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.     

Remote sensing evidence for the episodic transport of phosphorus from the littoral zone of a thermally stratified lake

• 1 Esthwaite Water in Cumbria is a small, thermally stratified lake fringed with beds of reed in the shallow littoral. In this study, we used a combination of in situ measurements and airborne remote sensing to investigate some of the physical processes influencing the transport of water and nutrients from the littoral zone.
• 2 The analysis of water samples collected from the reed beds, the littoral zone and the open water showed that significantly higher concentrations of dissolved reactive phosphorus (DRP) were frequently recorded in the reed beds in early summer.
• 3 Experiments with surface and near‐surface free‐running drogues demonstrated that the movement of water from the littoral zone was strongly influenced by the development of a secondary thermocline. When there was no secondary stratification, the surface currents generated by light winds seldom exceeded 2 cm s^‐1. When a secondary thermocline was present, surface current speeds of 5–10 cm s^‐1 were recorded even when the wind speed was less than 200 cm s^‐1.
• 4 A series of thermal surveys using a Daedalus Airborne Thematic Mapper (ATM) demonstrated that plumes of warm water frequently developed in the littoral zone when the weather was calm. Some of these plumes covered several hundred square metres and persisted for several hours. Others were only a few metres in width and acted as episodic ‘pumps’ that appeared and dispersed in less than an hour.

     

Pore water phosphorus and iron concentrations in a shallow,eutrophic lake — indications of bacterial regulation

Peak pore water SRP and iron(II) concentrations were found during summer in surface sediments in the shallow and eutrophic L. Finjasjön, Sweden, and the concentrations generally increased with water depth. The SRP variation in surface sediments (0–2 cm) was correlated with temperature (R² = 0.82–0.95) and iron(II) showed a correlation with sedimentary carbon on all sites (R² = 0.42–0.96). In addition, sedimentary Chla, bacterial abundances and production rates in surface sediments (0–2 cm) varied seasonally, with peaks during spring and fall sedimentation. Bacterial production rates were correlated with phosphorus and carbon in the sediment (R² = 0.90–0.95 and R² = 0.31–0.95, respectively), indicating a coupling with algal sedimentation. A general increase in sediment Chla and bacterial abundances towards sediments at greater water depth was found. Further, data from 1988–90 reveal that TP and TFe concentrations in the lake were significantly correlated during summer (R² = 0.81 and 0.76, in the hypolimnion and epilimnion, respectively). The results indicate that the increase in pore water SRP and Fe(II) in surface sediments during summer is regulated by bacterial activity and the input of organic matter. In addition, spatial and temporal variations in pore water composition are mainly influenced by temperature and water depth and the significant correlation between TP and TFe in the water suggests a coupled release from the sediment. These findings support the theory of anoxic microlayer formation at the sediment-water interface.     

Effects of common carp (Cyprinus carpio) on macrophytes and invertebrate communities in a shallow lake

1. We performed two field experiments using different cage sizes to determine the direct and indirect effects of carp on macrophytes and invertebrate community composition in a shallow lake. 2. The presence of carp produced a significant decrease in macrophyte abundance, changes in species composition and decreased abundance and diversity of invertebrates in small (2.5 × 2.5 × 2.0 m) cages. In large exclosures (15 × 5 m), only macrophyte stem length was significantly affected by carp. 3. There was considerable variation in the macrophyte and invertebrate responses to carp on different sides of the lake in the large exclosures, suggesting that wind and wave action are also important variables affecting macrophyte growth and persistence. 4. The data suggest that carp can significantly affect species abundance and diversity of macrophytes and some macroinvertebrates.     

Trophic dynamics in a whole lake experiment: size-structured interactions and recruitment variation

Horizontal and vertical heterogeneity as a result of size‐structured processes are important factors influencing indirect effects in food webs. In a whole‐lake experiment covering 5 years, we added the intermediate consumer roach (Rutilus rutilus) to two out of four lakes previously inhabited by the omnivorous top predator perch (Perca fluviatilis). We focused our study on the direct consumption effect of roach presence on zooplankton (and indirectly phytoplankton) versus the indirect effect of roach on zooplankton (and phytoplankton) mediated via effects on perch reproductive performance. The patterns in zooplankton and phytoplankton abundances were examined in relation to population density of roach and perch including young‐of‐the‐year (YOY) perch in the light of non‐equilibrium dynamics. The presence of roach resulted in changed seasonal dynamics of zooplankton with generally lower biomasses in May–June and higher biomasses in July–August in roach lakes compared to control lakes. Roach presence affected perch recruitment negatively and densities of YOY perch were on average higher in control lakes than in treatment lakes. In years when perch recruitment did not differ between lakes as a result of experimental addition of perch eggs, total zooplankton biomass was lower in treatment lakes than in control lakes. Phytoplankton biomass showed a tendency to increase in roach lakes compared to control lakes. Within treatment variation in response variables was related to differences in lake morphometry in treatment lakes. Analyses of the trophic dynamics of each lake separately showed strong cascading effects of both roach and YOY perch abundance on zooplankton and phytoplankton dynamics. Consideration of the long transients in the dynamics of top predators (fish) in aquatic systems that are related to their long life span involving ontogenetic niche shifts is essential for making relevant interpretations of experimental perturbations. This conclusion is further reinforced by the circumstance that the intrinsic dynamics of fish populations may in many cases involve high amplitude dynamics with long time lags.     

Macrophyte refuges, prey behaviour and trophic interactions: consequences for lake water clarity

Macrophytes may enhance grazing on phytoplankton by providing a refuge for zooplankton against fish predation. Loss of macrophytes can trigger sudden degradation of water clarity (regime shift) in lakes. However, the presence of piscivores may drive planktivorous fish to take refuge amongst littoral macrophytes. To address the possibility of regime shifts, I here constructed an empirically based model that combined population dynamics of organisms with game theory for optimal habitat selection, taking into consideration the trophic structure, lake size and eutrophication. The model showed that macrophytes generally acted as a refuge for zooplankton, rather than for fish. The model predicted that regime shifts were more likely in small, shallow lakes and that the presence of macrophytes raised the possibility of regime shifts. The present study demonstrated that the fast dynamics of animal behaviour could lead to regime shifts, in connection with slower variables such as nutrient loading.     

The impact of year-to-year changes in the weather on the dynamics of Daphnia in a thermally stratified lake

The factors influencing the seasonal dynamics of Daphnia in a thermally stratified lake (Esthwaite Water) are described and related to long-term changes in the weather. The Daphnia produced three cohorts in the year and the strength of the cohorts was determined by year-to-year variations in the physical characteristics of the lake and the abundance of edible algae. Food was most abundant in early summer when small, fast-growing flagellates were particularly common. In late summer, the phytoplankton community was dominated by large, inedible species but edible forms re-appeared when nutrients were entrained by wind mixing. Examples are presented to demonstrate the effect that year-to-year variations in the weather have on the growth of the phytoplankton and the dynamics of the Daphnia. In ‘good’ years, when the lake stratifies early and there are periods of episodic mixing in summer, there are two ‘pulses’ of edible algae and two strong cohorts of Daphnia. In ‘bad’ years when stratification is delayed and there is little episodic mixing, the growth of the edible algae is suppressed and the Daphnia produce two weak cohorts. The results are discussed in relation to the impact of intermediate disturbances on growth of phytoplankton and current theories of population regulation in Daphnia. The evidence suggests that the dynamics of the Daphnia in the lake are strongly influenced by seasonal variations in the mixing regime, the recycling of nutrients and the episodic growth of edible algae.     

Effects of a rapidly increasing population of common carp on vegetative cover and waterfowl in a recently restored Midwestern shallow lake

Although the common carp (Cyprinus carpio), an invasive benthic fish from Eurasia, has long been strongly implicated in the disappearance of vegetative cover and reduced waterfowl abundance in North American shallow lakes, the details of this relationship are obscure. This study documented ecological changes in a recently restored shallow lake (Hennepin and Hopper Lakes, IL, USA) at a time that it was experiencing a large increase in its carp population. We estimated the abundance and biomass of carp 7 years after this lake had been restored and then back-calculated carp population size across time while examining changes in the lake’s plant and waterfowl communities. We found that the biomass of carp remained below ~30 kg/ha for 5 years following restoration, but then increased to ~100 kg/ha in the sixth year following a strong recruitment event. Although a carp biomass of <30 kg/ha had no discernible effects on vegetative cover (which exceeded 90%) or waterfowl (which exceeded 150,000 individuals during fall censuses), the increase to 100 kg/ha was associated with a ~50% decrease in both vegetative cover and waterfowl. A further increase in carp biomass to over 250 kg/ha during the seventh year coincided with a decrease in the vegetative cover to 17% of the lake’s surface and a decline in waterfowl use to ~10% of its original value. These data suggest that the common carp is extremely damaging to the ecological integrity of shallow lakes when its density exceeds ~100 kg/ha. Since the biomass of carp in Midwestern shallow lakes commonly exceeds this value by 3–4 times, it seems likely that carp are responsible for the large-scale habitat deterioration described in many of these ecosystems. Handling editor: J. Cambray     

Influence of submerged vegetation and fish abundance on water clarity in peri-urban eutrophic ponds

Despite the presence of high nutrient concentrations, most ponds located around Brussels (Belgium) show a considerable variation in turbidity. The importance of submerged macrophytes in maintaining the clear-water state requires identification of the main factors determining macrophyte abundance and diversity in ponds and small lakes. In this study, the inter-relationships between submerged macrophyte cover, fish abundance and turbidity were investigated in 13 eutrophic peri-urban ponds. Along a turbidity gradient, vegetation switched from dominance by Stoneworts (Chara and Nitella spp.) in the clearest ponds, to dominance by Potamogeton pectinatus in ponds with a slightly lower water transparency. Despite the presence of both P. pectinatus and Stoneworts in each of the vegetated ponds, only one became dominant. Only a very low abundance (around 20%) of submerged vegetation was found in ponds of intermediate turbidity, while macrophytes were absent in turbid ponds. Multi- and univariate analysis showed a marked difference in chemical, physical and biological properties between ponds deliberately used for fish stocking and ponds that were not. Macrophyte cover was significantly negatively correlated with turbidity and plankti-benthivorous fish abundance. No such correlation was observed with piscivorous fish abundance, except for pike that were associated with a charophyte vegetation in the study ponds. The strong relationship found between fish abundance and turbidity, its negative effect on submerged vegetation cover, and the importance of submerged vegetation in controlling phytoplankton abundance, should be taken into account when selecting ponds for fish stocking. It also suggests that the study ponds have a good potential for ecological quality restoration by biomanipulation.     

Growth of pikeperch in relation to lake characteristics: total phosphorus, water colour, lake area and depth

The growth of pikeperch Sander lucioperca was studied in 41 lakes in central Finland. The backcalculated average total length of 3 year‐old pikeperch was used as an indicator of growth. The growth correlated positively with total phosphorus and water colour and negatively with lake area and depth. The reason for differences in growth may be differences in the amount of suitable food, foraging success or temperature dynamics in different lakes.     

Effect of intensive catchment and in-lake restoration procedures on phosphorus concentrations in a eutrophic lake

Lake Okaro is a small, warm monomictic lake in central North Island, New Zealand, which progressed from oligotrophic to eutrophic through the 1960s. Trends in phosphorus (P) concentrations in the lake are linked to multiple restoration efforts over a 5-year period (2003–2008). The restoration procedures include a 2.3 ha constructed wetland established in February 2006 and riparian margin protection to reduce external loading, as well as an Alum application in December 2003 and sediment capping using modified zeolite in September 2008 to reduce internal loading. The annual average total phosphorus (TP) concentration in the lake decreased by 41% from 2004–2005 to 2007–2008. Two predictive models based on external P loading data generally underestimated the measured TP concentrations in the water column due to internal P loading. The relatively rapid response of TP concentrations after reduction of the internal loading using modified zeolite suggests that this technique can effect a rapid decrease in lake water TP concentrations though the trophic state of Lake Okaro showed high resilience to the reduced P loading. It is concluded that the combined effect of all restoration procedures resulted in a relatively rapid decrease in TP concentrations in Lake Okaro, which may be prolonged by continued external load reduction.     

Effect of chlorophyll sampling design on water quality assessment in thermally stratified lakes

In order to adequately assess the ecological status of thermally stratified lakes based on chlorophyll, the sampling must cover all productive layers of the water column. Missing the deep chlorophyll maxima (DCM) that often occur in the meta- or hypolimnion of transparent lakes supported by sufficient illumination and good nutrient availability may cause serious underestimation of the productivity and lead to misclassification of the lake ecological status. There is no commonly accepted sampling design for stratified lakes, and various monitoring guides suggest controversial designs. Our aim was to find some robust criteria to assess the probability of occurrence of a DCM and estimate the differences in measured mean chlorophyll concentrations caused by various sampling designs. Our theoretical model showed that the probability of occurrence of a DCM increases with increasing water transparency and decreasing lake size. Empirical data from Italian and Estonian stratified lakes confirmed the results. Testing of different sampling designs on lakes with full measured chlorophyll profiles available showed that taking only surface layer samples will lead with a high probability to an underestimation of the chlorophyll concentration in the trophogenic layer. In order not to miss the Chl peak in stratified lakes, in most cases it would be more precautious not to limit the sampling to the well-mixed epilimnion but to extend it to the whole euphotic layer. Sampling the epilimnion instead of the euphotic zone could cause up to a 70% underestimation of the chlorophyll concentration, an error that would cause a misclassification of the lake by one or even two status classes in a 5-class assessment system. In most cases, the 2.5 * Secchi depths proved a suitable criterion of the sampling depth and only in the case of surface scums, would sampling of a 3 * Secchi depth layer be recommended in order not to miss the deep chlorophyll maximum.     

Retention of nitrogen, phosphorus and silicon in a large semi-arid riverine lake system

Lakes and reservoirs (impoundments) are often viewed as a sink for nutrients within the river continuum. To date, most studies on nutrient retention within impoundments are derived from the temperate climate zones of Europe and North America, only consider one nutrient, and are often short-term (1–2 years). Here, we present a long-term (17 year) data set and nutrient (nitrogen, phosphorus and silica) budget for two connected semi-arid lakes (the Lower Lakes) at the terminus of the River Murray, Australia. Most of the filterable reactive phosphorus and nitrate entering the lakes were retained (77 and 92%, respectively). Total phosphorus (TP) was also strongly retained (55% of the annual TP load on average) and the annual TP retention rates could be predicted as a function of the areal hydraulic loading rate (annual lake outflow/lake surface area). On average, there was a slight net retention (7%) of the annual total nitrogen (TN) load but a slight net export (6% of the load) of organic N. TN retention as function of the areal hydraulic loading rate was lower than expected from existing models, possibly because of high nitrogen fixation rates in the Lower Lakes. Silica was retained (39%) at similar rates to those observed in previous studies. There was also a marked increase in the TN:TP and TN:Si ratios within the lake (TN:TP~30 and TN:Si~0.67) compared to those entering (TN:TP~15, TN:Si~0.45), as a consequence of the relatively low net retention of nitrogen.     

在磷浓度不同的基质中种植密度对野菱生长的影响

研究了在3种不同磷浓度(低浓度27.56±0.78μg·g-1,中浓度52.85±1.30 μg·g-1,高浓度115.61±2.72μg·g-1)基质中,不同种植密度(11和32株·m-2)对野菱(Trapa incisa Sieb.et Zucc.)生长和不同部位磷含量的影响.结果表明,在磷浓度不同的基质中,野菱单株的根、茎和叶干质量及总干质量都随种植密度的提高而增加.种植密度的高低对野菱茎直径和叶数没有显著影响(P>0.05),但在3种基质中,种植密度高的野菱植株的叶数都高于种植密度低的野菱植株.种植密度埘野菱各部位干物质分配以及磷含量的影响因基质中磷浓度的不同而表现出不同的规律性;种植密度高的野菱植株的茎和叶片中的磷含量高于种植密度低的野菱植株,种植密度低的野菱植株的茎生物量比和根中的磷含量均高于种植密度高的野菱植株.研究结果显示,提高种植密度不仪能促进野菱个体的生长,且能促进磷在野菱体内的运输.     

Manipulation of chemistry and phytoplankton by hydrological intervention: a whole lake experiment in the northern Netherlands

SUMMARY. 1. The small Lake Negenmad, in the nature reserve 'De Oude Venen', was isolated from the canals and lakes of the Frisian lake system. Its chemistry and phytoplankton were monitored 1 year before (1984) and 2 years after (1985–86) isolation. These characteristics were compared with those of the adjacent Lake Veertigmad which was not manipulated.
2. The hydrological intervention prevented the inflow of chloride-rich IJsselmeer water into Lake Negenmad and made its water table more dependent on evapotranspiration, precipitation and upwelling water. In comparison with the unmanipulated lake. Lake Negenmad became less saline (c. 50%), more humic (c. 50%) and total dissolved (<0.2,μm) iron concentration increased dramatically.
3. During the 2 years of damming no marked differences in the total nutrient concentrations of either lake were observed. However, the maximum phytoplankton density in the dammed lake was half that in the unmanipulated lake. The lower density may have been caused by low P availability after formation of humus-iron phosphate species.
4. Preventing the inflow of water from the Frisian lake system favoured the occurrence in Lake Negenmad of flagellated species (Chrysophyceae, Cryptophyceae) at the expense of filamentous and nuisance-causing cyanobacteria (Oscillutoria, Anabaena, Aphanizomenon spp.).
5. The implications of these results for water quality management of reservoirs in peaty areas are briefly discussed.     

Occurrence of water phosphorus at the water-sediment interface of a freshwater shallow lake: Indications of lake chemistry

Better understanding of the occurrence of water phosphorus (P) at the water-sediment interface is vital to clarify P sources of origin in freshwater shallow lake ecosystems. This study focused on water-sediment interface systems and explored implicit indications of lake chemistry on water P based on a case study of Baiyangdian Lake, North China. 20 variables from 14 sampling sites collected for six months in two years were investigated, including sequentially extracted P fractions. Exploratory data analysis with multivariate statistical techniques and the index of P maximum solubilization potential were employed to examine interactions of water P and coexisting chemicals, and to accomplish pattern recognition of water-sediment interface systems. Results showed that nine key variables (temperature, conductivity, ammonium nitrogen, total nitrogen, sediment total P, metallic oxide bound P, organic P, aluminum and ferrum) were identified and ranked into four latent parameters (physical factors, nutrients, P species, and metals), accounting for 81% of water P variation. Accordingly, the recognized three patterns of water-sediment interface unraveled spatial partitioning for the domination of external or internal P sources. Four variables (temperature, sediment total P, metallic oxide bound P and organic P) were competent to classify patterns of water-sediment interface with 100% correct assignment of cases. Using two parameters (organic P and metallic oxide bound P), discriminant functions produced 85.7% correct assignations, indicating the importance of the two P species in explaining spatial heterogeneity of water P under oxic and alkaline circumstances. This study provides an operational zoning frame and implications for eutrophication management applicable to freshwater shallow lakes.     

Response of epiphytic algae to nutrient loading and fish density in a shallow lake: a mesocosm experiment

Studies on the effect of eutrophication on the ecology of shallow lakes, usually pay scant attention to changes within the epiphytic algal community, though the contribution of this to the ecosystem dynamics is transcendental. In order to test the influence of nutrient loadings and fish densities in the structure of algal epiphyton in a shallow lake, an experiment was performed using in situ mesocosms. Nutrient additions were related to significant decreases in the total epiphyton biovolume and that of bacillariophyceans and zygnematophyceans, but with increases in the abundance of cyanobacteria. The different response of algal groups at the higher nutrient concentrations (increases or decreases in their abundance and/or biovolume) can be related to their ecophysiological constraints such as different resistance to toxicant ammonium accumulation. Plant-associated macroinvertebrates numbers were positively correlated with total numbers of epiphyton. The presence of planktivorous fish enhanced the abundance and biovolume of all algal groups, except cyanobacteria. Fishes enhanced the abundance of plant-associated animals and of total epiphyton. Fish indirect effects (e.g., nutrients release) and their dietary particularities were among the factors that together with nutrients influenced epiphyton growth. The role of indirect effects of fishes and the importance of their dietary particularities are stressed as key factors to understand the processes controlling epiphyton ecology and the food web structure of shallow lakes. Handling editor: D. Ryder.     

Effects of nutrients and grazers on periphyton phosphorus in lake enclosures

SUMMARY. 1. Periphyton. measured as particulate phosphorus (PP) and expressed as periphyton PP, growing on vertically oriented substrata (polyvinyl impregnated nylon) under different nutrient loadings, light intensities (exposures), and grazer communities was examined in eight large enclosures (750 m3) where nutrients (N and P) and planktivorous fish (1+yellow perch) were added in a 2x2 factorial design.
2. During the first 3 weeks of the experiment (25 June to 15 July), there was a significantly higher accumulation of phosphorus into periphyton (periphyton PP) with fertilization, but fish addition had no effect. During the fourth to seventh weeks (16 July to 12 August), addition of fish was associated with lower abundance of amphipods and chironomids and higher concentration of periphyton PP. In the enclosures without fish, these invertebrates were over 25 times more abundant, and periphyton PP decreased substantially compared to the June-July period. Fertilization increased periphyton PP only at high exposures in the enclosures with fish.
3. Exposure had a significant effect on periphyton PP. In the enclosures with fish, high abundance of nanoplankton reduced water transparency, and periphyton PP was lower in the deeper waters which may have been due to limitation by low light. Lower periphyton PP was also observed at the surface on sunny sides of enclosures without fish, and therefore with high water transparency. This pattern may have been due to inhibitory effects of high light intensity.
4. Periphyton communities in the enclosures with fish had higher uptake rates for planktonic phosphorus, and lower rates of phosphorus release, suggesting that periphyton with high phosphorus demand may have high internal cycling of assimilated phosphorus.