optical properties of the coastal and oceanic waters off ... · 590 new zealand journal of marine...

New Zealand Journal of Marine and Freshwater Research, 1995: Vol. 29: 589-6020028-8330/95/2904-0589 $2.50/0 © The Royal Society of New Zealand 1995

589

Optical properties of the coastal and oceanic watersoff South Island, New Zealand: regional variation

CLIVE HOWARD-WILLIAMSNational Institute of Water and Atmospheric

Research LtdP. O. Box 8602Christchurch, New Zealand

ROB DAVIES-COLLEYNational Institute of Water and Atmospheric

Research LtdP.O.Box 11-115Hamilton, New Zealand

WARWICK F. VINCENTDepartement de BiologieUniversite LavalQuebec G1K7P4, Canada

Abstract A study of the optical properties of thevaried water masses in inshore and oceanic watersaround the South Island of New Zealand was made.This was a contribution to a larger effort to modelthe productivity of the southern sector of NewZealand's Exclusive Economic Zone. A combin-ation of spectroradiometry, PAR sensor deploy-ments, transmissometry, and yellow-substancemeasurement was used at 30 stations to characterisethe optical properties of the region. The waterswere classified according to Jerlov's system forocean waters. Inshore sites on the west and eastcoasts and Foveaux Strait fell into optical typesCoastal I and Oceanic III. Oceanic II watersoccurred in the Subtropical Convergence Zone(STCZ). Subantarctic waters and those south ofthe STCZ were in the clear categories Oceanic Iand IB. Attenuation coefficients (K) varied morethan three-fold from 0.05 to 0.16 m-1. Reflectancewas lowest off the Fiordland coast where light-absorbing yellow substance was maximal. An indexof the average lighting of the mixed layer (Eo) for

M95026Received 27 April 1995; accepted 28 September 1995

phytoplankton photosynthesis was calculated fromthe light-depth profile. Highly varying mixed-layerdepths, combined with a 3-fold difference in K,caused Eo to vary by as much as 11, suggestingthat the growth of oceanic phytoplankton in theregion may vary far more than predicted by presentmodels.

Keywords absorption; scattering; light climate;spectral attenuation; reflectance; mixed layer

INTRODUCTION

The waters off the South Island of New Zealand(spanning latitudes 40-50°S) display a high degreeof physical variability induced by boundarycurrents, upwelling, and other phenomena (Heath1976, 1985) and a range of incident solar radiation.The variability of these factors in combination,create a diversity of conditions for ocean planktoniccommunities. Coastal Zone Colour Scanner (CZCS)satellite images of phytoplankton concentrationsover the Southern Ocean show a five-fold variationin mean summer chlorophyll concentrations aroundthe South Island (Sullivan et al. 1993). Synopticwater column sampling and satellite sea surfacetemperature imagery used by Vincent et al. (1991)similarly testify to the environmental variability inthis region. This latter work emphasised the strongnorth-south and east-west gradients in the oceanicenvironment, with related spatial patterns betweennutrients and temperature, and a strong influenceof freshwater inputs around the coast. Large site-to-site differences were detected in fluorescenceyield of phytoplankton per unit of chlorophyll aconcentration. That study also showed the depth ofthe mixed layer to vary widely with location, suchthat surface chlorophyll a concentrations (mg m~3)were only weakly related to total water columnchlorophyll (mg m"2) and, presumably, to depth-integrated productivity.

Vincent, Wake et al. (1989) developed a simplemodel to examine the influence of temperature andirradiance on daily primary production as a function

590 New Zealand Journal of Marine and Freshwater Research, 1995, Vol. 29

NEWZEALAND'

-40"S

Karamea) C a Pl F a r e w e "Bight

Fig. 1 Map of the South Islandof New Zealand showing marinestations sampled on Voyage 2027of RV Rapuhia (16-28 May1989).

of latitude in the oceanic waters of New Zealand'sExclusive Economic Zone. A central parameter inthe irradiance equations in the model was the diffuseattenuation coefficient for irradiance (K), whichwas assumed to take a single value (K= 0.07 irr1)across all water masses. However there are reasonsfor expecting appreciable variation in K (Vincentet al. 1991; Davies-Colley 1992). The variedbiological communities and chlorophyll aconcentrations in the oceanic waters around theSouth Island, together with suspended matter anddissolved organic matter from rivers, all affect theunderwater light field offshore.

The aim of this study was to provide the firstdetailed report on optical properties for the diverserange of waters around the South Island. Weconducted depth-profile measurements in a rangeof contrasting water masses which included: inshorewaters influenced by land run-off and fiordicexchanges, the West Coast upwelling area, the Sub-Tropical Convergence Zone (STCZ), sub-antarcticwaters, Foveaux Strait, and Cook Strait. Here weexamine the correlations between the optical pro-

perties and other biological and physical character-istics of the waters, and classify the water massesaccording to Jerlov's optical classification (Jerlov1951, 1976). The optical data presented here willaid the early interpretation of optical imagery (e.g.,SeaWiFS) of the ocean around New Zealand. Thework is a contribution to the goal of modelling pro-ductivity of New Zealand's Exclusive EconomicZone.

METHODS

All measurements were conducted during voyage2027 of RV Rapuhia between 16 and 29 May1989. The voyage track encircled the South Island(Fig. 1) and included seven stations off the SouthIsland west coast, five south-west of the SouthIsland, six through Foveaux Strait (between theSouth Island and Stewart Island), and seven acrossthe Chatham Rise on the eastern side of the SouthIsland, as well as additional stations to samplespecific coastal (e.g., river plumes) and oceano-graphic features (e.g., Subtropical Convergence).

Howard-Williams et al.—Optical properties of water off New Zealand 591

For conductivity and temperature profiling, aGuildline Model 8755 Conductivity-Temperature-Depth instrument was used, mounted on the rosettesampler. The thermistor was calibrated immediatelyafter the cruise with a PRT bridge (AutomaticSystems Laboratories Model F6). Salinity sampleswere subsequently measured with a GuildlineAutosal Laboratory Salinometer, Model 8400A.The absolute error in the CTD measurements aftercalibration correction is estimated as less than±0.025 psu (practical salinity units), ±0.010°C(temperature), and + 1 m depth.

Chlorophyll a fluorescence was profiled with aSeamartek in situ fluorometer that was shieldedfrom ambient light and mounted at the same levelas the CTD on the rosette sampler. For absolutemeasurements of chlorophyll a, duplicate one-litresamples from each depth sampled for nutrientswere filtered on to 4.25 cm (diameter of filterclearing area) Whatman GF/F filters and storedfrozen. Subsequently the filters were ground with90% acetone in a Teflon tissue grinder, the extractwas cleared by centrifugation, and then assayedfor chlorophyll a by fluorometry both before andafter acidification (Strickland & Parsons 1972).Calibration of the Aminco spectrofluorometer wascarried out using standard pure chlorophyll a(Sigma Chemical Inc.) in acetone.

Duplicate samples for particulate nitrogen andphosphorus analysis were filtered through acid-washed, precombusted Whatman GF/F filters.These were subsequently Kjeldahl-digested andmeasured by auto-analyser as described in Vincent,Chang et al. (1989). The filtrates were analysedimmediately for dissolved nutrients (nitrate plusnitrite, ammoniacal-N, urea) or were stored frozenfor up to 2 months before analysis (silica, dissolvedreactive phosphorus), using the auto-analyticalmethods detailed in Vincent, Chang et al. (1989).

Yellow substance was measured on samplescollected from 10 m depth by absorption at 440 nmof 0.22 \xm filtered (Sartorius membrane filters)samples in 40 mm matched cuvettes in a PU88OOPye-Unicam spectrophotometer (Bricaud et al.1981 ; Davies-Colley 1992). The reference was pre-filtered high-quality double glass-distilled water.

Symbols and units of the measured opticalproperties are given in Table 1. The shipboardprocedure for all measurements was as follows:the ship was manoeuvred so that the sun was onthe side where the measurements were to be taken.The instruments were lowered from a cranestretched out 10 m from the ship's side. Threeinstruments were deployed, a PhotosyntheticallyAvailable Radiation (PAR) sensor array, aspectroradiometer, and a transmissometer. The PAR

Table 1 Symbols and units referred to in the text. Beam attenuation (1) was measured directlywith the Martek transomissometer, beam attenuation (2) was calculated from a + b derived fromPAR data.

Variable Symbol Unit

Scalar irradiance (1 ) 400-700 mmDownward irradiance 400-700 nmUpward irradiance 400-700 nmDownward spectral irradiance 5 nm intervals 350-750 nmUpward spectral irradiance 350-750 nmSea stateCloud coverAttenuation coefficient for scalar irradianceAttenuation coefficient for downward irradianceAttenuation coefficient for upward irradianceEuphotic depthReflectance (400-700 nm)Average cosine (at the midpoint of the Euphotic zone)Absorption coefficient (400-700 nm)Scattering coefficient (400-700 nm)Beam attenuation (1)Beam attenuation (2)Spectral attenuation coefficient for downward irradianceSpectral reflectanceSun angleAngle of the solar beam

umol m l s-2 . - I

r d

£da>EU(X)SeaCloud

r d

ZeuR|X

abc(53O)cKd(X)^(?L)Sun90-B

umol ITT2 sT2

umol n f 2 s~'W m"2 nm"1

W m^2 nm"'Dimensionless/8

m"1

mDimensionlessDimensionlessm"1

m"1

m"'ITT1

m"1

Dimensionless


instrument array consisted of a Licor ScalarIrradiance probe (Model LI 193 SB) and two Licorcosine response probes (Model LI 192): one facingup and one down, to provide almost simultaneousreadings of Eo, EA, and Eu.

Downward and upward facing deployments ofthe LiCor 1800UW scanning spectroradiometerwere made consecutively. Efforts were made toensure that irradiance spectra were not distorted bychanges in ambient irradiance during scanning.The transmissometer was a Martek XMS with a0.25 m path and a filter peak wavelength of 530 nm.(A 1 m path length instrument would have beenpreferable for clear ocean water but was notavailable at the time.) A LiCor LI-1000 data loggercontinuously recorded solar radiation in thewaveband 400-700 nm (using a LiCor LI-190S Aquantum irradiance sensor).

Attenuation coefficients for the downwellingvector irradiance (K^) and for scalar irradiance

(A"o) were calculated from the slope of the linearregression of the natural log-transformed irradiancemeasurements versus depth. The euphotic depth(depth to 1 % of surface light level) was estimatedas 4.6/A^. Reflectance (EJE^) and the averagecosine (\x) (Kirk 1981) were used to compute thePAR band averaged absorption coefficient andscattering coefficients according to Kirk's (1981)method. Spectral attenuation coefficients (K^(k))were calculated from:Kd (X) = In [Ed (X,z)/Ed (X,z + Az)} / Az (1)

for the depth interval Az. Ku(k) was calculatedfrom the equivalent equation for Eu.

Spectral Reflectance R(X) was calculated from

R(K) = Eu(X)/Ed(X). (2)

PAR-band integrations of the spectral irradiancewere used to check that the spectral irradiancesappearing in Eq. 1 and 2 had not been obtainedunder different lighting conditions.

Table 2 Weather conditions and sea state at all stations. Units and symbols are: Cloud, visually estimated inoctets; Sea, visually assessed on Beaufort scale; Sun, sun angle, degrees calculated from time (NZST) and 40°latitude nomograms of Walker & Trexler (1977) with corrections for latitude at each station; 90-B, angle incident ofthe direct solar beam where 6 is solar altitude (90-6 = 43 under occluded sun, OC).

Station

387388389390391392393400401402403404405406407408409410411412413414415416417418419420421422

Site

NarrowsOuter Westport LineMid-shelf, Westport LineInshore, Westport LineHokitika inshoreAbut HeadOff Thompson SoundOff Doubtful SoundPuysegur PointPuysegur/Snares TrackPuysegur/Snares TrackPuysegur/Snares TrackSnaresStewart Island TrackStewart Island TrackStewart Island TrackFoveaux StraitFoveaux StraitFoveaux StraitFoveaux StraitFoveaux StraitSub-antarctic zoneClutha offshoreClutha inshoreChatham Rise SouthChatham RiseChatham RiseChatham RiseChatham RiseChatham Rise

Date

16 May 8917 May 8917 May 8917 May 8918 May 8918 May 8919 May 8921 May 8922 May 8922 May 8922 May 8922 May 8923 May 8923 May 8923 May 8924 May 8924 May 8924 May 8924 May 8924 May 8924 May 8925 May 8925 May 8925 May 8926 May 8927 May 8927 May 8927 May 8928 May 8928 May 89

Time

15.1514.3020.3022.0008.3015.1511.2515.0009.0014.1516.3522.0009.5015.0522.3008.4511.4013.2015.0017.3019.3009.0016.2019.0021.0008.3013.0017.4008.1012.30

Cloud

3/87/8

NightNight

0/80/88/87/87/85/86/8

Night0/88/8

Night8/88/88/88/8

NightNight

0/80/8 hazNightNight5-6/85-6/8Night5-6/8

5/cirrus

Sun

20OC/26NightNight8-1320OCOCOC2410

Night13

OCNightOCOCOCOC

NightNight

78

NightNightOC

OC/28NightOC29

90-6

7043/64NightNight77-82

704343436680

Night7743-43434343

NightNight

8382

NightNight

4343/62Night

4361

Sea

14--21?

5-65-62-32-3

32-33-4

3-4

4-56

3-42-3-35-

7-877-6

4-6

Notes

Patchy cloudNightNight

Night

Night

NightNight

Stratified waterNightNightVariable lightVariable lightNightPatchy cloudPatchy cloud


The depth of the mixed layer (Zm) was definedfrom the first major density discontinuity detectedin the CTD profiles, and the average lighting overthe mixed layer was estimated from the averagequantum scalar light intensity for the mixed layerEo defined as:

£o=—poWdz (3)m m

which can be simplified (Vincent 1983) to:

£0 = Eo(0)/(Ko Zm) (4)

RESULTS

Sea, weather, and solar conditions at the time ofsampling are given in Table 2.

Water masses

The water masses encountered on this cruise havebeen grouped by Vincent et al. (1991) into thefollowing types on the basis of their origins,temperatures, salinities, and other properties:inshore coastal, West Coast oceanic, subtropicalconvergence, sub-antarctic. Table 3 gives data ontemperature, salinity, mixed depth, chlorophyll a,#440 and PN for stations so grouped.

Inshore/coastal

This includes inshore west coast, inshore east coast,and Foveaux Strait (Table 3). Mixed-layer depthswere generally shallow (< 40 m) and salinity andtemperature comparatively variable as might beexpected. Moderate chlorophyll a concentrationswere found, varying between 0.26 and 0.71 mg i r r \

Table 3 Temperature, salinity, mixed layer depths (Zm), chlorophyll a, yellow substance (g44o), and an index ofpaniculate matter (PN) at all sites; analyses from 10 m depth. STCZ, Subtropical convergence zone.

Region (type)

Inshore/coastal (west)

Inshore/coastal (east)

Foveaux Strait

West Coast

STCZ:South-western oceanic

STCZ:Chatham Rise

Sub-antarctic

Station

390391393400415416409410411412413387388389392

401402403404405406407408

418419420421422414417

Temp. (°C)

14.6014.4914.5614.1711.1711.3712.4711.9612.1411.9511.6115.4115.7215.1914.94

13.7313.8213.1312.3711.9712.0413.2212.98

9.8211.7313.0813.0512.749.59.73

Salinity(psu)

34.6234.3134.0934.2833.7933.9034.7634.5434.7034.6234.7234.9234.8534.6134.68

34.8934.8834.9234.8234.7734.7734.8934.82

34.3434.5134.7234.7134.6334.3634.25

Zm (m)

36352510t10tiot48283423553545

30t

959595

1201981108052

6055

10010070

10060

Chi. a(mg ITT3)

0.410.710.530.520.290.620.380.340.262.290.330.300.290.430.70

0.320.460.420.240.170.220.140.47

0.301.460.510.270.620.220.24

#440(irr1)

0.0900.0400.0830.1090.0580.060

0.052

0.0850.0700.0640.042

0.0490.0230.0450.0620.091

0.102

0.0140.0300.0220.0850.0340.0270.031

PN(mg rrr3)

1.010.971.071.111.061.410.640.430.520.520.720.910.780.760.92

0.580.910.610.500.420.380.481.10

0.721.150.740.060.770.650.85

tcomplex structure


Levels of yellow substance were relatively highwith g44Q ranging from 0.04 to 0.11 nr1 .

West CoastWater temperatures were warm (14.9-15.7°C),salinities varied little (34.61 and 34.92 psu), andmixed-layer depths were shallow (30-45 m: Table3). Yellow substance level as #440 varied between0.04 and 0.085 m"1, and chlorophyll a concentrationwas low to moderate (0.29-0.7 mg irr3).

Subtropical convergence(i) South-western oceanicTemperatures ranged from 12.37°C at the south(Snares Island) to 13.82° at the north with a narrowsalinity range in the surface waters (34.77-34.92psu). Deep mixed layers (> 90 m) were a feature ofthese waters. g440 levels were low and rangedbetween 0.023 and 0.062 rrr1. Chlorophyll aconcentration was low, always less than 0.5 mg nr 3

(Table 3).

(ii) Chatham RiseMixed-layer depths varied from 55 to 100 m withthe shallowest mixed layer near the top of theChatham Rise. Temperatures increased from southto north: 9.7° at Stn 417 (latitude 45°00'S) to

13.08° on the R:ise itself (latitude 43° 19.9').Slightly cooler waters (c. 12°C) were recordednorth of the rise associated with the large eddiesidentified in Vincent et al. (1991). Chlorophyll aconcentration was highly variable with low values(0.3 mg rrr3 or less) south of the Rise and highervalues (up to 1.46 mg nr3) on the Rise and to thenorth. Yellow substance level was low (< 0.04 m~') at all sites throughout the transect, except Stn421 to the north of the Rise.

Sub-antarctic watersThese are represented by Stn 414 (Fig. 1) and Stn417. The temperature at Stn 414 was 9.5°, salinitywas low (34.36 psu), and nutrient concentrationswere high compared with elsewhere (Table 3).These waters were low in chlorophyll a content(0.22 mg m~3) in spite of being nutrient-rich. Yellowsubstance level was low (< 0.03 ITT1).

Optical propertiesIrradiance attenuation coefficients (K) weregenerally higher off the west coast than off the eastcoast (Table 4). The clearest waters were those ofthe sub-antarctic water mass (e.g., Stn 414 with Ko

= 0.05 rrr1, and a corresponding euphotic zone of104 m).

Table 4 Selected optical properties in the PAR wave band at the major water masses of the South Island.

Water mass

Inshore/coastal

West Coast

STCZ: South-western oceanic

STCZ:Chatham Rise

Sub-antarctic

Station

391393400409410411415387392

401402403405406408

418419421422414

Kd(m-1)

0.1400.166

0.122

0.144

0.0840.099

0.068

0.106

0.0650.112

0.1030.048

Ko(m-1)

0.1580.1390.1590.0790.1150.1260.1080.1040.144

0.0940.0670.0640.0670.092

0.1210.0590.0940.050

Zeu(m)

253328584038434532

554766706643

68417854

104

E(%)

18.128.862.926.331.123.392.627.523.2

11.215.77.9

13.620.9

26.015.016.915.220.0

R(%)

3.41.62.2

3.84.2

3.3

2.83.02.03.3

2.1

4.13.1

3.44.0

a(rrr1)

0.0980.1010.114

0.0780.064

0.086

0.0560.0610.0530.042

0.073

0.0410.072

0.0660.033

b(1TT1)

0.3120.1810.298

0.3120.266

0.294

0.1680.1900.1010.135

0.183

0.1770.229

0.2170.113

c(nr 1 )

0.4110.2820.412

0.3900.330

0.380

0.2240.2510.1680.177

0.256

0.2170.301

0.2830.148

c (530)(nr 1 )

0.4750.3640.5570.2390.338

0.520

0.3680.350

0.151

0.364

0.2820.3600.2740.3770.083


The attenuation coefficients varied more thanthreefold from 0.05 to 0.17 rrr1. As expected theinshore coastal waters generally had the highest Kd

but the offshore waters also included some highvalues among a wide variation in K^.

Inherent optical properties shown in Table 4are the PAR band average absorption coefficient(a), the scattering coefficient (b), and beamattenuation coefficient (c).

Maximum values of a and b and c were foundat the inshore coastal stations as would be expected.For oceanic waters, values of a were greater off thewest coast than on the east side of the South Island.The lowest values were recorded at Stn 405 (0.042nr1), 418 (0.040 rrr1). and 414 (0.030 m - ' ) in sub-antarctic waters. No data were available for Stn417 (sampled at night, Table 2) which, also being acold "sub-antarctic" water, is expected to be clear.Lowest scattering coefficients (b < 0.14 ITT1) andbeam attenuation coefficients (c < 0.18 m"1) wereat the southernmost stations where recordings weremade (Stn 403, 405,414).

The ratio of scattering to absorption, bla, whichcorrelates closely with reflectance (e.g., Kirk 1994),was near constant in the range 3.1-4.4 for allstations other than the two inshore stations 393 and400 on the west coast off Fiordland. Low ratios of1.8 and 2.6, respectively, were recorded here, owingto the relatively high levels of yellow substanceand its consequent influence on the absorptioncoefficient (Tables 3, 4).

Spectral characteristicsThe various water masses could be distinguishedby their spectral attenuation properties. This isillustrated in Fig. 2A by the KA(X) plots for fourrepresentative stations in contrasting waters: Stn391, West Coast; Stn 400, inshore/coastal, WestCoast; Stn 414, sub-antarctic; and Stn 422, ChathamRise. Station 400 showed a relatively highattenuation at the blue end of the spectrum owingto a relatively high yellow-substance concentration(g440 = 0.11 rrr1), with a minimum between 500and 570 nm, a green "window". The curve for westcoast oceanic water (Stn 391 ) was similar but withlower blue attenuation. Clear waters with a distinctblue-green window (minimum K^X) = 500 nm)were characteristic of the north Chatham Rise(Stn 422). The sub-antarctic water (Stn 414), withits low attenuation at the blue end of the spectrumand a blue window between 410 and 470 nm wasclose to optically pure sea water (Smith & Baker1981).

0.6 T

•Ç 0.5-

1 0.4-oco

I o-3 Hc

z a2-0.1-

0.0

0.08-

0.06-

0 0.04

'She

0.02

A

400391 „ \

422 s X \

414*"*-

^ _

cJ Ja-i

Z\.'-'''Js ^ Pure water

0.0

B

414''

422 v"

400—i—

\^

391 / ' V"'"\

1 1 r300 400 500 600

Wavelength (nm)

70C

Fig. 2 A, Spectral attenuation Kj (X) values for fourcontrasting stations. Spectral K for purest ocean waters(after Smith & Baker 1981 ) is shown for comparison.B, Spectral reflectance R (X) for the same four contrastingstations.

Spectral reflectance (R(X) followed a broadlyinverse trend (Fig. 2B) with maximum reflectanceat 520 nm for Stn 400, 500 nm for Stn 391, 480 nmfor Stn 422, and 400 nm for Stn 414. Most of theoff-shore waters showed maximum reflectance inthe 450-500 nm range, and appeared blue to green.Sub-antarctic water had maximal reflectance atshorter wavelengths and appeared blue in hue.

Contributions to absorptionThe contribution to the total absorption coefficientof yellow substance (ay), paniculate matter (ap),chlorophyll (achia), and water itself (aw) at 440 nm


was estimated. The total absorption coefficienta(440) was calculated from irradiance data where:o(440) = n(440) Kd(440) (Kirk 1981) and \I wascalculated from spectral reflectance, R(440) (Kirk1981). g44o data were used for absorbance by yellowsubstance at each station, and that due tophytoplankton pigments at 440 nm (achia) calculatedfrom the method of Bricaud & Stramski (1990)using the lower end of the range of publishedvalues for the specific absorption coefficient forphytoplankton (Prieur & Sathyendranath 1981).The values of a^^ were obtained by multiplyingthe absorption correction of Achia = 15 m2 g"1 bythe measured values of chlorophyll a recorded inTable 2. The remainder was attributed to absorptionby non-chlorophyll particulate matter ap, where:

tfp = a(440) - (tfw + g + )

all at 440 nm, where aw is the absorption due towater itself (Smith & Baker 1981).

Data for a selected range of stations (Table 5)show that the contribution of pure water to thetotal absorption (i.e. #w/a) at 440 nm is high in thesouth-western oceanic (Stn 402) and southern sideof the STCZ (Stn 418): 32 and 40%, respectively.The contribution of yellow substance at thiswavelength varied between 37 and 66%, but thatdue to phytoplankton pigments was generally lessthan 20%. One exception was at Stn 419 in theeddy on the Chatham Rise where 31% of theabsorption coefficient at 440 nm was attributableto phytoplankton. Chlorophyll a concentrationswere used in this calculation for absorption due tophytoplankton. Although chlorophyll a is thedominant pigment it is only one pigment in anassemblage of different pigments in the phyto-plankton (Hoepffner & Sathyendranath 1992) sochlorophyll a alone may underestimate theproportional contribution by phytoplankton.

Absorption due to non-living particulate matterwas a highly variable contributor to total absorption(negligible up to 39%).

Average irradiance in the mixed layerThe average irradiance that a_circulating cell in themixed layer is subjected to ( Eo), was calculated asa percentage of subsurface irradiance (Table 3) toprovide an index of the degree of illumination ofthe mixed layer. Relatively low values (< 15%)were found in the deep mixed layers of the STCZ(Stn 405, 406). While a deep mixed layer (Zm =100 m) occurred in the sub-antarctic Stn 414, thehigh clarity of this water (Zeu = 104 m) more thancompensated, giving a relatively high Eo of 20%.A high Eo value of 92% at Stn 415 resulted fromthe very shallow mixed layer (c. 10 m) in thishighly stratified coastal water mass, affected by afreshwater plume from the Clutha River.

General correlations between propertiesSeveral significant correlations were observedbetween optical variables and certain chemical,and biological variables (Table 6). Ko was correlatedto chlorophyll a (r = 0.504, P < 0.05), yellowsubstance (r = 0.504, P < 0.05), and particulatematter expressed as particulate nitrogen (r = 0.62,P < 0.05) and was also correlated with watertemperature and negatively with distance from land.(Note that K¿ was very similar numerically to Ka

but good measurements of K& were available fromfewer sites so correlations were made with Ko

only) Eo was only weakly related to chlorophyll,yellow substance, or particulate matter, as mixedlayer depth was a primary controlling variable on

KA multiple regression of A o

o n chlorophyll a,yellow substance, and particulate matter showed

Table 5 Absorption coefficients (rrr1) at 440 nm for various components ofthe water mass. Data in parentheses are the % contribution to total a(440).aw, absorption by pure water; ay, absorption due to yellow colour = £440,ach\a absorption due to chlorophyll a; ap, absorption due to non-chlorophyllparticulate matter.

Station

391400402418419422

a(440)

0.1080.1650.0430.0350.0700.078

aw

0.014(13)0.014 (8)0.014(32)0.014(40)0.014(20)0.014(18)

ay

0.040 (37)0.109(66)0.023 (53)0.014(40)0.030 (43)0.034 (44)

«chk

0.01 (10)0.008 (5)0.007(16)0.004(11)0.022(31)0.009(11)

ap

0.043 (39)0.034(21)negl (negl)0.003 (9)0.004 (6)0.021 (27)


the following equations:

For all data:Ko m"1 = 0.023 + 0.057 PN + 0.033 Chl.a + 0.281 g440

(r= 0.71, P< 0.02).

For oceanic sites:Ko m"1 = 0.031 + 0.042 PN + 0.035 Chl.a + 0.159 g440

(r = 0.78, P< 0.02).There was a significant correlation between

chlorophyll a and particulate matter (r = 0.64,P < 0.05) in the oceanic waters and for the full dataset (r = 0.58, P < 0.05), but removal of particulatematter from the multiple regression equationsresulted in a reduced r. This was 0.58 (P < 0.05)for all sites and 0.69 (P < 0.05) for oceanic sites.

There was a moderate correlation (r = 0.48,P < 0.1) between the absorption coefficient for thePAR waveband and yellow substance, and astronger correlation between a (PAR) with organicparticulate matter (r = 0.64, P < 0.01) suggestingthat absorption by non-living particulate matterwas more important than by yellow substance forthe full PAR waveband in these waters. This arisesbecause yellow substance absorbs mainly in theblue region of the PAR waveband, whereasabsorption by particulate matter is less spectrallyselective and contributes more attenuation rightacross the PAR waveband. In the full data set, Ko,a, and c were negatively related to distance fromland, but less strongly when the inshore coastalstations were omitted. There was a positivecorrelation with temperature as the colder waters

of the subantarctic and subtropical convergencewere the clearest.

Chatham Rise transectA highly variable transect across the Chatham Riseis used to illustrate the relationships between theoptical parameters. The temperature structure (fromVincent et al. 1991) shown in Fig. 3 revealed acentral cell of slightly cooler water in a warmwater eddy over the Rise. A sharp temperaturegradient to the south of the Rise led to sub-antarcticwater. A peak in chlorophyll a and particulatematter at Stn 419 was followed by low values inthe eddy on the Rise. Beam attenuation and Ko

followed this trend, varying by a factor of almost3. Eo fell between Stn 418 and 419, increased onlyslightly in the clearer waters of the eddy as a resultof a deepening of the mixed layer there, and thenfell again to the north of the Rise.

DISCUSSION

The negative relationships of Ko, a, and c to distancefrom land may arise for two reasons: firstly becauseland drainage is the source of much light-attenuatingmatter, and secondly, because nutrients advectedinto the mixed layer from coastal upwelling (e.g.,west coast: Vincent, Chang, et al. 1989; Viner1990) result in increased planktonic production.This would have consequently increased lightattenuation due to increased particulate anddissolved organic matter concentrations.

Table 6 Pearsons correlation coefficients for Ko, a, b, and c against several environmental parameters for the totaldata set and for oceanic sites only (data are from 10 m samples where appropriate). *, P < 0.05; **, f < 0.01 ;***,/>< 0.001.

All dataChlorophyll a (mg m~3)Temperature (°C)Yellow substance (rrr1)Particulate matter (mg PN m~3)Distance from land (km)n

OceanicChlorophyll a (mg m~3)Temperature (°C)Yellow substance (irr1)Particulate matter (mg PN irr3)Distance from land (km)n

Kof (nr1)

0.504*0.496*0.504*0.621**

-0.787***21

0.626**0.493*0.0710.582**

-0.47616

a (irr1)

0.4020.747***0.4680.640**

-0.777***15

0.641**0.767***0.1640.713***

-0.48111

b(Tcrx)

0.3130.3750.0900.428

-0.552*15

0.605*0.4840.1550.633**

-0.19711

c(m-')

0.3560.5000.1920.505**

-0.647**15

0.644**0.5820.0930.676**

-0.29211

£„/£,,(-0)

0.1200.0680.2700.390

0.0480.5290.2190.346

•\K¿ for sites 401 and 418 included


Station417 418 419 420 421 422

(m)

.c

Dep

100

200

300

400

' " " • — ~ ~ ~ " " " - -

""80.

\\ yI - ^ l

\ 1 /

\X13 4 1 3 0 ' ,

v IV/ 1 1

^— 0.12--§ 0.10.o^ 0.08

0.06T— 25

% 20I Uj iq

TE 0.40

S 0.35

Hl 0.30

0.25III

HI-

IB

S «

^

5

?— 1.4-

í 1.0-¿ 0.6-

0.2?~ 1.2-I 1.0-E, 0.8-

0.6-

50 100 200 300Distance (km)

50 100 200Distance (km)

300

Fig. 3 Transect across the Chatham Rise showingtemperature structure, the eddy feature on the Rise, andoptical properties across the Rise.

The clearest waters (Ko < 0.06 m ') weregenerally found south of the subtropicalconvergence, where low levels of chlorophyll aand yellow substance prevailed. The influence onthe optical properties of land-derived material,especially yellow substance, was strikingly evidentoff the Fiordland coast. Reflectance was lowest atthese sites which also shared maximal attenuationK(X) at the blue end of the spectrum. In contrast,the cold, nutrient-rich, but chlorophyll a-poorwaters of the subantarctic were "blue" (low spectralK and high spectral R in the region 350-450 nm).Large-scale remote sensing analyses of southernocean biomass (Sullivan et al. 1993) showed thatrelatively high chlorophyll a (> 1.0 mg m"1) couldoccur in summer in regions occupied by the sub-antarctic water mass.

The data presented in the present paper reflectautumn-winter conditions. A time series of satellitecomposite images of sea-surface temperatures overthe area reveals a regular increase in temperatureacross and south of the STCZ in summer. Resultsfrom a subsequent summer voyage to the STCZ atthe Chatham Rise and south to sub-antarctic water(February 1995) show a mixed layer depth of 20-40 m across this area (Singleton 1995). Mixed-layer water temperatures ranged from 18°C in theSTCZ to 14°C in subantarctic water. Opticalproperties in the waters around the South Island insummer time may therefore be appreciably differentfrom those measured in autumn-winter as reportedhere. In particular the parameter Eo may be lessvariable in summer as a result of surface warmingof the ocean and a more-nearly "uniform" shallowmixed-layer depth.

Our data indicate that chlorophyll a is a smallcontributor (< 20%) to the absorption coefficient a(Table 5). Kirk's (1981) empirical equation:

Kd (Zm) = (a2 + 0.256 a b) 1/2

shows that irradiance attenuation is primarilydetermined by the absorption coefficient. In theopen ocean (Stn 401-422) across the STCZ,absorption (and hence K) appears to be depen-dent on yellow substance and particulate matter,presumably derived from old primary production.This would therefore not be expected to co-varywith present chlorophyll a values. Fenton et al.(1994) have recently studied optical properties inthe surface waters of part of the southern oceanand similarly found that absorption by yellowsubstance provided a major contribution to Ko.

Ho ward-Williams et al.—Optical properties of water off New Zealand 599

Optical classification

Jerlov (1951, 1976) provided a scheme for opticalclassification of ocean waters based primarily onthe scattering coefficient (b) and the irradianceattenuation coefficient at 465 nm. This wavelengthwas assumed to be close to the minimum of theattenuation coefficient for the clearest ocean waters(Jerlov 1951 ). The widely used Jerlov classificationhas been modified in recent years by Pelevin &Rutkovskaya (1977) and further refinements havebeen made by Smith & Baker (1978, 1981).However, these changes have only altered theclassification in coastal waters and regions whereattenuation is strongly influenced by yellowsubstance, resuspended sediments, or terrigenousinflows. We have followed Jerlov's classificationso as to relate optical character to the autumn-winter physico-chemical patterns around the SouthIsland on the basis of temperature, salinity, andother properties as set out in Table 3. We recognisethat the optical classification of surface water at aparticular site may vary with season.

The data for Kd (465) and scattering coefficientare given in Table 7 together with the inferredoptical type. The inshore/coastal waters all fallinto optical types Coastal I and Oceanic III. Theother extreme is seen by sub-antarctic water at Stn414 which is in the clearest optical categoryOceanic 1. The only West Coast station whereadequate data are available, Stn 392, was classified

as Coastal I. This site was off Abut Head only22 km offshore on the continental shelf. The watersof the sub-tropical convergence south-east of theSouth Island were clear waters in categoriesOceanic I(B) and Oceanic II. Across the ChathamRise the clarity increased from north to south withclassification Oceanic III to the north and OceanicI(B) in the south (Table 7, Fig. 4).

The optical classification is mapped on a broadscale on Fig. 4. Type Coastal 1 predominated alongthe West Coast. Oceanic II waters occurred in theSTCZ to the south of the South Island and on theChatham Rise. Sub-antarctic waters and those southof the STCZ were types Oceanic I and IB. Sharpchanges across the STCZ at the Chatham Riseshow the transition from Oceanic I(B) to OceanicIII across the Rise.

Prieur & Sathyendranath (1981) pointed outthat the classifications of Jerlov (1976) and Smith& Baker (1978) were dependent on an apparentproperty of the water, the attenuation coefficient.They proposed a more rigorous system based onthe absorption coefficient, an inherent opticalproperty. Their classification was based on therelative importance of three optical coefficientsC", Y, and P', expressing the different spectralabsorption characteristics of chlorophyll, yellowsubstance, and non-chlorophyllous particles,respectively. However, in the range of water typescovered in this study, the three coefficients are

Table 7 K¿ (465), scattering coefficient and Jerlov water types for the majorwater masses off the South Island.

Water mass

Inshore/coastal

West CoastSTCZ: South-western oceanic

STCZ:Chatham Rise

Sub-antarctic

Station

391393400411415392

401402403405408

418419422414

Kd (465)(nr1)

0.1370.1300.1680.0930.0820.132

0.0650.0490.0700.0500.067

0.0570.0690.0810.035

b (PAR)(nr1)

0.3120.1810.2980.3120.2660.294

0.1680.1900.1010.1350.183

0.1770.2290.2170.113

Jerlovwater type

Coastal 1Coastal 1Coastal 1Oceanic IIIOceanic IIICoastal I

Oceanic IIOceanic 1(B)Oceanic IIOceanic I(B)Oceanic II

Oceanic I(B)Oceanic IIOceanic III

Oceanic I


i, ii Jerlov class(0.11) Ko

— Jerlov class

B 35(28)

E. < 20%

Fig. 4 Map of the sector of the Exclusive EconomicZone where optical measurements were made showing:A, the Jerlov classification values, and A"o; and B, themixed-layer depths (Zm) and average irradiance E inthe mixed layer.

very low (i.e., Jerlov Oceanic I and IA havenegligible coefficients) and hence too impreciselymeasured to permit application of this classification.

Applications to models of primary productionFor the purposes of modelling ocean primaryproduction and for remote sensing of waters forregional estimations of productivity, apparentoptical properties Kd (or Ko), R, and the attenuationcross-sections for the constituents affecting thelight field (Topliss et al. 1989; Fenton et al. 1994)are used more often than inherent properties.

The waters considered in this study span afairly narrow range of optical types at the highclarity end. In spite of this, K^ (and Ko) varied byas much as 3.5-fold. A model of marine food chain

dynamics in a South Island west coast ecosystem(Kumar et al. 1991 ) was highly sensitive to changesin the light limitation parameter (a) for pico-plankton and was also sensitive to changes in Kd.Running the model with a ten-fold change in Kdfrom 0.07 to 0.7 nr 1 to a steady state (30 days)resulted in a four- to fivefold decrease in thebiomass of picoplankton, nanoplankton, and netplankton components, a fivefold increase inmicrozooplankton biomass, and a correspondingdecrease in macrozooplankton biomass. Evidentlylight availability strongly influences the West Coastplankton ecology. Kumar et al. (1991) suggestedthat a shallowing of the mixed layer (Zm) couldpartially offset such a decrease in light penetration.Indeed, the average irradiance that a circulatingphytoplankton cell is subjected to (Eo) wasconsistently higher in the waters with the highestattenuation coefficients (Table 3). In all thesewaters Zm was small (c. 30 m or less). AlthoughEo is a valuable indicator of light availability inthe mixed layer (e.g., Vincent 1983) the strongnon-linearity of production vs irradiance curvesprecludes direct use of this index in models(McBride 1992).

The significance of the mixed-layer depth inprimary production may be affected by photo-adaptation by the phytoplankton. For instance theparameters £k (saturating light intensity) and Pmax

(maximum rate of photosynthesis) are known to bevariable (Kirk 1994). Present models of productivityin New Zealand's EEZ have assumed a constantrelationship of photosynthesis to irradiance. Studiesare currently underway to examine photosyntheticparameters in New Zealand marine waters(I. Hawes, NIWA, pers. comm.).

The EEZ model of Vincent, Wake et al. (1989)showed a three-fold variability in primaryproduction (g C nr 2 yH) from top (latitude 25°S)to bottom (latitude 55°S), primarily owing tovariation in incident irradiance and temperature.The model was based on a constant Kd of 0.07 m~'and mixed layer depth was not considered. Thepresent study has shown not only a 3.5 folddifference in Kd and Ko from latitude 40-50°S'alone, but also that Eo in the mixed layer can varyby a factor of 11 (Fig. 4). Thus the productivity ofoceanic phytoplankton in New Zealand's EEZ maybe expected to vary more than predicted by Vincent,Wake et al. (1989). An improved understanding ofthe factors controlling spatial variability inphytoplankton productivity and biomass in NewZealand's EEZ is a major goal for future research.


ACKNOWLEDGMENTS

We thank Anthony Cole, Stuart Pickmere, Julie Hall,and Lynelle May for assistance on the ship. ShaunFalconer and Anne-Maree Schwarz collated andprocessed the optical data set. The study was funded bythe Foundation for Research, Science and Technologyunder contract No. CO 1214 and CO 1304. The manuscriptwas typed by Carol Whaitiri with figures by PeterBennett. Dr John T. O. Kirk provided useful commentson a draft of this paper, which was further improvedfollowing referee comment by Dr J. Priddle and Dr S.Sathyandranath.

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