Optical monitoring of coastal waters: Photic depth estimates

Marine EnrironmentalResearch7 (1982)295-308

OPTICAL MONITORING OF COASTAL WATERS: PHOTIC DEPTH ESTIMATES

B. J.ToPLISS*

Department of Physical Oceanography, Marine Science Laboratories, Menai Bridge, Anglesey, Great Britain (Received: 1 May, 1982)

ABSTRACT

Measurements of underwater irradiance, transparency and Secchi disc depth were carried out in Liverpool Bay during 1974. The depth of the photic zone measured in the green portion of the spectrum rangedfrom 5 m to 50 m, depending on location and season. Inshore values of photic depth were smaller than those encountered offshore. By mid-summer the depth of the photic zone had reached or exceeded the depth of the sea bed over the whole study area. Consideration was given to the error terms associated with the optical measurements and to the problems of using standard optical relationships in coastal waters. The relationship between Secchi disc depth and the depth of the photic zone could not be taken as constant but was a function of the absorption and scattering properties of the water mass.

NOMENCLATURE a

Absorption coefficient. b Scattering coefficient. c, c(z) Collimated beam attenuation coefficient; c as a function of depth. Collimated beam attenuation coefficient averaged over profile. Inherent contrast of Secchi disc against background. Co c, Apparent contrast of Secchi disc at disc depth. E(o) Downwelling irradiance at surface. E(z) Downwelling irradiance at depth, z. E(z~) Downwelling irradiance at depth, zsd* Present address: Atlantic Geoscience Centre, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, B2Y 4A2, Canada.

295 Marine Environ. Res. 014i-1136/82/0007-0295[$02.75 © Applied Science Publishers Ltd, England, 1982 Printed in Great Britain

296

B. J. TOPLISS

k k' N

lrradiance attenuation coefficient. Irradiance attenuation coefficient due to depth changes in transparency. Number of data points in a profile. r Correlation coefficient. 2 Depth. Depth of Secchi disc disappearance. 2sd 21% Depth of 1% of surface irradiance. 210% Depth of 10 % of surface irradiance. Ac Standard error on 6. Ak Standard error on k. Azt% Standard error on z1%. # Average cosine. Water mass dependent constant.

INTRODUCTION

The transmittance of sea water in the visible spectrum indicates whether natural radiation will reach photosynthetic organisms and marine animals which need light to illuminate and regulate their activities. The bulk of primary production in the marine environment is generally considered to occur in the photic or euphotic zone. This zone is defined as the depth penetrated by 1% of surface daylight. Below this depth photosynthetic activity can still occur and has been observed by several workers, Steemann-Nielsen (1974). The optical properfies of sea water and their regional characteristics have been extensively investigated by Jerlov (1951, 1976). In clear oceanic water, where the depth of the photic zone may reach 140 m, maximum daylight penetration occurs in the blue portion of the spectrum (475 nm). For clear coastal waters maximum transmission occurs around 525 nm, with the depth of the photic zone at these wavelengths being typically 35 m. As coastal waters become more turbid, light penetration is reduced and maximum wavelength penetration occurs around 575 nm with photic depths falling as low as 7 m (Jerlov, 1976). One characteristic feature of coastal waters is the presence of large quantities of scattering material. This results in large values of the collimated beam attenuation coefficient, c, which is related to the absorption, a, and scattering, b, coefficients via: c=a+b

(1)

The attenuation of daylight with depth is dependent on the irradiance attenuation coefficient, k, and: E(z) = E(o) e - k~

(2)

where E(z) and E(o) are the downwelling irradiance at depth, z, and in the water

OPTICAL MONITORING OF COASTAL WATERS

297

surface, respectively. For practical reasons, however, this latter measurement is normally taken above the surface. The coefficient, k, is directly dependent on the absorption properties of the water but is also indirectly connected to the scattering properties via a water mass dependent parameter, /1, called the average cosine. Neglecting contributions from upwelling irradiance we have: a

k =-

/1

(3)

The depth of I ~o of daylight penetration, z ~ (i.e. the depth of the photic zone) can easily be calculated from eqn. (2) to give the relationship: 4.6 zl~° = T

(4)

It has been common practice for biological oceanographers to estimate the depth of the photic zone by multiplying the Secchi disc depth by three (Atkins & Poole, 1929, Otobe et al., 1977). Tyler (1968) and Williams (1968) have shown that the vertical depth of disc disappearance, zsa, is dependent on the sum of c and k so that: zsd =

In (Co~C,) c+k

(5)

where Co is the inherent contrast of the disc against the background and C, its apparent contrast at z,a, respectively. In oceanic areas, values of C o = 40 and C, = 0-066 have been selected by Tyler (1968) and Otobe et al. (1977) to give a value for In (Co~C,) of 6.4. For turbid coastal waters, Holmes (1970) obtained average values of C o = 11.03, C, = 0.0014 and In (Co~C,) = 9.4. In this paper, data for beam attenuation, irradiance attenuation and Secchi disc depth are presented for Liverpool Bay, a coastal area ranging from tidally exposed shorelines'to depths of 50 m. Liverpool Bay is a nutrient rich area with strong tidal influences and several major run-off sources. The sea-bed composition within the bay varies from mud flats and sand-banks to coarse gravel beds. Optical measurements in such an area are therefore subjected to a wide range of influences and represent an interesting extension to the coastal work already covered by Jerlov (1976). METHOD OF MEASUREMENT

Measurements of underwater irradiance and transparency were made in the western part of Liverpool Bay in May, late August and early December, 1974 (Topliss, 1977). Throughout this period, continuous recordings of irradiance and transparency were also taken at a near-shore site off the north-east coast of Anglesey and have been fully described by Topliss (1977). The instruments used for the survey and for the continuous work were identical and were all fitted with broad band green

298

B.J. TOPLISS

filters, thus maximally utilising the spectral transmittance properties of coastal waters. The overall spectral response of the irradiance meters was peaked at 542 nm with a 150 nm half bandwidth. Underwater irradiance measurements are influenced by a wide range of factors, including surface lighting conditions, sea surface conditions and the degree of optical inhomogeneity. Some of these factors are more dominant in coastal waters (e.g. shallow water wave action, optical inhomogeneities, etc.) and hence present the oceanographer with additional difficulties in obtaining accurate estimates of underwater irradiance. Irradiance profiles conducted with broad band filters exhibit rapid and varying attenuation in the surface layer, due to such factors as wave action, directional sunlight and selective wavelength absorption. In order to minimise all such effects, the first four optical depths (defined as the product of c and depth) of all profiles were ignored. In coastal areas strong tidal currents may cause a profiling meter to be tilted from the vertical. This results in errors because the meter is no longer recording true downwelling irradiance. To eliminate this source of error, stabilising fins were attached to the body of the instrument, allowing accurate measurement in currents of several knots. The value of the diffuse attenuation coefficient, k, can be obtained by an ordinary least squares calculation of the natural logarithmic ratio of underwater to surface irradiance fitted against depth. A constant term allows for any bias in the data, such as occurs with instrumentation drift, etc. In ( E ( z ) ~ = - k z

\e(o)/

+ constant

(6)

This simple model will adequately explain the variance in underwater irradiance, providing homogeneous optical conditions exist, lnhomogeneities in the water column will result in changes in k giving a variation + Ak to the estimates. If these inhomogeneities possess a definite relationship to the changes in k (e.g. as with an optically stratified water column) their effect on the underwater light field can be extracted. For a given water mass we can assume that a change, c ( - ) - ~ , in transparency, produces a change in attenuation, k': k' = ~(c(z) - c-)

(7)

where: c(z) = beam attenuation coefficient at depth, ( = mean c(z) over profile

= a water mass dependent constant which then becomes an additional term, k ' z , in eqn. (6). Changes in the underwater light field directly related to the transparency profile

OPTICAL MONITORINGOF COASTAL WATERS

299

can be accommodated by the addition of a In (c(z)) term into eqn. (6). If neither k'z nor in (c(z)) makes any significant statistical contribution to the variance of the underwater light field, it is automatically dropped from the computer analysis, leaving the simple analysis given by eqn. (6). These regression analyses were calculated between two depth limits. The upper depth limit was taken as four optical depths, whilst the lower depth limit of the regression was set by the low light level sensitivity of the irradiance meter. In virtually all cases changes in k resulting from alterations in these regression limits were far smaller than changes in k between stations. Of 55 stations analysed to a 0.1% significance level, 45 analyses automatically reduced to the simple form given in eqn. (6). In only three of the remaining ten stations did the new estimates of k differ by more than 4 % from estimates obtained by eqn. (6). These analyses, in conjunction with the lack of structure exhibited in the transparency profiles, confirmed the fact that no easily definable optical structure could be detected in the underwater light regime. Errors on estimates Although optical homogeneity has been assumed for the purposes of data analysis, random optical inhomogeneities in the water column will ultimately limit the accuracy to which optical measurements may be made. However, instrumental, optical and non-optical environmental effects will all contribute to the error term on an optical measurement. The error on the mean beam attenuation coefficient, 5, was given by the standard error on the mean, Ac. It was found that Ac was correlated with ~ (r = 0.77) and increased linearly with increasing attenuation. Similarly, the standard error, Ak, was positively correlated with k (r = 0.80). Hence, greater turbidity reduced the accuracy with which optical measurements could be made, based on homogeneous assumptions. Both Ac and Ak produced no significant correlation coefficient with environmental parameters such as sea state, wind, cloud cover, station depth, etc. The error term Ak can, however, be related to Ac in the following manner:

Ak = 0 . 3 1 A c - 0-00014N + 0.018

(r = 0.8)

(8)

where N = number of data points in profile. In this expression Ncan be taken as representing an error term due to instrument limitations and Ac representing an optical inhomogeneity term. The term Ac itself was not significantly correlated with N (r = - 0.2) and hence, in this analysis, gave a better indication than Ak of random optical inhomogeneities. The error term on the photic depth estimates can be derived from Ak by error theory (Topping, 1956) as follows: 4.6 Ak AZt%-- k2 (9)

300

n.J. TOPLISS

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Fig. I. (a) Cruise l--depths ofthe photic zone in metres during May. (b) Cruise 2~depths of the photic zone in metres during late August. (c) Cruise 3--depths of the photic zone in metres during early December.

301

OPTICAL MONITORING OF COASTAL WATERS

The resulting average standard error for all the photic depths calculated was + 0 . 6 m with a range from +3-8m to +0.08m. Hence, since the average photic depth for the data set was 21 m, for most purposes the error term on zt~ was negligible. CRUISE DATA

Figures l(a) to l(c) illustrate the variations in the depth of the photic zone in Liverpool Bay for three cruises. For Fig. l(a), a cruise in May, photic depths at the centre of the bay were comparable with the depth of the sea bed. A turbid region was found along the coast of Anglesey where the local Phaecoystis bloom was expected to be most intense at this time of the year (Lennox, 1979). On the August cruise (Fig. l(b)) this turbid coastal patch was replaced by clear water. In the inshore region the photic zone exceeded the depth of water present and between l0 ~o and 1% of surface light might be expected to reach the near coastal sea bed. On average, the photic zone extended to the sea bed throughout the study area, indicating the presence of clearer water throughout. TABLE I VALUES OF Zt.y, AND c FOR CRUISES UNDERTAKEN IN LIVERPOOL BAY IN 1974 M a x(m~rZ~ .

Min. Zrz° (m)

Mean z t % (m) *

Mean c (m-t)

All stations Cruise 1: May Cruise 2: Aug. Cruise 3: Dec.

48 54 23

6 27 3

21.6 35.9 12.0

2-28 0.80 2-16

Coastal stations only (Puffin Island to Point Lynas) Cruise I: May Cruise 2: Aug. Cruise 3: Dec.

25 45 16

6 27 6

13 33 12

2.44 0.82 1.70

In early December (Fig. l(c)) the photic zone was greatly reduced so that, in most areas, only about 0.1% of surface light reached the sea bed. Table 1 shows that the same near shore stations on the May and December cruises had almost the same depth of photic zone (12 m and 13 m, respectively), but that the mean value of c indicated that the water was 40 % more turbid in May. TIME SERIESDATA Daily values of :t% were obtained from an irradiance meter situated in approximately 7 m of water, about 150 yards off Moelfre on the north-east coast of Anglesey (Topliss, 1977). Figures 2 and 3 give the two time series of measurements

302

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Z~% ira)

?

"

_Z1% (m)

T,

'-c~_.

T : : : ";' : '

Apr. T qov 1

June

I

__S Fig. 2. Depths to which 10% (upper) and 1% (lower) of surface irradiance penetrated from 15 March to 22 September, 1974, at the continuous monitoring site off Moelfre.

lec

1

an

1

~4

Fig. 3. Depths to which 10% (upper) and 1% (lower) of surface irradiance penetrated from 19 October, 1974, to 25 January, 1975, at the continuous monitoring site off Moelfre.

made in the summer of 1974 (from 15 March to 22 September) and in the winter (from 19 October to 25 January). The upper portion of each Figure gives the depth to which 10% of surface light penetrated, i.e. Z~o% and the lower portion, z1%. Comparison with the cruise data at the corresponding periods in the time series revealed the offshore site to be more turbid than the nearest cruise stations. At Moelfre average zt~ values recorded at the times of the three cruises were 9.6 m, 15-3m and 8.5m, respectively, seasonally comparable with the cruise data (cf. Table 1). Table 2 gives mean values of zt~, collimated beam attenuation coefficient and underwater and surface irradiance durations at the near shore site at Moelfre. Throughout May, turbid conditions can be seen to exist at a time when a local Phaeocystis bloom was known to occur. In early June, when the bloom had finished, clear water doubled the depth of the photic zone. In late August and early September the depth of the photic zone steadily increased as clear, high salinity water entered Liverpool Bay from the Irish Sea (Hunt, 1978).

303

OPTICAL MONITORING OF COASTAL WATERS

TABLE 2 VALUES OF ,71V., c AND DURATIONOF IRRADIANCEATTHE DEPTH OF THE IRRADIANCEMETERAND AT THE SURFACE FOR THE CONTINUOUS MONITORINGSITE OFF MOELFRE DURING 1974/5 Mean z ~ (m)

Mean c (m - 1)

Mean underwater duration*

Mean surface duration*

Ch)

Summer: 1974 15 M a r c h - I 1 April 12 April-9 May 10 M a y - 6 June 7 June-4 July 5 July-I Aug. 2 Aug.-29 Aug. 30 Aug.-22 Sept. Winter: 1974/75 19 Oct.-15 16 Nov.-13 14 Dec.-10 11 Jan.-25

Nov. Dec. Jan. Jan.

(h)

13.9 10.4 8.5 16.0 1 I.i 14.5 19.3

1.94 3.15 5.74 1.82 2.52 1.71 2-09

10.7 11.4 13.8 14.3 12.6 12.8 11.6

11-8 13.3 15.0 16.0 15-9 14.3 12-3

5.3 7.5 7.4 3-9

5.67 4.09 4.20 6.07

5-9 6.3 6.6 4.9

8-9 7.8 7.4 7-9

* Duration defined as period for which irradiance exceeds 20.1 W m -2.

Figure 2 shows that, for a sea bed situated at a depth of 7 m below mean sea level, light in excess of I ~ of surface values reached the sea floor for most of the summer. Only during the Phaeocystis bloom did light intensities fall persistently below the 1 ~ level, a possible example of self-shading by marine life. For approximately 50 of the time, light reaching the sea bed was greater than 10 ~ of the surface value. In winter the situation changed; for virtually the whole of the period, 10 ~o of surface light never reached the sea floor, whilst even 1 ~o of surface light only reached marine life on the sea bed for approximately 30 ~o of the days recorded. It should be noted that at this shallow coastal site the tidal cycle will have an appreciable effect on the depth of water over the sea bed. Hence, at some stage in the tidal cycle and depending on transparency conditions, 1 ~ of surface light is liable to penetrate to the sea bed. The values ofzlo % and z1% given in Figs 2 and 3 are those averaged over all hours of daylight for each day, as recorded in the green portion of the spectrum alone. The use of relative light levels throughout the year can, however, be misleading. The absolute value of 1 ~ of surface irradiance, in terms of radiant energy, will change on a seasonal basis. Similarly, the duration for which this surface daylight energy is available will also vary on a seasonal basis. The durations given in Table 2 are based simply on the lowest, accurately recordable, light level obtainable with the given irradiance meter (i.e. 0.1 W m - 2). It can be seen from Table 2 that although, during the spring bloom in May, the average depth of the photic zone was low, no reduction occurred in the average underwater duration. The duration of underwater irradiance in summer basically followed the seasonal trend of varying daylength.

304

B.J. TOPLISS

The shortest underwater durations in winter, however, were not related to the shortest surface duration (cf. Table 2), but to the occurrence of the most turbid optical conditions. At the Moelfre site, the combined effects of high summer surface intensities and durations with clear optical conditions gave daily totals of underwater intensity of up to 1000 times those recorded in mid-winter. Clearly, the use of relative light levels alone is insufficient to determine the quantity of biologically available light over the course of a season. SECCHI DISC MEASUREMENTS

Secchi disc measurements were made according to the suggestions of Tyler (1968) and Holmes (1970), that is: (a) (b) (c)

A 30cm diameter disc was used. Only truly vertical disc sightings were recorded. Measurements were made on the sunny side of the vessel. (The majority of readings in practice were made under cloudy or overcast conditions.)

Figure 4 represents a plot of Z~d versus l/(c + k) and a linear regression analysis on the data gives the equation: 4.81 z~d= c - ~ + 0"66 (0"27)

(0-14)

(10) standard errors

r = 0-94. Degrees of freedom = 40. This analysis yielded a value of In (Co/C,) equal to 4-81 + 0.27. Equation (10) possesses a sufficiently high degree of correlation to confirm that the measurements are consistent with the theoretical relationship as given in eqn. (5). It is when approximations are made to this simple theoretical relationship that problems arise. Using all the data collected in Liverpool Bay during 1974 a correlation coefficient of 0.84 (degrees of freedom = 40; more than 0-1% significant) was obtained between z1% and the Secchi disc depth, Z,d. The percentage of surface light present at Z,d varied between 22 % and 66 9/0for stations on the different cruises. The data from the three cruises was grouped in I0 m classes according to the values of z~% recorded at each station. Table 3 gives the mean values of optical parameters within these groups. It also includes, for comparative purposes, mean oceanic values as recorded by Otobe et al. (1977). The relationship between zt% and z,d was seen to vary for the different groups. Only in group 1, the most turbid coastal group, did the relationship approximate to the commonly used oceanic expression of ztx = 3z,d. Group 1 also yielded mean percentage light levels at the Seccki disc depths most similar to the oceanic values of Otobe et al. (1977). Values ofk and ztx comparable

OPTICAL MONITORINGOF COASTAL WATERS 10

•

305

•

8.

~'

vv

~6"

N

•

4

•

•

•

yew 2

•

I¢ 0

Fig. 4.

i

I

0"5

1"0

I

I'5 11 (C * K)

I

I

2.0 metres

2"$

Plot of z~ versus l/(c + k) for Liverpool Bay, cruises 1 to 3 inclusive.

with the oceanic results, however, appeared in group 4, the clearest coastal group. In order to understand these apparent anomalies in Table 3 it is first necessary to consider how the Secchi disc depth is related to light intensity levels. Optical relationships A relationship between z~% and Z~dcan easily be obtained by combining and rearranging eqns (4) and (5) to give: zl~ Ld

c 4-6 4.6 k In (Co~C,) + In (Co~C,)

(11)

TABLE 3 MEAN VALUES OF SPECIFIED OPTICAL PARAMETERS IN" LIVERPOOL BAY TOGETHER WITH OCEANIC VALUES

FROMOTOBEetaL (1977)

Zt~ (m) . Mean k (m- ~) Mean z ~ (m) Mean z~ (m) Mean % irradiance at zs~

Mean E(m- ~) Mean z,~/z,d

Group 1

Group 2

Group 3

Group 4

Otobe ¢t al. (1977)

0 ~ I0 0-75 6.1 1.8

I0 ~ 20 0.35 13.3 2.9

20 < 30 0.183 25.1 5.0

<30 0-116 39.7 7.0

19-65 0.113 45 14

26 3.96 3.4

36 1.73 4.6

40 1.15 5.0

44 0.70 5.7

25 0.339 3.2

306

B. J. TOPLISS

IC

v VV

V

v

V v v

v V VV V V V V ~V VV v

4' V V

0 i

VV V

V v

l

I

l

l

I

2

4

6

I

10

elk

Fig. 5.

Plot of

zt%/z,~ versus c/kfor

Liverpool Bay, cruises 1 to 3 inclusive.

Figure 5 represents a plot of zl~/zsa versus c/k from the Liverpool Bay data. The oceanic value of 3 for the ratio z~/zsd can be seen from Fig. 5 to hold only for a water mass with a c/k ratio of -,-3. A range of c/k from 2 to 10 was, however, obtained from the combined summer and winter local data and hence a constant value for the parameter z~/zsa cannot be used for such waters. The 1 ~ intensity level relationship is only a special case ofthe general formula for estimating the percentage of surface light at the Secchi disc depth. This formula can be obtained by combining eqns (2) and (5) to give:

In :Ezsl)

l

c

1

1

\ E(o) ] = k In (Co~C,) + In (Co~C,)

(12)

As with eqn. (11), the above relationship is dependent on the ratio elk. Hence, as elk increases, an increased percentage of surface light is required to fall on the Secchi disc in order to make it distinguishable against relatively increased background scattering. Incident light levels between 22 ~o and 66 ~ of surface values were needed to observe the Secchi disc under different local conditions when c/k ranged from 2 to 10. Such intensity levels were consistent with eqn. (12) and a value for in (Co~C,) of 4.8 as obtained from eqn. (9). However, the local regions of low c/k (2 to 3) did not yield intensity levels as low as those obtained in clear oceanic waters with similar elk values (Tyler, 1968). Tyler (1968) used a value for In (Co~C,) of 6.4 and Holmes (1970), working in coastal waters, obtained a value for In (Co~Q) of 9-4. Neither

OPTICAL MONITORING OF COASTAL WATERS

307

result appears consistent with the other or with the value obtained in this survey and this clearly indicates the need for a closer examination of factors lifely to affect both Co and C,. Variations in the scattering and absorption properties, and hence in the c/k ratio for a water mass, can be used to explain the apparent anomalies in Table 3. Group 4 contains data predominantly from summer months when biological activity was present. The low value of k, comparable with the clear oceanic waters examined by Otobe et al. (1977), indicates the presence of low absorption processes. A high value of c, more than double that estimated for the oceanic study, suggests the presence of large quantities of scattering material. Both eqns (11) and (12) show that the resulting higher value of c/k in this coastal group 4 must produce optical relationships inconsistent with those found in oceanic areas. Group 1, predominantly winter data, possesses the lowest average value of c/k and optical relationships most consistent with those obtained in oceanic areas with similar low c/k ratios. Group 1, however, has the highest absolute value of c and k, indicating much larger quantities of absorption and scattering material in the coastal waters than is likely to occur in oceanic waters. Oceanic areas may frequently be considered to contain only a negligible amount of scattering material; this approximation then enables their optical properties to be characterised by any single absorption-dependent parameter. The local coastal water has been seen, however, to be subject to large and variable changes in both its scattering and absorption content (as indicated by c and k). Hence, at least two independent optical measurements must be made when comparing oceanic and coastal data, or different coastal data sets. Only by considering c and k together was it possible to understand the differences in Table 3. This need to consider both c and k in coastal waters is also illustrated in Table 1, which shows that, along the Anglesey coastline, comparably small photic depths were encountered on cruises in both May and December. The larger mean value of c for the May cruise, however, indicated that this reduction of the photic zone was the result of the presence of a large quantity of scattering material, namely a bloom of Phaeocystis pouchetii. In December, a similar depth of photic zone was determined by non-biological material of a relatively higher absorption to scattering nature. The identification of the actual dissolved and suspended constituents of a water mass would only be possible using full spectral optical information.

CONCLUSIONS

Coastal waters can rarely be considered as optically homogeneous, but, in the absence of clearly definable optical structure, homogeneous formulae and parameters may be used. Random optical inhomogeneities will, however, determine the ultimate accuracy of optical measurements. A rough estimate of this inherent

308

B . J . TOPLISS

optical variability of coastal measurements may be obtained from Ac, the standard error associated with the mean beam attenuation coefficient for a profile. Workers are generally agreed that c is not a fixed multiple ofk (Tyler, 1968) but in certain waters, where only a narrow range of c/k is found, such an assumption has often been convenient. This has, however, led to the introduction of numerous relationships that are invalid in coastal waters with large ranges of c/k. It has been shown that for Liverpool Bay the ratio c/k can range from 2 to 10 throughout the year. This means that the depths of specified underwater intensity levels, including the depth of the photic zone, should be measured directly rather than estimated from empirical relationships for the Secchi disc depth. The spatial and temporal variability in the content of coastal waters makes comparison of optical data difficult. However, the ratio c/k can act as an indicator for the relative scattering to absorption properties of material found in coastal waters and the magnitudes of c and k as indicators of the quantities of such material present. Optical comparisons between coastal areas can only be made when information is available on the scattering and absorption properties of the water content. Too frequently, however, such relevant information is lacking in the literature concerning the optical properties of coastal waters. ACKNOWLEDGEMENTS

The work was conducted under an NERC research studentship. Special thanks go to the officersand crew of the R. V. Prince Madog who gave assistance during the collection of data. Thanks go to Dr J. R. Hunter and E. Pritchard for giving constructive criticism of this manuscript. REFERENCES ATKINS, W. R'. & POOL~, H. H. (1929). Photoelectric measurements of submarine illumination throughout the year. J. Mar. Biol. Ass. U.K., 16, 29%324. HOLMES,R. W. (1970). The Secchi disc in turbid coastal waters. Limnol. Oceanogr., 15(5), 688-94. HUNT, D. T. E. (1978). Factors affecting silicate and trace metal concentrations in natural waters. PhD Thesis, University of Wales. JERLOV,J. G. (1951). Optical studies of ocean water. Rep. Swedish Deep-Sea Exped., 3, 1-59. JERLOV, N. G. (1976). Marine optics. Elsevier Oceanography Series, 14. Elsevier Publ. Co. LENNOX, A. (1979). Studies onthe ecology and physiology ofphaeocystis. PhD Thesis, University of Wales. OTOSE, H., NAKAI,T. & HxrroRI, A. (1977). Underwater irradiance and secchi disc depth in the Bering Sea and the northern North Pacific in summer. Mar. Sci. Comm., 3(3), 255-70. ST~MAN-NIELSEN,E. (1974). Marine photosynthesis. Elsevier Oceanography Series, 13. Elsevier Publ. Co. TOPLISS,B. J. (1977). A study of optical irradiance in coastal waters. PhD Thesis, University of Wales. TOPPING, J. (1956). Errors of observation and their treatment. Chapman & Hall. TYLER,J. E. (1968). The Secchi disc. Limnol. Oceanogr., 3(1), 1-6. WILLIAMS,J. (1968). The meaningful use of the Secchi disc. Technical Report 45. Ref. 68-15. Chesapeake Bay Institute.