The Hydrography of the Chupa Estuary, White Sea, Russia

Estuarine, Coastal and Shelf Science (1999) 48, 1–12 Article No. ecss.1999.0399, available online at http://www.idealibrary.com on

The Hydrography of the Chupa Estuary, White Sea, Russia R. J. M. Howlanda, A. N. Pantiulinb, G. E. Millwardc and R. Pregod a

NERC Centre for Coastal and Marine Studies, Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 3DH, U.K. b Department of Oceanology, Geographical Faculty, Moscow State University, 119899 Moscow, Russia c Department of Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, U.K. d CSIC Instituto de Investigacions Marinas, Eduardo Cabello 6, 36208 Vigo, Spain Received 22 December 1997 and accepted in revised form 15 July 1998 This study was undertaken to determine the fate and fluxes of materials from Arctic estuaries to the coastal zone. The paper is the first of a series addressing questions relating to the physics and chemistry of the region. Three seasonal cruises were undertaken in the Chupa Estuary, White Sea, Russia; in summer (July 1994), autumn (September, 1995) and spring (May/June 1997). The Chupa is a fjord type estuary about 37 km long with several deep troughs, connected by shallow sills, situated south of the Arctic circle on the western shore of Kandalaksha Bay. Vertical profiles were carried out on an axial grid of 10 stations, at spring and neap tidal states, during which measurements were made of salinity, temperature, current speed and direction, the concentration of suspended particulate material (SPM), pH, dissolved oxygen, nutrients (phosphate, silicate, nitrate, nitrite and ammonia) and particulate trace metals in sediments and SPM (Millward et al., 1999, Estuarine, Coastal and Shelf Science, 48, 13–25). Additionally, sampling for the hydrodynamic determinants was carried out on diurnal anchor stations at key points in the estuary. Vertical stratification was pronounced, particularly in summer, with sharp gradients in temperature and salinity. A three layer vertical structure was observed with surface (0–5 m) and deep (20–65 m) water layers providing net down-estuary transport while the intermediate (5–20 m) water layer drives net up-estuary transport and advection of more saline waters into the estuary. Strong internal waves were observed in the seaward half of the estuary at certain tidal states. The mechanism for renewal of deep waters in the troughs was investigated, this being considerably slower than in the near surface waters. Evidence of under-saturation of dissolved oxygen was evident in the deep waters of the troughs during the three surveys.  1999 Academic Press Keywords: hydrography; estuarine circulation; Chupa Estuary; Arctic; Russia

Introduction The fate and flux of material discharged from the continent to the ocean is determined, to a large extent, by the biogeochemical and sedimentological processes occurring in estuaries and coastal seas (Martin et al., 1993). The contribution of Arctic estuaries to global biogeodynamics is unknown. The generalized concepts of estuarine biogeochemistry fashioned from extensive studies of temperate zone estuaries may not be applicable to Arctic estuaries, thus specific studies of these estuaries are required. In addition, their dynamic ice cover may create seasonal variations in the fates and fluxes of particles and biogeochemical species through these estuaries. Research into the biogeochemistry and chemistry of Arctic estuaries and their effect on the Arctic Ocean has centred on the larger estuaries of the Ob, Lena and Yenissei (Gordeev & Sidorov, 1993; Martin et al., 1993; Coquery et al., 1995; Rovinsky et al., 1995). The 0272–7714/99/010001+12 $30.00/0

importance of the smaller Arctic estuaries on the hydrology, chemistry and biology of the Arctic Ocean still needs to be investigated. This has been the purpose of this study which forms part of the work carried out during the ARIES (Anglo-Russian Interdisciplinary Estuarine Study) project, a collaboration between European and Russian workers, centred on a number of medium and small estuaries in Kandalaksha Bay, White Sea, Russia. The White Sea is a semi-enclosed Arctic sea, which occupies a long gulf south-east of the Kola Peninsula and is joined to the Barents Sea by a strait between Cape Kanin and the Kola peninsular (Figure 1). The White Sea covers approximately 95 000 km2 and has four large bays (Mezen, Dvina, Onega and Kandalaksha) and numerous islands (Klenova, 1966). During the long winters (October–April) the bays of the White Sea freeze over (Klenova, 1966) and freshwater runoff through the estuaries to the White sea is restricted. Seasonal extremes of runoff are seen, for  1999 Academic Press

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Barents Sea

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example, the Lena River Estuary which has a minimum runoff into the Laptev Sea of 1220 m3 s
1 in April and a maximum of 73 700 m3 s
1 in June (Martin et al., 1993). The May/June melt brings a short period of extreme flood, which slowly decreases to the winter values (Gordeev & Sidorov, 1993). The Barents Sea exports ice to the Arctic basin in winter and imports ice in summer (Pfirman et al., 1995), thereby providing a transport system for particulate material and biogeochemical species into and out of the White Sea. The White Sea surface waters are considerably fresher (salinity 24–26) than the bottom waters (salinity 30–30·5) and consequently the overflow of fresh water enters the Barents Sea. Exchange of bottom waters is less frequent due to a

shallow sill across the narrow ‘ neck ’ (mean depth 50 m) which restricts the exchange of waters between the Barents and White Seas. The central basin (maximum depth 350 m) and Kandalaksha Bay (maximum depth 300 m) are the deepest parts of the White Sea. Seasonal variation of surface water temperature in the White Sea is about 20 C but the bottom waters have a constant temperature of about
1·5 C (Klenova, 1966). Average water temperature in Kandalaksha Bay in summer is 14–15 C with a salinity of 23–24. In winter, temperatures are below zero throughout the water column, with the minimum temperatures occurring in the near surface layer. The widespread occurrence of permafrost in the catchment area of Arctic river-estuary systems has

Hydrography of the Chupa Estuary 66° 24' N

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F 2. (a) Map of the Chupa Estuary showing station positions and (b) the bed topography of the axial profile.

been shown to decrease the intensity of chemical and physical weathering processes (Ugolini, 1986). Arctic estuaries may generally have low annual chemical denudation rates, such as 19·7 t km
2 for the Lena River Estuary, which is half the current annual world average (Gordeev & Sidorov, 1993). Groundwater runoff increases in importance during winter, creating a seasonal oscillation in the ionic composition of the runoff (Gordeev & Sidorov, 1993). It may be assumed that these are widespread phenomena for Arctic estuaries. Chupa estuary: morphometry, climate, freshwater input and tides The Chupa [Figure 2(a,b)] is the largest estuary in the Kandalaksha Bay and is situated close to the Arctic Circle. It is 37 km long, has a typical width of 1–2 km and an overall area of 57 km2. Chupa town is situated near the head of the estuary and has a population of

approximately 6000. An active China clay works near Chupa town and two disused muscovite mines, one near Kartesh Station (Station 2) and the other near Station 6 (Figure 2) are the only evidence of industry along the estuary. The Chupa is a fjord type estuary but does not have the steep sides generally associated with fjords. Along its axis there are two deep basins (average depth 65 m) connected by a sill (mean depth 20 m) about 7 km long. Within the sill area there are a number of hollows with a maximum depth of 35 m. At the seaward end of the estuary there is another sill (average depth 50 m) and a number of islands. The area is in the sub-Arctic climatic zone with, predominantly, cyclonic weather. The air temperature is below zero from mid November to mid May and the estuary is covered by ice, of approximately 2 m thickness. Mean annual precipitation is 400–450 mm, of which approximately half falls as rain and half as snow. The freshwater input to the Chupa originates from three sources, mainly from the rivers Pulonga and

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Keret [Figure 2(a)]. There are no historical flow data for the freshwater inputs but, our measurements show flows of 3–5 m3 s
1 from Middle Spring near the head of the estuary and about 7 m3 s
1 from the River Pulonga in mid-estuary. The input from the River Keret varies between 13–34 m3 s
1 but, only a small proportion of this flows into the Chupa estuary as brackish water (salinity 19–20), through the Keret Strait [Figure 2(a)] during the flood tide. Tides in the Chupa estuary are semi-diurnal but the tidal cycle is markedly asymmetrical with an average flood tide of 5 h and ebb tide of 7 h duration. The tidal range in the estuary varies between 1·7 m at springs and 0·7 m at neaps. The tidal prism is approximately 10107 m3 at spring tide, decreasing to 4107 m3 at neaps. Methods Three surveys were carried out, in July 1994, September 1995 and May 1997. Each survey consisted of axial transects with vertical profiling at fixed stations along the estuary [Figure 2(a)], and 24·5 h (or 12·25 h) stations at key points in the estuary. An axial depth profile of the estuary is shown in Figure 2(b). The axial transects were carried out at spring and neap tidal conditions. The July 1994 and September 1995 surveys were conducted using R.V. Professor Vladimir Kuznetzov, a research vessel from the St Petersburg Institute’s Kartesh Station, situated near the mouth of the Chupa Estuary. During the May 1997 survey the M.V. Surf, a vessel from the Moscow State University White Sea Biological Station, was used. Throughout the surveys the weather was very favourable with wind speeds never exceeding force 2 on the Beaufort Scale. An important logistical issue for the estuarine surveys was that the duration of ebb and flood tides were approximately the same as the time taken to carry out the axial transects (5–7 h). Therefore, the data from each station contains information about spatial and temporal variability. Generally, axial transects were carried out starting at Station 1 at the mouth of the estuary and moving up-estuary to Station 10 [Figure 2(a)]. Station 11 could only be sampled using a dinghy, which we carried with us on the research vessel. During the summer 1994 survey two axial up-estuary transects were undertaken at spring tide and two at neap tide, the first starting at low water and the second at high water on consecutive days, with one complete tidal cycle between the pairs of transects. These transects provided data at approximately the same tidal phase for each station. Additionally, on the autumn 1995 survey a double transect was carried out starting at Station 1, moving

up-estuary to Station 10 and returning down-estuary to finish at Station 1. This transect was started at low water, reached the head of the estuary at high water and returned to Station 1 at approximately low water. This method of traverse provided data with a minimal tidal phase displacement of zero at the head of the estuary, approximately 180 in mid-estuary (on the up-estuary and down-estuary transects) and 360 at the mouth (at the beginning of the first and end of the second transects). This strategy, designed to characterize temporal and spatial variation in estuaries is described in Morris et al. (1982). At each station, during the axial transects, the ship was anchored while sampling was undertaken. Physical measurements were made on all stations using a calibrated Applied Microsystems Limited, Canada, EMP 2000 multi-parameter (salinity, temperature, optical back-scatter, pH and depth) monitoring probe and a Valeport Limited, U.K. BFM 108 MkII, direct reading current meter (current speed and direction). Results and discussion Main features of the salinity and temperature structure The Chupa estuary is stratified, with marked vertical differences in salinity and temperature. In summer salinity varies by 5–7 and temperature by 13–15 C from surface to bottom. In autumn the differences are less, typical salinity and temperature variations being 2–3 and 3–5 C, respectively. In spring the salinity differences are greatest, reaching 8 in the outer estuary and 21 at the head of the estuary. For over 20 years a data station has been maintained (Babkov, 1982) close to Kartesh Biological Station (Station 2 on our transect). Salinity and temperature were measured at 10 day intervals. Monthly averaged data for the whole period are shown in Figure 3. Salinity and temperature data from our summer (July 1994), autumn (September 1995) and spring (May 1997) cruises, taken from our vertical profiles at Station 2, close to the station used by Babkov, are in close agreement with his monthly averaged data. Thus, we consider that data reported here are wholly representative of mean conditions in the estuary. Despite the fact that there are freshwater inputs in mid-estuary and near the mouth the horizontal distributions of salinity increase down-estuary. This applied to the whole of the water column, for all seasons which is largely due to the fact that the Chupa Estuary has comparatively small freshwater inputs, about 0·3– 1·0% of the tidal prism. Almost all the water volume

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F 4. Contour plots of Temperature and Salinity in the Chupa estuary during the Spring survey, shortly after the ice melt. The thick line denotes the bed of the estuary.

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tive processes in the whole estuary and not grossly influenced by an individual freshwater input.

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F 3. Salinity and Temperature variation at Kartesh Station, monthly averaged over a 20 year period. Data from Babkov, 1882. 65 m ( ); 25 m ( ); 10 m ( ); 0 m ( ); Ice period (*).

in the estuary had a salinity between 20–27 (summer), 24–27 (autumn) and 20–28 (spring), which was close to the seawater value in the immediate coastal zone. Low salinity water is limited to the immediate vicinity of the freshwater inputs. Thus, for the largest input, Pulonga, the freshwater/seawater mixing zone (salinity 0–20) extends less than 1 km from the point at which the fresh water enters the estuary. This means that the observed salinity distributions in the Chupa Estuary are generated by the interaction of mixing and advec-

Water structure and seasonal variability The Chupa is a fjord type estuary and has the threelayer vertical structure typical of fjords; a thin surface layer (fresh and warm), an intermediate layer (to the depth of the sill) and a deep layer (below the depth of the sill) as described by Farmer and Freeland (1982). The layers in the Chupa Estuary are: surface, 0–5 m; intermediate, 5–20 m and deep, 20–65 m. The highest freshwater runoff is experienced in spring but, its influence is limited to a thin surface layer (about 1 m) (Figure 4). After the spring ice-melt period this layer rapidly increases in temperature due to insolation; a process stimulated by the adsorptive properties of humic-rich fresh water. The spring survey was conducted as soon as the estuary was free from ice but, already the temperature at the water

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surface had reached 8–10 C at a salinity of 4–10. The structure of the intermediate layer, which started below a sharp pycnocline, varied in thickness along the estuary and between salinities of 26·0 and 27·5. The spring situation in the deep layer gives an insight into estuarine processes during the previous winter. The deep waters are characterized by an annual maximum in salinity and minimum in temperature but there are dissimilarities between the two deep basins. Respective salinities and temperatures in the up-estuary basin were 27·65 and
0·68 C and in the seaward basin, 28·62 and
1·17 C. The temperature in the up-estuary basin was 0·8 C above freezing and in the seaward basin 0·4 C above freezing point. This suggests that winter convection does not affect the bottom of either of the deep basins and that deepwater renewal is promulgated by advective processes as observed during the other seasons. The summer structure is characterized by the development of the surface layer: thickness 5–7 m, maximum temperature 17 C, but the salinity increases to 20 [Figure 5(a–d)]. The mean depth of the pycnocline is 7–10 m. The basic structure of the intermediate layer remained the same as during the spring survey. The characteristics of the deep water varied between spring and summer, the salinity decreased and the temperature increased but differences between the bottom waters in the two deep basins were still retained. The autumn structure is characterized by a subsurface maximum in temperature at the seaward end of the estuary as a result of intensive cooling from the surface [Figures 6(a–c), 7(a)]. The temperature of the deep water increases and reaches an annual maximum which is accompanied by a small rise in salinity [Figure 7(a,b)]. Tidal, neap-spring variability Our data allow us to analyse this variability for two seasons: summer and autumn. The most interesting results were collected during the summer seasonal cruise in July 1994. It was found that there were two modes of variability. The first was associated with neap tidal conditions [Figure 5(a,b)] and consisted of variation in the thickness of the surface layer with a displacement of the pycnocline of 3–5 m. A different structural transformation was evident during spring tidal conditions [Figure 5(c,d)], when an intermediate

layer intruded into the estuary from the seaward end at between 5–15 m. The advective origin of this layer was confirmed by measurements of current speed and direction. The autumn regime was characterized principally by variation in the intermediate layer with advection up-estuary from the mouth during spring tides and recirculation during neap tides (Figure 6). Tidal, semi-diurnal variability Analysis of the data at the diurnal/semi-diurnal stations at three points in the Chupa estuary has shown vertical oscillations of the temperature and salinity isohalines at tidal frequencies. The most distinct oscillations are noticeable during autumn, when two diurnal (24·5 h) stations were carried out in tandem; the first in the seaward deep basin and the second over the sill, in mid-estuary. Internal waves close to Station 2 were evident below the layer of the temperature maximum and had a semi-diurnal period and a height of around 5 m [Figures 7(a,b)]. The crest of the wave coincides with the ebb stage in the estuary. At Station 6 two systems of internal wave oscillation were detected [Figure 8(a,b)]. The first was in the same layer and had a similar phase as at Station 2 (Figure 7). The wave height was about 10 m and was situated close to the bottom in a local depression where the depth reaches 35 m. The second internal wave oscillation occurred in the 5–15 m layer and had a height of about 5 m. An interesting feature of these two sets of internal waves was their approximate 180 phase difference. The summer survey included two semi-diurnal (12·25 h) stations, one at Station 2 and one in the Keret Strait at which internal oscillations were found to have similar characteristics to those of the Station 2 autumn data (Figure 7). Tidal and residual currents The Chupa estuary has moderate tidal dynamics, the tidal range varying between 1·7 m and 0·7 m. Our measurements for 3 years in different seasons gave maximum tidal velocities of around 0·40 m s
1 but, typically velocities in the range 0·05–0·30 m s
1. The highest velocities were measured within the intermediate layer (5–20 m) during flood tides. The most detailed analysis of current speed and direction was

F 5. Temperature and Salinity distributions along the Chupa Estuary during Summer 1994; (a) Neap tidal conditions, axial up-estuary transect starting at low water, 18 July 1994; (b) Neap tidal conditions, axial up-estuary transect starting at high water, 19 July 1994; (c) Spring tidal conditions, axial up-estuary transect starting at low water, 23 July 1994; (d) Spring tidal conditions, axial up-estuary transect starting at high water, 24 July 1994. The thick line denotes the bed of the estuary.

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F 6. Temperature and Salinity distributions along the Chupa Estuary during Autumn 1995; (a) Spring tidal conditions, axial up-estuary transect starting at low water, 14 September 1998; (b) Spring tidal conditions, axial down-estuary transect starting at high water, 14 September 1995; (c) Neap tidal conditions, axial up-estuary transect starting at low water, 19 May 1995. Note that the ‘ anchor station ’ data is shown in Figure 7. The thick line denotes the bed of the estuary.

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F 7. Temperature (a) and Salinity (b) variation during a diurnal anchor station at Kartesh (Station 2) on 15–16 September 1995.

carried out on diurnal (24·5 h) stations during the autumn seasonal survey in 1995. The most significant features were observed at Station 2 (Figure 9). These were:

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F 8. Temperature (a) and Salinity (b) variation during a diurnal anchor station at Pulonga (Station 6) on 17–18 September 1995.

(i) The predominance of ebb or flood currents in different layers—the ebb current is stronger in the surface (0–5 m) and deep (25–45 m) layers but

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F 9. Current velocities (a) and Residual currents (b) (calculated for each of the two tidal cycles) during a diurnal anchor station at Kartesh (Station 2) on 15–16 September 1995. Positive current values to seaward.

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the flood current is stronger in the intermediate (5–25 m) layer; The total absence of ebb currents in the 10–15 m layer throughout two tidal cycles. The current structure on the Keret Strait station shows similar features (Figure 10); The currents on the mid-estuary, sill, station (Station 6, Pulonga) change synchronously over the whole water column but retain the flood/ebb predominance in the different layers (Figure 11); The irregular character of the currents in the Chupa estuary may be attributed to the superposition of tidal, wind and density currents together with non-linear tidally-induced currents; Residual currents, calculated as described by Uncles and Jordan (1979), were estimated for summer and autumn conditions at the mouth (Station 2, Figure 9), Keret Strait (Figure 10) and in mid-estuary (Station 5, Figure 11). All three stations exhibit a three layer structure of residual currents: the surface layer current downestuary, the intermediate layer current up-estuary and deep layer current down-estuary [Figures 9(b), 10(b) & 11(b)]. The velocities of the residual current were in the range 2–12 cm s
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Deep water renewal It is known that there are several mechanisms influencing deep water formation in fjords: advection from the sea, advection with mixing on a sill and local winter convection. Our data indicate that the seaward deep basin has comparatively free connection with the open sea and that its temperature/salinity structure is similar to that of its immediate coastal zone. The deep-water formation and renewal in the upestuary basin, during all seasons, occurs by advection of seawater via the sill, without effective mixing. Temperature and salinity differences exist between deep waters of the two basins in the Chupa throughout the year. However, the deep water characteristics in the up-estuary basin correlate closely with those of the seaward basin at the level of the sill mean depth (20 m) and to a depth of 5–10 m below it (Figures 5, 6 & 7). Comparing the spring and neap tidal situations shows that the advection of this water into the up-estuary basin is more likely to occur during the neap tide periods in summer and during spring tide periods in autumn. Thus, the renewal time for the up-estuary trough is approximately two weeks. The distinction between summer and autumn may be

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F 10. Current velocities (a) and Residual currents (b) during a semi-diurnal anchor station in Keret Strait on 16 September 1995. Positive current values into Chupa Estuary. Note that the intermediate layer (5–20 m) moves up-estuary throughout the period.

explained by different stratification conditions. In summer the stratification is more intensive and intrusion of the intermediate water into the estuary produces a sinking of the pycnocline which bars water advection from the seaward side [Figure 5(c,d)]. During autumn, with weak stratification, the intermediate advection entrains the deep water and stimulates its intrusion into the up-estuary basin [Figure 6(a,b)]. The ‘ new ’ body of water from the seaward end of the estuary can only penetrate up-estuary over a number of tidal cycles because the sill length is about 7 km and the mean axial displacement of water volumes during the flood is of the order of 1–3 km. The spring seasonal transect was carried out at neap tide (Figure 4) and it can be seen that the seawater intruded via the sill up-estuary and filled the top basin. It was surprising that there was no evidence of winter advection of seawater into the up-estuary basin. An interesting question remains, as to how the more dense spring (or winter) waters are replaced by summer water. Conclusions The surveys in Chupa over three years have provided an insight into the seasonal factors controlling

the hydrodynamics, as well as the distribution and transport of chemical material in the estuary. The question of the position of the Chupa in estuarine classification is debatable. Morphometrically, it is a fjord type estuary but, a shallow version. In the Chupa the up-estuary deep basin has been shown to be not deep enough for stagnation and the sill not shallow enough for vigorous mixing. This view is supported by the estuarine classification scheme of Hansen and Rattray (1966). The stratification and circulation parameters, estimated for the middle part of the estuary, respectively vary between 0·08–0·18 (0·85-spring) and 170–430, which corresponds to a ‘ softly ’ stratified fjord. The Chupa Estuary differs markedly from typical fjords in some features, the most important of which is the nature of the threelayered structure which was clearly evident during each of our surveys in the distributions and variability of temperature and salinity, tidal circulation and residual currents. On average, down-estuary transport prevails in the surface (0–5 m) and deep (20–65 m) layers. The intermediate layer (5–20 m) drives mainly up-estuary transport and advection of seawater into the estuary. In addition, the deep-water formation and renewal in the top basin are an unusual feature of the

Hydrography of the Chupa Estuary (a)

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F 11. Current velocities (a) and Residual currents (b) (calculated for each of the two tidal cycles) during a diurnal anchor station at Pulonga (Station 6) on 17–18 September 1995. Positive current values to seaward.

estuary. The events of internal oscillations with tidal period are also of interest. The Chupa Estuary offers a test bed for further studies of Arctic systems, especially in the development of empirical modelling methods. There are clear seasonal differences in the water structure which suggest that relatively small changes in the seasonal temperature regime, through climate change, may more markedly affect its hydrological structure. Additionally, its relatively pristine nature forms a vital baseline for comparative biogeochemical and hydrological studies of temperate and Arctic estuaries. Small Arctic estuaries play a minor role in the land-ocean exchanges of materials. However, they may serve as surrogates for larger estuaries which are less accessible. The comparison of small Arctic estuaries with those in temperate regions (e.g. the Tamar and Tweed Estuaries, U.K.) may establish regional backgrounds against which the impact of climate change on estuarine processes can be assessed. Of particular importance is the role of ice cover and the freeze-thaw cycle on estuarine circulation and biogeochemical processes. More international collaborative studies are required to investigate these gaps in knowledge.

Acknowledgements The authors wish to express their gratitude to the Royal Society (London) for their support over 4 years through a project grant and to INTAS (Brussels) for their support under Co-operation Agreement Number 1010-CT93-0019. The support and advice given by Dr Alan Morris and Dr Reg Uncles is gratefully acknowledged. We also acknowledge the great assistance provided by the Captain and crew of R.V. Professor Vladimir Kuznetzov.

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