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In natural lakes, where thermal stratification hinders complete mixing, the theoretical value T0 of the water renewal time provides a low-order approximation to the time T37 when 37% of the original water is still present within the lake; this time could be operatively regarded as the actual value of the water renewal time. In this paper, we present a simple nonparametric model to estimate the age distribution of water within stratified natural lakes, taking into account fundamental aspects of its mass exchange and thermal evolution. This distribution provides a straightforward way to compute T37. The model is presented as a system of ordinary differential equations along with a MATLAB script for its numerical solution, so that it can be easily applied to lakes where a minimum of limnological data are available, without the need of extensive meteorological data set and modeling expertise that an hydrodynamic model would require to the same purpose. The case of a deep oligomictic Italian prealpine lake (Lake Iseo) is considered: after a positive comparison with the results obtained using a 1-D lake hydrodynamic model, the reiterated application to the available time series allows to approximate the water age probability distribution. This distribution is used to compute the actual value of the water renewal time, that resulted T37 = 1.6T0.

This note analyzes the change in water renewal time characteristics based on reservoir action and then establishes calculation models for the water renewal time in the Yellow River mainstream. The results indicate that the amount of renewable water with reservoir action can meet the annual water demand and that water flows naturally at the Lijin station near estuary. Initial storage dynamics is an important factor in water resource renewable capacity at a certain time, and rational reservoir action can promote sustainable water resource utilization. When the initial storages in the Longyang Gorge reservoir are 9.343 and 5.343 billion m3, the water renewal times are 28 and 33.9 d, respectively. Flow stoppage appears in April and May.

Integrated classification maps were produced by combining sediment grain-size and hydrological data (water renewal time, WRT) from two Mediterranean lagoons, Lesina (LL) and Varano (LV), Italy. The geophysical characteristics of the two basins, derived from detailed bathymetric charts, are quite distinct: ∼30% of LL (mean depth ∼1 m) but only 3% of LV (mean depth ∼3 m) is shallower than 1 m. The sediments of both lagoons are mainly composed of mud (∼80%). A detailed multivariate analysis of grain-size data by EntropyMax classified the lagoon beds of LL and LV into five sedimentary facies. WRT data, computed by a hydrodynamic model, indicated different hydrological conditions in the two lagoons: LL showed a sharp west–east gradient, with a basin-wide average of ∼190 days, whilst LV showed a fairly uniform distribution and a higher basin-wide average (∼260 days). The distribution of sedimentary facies and water renewal times were combined in a composite map representing the distribution of environmental patterns. The approach outlined in this study can be used to improve zonation schemes by providing a hydromorphological perspective on transitional and coastal environments., This is the accepted version/post-print of the article Molinaroli, E.; Sarretta, A.; Ferrarin, C.; Masiero, E.; Specchiulli, A.; Guerzoni, S. Sediment grain size and hydrodynamics in mediterranean coastal lagoons: Integrated classification of abiotic parameters. Journal of Earth System Science 2014, 123, 1097-1114. Available online: https://doi.org/10.1007/s12040-014-0445-9

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 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 concentration of total phosphorus in Lake Washington before and after the sewage diversion project was simulated using first-order linear dynamics. Fluctuation in total phosphorus in the lake occurred as a forced response to changes in inflow phosphorus concentration. The dynamics of total phosphorus in Lake Washington was adequately represented by two independent time scales based on water renewal and sedimentation. The water renewal time scale was modelled as a time dependent process. Sedimentation of total phosphorus, on the other hand, appeared constant over the 16-yr period that data were available. A marked increase of total phosphorus in the lake occurred during two flood periods when high concentrations of total phosphorus corresponded to small values of the water renewal time scale (i.e., high flows). At other times, peak concentrations of total phosphorus in the inflow coincided with large values of the water renewal time scale and the lake was dynamically unable to respond to these peaks.

Lake Bolsena, the fourth Italian lake for volume (9.2âEUR?×âEUR?109âEUR?m3), must be considered as highly sensitive to eutrophication for its extremely long water renewal time. In this paper, temperature and chemical characteristics of the lake measured in the period 2003-2017 are used to discuss the mixing pattern and the variation in the oxygen and algal nutrient concentrations, as indicators of the trophic level. In the analysed period the lake showed oligomictic characteristics, reaching the full overturn, with homogenization of the chemical profile over the whole water column, only in 4 out of the 15 considered years. A regular decrease of oxygen and increase of phosphorus concentrations in the deepest layers has been observed in the non-circulating multi-year periods. The mean total phosphorus concentration showed a regular increase, reaching values close to 16âEUR?μgâEUR?P Lâ^'1 in early spring 2017, mostly because of the urban discharge from the watershed, not adequately collected from an existing sewage pipe. Chemical and mixing patterns are discussed in relation with a previous study, carried out in 1966-1971, confirming the recent increase of phosphorus concentrations and the lower frequency of full circulation. The progressive deterioration of lake water quality indicates the need of prompt actions to reduce the external nutrient load and of further studies on the physical and biological characteristics of the lake, still strongly missing.

A bay's capacity to buffer fluvial fluxes between the land and sea is sensitive to hydrological changes that can affect its water renewal rates. In Cartagena Bay, Colombia, pollution issues have been associated with freshwater fluxes which are projected to increase in future years. This has led to plans to reduce freshwater flows by constructing upstream hydraulic doors. Given the influence of freshwater discharge on coastal water renewal, it is important to assess how these upstream changes will affect the bay's hydrodynamic processes. This study calibrated the 3D MOHID Water model, configured with a high-resolution mixed vertical discretization to capture the bay's characteristic processes of vertical stratification and mixing. A Lagrangian transport model was used to analyze the flow of passive particle tracers and calculate water renewal time scales. Mean residence times of 3-6 days and flushing times of 10-20 days for canal water were found, while mean residence times of 23-33 days and flushing times of 70-99 days were calculated for the bay's complete water volume. An assessment of future scenarios showed that increases in freshwater runoff would result in faster water renewal in the bay, while plans to decrease freshwater discharge would result in slower water renewal in the bay. It is therefore imperative that any plans for reducing fluvial fluxes into the bay be accompanied by the control of local pollution sources, which are abundant and could worsen the bay's water quality issues should water renewal times become longer. International Development Research Centre, Ottawa, Canada [108747-001] EAFIT University Corporacion Autonoma Regional del Canal del Dique (CARDIQUE) [15601] Erasmus Mundus Doctoral Programme in Marine and Coastal Management (MACOMA) Fundacion Hernan Echavarria Olozoga Escuela Naval de Cadetes "Almirante Padilla" University of Wyoming's surface observation service U. Lisboa's Marine Environment Technology Center (MARETEC) info:eu-repo/semantics/publishedVersion

Data for over 100 watershed properties, including aspects of topography, hydrology, geology, soils, vegetation, lake morphometry and input precipitation chemistry, have been developed since 1980 for 316 watersheds in northern Wisconsin. The hypothesis being evaluated for this lake population is that the observed water chemistry, can be accounted for as a function of antecedent water and chemical inputs, after considering exchange processes in the lake and watershed and the lake/groundwater interactions. The variables found by regression analysis to explain observed variability in color, sulfate, and acid neutralizing capacity (ANC) levels in Wisconsin lakes are: for color, vegetative characteristics, mean depth, and water renewal times; for sulfate, precipitation concentration of sulfur, evaporative concentration, and lake water renewal time; ANC appears to be controlled by the size of the watershed, lake depth or water renewal time, and the intensity of anthropogenic inputs and cultural developments in the watershed. These results differ from previous studies in Wisconsin and nearby areas of Michigan and Minnesota by indicating that in some lakes acidity may not be in equilibrium with current precipitation chemistry.

To improve the water environment, manual water diversion projects were scheduled to bring freshwater to an urban artificial lake to dilute and divert pollutants out of the lake. A three-dimensional numerical model was used to study the effect of diversion schemes on the transport of dissolved substances by using concepts of water renewal time and water age. The model results showed that the impact of diversion schemes on transport processes was strongly influenced by hydrodynamic conditions induced by inflow/outflow discharges. Shorter water timescales occupied the west and east lake, implying a faster water renewal occurrence therein. Water ages were highly variable, both spatially and temporally. Longer water age was exhibited in the inner lake and bays. An exchange rate of 89.3% was obtained at the 5 m3/s discharge condition under 60 days' duration, with integral renewal time of approximately 50 days. The distribution of water age showed that it only took less than 10 days for the substance discharged at the inlets to be transported to the outlets. The results offer useful information for understanding the efficiency of water exchange by diversion projects and can be used to estimate the water renewal capacity for environmental assessment purposes.

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.

Relationships between groundwater and lake ecology are often overlooked, but they may be strong, particularly in seepage lakes. As a result, the nature and degree of groundwater effects on lakes are usually neglected. In this study interactions among rainfall, groundwater and surface water and their limnological effects were traced seasonally for two years of changing rainfall in a Spanish flowthrough, seepage lake complex. Cumulative rainfall dictated recharge of groundwater with delays of nine months. Groundwater discharge, in turn, increased surface discharge downstream. Mediated by the geographical setting of lakes, both fluxes impinged on lake water renewal time, but effects of the latter on limnological variables were much stronger at the district scale than at the single lake scale. These water-renewal effects included the following: decreasing salinity, total phosphorus concentration and phytoplankton biomass and increasing water transparency and total nitrogen concentration as water renewal shortened, the nitrogen effect arising because of nitrate-rich water entering the lakes as groundwater levels rose. This complex response of a Mediterranean lake district to water availability may also be expected in cold temperate lakes as climate change effects become stronger. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

With the development of regional economies, the water use environment in the Yellow River Basin, China, has changed greatly (Fig. 1). The river is well known for its high sediment content, frequent floods, unique channel characteristics in the downstream (where the river bed lies above the surrounding land), and limited water resources. This region is heavily irrigated, and combinations of increased food demand and declining water availability are creating substantial pressures. Some research emphasized human activities such as irrigation water withdrawals dominate annual streamflow changes in the downstream in addition to climate change (Tang et al., 2008a). The North China Plain (NCP), located in the downstream area of the Yellow River, is one of the most important grain cropping areas in China, where water resources are also the key to agricultural development, and the demand for groundwater has been increasing. Groundwater has declined dramatically over the previous half century due to over-pumping and drought, and the area of saline-alkaline land has expanded (Brown and Halweil, 1998; Shimada, 2000; Chen et al., 2003b; Nakayama et al., 2006). Since the completion of a large-scale irrigation project in 1969, noticeable cessation of flow has been observed in the Yellow River (Yang et al., 1998; Fu et al., 2004) resulting from intense competition between water supply and demand, which has occurred increasingly often. The ratio of irrigation water use (defined as the ratio of the annual gross use for irrigation relative to the annual natural runoff) having increased continuously from 21% to 68% during the last 50 years, indicating that the current water shortage is closely related to irrigation development (Yang et al., 2004a). This shortage also reduces the water renewal time (Liu et al., 2003) and renewability of water resources (Xia et al., 2004). This has been accompanied by a decrease in precipitation in most parts of the basin (Tang et al., 2008b). To ensure sustainable water resource use, it is also important to understand the contributions of human intervention to climate change in this basin (Xu et al., 2002), in addition to clarifying the rather complex and diverse water system in the highly cultivated region. The objective of this research is to clarify the impact of irrigation on the hydrologic change in the Yellow River Basin, an arid to semi-arid environment with intensive cultivation.

The stable oxygen isotopic signature delta O-18 of water has been proved to be a useful marker in hydrological lake studies. However, it is applied only sporadically to determine the extent of mixis and stratification, which is vital for all hydrological and limnological studies. We present a study of the changes in delta O-18, temperature and Schmidt stability in the water column of three adjacent lakes of very different mixis types (polymictic to meromictic) over almost one year (May 2008 to April 2009). The response of delta O-18 to important hydrologic processes (entrainment, stratification, circulation, groundwater inflow) and weather influences (cooling period, ice cover and melt water inflow) is discussed. The lakes are part of the Osterseen chain of lakes south of Munich, Germany. Although hydrologically connected, these lakes show various mixis types (polymictic to meromictic) due to large differences in size, groundwater inflow and water renewal time. Polymixis and the strong subsurface inflow of groundwater in Lake Waschsee (25.6 x 10(3) m(3)) were indicated by the same trends in the delta O-18 signature throughout all water layers and by the mean overall signature (-9.94 parts per thousand) being very close to delta O-18 of local groundwater (-10.01 parts per thousand +/- 0.06). delta O-18 signatures of the larger dimictic Lake Fohnsee (2298.3 x 10(3) m(3)) revealed a highly significant trend towards lower values of delta O-18 in its hypolimnion, indicating inflow of groundwater. A cooling period during the summer stratification characterised by high wind speeds resulted in a considerable drop of lake surface temperatures and Schmidt stability (up to 25%) in lakes Fohnsee and Eishaussee and was followed by a deepening of the mixed upper water layer and entrainment of hypolimnetic water layers. This was clearly shown by a signal change in deeper water layers formerly constant in delta O-18. The permanent meromixis present in Lake Eishaussee (297.0 x 10(3) m(3)) could also be confirmed by isotopic signatures, as bottom water layers remained significantly isolated in delta O-18 from the remaining water column over the whole study period. We summarize that the oxygen isotopic signature of water is an easy to interpret, excellent indicator of important hydrologic processes in a lake and can readily be integrated into routine sampling. The present findings will further contribute to the analysis of hydrological data as well as to the interpretation of paleoclimatic reconstructions using proxies of lake water delta O-18.

This report summarizes results from the fifteenth year (2006) of nutrient additions to the North Arm of Kootenay Lake and three years of nutrient additions to the South Arm. Experimental fertilization of the lake has been conducted using an adaptive management approach in an effort to restore lake productivity lost as a result of nutrient uptake in upstream reservoirs. The primary objective of the experiment is to restore kokanee (Oncorhynchus nerka) populations, which are the main food source for Gerrard rainbow trout (Oncorhynchus mykiss) and bull trout (Salvelinus confluentus). The quantity of agricultural grade liquid fertilizer (10-34-0, ammonium polyphosphate and 28-0-0, urea ammonium nitrate) added to the North Arm in 2006 was 44.7 tonnes of P and 248.4 tonnes of N. The total fertilizer load added to the South Arm was 257 tonnes of nitrogen; no P was added. Kootenay Lake has an area of 395 km{sup 2}, a maximum depth of 150 m, a mean depth of 94 m, and a water renewal time of approximately two years. Kootenay Lake is a monomictic lake, generally mixing from late fall to early spring and stratifying during the summer. Surface water temperatures generally exceed 20 C for only a few weeks in July. Results of oxygen profiles were similar to previous years with the lake being well oxygenated from the surface to the bottom depths at all stations. Similar to past years, Secchi disc measurements at all stations in 2006 indicate a typical seasonal pattern of decreasing depths associated with the spring phytoplankton bloom, followed by increasing depths as the bloom gradually decreases by the late summer and fall. Total phosphorus (TP) ranged from 2-7 {micro}g/L and tended to decrease as summer advanced. Over the sampling season dissolved inorganic nitrogen (DIN) concentrations decreased, with the decline corresponding to nitrate (the dominant component of DIN) being utilized by phytoplankton during summer stratification. Owing to the importance of epilimnetic nitrate that is required for optimal phytoplankton growth discrete depth water sampling occurred in 2006 to measure more accurately changes in the nitrate concentrations. As expected there was a seasonal decline in nitrate concentrations, thus supporting the strategy of increasing the nitrogen loading in both arms. These in-season changes emphasize the need for an adaptive management approach to ensure the nitrogen to phosphorus (N:P) ratio does not decrease below 15:1 (weight:weight) during the fertilizer application period. Phytoplankton composition determined from the integrated samples (0-20m) was dominated by diatoms, followed by cryptophytes and chrysophytes. The contribution of cryptophytes to total biomass was higher in 2006 than in 2005. Cryptophytes, considered being edible biomass for zooplankton and Daphnia spp., increased in 2006. Phytoplankton in the discrete depth samples (2, 5, 10, 15 and 20m) demonstrated a clear north to south gradient in average phytoplankton density and biomass among the three stations sampled, with highest values at the North Arm station (KLF 2) and lowest values in the most southern station in the South Arm (KLF 7). Populations were dominated by flagellates at all stations and depths in June and July, then dominated by diatoms in August and September in the North and South arms of the lake. There were no large bluegreen (cyanobacteria) populations in either arm of the lake in 2006. Seasonal average zooplankton abundance and biomass in both the main body of the lake and in the West Arm increased in 2006 compared to 2005. Zooplankton density was numerically dominated by copepods and biomass was dominated by Daphnia spp. The annual average mysid biomass data at deep stations indicated that the North Arm of Kootenay Lake was more productive than the South Arm in 2006. Mysid densities increased through the summer and declined in the winter; mean whole lake values remain within prefertilization densities. Kokanee escapement to Meadow Creek declined in 2006 to approximately 400,000 spawners. The Lardeau River escapement also declined with approximately 100,000 spawners. Escapement in the South Arm tributaries in 2006 was similar to 2005 with virtually no spawners. The mean size of female and male kokanee from Meadow Creek was larger than the past three years (24.9 cm and 25.2 cm, respectively) and larger than the long term average (25.2 cm). This is indicative of a density dependent growth response as a decline in in-lake abundance and returning spawners occurred. Spring hydroacoustic estimates indicated low densities of South Arm kokanee with most of the North Arm fry remaining at the most northerly stations. By the fall, the distribution of kokanee from the North Arm to the South Arm was fairly uniform, a normal occurrence in Kootenay Lake. Fall hydroacoustic estimates in 2006 increased to approximately 22 million kokanee.

A simplified model of the lake-sediment system is proposed, which aims to predict the long term evolution and the recovery time of eutrophied lakes. The model represents the state of lake and sediment each by means of a single variable. Having recourse to a well-defined set of assumptions as for sediment processes, and adopting time independent coefficients, the model balance equations can be solved analytically, and the evolution of the state variables towards steady state conditions, can be discussed in terms of characteristic times of the considered system. Particular attention is devoted to the consequences of P saturation of the solid phase in surface sediment layers, i.e. to a situation which is frequently present in eutrophied lakes. The model is applied to the case of the highly eutrophied Lake Varese (Northern Italy), in which internal P loadings from sediments appear to be substantial. Taking into account the coupling between lake and sediments, the time needed to attain the steady state P concentration compatible with the present P loadings is evaluated as ∼ 5 times the water renewal time of the lake. It is also shown to what extent the assumptions on sediment dynamics affect the model predictions.

This report examines the results from the eleventh and twelfth years (2002 and 2003) of the Kootenay Lake fertilization experiment. Experimental fertilization has occurred with an adaptive management approach since 1992 in order to restore productivity lost as a result of upstream dams. One of the main objectives of the experiment is to restore kokanee (Oncorhynchus nerka) populations, which are a main food source for Gerrard rainbow trout (Oncorhynchus mykiss). Kootenay Lake is located between the Selkirk and Purcell mountains in southeastern British Columbia. It has an area of 395 km2, a maximum depth of 150 m, a mean depth of 94 m, and a water renewal time of approximately two years. The quantity of agricultural grade liquid fertilizer (10-34-0, ammonium polyphosphate and 28-0-0, urea ammonium nitrate) added to Kootenay Lake in 2002 and 2003 was similar to that added from 1992 to 1996. After four years of decreased fertilizer loading (1997 to 2000), results indicated that kokanee populations had declined, and the decision was made to increase the loads again in 2001. The total load of fertilizer in 2002 was 47.1 tonnes of phosphorus and 206.7 tonnes of nitrogen. The total fertilizer load in 2003 was 47.1 tonnes of phosphorus and 240.8 tonnes of nitrogen. Additional nitrogen was added in 2003 to compensate for nitrogen depletion in the epilimnion. The fertilizer was applied to a 10 km stretch in the North Arm from 3 km south of Lardeau to 3 km south of Schroeder Creek. The maximum surface water temperature in 2002, measured on July 22, was 22 C in the North Arm and 21.3 C in the South Arm. In 2003, the maxima were recorded on August 5 at 20.6 C in the North Arm and on September 2 at 19.7 C in the South Arm. The maximum water temperature in the West Arm was 18.7 C on September 2, 2003. Kootenay Lake had oxygen-saturated water throughout the sampling season with values ranging from about 11-16 mg/L in 2002 and 2003. In both years, Secchi depth followed the expected pattern for an oligo-mesotrophic lake of decreasing in May, June, and early July, concurrent with the spring phytoplankton bloom, and clearing again as the summer progressed. Total phosphorus (TP) ranged from 2-11 {micro}g/L in 2002 and 2-21 {micro}g/L in 2003. With average TP values generally in the range of 3-10 {micro}g/L, Kootenay Lake is considered to be an oligotrophic to oligo-mesotrophic lake. Total dissolved phosphorus (TDP) followed the same seasonal trends as TP in 2002 and 2003 and ranged from 2-7 {micro}g/L in 2002 and from 2-10 {micro}g/L in 2003. Total nitrogen (TN) ranged from 90-380 {micro}g/L in 2002 and 100-210 {micro}g/L in 2003. During both the 2002 and 2003 sampling seasons, TN showed an overall decline in concentration with mid-summer and fall increases at some stations, which is consistent with previous years results. Dissolved inorganic nitrogen (DIN) concentrations showed a more pronounced declining trend over the sampling season compared with TN, corresponding to nitrate (the dominant component of DIN) being used by phytoplankton during summer stratification. DIN ranged from 7-176 {micro}g/L in 2002 and from 8-147 {micro}g/L in 2003. During 2003, discrete depth sampling occurred, and a more detailed look at the nitrate concentrations in the epilimnion was undertaken. There was a seasonal decline in nitrate concentrations, which supports the principle of increasing the nitrogen loading and the nitrogen to phosphorus (N:P) ratio during the fertilizer application period. Chlorophyll a (Chl a) concentrations in Kootenay Lake were in the range of 1.4-5.1 {micro}g/L in 2002 and 0.5-4.9 {micro}g/L in 2003. Over the sampling season, Chl a at North Arm stations generally increased in spring corresponding with the phytoplankton bloom, decreased during the summer, and increased again in the fall with mixing of the water column. The trend was similar, but less pronounced, at South Arm stations in these years, and spring Chl a concentrations were lower. During 2002, total algal biomass averaged during June, July and August was lower in the North Arm than the South Arm. This was the first time this occurred since the commencement of the North Arm fertilization experiment. Results in 2002 indicated Kootenay lake continues to be a diatom dominated lake (80 to 89% of the total average biomass). The overall trend observed throughout the 2003 sampling season was one of a slight decline in algal biomass from the North Arm stations towards those in the South Arm. Kootenay Lake continued to be a diatom-dominated lake (76-83% of total average biomass). Synedra spp. and some Asterionella, as in the previous three years, dominated the early biomass increase in 2003, but the peak biomass in July was largely due to Tabellaria.

The role of oceanic tide, wind stress, hoa inflow and stratification in the long-term circulation in Mururoa lagoon is investigated using a sensitivity analysis carried out by means of a three-dimensional model. Water renewal time scales are estimated. Wind stress is shown to be the dominant forcing. The hoa inflow slightly increases the turnover time, while stratification enhances the impact of motion in vertical planes parallel to the wind stress. The modelled turnover time is approximately 100 days, and becomes much larger than one year whenever the wind stress is disregarded.

The stable oxygen isotopic signature δ18O of water has been proved to be a useful marker in hydrological lake studies. However, it is applied only sporadically to determine the extent of mixis and stratification, which is vital for all hydrological and limnological studies. We present a study of the changes in δ18O, temperature and Schmidt stability in the water column of three adjacent lakes of very different mixis types (polymictic to meromictic) over almost one year (May 2008 to April 2009). The response of δ18O to important hydrologic processes (entrainment, stratification, circulation, groundwater inflow) and weather influences (cooling period, ice cover and melt water inflow) is discussed. The lakes are part of the Osterseen chain of lakes south of Munich, Germany. Although hydrologically connected, these lakes show various mixis types (polymictic to meromictic) due to large differences in size, groundwater inflow and water renewal time. Polymixis and the strong subsurface inflow of groundwater in Lake Waschsee (25.6 × 103 m3) were indicated by the same trends in the δ18O signature throughout all water layers and by the mean overall signature (-9.94‰) being very close to δ18O of local groundwater (-10.01‰ ± 0.06). δ18O signatures of the larger dimictic Lake Fohnsee (2298.3 × 103 m3) revealed a highly significant trend towards lower values of δ18O in its hypolimnion, indicating inflow of groundwater. A cooling period during the summer stratification characterised by high wind speeds resulted in a considerable drop of lake surface temperatures and Schmidt stability (up to 25%) in lakes Fohnsee and Eishaussee and was followed by a deepening of the mixed upper water layer and entrainment of hypolimnetic water layers. This was clearly shown by a signal change in deeper water layers formerly constant in δ18O. The permanent meromixis present in Lake Eishaussee (297.0 × 103 m3) could also be confirmed by isotopic signatures, as bottom water layers remained significantly isolated in δ18O from the remaining water column over the whole study period. We summarize that the oxygen isotopic signature of water is an easy to interpret, excellent indicator of important hydrologic processes in a lake and can readily be integrated into routine sampling. The present findings will further contribute to the analysis of hydrological data as well as to the interpretation of paleoclimatic reconstructions using proxies of lake water δ18O.

Partial pressures of CO2 and CH4 were measured directly or calculated from pH and alkalinity or DIC measurements for 25 lakes and 4 rivers on the North Slope of Alaska. Nearly all waters were supersaturated with respect to atmospheric pressures of CO2 and CH4. Gas fluxes to the atmosphere ranged from -6.5 to 59.8 mmol m - 2 d - 1 for CO2 and from 0.08 to 1.02 mmol m - 2 d - 1 for CH4, and were uncorrelated with latitude or lake morphology. Seasonal trends include a buildup of CO2 and CH4 under ice during winter, and often an increased CO2 flux rate in August due to partial lake turnover. Nutrient fertilization experiments resulted in decreased CO2 release from a lake due to photosynthetic uptake, but no change in CO2 release from a river due to the much faster water renewal time. In lakes and rivers the groundwater input of dissolved CO2 and CH4 is supplemented by in-lake respiration of dissolved and particulate carbon washed in from land. The release of carbon from aquatic systems to the atmosphere averaged 24 g C m - 2 Y - 1, and in coastal areas where up to 50 % of the surface area is water, this loss equals 1/5 to 1/2 of the net carbon accumulation rates estimated for tundra.

Individual components of lakewater budgets in the Experimental Lakes Area (ELA) are highly variable through time. Direct field measurements combined with empirical relationships for unmeasurable components are required to achieve balanced lakewater budgets over short intervals of several hours or days. However, annual variations in the hydrological regime are related to a few simple characteristics of the lake and surrounding watershed. In the 1969–77 period the annual ratios of terrestrial evapotranspiration/precipitation and lake evaporation/precipitation showed little variation and were of a similar magnitude. As a result, the potential for water renewal or replacement in the lake basins annually can be characterized by only four factors: the annual precipitation, the tributary drainage basin area, the lake area, and the lake volume. The potential has been expressed as the Annual Watershed Efficiency (AWE), the reciprocal of the water renewal time, to obtain a linear relationship between the four variables. The AWE i a measure of the extent to which a particular watershed and lake combination can renew the lake water under various annual precipitation conditions. A general graphical relationship for the AWE has been prepared for small headwater lakes similar to those of the ELA for a range of annual precipitations.Key words: watershed hydrology, water renewal, water residence, precipitation/runoff ratios, water budgets, evaporation, watershed efficiency

The relationship between lake sensitivity to atmospheric acidic inputs and the neutralization capacity of watersheds is examined for 267 lakes in northeastern Minnesota. Three water chemistry/sensitivity measures (color, sulfate, and alkalinity) are correlated with variables representative of precipitation and sulfate inputs, hydrology, and the acid neutralization capacity of various watershed components. An ordinal scale for ranking bedrock and surficial deposit neutralization capacity is presented. The watershed variables found to account for the largest percentages of the variability in measured color, sulfate, and alkalinity levels are determined. Color is strongly related to the presence of peat or marsh and hydrologic renewal time, whereas sulfate is primarily related to atmospheric deposition, evaporative concentration, bedrock type, and the presence of coniferous forest. Variation in alkalinity is the most difficult of the water chemistry measures to explain; for headwater lakes, atmospheric sulfate input, water renewal time, the presence of deciduous forest, and the weatherability of underlying bedrock determine much of its variability. The results illustrate important averaging properties of watersheds from small headwater systems to large drainages and the difficulty in obtaining correlations for some water quality measures (e.g., alkalinity) when some variables, such as soils and land cover, are available only as large-area averages.

Water renewal may influence reservoir zooplankton communities through the mechanisms of dilution, transport (selective removal or addition), or through localized modifications of limnological conditions (e.g., light penetration, food quality, temperature) which then provide the setting for alteration of zooplankton community structure. An inverse relationship between reproduction in cladoceran zooplankton and water renewal time was found for six reservoirs sampled during summer 1980. This relationship suggests a strategic response in resource allocation by cladoaerans to dilution effects associated with water renewal. However, analysis of variation in cladoceran demography at different sites within each of the reservoirs and over a four-month period at a single site in one reservoir demonstrated that repoductive variation could be explained by spatial or temporal variation in limnological conditions and independently of transport or dilution processes associated with water renewal.

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.