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We apply the three-dimensional COupled Hydrodynamical Ecological model for REgioNal Shelf seas (COHERENS) to compute water renewal time scales for Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia. Water renewal time scales are not directly observable but are derived indirectly from computational studies. Improved knowledge of these time scales assists in evaluating the water quality of coastal environments and can be utilised in sustainable marine resource management. Results from simulations with climatological September forcing are presented and compared to cruise data reported by Ribbe (2006. A study into the export of saline water from Hervey Bay, Australia. Estuarine Coastal and Shelf Science 66, 550-558). A series of simulations using idealised forcing provides detailed insight into water renewal pathways and regional differences in renewal timescales. We find that more than 85% of the coastal embayment's water is fully renewed within about 50-80 days. The eastern and western shallow coastal regions are ventilated more rapidly than the central, deeper part of the domain. The climatological simulation yields temperature and salinity patterns that are consistent with the observed situation and water renewal time scales in the range of those derived from idealised model studies. While the reported simulations involve many simplifications, the global assessment of the renewal time scale is in the range of a previous estimate derived for this coastal embayment from a simpler model and observational data.

The Taranto basin is a shallow water marine system in the South of Italy characterized by the presence of a lagoon environment together with a semi-enclosed bay connected to the Ionian Sea. This marine system experienced over the last few decades strong biochemical pollution and environmental degradation, and it is considered a hotspot study site for economic, ecological and scientific reasons. The aim of this study was to examine, on an annual temporal scale and with high spatial resolution, the main hydrodynamical processes and transport scales of the system by means of a 3D finite element numerical model application, adopting the most realistic forcing available. The model allowed us to assess the role played by baroclinic terms in the basin circulation, describing its estuarine nature. In particular, the main features of water circulation, salinity and temperature distribution, water renewal time and bottom stress were investigated. Our results allowed us to equate this system dynamic to that of a weakly stratified estuary, identifying the main driving sources of this mechanism. The vertical stratification over the whole year was proved to be stable, leading to a dual circulation flowing out on the surface, mainly through Porta Napoli channel, and inflowing on the bottom mainly through Navigabile channel. This process was responsible also for the renewal time faster on the bottom of the Mar Piccolo basin than the surface. Due to the great importance of the Taranto basin for what concerns sediment pollution, also the effect of currents in terms of bottom stress was investigated, leading to the conclusion that only in the inlets area the values of bottom stress can be high enough to cause erosion.

In this study we investigated the hydrodynamics and the interaction between riverine and marine waters in the Po River-Delta-Sea (RDS) system (Italy). Through the application of an unstructured 3D numerical model to a domain comprising the Po river branches, seven coastal lagoons and the shelf sea, many of the hydrodispersive phenomena evolving from the interaction of the different water bodies of the RDS system were described. The model was successfully calibrated and validated for water levels, fluxes at the inlets and at the river branches, and water temperature and salinity observations. The use of such a comprehensive approach allows the characterization of the general hydrodynamics of all the components of the Po RDS system, of the water exchange between different water bodies as well as the Po river discharge distribution among all branches. The analysis of calculated coastal current patterns in the prodelta of two different years and during a flood event confirms that the Po River is the main driver of the baroclinic coastal sea circulation. However, when strong winds occur (Sirocco and Bora), the surficial circulation of the shelf area is significantly modified. Generally, the lagoons of the Po RDS system can be considered hydrodynamically active since their tidal prisms represent a relevant fraction of each lagoon's volume. The natural consequence is that the lagoons show an active flushing with water renewal times in the order of few days. The hydrodynamics is mainly forced by the tide and is locally enhanced by riverine inputs and wind. Where freshwater inputs are significant, they are responsible of the lagoons salinity patterns that show high spatial gradients (not all the lagoons are well mixed) and temporal variability. The variability of the freshwater discharges and the complex morphology of the Po RDS allows the presence of both marine and riverine environments.

A multidisciplinary approach that combines field measurements, artificial neural networks, water balance analyses and hydrodynamic modelling was developed to investigate the water budget and renewal capacity of semi-closed coastal systems. The method was applied to the Lesina Lagoon, a micro-tidal lagoon in the southern Adriatic Sea (Italy). Surface water flux between the lagoon and the sea was determined by neural network prediction and used as input in the analysis. Strong seasonal variations in the water budget equation were predicted. Fresh water inputs estimated by the water balance analysis were used as forcing by a calibrated finite element model to describe the water circulation and transport time scale of the lagoon's surface waters. The model highlighted the spatial heterogeneity of the renewal behaviour of the system, with a strong east-west water renewal time gradient. Knowledge of spatial distribution of water renewal times is crucial for understanding the lagoon's renewal capacity and explaining the high spatial variability of the biogeochemistry of the Lesina Lagoon.

A comparison study between 10 Mediterranean lagoons has been carried out by means of the 3-D numerical model SHYFEM. The investigated basins are the Venice and Marano-Grado lagoons in the Northern Adriatic Sea, the Lesina and Varano lagoons in the Southern Adriatic Sea, the Taranto basin in the Ionian Sea, the Cabras Lagoon in Sardinia, the Ganzirri and Faro lagoons in Sicily, the Mar Menor in Spain, and the Nador Lagoon in Morocco. This study has been focused on hydrodynamics in terms of exchange rates, transport time scale, and mixing. Water exchange depends mainly on the inlet shape and tidal range, but also on the wind regimes in the case of multi-inlet lagoons. Water renewal time, which is mostly determined by the exchange rate, is a powerful concept that allows lagoons to be characterized with a time scale. In the case of the studied lagoons, the renewal time ranged from few days in the Marano-Grado Lagoon up to 1 year in the case of the Mar Menor. The analysis of the renewal time frequency distribution allows identifying subbasins. The numerical study proved to be a useful tool for the intercomparison and classification of the lagoons. These environments range from a leaky type to a choked type of lagoons and give a representative picture of the lagoons situated around the Mediterranean basin. Mixing efficiency turns out to be a function of the morphological complexity, but also of the forcings acting on the system. Key Points Comparative study of 10 Mediterranean lagoons using a 3-D hydrodynamic model Characterization of flushing, mixing efficiency, and water renewal Classification of lagoons based on water renewal time and exchange rate © 2014. American Geophysical Union. All Rights Reserved.

The Lagoon of Nador (Morocco) has been subject to relevant man-made changes in recent years, which consisted of closing the inlet and opening a new one, wider and deeper, with long lateral jetties. In this paper, we study some of the consequences of this modification, using a 3D hydrodynamic model that allows a comparison of the two configurations (old and new inlet). This would be not possible analysing only the observations and the approach we followed is a general method that can be applied wherever man-made or natural changes affect a coastal system. We have created two numerical grids, one with the old inlet and one with the new inlet, both covering the entire lagoon and a wide part of the coastal area. Using these two configurations, fully baroclinic simulations were run for one year. The results show a significant increase of the tidal amplitude and of the water circulation inside the lagoon, with a tidal prism four times larger. The change of the tidal asymmetry from positive to negative values (−0.7 in the inlet) causes a shift toward the ebb dominance, with implications for the mid-term morphological evolution. With the new inlet the water renewal time of the lagoon decreased from 57 to 15 days and the water temperature got warmer in winter (+1 °C) and colder in summer (−2 °C). As a consequence of this, the evaporation lowered and the salinity reduced by 0.8 PSU. The seasonal variability of the water renewal time shows that the tidal forcing, the baroclinic circulation and the wind contribute to the mixing processes. The mixing capacity of the lagoon (0.7) did not change because it is an intrinsic property of the lagoon morphology and of the atmospheric forcing. Finally, we found that the change of the water renewal time, with the new inlet, is higher than the change predicted, with the old inlet, under the RCP8.5 climate change scenario.

The aim of this study is to estimate the water renewal time in the Boughrara lagoon. This is a key parameter for ecological studies of the lagoon. The method used consists of the injection of a soluble tracer which has a concentration of zero at the open boundaries of the domain (C0 = 0) and is equal to one inside the lagoon (C1 = 1). In this study, we use numerical simulation modules developed by Deltares: the hydrodynamic Delft-Flow module and the Horizontal Large Eddy Simulation (HELS) module. We compute the horizontal advection and dispersion and get estimates, at every mesh of the computational grid, of the time needed for the tracer's concentration inside the lagoon to drop from 1 (C1) to 0.38. This time is known as the Local Renewal Time (LRT). A spatial distribution analysis of the LRTs enables a subdivision of the domain based on the similarity of values near the LRT (at every mesh of the domain) and on the physical configuration of the domain (e.g. bay, deep/shallow zone, harbor). Then it becomes simple to compute a renewal time for each region in the domain, which is the Integral Renewal Time (IRT). If we want to take into account the whole domain, we consider the maximum value of the IRT. The numerical simulations were run taking into account the effect of the predicted tide. The computational grid has a horizontal mesh size of 150 m × 150m. The LRT under only the effect of the tidal forcing at the zone where an aquaculture farm is located is estimated at one week. Moreover, in the region of the mini-gulf of Guellala, this LRT is estimated to be one month. The encounter between the waters coming from the two channels of the lagoon occurs after a period of four months in the zone bounded to the south by the island of Djilij and to the north by the mini-gulf of Guellala. South of the fishing port of Boughrara and Djilij Island, the local water renewal time varies between 5.5 and 6.5 months.

The hydrological consequences of historical, contemporary and future human activities on a coastal system were investigated by means of numerical models. The changes in the morphology of the Lagoon of Venice during the last century result from the sedimentological response to the combined effects of human interventions on the environment and global changes. This study focuses on changes from 1927 to 2012 and includes the changes planned for the protection of the city of Venice from storm surges and exceptional tides under future sea level rise scenarios. The application of a hydrodynamic model allowed for the analysis of the morphological effects on the lagoon circulation, the interaction with the sea and the internal mixing processes. The absolute values of the exchange between the lagoon and sea increased from 1927 to 2002 (from 3900 to 4600 m3 s−1), while the daily fraction of lagoon water volume exchanged decreased. At the same time, the flattening of the lagoon and loss of morphological heterogeneity enhanced the internal mixing processes driven by the tide and wind, reducing thus the overall water renewal time from 11.9 days in 1927 to 10.8 days in 2002. Morphological changes during the last decade reduced the water exchange through the inlets and induced an increase of the basin-wide water renewal time of 0.5 day. In the future, Venice Lagoon will evolve to a more restricted environment due to sea level rise, which increases the lagoon volume, and periodical closure of the lagoon from the sea during flooding events, which reduces the communication with the open sea. Therefore, the flushing capacity of the lagoon will decrease considerably, especially in its central part. Furthermore, some considerations on the impact of the hydromorphological changes on the ecological dynamics are proposed.

The Red Sea is a deep marine basin often considered as small‐scale version of the global ocean. Hydrographic observations and ocean‐atmosphere modeling indicate Red Sea deep water was episodically renewed by wintertime open‐ocean deep convections during 1982–2001, suggesting a renewal time on the order of a decade. However, the long‐term pacing of Red Sea deep water renewals is largely uncertain. We use an annually resolved coral oxygen isotope record of winter surface water conditions to show that the late twentieth century deep water renewals were probably unusual in the context of the preceding ~100 years. More frequent major events are detected during the late Little Ice Age, particularly during the early nineteenth century characterized by large tropical volcanic eruptions. We conclude that Red Sea deep water renewal time is on the order of a decade up to a century, depending on the mean climatic conditions and large‐scale interannual climate forcing.

Cyclonic frontal eddies are distinguishable from the surrounding water due to their unique biological and physical characteristics and have been observed in all western boundary current regions. These eddies spawn from cut-off meanders and are found on the landward side of the current. Here, we report for the first time observed frontal eddies for the intensification zone (north of 28°S) of the East Australian Current (EAC) off Southeast Queensland, Australia, by analysing remotely sensed sea surface temperature (SST) and chlorophyll-a (Chl-a) data. The frontal eddies were detected initially in the analysis of satellite tracked surface drifters. The shelf-crossing cyclonic drifter pathways indicated the presence of drifter-trapping cyclonic frontal eddies. The subsequent analysis of satellite images allows to quantify key eddy characteristics, cross-shelf volume transports associated with eddy filaments, eddy-driven shelf water renewal time scales, and export of total Chl-a and carbon per day. The observed frontal eddies have core radii of approximately 13 km and 15 km. The cold core surface SST anomaly and elevated chl-a indicates eddy entrainment of shelf water. The translational or core displacement velocity is estimated with 0.17 m s−1 or 15 km per day and the tangential velocity quantified from tracking surface drifters is 0.28 m s−1 to − 0.5 m s−1. This results in a rotational period of 1.9 days to 3.9 days. We use maximum Chl-a and SST gradients to approximate the width of importing and exporting filaments associated with the frontal eddy to derive volume transports of 1.5 Sv and 1.9 Sv (import) and 0.3 Sv and 1.8 Sv (export), respectively. Chl-a concentrations of the exporting filaments are about 0.4 mg m−3 to 0.6 mg m−3 yielding a total export of 13–78 t of Chl-a per day. The frontal eddy induced on-shelf transport of 130 km3–160 km3 per day represents between 18% and 22% of the shelf volume. Therefore, it would take approximately five days to renew all shelf water. We conclude that the observed frontal eddies of the northern intensification zone of the EAC potentially play an important role in determining cross-shelf exchanges, contribute to on-shelf marine conditions, enhancing coastal primary productivity and are possibly important to the export of shelf water properties such as the fish larvae of subtropical species via entrainment.

In this study, we explored the impact of eutrophication and stratification on hypoxia in deep waters of moderately warm Croatian marine lakes. Although the Mljet Lakes (MLs) are predominantly oligotrophic, mesotrophic conditions are present at depths below 20 m in the Small Lake (SL) and below 30 m in the Big Lake (BL), along with higher apparent oxygen utilization (AOU). Hypoxia at depths ≥ 25 m in SL and and ≥ 40 m in BL was observed between October 2009 and January 2010, and in SL in summer (July and September 2010). Significant differences (p < 0.05) in several physical, biological and chemical parameters were detected between the lakes, while AOU, derived oxygen utilization rate (OUR) and organic carbon remineralization rate (OCRR) were not significantly different (p >0.05) between the lakes. An intense and persistent pycnocline throughout the year, comparatively high water temperature, extended water renewal time and summer phytoplankton bloom were identified as physical and biological parameters which might have significantly contributed to increased frequency of hypoxic events in a shallow SL. Significantly (p < 0.05) higher ammonium concentration in SL, especially in its deep water, seems to be a long-term chemical feature related to the poor ventilation and higher sediment oxygen demand. At the current level of eutrophication and the present climate change trends, the MLs and similar systems may experience more persistent and intense stratification, which could further prevent mixing between upper and deep waters, likely leading to increasing duration of hypoxia and its negative impacts on the biodiversity of benthic communities. En este estudio, exploramos el impacto de la eutrofización y la estratificación sobre la hipoxia, en aguas profundas de los lagos marinos croatas moderadamente cálidos. Aunque los lagos Mljet (LsM) son predominantemente oligotróficos, las condiciones mesotróficas están presentes a profundidades inferiores a 20 m en el Lago Menor (LMe) y a 30 m en el Lago Mayor (LMa), junto con un mayor uso del oxígeno aparente (UOA). Fue observada hipoxia a profundidades ≥ 25 m en el LMe y ≥ 40 m en el LMa entre octubre de 2009 y enero de 2010, y en LMe en verano (julio y septiembre de 2010). Fueron detectadas diferencias significativas (p < 0.05) en varios parámetros físicos, biológicos y químicos entre los lagos, mientras que el UOA, la tasa de utilización de oxígeno derivado y la tasa de remineralización de carbono orgánico no fueron significativamente diferentes (p > 0.05) entre los lagos. La intensa y persistente picnoclina a lo largo del año, la temperatura comparativamente alta del agua, el tiempo prolongado de renovación del agua y la floración estival del fitoplancton se identificaron como parámetros físicos y biológicos que podrían haber contribuido significativamente incrementando la frecuencia de eventos hipóxicos en un LMe superficial. Una concentración de amonio significativamente mayor (p < 0.05) en LMe, especialmente en sus aguas profundas, parece ser una característica química a largo plazo relacionada con la escasa ventilación y la mayor demanda de oxígeno en los sedimentos. Al nivel actual de eutrofización con las tendencias actuales del cambio climático, los LsM y sistemas similares podrían experimentar una estratificación más persistente e intensa, lo que podría prevenir aún más la mezcla entre aguas más superficiales y profundas, conduciendo posiblemente a un incremento en la duración de la hipoxia y a los impactos negativos sobre la biodiversidad de las comunidades bentónicas.

This paper examines the on-shelf circulation of the eastern Australian continental shelf for a region off southeast Queensland. We identify a characteristic seasonally reoccurring wind-driven cyclonic flow. It influences the cross-shelf exchange with the East Australian Current (EAC), which is the western boundary current of the South Pacific Ocean. We refer to this cyclonic circulation as the Fraser Gyre. It is located south of Fraser Island between about 25 °S and 27 °S. The region is adjacent to the intensification zone of the EAC where the current accelerates and establishes a swift, albeit seasonally variable southward boundary flow. Through the analysis of several data sets including remotely sensed sea surface temperature and sea surface height anomaly, satellite tracked surface drifters, ocean and atmospheric reanalysis data as well as geostrophic currents from altimetry, we find that the on-shelf Fraser Gyre develops during the southern hemisphere autumn and winter months. The gyre is associated with a longshore near-coast northward flow. Maximum northward on-shelf depth averaged velocities are estimated with about 0.15–0.26 ms-1. The flow turns eastward just to the south of Fraser Island and joins the persistent southward EAC flow along the shelf break. The annual mean net cross-shelf outward and inward flow associated with the gyre is about -1.17 ± 0.23 Sv in the north and 0.23 ± 0.13 Sv (1 Sv = 106 m3s−1) in the south. Mean seasonal water renewal time scales of the continental shelf are longest during austral winter with an average of about 3.3 days due to the Fraser Gyre retaining water over the shelf, however, monthly estimates range from 2 to 8 days with the longer timescale during the austral autumn and winter. The southerly wind during austral autumn and winter is identified as controlling the on shelf circulation and is the principal driver of the seasonally appearing Fraser Gyre. The conceptual model of the Fraser Gyre is consistent with general physical principals of the coastal shelf circulation. A southerly wind is associated with surface layer flow toward the coast, a near coast positive SSHa with a current in the direction of the wind, down-welling and export of shelf water. The Fraser Gyre influenced cross-shelf exchanges are possibly facilitating the offshore transport of fish larvae, sediments, nutrients, river discharges, and other properties across the shelf break and into the southward flowing EAC during the austral autumn and winter.

International audience; Globally, there has been a decrease in dissolved oxygen in the oceans, that is more pronounced in coastal waters, resulting in more frequent hypoxia exposure for many marine animals. Managing hypoxia requires an understanding of the dynamics of dissolved oxygen (DO) where it occurs. The French coast facing the Bay of Biscay (N-E Atlantic Ocean) hosts at least a dozen tidal and turbid estuaries, but only the large estuaries of the Gironde and the Loire, are subject to a continuous monitoring. Here, we compared the DO dynamics in these two systems, in order to evaluate to what extent it is possible to predict the sensitivity to hypoxia of a tidal and turbid estuary based on its most common and easily accessible characteristics (the estuary surface area and length, liquid and solid discharge, suspended particulate load, water renewal time, presence of cities). Whereas the hyperturbid Gironde estuary is the longest (170 km), implying longer water and particle residence times, and is bordered by a large metropole (>750,000 inhabitants), only episodic summer hypoxia events (DO < 2 mg L−1) were recorded in its urban Garonne subestuary. In contrast, the turbid Loire estuary, smaller in terms of surface area and length, experiences permanent summer hypoxia. This demonstrates the inability to evaluate the susceptibility of a tidal and turbid estuary to hypoxia only by considering its general characteristics. We urge that there is a need for a pertinent DO survey strategy based on an initial assessment of DO in a specific estuary based on longitudinal investigations, in particular during the warm season (as higher water temperatures decrease DO solubility), for detecting the potential zones of minimum DO.

A temporally and spatially detailed historical (1985-2018) analysis of cyanobacteria blooms was performed in the Curonian Lagoon (Lithuania, Russia), the largest coastal lagoon in the Baltic Sea. Satellite data allowed the mapping of cyanobacteria surface accumulations, so-called "scums", and of chlorophyll-a concentration. The 34-year time series shows a tendency towards later occurrence (October-November) of the cyanobacteria scum presence, whereas the period of its onset (June-July) remains relatively constant. The periods when scums are present, "hot moments", have been consistently increasing in duration since 2008. The differences in the starting, ending and annual duration of cyanobacteria blooms have been significantly altered by hydro-meteorological conditions (river discharge, water temperature, and wind conditions) and their year-round patterns. The most important environmental factors that determined the temporal changes of the scum presence and area were the standing stock of cyanobacteria and the ambient wind conditions. The "hotspots", the areas where the blooms most likely occur, were distributed in the south-southwestern and central parts of the lagoon. The least affected areas were the northern part, which is connected to the coastal waters of the Baltic Sea, and the Nemunas River delta region. The longstanding, well-established spatial patterns of cyanobacteria blooms were linked to hydrodynamic features, namely water renewal time and current patterns, and to potential nutrient sources that included muddy sediments and the locations of colonies of piscivorous birds. Our findings confirmed that the annual and seasonal variations of cyanobacteria blooms and their regulation are a complex issue due to interactions between multiple factors over spatially and temporally broad scales. Despite great progress in the prevention and control of eutrophication and cyanobacteria blooms, the lagoon is still considered to be in a poor ecological status. This work provides a new and missing understanding on the spatial and temporal extent of cyanobacteria blooms and the factors that govern them. Such an understanding can help in planning management strategies, forecasting the magnitude and severity of blooms under changing nutrient loads and potential climate scenarios.

The aim of this study was to investigate the variability of the water exchanges in the Curonian Lagoon based on the hydraulic regime and the atmospheric forcings. A finite element hydrodynamic model has been applied to the Curonian Lagoon to simulate the circulation patterns for ten years. With the help of a transport-diffusion model the salinity distribution and the renewal times of the Curonian Lagoon have been investigated when forced by river runoff, wind and Baltic Sea level fluctuations. The hydrodynamic model has been validated using in situ salinity measurements. Model results show that the variability depends mainly on seasonal changes in hydrographic forcing and on the dominant wind regimes that prevail over the Curonian Lagoon. Exchanges between the southern and the northern part of the lagoon are mostly depended on the wind forcing and are much less influenced by the river discharge. However, when looking at the water renewal time, the most important factor is the river discharge into the lagoon. Other physical forcings are only marginally determining the renewal time, and not even ice cover is able to influence it. Even if ice cover is strongly inhibiting the exchanges between southern and northern lagoon, it is basically not able to change the absolute value of the renewal times.

New observations of physical, biological, and chemical parameters were obtained between July 2003 and February 2005 in Nitinat Lake, British Columbia, a permanently anoxic seawater fjord. These observations include continuous profiles of nitrate and bisulfide using a new instrume nt for in situ ultraviolet spectrophotometry. Anoxic seawaters in this 200-m–deep basin can be found as shallow as 7 m. However, Nitinat Lake is surprisingly dynamic, with a deep-water renewal time of only about 5 yr and significant spatial and temporal variability. Exchange with the ocean is limited by the size of the linking channel, and during low tides the outflow is subject to an hydraulic control. This nonlinearity causes a relatively large fortnightly cycle in water level and explains flooding events associated with heavy rainstorms. In addition, the restricted flow makes deep-water renewal gradual so that anoxic conditions are preserved. Intermediate waters (5–40 m in depth) are renewed alternately by oxygenation from surface mixing in winter and by upwelling of anoxic water and horizontal inflow of subducted ocean water in summer. The horizontal inflow results in a subsurface region of suboxic water in mid-fjord during summer. No suboxic regime is evident at either the river end of Nitinat Lake (where oxygen and sulfide–rich regions adjoin in a nitrate-depleted water column) or at the ocean end of Nitinat Lake (where oxygen and nitrate–rich regions sometimes overlap with sulfide-rich regions). Maximum phytoplankton levels near the surface are high in both winter and summer. A second peak in biological activity appears near the anoxic boundary, possibly as a result of sulfur-oxidizing bacteria. Sulfate-reducing bacteria are also present in anoxic water in both seasons and just above the oxic/anoxic interface in winter and just above the suboxic/anoxic interface in summer.

Tidal and wind renewal of water in the coastal zone is important in the transport of natural and anthropogenic particles and solutes. Models of these systems require detailed parameterization of exchange processes. Renewal of seston food supplies for cultured suspension-feeding shellfish is one such application. This study examines the sensitivity of food limitation estimates of an inshore region for shellfish aquaculture to the methods used in calculating the region's water renewal time. The region is the Indian Island oyster culture site in the Richibucto estuary, New Brunswick, Canada, and the methods considered are the volume advection and the local conservative tracer methods. In both cases, water renewal is calculated from field time series measurements of hydrodynamic parameters and outputs of coupled hydrodynamic and advection–dispersion three-dimensional (3D) numerical models. A comparison of oyster food demand and supply is then made to estimate the food limitation risk in the region using field measurements of biological parameters and the renewal times previously estimated. It is found that water renewal by tides is efficient but it can be cancelled altogether by meteorological forcing. Contrary to the conservative tracer method, the tidal prism method proves to be inadequate when estimating water renewal in estuarine tidal channels and was replaced by another volume advection method. The preliminary results obtained using the depletion index estimates indicate that the Indian Island region could sustain a high oyster biomass. These simple calculations also show the importance of water renewal estimate for carrying capacity studies. Hence, the tidal prism method or other volume advection estimates should be used with great caution or be avoided altogether in such studies.

The Arctic Ocean likely impacts global climate through its effect on the rate of deep-water formation and the subsequent influence on global thermohaline circulation. Here, the renewal of the deep waters in the isolated Canadian Basin is quanitified. Using hydraulic theory and hydrographic observations, the authors calculate the magnitude of this renewal where circumstances have thus far prevented direct measurements. A volume flow rate of Q = 0.25 ± 0.15 Sv (Sv ≡ 106 m3 s−1) from the Eurasian Basin to the Canadian Basin via a deep gap in the dividing Lomonosov Ridge is estimated. Deep-water renewal time estimates based on this flow are consistent with 14C isolation ages. The flow is sufficiently large that it has a greater impact on the Canadian Basin deep water than either the geothermal heat flux or diffusive fluxes at the deep-water boundaries.

Journal article We applied a three-dimensional general ocean and coastal circulation model to the Irish Sea in order to determine water renewal time scales in the region. The model was forced with meteorological data for 1995, a year with relatively warm summer and when extensive hydrographic surveys were conducted in the Irish Sea. We investigated intra-annual variability in the rates of net flow through the Irish Sea and carried out several flushing simulations based on conservative tracer transport. The results indicate that the net northward flow of 2.50 km(3)/d is seasonally highly variable and under certain conditions is reversed to southward. The variability in obtained residence times is high; baroclinic effects are significant. Obtained results point at the importance of spatial and temporal consideration for transport of pollutants in the shelf seas. Implications for management are numerous and involve activities such as transport, fishing, use of resources, nature conservation, monitoring, tourism and recreation. Environmental Protection Agency (Ireland) peer-reviewed

Anodonta grandis grandis is found in about half of 50 Experimental Lakes Area lakes surveyed but is abundant in only some of these lakes, including lake 377. Lake 377 is a typical small Precambrian Shield lake, 27.7 ha in area and 17.9 m in maximum depth, with [Ca2+] of [Formula: see text], conductivity of 25 μmho∙cm−1 (1 mho = 1 S), and alkalinity of [Formula: see text]. The water renewal time of approximately 187 days is shorter than that of most Precambrian Shield lakes. Bottom sediments in the sublittoral zone ranged from fine sand through granules to cobbles and boulders. Several species of possible glochidial host fish including yellow perch were collected from lake 377. The size of the mussel population, estimated by depth-stratified random sampling, was 36 800 ± 12 000 (± 95% confidence interval). Mean density was 0.133 mussels/m2 lake surface, and maximum density was 4.3 mussels/m2. Mussels were most abundant in the 1.5- to 3.1-m depth stratum. Mean lengths and weights in collections ranged from 77 to 87 mm and from 43 to 56 g, respectively. Maximum length and weight were 117.9 mm and 109.6 g, respectively. Based on external annuli, mussels live to 15+ years in lake 377. Flesh and shell averaged 25.1 and 23.2% of live weight, respectively. Calcium constituted 44.7% of the ash weight of shell. We estimated a standing dry weight biomass of mussels of 330–390 mg∙m−2 and dry weight production of 60 mg∙m−2∙year−1. This is [Formula: see text] of the estimated annual dry weight algal production. The shells of live mussels contain [Formula: see text] of the total calcium in lake 377. Despite oligotrophic conditions and low [Ca2+], lake 377 supports a substantial population of A. g. grandis growing at a moderate rate. Lake 377 may be a favourable habitat for this species because of its short water-turnover time.

This study describes some physical oceanography features of an estuary maintained by fresh groundwater inflow. The mesoscale variability of currents, temperature and salinity are documented from moored current meter observations of 12 days duration at three stations. Current speeds exceeded 0.7m.s–1 during spring tides and decrease to 0.1m.s–1 during neap tides. Current directions followed the major channel axes, reversing by 180 degrees between the flood and the ebb tides. Harmonic analysis indicated that tidal currents were mainly semi-diurnal, with a predominance of the M2 harmonic, but included M4 and M6 sub-harmonics, possibly resulting from non-linear interactions of the flow with the shallow bottom and channel curvatures. Close to the lagoon entrance the water is of marine character but, further upstream, salinity decreased significantly. In the absence of river runoff, this decrease must result from some yet ndisclosed underground freshwater inflow. The water renewal time estimated using the tidal prism method was at least two tidal cycles. Using the fresh-water fraction method, the groundwater inflow was estimated to be 2.3m3.s–1 at most. Coupled 2D hydrodynamic and advection-dispersion models (MIKE21) were then used to verify some renewal times. The tidally averaged renewal time for the whole lagoon was found to be 7 days, while the local renewal time at the upstream end of the lagoon was 25 days. Finally, an inverse modelling approach using MIKE21 was used to estimate the groundwater inflow required to produce the observed salinities. That inflow was found to be 0.25m3.s–1 for the period under consideration, showing that the tidal prism method gave flows that were far too high. African Journal of Aquatic Science 2005, 30(1): 1–10

To estimate the water-renewal time of the Gulf of Finland, a numerical-model simulation comprising the years 1987 – 1992 was carried out for realistic meteorological forcing. The results concerning water age as well as renewal index proved to reflect properties of the mean circulation system. The highest water ages (of around 2 yr) were found in the south-eastern part of the Gulf. The water-age distribution after the 5-yr simulation proved to be almost identical to that obtained after 3 yr, indicating that the circulation in the Gulf may be rather stable. The analysis of the renewal-index results showed that it takes around 5 yr to renew 98% of the water masses of the Gulf. When local maxima of the renewal index are found in conjunction with high water ages, this may indicate that the region is particularly vulnerable to eutrophication.

Wintertime deep convection was studied in the Gulf of Lions (northwestern Mediterranean) using the chlorofiuoromethane (CFM) distributions (components F11 and F12), from December 1991 and February–March 1992. Convection occurred on small spatial and temporal scales and it was not complete: only convection to 1900 m depth was observed. The vertical mixing of the water column above the convection depth turned out to be of major influence regarding the characteristic of newly formed deep water and regarding the boundary conditions for CFMs in modeling deep water formation. Air-sea gas exchange and entrainment were found to be negligible. These findings are important in estimating deep water formation rates for all water masses renewed by convection (e.g., Greenland Sea Deep Water, Labrador Sea Water) and in evaluating the spreading of deep water masses along the western boundary current. A box model is presented which uses CFM and tritium distributions to estimate formation of Western Mediterranean Deep Water below 1000 m. The model starts with zero tracer concentrations in 1945, the integration step is 1 year, and the deep water formation rate is assumed to be constant from 1945 to 1992. The best fit between CFM model concentrations and measurements were obtained at a mean deep water renewal time of 5–7 years, equivalent to a conversion of 0.46 Sv Modified Atlantic Water (0- to 150-m depth) and 0.76 Sv Levantine Intermediate Water (150- to 400-m depth) into deep water. However, the tritium measurements are only partly matched by the model, by assuming that convection stopped in the years 1964–1968.

A regression analysis of global data for freshwater phytoplankton production, chlorophyll, and various nutrient parameters revealed the following: A high proportion of the variance in both annual phytoplankton production and mean annual chlorophyll could be explained by annual phosphorus input (loading), once a simple correction for water renewal time was applied. Good relationships were also found between phosphorus loading and mean total phosphorus concentration, and between total phosphorus concentration and chlorophyll. The slope of the regression of total phosphorus on phosphorus loading for stratified lakes was not significantly different from that for unstratified lakes, suggesting that the effect of stratification on phosphorus concentration is insignificant compared to external sources of the element. Nutrient input, which was unavailable in previous analyses, appears to be an important factor in controlling freshwater production. There is some evidence for a correlation between latitude and nutrient input, and it is possible that this may explain the good correlation between latitude and production observed by earlier investigators. In an earlier synthesis of IBP freshwater primary production results, Brylinsky and Mann (1973) and Brylinsky (in press) concluded that variables related to energy availability had a greater influence on phytoplankton production than those related to nutrient availability. They found that 57% of the variability in global primary production could be explained by latitude alone. Brylinsky and Mann, however, recognized the scarcity of good data for nutrient input to lakes in the IBP data set. They suggested (p. 12) that some of the influence attributed to energy might be due to its driving of circulation systems, so that nutrient availability might be affected directly. Recently, more data on nutrients, including input and output dynamics, have accumulated, chiefly as the result of investigations of the eutrophication of lakes. These data, plus our own experimental studies in lakes, suggested that nutrients played a preeminent role in controlling phytoplankton production and standing crop. Of all the variables which may control aquatic production and standing crop. only nutrients are amenable to control by humans. It therefore seemed worthwhile to repeat Brylinsky and Mann’s analysis, including this new information (Schindler and Fee 1974). The following analysis is highly biased toward glacial lakes in north temperate and subarctic regions, where most limnological activity has been concentrated. More complete data are sorely needed for water bodies in tropical and arctic regions, in the southern hemisphere, and in unglaciated areas before a definitive global analysis of production and factors affecting it can be made. Increases in the annual input of nu

River input has long been considered as the major source of chemical elements to the ocean system. However, in recent years, it has become increasingly apparent that atmospheric transport plays a central role in the supply of material to the ocean. This atmospheric input is especially important in shelf seas and semi-enclosed seas such as the Mediterranean, which is close to potential pollution sources in Northern Europe and subjected to the well-known influx of Sahara red dust. This paper summarizes the work performed by the Land-Sea Interactions Group of CNRS (1984–1987) and the EROS-2000 programme (1987–1988) on a variety of elements and compounds in this area, where a combined study of river and atmospheric fluxes has been carried out. It is shown that the atmospheric input of red dust is of the same order of magnitude as the annual downstream flow of rivers discharging to the Western Mediterranean. As far as trace metals are concerned, the atmospheric flux of Cu, Pb and Cd exceeds river input by one to two orders of magnitude. However, the incoming flux through the straits of Sicily and (mainly) Gibraltar might be as important as the atmospheric flux; the residence times of these three metals are either shorter than the water renewal time (Pb, Cu) or equivalent to it, so that their accumulation in the water column is unlikely. As far as artificial radionuclides are concerned, atmospheric input is predominant for 239 Pu + 240 Pu and 137 Cs , whereas 238 Pu is mainly of riverine origin. The fluxes of nitrogen supplied by rivers and rain are approximately equivalent. The biological significance of this nitrogen input is discussed; it is estimated that the atmospheric nitrogen corresponds to 10% of the average ‘new’ production and may reach values as high as 50% during oligotrophic periods.

Lake Alexandrina is a shallow, well-mixed freshwater system at the mouth of the River Murray. Median water renewal time is 109 days. The lake is characterized by high turbidity and high light extinction, with mean values of 93 NTU and 6.54 In units m-1, respectively. Background extinction is high and the algal biomass contributes little to light extinction. Changes in turbidity reflect disproportionate contributions from the Murray and its major tributary the Darling. Based on total phosphorus and total Kjeldahl nitrogen levels, the lake is chemically eutrophic-hypertrophic but the nitrogen to phosphorus ratio is low and levels of filterable and soluble reactive phosphorus and especially NO3/NO2-N are often low, suggesting that most of the nutrients are in particulate form and perhaps unavailable for plant growth. From 1975 to 1978, the ulotrichous green alga Planctonema lauterbornii generally accounted for more than 95% of algal cells. It was growing under conditions of low average irradiance in the mixed water column. Chlorophyll a varied seasonally, with peak values in late spring. The summer drop in chlorophyll a correlated with low availability of nutrients, especially NO3/NO2- N. Low light availability appeared to preclude blue-green algae during the present study although blooms were recorded in 1965-1967 and 1972-1973.