The Loch Eil project: Chlorophyll a and nutrients in the water column of Loch Eil

J. exp. mar. Biol. Ecol., 1981, Vol. 55, pp. 283-297 ElsevierlNorth-Holland Biomedical Press

THE

LOCH

EIL PROJECT: IN THE

CHLOROPHYLL

WATER

BRIAN Scottish

Marine Biological Association,

COLUMN

u AND NUTRIENTS

OF LOCH

EIL

GRANTHAM

Dunstyffnage

Marine Resc~arcll Laboratory,

Ohan, ArgJN, Scotlund

Abstract: Monthly measurements of temperature, salinity, chlorophyll a, phosphate, nitrate, and ammonia were made at three stations in Loch Eil and one station in the Lynn of Lorne, on the west coast of Scotland, during 1976 and early 1977. Dissolved inorganic phosphate ranged from co.03 to 0.91 pg-at. PO,-P/I, dissolved inorganic nitrate from co.05 to 8.4 pg-at. NO,-N/I, ammonia from < 0. I to 2.8 pg-at.NH,-N/I, and chlorophyll a from 0.02 to 5.46 mg/m’. The main freshwater input to Loch Eil is at the seaward end of its mouth. This gives unusual hydrographic characteristics with an increased proportion of freshwater in the water column. Compared with Loch Creran, of similar dimensions and freshwater input, Loch Eil has a lower standing crop of phytoplankton, higher light attenuation from humic compounds, and a deeper surface mixed layer. There were no significant differences between stations in Loch Eil except for chlorophyll a at 15 m depth and below which decreased with distance from.the mouth. Many of the changes in the water column of Loch Eil appear linked to changes in rainfall which affect water column stability and loch circulation.

INTRODUCTION

Phytoplankton production and nutrient cycling in Scottish sea lochs depend largely on local weather which, although highly variable, is dominated by a high rainfall (average of 2000 mm per annum, Fleming & Walker, 1981). Rainfall produces freshwater run-off, which stabilizes near-surface layers thus allowing phytoplankton to remain in the photic zone (Solorzano & Grantham, 1975). Excessive run-off might, however, lead to dilution of the phytoplankton and produce “wash-out”. Although run-off is deficient in certain nutrients such as phosphate (Jones, 1979; Solorzano & Ehrlich, 1979) the two-layer circulation engendered by run-off in a fjordic sea loch might renew surface nutrients. Nutrient distribution

in deep water

is affected

by the deep water renewal

process

which

is

often intermittent and is controlled largely by the effects of run-off on the sea water supply to the loch in question (Edwards & Edelsten, 1977; Edwards et al., 1980). Loch Eil is unusual in that the amount of fresh water entering the loch directly from rivers and streams is small (on average 7.3 x lo5 m’/wk), but a far greater amount (90-220 x lo5 m’/wk) enters at the head of Loch Linnhe (Johnston & Topping, 1972) and may enter Loch Eil through the Annat Narrows after mixing. Loch Eil might thus be expected to differ from more typical sea lochs in its phytoplankton production and in some aspects of nutrient cycling. In view of the possible 0022-098 1/81/0000-0000,/$02.50

0 1981 ElsevieriNorth-Holland

Biomedical

Press

BRIAN GRANTHAM

284

importance ation,

of algal input

the distribution

to the benthos,

of nutrients

and of the benthos

and phytoplankton

in nutrient

biomass

Loch Eil between November 1975 and March 1977, in parallel described in this series and introduced by Pearson (198 1). This paper presents results of monthly measurements dissolved inorganic nutrients, temperature and salinity and at a comparative station in the Firth of Lorne.

regener-

was investigated

in

with the other studies

of planktonic chlorophyll a, at three stations in Loch Eil

METHODS Surveys at approximately monthly intervals were carried out over the period November 1975 to March 1977. Three stations were qmpled in Loch Eil (E2, E70, and E24). A fourth station (LYI), located in the Lynn of Lorne, north of Oban, was sampled as a control station. A map of the area showing the station positions is given in Pearson (1981). TEMPERATURE AND SALINITY Temperature and salinity were measured in situ using an E.I.L. T/S bridge, model M.C.5. This was fully calibrated every 6 to 12 months in the laboratory against a Hewlett-Packard Quartz Thermometer and an Autolab Precision Salinometer. In the field the accuracy of each T/S profile was checked by using paired protected reversing thermometers at selected depths and by taking water samples for measurement on the Precision Salinometer. Unreliable bridge data were rejected. Station LYl was also sampled for temperature and salinity at irregular intervals from March 1977 to February 1980 as part of a study of Loch Creran. Some of this data has been included, by kind permission of Dr. P. Tett, in order to show the year-to-year changes that have occurred at this station. Dunstaffnage rainfall data was provided

by N. Pascoe;

Loch Eil and probably catchment area.

the Laboratory

receives

is 5 km from Station

only 75% of the annual

rainfall

LY 1,43 km from over the Loch Eil

NUTRIENTS AND CHLOROPHYLL a Water

for nutrient

analysis

was taken

using

N.I.O.

polypropylene

sampling

bottles. The sampling depths were generally 0, 5, 10, 15, and 30 or 40 m at all stations plus 60 m at Stations E2 and E70. There was some variation in the depths of the deepest samples from one sampling trip to another depending on the maximum depth found. Generally the deepest samples were taken within 5 to 10 m of the bottom. Each sample was divided into two parts and then filtered through Whatman GFjC filters which had been pre-baked at 500°C for 30 min. One filter was used

CHLOROPHYLL

for chlorophyll analysis

a AND NUTRIENTS

a determination,

the results

filter was separately

the other

of which will appear wrapped

in pre-baked

for particulate

in a separate aluminium

then freeze dried at the earliest opportunity. The filtrate was transferred to polyethylene means of “Dri-Kold” (solid CO,) for transfer

285

IN WATER COLUMN

paper.

carbon After

and

nitrogen

filtration

each

foil and stored deep frozen,

bottles and was rapidly frozen by back to the laboratory deep-freeze.

A number of workers have reported marked changes in nutrient concentrations of sea-water samples during storage (e.g., Gilmartin, 1967; Grasshoff, 1976). The adoption of a standardized procedure for filtration, freezing, and storage helped to minimize the variability between samples due to these changes. Analyses were carried out within one month of sampling. Dissolved inorganic phosphate was determined by the method of Murphy & Riley (1962) nitrite by the method of Bendschneider & Robinson (1952), and nitrate first by reduction to nitrite with a copper-cadmium column (Wood et al., 1967) and then by determination of the nitrite. The original nitrite concentration was subtracted. Ammonium chloride was used as a pH buffer. Ammonia was determined by the phenol-hypochlorite method of Solorzano (1969). All these procedures are described by Strickland & Parsons (1972) which was used as the main source of experimental detail. Chlorophyll a was determined by extracting the filters in 90% acetone and measuring the fluorescence of the extract, before and after acidification, with a Turner model III fluorometer (Tett & Wallis, 1978). The fluorometer was calibrated annually against a Unicam SP 500 spectrophotometer using an extract of pure chlorophyll a (Sigma Chemical Co.). SECCHI DISC

A Secchi disc 27 cm in diameter, estimates

of the depth of the euphotic

and painted

matt white was used in obtaining

zone. According

to Holmes

(1970) the factor

relating the Secchi disc depth to the depth of the euphotic zone (taken as the depth reached by 1% of the ambient radiant energy) itself varies with the Secchi disc depth, being about 3.5 for depths < 5 m and x2.0 for depths between 5 and 12 m, for turbid inshore waters. For the best results it is necessary to calibrate the disc against a photometer and to use it under standardized conditions (Tyler, 1968). The disc used in this study was, however, not calibrated and was used under a variety of different lighting and sea surface conditions. The results, therefore, give only a rough indication of the transparency of the water.

BRIANGRANTHAM

286

RESULTS TEMPERATURE,SALINITYAND TRANSPARENCY In Loch Eil the pycnocline was usually salinity-determined except during dry spells in summer. At Stations E70 and E2 it varied between 10 and 15 m, but was slightly shallower at Station E24, at the head of the loch. There was little variation otherwise between the three stations in Loch Eil and Figs. 1, 3, and 5 give results from E70 only.

0’ j75176 ’ u Fig. 1, The annual

temperature

0’ ” 75176” Fig. 2. The annual

s n

”

temperature

L

I

L I

cycle at Station

”

”

cycle at Station

4 c I 76’77

’

E70: gaps are due to insufficient

j

” 76177

”

data

”

LY 1 : gaps are due to insufficient

data

CHLOROPHYLL

a AND

NUTRIENTS

IN WATER

287

COLUMN

Fig. 3. Salinity

minimum,

median

(thin central

line), and maximum

at Station

E70.

Fig. 4. Salinity

minimum,

median

(thin central

line). and maximum

at Station

LY

1.

BRIAN

288

GRANTHAM

Although the annual temperature cycle at Station LYl (Fig. 2) was similar to that in Loch Eil, with differences less than the year-to-year variation at LYl 300r

I

’ Fig. 5. Monthly

I

III

I,

j,

N$JJFMAMJJ

rainfall

I,

I,,

I,.

i

ASONL&FMAMJ

’

at Dunstaffnage (correct horizontal scale) and Secchi disc depths at Stations and LYI (displaced 1 month to the right, to show lag).

E70

TABLE I Temperature

(“C) maxima

and minima at Stations E24, E70, and E2 for 1976 and and at Station LYI for 1976 to 1978.

1977 (minima

only)

Station LYl Year

Depth (m)

E2

E70

E24

Max.

Min.

Max.

Min.

Max.

Min.

Max.

Min.

13.6 13.4

5.7 7.0

13.0 13.3

4.9 7.2

12.4 13.0

5.6 7.2

6.3 6.6

_ _

6.9 6.5

1976

0 40

12.9 13.8

6.6 7.3

1977

0 40

14.2 11.9

6.5 6.7

1978

0 40

13.4 12.7

6.0 6.3

7.0 6.5

(Table I), mean salinities at LYl (Fig. 4) were greater than those in Loch Eil and a pycnocline was often absent. Secchi depth was deeper at LYl than at E70 and corresponded to rainfall only at E70 (Fig. 5).

%o 35TZdinity

Fig. 6. Monthly rainfall at Dunstaffnage, salinity at Station E70. and isopleths of phosphate (P), nitrate (N), ammonia (A), chlorophyll a (C), and acid ratio (AR) at Station E70: P, N and A. pg-at./l; C, mg/m3; monthly rainfall, mm; depth, m.

290

BRIAN

DISSOLVED

INORGANIC

In general,

GRANTHAM

NUTRIENTS

levels of phosphate

and nitrate

were the same at Stations

and LY 1 (Fig. 7). At E70 there was, however,

greater stratification

nitrate.

deep water and winter

At LYl there were somewhat

higher

E70 (Fig. 6)

and lower surface nitrate

concen-

trations, reaching >8 ,ug-at. NO,-N/l compared with < 7 at E70. Ammonia concentrations were often greater at E70 than at LY 1. In 1976 surface minima of phosphate and nitrate occurred in May at E70 ( ~0.1 pg-at. PO,-P/l, < 1 fig-at. NO,-N/l) but not until June at LYl (~0.03 pg-at. PO,-P/l, ~0.05 pg-at. NO,-N/l). At both stations deep water minima generally occurred later in the year; a phosphate minimum of ~0.4 pg-at. PO,-P/l was in July at E70, and nitrate minima of < 1 pg-at. NO,-N/l was in August at E70 and LY 1. Ammonia was highest between .4pril and November ( > 1 pg-at. NH,-N/l) in E70 deep water, and between March and September ( >0.6 pg-at. NH,-N/l) in LYl deep water. Some special features

Fig. 7. Isopleths

of phosphate (P), nitrate (N). ammonia (A), chlorophyll n (C), and acid ratio (AR) at Station LYl: P, N and A, pg-at.J; C, mg/m’; depth, m.

CHLOROPHYLL

were shown April

and

a AND NUTRIENTS

in deep water phosphate September.

Related

IN WATER

at E70, which reached

events

in autumn

291

COLUMN

secondary

1976 are shown

maxima

in more

in

detail

in Fig. 9 (see p. 295). CHLOROPHYLL

a

The range of chlorophyll a values found in Loch Eil was considerably greater than that found at Station LY 1. At Station E2, for instance, surface values ranged from 0.05 mg/m3 in winter to a peak of 5.5 mg/m’ in May. At LYl the corresponding range was 0.13 mg/m3 to 2.64 mg/m’ (Figs. 6 and 7). With a sampling interval of one month, however, short period fluctuations went unrecorded and some extreme values were probably missed. During most of the spring bloom period surface chlorophyll a values at Station LYl were lower than those at 10 m and the corresponding acid ratios were lower also. In Loch Eil surface chlorophyll a values were almost always higher than those in deeper water. This difference was least during the spring bloom period when chlorophyll a values in the deep water approached surface values and had high acid ratios. At other times in Loch Eil, however, the chlorophyll a values in water below

10 m were considerably

lower than those at the surface.

DISCUSSION HYDROGRAPHY

The hydrography of Loch Eil has been outlined by Milne in more detail by Johnston & Topping (1972). An important

(1972) and discussed feature is the source

of the freshwater supply to the loch. In a typical sea loch such as Creran (Landless & Edwards, 1976) the major freshwater input is at the head where it forms a thin surface layer. This moves seawards and tends to deepen as it entrains saline water from below. In contrast, Loch Eil receives only minor amounts of fresh water directly from rivers, the major amount Linnhe and enters Loch Eil at its mouth

comes from sources at the head of Loch through the Annat Narrows on flood tides.

This gives a partially mixed brackish layer 10 to 15 m deep in the loch. Because the surface water outside the loch may be more brackish than that inside brackish conditions tend to persist in Loch Eil and the effect of the fresh water is, therefore, considerably accentuated. The water column structure in Loch Eil bears more resemblance to that in lower Loch Etive (Wood et al., 1973) than Loch Creran despite Loch Etive receiving six or seven times more freshwater run-off in relation to its mean tidal volume than Lochs Creran or Eil. The ratio of mean freshwater inflow to mean tidal volume is, for Loch Etive, 1 : 8 (Wood et al., 1973), for Loch Creran, 1 : 60 (Tett & Wallis, 1978) and for Loch Eil, 1 : 42 (assuming that half of the run-off given by Johnston & Topping (1972) for Loch Linnhe enters Loch Eil).

BRIAN GRANTHAM

292

The density column,

stratification

produced

by the fresh water gives stability

but in Loch Eil the water above the pycnocline

mixed as a result of the strong the winter

the higher

rainfall

tidal movements increases

water

tends to be at least partially

in the entrance column

to the water

stability

to the loch. During and wind mixing

is

restricted to the surface layers. A period of low rainfall (as in December 1976) can very occasionally reduce the stability and allow mixing of the whole water column in the following months. This can be seen by comparing the phosphate, nitrate, and chlorophyll a isopleths for the period December 1975 to February 1976 with those for the same period a year later (Fig. 6). Stratification evident in the first winter was absent in the second. PHYTOPLANKTON

The growth of phytoplankton in sea lochs is governed largely by water column The stability comes from density stability, nutrient supply, and illumination. stratification which in Scottish sea lochs is almost wholly due to the freshwater input. This stability affects growth in two opposing ways. By reducing mixing of the surface layers it allows the phytoplankton near the surface to maintain their position and receive illumination, and by reducing mixing it also reduces the supply of nutrients from the deeper water when the surface nutrients have been depleted, thereby limiting the phytoplankton growth. In a fjordic circulation system (Edwards & Edelsten, 1976) the seaward flowing surface layer carries the surface phytoplankton out of the loch. Entrainment of deep water might enrich surface nutrients, but if there is excessive run-off the phytoplankton will be diluted and the dispersion will be increased. West coast weather is wet but variable. Freshwater run-off and subsequent surface stability conditions often fluctuate rapidly, in response to the weather pattern. An alternation

of low and high stability

might provide

both for good mixing of nutrients

to the surface and also stable surface layers at some (but not the same) time during each cycle. Thus it is possible that fluctuating conditions may actually benefit the phytoplankton production of a sea loch. In Loch Eil, however, surface salinities probably do not change as rapidly as those in typical lochs (for which Edwards et al. (1980) suggest a response time of about a week) because of Loch Eil’s unusual hydrography. Studies by Johnston & Topping (1972) of dye injected into the Annat Narrows through the pulp mill effluent pipe show a retention time of x 10 days for the upper 10 m at stations near the lower end of the loch and retention times of 2 to 3 wk at stations near the head. In view of the enhanced effect of fresh water in Loch Eil it is not surprising that surface chlorophyll a values compare more with those in the high run-off Loch Etive for Station E6 (Wood et al., 1973) than with those found in the moderate run-off Loch Creran by Tett & Wallis (1978). With a sampling interval of one month it is, however, likely that a considerable part of the spring bloom, including the peak,

CHLOROPHYLL

may have been chlorophyll

missed

a values

a AND

in Loch

NUTRIENTS

Eil. It is not valid,

for Loch Eil with those

has been more (or less) frequent.

IN WATER

Instead

therefore,

for other

293

COLUMN

to compare

lochs where

peak

the sampling

values for Loch Eil are compared

in Fig. 8

-22

JFMAMJJASUND

Fig. 8. Envelope (between continuous lines) of all chlorophyll data from Loch Creran l-8 zone from 1972 to 1976 (from Tett & Wallis, 1978) with Loch Eil data superimposed

m depth

with a plot of the general annual cycle of phytoplankton standing crop in Loch Creran, 1972-1976 (Tett & Wallis, 1978). The upper and lower limits of the envelope are drawn so as to include 95% of the Creran data. Tett & Wallis selected data from the 1 to 8 m depth range as being representative of the brackish water layer in Loch Creran. The brackish layer in Loch Eil is generally deeper than that in Loch Creran and more mixed, with less stratification at the surface. The data from the depth zone 0 to 10 m from Stations E2 and E70 were, therefore, taken as being representative of the brackish layer in Loch Eil. Fig. 8 shows that the spring increase started considerably later in Loch Eil than in Loch Creran and seems to have peaked later. Midsummer values in Loch Eil were similar to those in Loch Creran, but autumn and winter values were distinctly lower in Loch Eil. These differences do not seem related to differences in inorganic nutrient supply between the two lochs. Winter nutrient values in Loch Creran (Jones, 1979; Solorzano & Ehrlich, 1979) were

comparable

with

those

in Loch

Eil.

The

drop

in

surface

nitrates

and

phosphates in the spring was two months later in Loch Eil than in Loch Creran and the return of the surface nitrates started about the same time (September) in both lochs. Thus, the supply of inorganic nutrients in Loch Eil seems adequate for phytoplankton growth in both spring and autumn. It is more likely that the differences in standing crop between the two lochs are due to differences in water column stability and illumination, both of which are related to the freshwater run-off. The brackish surface layer in Loch Eil is deeper than that found in Loch Creran and is less stratified. Therefore, although the overall

294

BRIAN

stability

of the water column

the stability would The

within

not matter phytoplankton

is greater

the surface if the depth

GRANTHAM

in the former

because

mixed layer is probably of the photic

in the brackish

layer

zone exceeded would

of the strong halocline

less than in the latter.

then

This

that of the halocline.

always

receive

sufficient

illumination for growth, but this only happens infrequently in Loch Eil. The depth of the photic zone estimated from the Secchi disc readings usually lay between 6 and 11 m compared with the pycnocline depth of 10 to 1.5 m. In this situation the phytoplankton will spend part of the time below the photic zone where there is no growth. The transparency of the surface water is related to the presence of humic compounds brought in by the freshwater run-off. Fig. 5 illustrates the difference in Secchi disc readings between Station E70 in Loch Eil and Station LYl which is much less affected by run-off. The water in Loch Eil is clearly less transparent than that at Station LY 1. Surface layer salinity reductions in Loch Creran are similar to those at Station LYl, representing a dilution by fresh water of only 5-10%. In Loch Eil the surface layer salinity reductions represent a dilution by fresh water of 20% or more. Assuming that there is a proportional relationship between the freshwater volume and the amount of humic compounds (Solorzano & Ehrlich, 1977, 1979) it is likely that the surface waters in Loch Eil contain considerably more humic compounds than those in Loch Creran and will have a correspondingly higher attenuation coefficient. In addition, the more mountainous terrain surrounding Loch Eil might affect the amount of cloud cover and thereby decrease the amount with Loch Creran, but there are no data to confirm this.

of illumination

compared

NUTRIENTS

An adequate supply of nutrients is essential for phytoplankton growth and again freshwater run-off is the principal factor in controlling the distribution of both surface and deep nutrients. Freshwater input to Loch Etive (Solbrzano & Ehrlich, 1977) and to Loch Creran (Jones, 1979; Solorzano & Ehrlich, 1979) is deficient in phosphate

but contains

variable

amounts

of nitrate

and ammonia.

It is likely that

the run-off to Loch Eil is essentially similar and there appeared to be some association between low surface phosphates and low salinities. The reduction in all surface nutrients during the spring and early summer seems likely to have been largely the result of uptake by the phytoplankton. Growth could only take place in the surface layers, therefore, the gradual reduction in nutrients in the deeper water must have resulted from mixing processes in the water column, and to some extent from inflows of nutrient-depleted water from outside the loch. The regeneration of nutrients in the sediments of the loch and their subsequent return to the water column are important processes in the supply of nutrients to the phytoplankton. With regular renewal of the deep water (Edwards et al., 1980) there is no build up of nutrient levels in the water overlying the sediment. When

CHLOROPHYLL

renewal

ceases or slackens

bottom

water.

a AND

NUTRIENTS

for a long enough

In September

to November

IN WATER

period

distinct

1976 increasing

295

COLUMN

changes

rainfall

occur

reduced

in the or may

even have stopped deep water renewal in Loch Eil. Phosphate and nitrate values increased, ammonia values increased then fell slightly, and acid ratios fell indicating senescence of the phytoplankton (Fig. 9). In December the rainfall dropped to less

Fig. 9. Changes in phosphate water (60 m) at Station

(PO,), nitrate (N03), ammonia (NH,) and acid ratio (AR) in the deep E2 during August to December 1976: PO,, NO;, and NH,. pg-at.4.

than half the November figure, and a reversal of the previous trends in the deep nutrients was found. Phosphate and ammonia values fell, the ammonia to below the level of detection. Acid ratios rose indicating an influx of more healthy phytoplankton. Nitrate continued to rise. These changes did not result from an increase in the rate of deep water renewal, for this would have increased salinities in the deep water. Instead salinities fell and it is apparent from changes in the upper part of the water column that mixing of the water had taken place. It is presumed

that

the drop

in rainfall

during

December

reduced

the water

column

stability and allowed the winter gales and tidal forces to mix the whole column of water. A similar increase in nutrients was observed by Mortimer (1971) in Esthwaite Water during the summer and autumn stratification of the water column. The regeneration of inorganic phosphate and ammonia proceeded rapidly as the oxygen concentration fell below 1 mg/l. The nitrate levels remained low until the overturn supplied sufficient oxygen for the oxidation of the ammonia. With the overturn phosphate and ammonia rapidly fell. In Loch Eil the nitrate values steadily increased throughout the autumn and winter and it is likely, therefore, that oxygen was not completely depleted.

296

BRIAN GRANTHAM

PHYTOPLANKTON

AND NUTRIENT DISTRIBUTION

WITHIN THE LOCH

Benthic fauna1 differences between Stations E24 and E2 have been reported by Pearson (1971). No major differences in nutrients, temperature, and salinity were, however, found in this investigation between the three Loch Eil stations. The only differences, in the bottom water, are due to vertical distribution. In general, nutrients increased with depth and hence the deeper stations had higher nutrient levels in their overlying water. At the same depth there was no significant difference between stations. Differences were found, however, in the phytoplankton distribution. At the 15-m level there was a significant difference between both the logarithmically transformed (Barnes, 1952) standing crop (t-tested for paired comparisons, F = 5.5,d.f. = 1, 14, P < 0.05) and acid ratio (F = 31.2, d.f. = 1, 14, P < 0.001) for Stations E24 and E2. The standing crop and acid ratios at Station E24 were lower than those at Station E2. These differences cannot be attributed to a higher surface standing crop at E2 because there was no significant difference in surface chlorophylls or acid ratios between the two stations. It is likely that the phytoplankton at 15 m and below originate outside Loch Eil and the differences between the two stations reflect the time lag (of several weeks) in water movement towards the head of the loch during which the phytoplankton tend to die and settle.

ACKNOWLEDGEMENTS

I am indebted to Miss Lucia Sol6rzano for initiating the chemical study and for her encouragement. I also thank P. Tett for his advice, for critically reading the manuscript, and for supplying some T/S data, N. Pascoe for supplying rainfall data, J. Shaw for drawing the figures, and L. MacNaughton for T/S analysis.

REFERENCES BARNES, H., 1952.

The use of transformations

in marine biological statistics. J. Cons. perm. int. Explor.

Mer, Vol. 18, pp. 61-71. BENDSCHNEIDER,K.

& R. J. ROBINSON,1952. A new spectrophotometric method for the determination of nitrite in sea water. J. mar. Res., Vol. 11,pp. 87-96. EDWARDS,A. & D. J. EDELSTEN,1976. Marine fish cages the physical environment. Proc. R. Sot. Edinb. Ser. B., Vol. 75, pp. 207-221. EDWARDS, A. & D. J. EDELSTEN, 1977. Deep water renewal of Loch Etive: a three basin Scottish fjord. Estuar. cstl mar. Sci., Vol. 5, pp. 575-595. EDWARDS,A., D. J. EDELSTEN,M.A. SAUNDERS& S. 0. STANLEY,1980. Renewal and entrainment in Loch Eil; a periodically ventilated Scottish fjord. In, Fjord oceanography, edited by H. J. Freeland, D. M. Farmer & C. D. Levings, Plenum Press, New York, pp. 523-530. FLEMING,G. & R. A. WALKER,1981. The Loch Eil Project: simulation of the hydrography and sediment inputs to Loch Eil. J. exp. mar. Biol. Ecol., Vol. 55, pp. 103-113. GILMARTIN,M., 1967. Changes in inorganic phosphate concentration occurring during seawater sample storage. Limnol. Oceanogr., Vol. 12, pp. 325-328.

CHLOROPHYLL

a AND

GRASSHOFF, K., 1976. Melhods of seawater

NUTRIENTS

IN WATER

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297

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