Deep-water Renewal Events Research Articles

Internal tidal waves and deep-water renewal in a tropical fjord: Lessons from Ambon Bay, eastern Indonesia

Seasonal-varying internal tidal dynamics have a critical role in deep-water renewal in tropical shallow-silled fjords (e.g. Ambon Bay, Indonesia). Seasonal- and tidal-based longitudinal CTD casts from the slope of outer Ambon Bay (OAB) to the sill of Ambon Bay coupled with bottom-mounted current meters at the sill and in inner Ambon Bay (IAB, the fjord basin), were employed to investigate the internal tidal-sill slope interaction and the characteristics of deep-water inflow. Weaker stratification in the easterly monsoonal season than the transitional monsoonal season (dT/dz = ~0.02 °C/m cf. dT/dz = ~0.06 °C/m) drives the predominant sub-critical slope condition at the OAB slope causing more frequent tidal upwelling events in the easterly monsoonal season (13 events in a spring-neap sequence) than in the transitional monsoonal season (5 events). The magnitude of tidal upwelling in Ambon Bay, as measured by the depth from which water upwells, is stronger in the easterly monsoonal season (up to 200 m) than in the transitional monsoonal season (maximum: 115 m). The influx of upwelled water across the sill was controlled by the tidal excursion of the deep-water plume in the easterly monsoonal season and by the deep-water/sill density difference in the transitional monsoonal season. A series of deep-water renewal events in IAB within a spring-neap sequence (~2 weeks) in the easterly monsoonal season supplied a total inflow volume of 0.09 ± 0.02 km3 which can replenish approximately 80% volume of the IAB deep layer. This total inflow volume was smaller in the transitional monsoonal season (0.05 ± 0.01 km3), hence, only flushing 40% volume of the IAB deep layer. The knowledge of internal tidal waves and deep-water inflow in Ambon Bay presented here will be a key reference for future studies focused on water transport in Ambon Bay and other shallow-silled tropical fjords.

Dissolved methane in the water column of the Saguenay Fjord

Near-shore environments are a significant source of atmospheric methane but the size of this source is poorly constrained, particularly for fjords and fjards. This study investigated the methane emission rates and the drivers controlling the dynamics of dissolved methane in the Saguenay Fjord, a deep, stratified, and well‑oxygenated subarctic fjord system in eastern Canada. Dissolved methane concentrations ([CH4]) in the water column were measured in October 2016 and June, October, and November 2017, with stable carbon isotope composition of methane (δ13CCH4) analyzed during the November 2017 survey. Surface-water [CH4] ranged from 16 to 184 nmol L−1 and decreased with increasing salinity in a bi-segment linear manner, inferring a temporally constant marine endmember but a freshwater discharge-dependent river endmember. The multi-cruises dataset yields a mean [CH4] saturation ratio of 12.7 (range: 4.5–48.7) and a mean emission rate of 53.4 μmol m−2 d−1 (range: 16.4–256.9 μmol m−2 d−1). [CH4] was generally higher in surface water than in deep water. However, sill-induced mixing could homogenize [CH4] near the mouth of the fjord and sedimentary input of biogenic methane (δ13CCH4: −57.660‰) in the fjord's head region increased [CH4] in the overlying bottom water up to 459 nmol L−1. The longitudinal pattern of [CH4] below the surface layer was primarily controlled by deep-water renewal events. Deep-water [CH4] declined with rising apparent oxygen utilization, suggestive of aerobic microbial methane oxidation at rates estimated to be <0.1 nmol L−1 d−1. The δ13CCH4 data yields a carbon isotopic fractionation factor of 1.08 in both the surface and deep waters that points to microbial oxidation dictating the carbon isotopic fractionation of methane in the fjord. Mass-balance budgeting reveals that river runoff accounts for 81% of the total methane input to the fjord (12.13 × 106 mol year−1) and that microbial oxidation of methane (4.45 × 106 mol year−1) is comparable to emission to the atmosphere (4.27 × 106 mol year−1). This study demonstrates the important roles of river runoff and deep-water renewal in controlling the dynamics of [CH4], δ13CCH4, and methane emission to air in fjords receiving large terrestrial freshwater discharges and experiencing frequent deep-water renewals. The areal methane emission rates for deep fjords obtained by this and earlier studies are one to two orders of magnitude higher than the mean flux estimate for global coastal oceans, placing fjords and fjards as a potentially significant contributor to coastal methane emission.

Paleo-environmental changes during the Middle–Late Ordovician transition on the Yangtze Platform, South China and their ecological implications

Rocks of Darriwilian and Sandbian age (Middle–Late Ordovician) record the extraordinary peak of the Great Ordovician Biodiversification Event (GOBE) and significant global environmental changes. To track paleo-environmental changes through this critical time, we have analyzed iron speciation, trace elements and total organic carbon in samples of the Miaopo Formation from an intra-shelf basin in South China. The results reveal moderate primary productivity and possible fluctuations in benthic redox. Upwelling during marine transgression is thought to have increased the availability of nutrients in surface waters, sustaining the biological pump. Enhanced export of organic matter would have consumed dissolved oxygen at depth, driving the system toward anoxia. However, it is also possible for deep-water renewal events to transfer more dissolved oxygen into the bottom water. Those proxies suggests high primary productivity and varying sea level (global and local), not benthic anoxia, as the driver of accumulated “black shale”. Thus, the local marine ecosystem thrived in this oxic intra-shelf basin, challenging the preexisting paradox that the high-diversity community flourished in an oxygen-depleted environment.

Deep-Water Renewal Events; Insights into Deep Water Sediment Transport Mechanisms

Deep-water renewal (DWR) events are characterized in the Strait of Georgia, Canada using 11 years of real-time physical and chemical oceanographic data and seafloor videos. At least 6 DWRs occur per year at 300 m water depth and each event continues for over 3 days. They initiate during neap tides and are associated with increased turbidity. In the spring, DWRs introduce cold, oxygenated and nutrient-poor waters, and in the fall they introduce warm, oxygen-depleted, nutrient-rich and saline waters. Although the timing and magnitude of DWRs differ from year to year, we demonstrate that they are not restricted to two seasons, but continue throughout the year. High-resolution videos of DWRs show that these events comprise a plume of high suspended sediment concentration that flows parallel to the basin axis and deposits approximately 1.5 cm per event.

Hydrographic maintenance of deep anoxia in a tidally influenced saline lagoon

Low dissolved oxygen concentrations are of increasing concern in aquatic ecosystems, particularly at the interface between freshwater and marine environments. Oxygen depletion occurs naturally in many perennially stratified systems and it remains to be seen how climate change will affect these habitats. This is due, in part, to a lack of high-resolution, long-term data describing interannual variability in dissolved oxygen concentrations within stratified basins. Physicochemical parameters for Lough Furnace, an ecologically important tidal lagoon, were assessed using daily measurements (2009–14) from an undulating CTD (conductivity, temperature and depth) profiler and observations of tidal exchange flow. Continuous vertical saline stratification existed, with anoxia (&lt;0.1mgL–1) typically persisting below 6m. Tidal inflows were generally restricted, with deep-water renewal events by intrusions of denser spring tidal water occurring episodically (three times in 6 years), following prolonged periods of low freshwater input. Although wind forcing alone was not sufficient to generate basin-scale mixing, the conditions that led to deep-water renewals may also be conducive to wind-driven upwelling events in nearshore areas. These findings have wider application to larger-scale two-layered stratified systems with deep anoxia because the ability to forecast such dynamic events is important for assessing the ecological implications of dissolved oxygen depletion.

Modelling sensitivities to mixing and advection in a sill-basin estuarine system

This study investigates the sensitivity of a high resolution regional ocean model to several choices in mixing and advection. The oceanographic process examined is a deep water renewal event in the Juan de Fuca Strait–Strait of Georgia sill-basin estuarine system located on the west coast of North America. Previous observational work has shown that the timing of the renewal events is linked to the spring/neap tidal cycle, and in turn, is sensitive to the amount of vertical mixing induced by tidal currents interacting with sills and complicated bathymetry. It is found that the model’s representation of deep water renewal is relatively insensitive to several mixing choices, including the vertical turbulence closure and direction of lateral mixing. No significant difference in deep or intermediate salinity was found between cases that used k−ϵ versus k−ω closures and isoneutral versus horizontal lateral mixing. Modifications that had a stronger effect included those that involved advection such as modifying the salinity of the open boundary conditions which supply the source waters for the renewal event. The strongest impact came from the removal of the Hollingsworth instability, a kinetic energy sink in the energy-enstrophy discretization of the momentum equations. A marked improvement to the salinity of the deep water renewal suggests that the removal of the Hollingsworth instability will correct a fresh drift in the deep and intermediate waters in an operational version of this model.

In situ O2 and N2 measurements detect deep-water renewal dynamics in seasonally-anoxic Saanich Inlet

We interpret 2.5 years of dissolved oxygen and N2 excess concentrations from in situ observations at mid-depth in Saanich Inlet, a seasonally anoxic fjord. N2 excess was derived by combining measurements of total dissolved gas pressure from a gas tension device (GTD) with dissolved oxygen measurements and comparing to expected equilibrium concentrations. We present an important and previously ignored correction for the effects of hydrostatic pressure on GTD deployments at depth. Calibration of the dissolved oxygen measurements was the largest uncertainty for our data. During late summer/early fall deep-water renewal events, we observed rapidly declining oxygen and increasing N2 excess for several days, which we interpret as deep anoxic waters being pushed to mid-depth, where we observed them, by the entering dense waters that flow to the bottom. High rates of denitrification in the bottom waters over most of the year provided the N2 excess. We describe a recovery period lasting a few days following each active renewal event during which oxygen increased and N2 fell again. The recovery period appears to be caused by lateral mixing with new waters entering at shallower depths on the western side of the inlet.

Fresh oxygen for the Baltic Sea — An exceptional saline inflow after a decade of stagnation

The ecological state of the Baltic Sea depends crucially on sufficiently frequent, strong deep water renewal on the periodic deep water renewal events by inflow of oxygen rich saline water from the North Sea. Due to the strong density stratification these inflows are the only source for deep water ventilation. Since the early eighties of the last century the frequency of inflow events has dropped drastically from 5 to 7 major inflows per decade to only one inflow per decade. Wide spread anoxic conditions became the usual state in the central Baltic. The rare major Baltic inflow (MBI) events in 1993 and 2003 could interrupt the anoxic bottom conditions only temporarily. After more than 10years without a major Baltic inflow events, in December 2014 a strong MBI brought large amounts of saline and well oxygenated water into the Baltic Sea. Based on observations and numerical modeling, the inflow was classified as one of the rare very strong events. The inflow volume and the amount of salt transported into the Baltic were estimated to be with 198km3 and 4Gt, respectively. The strength of the MBI exceeded considerably the previous 2003 event. In the list of the MBIs since 1880, the 2014 inflow is the third strongest event together with the MBI in 1913. This inflow event will most probably turn the entire Baltic deep water from anoxic to oxic conditions, with substantial spread consequences for marine life and biogeochemical cycles.

Oxygen in the deep Strait of Georgia, 1951–2009: The roles of mixing, deep‐water renewal, and remineralization of organic carbon

The concentration of oxygen in the deep Strait of Georgia has declined at a rate of 0.45–1.3 µmol L−1 yr−1 since 1971 and now seasonally approaches thresholds of biological tolerance. The decline has resulted principally from the increasing hypoxia of upwelled Pacific Ocean water. Local anthropogenic loadings have little potential to reduce bottom water oxygen concentrations. Hypoxic, upwelled water mixes vigorously with surface water in the tidal passages of Haro Strait before entering the deep Strait of Georgia during a series of deep‐water renewal events in late spring and late summer. Remineralization of at least 22 µmol L−1 yr−1, together with diffusive mixing, reduces the bottom water oxygen concentration to a winter minimum of 90–110 µmol L−1. Linear extrapolation of the long‐term trend indicates that parts of the Strait could become episodically hypoxic (&lt; 62.5 µmol L−1; &lt; 1.4 mL L−1) as early as 2042. However, water mass modeling shows that the mixing with surface water in Haro Strait limits the potential of the shelf water to reduce the oxygen concentration in the deep Strait; even should the shelf water become completely anoxic, the concentration of oxygen in bottom waters would level off just above 90 µmol L−1 after 3 yr. Increasing surface water temperature will reduce the solubility of oxygen, but this effect is projected to cause a further decline of only 2.2 µmol L−1 over the next 25 yr. Despite its restricted circulation, the deep Strait of Georgia is less threatened by hypoxia than are many other coastal areas, because of the combination of light‐limited primary production and intense tidal mixing in the approaches, which replenishes the deep‐water oxygen annually.

A box model of the seasonal exchange and mixing in Regions of Restricted Exchange: Application to two contrasting Scottish inlets

We present a time-dependent box model of exchange and mixing processes in Regions of Restricted Exchange (RREs). The model is applied to two contrasting Scottish inlets, but can potentially be applied to a wide range of stratified inshore water bodies. The model represents the vertical structure of the inlet using up to three horizontally uniform layers, representing surface, intermediate and deep basin water respectively, and calculates the daily volume, thickness, salinity and temperature of each layer over an annual cycle. The model is forced by observed time series of daily-mean wind stress, river discharge, surface heat flux, and depth-profiles of the external coastal temperature and salinity. Advective and diffusive fluxes between the layers, and between the inlet and adjacent coastal ocean, are calculated from parameterisations of known physical processes in inshore stratified waters, utilising published analytical solutions and empirically-based formulae. The goal is for the model to be generally applicable to RREs to support improved resource management. As such, the number of free parameters within the model has been kept to a minimum.The model predictions of layer temperature and salinity are quantitatively compared to observations from year-long sampling programmes in two contrasting Scottish inlets, Loch Creran and Loch Etive. The model showed good agreement with the observed data, reproducing the broad scale seasonal cycle and other shorter-term fluctuations. RMS errors for temperature were less than 1 °C, and for salinity typically of the order 1. The model results suggest that Loch Creran behaved as a well-mixed box, with weak stratification maintained by strong vertical diffusion, and volume exchange between the inlet and coastal waters dominated by tidal forcing. In contrast, Loch Etive exhibited classical fjordic three-layer hydrography and dynamics, with strong stratification, weak vertical diffusion and a density-driven circulation comparable in strength to tidal exchange. The effects of a deep water renewal event on water properties were successfully reproduced by the model. The potential for wider application of the model to resource management issues and to other types of RRE is discussed.

Impact of deep-water renewal events on fixed nitrogen loss from seasonally-anoxic Saanich Inlet

We interpreted profiles of N 2/Ar ratios, δ 15 N – N 2 , and O 2 concentration collected in Saanich Inlet, British Columbia, Canada over an annual cycle. Our measurements and data from a regional cabled observatory indicated that four deep- or bottom-water renewal events occurred over our study period. Each event was correlated with a period of weak tidal currents, such that very low tidal mixing allowed inflowing water to retain its high density as it moved across the sill and into the deeper basin. By quantifying the concentration of excess N 2 in each month and the vertical diffusion rate, we determined that the N 2 production rate ranged from 1.7 ± 0.3 mmol N 2 m − 2 d − 1 in summer to 8.1 ± 2.8 mmol N 2 m − 2 d − 1 in winter. This depth-integrated estimate accounts for all pathways resulting in fixed (bioavailable) nitrogen loss as N 2 gas, including denitrification and anammox, and incorporates any benthic production of N 2 that diffuses into the overlying water column. In spring and summer, the maximum N 2 excess corresponded to the maximum δ 15 N – N 2 , indicating that denitrification approached completion. In these months, the average isotopic composition of the fixed N consumed was 7.5 ± 1.2‰. Following bottom-water renewal in fall, which brought in nutrient-rich, low-N 2 water, the N 2 concentration increased and became progressively more enriched in 15N. The high rates of N 2 production in Saanich Inlet likely exist in other anoxic basins that undergo periodic deep-water renewal by nitrate-rich waters.

Depth distribution during the life history of Neocalanus plumchrus in the Strait of Georgia

Neocalanus plumchrus is the most common large copepod in the Northeast Pacific, and plays an important role in the ecosystems of that area; it is particularly common in the Strait of Georgia. In this study, seasonal changes in the vertical distribution of late-stage N. plumchrus at a single site in the Strait of Georgia were measured with an optical plankton counter (OPC) and depth-stratified net samples. Particles measured in situ by the OPC were compared to an empirically measured size distribution for live N. plumchrus. Below 200 m (primarily overwintering individuals), OPC estimated abundance usually corresponded to abundance estimates from net samples. Abundance in the top 100 m was consistently overestimated by the OPC, relative to results from net samples. During overwintering, the vertical distribution shifted downward over time, coinciding with deep-water renewal events in the Strait, and was followed by a partial upward spawning migration by newly moulted adults. Spatial structure at several scales was inconsistent, but significantly different from random. A simple simulation model of depth as a function of time showed that both hydrographic changes and endogenous changes within the animals may influence depth distributions, and suggests that maintenance of position in the water column during diapause is not a passive process.

The Holocene record of Loch Etive, western Scotland: Influence of catchment and relative sea level changes

Two sediment cores from inner Loch Etive, a deep fjord basin on the west coast of Scotland, reveal a continuous sediment sequence spanning the last 10,000 yr. Benthic foraminiferal assemblages indicate that marine conditions prevailed in Loch Etive throughout the Holocene. However, changes in sediment grain size composition and magnetic susceptibility suggest that the strength and frequency of deep water renewal events has varied through time. This exchange with coastal water is controlled mainly by relative sea level changes (influencing sill depth) and the influx of freshwater to the loch. The very early Holocene sediments are well-sorted coarse silt–fine sands, with a diverse assemblage of calcareous foraminifera indicating frequent renewal of bottom waters. During this period, immediately following the withdrawal of Younger Dryas glaciers, denudation rates were high in the catchment area. The period 9–7 ky BP, during which relative sea level rose in this part of W Scotland, is characterised by highly variable environments. The grain size data indicate an enhanced transport of sandy bed load material from the ‘Bonawe Sill’ region to the basin deep during the mid-Holocene relative sea level high (8–9 m higher than present). High relative sea levels at this time would explain the occurrence of frequent, vigorous deep water renewal events and an increased tidal current regime in the loch. The late Holocene record is characterised by an upward fining of sediment grain size, disappearance of calcareous benthic foraminifera and increasing organic content; suggesting a development towards a more restricted fjordic circulation in response to the progressive lowering of relative sea level. A sudden acceleration in this late Holocene trend took place during the last millennium, during which time human land use and deforestation have influenced the sediment and organic material flux to Loch Etive. It appears that average Holocene sediment accumulation rates in the ‘Bonawe Deep’ were about 0.01–0.03 g cm − 2 yr − 1 , which is far below modern estimates for the site. This can be explained by vigorous bottom current activity during much of the Holocene, leading to a low net depositional rate.

On Deep-Water Renewals in Indian Arm, British Columbia: Sensitivity to the Production of Turbulent Kinetic Energy Caused by Horizontal Variations in the Flow Field

Abstract A two-dimensional (i.e., laterally averaged) numerical model of the circulation in Burrard Inlet and Indian Arm near British Columbia, Canada, is used to examine the sensitivity of deep-water renewal events in Indian Arm to the turbulent mixing in the lee of the narrow sills in Burrard Inlet. Horizontal variations in the flow field can have an important influence on the production of turbulent kinetic energy near the sills and therefore also on the renewal events in Indian Arm. An ad hoc modification to the expression for the production of turbulent kinetic energy, required to obtain an acceptable simulation downstream of Second Narrows in Burrard Inlet, also results in a reasonable simulation of the observed circulation in Indian Arm. The modified laterally averaged model can reproduce the main features of the circulation away from the narrow sills. However, it seems that a three-dimensional model will be required if the circulation is to be simulated with greater accuracy and without the ad hoc modification, which has a free parameter.

A preliminary investigation of basin water response to climate forcing in a Scottish fjord: evaluating the influence of the NAO

The sea lochs (fjords) of NW Scotland bridge the land–ocean interface in a region of Europe which is particularly well situated to monitor changes in westerly air streams. Inter-annual atmospheric circulation changes at this latitude are largely governed by the North Atlantic Oscillation (NAO). Comparing two recent extreme NAO years, a two-dimensional model of Loch Sunart, NW Scotland, is used to examine the potential effects of climate oscillations on the magnitude and frequency of deep-water renewal events and the resulting water properties in the fjord basins. In the upper basin of the fjord, meteorological forcing during the high NAO index year (1988–89) resulted in less-frequent deep-water renewal, greater variability in basin salinity and a lower annual-mean salinity (by 0.52) than that predicted for the low NAO index year (1995–96). In the main basin, variations in meteorological forcing had much less effect on basin water properties. In both basins, predicted deep water inflow was significantly greater during the negative phase NAO, with annual inflow to the upper and main basins being respectively 50% and 300% greater during 1995–96 relative to 1988–99. Through a sensitivity analysis, the NAO is shown to affect upper basin water properties through the influence of low-frequency anomalies in the meteorological forcing, particularly the enhanced westerly wind stress associated with positive phases, which inhibits deep water renewal over the winter months. The salinity of the main and upper basins respond differently to the boundary forcing due to differential tidal mixing above the respectively sub- and super-critical entrance sills. Predictions of basin water isotope ratios are made by applying the salinity: δ 18O water mixing line for the region to the salinity results; the combination of the weak dependence of δ 18O water on salinity and the stable salinity predicted for the main basin suggests that some fjord basins may provide ideal sites for palaeotemperature studies.

Deep-water renewal in a Scottish fjord: temperature, salinity and oxygen isotopes

The sea lochs (fjords) of north-west Scotland are located in a region of Europe particularly well situated to monitor changes in westerly air streams. The moisture transported in these air streams has a profound effect on regional precipitation, freshwater run-off and, in turn, sea loch circulation. The gentle slope of the regional salinity:?18O mixing-line, defined as 0.18 ‰ per salinity unit, suggests that the temperature: ?18O relationship may be readily resolved in these coastal waters. Deep-water renewal events, both observed and predicted from empirical models, in the bottom-waters of Loch Etive provide an opportunity to assess the temperature, salinity and ?18O relationship. Predicted changes in ?18Ocalcite as a function of changing salinity (?S) and changing temperature (?T) during deep-water renewal events suggest that >80% fall above analytical detection limits. The theoretical likelihood of recording such renewal events in the “palaeoclimate” record appears to be promising, but temperature and salinity change during renewal events may have either sign. Scottish fjords, because of the relatively small impact which salinity has on ?18Owater, may provide useful study sites in palaeoclimate research, particularly where palaeotemperature is the primary record of interest.

Deep Water Renewal in the Strait of Georgia

The Strait of Georgia is a semi-enclosed basin on the Canadian west coast in which exchange with the shelf is restricted by narrow constrictions and shallow sills. The local dynamics are mostly dominated by the mixed tides and by the estuarine circulation that is forced mainly by discharge of fresh water from the Fraser River. The intermediate and deep water of the Strait are renewed through discrete deep water renewal events during which dense water flows over the sills and into the interior basin. Several data sets are closely examined to better understand the nature and variability of the deep water renewal process over a wide range of time scales. Measured deep water renewal (DWR) events are clearly identified, as well as their effects on the water properties in the deep basin. It is found that the events can be classified into two categories: late winter intrusions bring cold, oxygen rich water, and late summer events bring warm, saline, low oxygen water. These two DWR seasons determine the annual cycle of the deep water properties in the Strait of Georgia.For both seasons, the DWR events always occur following a neap tide, at which time the density of bottom water in the sill area peaks. However, not all neap tides are followed by a DWR event. During the DWR seasons, discrete events are found to occur every second neap tide. It is believed that this monthly periodicity is required to allow enough time for the salinity (density) of the deep water near the sill to increase sufficiently following the flushing of dense local water by the previous DWR event. Also, the timing of the two DWR seasons is explained in terms of the yearly cycle of the surface and bottom water density in the estuary: DWR events are more likely to happen at the beginning (spring) and end (fall) of the coastal upwelling season, when the Fraser River discharge is not too large. Finally, it is shown that, during El Niño years, unfavorable conditions develop that can shut down the late winter DWR season, leading to much warmer deep water during the following winter.

Modeling and Observations of Deep Water Renewal and Entrainment in a Swedish Sill Fjord

A baroclinically forced deep-water renewal event in a small Swedish sill fjord is investigated using a one-dimensional numerical model and a dataset that resolves temporal variations in salinity and oxygen. The observations indicate an almost complete renewal of the basin water within a period of 2-3 weeks. The details of the renewal process are emphasized by modeling the sill flow as well as the resulting dense bottom plume, with various rates of entrainment. It is found that sill mixing is relatively unimportant, but that entrainment increases the deep-water inflow by a factor of 2-4. Different formulas for calculating plume entrainment are compared and the model's sensitivity to variations in sill flow and bottom friction is investigated. It is shown that even weak entrainment, occurring at shallow depths where the density difference between the plume and the resident water is large, has a significant impact. Entrainment prolongs the time it takes for a complete renewal (i.e., to flush out all resident water) and, on the moderate timescales considered here, yields lower post-renewal salinity and oxygen concentrations. This implies that entrainment (and mixing) during renewal may be as important as basin water diffusion in setting the timescale for forthcoming renewal events.

The Roles of Hydrographic and Biogeochemical Processes in the Distribution of Dissolved Inorganic Nutrients in a Scottish Sea-loch: Consequences for the Spring Phytoplankton Bloom

Observations taken in Loch Linnhe, a fjord on the west coast of Scotland in 1992, indicate that nutrient distributions are controlled by physical mixing processes (conservative behaviour), temporally varying end-member concentrations (apparently non-conservative behaviour) and biogeochemical processes (real non-conservative behaviour). One-dimensional steady-state mixing diagrams conventionally employed for estuarine nutrient studies cannot be used for Loch Linnhe because of the complex hydrography observed. A one-dimensional box model has been developed which accurately reproduces the hydrography of the system and has been used to isolate and quantify the different types of nutrient behaviour in the system: non-conservative behaviour is identified in the isolated bottom-waters of the basin and apparently non-conservative behaviour is identified at depths greater than 60m. Chlorophyll data show the onset of a phytoplankton bloom coinciding with a deep water renewal event. A similar sequence of events was also noted in 1990 and 1991 and it is suggested that these events are linked.

A Three-dimensional Model of Deep Water Renewal and its Influence on Residual Currents in the Central Strait of Georgia, Canada

Deep water renewal in the Strait of Georgia, British Columbia, Canada, has been modelled. Renewal occurs in the model, during summer, once a month. The phase and amplitude of renewal events compare well with those observed in the Central Strait of Georgia. A deep water renewal event starts at extreme neap tides, the time of minimal mixing, when maximum stratification is reached at Boundary Passage and the denser waters of Juan de Fuca Strait penetrate past the sill at Boundary Passage and reach the bottom of the deep basin in hte Central Strait of Georgia as a gravity current.