Episodic nearshore-offshore exchanges of hypoxic waters along the north shore of Lake Erie

We investigate the nearshore-offshore exchange of hypoxic waters during episodic coastal upwelling events in the nearshore waters of northern Lake Erie using intensive field observations and a validated hydrodynamic and water quality model. We observe wind-induced coastal upwelling events to be the dominant nearshore physical process in the lake which are energized every 5–10 days. When the winds were predominantly blowing from the west or south-west, epilimnetic waters were transported to the offshore bringing in hypolimnetic waters with low temperature (8–10 °C), dissolved oxygen (DO: 0–6 mg L^?1) and pH (6–7) to the nearshore zones. During these events, vertical diffusivity coefficients decreased from 10^?2 m² s^?1 to values as low as ~ 10^?7 m² s^?1. In late summer, the coastal upwelling events in the nearshore waters lower the near bottom DO to hypoxic levels (DO < 2 mg L^?1). Lake-wide observations of DO and pH show that they are positively and linearly correlated while in the nearshore DO and pH experience spatial and temporal variability where upwelling events were developed, which were further assessed using a three-dimensional model. The model accuracy to reproduce offshore hypoxia was first assessed on a lake-wide basis using a coarse resolution model for a five-year period (2008–2012) and in nearshore waters using a higher resolution model for 2013. We use the model results to delineate the near bottom areas experiencing hypoxia at time scales longer than 48 h.     

Chronicles of hypoxia: Time-series buoy observations reveal annually recurring seasonal basin-wide hypoxia in Muskegon Lake – A Great Lakes estuary

We chronicled the seasonally recurring hypolimnetic hypoxia in Muskegon Lake – a Great Lakes estuary over 3?years, and examined its causes and consequences. Muskegon Lake is a mesotrophic drowned river mouth that drains Michigan's 2nd largest watershed into Lake Michigan. A buoy observatory tracked ecosystem changes in the Muskegon Lake Area of Concern (AOC), gathering vital time-series data on the lake's water quality from early summer through late fall from 2011 to 2013 (www.gvsu.edu/buoy). Observatory-based measurements of dissolved oxygen (DO) tracked the gradual development, intensification and breakdown of hypoxia (mild hypoxia <4?mg DO/L, and severe hypoxia <2?mg DO/L) below the ~6?m thermocline in the lake, occurring in synchrony with changes in temperature and phytoplankton biomass in the water column during July–October. Time-series data suggest that proximal causes of the observed seasonal hypolimnetic DO dynamics are stratified summer water-column, reduced wind-driven mixing, longer summer residence time, episodic intrusions of cold DO-rich nearshore Lake Michigan water, nutrient run off from watershed, and phytoplankton blooms. Additional basin-wide water-column profiling (2011–2012) and ship-based seasonal surveys (2003–2013) confirmed that bottom water hypoxia is an annually recurring lake-wide condition. Volumetric hypolimnetic oxygen demand was high (0.07–0.15?mg DO/Liter/day) and comparable to other temperate eutrophic lakes. Over 3?years of intense monitoring, ~9–24% of Muskegon Lake's volume experienced hypoxia for ~29–85?days/year – with the potential for hypolimnetic habitat degradation and sediment phosphorus release leading to further eutrophication. Thus, time-series observatories can provide penetrating insights into the inner workings of ecosystems and their external drivers.     

风浪对浅水湖泊水质的影响机制

以太湖重要的浅水湖湾——东太湖为例,应用经典风浪经验公式,将风速转化为浪高,以直接体现风浪强度,在分析其与污染底泥卷起、水质指标相关关系的基础上,对风浪影响湖区水质的机制进行进一步探究。结果表明:随着风速、浪高的增加,水体浑浊度增加,偏北风作用下湖区浑浊度更高;DO的质量浓度呈降低趋势,全年浓度呈现冬季偏高而夏季偏低的规律;COD的质量浓度呈增加趋势,全年浓度呈现冬季偏高而夏季偏低的规律;TN的质量浓度与风速相关关系不明显,全年浓度呈现夏季偏高而冬季偏低的规律;浑浊度与COD存在较好的相关性。确定风浪对湖区水质的影响机制,旨在为针对性地制定湖泊水质改善措施提供技术支撑。     

Stratification and mixing in large floodplain lakes along the lower Amazon River

Large, shallow lakes are common in the extensive floodplains throughout the tropics. To determine controls on their mixing dynamics, we instrumented 5 stations in two shallow, connected tropical lakes on the lower Amazon floodplain with meteorological and temperature sensors. A tight relation between changes in thermal structure and L_MO/h (the ratio of the Monin-Obukhov length scale to the depth of the actively mixing layer) indicates the sensitivity of thermal structure to wind speed relative to heating and cooling. Four regimes led to variations in mixing: (i) high solar radiation with light winds in the mid-morning to early afternoon resulted in shallow stratification, 0?<?L_MO/h?<?1; (ii) afternoons with higher winds caused the diurnal thermocline to downwell and heat to mix to deeper layers, L_MO/h?>?1; (iii) by late afternoon, buoyancy flux became negative and L_MO/h?<??1 and with u_1W and w₁ both >0.06?m?s^?1, mixing from wind and cooling co-occurred; and (iv) convection dominated mixing on nights with light winds, ?1?<?L_MO/h?<?0. Pattern (ii) occurred mid-day if winds were higher. When winds were intermittent and regime (i) predominated mid-day, changes in heat content were primarily determined by one dimensional processes of heating and cooling. When easterly winds were sustained and regime (ii) occurred mid-day, heat was transported west in the day, and colder water upwelled to the west or was advected to the west and north at night. Subtle differences in wind speed determined the extent to which advection moderated the thermal structure.     

The Impact of Surface Heat Flux and Wind on Thermal Stratification in Portage Lake,Michigan

Portage Lake is situated near the center of the Keweenaw Peninsula and is connected to Lake Superior via lengthy (> 7 km) navigation channels. Using moored thermistor records and meteorological data, we examine how changes in lake stratification are related to surface winds and heat flux. Frequent episodes of full water column mixing are observed throughout the summer. Convective mixing through surface cooling appears to be an important agent responsible for these events, as all occur during cold air outbreaks and when the net heat flux is directed out of the lake (negative). However, wind-induced mixing is also implicated in contributing to some vertical mixing events, as evidenced by two events initiated during a period of strong winds and declining, but not yet negative, heat fluxes. Our analysis indicates that each time the water column restratifies, it tends to become more susceptible to convectively-driven overturn during cold air outbreaks. This tendency is quantified by the estimated time over which surface cooling due to a specified set of conditions, characteristic of a cold air outbreak, would reduce the temperature contrast between the upper and lower layers by half. This time declines by more than an order of magnitude for successive restratification events observed in the summer of 1999. Our analysis also reveals successive formation and dissolution of a diurnal surface mixed layer in an otherwise homogeneous water column during a 10-day period of August 1999. This is attributed to the combination of relatively light winds and negative daily net heat fluxes.     

The influences of typhoon‐induced mixing in a shallow lake

Vertical mixing affects the vertical transport of biochemical materials and modifies the characteristics of basin‐scale internal waves in subtropical, subalpine Yang Yuan Lake (YYL), in the north central region of Taiwan. Vertical mixing in YYL is generally caused by typhoons (strong winds and heavy rainfalls) during the spring, summer and early fall, or by cooling of the water column during late fall and winter. Vertical mixing caused by typhoons significantly affects internal thermal dynamics and biochemical processes, with basin‐scale internal waves enhanced before complete mixing occurs, and temporal variations of water temperature dramatically changing after typhoon events. This study quantitatively determines when the complete mixing (defined as homogenous temperature in a water column) occurs on the basis of the balance between kinetic energy and potential energy and also shows what meteorological conditions contribute to the vertical mixing. Data on water column thermal profiles and meteorological variables were collected by a wireless, instrumented buoy in the deepest location of YYL and from a nearby meteorological station, from spring 2004 to summer 2006. This study also investigated how a physical process (internal waves) is affected by the mixing associated with typhoons. Signal processing and a two‐layer model of the water column are used to understand the characteristics of basin‐scale internal waves.     

Numerical study of hydrodynamic process in Chaohu Lake

In this paper, the hydrodynamic characteristics of water flow in Chaohu Lake are studied by using the finite volume coastal ocean model(FVCOM), which is verified by the observed data. The typical flow field and the 3-D flow structure are obtained for the lake. The flow fields under extreme conditions are analyzed to provide a prospective knowledge of the water exchange and the transport process.The influence of the wind on the flow is determined by the cross spectrum method. The results show that the wind-driven flow dominates most area of the lake. Under prevailing winds in summer and winter, the water flows towards the downwind side at the upper layer while towards the upwind side at the lower layer in most area except that around the Chaohu Sluice. The extreme wind speed is not favorable for the water exchange while the sluice's releasing water accelerates the process. The water velocity in the lake is closely related with the wind speed.     

Estimation of Heat and Hydrologic Budget of Upper Klamath Lake Oregon,USA Using Updated DLM-WQ Model

Estimation of lake hydrologic budgets is essential for sustainable water management due to increasing water demand and uncertainties related to climate change. The updated turbulent diffusion transfer algorithms were developed and incorporated in the DLM-WQ model developed at UC Davis to estimate the Upper Klamath Lake’s dynamics, and heat and hydrologic budget. The exchange coefficients for latent heat (C_EN), sensible heat (C_HN), and wind drag (C_D) of the turbulent diffusion model were calibrated using coefficient of correlation as the objective function. The agreement between estimated and measured lake water elevation and temperature are found to be excellent with correlation coefficients 0.99 and 0.95, respectively. The heat and hydrologic budgets are more sensitive to evaporative heat loss (35 %) than sensible heat exchanges (11 %). The stream inflow and lake outflow dominate the hydrologic budget with approximately 47 % due to stream inflow and approximately 44 % due to lake outflow. Precipitation directly on the lake and evaporation from the lake are only 3 % and 6 %, respectively. The lake mixes to the bottom sporadically during spring and summer. Estimated deep mixing for the period 1994–1996 shows the lake’s increasing stability without a deep mixing event for approximately 4 months during summer in 1996. Prolonged stratification in the hypereutrophic lake is expected to lead to hypoxia near the sediment surface resulting in exacerbation of existing ecological problems. The DLM-WQ model can be applied to a broad range of lakes/reservoirs with selection of appropriate C_EN and C_HN value for lake/reservoir dynamics and water resources planning evaluation.     

Quantifying the influence of cold water intrusions in a shallow,coastal system across contrasting years: Green Bay,Lake Michigan

We present water column thermal structure for two climatically different years: 2012, which experienced abnormally warm spring and summer air temperatures preceded by a relatively low ice winter and 2013, which experienced cooler than average spring and average summer air temperatures and preceded by average ice conditions. Mean bottom water temperatures for the season and during cold water intrusions were significantly warmer in 2012 than 2013 leading to a significantly reduced stratified season in 2012. Cold water intrusions were driven into southern Green Bay by southerly winds while intrusions were terminated when winds switched to persistent northerly winds. 2012 observed a significant increase in northerly winds relative to 2013, decreasing cold water intrusion presence and duration but winds did not fully explain the difference in thermal conditions for southern Green Bay. These cold bottom waters drive stratification in polymictic southern Green Bay while dimictic waters were found to have significantly warmer bottom temperatures during 2012 and a deeper mixed layer. Our observations suggest that relatively shallow (<20?m), seasonally stratified systems may not increase in stratification strength and duration under a warming climate; rather, changing wind climatology and surface heat flux can inform the degree to which the mixing regime can be expected to change and impact stratification and thermal structure of coastal systems. We discuss the biogeochemical implications of different thermal regimes, particularly within the context of multiple drivers of physical water column structure in eutrophic, stratified coastal systems.     

Documenting Complex Surface Temperature Patterns from Advanced Very High Resolution Radiometer (AVHRR) Imagery of Saginaw Bay,Lake Huron

Interannual differences in nutrient concentrations, phytoplankton densities, and the dispersal of zooplankton and fish life history stages (e.g., planktonic larvae, resting eggs) in embayments often are contingent upon bay and coastal water exchange and movements. However, predicting water mass exchange is difficult in that many factors influence the circulation of waters within bays. The central theme of this contribution is that circulation phenomena derived from simple models can be identified and classified using satellite-obtained lake surface temperature maps, and the incidence of events related to gross temperature regime (season) and winds (shorter-term perturbations). To illustrate this potential, the high sampling frequency of AVHRR imagery was used to examine seasonal surface temperature patterns between Saginaw Bay and Lake Huron over a 3-year period. Linear regressions of CoastWatch IMGMAP and OCNMAP daytime sea surface temperature (SST) algorithms against shipboard bulk temperatures were highly significant, with r² values of 0.98 and 0.94. Synoptic reconnaissance of lake surface temperatures from AVHRR verified many previously known general seasonal events, yet provided much better spatial coverage. The thermal bar persisted in the bay for approximately 6 weeks from late April until mid-June. The combination of shallow depths and impounded river discharges caused inner bay waters to warm more rapidly than outer bay and open lake waters. A thermal gradient of 6 to 10°C persisted between the inner bay and Lake Huron waters throughout the summer. The persistence of major spatial thermal gradients restricted mixing of inner bay waters with the outer bay and open lake, whereas inner and outer bay temperatures converged during fall months, increasing the likelihood of mixing. However, frequent wind-induced effects can cause circulation reversals and move surface waters in quite complex patterns across the bay. AVHRR image classification confirms the sensitivity of inner bay waters to wind stress-related circulation, aiding interpretations of historical data sets.     

Characteristics of declining dissolved oxygen concentrations in Lakes Kasumigaura and Kitaura,two shallow polymictic eutrophic lakes in Japan

To elucidate the patterns and characteristics of dissolved oxygen (DO) decline near bottom in shallow polymictic eutrophic lakes, we conducted a statistical analysis of monthly measurements for 12 years at 14 stations and hourly observation for 8 years at two stations in Lakes Kasumigaura and Kitaura. Results indicated that DO decline (hereafter, <2 mg/L) was negligible at stations with a depth of <5 m. Thus, we consider this depth to be critical depth for studying DO decline conditions in these lakes. The collected data revealed that DO decline events lasting more than 2 days occurred a few times a year. During these events, wind blew weakly (usually, <2 m/s), and diurnal stratification was formed by solar radiation. The averaged DO declining rate leading up to these events was 4.3 (±2.7) mg L^?1 d^?1, suggesting that within a few days, DO can fall into a declined state. The occurrence probability of DO declined events in the respective summer increased significantly with the proportion of weak wind hours (<2 m/s). Significant correlations between wind velocity and DO change rate (1 hr difference in DO) supported the importance of wind on DO changes in polymictic lakes. The influence of mean air temperature on DO decline events was insignificant.     

Nearshore-offshore exchanges by enhanced turbulent mixing along the north shore of Lake Ontario

Seasonal nearshore-offshore exchanges by coastal upwelling events in large lakes can play a significant role in nearshore nutrient dynamics, affecting lake productivity and water quality. We analyzed field observations along the north shore of Lake Ontario, collected in the summer of 2018, focusing on the littoral zone and specifically the Cladophora habitat zone (<15 m), to investigate episodic enhanced vertical mixing by coastal up/downwelling events. Vertical turbulent diffusivity (K_z) based on the buoyancy Reynolds number above the metalimnion layer during downwelling events was generally higher than those below the metalimnion layer during upwelling events; while K_z at the metalimnion layer can increase by ∼ two orders of magnitude during upwelling events. Our results suggest that K_z based on the Richardson number parameterization, which only accounts for large-scale current shear and stability, and incorporates an adjustable parameter is ∼ ten times higher than the K_z based on the buoyancy Reynolds number. Analysis of historical wind records indicates that the frequency of coastal upwelling favorable winds on the north shore of Lake Ontario has increased by > 45% over the last thirty years - suggesting an increasing trend of nearshore-offshore nutrient exchanges as a contributing factor for the nearshore water quality management.     

Long-term changes in hypolimnetic dissolved oxygen in a large lake: Effects of invasive mussels,eutrophication and climate change on Lake Simcoe, 1980–2012

Lake Simcoe has been influenced by multiple environmental drivers over the past decades, especially by reductions in phosphorus (P) loading, climate change, and invasive species such as dreissenid mussels (DM) which became firmly established in 1996. We examined the cumulative impact of these drivers on the volume-weighted hypolimnetic dissolved oxygen concentration (VWHDO) below 18?m at station K42 in Kempenfelt Bay during ice-free seasons from 1980 to 2012. Hypolimnetic DO depletion began in early spring when thermal stratification was observable but weak and continued throughout the ice-free season until cooling sufficiently lowered water column stability. In comparison to the pre-DM invasion period (1980–1995), mean annual VWHDO_init was 2.4?mg?L^?1 higher in the post-DM period (1996–2012), VWHDO_min was 1.54?mg?L^?1 higher and the mean duration of the depletion period (L) was 16?days longer. Mean DO depletion rate (DR) and temperature adjusted DO depletion rate (DR_adj) were slightly lower (7% and 5%, respectively) after 1996. P controls and DM had a positive effect on VWHDO, presumably by lowering productivity and diverting organic matter away from the hypolimnon. However, longer L apparently offset improvements in VWHDO_min. If lengthening of L associated with regional warming continues, then additional efforts to reduce P loads will be necessary to achieve the goal of maintaining VWHDO_min above the target of 7?mg?O₂?L^?1 throughout the summer and fall.     

Relationships between actinomycete populations and organic matter degradation in Lake Mulwala,southeastern Australia

This communication examines the proposal that actinomycete populations, and thus the occurrence of taste and odour problems in potable water supplies, are related to the input of organic matter to reservoirs. The number of actinomycetes in Lake Mulwala, a man-made lake in southeastern Australia, ranged from 2–80 ml^?1 in the water column, and 2.7–12 × 10⁴cm^?3 in the sediments. Population densities in the water column and sediments were greatest in winter and summer, respectively. They were not highly correlated with protein or polysaccharide degradation. The weak relationship between actinomycete numbers and the rate of organic-matter breakdown was probably due to metabolic inactivity of the bacteria, since actinomycetes in the sediments occurred mainly as spores rather than as vegetative cells. Limiting the input of organic matter, for instance by the removal of fringing vegetation, will have little effect on actinomycete populations in this reservoir.     

THRESHOLD FLOWS FOR THE BREAKDOWN OF SEASONALLY PERSISTENT THERMAL STRATIFICATION: SHOALHAVEN RIVER BELOW TALLOWA DAM,NEW SOUTH WALES,AUSTRALIA

Reduced mixing of deep pools attributable to river regulation and downstream flow suppression can lead to an increase in the magnitude, frequency and duration of thermal stratification in riverine pools over summer. This study monitored hourly temperature profiles with five thermistor loggers in a 15 m deep natural pool over 12 months from May 2005. Detailed bathymetric and topographic survey data and HEC‐RAS hydraulic modelling of layer Richardson numbers were used to extend thermistor observations of flow‐related stratification breakdown in this single deep pool to a 20 km long pool‐riffle dominated river reach below the dam. Reach‐wide breakdown of persistent thermal stratification in deep pools over spring and summer was likely to be achieved by a flow rate of 3000 ML day^?1. This flow rate approximates the long‐term mean annual natural flow (2860 ML day^?1) and the 16th flow duration percentile (mean daily flows equalled or exceeded for 16% of time), indicating that thermal stratification of the deepest pools in the Shoalhaven River is a common, natural phenomenon not solely attributable to river regulation. Should reasonably consistent hydraulic geometry relationships exist between low salinity rivers in similar climatic, hydrologic and geomorphic settings, then we suggest that the mean annual natural flow is likely to achieve widespread breakdown of thermal stratification across lengthy reaches of similar pool‐riffle sequence rivers elsewhere. Hourly mean wind speeds of up to 65 km h^?1 recorded at an automated weather station 25 km from the study site were found to suppress of the degree of thermal stratification in the study pool but did not achieve deep mixing of persistent seasonal thermoclines. Large, rapid and sustained air temperature decreases associated with the passage of cold fronts across southeastern Australia in summer were found to be more effective than wind and achieved mixing to depths of at least 4.2 m. Copyright © 2011 John Wiley & Sons, Ltd.     

定常风场下的太湖风生流水动力特征研究

太湖的主要流动类型为风生流,本文采用数学模型模拟了四种定常风场作用下的太湖风生流水动力特征,总结了风向对太湖风生流分布的影响。研究结果指出：东南风与西北风作用下太湖主湖区存在西部环流、中部环流和北部环流三个大尺度环流,北风与南风作用下太湖主湖区仅存在西部环流和中部环流两个大尺度环流。东南风和西北风作用下,太湖北部竺山湾东北部、梅梁湾东南部和西北部及贡湖湾东北部存在大尺度环流。东风与南风作用下,竺山湾西部、梅梁湾东南部和中部南北两岸及贡湖湾东部存在大尺度环流。定常东南风作用下,太湖表面流向与风向相同,底层流向与风向相反,流向拐点一般出现在相对水深hc=0.8处。     

Temporal dynamics and drivers of lake ecosystem metabolism using high resolution observations in a shallow,tropical, eutrophic lake (Laguna Lake,Philippines)

The physical, chemical, and biological dynamics under changing atmospheric conditions of Laguna Lake, Philippines were analyzed from intensive observations. Diel measurements were conducted for 48 continuous hours for both dry and wet tropical seasons in addition to fine resolution long-term monitoring. Results revealed significant vertical and diel variations of lake variables in spite of a shallow water depth (2.5 m) caused by the intense surface heating from solar irradiance (~800 W m^?2) and accentuated by the lake's high turbidity (16–32 ftu). Late afternoon land-lake breeze (~5–7 m s^?1) regularly breaks daytime thermal stratification, and convective cooling at night maintains isothermal condition until dawn of the next day. The stratified condition demonstrated a wind-driven, density-induced 2-layer current pattern with a windward moving epilimnion (~4 folds faster) and a compensating hypolimnetic flow in the general lake circulation direction. Laguna Lake was observed to have a dominating diel cycle but also undergoes significant seasonal limnological variations brought primarily by climate, hydrology, and its interaction with the adjacent sea. Significantly correlated variations of pH, chlorophyll-a and DO in the dry season were indicative of the higher biological activity associated with the intrusion of polluted waters from Metro Manila. The non-occurrence of thermal over-turn was observed to be regularly followed by bottom hypoxic conditions (2–4 mg L^?1), indicative of the eutrophic condition of the lake and the importance of diel wind-induced mixing in the bottom supply of DO. Laguna Lake was found to be predominantly net heterotrophic (GPP:R < 1, NEP < 0).     

Spatial,seasonal, and historical variation of phytoplankton production in Lake Michigan

There have been few direct measurements of phytoplankton production made in Lake Michigan since invasive dreissenid mussels became established in the lake. Here we report the results of 64 measurements of phytoplankton primary production made in Lake Michigan during 2016 and 2017. We conducted two lake-wide surveys, one in the spring 2016 isothermal period and one after summer stratification in 2017 and examined seasonal production with bi-weekly sampling between May and November 2017 at an offshore station in the southwestern part of the lake. We assessed nearshore-offshore gradients by sampling at three transect locations on three occasions in 2017. Spring 2016 production and production:biomass (P:B) ratios (reflective of growth rates) were similar across the lake and were higher than those reported before dreissenid mussels became established, suggesting that despite decreases in phytoplankton biomass, growth rates remain high. Summer 2017 production and growth rates increased from south to north. Areal production in 2017 peaked in late summer. Mean 2017 summer production (499 ± 129 mg C m^?2 day^?1) was lower than values reported prior to the mussel invasion, and the fraction of total production occurring in the deep chlorophyll layer was about half that measured pre-mussels. At the offshore site picoplankton accounted for almost 50 % of the chlorophyll. As spring P:B ratios have increased and summer P:B and seston carbon:phosphorus ratios have not changed, we conclude that the decrease in phytoplankton production in Lake Michigan is due primarily to grazing by mussels rather than to stronger nutrient limitation.     

基于多元统计分析的滆湖水质时空变化特征及原因解析

为探究滆湖水环境质量的时空变化特征,在全湖布设20个采样点,开展逐月监测并分析水环境时空变化现状,对滆湖水体进行营养状态评价,通过多元统计分析的手段识别水质时空变化特征。结果表明:春季DO、Chl-a、TN较高,DO最高月均值为11.90 mg/L,Chl-a最高月均值为50.46 mg/m~3,TN最高月均值为3.41 mg/L;夏季藻密度、TP较高,藻密度最高月均值为5 462.3万个/L,TP最高月均值为0.29 mg/L;春冬季的SD较高,春冬季SD均值为31.1 cm;春夏季的pH、EC较高,春夏季pH均值为8.063,春夏季EC均值为358.9μS/cm;北部湖区SD、水温、DO、TN、TP高于其他两个湖区,中心湖区的pH、Chl-a和EC高,南部湖区的藻密度高。滆湖水体综合营养状态指数为中营养状态与水环境因子有显著相关性。将滆湖9个水质指标数据,可概括为3个主成分,聚类分析将20个样点按时间尺度上分为2组;按空间尺度上分为2组。DO、水温、TP是导致月份差异的主要指标,SD、DO、EC是导致空间差异的主要指标。通过对水质时空变化的进行客观全面的分析更有助于界定水质特征,为湖泊修复提供理论依据。     

Physicochemical gradients and water fluxes between Nyanza Gulf and main Lake Victoria,East Africa: Tracing dynamics of gulf-main lake interaction

Variability in physico-chemical water column structure along a gradient between Nyanza Gulf and Lake Victoria was characterized, and the exchange between the two basins estimated. Specific conductivity (SC) measurements between March 2005 and March 2006 identified three morphologically distinct zones, the inner-gulf, the connecting Rusinga Channel and the main lake basin. The relatively shallow, wide and river-influenced inner-gulf had higher turbidity and SC compared to the channel and the main lake. The deep, narrow and more wind-exposed mid-channel area was significantly cooler than the rest of the study areas. Observational data revealed “density-driven” underflow of the cooler channel water into the main lake and compensatory surface flow into the gulf as well as seasonal patterns of gulf-main lake exchange in relation to variation in tributary input. The wind field was dominated by a diurnal lake-land breeze which resulted in wind-driven diurnal reversal of water flow in and out of the gulf, where the constriction of flow within the channel induced strong currents. The exchange between the gulf and the main lake was estimated using a box model, with SC as a conservative tracer. A net annual residual water flow of 35?m³?s^?1 occurred from the gulf into the main lake although during months with low tributary discharge the flow was towards the gulf. The daily flow in and out of the gulf resulted in relatively high exchange flux compared to residual flow across the Rusinga Channel leading to the development of the well-defined physicochemical gradient along the Rusinga Channel.