic progressíon from the north-western end where the diversion waters enter (fig 1 ). Initially 5 sites \¡¡ere sampled along each transect. This was reduced to three per transect (i.e. one central site and thto "nearshore" sites in about 20 m depth of water) after the first sampling occasion which demonstrated redundancy of information. It was intended that each site be sampled rnonthly commencing SeptemÞer 1986. However, due to the rough seas that develop on the lake in windy conditions, sampling became less regular, with 1 1 occasions being successful over 1 2 months.

At all sites Secchi disk depth (%Ol vras measured on the sunny side of the boat as recommended by Tyler (1968) usj-ng a 2OO mm black and white disk, and viewing box. Surface $rater samples (t litre) were also col-Iected for turbidity (Turb.¡ analysis in Èhe laboratory. At four of the sites (2/3, 5/3, 7/3, g/3 - hereafter called the "main" sampling sites) 3 x 21, on the first sampling occasion, and on subsequent samplings 3 x 5L, samples were collected for total suspended solids (TSS) analysis and a further 3 for chlorophyll a (ChI. a). When Secchi depth was less than 7.0 m at any of the sites on transect 1, triplicate 59" samples were also collected from Lhose sites so as to permit a more precise quantification of the relationship beÈween Secchi disk clarity, and TSS and ChI a.

Profiles were also taken at the maín sites of upwelling (gr) and downwelling (9d) quantum irradianc" (3åE) using a Li-cor PAR sensor and meter, and with measurements at 2 m intervals for 30 m down the water column. Most measurenents were taken between 1000 and 1500 h NZST. These data \â/ere used to calculate the vertical attenuation co-ef f icient, S, (where K. = -LLnQ^/Lz, with z = depth of measurements); the depth of the < -(-¡,' euphotic zerrat z ur (where z r, = a.6/!ì¡ and the refl-ectance coefficient, E, (where I = !*r/gu at the rnid-point of the euphotic zone). The nomograms of Kirk (1981) were used (with R and 5d) to estimate the PAR-band scattering and absorption coefficient.s of the waters.

In the laboratory, on the day following sarnpling, turbidity was measured using a Hach 21004 turbidimeter; TSS by drying material collected on a

'r rtdttrrt r:,r ¡ , ¡i 0

Whatman GF/C filter from each 5.0 replicate at 105oC for 24 hours, cooling in a desiccator and weighing; ChlorophyJ-I a by extracting the phytoplankton collected on a Whatrnan GF/C fil-ter from each 5.Q, replicate in boiling (7BoC) gOZ ethanol and steeping for 24 hr at 4oC then reading on a Varian spectrophotometer at 665 and 750 nrn. An absorption coefficient of 3 2A.66 mg Chl a/m- per absorbance unit ( 1 Omm cuvette ) was used and all values were corrected fJ trr. presence of phaeopigments (Sartory 1982).

Absorbances at 27O nm of both filtered and unfiltered water samples r/ere measured by spectrophotometer in a 1 0 mm cuvette to indicate absorption by dissolved and total organic matter. VoIatiIe suspended solids \^/ere initially determined on the TSS filters as a measure of particulate organic matterr but were discontinued after the first 2 sampling occasions because levels v¡ere very low (within the error range for the method).

Data hrere logged on a VAX mini computer using the AQUi\-t data entry programme and transferred to SAS (SAS 1985) for statistical analyses.

On one occasíon (September 23,1986) water samples were obtained from sites 2/3 and 8/3 for full spectrophotometric analysis following the procedures of Davies-Colley (1983). These analyses were performed on a Pye-Unicam pU8800 spectrophotometer. The absorbances of the fiftered water sample, and of the particul-ates concentrated frorn the water by filtration on a O.2 Um membrane filter, r¡rere scanned through the visible spectrum and, subsequently, spectral absorption and scattering coefficients were estimated. RESULTS eppendix 1 gives a computer listing of the rav¡ data collected from the four main sites, and Appendix 2 lists the complete dataset for chlorophyll !r Secchi disk, TSS, and turbidiÈy. Table 1 summarises data (means and coefficients of variatj-on) for the 4 main sites.

Lakewater colour

The colour of water, viewed from above, is a manifestation of the spectral scattering and absorption properties of ttre water.

Figure 2a shows spectral absorption and scattering coefficients (plots of absorption and scattering against wavelength) for síl.e 2/3 on 23 September, 1986. The total absorption in the \¡rater is noteworthy in being low and dominated by the absorption of (pure) water throughout much of the visibLe spectrum. The water contains very little so-called "yellow substance'l (Iight-absorbing dissolved organic matÈer) although this is still the main absorbing component in the short wave visible and ultraviolet (UV) region ( À ( 450 nm) . The absorption by particulate constituents of water is low and is probably nainly attributable to the Low phytoplankton bionass and to sma1l amounts of detritus (particulate organic matter) derived from organic production by phytoplankton.

Also shown in Figure 2a is the sgectrum of scattering. the scattering is entirely do¡ninated by particul-ate constiÈuents of the water, mainly mineral sediment. The scattering coefficient, averaging about 0.3/m ín the middle of the visible spectrum, is consistent with the turbidity measured by tj¡e Hach 21004 nephelometer on this occasion (0.4 FTU). (Kirk 1981, 1983 has shown that turbidity measured on a HACH 21 004 instrument, by a fortunaÈe co-incidence, is numerically similar to the scattering coefficient).

Figure 2b shows the normalised reflectance spectrum for site 2/3 predicEed from the Èotal absorption coefficient (g( À) ) and scattering coefficient (g(À)) given in fig. 2a. It is the reflectance spectrum which determines the water colour. The simulated refLectance was calculated: Table 1: Summary of mean optical property determinands for the four main sites in Lake CoJeridge (n = 1 1 ). Coefficients of variation are given in parentheses as percentages.

Variable Site 2-3 5-3 7-3 8-3

Secchi (m) 7.5 (34) 8.6 (32) 8.8 (2s) e.0 (22) 3) TSS (grh 0.e9 (39) 0.78 (44) a.72 ( 38 ) o.74 (34) Turb. (rTu) 0.80 (38) o.15 (44) 0.65 (2e) 0.6e (2e) 3) ChI a (mg/m 0.37 (92) o.29 ( e3 ) o.42 (107) o.42 (1 30) }!u (x/m) o.162 (ZS) 0.1 s6 (22) o.152 (17) 0.150 (15)

(m) (16) (15) +uz 30.1 (25) 30.7 (21 ) 31 .0 31.4

R (%) 12.5 (26) 12.3 (25) 12.2 ( 20 ) 11.4 (tt ¡ *ABS27OF 0.004 (es) 0.003 (tzt7 0.003 (120) 0.004 (a¡) a (1/m) 0.075 (20) o.o74 (1e) o.o72 (13) o.o72 (17)

( (31 (3s) I ( 1/m) 1.03 (+Z¡ o.ee ¡e) 0.e5 ) 0.8s4 (zS)- (23) (34) Wz 1 3.s (2e) 1 3.3 13.1 12.1

Note *AtsS27Ol = Absorbance (-IoS.l g Transmittance) measured by spectrophotometer at 270 run on a filtered water sample in a 10 mm cuvette. Fig.2

VISIBLE

u.v.-'- - B LUE- - - G R E EN-- -y E LLOW- - B ED +-'-1.R.

N oF40 x (scattering coeff.) E b ì.o a Ë 4v yellow \ substance

dp particulates

4OO 45O 5oO 55O 600 B.

3', èeo

1(ú -- 60 G É,

4so 500 550 600 650 Wavelength (nm )

Fig, 2 a Absorption and scattering spectra of water from site 2/3, Lake Coleridge, 23 September 1986. Separate curves are shown for yeIIow substance, particulate constituents and water itself. Simulated reflectance spectra for take Coleridge on 23 Septenber 1986 at sites 2/3 and 8/3. 12

a(^) following }4orel and Prieur (1977) (see also, Davies-Col1ey, 1983) where (assumed <)b*(À) is the backscaÈtering coefficient to be about 1.5% of total scattering). In line with intuition, the reflectance is inversely proportional to absorption but directly proportionaJ- to scattering at a given waveLength. AIso shown in Figure 2b is the reflectance spectrum for site 8/3 sampled on the same date. Both reflectance spectra peak in the blue region of the spectrum at about 490 nm. this is because most of Èhe absorption is of red light, by water itself rather than its constituents. It is evident, therefore, that the notable blue hue of Coleridge is attributable to the bl-ue colour of water itself. The only significant influence of the water constituents on the lake vrater colour is the enhanced brightness due to backscattering by the predominantly inorganic suspended sedíments. This backscattering also tends to make the water colour appear "glreyer" than pure water, due to flattening of the reflectance peak, depending on the concentration of suspended mineral sediments. The reflectance of the water, at the time of sampling for the spectrophotometric analysis given Ln Flg. 2, \^ras relatively low (about 7%) compared to an average of 12% - see below.

Visual clarity

Average Secchi depths (lr'l ranged from 7.5 m at sil-e 2/3 to 9.0 rn at 8/3 (Table 1) and represent "fairly high" visuaf clarity (Davies-Colley 1987). These values are similar to those of some other South Island glacial lakes e.g. L. ohau (r,ivingston et. al. 1986).

Secchi disk clarity fluctuated widely over the year at the four main sites (rangez 2.7 Eo 12.6 n). Fluctuations from I m to 10.8 rn occurred in the space of 4 days at site 1/1 at the head of the lake (fig. 3) Turbid water masses appear to clear rapidly following high inflow events. Periods of highest clarity occurred in December and September, hTith low clarity in late summer (January to March). Clarity at all sites varied, more-or-less, in sympathy with that at site 2/3 at the head of the lake although there 13 was a gradient of increasing clarity down the Lake (nig 3, Table 1 ). This gradient v¡as very pronounced on some sarnpling dates (fig 4). overall Secchi disk clarity was significantly lower at Èransects 1 and 2 compared with transect 8, farthest a\¡ray from the diversion inflows (Table 2), (as were Turb. âDd TsS). There \¡rere no significant changes in ChI a down the Iake.

Transverse differences in visual clarity vrere also evident on 28 January 1987 and 16 March 1987 (Fig 5). These results ippty that proximity to the Iake edge and lake water current patterns can also be important factors influencing the clarity of the lake depending on local wave conditions and the lake level at the times of sampling.

Light attenuation and reflectance

The irradiance (PAR) attenuation coefficient, S, which measures penetration of diffuse light i-nto waÈer, r¡ras Iow in Lake Coleridge inplying low concentrations of light-absorbing constituenÈs. [u in Ne\^I Zealand waters ranges over three orders of magnitude from about 0.03/m in the clearest ocean waters to at least 3O/m, as reported for extremely turbid Lake Wairarapa by Howard-Williams and Vincent (1985). Thus the average & values given for Lake Coleridge (Table 1 ) are near the clearer end of the scale and are not much higher than in Lake Taupo (5d - 0.1/m). The low q values in Coleridge imply deep penetrat,ion of Iight a€t meas¡ured by the euphotic depth, %u (depth to which 12 of the surf ace light penetrates - a useful index of the depth to which plants can grow, -?eu = In 1 00/Kd = 4.6/Kd') . Table 1 shows that aver.9. %' was about 30 m. The depth of ttre euphotic zone (%,r) fluctuated markedly during the year (Fig. 6). During December 1986 the 3,, reached a maximum of over 43 m at the head of the lake and decreased to as low as 19.3 rn at the same siÈe the following monbh. Average values of 30 to 31 m v/ere recorded at all sites

(table 1 ).

The reflectance coefficient for quantum irradiance in Lake Coleridge averaged about 122 (tabte 1 ) which is "moderately high" implying fairly bright. water colour. However, like other optical properties, the Fig.3

12

11

t- ¿q--A- K / ñ ? 7 I E Ê.06 ït ^ Eso 34o

sep Oct ' Nov Dec Jan Feb Mar Apr May Jun Jul Aús sep Oct 1986 1987

in Lake coleridge at the four main sites in Fig. 3 secchi disk clarity '7/3, 1986-1987 (¡ = site 2/3, + = site 5/3, o = site = site 8/3). The inset gives results for a short period of intens^ ive sampling of site 1/1, near the head of the lake' Fig.4 '.r

16-3-87

Relative location of transects down lake

Fig. 4 Changes in mean secchi disk clarity at the eight transects do\^tn Lake Coleridge on tÏrree sampling occassions. Tabl-e 2 : Comparison (t-tesÈ) of differences in the four main water clarity determinands between transects I & 2, and I & 1.

Determinands Range Mean S.D. t dfP

a ) Secchi-8 34 6.4-12.6 9.0 1.92 vs Secchi-2 33 2.4-12.1 7.3 2.85 2.90 55.9 0.0054 Secchi-1 33 1 .0-11 .8 6.7 2.93 3.79 5s.0 0.0004 b) TSS-8 15 0.37-1.05 0.72 0.218 vs TSS-2 15 0.25-1 .62 0.94 0.443 -1 .63 20.4 0.1200 TSS-1 21 0.51-13.45 2.63 3.50 -2.4A 20.2 0.0220 c ) Turb-8 34 0.33-1.40 0.66 0.220 vs Turb-2 33 0.34-2.59 0.98 0.653 -2.62 39.0 0.0120 Turb-1 33 0.35-2.85 1.56 1.75 -2.91 33.0 0.0065 d) Chl a-8 15 o.o5-2.17 0.58 0.739 vs ChI a-2 15 O.O5-1.29 O.44 0.431 0.63 22.5 0.5400 chr ã-1 22 0.04-1 .1 1 0.37 0.320 1 .04 17.6 0.3100 Fig.5

2A-r87 16-3-87 Transects

4 3 1

5 2

5 3 4

EÁ 6 4 5 Ë.4o !t E o E6 6 Ø 5 5 5

7 6 6

7 v 7

I I 7

2 2 Sites Sites

Fis. 5 Changes in secchi disk clarity at three stations across Lake Coleridge on each of eight transects, on two of the sampling occasions. Fig. 6

40

38

tsge ã ãs¿ N o ã3232 o G 9so30 o o28 E Ëzt o

Jul Aug sep Oct sep Dec Jan Feb Mar Apr

(%u) Coleridge at the four Fig. 6 Depth of the euphotic zone in Lake main sites in 1986-1987 (. = site 2/3, + = site 5/3, o = site 7/3, À = site 8/3). 14 reflectance varied widely (Fig. 7). At relatively low reflectance, around |eo or lower, the lake water appears fairly deep blue but at high reflectances, up !o 2Qeo or more, the water appears bright turquoiset similar to the water in Lake Tekapo which has a reflectance around 30% (Vant & Davies-CoIIeY 1984). Table 3 : Correlation matrix of optical property deterrninands for Lake Coleridge. Values above the diagonal are from linear correl-ations and those below are from correlations using Iog (base-e) transformed data. For each combination of variables is given correlation coefficient (r), significance level (P) and number of observations (n)

Secchi TSS Turb. Chl a

Secchi- -0 .5 96 -o.652 0.1 43 -0.875 -0.858 -0.738 0.000 1 0 .0001 0.1 563 0.0001 0.0001 0;0001 100 263 100 43 43 41

TSS -0.806 0.91s -0.208 -0.589 -O.562 0.755 0.0001 0 .0001 0.0383 0.0001 0.0001 0.0001 100 100 100 43 43 41

Turb. -0 .889 0.89 3 -o.265 0.705 -0.630 0.768 0 .0001 0.0001 0.0076 0.0001 0.0001 0.0001 263 100 100 43 43 41

ChI a 0.071 6 -0.366 -0.386 o.282 -0.328 -0.375 0.4788 0.0002 0.000'l 0.0661 0.031 8 0.01 56 100 100 100 43 43 41

K- --d -0.864 0.550 0.581 o.452 -0.979 0.558 0.0001 0.0002 0.0001 0.0023 0.0001 0.0002 43 43 43 43 43 41

-æu 0.864 -0 .5 50 -0.581 -o.452 -1.000 -o .51 2 0.0001 0.0002 0 .0001 0.0023 0.0001 0.0006 43 43 43 43 43 41

R -0.665 o.7 24 0.799 -o.260 o.442 -O.442 0 .0001 0.0001 0.0001 0.1 01 0.0038 0.0038 41 41 41 41 41 41 Fig. 7

18

17 Ït 16 ->l-/ \ i \ \-¡r+\ \\ / \ N.- i r d 15 o 14 "\\ i 2..' o \\ tr ", \ o ')\ i/ o 13 I o 12 E

11 )

10 X X 9 I I I

7

t! sep Oct sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug 1986 1987

Fig.7 percentage tight reflectance in the water column at Èhe four main sampling sites in Lake coleridge, 1986-1987 (J = sj-t,e 2/3, + = siÈe 5/3, o = sLle 7/3, Â = siÈe 8/3). 15 vÍater composition and optical character

There are four main constituents of natural waters which significantly attenuate light: raineral sediment, phytoplankton, organic detritus and yellow substance (yellow coloured dissolved organic matter). In Lake Coleridge the indications are that organic contributions to optical character are low. In particular, the water colour, as noted above, is essentially that of Pure lvater.

An attempt v¡as made to gain an impression of the concentration of organic detritus from volatile suspended sediment measurements but these \¡¡ere effectively below detection level. Thus the parti-culate organic contenÈ of water in Lake Coleridge is very Iow and probably negligible as regards light attenuation (with the exception of the algal biomass - see below).

Attempts were made to measure the contribution of yellow substance as the absorption at 27O nm in the UV of f iltered \"/ater samples. These measurements (absorbances ABS270Fr Table 1 ) !ìIere very low and also quite variable suggesting that fine particulate material sometimes passed Èhe membrane filters (0.45 um Pore size) in amounts sufficient to bias absorbances (high) by the effect of light scattering. However, the fact that the absorbances are so low in absolute terms indicates that the yellow substance does not contribute significantly to the optical character of Lake coleridge. Kirk (lglø) has suggested that the absorption coefficient of filtered water samples at 44O nm (gOnO) is a useful index of yellow Substance concentration. Absorbances at 44O nm were too low for direct ¡neasurement but since the spectral shape of yellow substance absorption is f airly universal (Davies-Colley & Vant 1987 ) !{e can estimate fu,qO from Atss27OF measurements . This suggests %,ao o.o2/m, a very Iow value similar to that for clear ocean water'

The Èwo main constituents affecÈing the optical character of Lake Coleridge, apart fron the water itself, are thus likely to be the suspended mineral sediment, quantified by TSS, and the phytoplankton biomass, quantified by ChI a. Of these Secchi clarity correlaÈed most closely with TSS, and its surrogate Turb., with chl a not being significantly correlated with Secchi depth (table 3). This suggests that the (small) influence of 16 chlorophyll-containing algae on the clarity is being masked by the influence of inorganic sediments. R A range of other variables vÍas also correlated with, and thus probably influenced by, concentrations of TSS (Table 3). These include Turb., ChI a, <'K,, æu'z^.-, and R. Turb., {'K., and +uz^-- were also significantly correlated with Chl a indicating that these variables are affected by both TSS and Chl a, but \^/ith TSS being the dominant consÈituent.

From these relationships it is possible, 1n principle, to de..velop a multiple regression model illustrating the dependence of Secchi disk clarity on the constituents and thus demonstraÈe their relative importance. TSS expJ-ained 63.7% of the variance in Secchi depth at the 4 main sites, with other constituents contributing little nore to the explanatory power (Tabte 4). suspended solids thus appear to have the single major influence on Èhe clarity and optical properties of Lake Coleridge.

Absorption and Scattering of Light

Linear regression models relating the Secchi depth to the water composition, while useful as an indicator of trends, are inevitably fJ-awed. Established optical theory (e.9. Kirk 1983) shows that, the Secchi depth, and other so-called apparent optical properties, are not linear functions of the water composition. Hovrever the so-caIled inherent optical properÈies, the absorptíon coefficient, lt and scattering coefficient, \, are linear functions of the concentrations of water constitutents. Given the inherent opÈical properties of a \¡rater, the apparent optical properties of management concern (4Or Id, E) can be estimated by optical models as is discussed below.

Table 1 gives average values of a and ! and of their ratío þ/a. The absorption coefficient val-ues can be considered "Iow" whilst scattering coefficients are higher (although not "highu as lake waters go) and more variable. The ratio of scattering to absorption is quite variable and closely related to the reflectance coefficient of the water. Kirk (1981) has shown that refl-ectance (in Z) is almost numerically equal to the ratio b/a in the range 0 < 3 < 12e", although R falls below equality with þ/a at higher values. 7

Fig. 8 shows a and b plotted against the TSS. Absorption shows no significant relationship wÍth TSS (t2 = 0.058) suggesting, as expected, that the sediment in Lake Coleridge is mainly inorganic material (highly scattering but not absorbing of fight) and particulate organic (light-absorbing) matter is very low. Scattering is fairly strongly (t2 = 0.557, p ( 0.000'l ) related to TSS and the regression line is shown in the figure (b = 0.779 TSS + 0.335). Theoretically, when particulate matter (tSS¡ concentration is zero, scaÈtering should also be zero (scattering by non-particul-ate water constituents being negligible). The positive intercept in the regression equation arises r Dot merely because of imperfect correlaLion, but because of trend in grain size with concentration of scattering particulates in Lake Coleridge. At high TSS Ievels relativel-y coarse particles may þe present. which would scatter less per unit mass concentration than very fine particles present at times of low TSS. Forcing the regression line to pass through the origin produces a Iine of very different slope (Þ = 1.138 TSS). The correlation of b and TSS is not as close as mighÈ be expected (e.g. Vant & Davies-Colley 1984), possibly also reflecting variation in grain size and thus scattering efficiency of the particulates. Another possible reason for the imperfect correlation is that both the TSS and b datasets are degraded to some extent by analytical error which nay be significant at the relatively low concentrations encountered.

The total suspended sedimenÈ (tSS¡ in Lake Coleridge is not entirely inorganic, in spite of very low volatile sediment content (see above). The chlorophyll-a content of algae is typically about 1% therefore the mean algal biomass corresponding to the mean Chl-a (0.37 mg/m3 Table 1) is about 0.037 g/n3 which is about 52 of the total mean TSs (- 0.806 9/m, Table 1 ). Scattering by different phytoplanktonic algae is quite variable but averages about IOO/n2/g ChI-a (Davies-Colley et aI. 1986). Therefore at the mean Chl-a level, the algal scattering coef f ici"tt %fS - O.O37 /n which is about 4eo of the total average scattering coefficient of O.955/m (Table 1 ). This analysis confirms that the contribution of the phytoplankton to reduction of visual clarity (mainly by scattering) in Lake Coleridge is small. Table 4 : l"lultiple regression models for secchi disc water clarity (n=40) and Iake water suspended solids concentrations (n =83) in Lake Coleridge developed from environnental predictor variables (abbreviations previously undefined are TSS-inflow=suspended sediment mass inflow from the Oakden canal over the previous 30 daysi dn = d.istance fron north shore, ds = distance from south shore, d.Q, = distance d.own lake, L = mean lake }evel over previous 30 d,ays).

Dependant Variable Predictors Coefficients Cumulative 12 p>F

l:IN Secchi c 1.9898 In(TSS) -o.4667 0.6370 0 .0001 AtsS27OF -17 .8473 0.6860 0.0039 Chl .a -0.0 1 87 0 .6 864 o.7 834

2 : TS51,¡ c 272.2382 TSS-inflow 0.0 052 o.0642 o.0225 dn -0.5859 0.1 038 0.0670 dg -0.0 684 0.1 507 o.0427 ds -o.2128 0.1 581 0 .41 60 L -o.5322 0.1595 o.7206 Fig. I

o.8 1.o 1.2 TsS (g/m3)

o.2 o-4 o.6 o.8 1.0 1.2 1.4 TSS {s/m3)

Fig. 8 a Relationship between the absorption co-efficient, !, and total- suspended soilds, TSS. The line shown is for an average a = O.O73/n. b Relationship between scattering coefficient, b, and TSS. The regression Iine shown is b = 0.729 TSS + 0.33-5 (r2 : 0:557, p <0.0001 ) . 18

In contrast to TSS, Chl a is significantly related to the absorption coefficient, a (t2 = O.373, p< 0.0001 ) ¡ut not to the scattering coefficient b (t2 = 0.007) (n:-g. 9). Thus it would appear that the phytoplankton is a significant Iight-absorbing constituent in Lake Coleridge - a finding which is consistent with the spectrophotonetric analysis of lake water colour reported above. At Site 8/3, most remote from the turbid inflows, where the TSS is relatively low (Table 1 ) and the Chl- a relatively high, the absorption coefficient was more closety related to Chl a than at other sites (t2 = 0.724, p < 0.002). Or¡ average, however, the algal contribution to total absorption of visible tight in Coleridge is low (about 1O-2Qe"r, most of the remainder of the absorption being attributable to water itself.

Infl-ow of suspended sediments

Suspended solids mass infl-ow from the oakden Canal, calculated from discharge monitoring and turbidity/fSS monitoring for each month during the sampling period, shows a broad relationship with lake water TSS (fig 10). There is some indication from this figure that in-lake TSS levels lag behind peaks of sediment input. It may take a considerable time for fine sediments entering the lake (mainly as underflows, Jo$¡ett 1984) to be entrained into surface waters by lake mixing processes. OnIy 15.5% of the variance in in-Iake TSS concentrations could be explained by inflow TSS mass flow, distance across the lake, and distance down the lake (Table 4). Good inflow mass records \¡rere not available for the Harper diversion, and a better relationship might be expected if these could be included. Fig.9

ßlo.1o

o.a r.o 1.2 1.4 Chlorophyll a (mg/m3)

+ + 4* + ++ ! t'i+1 +

0.6 0.8 l.o 1.2 1.4 ChloroPhYll a (mg/m3 )

Fig. 9a RelationshiP between a and chlorophyll a concentration' The 0.0667 (r2 = 0'373, regression line shown î" a = 0.01e0 "itr.l* p<0.0001 ). chlorophyll b Rel-ationshiP between scattering coefficient, Þ, and a concentration. The line shown is for average Þ = O '955/n' Fis. 10

_G- -

¡ ?300 o o ç s Ê200 o tr o 5 !t ú) 100 lÉ q, oo sep Oct Nov Jan Feb Mar Apr May Jun Jul Aug sep Oct 1986 't987

Fig.10 Lake water TSS concentrations and monthly inflow suspended solids mass from the Oakden canal, Lake Coleridge, 1986-1997 (o sLte 2/3, + = site 5/3, o = síLe 7/3, À = site 8/3) Table 5 : Comparison (t-test) of differences in Secchi disc clarity records between (a) august 1986 to February 1987 (Secchi-86) and August 1965 to February166 (Secchi-65). and.; (b) January 1987 to October 1987 (secchi-87) and January 1967 Eo October 167 ( Secchi-67 ) .

Variables Range (m) Mean (m) s.D. t df p a ) Secchi-86 6 5.5-12.1 8.6 2.s6 Secchi-65 13 8.0-1 8.0 13.4 2.91 -3.67 11 .1 0.0037 b ) Secchi-87 7 5.5-12.2 8.0 2.54 Secchi-67 I 6.0-15.5 13.4 3.22 -3.65 12.9 0.0033 19

DISCUSSION

Effect of ttre diversion waters on visual clarity

The water clarity and optical properties of Lake Coleridge are largely determined by inorganic suspended solids, mainly derived from the Vlilberf orce and Harper River diversion \^Iaters.

Although the impacts on vrater clarity of these discharges are most severe at the head of the lake the impacts are not confined to this zone. It is evident that fine silts and. clays become distributed throughout the water column and down the full length of the lake basin. Significant sediment inputs probably also occur naturally from the Ryton River (e.9., causing the perturbation in TSS at Site 7, June 1987 refer Fig 10) however, the overall effects of such inputs appear to be minor.

It is not known what the impact of the Harper diversion alone has been as it \ras commissioned before any water clarity record.s were taken (1921). Ho!'/ever, from the results of this study it can be demonstrated that the addition of the l,filberforce diversion in 1977 ]:las significantly reduced the visual clarity of the main lake frorn when just the Harper diversion was operating. Pre-Wilberforce Secchi disk record,s taken in 1965 and 1967 from near site 6/3 (l,t Flain, FRD-pers.comm), using the same method as in this stud.y, are compared with values at similar times of the year in 1986-87 in Table 5. Highly significant differences in the mean Secchi disk clarity occurred for both comparisons, with means changing from 13.4 m to 8.6 m, and with rnajor shifts in the range being evident. These figures also suggest thaÈ mean lake clarity has become considerably lower than the mean of 10.6 rn (range of 5-17 m) predicted by Jowett (1984) for the central lake using a multiple linear regression model (based on residence time and percent glaciation).

The one year of data collection in this study does not show whether the Iake clarity has reached a ne\¡/ steady state or is still changing (say, in response to higher average Iake levels and consequent shoreline erosion). Hov¡ever, most tikely a steady-state, such as ttraÈ which is implicit in Jowettrs model (Jowett 1984), has been reached over the 10 years of scheme 20 operation $¡hich reflects catchment type, flow history' and lake hydro-dynamics (including the effects of higher than pre-diversion lake Ievels). If this condition has been reached then no further trend in clarity is expected if inflow management Practices are maintained as at present.

The impact of the reduction in visual clarity represented by the finding of a statistically significant drop in Secchi depth is difficult to assess. !ûe lack well-established criteria for visual clarity of waters as regards both aesthetics (human vision) and vision of aquatic fauna. Visual range of low contrast objects such as fish in Lake Coleridge would be expected Èo be around 5 m based on correlation of Secchi depths with black disk ranges (navies-colley this in press 1988).of rt significantly seems range sun likely visual could that for a variety of order waters impact habitat of fish and other aquatic fauna. Thus it seems tl.at the only important impact is likely to be on aesthetic vatue of the water. Undoubtedly a water with an 8.6 m average Secchi depth is less valuable than a water of 13-4 m average Secchi depth, other features being equal, but how much less? An 8.4 m Secchi depth still rePresents "high" clarity according to Davies-Colleyrs (1987) classification of lake water clarity. This drop in visual clarity seems unlikely to constrain any \¡Iater use in Lake Coleridge with the possible exception of diving. possibly a more useful assessment of the impact of the diversion on the visual quality, and thus recreational uses, of Lake Coleridge would consider the distriþution of clarity rather than mean clarity. The frequency of occurrence of relatively low clariÈy in Lake Coleridge (say < 5 m at the t bound.ary between ,,moderate" and "high" clarity in Davies-Colley s ( 1 987 ) classification) has undoubtedly increased, post diversion, particularly at the northvrestern end of the lake (see the discussion in Mitchell 1984, p' e).

Rec¡ression l,lodels

The close correlation between Secchi disk clarity and, Èhe other measures of optical character (table 3) permits the development of Table 6 : Regression models for Lake Coleridge optical property determinands as predicted from Secchi disk deptft

DependenÈ Variable Predictor 12

e 2.256 Lnz-€D -1.225 0.650 0.0001 100 c 1.918 ltfuo -1.096 0.789 0.0001 263

3 InK-< c 0.81 7 ]-nz+D 0.51 1 0.7 47 0.0001 43 4 Lnz+u c 2.343 rt%p 0.51 1 o.7 47 0.0001 43 r.E Itg.uo 3.576 -0.534 o.442 0.0001 41 21 regression models (Table 6) for calculating changes in these determinands over the period of operation of the Ì,Iilberforce diversion. The po\der of prediction of ttrese equations within the lake is fairly high (as shown by the r 2-varues rn Table 6). Table 7 Iists the recorded pre-diversion and post-diversion Secchi disk averages for the rniddle of Lake Coleridge (near site 6/3) (t'iq 1 ) and. the calculated vafues of other variables for which satisfactory relationships were developed. Of considerable interest is the estimated 7Or" increase in TSS, and secondarily the 2OZ reduction in the depth of the euphotic zone attributable to tJre V,lilberforce diversion.

The estimaÈed increase i-n TSS is roughly consistent with the increase expected from consideration of catchment loads. Jo\n/ett (1984) estimated that the ïlilberforce diversion increased sediment loading on the lake from 34rOOO-134r000 tonnes/yr. Of course much of the sedimenÈ entering Lake Coleridge is rapidly settling material- (coarse silt and sand) which does not contribute to open water TSS Ievels.

It appears that a small decrease in light penetration in Lake Coleridge can be attributed to the llilberforce diversion. This change in light penetration has probably not affected primary production by phytoplankton in the lake since it is associated with inputs of light-scattering rather than light-absorbing material (i.e. the diversion water TSS does not compete with the phytoplankton for available light). Phytoplankton production in oligotrophic Lake Coleridge is severely restricted by 1ow nutrient (nitrogen and phosphorus) concentrations which indeed, may have increased slightly due to d.iversion inputs (Mitchell 1984). However, there is no indication of eutrophication as evidenced by the very 1ow concentrations of chlorophyrr (Tabre 1 ) '

The compression of the euphotic zor,e may well have reduced the area available for colonization of the littoral (shallow water) lake bed by aquatic macrophytes. Vant et aI. (1986)'have shown that depth ranges of macrophytes in New Zealand Iakes are largely determined by light penetration; the maximum depth, % (Spence 1982), averaging 4.34&. Clayton (1984) has reported that vascular macrophytes (native milfoils and pondweeds and introduced Elodea canadensis and Ranunculus j!$!!g) i" Lake Coleridge grow at low cover levels down to 8 rn depth with charophytes Table 7 : Average Secchi disk values for site 6/3 and the values for other determinands calculated fro¡n !q,-., using tTre regression models in lable 6, and tTre optical model discussed in the Ëéxt (see Figrure 11).

Variable Pre-diversion Post-diversion % Change (1965,66,68) (1986,87) a ) ì4easured Secchi (m) 13.4 8.6 36% reduction

Calculated }{easured b) Calculated from the regression models of Table 6 rss (g/n) 0.40 0.68 0.78 70% increase Turb. (FTU) 0.40 o.64 o.75 60% increase 5d ( 1/¡n) 0.117 o.1 47 0.1 56 26% increase *) 39.2 31 .3 30.7 20c" reduction ftt*f 8.94 11 .32 12.3 27% increase c) Calculated from LLre optical model discussed in the text (figure 11)

rss (g/n3) o.52 0.91 0.78 75% increase ç (1/n) 0.1 35 0.1 63 0.1 56 21% increase -4" (m) 33.s 28.0 30 .7 16% reduction R (%) 8.7 14.1 12.3 62% increase )) extend.ing to about 35 n depth. The predicted % in Coleridge (using K¡ values from Table 1 ) is about 30 m, consistent with the average n charophyte depth limiÈ. However bryophytes (water mosses), which are particularly shade tolerant, grohr down to 7o m (Clayton 1984)' The vascular macrophytes, which probably represent a major habitat area for aquatic fauna in Lake Coleridge, are unlikely to have been restricted in range due to reduction in euphotic depth, since they are probably depth-limited by the pressure gradient rather than the Iight gradient at their Iower depth range in Coleridge. (It should be noted, though, that Clayton (1984) reported a very localized, impact on the tall-growing vascular macrophytes at the head of the lake, presumably due to light-shading by turbid sraters and also sediment insÈability). optical lilodel

Althoughli-nearregressionmodels(table6)appeartoworkwellinLake Coleridge for the estimation from Secchi depth of other optical properties, such linear models are not in accordance with established optical theory. For this reason a more rigorous model was constructed based on published relationships between the inherent optical properties (a and b) and (5d, that the apparent optical propertie" %u, R). It vras assumed absorption coefficient was constant at the average value of 0.073 and that the scattering coefficient was proportional to TSS (fig. 8) \^¡ith b = 0 at TSS = 0.

Kirk (1 9g4) has given the following expression for the attenuation coefficient as a function of a and b:

fo(z^) = {1 + (0.425 vo - 0.1eO)g/s}1 /2!ro where Io is the cosine of the angle to the vertical of light al zeto depth (dependent mainly on sun angle). This expression shows that, as expected, absorption, but also depends on scattering s is mainly dependent on !, as it affects coefficient, Þ, and (rather weakly) on solar altitude uo. \u, calculated according to the above equation, is plotted as a function of TSS in Figure 11 for a sun angle of e - 45o (Io = cos{sin-11"in O/1.33)} = 0.847). Clearly the relation between 5d and TSS is non-Iinear' Also 23

ptotted on Fi-gure 1 1 is the reflectance coefficient R(z-) interpolated for - -{n a sun angle of 45" from tabulated data supplied to one of us (no-c pers. comm. Dr JTO Kirk, CSIRO, Canberra). preisendorfer (1986) has given an expression relating the Secchi depth, 4o, to various oPtical ProPerties

rn{(ez - e)/o.oooni åSD g+Þ+5 where R is the reflecÈance, and K is the diffuse attenuation coefficient of the water, both quantities referring to the spectral sensitivity of the human eye rather than to that of a PAR sensor. In Figure 11 the Secchi depth is plotted, ignoring the (small) influence of spectral sensitivity.

Figure 1 1 presents a self-consistent model of the optical response of Lake Coleridge to changed in-Lake TSS concentrations. The figure shows at a TSS Èhus b. The glance how 3eu, kf¡, & and R trend with change in and average pre- and post-diversion conditions are indicated on the figure at the recorded average Secchi depths of 1 3.4 and 8.6 m, Pre- and post-diversion respectively. The estirnated values of Tss, h, 4' and R (given in Table 7) are in fair agreement with average values measured in this study, and also in reasonable agreement with the values predicted from Ehe regreeel.on models of Table 6, denonoeråttnE the Eelf-conslstenoy of the model. In principle Figure 1 1 could be used to predict the average Secchi depth, reflectance and attenuation coefficient (and thus euphotic depth) which would result from a given change in average TSS. The problem now is how to refate the in-lake lSS levels to sediment loads to the Iake including those frorn the diversions. Fig. 11

',-o,'\ì.*"u (x ro) 3.O c bla (x ro) rO .tt i:- 2.5 lL .ir is El b,arKd rõ *. 1 aY (m{) lo ool 2.o ol -/ -¿ Zeu,Zso lo. r:/ ,/ (m) I 1.5 3

1.

o.5

o.o T--f rrrrl..,,r¡r¡r1.... l'.,..... I ob o.s 1.o 15 2-o

rss ( g/m3)

Fig. 11 optical model for Lake Coleridge. The scattering coefficient, b, is assumed proportional to Tss and the absor.otion -co-efficient, !, is assumed to be constant (=0.073/m). The other optical quantities are calculated from a and b using publ_ished relationships (see text). The multlpliers -on the plotÈed curves should be used on the vertical axis scale values to achieve the correct numbers for the associated parameter (e.g. a = 0.73/mx10-1 = 0.073/m and pre-d.iversion ZSD = 1 .34mx1 0=1 3.4n) .

24

Relation of In-lake Sus nded Sediments to Sediment Inflow

Id.eally, we should be able to predict the suspended solids concentration (and thus the Secchi disk clarity) at any location in the lake given the sediment mass inflow and the distance down/across the lake. Howeverr only poor results were obtained attempting this using multiple linear regression with the existing data set (t2 = 0.160, Table 4). TSS mass inftow and distance down the lake hrere the only significant predictor variables. Further effort would be reguired on this problem if lake management changes are planned. probable sources of noise in the regression include the time of travel of sediments from the diversion waters to the centre of the lake, the Iack of inflow estimates for the Harper, poor estimates for the V,lilberforce, the lack of good wind run records for the lake and, finally, the time lag between inflow of the sediment and entrainment into the surface waters. An attenpt to improve estimates of sediment mass inflow based on flow records in the Harper and lfilberforce Rivers was considered, but rejected. This was because flows in the Oakden Canal- (and sediment concentrations) are influenced by the Harper flows, !{ilberforce flows, the proportion of the !{ilberforce being diverted, the amount of water being split by the Wilberforce and Harper overflow spillwaysr the relative timing of the flows in the two rivers, and the timing and size of breaches in the Wilberforce and Oakden banks. (ttt.¡. Duncan, Hydrology Centre, pers. comm) .

A more fundamental lirnitation of such modelling of in-lake TsS concentrations from the inflows is the complexity of the hydraulic behaviour of the inflow waters in the Iake (dependent on temperature/density differences between water masses ) and the wide range of sediment grain sizes and thus fall velocities (and scattering efficiencies) in the diversion waters (refer Jowett, 1984 p. 10). It would probably be very dif f icult to mod'el the dynamics of r¡/ater and sed'iment f lows in the lake in sufficient detail to permit accurate pred,iction of in-lake (near surface) tSs from known sediment loads, even if grain size distributions were known. 25

CONCLUSIONS

The blue colour of Lake Coleridge is essentially thaÈ of pure \n¡ater modified slightly (brighter and greyer) by the light-scattering effect of varying concentrations of inorganic suspended sediments.

Light beam attenuation (and thus attenuation of image-forming light rays ) in Lake Coleridge is dominated by the scattering of inorganic suspensoids. The low phytoplankton biomass in this oligotrophic lake, although an important constituent as regards light absorption, has comparatively little influence on overall optj-cal character.

Íhe attenuation of diffuse ambient light in Lake Coleridge is low (nearly as low as that in Lake Taupo) and consequenÈty the euphotic depth (depth to which sufficient light. penetrates for plant growth) is deep, averaging about 30 m.

The visual clarity of Lake CoJ"eridge (as measured by Secchi disk depth- which averages about 8.6 m in mid-lake), is fairly high but has been sígnificantly reduced by diversion to the lake of turbid wa¡ers from the Wilberforce River to augment hydroelectric Po\^ter generation. It cannot be determined at present whether clarity is continuing to trend downwards, although it is thought that this is unlikely. The reduction in visual clarity may have had a small ecologlcal impact (e.g. on físh vision) but the maLn impact has been on the aesthetic character of the lake with reduced visibilit'y of the bottorn in shoreline areas and temporal shifts in colour - an impact which is difficutt to qualify in the absence of criteria for aesthetic quality.

The light climate in Lake Coleridge has been less markedly altered by the inflow of díversion waters than the visual clarity, because the inorganic suspended sediments (silts and clays) are light scattering rather than light absorbing. The reduction in euphotic depth has probably slightly reduced production by benthic plants in the lake but is likely to have caused little, if arYr change in habitat for aquatic animals or reduction in phytoplankton production. 26

I{odels have been developed which pernit prediction of the optical properties of Lake Coleridge (including gecchi depth) gl-ven in-lake concentrations of suspended sediments. Another model which relates in-Iake suspended sediment concentrations to sedi¡nent inflows to the lake would be required if predictions were to be nade concerning lake optical response to changed diversion flows. Such modelling would require detailed consideraÈion of the complex hydraulics of the lake and inflows (dependent on temperatures and thus densities of water masses), and of the grain sLze distributions of thê suspended materials and thus fall velocities änd efficiency of light scattering" 27

RECOMMENDATIONS FOR FUTURE IúORK

Future investigations and monitoring of the water clarity of Lake Coleridge could be carried out at t$¡o levels, depending on management plans for the lake. First,ly, íf changes in the managernent of inflows are being contemplated, and it is desired to predict the effecÈs of these changes, then iÈ would be necessary to carry out detailed studies relatíng optical properties to the quantity, size distribution, and hydraulic behaviour of sediment inflows, and subsequently rnodel these relationships. However, if no plans for changes in management of the lake are proposed then a lovt level of monitoring over the full life of the water right would probably be sufficient for managment needs. Such a programme would:

(a) track any fuÈure changes (e.g. continued degradation) in lake optical quality and; (b) provide data against, which to gauge the effects of any future nanagement changes including change in sediment loading.

Study Outtine: Prediction of Optical Properties from Inflow Sediment Loads

Grain size range of diversion water sediments in relation to optical properties (scattering per unit mass concentration) and settling velocities.

ttydraulle behavíour of dlversion waters ln the lake as a function of densities of different water masses.

Pathways of different grain size ranges of sediments in the lake. This could be studied enpirically by surveying sediment movement through the lake using a beam transmissometer (a device which measures attenuation of a tight beam by absorption plus scattering) to trace turbid water masses (e.g. Irwin c Pickrill 1982).

Changed clarity and other optical features consequent on changed diversion loading could be studied empirically by doing an experiment (i.e. actually changing the loading for an experimental period, as suggested by Mitchell (1984) , P. 1 1 ). 2A

Study Outline: Long Term Monitoring

Lake Coleridge, a large wind-exposed lake, is often too rough to permit safe access by smal-l boat. Sampling must therefore be weaÈher-dependent. However, as far as possible it is recommended that visits be rnade monthly, by boat to a central site on the lake, (say site 6/3) in order to sample water and take clarity readings.

Secchi depth readings must be the focus of tTre long-term monitoring and these should be plotted as a time series for immediate comparison with historical data. Black disk visual ranges (in both horizontal and vertical directions) would no\¡t be a preferred visual clarity index (Davies-Colley

1 988 in press ) but would require the services of a snorkel diver presenting logistical problems. More importantly, it would be difficult to compare black disk clarity with historical- Secchi measurements. As a back-up to Secchi data, large samples (10 l,) of water should be obtained on each visit for the analysis of suspended sediment and for measurement of nephelometric turbidity (ttris is a check on consistency of data). The suspended sediment and turbidity data, Iike the Secchi data, should be plotted as a time series in order to indicaÈe any trends in lake character \ntith time.

ACKNOITLEDGEMENTS hle are grateful to Pete Mason and Albert Ledgard (Water Resources Survey, DSIR) for their assistance with the field $/ork and to Penelope l"foffit, Greg Tod and Jenny Templeton (uydrology Centre, DSIR) for their assistance with the laboratory analyses. Thanks also to Graeme Davenport (!'iater Resources Survey, DSIR) for supptying data on lake inflow volumes and. sedirnent concentrations. 29

REFERENCES

Bowden, M.J. 1983. The Rakaia River and Catchment. A Resource Survey. Resource InvestigaÈions Division Report, North Canterbury Catchment Board, Christchurch. Clayton, J.S, 1984. Lake Coleridge submerged vegetation and the Wilberforce diversion. Report to Electricity Division, Ministry of Energy. Ruakura SoiI and Plant Research Station, MAF, Hamilton. December 1984. 4 p. Davies-Colley, R.J. 1983. optical properties and reflectance spectra of three shallow lakes obtained from a spectrophotometric study. New Zealand journal of marine and freshwater research 17: 445-459. Davies-Colley, R.J. 'l 987. Water appearance. In: Lake Managers

Handbook. v,f .N. Vant (ed. ). p. 66-78. Water and Soil Miscellaneous Publication. Davies-Colley, R.J.; Pridmore, R.D. and Hewitt, J.E. 1986. optical properties of some freshwater phytoplanktonic algae. Hydrobiologia 'l 1 33: 65-1 78. Davies-Colley, R.J.; Vant, W.N.1987 Absorption of light by yellow substance in freshwater lakes. Limnology and oceanography 322416-425. Davies-Colley, R.J. in press 1988. Tlater ctarity measurement using a black disk. Limnol-ogy and oceanography. Graynoth, E. 1987. Effects of hydroelectric development on the fish stocks and fisheries of Lake Coleridge - Fisheries Research Proposal. Fisheries Research Centre Report, Christchurch, MAFish.

Howard-l'IilIiams, C. & Vincentr r¡t.F. 1 985. optical properties of New Zealand lakes. II: Under!,rater spectral characteristics and effects on PAR attenuation. Archives für hydrobiologie 1 04: 441-457. lrwin, J. 1975. Checklist of. New Zealand lakes. NZ O"eanog@. Institute Memoir 74. 161 p. Irwin, J. and Pickrill, R.A. 1982. I,üater temperatures and turbidity in glacially-fed lake TekaPo. New Zealand journal of marine and f reshwater research i9r 1 89-200. Jowett, I.G. 1984. Lake CoJ-eridge. Report on the discharge of turbid water into Lake Coleridge. Report to Electricity Div., Ministry of Energy. Po\^Ier Directorate, lvlWD, Vfetlington July 1984. 18 p' 30

Kirk, J.T.O. 1976. Yellow substance (gelbstoffe) and its contribution to the attenuation of photosynthetically active radiation in some inland and coastal south-eastern Australian h¡aters. Australian journal of marine and freshwater research 27: 61-71. Kirk, J.T.O. 1981. EstinaÈion of the scattering coefficient of natural waters using underwater irradiance measurements. Australian rournal of marine and freshwater research 32: 533-539. Kirk, J.T.o. 1983. Light and photosynthesís in aquatic ecosystems. Cambridge University Press. 401 p. Kirk, J.T.o. 1984. Dependence of relationship between inherent and apparent optical properties on solar altitude. Limnology and oceanography 29: 350-356.

Kirk, J.T.o. 1 985. Physical effects of suspensoids (turbidity) in aquatic ecosystems . Hydrobiologi a E, 1 95-208. Livingston, M.E.; Biggs' B.J. and Gifford, J.S. 1986. Inventory of New Zealand takes. Water and Soil Miscellaneous Publication No. 80. Mitchell, S.F. i984. Ecological effects on Lake Coleridge of diversion inflows. Report to l4inistry of Works and Development. Department of ZooLogy, University of Otago, Dunedin, October 1984. 20 p. Morel, A. and Prieur, L. 1977. Analysis of variations in ocean col-our. Limnology and oceanography 22: 7O9-722-

Preisendorfex, R.!9. 1 986. Secchi disk science: visual optics of natural waters. Limnology and oceanography 31: 909-926. Sartory, D.P. 1982. Spectrophotometric analysis of chlorophyll a in freshwater phytoplankton. Hydrological Research fnstitute, Department of Environment Affairs, Pretoria, South Africa, Technical Report ÎR1 '15. SAS, 1985. SAS Users Guide: Statistics Version 5 Edition. SAS Institute Inc. Caty, North Carolina USA. 956pp. Spence, D.H.N. 1982. The zonation of plants in freshwater lakes. In: Macfadyen, A. & Ford, E.D. (eds). p. 37-125. Advances in Ecological Research. Academic Press, London. Tyler, J.E. 1968: The Secchi disk. Limnology and oceanography 13: 1-6. Vant, V{.N. and Davies-Colley, R.J. 1984. Factors affectng clarity of New Zealand Lakes. New zealqn

research 1 8: 367-377. 1

Vant, W.N.; Davies-Colley, R.J.; CJ_ayton, J.S. and Cof f ey, B.T. 1986. Macrophyte depth limits in Ne\^/ Zealand lakes of differing clarity.

Hydrobiol-ogia 1 37: 55-60. Appendix 1 Summary of Data from the Four Main Sites Lake Colferidge, l_98 6-87

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