The effect of wave-reduction engineering on sediment resuspension in a large, shallow, eutrophic lake (Lake Taihu)

Frequent resuspension of sediments is recognized as an important process in large shallow lakes, impeding the recovery of eutrophic lakes. A large-scale project, including a wave barrier (3.3 km long) and a soft enclosure, was implemented to reduce wave energy and sediment resuspension in Lake Taihu, eastern China. The effects of the wave-reduction engineering on sediment resuspension and internal nutrient loading were investigated. Results showed that sediment resuspension rates as well as suspended solids (SS) in the areas protected by the wave barrier and the soft enclosure were significantly lower than in the unprotected areas. There was a positive relationship between total phosphorus (TP) and SS; thus internal loading of phosphorus was significantly reduced by the wave-reduction structure. High nutrient levels and phytoplankton biomass persisted during the experiment period, suggesting that additional measures, such as re-establishment of the macrophyte community, must be included to help restore the water quality in such a large, shallow and eutrophic lake.     

Effects of hydrodynamics on phosphorus concentrations in water of Lake Taihu,a large,shallow, eutrophic lake of China

To understand the effect of hydrodynamical process on water phosphorus concentration, wind, wave, and several water quality indices were observed in Meiliang Bay, a shallow and eutrophic bay locates in north of Lake Taihu. During the 7 day observation period, wind speed and significant wave height were recorded more than 3 h per day, and water samples were collected in five water-depth layers once a day. Hydrodynamical disturbance had no significant correlationship with the water quality at the top layer when the significant wave height was smaller than 30 cm, but it significantly increased suspended solids (SS) concentration of the bottom water layer. Concentrations of nutrients showed no positive correlationship with SS concentration in the water body. Intensive sediment resuspension may not have occurred when the hydrodynamic stress on sediment was only a little higher than the critical stress for sediment resuspension. A new method for confirming the critical stress for intensive sediment resuspension and nutrient release still needs to be developed. The range of the water quality indices was quite high during the seven days of observation. High variation seems to be a common character of large shallow lakes like Taihu.     

The Influence of Macrophytes on Sediment Resuspension and the Effect of Associated Nutrients in a Shallow and Large Lake (Lake Taihu,China)

A yearlong campaign to examine sediment resuspension was conducted in large, shallow and eutrophic Lake Taihu, China, to investigate the influence of vegetation on sediment resuspension and its nutrient effects. The study was conducted at 6 sites located in both phytoplankton-dominated zone and macrophyte-dominated zone of the lake, lasting for a total of 13 months, with collections made at two-week intervals. Sediment resuspension in Taihu, with a two-week high average rate of 1771 g·m^-2·d^-1 and a yearly average rate of 377 g·m^-2·d^-1, is much stronger than in many other lakes worldwide, as Taihu is quite shallow and contains a long fetch. The occurrence of macrophytes, however, provided quite strong abatement of sediment resuspension, which may reduce the sediment resuspension rate up to 29-fold. The contribution of nitrogen and phosphorus to the water column from sediment resuspension was estimated as 0.34 mg·L^-1 and 0.051 mg·L^-1 in the phytoplankton-dominated zone. Sediment resuspension also largely reduced transparency and then stimulated phytoplankton growth. Therefore, sediment resuspension may be one of the most important factors delaying the recovery of eutrophic Lake Taihu, and the influence of sediment resuspension on water quality must also be taken into account by the lake managers when they determine the restoration target.     

Quantifying measures to limit wind-driven resuspension of sediments for improvement of the ecological quality in some shallow Dutch lakes

Although phosphorus loadings are considered the main pressure for most shallow lakes, wind-driven resuspension can cause additional problems for these aquatic ecosystems. We quantified the potential effectiveness of measures to reduce the contribution of resuspended sediments, resulting from wind action, to the overall light attenuation for three comparable shallow peat lakes with poor ecological status in the Netherlands: Loosdrecht, Nieuwkoop, and Reeuwijk (1.8–2.7 m depth, 1.6–2.5 km fetch). These measures are: 1. wave reducing barriers, 2. water level fluctuations, 3. capping of the sediment with sand, and 4. combinations of above. Critical shear stress of the sediments for resuspension (V _crit), size distribution, and optical properties of the suspended material were quantified in the field (June 2009) and laboratory. Water quality monitoring data (2002–2009) showed that light attenuation by organic suspended matter in all lakes is high. Spatial modeling of the impact of these measures showed that in Lake Loosdrecht limiting wave action can have significant effects (reductions from 6% exceedance to 2% exceedance of V _crit), whereas in Lake Nieuwkoop and Lake Reeuwijk this is less effective. The depth distribution and shape of Lake Nieuwkoop and Lake Reeuwijk limit the role of wind-driven resuspension in the total suspended matter concentration. Although the lakes are similar in general appearance (origin, size, and depth range) measures suitable to improve their ecological status differ. This calls for care when defining the programme of measures to improve the ecological status of a specific lake based on experience from other lakes.     

Resuspension in a shallow eutrophic lake

The frequency and the importance of wind-induced resuspension were studied in the shallow, eutrophic Lake Arresø, Denmark (41 km², mean depth 3 m). During storm events in autumn 1988 lake water samples were collected every 2–8 hours by an automatic sampler at a mid-lake station. The concentration of suspended solids and Tot-P was found to increase markedly. During storms up to 2 cm of the superficial sediment was resuspended, and the concentration of resuspended solids in the water column rose to 140 mg l^–1. The resuspended particles had a relatively high settling velocity and on average, a relatively short residence time in the water column of 7 hours.A model which describes the concentration of resuspended solids as a function of wind velocity and of settling velocity of the resuspended particles is presented. Using additional wind velocity data from a nearby meteorological station, the model has been used to calculate the frequency of resuspension events and concentration of resuspended solids for the period from May to November 1988.These calculations show that resuspension occurred about 50% of the time. Average flux of suspended solids from the sediment to the water was 300 g m^–2 d^–1 and during 50% of the time lake water concentration of suspended solids was more than 32 mg l^–1. A relationship between concentration of suspended solids and Secchi-depth is presented. Because of resuspension, Secchi-depth in Lake Arresø is reduced to 0.5 m.Resuspension also had a marked effect on Tot-P concentration in the lake water, and P input to the lake water being totally dominated by resuspension events.     

Resuspension behaviour of aluminium treated lake sediments: effects of ageing and pH

Lake restoration with aluminium (Al) has been widely used in shallow lakes, but it is unknown how ageing of the Al floc affect resuspension behaviour, sediment stability and entrainment of Al and phosphorus (P). High primary production in shallow lakes can lead to high pH in the water column with a potential release of Al and P during resuspension events. A laboratory resuspension experiment at defined hydrodynamic conditions using a calibrated erosion chamber was performed with intact sediment cores (with intact vs. disturbed biofilm on the surface) from previously Al-treated shallow Lake Möllen, NE Germany. Newly applied Al reduced surface sediment stability, but ageing led to the same stability as untreated sediment within 2 months with an intact biofilm and within 4 months with a disturbed biofilm. Dissolved Al increased markedly at resuspension after 2 days and 2 weeks of ageing and with elevated pH (9–11) in the water. The Al floc were redissolved and dissolved Al increased with elevated pH and shear velocity. Dissolved P concentration was constantly low due to excess binding capacity of the Al floc. In conclusion, Al application to shallow lakes prone to resuspension and with a high production must be done in periods with less resuspension risk to allow 2–4 months time for floc stabilization. Otherwise, possible resuspension and high(er) pH may lead to elevated concentrations of dissolved Al in the water column.     

Phosphorus release from resuspended sediment in the shallow and wind-exposed Lake Arresø, Denmark

Wind-induced sediment resuspension occurs frequently in the shallow and eutrophic Lake Arresø, Denmark. The impact of resuspension on internal phosphorus loading was investigated by laboratory experiments studying P-release from the undisturbed sediment surface and by experiments simulating resuspension events.Phosphorus release from undisturbed sediment sampled in May and August was 12 mg and 4 mg m^–2 d^–1, respectively. During experimental simulation of resuspension, soluble reactive phosphate (SRP) increased by 20–80 µg l^–1, which indicates that a typical resuspension event in the lake would be accompanied by the release of 150 mg SRP m^–2. The internal P loading induced by resuspension is estimated to be 60–70 mg m^–2 d^–1, or 20–30 times greater than the release from undisturbed sediment.SRP release during simulation of resuspension was mainly dependent on the equilibrium conditions in the water column and was basically independent of the increase in suspended solids and the duration of resuspension. A second simulation of resuspension conducted 26 hours later, did not result in any further release of SRP from sediment sampled in May. In contrast, there was an additional SRP release from sediment sampled in August, indicating that an exchangable P pool, capable of altering equilibrium conditions, is built up between resuspension events.It is concluded that resuspension, by increasing the P flux between sediment and water, plays a major role in the maintenance of the high nutrient level in Lake Arresø. A relatively high release rate is maintained during resuspension because of the low Fe:P ratio and the high concentration of NH₄Cl-extractable P in the sediment.     

Phosphorus release from cyanobacterial blooms in Meiliang Bay of Lake Taihu, China

To investigate the relationship between cyanobacterial density and phosphorus release into a lake aquatic environment, in situ experiments with 2.5 L microcosms were conducted in Meiliang Bay, located in the northern part of Lake Taihu, China. The effects of different environmental factors on phosphorus release and the ways changes of water quality indexes are involved in phosphorus release were further examined. It was found that total dissolved phosphorus (TDP) concentration kept to low levels (around 0.488 mg L⁻¹) in the microcosm with the low cyanobacterial density (8.85 × 10⁷ cell L⁻¹) throughout the experimental period, whereas first-order kinetics of TDP release was observed in microcosms with intermediate (7.60 × 10⁸ cell L⁻¹) and high cyanobacterial density (3.65 × 10⁹ cell L⁻¹). Accordingly their TDP release rate constants were both approximately 0.8930 d⁻¹ in the latter two treatments. The dissolved inorganic phosphorus (DIP) concentrations also increased with the increase of cyanobacterial density in 4 days. However, the DIP decreased from 35.52 mg L⁻¹ on day 4 to 6.72 mg L⁻¹ on day 6 in microcosm with the high cyanobacterial density during the experiments. Temperature could remarkably improve phosphorus release, while disturbance and illumination had negative effects on it. In addition, both TDP and DIP concentrations were positively correlated with electronic conductivity, salinity and total dissolved solid, but negatively correlated to chlorophyll-a and cyanobacterial density when cyanobacterial density was more than 7.60 × 108 cell L⁻¹. Thus, more phosphorus can be released from cyanobacterial blooms at higher cyanobacterial densities in Meiliang Bay, which is also determined by high temperature. Higher dissolved phosphorus concentration in cyanobacteria-dominated lakes (regions) is mainly due to the decomposition of cyanobacteria during the outbreak of cyanobacterial bloom in Lake Taihu, especially in Meiliang Bay.     

Floating-leaved macrophyte (<Emphasis Type="Italic">Trapa quadrispinosa</Emphasis> Roxb) beds have significant effects on sediment resuspension in Lake Taihu,China

Trapa quadrispinosa Roxb is a common floating-leaved macrophyte in China. In this study, the effects of Trapa on sediment resuspension in Lake Taihu, a large, shallow, eutrophic lake in eastern China, were investigated using sediment traps. The study was conducted at stations with and without Trapa beds from 7 September to 18 October, 2003. Results showed that sediment resuspension rates at the station without Trapa were significantly higher than those at the station with Trapa. During the study period (41 days), 10,970 g dw m⁻² of surface sediment was resuspended within the Trapa beds and 29,903 g dw m⁻² in the pelagic zone. These equate to phosphorus fluxes of 7.4 g m⁻² to the water column at the station with Trapa and 16.1 g m⁻² at the station without Trapa. Floating-leaved macrophytes, such as Trapa, may be important in sediment resuspension and internal phosphorus loading in shallow lakes.     

鲤对浅水湖泊水质影响的围隔实验研究

鲤是我国浅水湖泊常见鱼类,鲤可以通过扰动底泥、牧食浮游动物、排泄营养盐等途径影响湖泊水质。为了研究鲤对浅水湖泊水质的影响,论文于2009年6月23日至8月18日在暨南大学明湖进行原位围隔实验。实验分为三个处理组(三组围隔):(1)在围隔中放养鲤(放鱼组);(2)在围隔底部放置塑料隔网,在隔网上部空间放养鲤(放鱼放网组);(3)围隔中不放隔网也不放鲤(空白对照组)。实验结果表明:鲤可以显著增加水体悬浮物、总氮、总磷和叶绿素a含量,显著降低了水体透明度。而鲤是否扰动底泥对水体悬浮物、总氮、总磷和叶绿素a含量等指标的影响不显著。 因此,该研究中鲤对浅水湖泊的影响可能主要是通过排泄等途径实现的,鲤扰动底泥对水质的影响不大。     

The effect of different submerged macrophyte species and biomass on sediment resuspension in a shallow freshwater lake

Our study aim was to elucidate the effects of different species of submerged macrophytes and biomass levels on sediment resuspension. For this purpose experiments were conducted in four different enclosures (Potamogeton maackianus enclosure-PE, Vallisneria spinulosa enclosure-VE, manipulated enclosure-ME and aquaculture enclosure-AE). A sediment trap method was employed and the experiments were conducted from summer to winter in a shallow freshwater lake located in central China. A total of 813, 1277, 613 and 693 g DW m⁻² of sediment was resuspended in VE, AE, ME and PE, respectively. Our results showed that P. maackianus was more effective than V. spinulosa in restraining sediment resuspension. Macrophytes reached their maximum effectiveness of reducing resuspension at a certain species-specific biomass threshold above which biomass effects on resuspension were negligible. The threshold biomass was estimated as 300 g m⁻² for P. maackianus. Accordingly, within a lake management and aquaculture aspect, we conclude that as long as biomass does not fall below this threshold its consumption will not influence sediment resuspension. In the mid-lower reaches of the Yangtze River macrophyte coverage protects the lake sediment against adverse effects of monsoon wind; if the vegetation is eroded aquaculture sediment resuspension increases significantly.     

DO PHYSICAL FACTORS REGULATE PHYTOPLANKTON DISTRIBUTION PATTERNS IN LARGE, SHALLOW LAKES?

Factors that regulate phytoplankton dynamics in shallow, productive lakes are poorly understood, due to their predisposition for frequent algal blooms and sediment resuspension events. In Lake Apopka, greatest phytoplankton biomass reflects wind-induced resuspension of algae (meroplankton) that exists on the aphotic lake bottom in a layer approximately 5 cm thick; this assemblage is dominated by diatoms (>60% of total biomass) that can occur in resting stages. Once exposed to moderate light, meroplankton are capable of growth and photosynthetic rates comparable with surface populations. In Lake Okeechobee, remote sensing was used to assess the basin-wide distribution of suspended particles. Satellite reflectance values agreed well with in situ particle densities at 20 in-lake stations (average r²; LANDSAT = 0.81, AVHRR = 0.53), and maps of algal blooms (r² = 0.79, p ≤ 0.01). The greatest chlorophyll concentrations occurred in the vicinity of tributary nutrient inputs at the lake's perimeter, while turbidity increased towards the center of the lake reflecting predominant water circulation patterns. These results underscore the importance of physical-biological interactions in lakes.     

Effects of resuspension and eutrophication level on summer phytoplankton dynamics in two hypertrophic areas of Lake Taihu,China

The composition of summer phytoplankton communities in two highly eutrophic areas of Lake Taihu, a large, shallow water body in China (area 2,338 km²; mean depth 2.0 m), was studied in relation to selected environmental variables. Both areas have multiple uses, including drinking water supply, fisheries, and recreation. The first area, Meiliang Bay, is much closer to central lake and subjected to wind-driven sediment resuspension, while the second, at Wuli Lake, is less affected by wave action but is considered more nutrient-rich due to its proximity to human inhabitation. By comparing two lake areas, we attempt to advance the understanding and management of different regions of shallow lakes, mainly differing in resuspension levels and eutrophication levels. A comparison of these two lake areas indicated a higher standing crop of phytoplankton in Wuli Lake than in Meiliang Bay. Canonic Correspondence Analysis indicated that nutrient status and sediment resuspension were the principal factors influencing phytoplankton composition in both areas. The dominant algal group in Meiliang Bay comprised immobile R-type strategists (Reynolds in Growth and reproductive strategies of freshwater phytoplankton, 1988), including filamentous diatoms and green algae (Aulacoseira granulate and Planctonema lauterbornii), and recruitment-beneficial RS-type strategists, mainly the colonial cyanobacteria (Microcystis spp.). The community of Wuli Lake was dominated by S-strategists, principally motile flagellates (Cryptomonas spp.) and C-strategists including small nonmotile centric diatoms and chlorococcal green algae.     

Modelling sediment transport in shallow lakes — interactions between sediment transport and sediment composition

In shallow, wind exposed lakes, the light conditions, the cycling of nutrients, heavy metals and organic micro-pollutants and changes in the local composition of the sediment top layer can be dominated by resuspension/erosion of bottom sediment and sedimentation of suspended solids. A 2 dimensional model for Sediment Transport, Resuspension and Sedimentation in Shallow lakes (STRESS-2d), based on an existing transport model, is discussed. In the model, mass balance equations for the water compartment and the bottom sediment are solved numerically. Up to 7 sediment fractions can be taken into account, each having a specific set of resuspension/erosion and sedimentation parameter values. Several options for modelling the changes in the bottom sediment composition are available.A simulation experiment for Lake Veluwe (The Netherlands), in which model options with and without the distinction of sediment fractions were used, showed that using sediment fractions to account for the variability in the sediment composition leads to an improvement of the model results, particularly the simulated phosphorus sediment-water exchange fluxes. For Lake Ketel (The Netherlands) two options for modelling changes in the bottom sediment composition are compared. It is shown that an option in which a thin water-sediment layer on top of the more consolidated bottom sediment is simulated provides an improvement in the simulation of the suspended solids concentration.     

A study of absorption characteristics of chromophoric dissolved organic matter and particles in Lake Taihu,China

Absorptions by non-phytoplankton particles and phytoplankton, and chromophoric dissolved organic matter (CDOM) were measured at 50 sites in large, shallow, Lake Taihu in winter and summer 2006 to study their seasonal and spatial variations, and their relative contributions to total absorption. The CDOM absorption was significantly higher in winter than in summer, due to degradation and release of fixed carbon in phytoplankton and submerged aquatic vegetation (SAV). The hyperbolic model was used to model the spectral absorption of CDOM, and the mean spectral slope of 6.38 nm⁻¹ was obtained. At most sites, the spectral absorption of non-phytoplankton particles was similar to that of the total particles, demonstrating that the absorption of the total particles is dominated by the absorption of non-phytoplankton particles. In summer, phytoplankton absorption increased markedly, due to frequent algal blooms especially in Meiliang Bay. In winter, the significant increase in non-phytoplankton particle absorption resulted from the increase of inorganic particulate matter caused by sediment resuspension. Strong linear relationships were found between a _d(440) and total suspended matter (TSM), organic suspended matter (OSM), and inorganic suspended matter (ISM). Strong linear relationships were also found between a _ph(440), a _ph(675) and chlorophyll a (Chl-a) concentration. The total relative contributions of non-phytoplankton particles over the range of photosynthetically active radiation (PAR) (400–700 nm) were 48.4 and 79.9% in summer and winter respectively. Non-phytoplankton particle absorption dominated the total absorption, especially in winter, in Lake Taihu, due to frequent sediment resuspension in the large shallow lake as a result of strong windy conditions. The results indicate that strong absorption by CDOM and non-phytoplankton particles at the blue wavelength has an impact on the spectral availability, and acts as a selection factor for the composition of the phytoplankton community, with cyanobacteria being the dominate species in Lake Taihu. Handling editor: L. Naselli-Flores     

Phosphorus dynamics at multiple time scales in the pelagic zone of a large shallow lake in Florida,USA

Phosphorus (P) dynamics in large shallow lakes are greatly influenced by physical processes such as wind-driven sediment resuspension, at times scales from hours to years. Results from long-term (30 year) research on Lake Okeechobee, Florida (area 1,730 km², mean depth 2.7 m) illustrate key features of these P dynamics. Variations in wind velocity result in changes in water column transparency, suspended solids, and total P (TP). In summer there are diurnal changes in TP associated with afternoon winds, and in winter, when strong winds occur for multiple days, monthly average TP remains high compared to summer. The magnitude of daily and seasonal TP changes can exceed 100 μg l⁻¹. Hurricanes and tropical storms also cause extreme changes in TP that are superimposed on seasonal dynamics. When a hurricane passed 80 km south of the lake in October 1999, mean pelagic TP increased from 88 to 222 μg l⁻¹. During large resuspension events, light attenuation is substantially increased, and this influences the biomass and spatial extent of submerged plants, as well as water column TP. In Lake Okeechobee, TP concentrations typically are ∼20 μg l⁻¹ when submerged plants are dense, and soluble reactive P concentrations are reduced below detection, perhaps by the periphyton and plant uptake and by precipitation with calcium at high pH. In contrast, TP exceeds 50 μg l⁻¹ when submerged plants and periphyton are absent due to prolonged deep water, and phytoplankton biomass and algal bloom frequency both are increased. In Lake Okeechobee and other large shallow lakes, complex models that explicitly consider wind-wave energy, hydrodynamics, and sediment resuspension, transport, and key biological processes are needed to accurately predict how lake water TP will respond to different management options.     

Effect of vegetation on alpha cellulose decomposition in littoral lake sediments

The effect of vegetation cover on the decomposition of organic matter (alpha cellulose) was studied at three sediment depths (5, 15 and 25 cm) in the littoral area of a small Lake Kiruvere (Estonia). The experiment was carried out in two adjacent sites, with and without vegetation, using the litter bag method. At all sediment depths decomposition was faster at the site covered with vegetation, and was highest at 5 cm sediment depth (decomposition rate k = 0.0037 day⁻¹) and lower at 15 and 25 cm depths (k = 0.0014 day⁻¹ and k = 0.0013 day⁻¹). Higher decomposition rates coincided with higher root mass in the sediment. Decomposition rates were similar at all sediment depths in the site without a vegetation cover (k = 0.0007-0.0009 day⁻¹). The presence of a vegetation cover also affected temperatures in sediments. Temperatures were several degrees higher at all sediment depths in the area with vegetation cover, than in the area without. Mean sediment temperature differences between the two experiment areas were 1.4 °C at 5 cm sediment depth, 2.5 °C at 15 cm depth and 3.1 °C at 25 cm depth. Higher decomposition rates in the site covered with vegetation can be explained by oxygen dispersion from young roots in the higher sediment layers and by higher sediment temperatures due to the internal gas flow enhancing the microbial activity in the lower sediment layers.     

Multivariate analysis of heavy metal and nutrient concentrations in sediments of Taihu Lake, China*

Taihu Lake is one of the largest freshwater lakes in China. The Lake is very shallow with a mean depth of 1.9 m and an area of 2428 km². Nutrient concentrations (Org-C, Tot-N and Tot-P) and heavy metal concentrations (Ag, As, Cd, Co, Cr, Cu, Hg, Ni, Pb, Sr, Zn, etc.) in the lake's surface sediments were sampled at 13 locations. This was done to determine if industrialized areas along the lake's coastline were impacting the nutrient and heavy metal distribution of the lake's surface sediments. Principal Component Analysis (PCA) was used to assess the degree of contamination and spatial distribution of heavy metals and nutrients in different areas of Taihu Lake.A distinctive spatial distribution of heavy metals and nutrients was observed. Sediments from a large embayment of Taihu Lake called Lake Wulihu had significantly higher nutrient concentrations (Org-C, 2.05–3.83%; Tot-N, 0.28–0.54%; Tot-P, 0.10–0.33%) than any other area of Taihu Lake. These high nutrient levels were associated with the input of untreated domestic sewage from the large (circa one million people) City of Wuxi, which discharges its effluents into the Liangxi River. As a result, Lake Wulihu is the most eutrophic area of Taihu Lake. The nearby Meiliang Bay suffered from the worst heavy metal contamination in Taihu Lake (e.g. As, 64.0; Ag, 4.2; Cd, 0.93; Co, 14.2; Cr, 155.0; Cu, 144.0; Hg, 0.25; Ni, 79.8; Pb, 143.0 and Zn, 471 mg kg^–1). These high heavy metal concentrations were ascribed to the discharge of untreated and partially treated industrial waste water from Changzhou and Wujin via the Zhihugang River. Surface sediment samples from the east basin of Taihu Lake were characterized by high Org-C (1.0–2.3%) and Tot-N (0.18–0.37%) and low Tot-P (0.048–0.056%) concentrations. It is likely that macrophytes removal accounts for a major reduction of phosphorus in the sediments of the east basin of Taihu Lake.     

Extreme water level decline effects sediment distribution and composition in Lake Alexandrina,South Australia

Water level decline affects the biophysical environment of shallow lakes. Unprecedented drought in Australia’s Murray–Darling Basin resulted in extreme water level drawdown in the large, shallow Lake Alexandrina at the end of the River Murray. Surface sediment was collected from 22 sites in the lake before and after water levels declined to assess the integrated limnological changes over the period of drawdown. Results indicate an increase in the proportion of organic particles in profundal sediments, as well as an increase of fine particles (<19.9 μm) in peripheral sediments. These changes to sediment composition corresponded to higher concentrations of suspended particles at low water levels. Increased autochthony and a shift in primary production from macrophytes to phytoplankton in Lake Alexandrina support these findings. Inorganic carbon and other nutrients were lost from sandy sediments most likely through carbonate dissolution driven by a localized decrease in pore water pH from increased mineralisation of organic matter.     

Invasion biology of Ponto-Caspian onychopod cladocerans (Crustacea: Cladocera: Onychopoda)

Lake Taihu is characterized by its shallowness (mean depth = 1.9 m) and large surface area (2,338 km²). Runoff sources are mostly from the mountainous west and southwest, and outflows are located throughout East Taihu. This causes shorter retention times in the south. In contrast, urban pollutants discharge into northern Taihu and result in poor water quality. Non-point pollution from rural areas and sewage wastewater is the primary pollution source. Water current velocity ranges from 10–30 cm s⁻¹, and surface currents normally follow wind direction. Bottom currents appear to be a compensation flow. Most wave heights are less than 40 cm, and underwater irradiance correlates to seston in the water column. Lacustrine sediment is distributed in littoral zones, mostly along the western shoreline, with almost no accumulation in the lake center. Intensive aquaculture in East Taihu caused eutrophication and hampered water supply in surrounding areas. In addition, development of marshiness in the eastern littoral zones and East Taihu has occurred. The function of flood discharging of East Taihu has been limited by flourishing macrophytes. The problems facing in Lake Taihu will be alleviated by improving the management of nutrient sources into the lake. Guest editors: B. Qin, Z. Liu & K. Havens Eutrophication of shallow lakes with special reference to Lake Taihu, China