Circulation and sedimentation of suspended particulate matter in New Zealand fjords

Marine Geology, 74 (1987) 21-39 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

21

CIRCULATION AND SEDIMENTATION OF SUSPENDED PARTICULATE MATTER IN NEW ZEALAND FJORDS R.A. P I C K R I L L * New Zealand Oceanographic Institute, Division of Marine and Freshwater Research, DSIR, Wellington (New Zealand)

(Received December 26, 1985; revised and accepted May 12, 1986)

Abstract Pickrill, R.A., 1987. Circulation and sedimentation of suspended particulate matter in New Zealand fjords. Mar. Geol., 74: 21-39. Circulation in the New Zealand fjords is estuarine, with a seaward-flowing brackish layer overflowing saline water. Despite an extremely high rainfall (>6000 mm yr-1), terrestrial sediment input is low and concentrations of suspended particulate matter (SPM) rarely exceed 1 mg 1-1. The terrestrial influence is barely detectable anywhere in the fjords; SPM in near-surface waters is dominated by phytoplankton. In surface waters, SPM components remain separate, as they settle they are clustered into larger groups by agglomeration, faecal pelletization and flocculation. Phytoplankton produce a secondary SPM maximum at the top of the deep zone. Deep waters are barren of living plankton with clustered SPM and broken phytoplankton settling from above. The low terrestrial SPM input and high organic/biogenic concentrations are reflected in low sedimentation rates and high organic carbon content of bottom sediments. The estuarine stratification is broken up in the fjord entrances by ocean waves which also resuspend sediment off the entrance sills thereby increasing SPM concentrations in bottom waters. High SPM concentrations are found in basins cut off from the main fjord by shallow sills which inhibit deep-water renewal.

Introduction R i v e r - t r a n s p o r t e d m a t e r i a l s c o n t r o l sedim e n t a t i o n in fjord estuaries. Rivers r a p i d l y lose e n e r g y on e n t e r i n g fjords a n d bedload is deposited on deltas. S u s p e n d e d load is j e t t e d o u t into the fjord as a b u o y a n t plume a n d is the m a j o r s o u r c e of s e d i m e n t in the r e m a i n d e r of the fjord. T r a n s p o r t of s u s p e n d e d p a r t i c u l a t e m a t t e r (SPM) w i t h i n the fjord is d r i v e n by the e s t u a r i n e c i r c u l a t i o n . S P M - r i c h fresh w a t e r e n t e r i n g the fjord overflows d e n s e r saline w a t e r a n d flows s e a w a r d as a t h i n skin,

*Present address: New Zealand Oceanographic Institute, Private Bag, Kilbirnie, Wellington, New Zealand. 0025-3227/87/$03.50

e n t r a i n i n g and m i x i n g w i t h the salt water. F a r r o w et al. (1983) liken t r a n s p o r t of S P M down-fjord to a p e r f o r a t e d c o n v e y o r belt in w h i c h t h e r e is a rapid d e c r e a s e in S P M a l o n g the s u r f a c e l a y e r a n d with depth. S e t t l i n g of S P M from the surface l a y e r is n o t a simple process a n d Syvitski and M u r r a y (1981) h a v e put f o r w a r d a model of p a r t i c l e d y n a m i c s w h e r e b y fluvial S P M e n t e r s s i n g u l a r l y , from w h i c h the c o a r s e r sediment m a y settle d i r e c t l y a n d the finer m a t e r i a l a g g l o m e r a t e a n d flocculate in the halocline. G r a z i n g by z o o p l a n k t o n i n c o r p o r a t e s m u c h of this m a t e r i a l into m i n e r a l - b e a r i n g faecal pellets w h i c h settle t h r o u g h the w a t e r c o l u m n at faster r a t e s t h a n the smaller i n d i v i d u a l particles. Outflow of surface w a t e r is b a l a n c e d by i n t r u s i o n s of

© 1987 Elsevier Science Publishers B.V.

22 denser higher salinity at depth which transport SPM back up fjord (Baker, 1984). Globally, fjords extend over a broad latitudinal range (40°-85°), and encompass a variety of sedimentological environments. At one extreme circulation and sedimentation in the Arctic fjords are strongly seasonal and driven by a very short but intense meltwater season and sea ice is an important sedimentological agent (Gilbert, 1983). Farther south, beyond the influence of sea ice, the extremes of a continental climate still drive a strongly seasonal cycle in fjord sedimentation (e.g. Farrow et al., 1983). In the southern hemisphere the fjords of New Zealand form the temperate extreme of fjord environments (Pickard and Stanton, 1980). Between 45 ° and 47°S they are closer to the equator than the northern hemisphere fjords while the island location produces a mild oceanic climate, with only weak seasonality and no significant glacial input to the fjords. Sand and gravel deltas and muddy basins (Pantin, 1964; Glasby, 1978) suggest sedimentation in the New Zealand fjords can be separated into that controlled by bedload and suspended load transport. However, little is known about SPM levels, circulation, and deposition. In this paper these processes are described and modeled and important differences between fine sediment deposition in these temperate fjords and the cooler fjords of higher latitudes are highlighted.

define the surface layer. Bottle depths of 0, 2, 4, 6, 10, 15, 20, 30, 50 m and then standard depths to near the bottom were generally used. Details of temperature, salinity and dissolved oxygen determination are given in Stanton (1984) and data summarised by Greig (1978, 1983, in prep.). SPM concentrations were only determined in the summer and winter surveys. One litre water samples were filtered through preweighed 0.8#m Nuclepore filters using a method described by Strickland and Parson (1968). SPM weights retained on the filters were low. However, polycarbonate membrane filters are extremely stable and experiments with blanks produced mean weight variations of 0.07 mg with a standard deviation of 0.04 mg (Pickrill et al., 1986). Scanning electron microscopy with a Cambridge 250 MKII SEM connected to a Link 860 X-ray dispersive analyser was used to identify and size SPM components. Detailed analyses of SPM composition were carried out on t w o sections in Preservation Inlet: a vertical profile in the deepest basin of Long Sound, and a longitudinal section in the shallow zone at 2 m depth. Samples from the other fjords were inspected but not subjected to the same rigorous counting procedure. Counting was done at 3000 x magnification on the SEM. The screen was overlain with a 10 × 10 grid and presence/absence counts made in each square. At least 2000 counts were made on each sample, over 40,000 counts in total.

Methods The New Zealand fjords Three seasonal surveys of water properties in the New Zealand fjords were carried out in December 1977 (spring), 23 March-7 April 1980 (late summer/autumn) and 12-21 July 1983 (winter). The 1977 cruise was the first systematic survey of all fjords (Stanton and Pickard, 1981). The 1980 and 1983 surveys only visited Milford Sound, George Sound, Thompson/ Bradshaw Sound and Preservation Inlet (Fig.l) as representative examples of the range of fjord types identified from the 1977 survey (Stanton and Pickard, 1981). Reversing bottle casts were made with the upper bottles closely spaced to

The southwest coast of South Island, New Zealand, is indented by 14 major fjords. The fjords, for the most part, incise gneisses and granites of the Fiordland Mountains which rise 2760 m steeply from the coast. Rainfall is among the highest in the world, and increases with elevation and exposure to the main rainbearing airflows from the west (Sansom, 1984). Mean annual rainfall at Milford Sound is 6267 mm, increasing to more than 8000 mm at higher elevations. Seasonality is weak, winter is the driest period (Fig.2), when some of the

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Fig.1. Map of Fiordland South Island New Zealand, showing the four fjords studied and stations sampled. precipitation is also locked up in high-altitude snow fields. There is only one small permanent ice field within the area. River catchments are small with largest inflow entering at the head of the fjords (Table 1). Only the Cleddau River, draining into Milford Sound has been gauged, and flows show a similar seasonal cycle to rainfall, with highest inflows in late summer/autumn (43 m 3

s-1, Fig.2, March, April) and lowest inflows in winter (15 m 3 s -1, June, July, August). The Cleddau catchment is considered to be the representative basin for the entire fjord region (Toebes and Palmer, 1969). The steep, small catchments respond rapidly to rainfall, with daily peak rainfalls coincident with peak river flows (Stanton and Pickard, 1981); high river flows can come at any time of the year. The

24 TABLE 1 Catchment areas of four New Zealand fjords

Milford Sound George Sound Thompson/Bradshaw Sound Preservation Inlet

19BO Survey

Total catchment (km 2)

Fjord-head catchment (km 2)

542 305 442 562

387 130 185 159

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Fig.2. Mean annual rainfall at Milford (Sansom, 1984) and mean monthly Cleddau River discharge 1967-1977 (excluding 1974) from Stanton and Pickard (1981). Times of the two surveys (1980, 1983) are marked.

high rainfall produces prolific vegetation and a dense forest extends from sea level to the tree line at approximately 1000 m. Run-off is rich in "gelbstoff", with principle components of fluvic and humic acid (Peake, 1978), giving a distinct yellow-brown colour to both river and surface waters in the fjords. In a comparison of Pacific fjords Pickard and Stanton (1980) show that the New Zealand inlets have typical fjord bathymetry, with deep basins behind shallow entrance sills (Fig.3). However, very shallow sills are rare, nowhere do they extend into the freshwater zone and hinder flow of saline water from outside. The smaller fjords tend to be single basins whereas the large/interconnected fjords may be broken into several basins by inner sills. The Fiordland coast marks the Alpine transform bound-

25.3 32.9 49.3 93.0

21.4 9.3 9.0 6.0

ary between the Indian and Pacific plates. As a result, development of the continental shelf has been minimal, with most entrance sills dropping away steeply into several thousand meters of water in the Tasman Sea (Van der Linden and Hayes, 1972). Only off the southernmost fjords (Figs.1 and 3) does the shelf broaden to more than a few hundred meters. Fetch is unlimited in the southern oceans and the fjord coast is exposed to prevailing west and southwest storm and swell waves. At the nearest wave recording site (40 km southeast of Preservation Inlet 46.6°S, 167.2°E) the prevailing deep-water wave is 3.5-4.5 m significant height and 10-12 s period (Pickrill and Mitchell, 1979). Deep-water offshore allows these large waves to reach the coast virtually unrefracted. In the narrow fjords wave energy is absorbed within the entrance. In the more open inlets swell may travel several kilometers up-fjord, well landward of the entrance sill.

Physical oceanography The physical oceanography of the fjords, and particularly of the four fjords outlined in this paper, have been described by Garner (1964), Stanton and Pickard (1981) and Stanton (1984, 1986). In summary, the inlets have typical fjord-estuary temperature and salinity structure. A relatively low-salinity surface layer, the s h a l l o w zone, overlies a marked halocline at 5-10 m depth. During most surveys salinity increased with depth through the shallow zone

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Fig.3 (continued). (d) Thompson/Bradshaw Sound in winter (1983). Note two different scales are used for a and d and b and c, yet the vertical exaggeration remains constant at approximately 50 x. (type 2 profiles of Pickard, 1967). S u r f a c e salinity is a f u n c t i o n of r e l a t i v e f r e s h w a t e r inflow; down-fjord decreases in salinity reflect steady inflow, w h e r e a s mid-fjord lows in salinity reflect earlier h i g h e r inflows m o v i n g down fjord. In summer, surface w a t e r s are warm (12-17°C) and t h e r e is a s h a r p t h e r m o c l i n e c o i n c i d e n t with the halocline. D u r i n g w i n t e r this t e m p e r a t u r e g r a d i e n t reverses, with a subsurface t e m p e r a t u r e m a x i m u m (12.0-12.3°C at 50-100 m), or t e m p e r a t u r e i n v e r s i o n surface to b o t t o m in the deep zone. Salinity and dissolved o x y g e n in the deep zone suggest t h a t the main basin of most fjords is well ventilated. However, in P r e s e r v a t i o n Inlet several shallow sills (Fig.3a, Adam Head) c o n s t r i c t the passage of deep water. In small fjord-head basins, cut off from the main fjord by shallow sills, o x y g e n depleted b o t t o m w a t e r was found (e.g. Figs.1 and 3, Deep W a t e r Basin Milford Sound; Precipice Cove B r a d s h a w Sound; Isthmus S o u n d P r e s e r v a t i o n Inlet).

SPM c o n c e n t r a t i o n Offshore S e a w a r d of the fjord e n t r a n c e s (Figs.1 and 3) SPM was n o r m a l l y highest n e a r the surface,

but c o n c e n t r a t i o n s were low and r a r e l y exceeded I mg l - 1. The depth of this l a y e r varied from 4 0 - 5 0 m off G e o r g e S o u n d down to 70-120 m off T h o m p s o n Sound. In deep w a t e r (100-1200 m) SPM was t y p i c a l l y less t h a n 0.3 mg l-1 a l t h o u g h it o c c a s i o n a l l y increased in b o t t o m waters. T h e r e was no obvious seasonality in c o n c e n t r a t i o n . Within the fjords The shallow zone F r o m the surface down to 10 m SPM concent r a t i o n s were low and only exceeded 1.0 mg l - 1 in a few s u m m e r samples from the h e a d of Milford S o u n d (Fig.3). In most fjords levels were a few points mg 1 1 h i g h e r in s u m m e r t h a n w i n t e r (Figs.3 and 4). With such low c o n c e n t r a t i o n s any along-fjord t r e n d s were v e r y weak. D e p t h - i n t e g r a t e d m e a n S P M e i t h e r increased or d e c r e a s e d seaward or showed midfjord highs or lows (Fig.4). No p e r s i s t e n t trends were present but, c o n c e n t r a t i o n s were even lower in the u n d e r l y i n g deep zone and SPM still forms an identifiable near-surface layer. T h e SPM:'~rich '' shallow zone t y p i c a l l y forms a surface wedge n o r m a l l y less t h a n 10 m but up to 20 m deep, t h i c k e s t at the h e a d of the fjords and t h i n n i n g t o w a r d the m o u t h (e.g.,

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Fig.4. Along fjord variation of depth integrated SPM in the shallow zone (0-10m) in winter (83) and summer (80). M= Milford; G= George; T/B = Thompson/Bradshaw;P = Preservation.

Fig.3 Preservation Inlet/summer), although in three surveys (e.g., Fig.3 George/winter; Thompson/winter/summer) this wedge reversed and thickened seaward. The shallow zone developed to greater maximum depths and exhibited a stronger wedge structure in summer t h a n winter. Within the wedge most fjords exhibited strong vertical zonation of SPM. The zone of maximum SPM was commonly at the surface, and SPM decreased with depth (Fig.3). However, mid-water maxima were also common (Fig.3) and found anywhere along the length of a fjord, it was not uncommon to find mid-water maxima at the head and surface maxima at the mouth and vice versa. In the shallow zone vertical zonation is probably complex and inadequately sampled by the 2 m bottle spacing. At the surface, plots of SPM and salinity show considerable scatter (Fig.5), with a higher range of SPM concentrations at lower salinities. The deep zone

In the deep zone SPM levels were very low, typically 0.2-0.4 mg l-1. In both summer and winter all four fjords had low SPM immediately below the shallow zone, usually 10-40 m (Fig.3). Below this depth SPM commonly

increased slightly before decreasing to minimum concentrations in mid-water. Highest SPM levels in the deep zone were normally found in bottom water, where concentrations near the fjord floor were often > 0.5 mg 1-1 and typically as high as in the shallow zone. In winter, Preservation Inlet proved the exception to this pattern with two of the five main basins (Fig.l, Otago Retreat and Isthmus Sound) with high SPM levels at the bottom but three others with maxima in mid-water and very low levels ( < 0 . 2 m g 1-1) at the bed. Highest SPM levels (>0.75mg 1-1) were in bottom waters of oxygen-depleted basins, cut off from the main fjord by shallow sills (e.g. Fig.3, Deep Water Basin Milford Sound; Precipice Cove Thompson Sound; Isthmus Sound Preservation Inlet). SPM-rich bottom water was also found on the slope of the Cleddau delta in summer, and on the landward slope of entrance sills in Preservation Inlet, Thompson and George Sounds (Fig.3).

SPM composition Five types of disaggregated SPM were identified; diatoms, coccoliths, and other phytoplankton (principally dinoflagellates/silicoflagellates), indeterminate organic matter and

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Fig.5. Variation of SPM in surface water as a function of salinity (dots: summer 1980; crosses: winter 1983).

mineral grains. The remaining SPM was clumped and the distinction made between faecal pellets, flocculated particles and agglomerated particles. Flocculated particles are formed by inorganic salt flocculation and particles held together by ionic bonding, in agglomerated clumps detritus is attached by surface tension and organic cohesion due to biological activity (Syvitski and Murray, 1981). Clumped particles and mineral grains were subdivided into five size classes, based on the length of the second largest axis of the particle. Within the shallow zone and upper waters of the deep zone phytoplankton are the major SPM component, typically making up 50-80% of the total (Figs.6 and 7). Inside the fjords diatoms predominate and Thalassiosira spp., difficult to speciate, make up 30-83% of the diatom population (Fig.8a). Chaetoceros simplex, Ch. similis and Thalassionema nitzchoides were the other dominant marine species; large numbers of Pleurosigma sp., were found down to 50 m depth and occasional CyclotteUa

sp. in surface waters. Chaetoceros dydimus, Ch. pseudocircoseatum, Stephanopyxis turris and Dictylum brightwellii were also common in winter. In summer, large numbers of the coccoliths Emiliania huxleyi and Syracosphaera pluchra were counted; silico-flagellates and dinoflagellates included Distephanum speculum, Dinophysis fucus and D. rotundata. In both summer and winter phytoplankton numbers were highest at the surface and decreased rapidly toward the base of the shallow zone. Below the shallow zone numbers increased to a subsurface maxima centred at approximately 20 m (Fig.6). Below 50 m diatoms made up a small percentage of the total SPM and were commonly in a broken and decayed state (Fig.8b). Most of the phytoplankton species were found throughout the sheltered water of the fjords. Diatom numbers decreased towards fjord entrances where most of the rarer diatom species were found whereas coccolith numbers generally increased seaward (Fig.7). In all fjords unattached mineral grains made

29

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Preservation Inlet Station M 6 0 I 5O

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20-

30-

40-

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g 100 •

150"

200 -

250 -

300 -

Fig.6. Distribution of SPM components through the water column in Long Basin, Preservation Inlet during summer (M601). Note the scale change at 50-100m. up a very small component of the total SPM. In Preservation Inlet cover was highest (7?/0) in the bottom water of Long Basin, elsewhere mineral grains contributed less than 5~o of the total SPM (Figs.6, 7 and 8). Even off river mouths grain counts were only marginally higher. Most unattached grains were clay and fine to medium silt (Fig.9, <4-16/~m). Some of

the mineral grains were incorporated into faecal pellets and flocculated/agglomerated particles but this material was principally biogenic in composition. In Preservation Inlet most of the clumps were agglomerated skeletal material, made up of broken phytoplankton, indeterminate organic matter and occasional mineral grains (Fig.8). Agglomerates take on a wide range of uncompacted forms. There were few flocculated clumps reflecting the low concentrations of mineral grains. Faecal pellets typically ranged from 4 to 32/~m. Pellets incorporated the full range of individual SPM components, suggesting that feeding was indiscrimate and pellets are presumed to be produced by neritic and coastal zooplankton resident in the fjords (Jillett and Mitchell, 1973). Lowest counts of clumped material were at the surface, numbers increased rapidly through the shallow zone to become the major component in the deep zone (Fig.6). Most clumps were in the silt range (4-64 #m) but increased in size down through the water column (Fig.9). In the shallow zone of the fjord entrances clump numbers were at similar concentrations to those in the deep zone (Figs.6 and 7). Indeterminate organic material is presumed to be largely terrestrial and marine plant matter and occasional pollen spores (Fig.8). Indeterminate matter forms a large part of the SPM in the deep zone, below 50 m, and at the base of the shallow zone (5-10 m): levels are low in near-surface water. Inspection of filters suggests SPM composition is similar in the other three fjords. However, two environments within the fjords appear quite different. Basins depleted of oxygen have a visibly high SPM cover on the filters (Fig.8c and d). In Precipice Cove (Thompson/Bradshaw Sound) and Isthmus Sound (Preservation Inlet) bottom waters have high concentrations of disaggregated coccoliths and broken diatoms. In Deep Water Basin (Milford Sound) filters clogged with a fine organic slime, similar to that in anoxic saline bottom water in nearby Lake McKerrow (Pickrill et al., 1981). On the inner slopes of entrance

30

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M601

M600

Long Burn River M651

DIATOMS

20 30

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Fig.7. Distribution of SPM components at 2 m depth ia longitudinal profile during summer in Preservation Inlet.

31

Fig.8.Scanning electronphotomicrographsof SPM samples.(A) M601, 0 m, Long Basin PreservationInlet,shallowzone diatoms, Thalassionema nitzschoides (TN), Thalassiosira sp. (T), Chaetoceros sp. (C/-/), the silico-flagellates Distephanum speculum (DS), and a pollen spore (PS) against a background of indeterminate organic material, mineral grains and coccoliths. (B) $483, 350 m, broken diatoms and agglomerated particles incorporating organic matter, diatoms and mineral grains. (Continued on next page.)

sills high S P M concentrations in bottom waters were also produced by a large number of broken diatoms and separated coccoliths. In the Long Burn at the head of Preservation Inlet, freshwater contains similar disaggregated particle types to those found in the fjord water (Fig.7). Phytoplankton, mineral grains and indeterminate organic matter are the major components, but coccoliths, clumped material and faecal pellets are absent.

Discussion Levels of S P M in New Zealand fjords are e x t r e m e l y low and m o r e typical of coastal and o c e a n i c w a t e r t h a n fjord e s t u a r i e s (e.g. E m e r y and Honjo, 1979). H i g h p r e c i p i t a t i o n all y e a r r o u n d and minimal s t o r a g e in s e a s o n a l or p e r m a n e n t ice fields e n s u r e s r e g u l a r freshw a t e r inflow t h r o u g h o u t the year. As a r e s u l t t h e r e is no s t r o n g s e a s o n a l i t y in SPM. In late summer, w h e n inflow r e a c h e s a slight maxi-

32 mum, SPM may be marginally higher and the wedge structure of the shallow zone most fully developed. However, these seasonal differences are minimal and insignificant when compared with those generated by the spring freshet in northern hemisphere fjords (e.g. Farrow et al., 1983). The predominance of biological components in the SPM suggest that this weak seasonality could just as easily be produced by variable productivity in the photic zone as by increased terrestrial sediment loadings. Short-period fluctuations within the shallow zone, produced by variable river discharge throughout the year, are probably more important than any seasonal trends. The freshwater influence may be either by direct entry of

terrigenous SPM, or, indirectly by providing conditions which encourage phytoplankton blooms. Salinity in the shallow zone responds within hours or days to increased river flow (Jillett and Mitchell, 1973; Stanton and Pickard, 1981) and variable but weak along fjord SPM gradients probably reflect pulses of earlier, higher freshwater inflow moving down fjord (Stanton, 1986). Other mid-fjord highs and lows are a response to mid-fjord inputs of freshwater, either from side streams or at fjord junctions. This is best developed at the confluence of Thompson and Doubtful Sounds (Fig.4). Doubtful Sound carries large volumes of freshwater (368m 3 s -1) diverted from catchments east of the main divide and discharged from the Manapouri Power Station tailrace in Deep

33

Fig.8 (continued). (C) M684,90 m, Precipice Cove, BradshawSound, disaggregatedcoccoliths and broken diatoms. (D) M594, 50 m, Deep Water Basin, MilfordSound, the filter is cloggedwith fine organic material.

Cove. The depth of the shallow zone and SPM concentrations are increased as this water flows into Thompson Sound through Pendulo Reach (Fig.l). Sampling of the shallow zone with water bottles at 2 m spacing identifies only broad trends through the water column. Divers have reported a microstructure within the shallow zone in which temperature and visibility change rapidly with depth across sharp interfaces (K.R. Grange, pers. commun., 1985). This complex vertical distribution probably results from spatial and temporal variability in inflows and from modification by wind wave mixing at the surface and diffusion and en-

trainment from below (e.g. Syvitski and Murray, 1981). Much of the freshwater enters through fjordhead rivers. However, the terrestrial influence is barely detectable at any season in either SPM composition or weight. Rainfall at Milford during the surveys was close to long-term monthly means, suggesting that these low SPM concentrations are the norm rather than the exception. Few of the major rivers flow directly to the sea. Most are intercepted by lakes (Irwin, 1975), which although small and of short residence time, probably filter out much of the coarser sediment. What little terrigenous SPM enters the fjords is fine

34 Bottom sediment

60 -- -/ ' ~

~

.f'~

/'

".-_.

.

Mineral grains 0 - 3 5 0 m

• ....... C l u m p s O-lOre --,--

C l u m p s 150-350m

\\ ,, .....-""

~'

"-....°,..,

Size (/=m)

Fig.9. Size distribution of clumped S P M in the water column, Long Basin, Preservation Inlet and in bottom sediment.

grained and stays in suspension long enough to be dispersed throughout the fjords, with no marked decrease in concentration away from the river mouths. The low terrestrial SPM input and conservative dispersal behaviour is reflected in the bottom sediments. The controlling influence of fjord head rivers on bottom sediment texture is confined to coarse sediments on the fjord-head deltas. Fine sediments are largely biogenic originating in the water column and from the surrounding forests (Fig.10). This SPM stays in suspension long enough to be transported all over the basins before settling as a uniform drape of massive silt (Pantin, 1964; Glasby, 1978). Composition of surface sediments in the basins reflects SPM settling from the deep zone. Cores from Milford Sound (Pantin, 1964) and Preservation Inlet have large numbers of faecal pellets preserved within the sediments which are also rich in organic carbon (10-15°//o dry weight). Analyses of these organic compounds from nearby Nancy Sound identified a strong contribution of total organic matter from land sources (Peake, 1978). Degradation of organic carbon is restricted by the productivity and chemical composition of the biogenic source rather than by degradation processes (Peake, 1978).

The low sediment input to the New Zealand fjords results in sedimentation rates of less than 0.1 cm y r - 1 (Glasby, 1978; Pickrill, unpublished t4C dates). By comparison sedimentation rates in northern hemisphere fjords are generally much higher (e.g., Clague, 1977; Smith and Walton, 1980) and as a result subaqueous slope failures on unconsolidated deltas are quite common (e.g. Holtedahl, 1975; Prior et al., 1984). In the New Zealand fjords delta failures are rare. The entrances to all four fjords are exposed to high-energy swell and storm waves. Seaward-flowing stratified water in the shallow zone is broken up by these waves before it reaches the entrance sill; diatom-rich surface water is mixed downwards and subsurface clumped SPM brought to the surface to produce a deep homogenous surface layer. This mixing of ocean and fjord waters near the entrance is manifest in a seaward decrease in organic carbon in bottom sediment (Pickrill, unpublished data). The estuarine stratification is totally broken down only a few kilometers seaward of the fjord entrances where the deep SPM-enriched surface layer corresponds to the depth of the photic zone off the west coast South Island (Chang and Bradford, 1985). High SPM concentrations on the landward slope of entrance sills suggest that sediment on these slopes is being resuspended off the bed. In fjords, with constricted sills and large tidal ranges, density currents generated by deepwater renewal resuspend sediment (Gade and Edwards, 1980; Baker, 1984). However, the New Zealand fjords have deep sills and a free connection to the open ocean (Pickard and Stanton (1980) and density currents are probably rarely generated. However, ocean waves are capable of stirring sediment on even the deepest sills. For example mean wave conditions off the Fiordland coast (height= 4.0 m, period = 12 s) would generate near bed orbital velocities of 17cm s -~ in 90m of water (Pickrill and Currie, 1983). Under these waves fine sand requires velocities of approximately 15 cm s-~ to be set in motion. Therefore even on deep entrance sills, such as in George Sound

35

Fig.10. Scanning electron photomicrograph of bottom sediment, Long Basin,-Preservation Inlet, showing broken diatoms, coccoliths, organic material and mineral grains.

(Fig.11), sand is frequently reworked by wave activity and SPM is prevented from settling. The effectiveness of ocean waves in transporting sediment decreases downslope into the fjord basins where sand deposition gives way to mud. Echograms from George Sound and Preservation Inlet show the landward slope of entrance sills to be slumped (Fig.12). High SPM concentrations on sill downslopes are probably produced by storm waves, either directly by resuspending sediment off the bed or indirectly by triggering slumping through landward transport o f sediment overloading the slope and/or by wave energy expended directly on the slope. Fiordland is a seismically active region, force 6 earthquakes and greater

can be expected every 10-20years (Smith, 1978), and these probably trigger slumping, resuspending sediment off the bed. While the processes whereby SPM is increased in bottom waters may be different from those previously described from the northern hemisphere (Gade and Edwards, 1980; Baker, 1984) the nett effect remains the same and SPM is available for upfjord transport in the intruding saline bottom water. Deep Water Basin developed SPM rich anoxic bottom waters that had probably been isolated for several years (Stanton, 1984). Development of these conditions in such a small shallow basin with high freshwater input shows that inflowing water overflows as a

36

Fig.ll. A medium sand bed with symmetrical wave formed ripples in 90 m of water on the entrance sill George Sound. The sea pens (sarcophyllum sp.) are typically 20-30 cm high.

b u o y a n t plume and t h a t densities w i t h i n the r i v e r w a t e r are probably r a r e l y low e n o u g h to g e n e r a t e underflows or deep mixing with a n d / o r d i s p l a c e m e n t of the saline b o t t o m water. Similarly s e d i m e n t a r y s t r u c t u r e s produced by downslope flows, such as c h a n n e l s typical of N o r t h A m e r i c a n and S c a n d i n a v i a n fjords (e.g. Holtedahl, 1975; Syvitski and Far-

row, 1983), h a v e not been found on deltas of New Zealand fjords. D o m i n a n t p h y t o p l a n k t o n w i t h i n the fjords are pelagic, coastal and s h e l t e r e d w a t e r species, m a n y of which h a v e been found in o t h e r New Zealand coastal inlets (Burns, 1977). This r e s i d e n t p o p u l a t i o n is the m a j o r S P M compon e n t in the u p p e r w a t e r column. Indiscrimi-

37

i

!

ill

"

L~

L

Fig.12. Dmboom seismic profile record across the entrance sill Otago Retreat, Preservation Inlet, showing slumping on the landward slope.

nate grazing of this very thin, rich, skin by pelagic zooplankton produces underlying water rich in faecal pellets. Flocculation and agglomeration clumps particles and accelerates the settling of SPM from near surface layers. The underlying water is subjected to '~fall out" of this clumped SPM from above but has a low resident diatom population, producing a subsurface minimum in SPM concentration near the base of the shallow zone. Diatom numbers increase at the top of the deep zone, producing the subsurface peak in SPM concentration found in most fjords and corresponding to the depth of a subsurface maximum in the chlorophyll a profile found in neighbouring Dusky Sound (Fig.l; Jillett and Mitchell, 1973). In temperate latitudes massing of phytoplankton near the surface and in deeper water, particularly at the base of the euphotic layer, is common (Raymont, 1980). The lower popula-

tion can be produced by a variety of conditions; at the optimum light intensity depth, at the thermocline or pycnocline, by shade-adapted cells, by self shading, by changes in the density of water in the column, by turbulent vertical mixing, by adjustment of buoyancy by the algae, by surface light inhibition and normal attenuation with depth, by sinking and accumulation, or, by variation in grazing intensity (Raymont, 1980). In the fjords causes for the subsurface maximum are difficult to isolate. However, in the shallow zone gelbstoff attenuates much of the light and phytoplankton are probably confined to a narrow vertical zone below the surface; the subsurface maximum probably extends to the base of the euphotic zone. Grazing by zooplankton enriches the water immediately below the SPM maximum with faecal pellets. Deep water is largely barren of living plankton. SPM concentrations

38

are low, and mainly clumped and indeterminate organics settling from above. Throughout this paper one of the recurrent themes has been the minor role that terrigenous SPM plays in the sediment budget. The non-conservative perforated conveyor-belt model of sediment-laden British Columbian fjords (Farrow et al., 1983), in which SPM concentration and sedimentation rates decay exponentially down-fjord, clearly is not applicable to normal inflow conditions to the New Zealand fjords. A two-tiered model, driven by biogenic SPM behaving as a conservative water property seems more applicable.

Summary and conclusions (1) The New Zealand fjords have typical fjord temperature and salinity structures. Predominate headwater inflow drives an estuarine circulation with a thin skin of low-salinity water flowing seaward in the shallow zone over oceanic salinity water in the deep zone. (2) Input of terrestrial SPM is small with only weak seasonality. Accordingly mineral grains form only a minor constituent. (3) Higher SPM concentrations in the shallow zone are largely produced in situ by a resident phytoplankton population and input of organic terrestrial detritus. (4) Diatoms produce a second SPM peak at the top of the deep zone. Processes of flocculation, agglomeration and pelletisation by zooplankton, clumps material in the SPM-rich layers to accelerate settling. (5) Deep waters are barren of living plankton, receiving SPM settling from above. (6) Highest concentrations in the deep zone are found in bottom waters (a) just inside the entrances where ocean waves resuspend sediment off the bed and (b) in small fjord-head basins where shallow sills restrict the renewal of saline water, allowing anoxic conditions to develop.

Acknowledgments I am grateful for the assistance of the Masters and crew and scientific compliment aboard R.V.

"Tangaroa", I am indebted to D.A. Burns for identifying phytoplankton and J.S. Mitchell and G. Walker for assistance on the S.E.M. The manuscript was improved by constructive criticism from L. Carter, J. Fenner, K.R. Grange and B.R. Stanton. A. Kelly drafted the figures and G.A. Marsden typed the manuscript.

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