Patterns and mechanisms of phytoplankton variability in Lake Washington (USA)

Temporal variability in lake phytoplankton is controlled largely by a complex interplay between hydrodynamic and chemical factors, and food web interactions. We explored mechanisms underlying phytoplankton interannual variability in Lake Washington (USA), using a 25-yr time series of water quality data (1975-1999). Time-series analysis and PCA were used to decompose chlorophyll data into modes of variability. We found that phytoplankton dynamics in Lake Washington were characterized by four seasonal modes, each of which was associated with different ecological processes. The first mode coincided with the period when the system was light limited (January-March) and phytoplankton patterns were driven by the amount of available solar radiation. The second mode (April-June) coincided with the peak of the spring bloom and the subsequent decline of phytoplankton biomass, and was largely controlled by total phosphorus levels and grazing pressure from cladoceran zooplankton. Evidence of co-dependence and tight relationship between phytoplankton and cladoceran dynamics were also found from July to October when a large portion of the phosphorus supply in the mixed layer was provided by zooplankton excretion. The fourth mode (November-December) was associated with the transition to thermal and chemical homogeneity and the winter phytoplankton minima (2-2.5 microg/l). Finally, we examined the effects of meteorological forcing and large-scale oceanic climate fluctuations (ENSO and PDO) on phytoplankton dynamics and assessed the significance of their role on the interannual variability in the lake.     

Nutrient and grazing factors in relation to phytoplankton level in a eutrophic shallow lake: the effect of low macrophyte abundance

The ability of submerged macrophytes to moderate the structure of food webs with respect to lake eutrophication management has been intensively studied in recent years. Many lake managers have adopted the option of increasing macrophyte abundance in order to restore eutrophic waters, with a view to improve water quality, increase water transparency and reduce phytoplankton biomass. These studies are mostly based upon the comparison of macrophyte-rich and macrophyte-free waters. There is less quantitative information on how different levels of macrophyte abundance, in particular very low levels, influence ecosystem structure, or vice versa, within the same ecosystem. This paper takes a multivariate statistical approach to examine ecosystem function with low macrophyte abundance (ca. 5%) in the context of nutrient-phytoplankton-zooplankton interaction in a eutrophic shallow lake. It shows that with low levels of macrophyte coverage, phytoplankton biomass was only limited by nutrient (TP and Si) availability, whilst nutrient (Si) as well as grazing (zooplankton and Daphnia) factors were both limiting phytoplankton levels in macrophyte-free waters. With respect to interactions between Daphnia and chlorophyll-a, the results suggest that low macrophyte cover does not result in a reduction of phytoplankton biomass due to the presence of Daphnia. Rather, low macrophyte cover provides a refuge that maintains constant Daphnia predation, so reducing chlorophyll-a fluctuation. These results add credence to the causal interpretation of different interactions amongst nutrients, phytoplankton and zooplankton at very low macrophyte densities.     

A preliminary modeling analysis of water quality in Lake Okeechobee, Florida: Diagnostic and sensitivity analyses

A deterministic mass balance model for nutrient and phytoplankton dynamics was previously developed and calibrated to a comprehensive set of field data for Lake Okeechobee. In the present study, diagnostic and sensitivity analyses were conducted with the calibrated model to better understand factors controlling phytoplankton and load-response dynamics in the lake. Phytoplankton growth rate limitation due to underwater light attenuation appears to be substantially greater than growth rate limitation due to non-optimal phosphorus concentrations. Phytoplankton biomass appears strongly controlled by the supply rate of dissolved available phosphorus to the water column. The dynamics of total phosphorus and chlorophyll a concentrations in the lake are strongly influenced by sediment-water phosphorus fluxes. There is a wide range of uncertainty in responses of total phosphorus and cholorophyll a concentrations to changes in tributary phosphorus loadings. Much of this uncertainty is due to a lack of quantitative understanding of sediment responses to changes in tributary loadings. Other important factors are inter-annual variability in hydrometeorological conditions and the potential influence of wind-induced resuspension of particulate phosphorus. Responses of total phosphorus and chlorophyll a concentrations for a given change in tributary loading depend not only on the magnitide of the loading change, but also on the time frame after the loading change due to a lag in sediment response. Load-response predictions for Lake Okeechobee must take into account changes in available phosphorus loadings to the water column, and must be premised on assumptions for changes in internal phosphorus loadings from the sediments. Results from this preliminary modeling analysis are provisional in that they do not include potential nitrogen limitation, potential interactions between phosphorus and nitrogen, or phytoplankton responses to potential nitrogen fixation.     

水体富营养化成因分析

基于富营养化发生的机理,从氮、磷营养盐水平,铁、硅含量,光照强度,温度,水体动力学(水体流态,风力)和水体更新周期等方面对水体富营养化成因进行分析。目的是为更好地维持水体生态平衡,控制水体污染,预防水体富营养化的发生提供参考。     

Phytoplankton growth control and risk of cyanobacterial blooms in the lower Senegal River delta region

Eutrophication, and the resulting proliferation of phytoplankton can affect water quality. Although environmental and ecological processes that lead to phytoplankton proliferations in temperate areas have received considerable attention, scientific background regarding this topic in tropical areas, especially West Africa, are scarce. In this study, bioassays in batch cultures were carried out in order to identify factors that may be stimulating or limiting the biomass of phytoplankton at various sites in the lower Senegal River delta region. Complete factorial designs were used to test several factors both alone and combined (nitrogen, phosphorus, sulfate, EDTA and trace elements, stirring). The effects of these factors on the growth of the whole phytoplankton communities were followed by the daily measurement of in vivo fluorescence. At all but two of the sites, enrichment with certain nutrients significantly stimulated phytoplankton biomass growth, with nitrogen and/or phosphorus appearing to be the primary limiting factor(s). Nitrogen was the primary limiting factor in coastal ocean waters and in the Senegal River estuary. In the freshwater ecosystems studied, nitrogen and phosphorus acted as alternating or concomitant limiting factors. We assessed the changes in the phytoplankton composition induced by nutrient enrichments in Lake Guiers, a drinking water reservoir for the population of Dakar. Our findings lead us to define the conditions of a possible proliferation of the potentially toxic cyanobacterium Cylindrospermopsis raciborskii mainly in response to phosphorus inputs.     

Which offers more scope to suppress river phytoplankton blooms: Reducing nutrient pollution or riparian shading?

River flow and quality data, including chlorophyll-a as a surrogate for river phytoplankton biomass, were collated for the River Ouse catchment in NE England, which according to established criteria is a largely unpolluted network. Against these data, a daily river quality model (QUESTOR) was setup and successfully tested. Following a review, a river quality classification scheme based on phytoplankton biomass was proposed. Based on climate change predictions the model indicated that a shift from present day oligotrophic/mesotrophic conditions to a mesotrophic/eutrophic system could occur by 2080. Management options were evaluated to mitigate against this predicted decline in quality. Reducing nutrient pollution was found to be less effective at suppressing phytoplankton growth than the less costly option of establishing riparian shading. In the Swale tributary, ongoing efforts to reduce phosphorus loads in sewage treatment works will only reduce peak (95th percentile) phytoplankton by 11%, whereas a reduction of 44% is possible if riparian tree cover is also implemented. Likewise, in the Ure, whilst reducing nitrate loads by curtailing agriculture in the headwaters may bring about a 10% reduction, riparian shading would instead reduce levels by 47%. Such modelling studies are somewhat limited by insufficient field data but offer a potentially very valuable tool to assess the most cost-effective methods of tackling effects of eutrophication.     

How green is my river? A new paradigm of eutrophication in rivers

Although the process of eutrophication is reasonably well understood in lakes, there is currently no conceptual understanding of how eutrophication develops in rivers. This issue is addressed here. A review of the main processes controlling the development of eutrophication in lakes has been carried out as a precursor to considering the effect in rivers. The importance of hydraulic flushing in controlling algal growth suggests that short-retention-time rivers will show different effects compared to long retention-time, impounded rivers. The latter are likely to operate like lakes, moving from macrophyte domination to phytoplankton domination whereas the former move to benthic and filamentous algal domination. Subsequently, a conceptual model of the development of eutrophic conditions in short-retention-time rivers is developed. Although there is general agreement in the literature that an increase in nutrients, particularly phosphorus, is a pre-requisite for the eutrophic conditions to develop, there is little evidence in short-retention-time rivers that the plant (macro and micro) biomass is limited by nutrients and a good case can be made that the interaction of hydraulic drag with light limitation is the main controlling factor. The light limitation is brought about by the development of epiphytic algal films on the macrophyte leaves. The implications of this conceptual model are discussed and a series of observable effects are predicted, which should result if the model is correct.     

Factors influencing lake recovery from eutrophication--the case of basin 1 of Lake Balaton

Lake Balaton is a large, shallow, and calcareous lake that was subject to a rapid eutrophication during the 1970s. Management measures taken from the mid-1980s decreased the phosphorus load to the lake from 0.5 to 0.3 g P m-2 yr-1. Using long-term load and water quality data, we analyse the response of the formerly hypertrophic Basin 1 of the lake by the means of simple empirical models. Several factors that are commonly neglected during studies of lake recovery modified the apparent settling velocity of total P and consequently, the biomass of the phytoplankton. These factors included the loads of calcium and suspended solids, the loading ratio of the dissolved to particulate phosphorus, and blooming of the dominant cyanobacterium, Cylindrospermopsis raciborskii. Due to the rapid immobilisation of the mobile phosphorus in the surface sediments, moderate reduction (45-50%) in the external load resulted in a surprisingly fast and significant improvement of the water quality in the hypertrophic southwestern basins of the lake.     

Is the future blue-green? A review of the current model predictions of how climate change could affect pelagic freshwater cyanobacteria

There is increasing evidence that recent changes in climate have had an effect on lake phytoplankton communities and it has been suggested that it is likely that Cyanobacteria will increase in relative abundance under the predicted future climate. However, testing such a qualitative prediction is challenging and usually requires some form of numerical computer model. Therefore, the lake modelling literature was reviewed for studies that examined the impact of climate change upon Cyanobacteria. These studies, taken collectively, generally show an increase in relative Cyanobacteria abundance with increasing water temperature, decreased flushing rate and increased nutrient loads. Furthermore, they suggest that whilst the direct effects of climate change on the lakes can change the timing of bloom events and Cyanobacteria abundance, the amount of phytoplankton biomass produced over a year is not enhanced directly by these changes. Also, warmer waters in the spring increased nutrient consumption by the phytoplankton community which in some lakes caused nitrogen limitation later in the year to the advantage of some nitrogen-fixing Cyanobacteria. Finally, it is also possible that an increase in Cyanobacteria dominance of the phytoplankton biomass will lead to poorer energy flow to higher trophic levels due to their relatively poor edibility for zooplankton.     

南四湖水体富营养化时空比较分析与评价

南四湖是南水北调东线工程重要的输水通道和调蓄湖泊之一,随着通水时间的临近,其水质安全和水体富营养化状况备受关注．根据2006、2007、2010和2011年4次32个采样点的南四湖水质空间分布监测数据,采用综合营养状态指数法进行评价．结果表明,4个监测年枯水期南四湖水体总体平均处于富营养轻度水平,在南四湖水质得到大幅度改善的情况下,各年营养水平变化不大;南阳湖和独山湖水体的综合营养状态指数出现下降趋势,而昭阳湖（下）和微山湖岛北水体的综合营养状态指数出现上升趋势,这与全湖总磷出现均一化趋势有关．2010-2011年除微山湖岛南水体处于中营养水平外,其他湖区水体均处于富营养轻度水平．对南四湖水质空间分布监测数据的分析表明,南四湖藻类水华的繁殖生长主要受水体中TP的制约．     

A preliminary modeling analysis of water quality in Lake Okeechobee, Florida: Calibration results

To gain understanding of nutrient and phytoplankton dynamics in Lake Okeechobee, Florida, we developed and applied a deterministic, mass balance, water quality model at the whole-lake spatial scale. The model was calibrated to a comprehensive set of field data for 1985–1986, and then used to simulate the period 1973–1992. The model represented the mean behavior of in-lake total phosphorus, dissolved available phosphorus, total nitrogen and chlorophyll a concentrations reasonably well during the calibration period. The model did not represent dissolved available nitrogen concentrations very well, nor did it capture much of the observed temporal variability during the calibration period. The model results identified important information needs to improve our understanding of the nitrogen cycle including, sediment-water nitrogen fluxes, denitrification and nitrogen fixation. Results from the 1973–1992 simulation indicated that model assumptions and/or calibration parameters were not uniformly applicable over this period. Total phosphorus concentration results from this model were compared with results from two site-specific, empirical loading models for the lake. None of these models represented annual average concentrations uniformly well over the entire 20-year period, and none captured much of the observed inter-annual variability. External total phosphorus loadings and lake hydrology are not sufficient to fully describe total phosphorus dynamics in Lake Okeechobee. Other important factors are diffusive sediment-water fluxes, wind-induced sediment resuspension, and the spatial heterogeneity in the lake.     

Effect of nitrate concentration in lake water on phosphate release from the sediment

Thirty-one Danish eutrophic lakes were investigated routinely over 1 year during the period 1978–1980. Nine lakes were dimictic with anoxic hypolimnia and 22 were very productive and shallow, polymictic lakes. Phosphate release from the sediment resulted in large increases in phosphate concentrations in anoxic hypolimnion, if concentrations of oxidized nitrogen in hypolimnion were less than about 0.1 g N m⁻³. If concentration of oxidized nitrogen (mainly nitrate) in hypolimnion was about 1 g N m⁻³ or higher, no release of phosphate from the sediment to anoxic hypolimnion occurred. In lakes with no summer stratification a release of phosphate from the sediment to the well oxygenated water resulted in summer maxima of phosphate in the lake water, when nitrate concentration in the water was less than about 0.5 g N m⁻³, but no release took place if nitrate concentration exceeded about 0.5 g N m⁻³. This effect of oxidized nitrogen in preventing phosphate release from the sediment demonstrates the ability of oxidized nitrogen to buffer the redox potential of the surface sediment at a level high enough to prevent a release of phosphate. Thus, among the efforts to limited phytoplankton biomass in lakes an artificial enrichment with nitrate may in some cases be an important supplement to the usually necessary reduction in phosphorus loading, but the possibility of a stimulation of phytoplankton growth through the addition of nitrate must be carefully considered.     

不同水源补给对景观水体水质及浮游动物的影响

通过定期监测由再生水补给的桃花潭和由地表水补给的兴庆湖两个景观水体中水质和浮游动物的历时变化,探究不同水源补给对水体水质和浮游动物的影响及其相关性。结果表明,桃花潭水体中TN、TP、TCOD和Chl-a浓度年均值分别为4.65、0.22、29.24 mg/L和92.81μg/L,分别为兴庆湖对应值的1.95、2.29、1.49和2.30倍,分析可知再生水补给水体TP浓度增加引起浮游藻类增加,最终导致水体TCOD浓度上升。两水体检测到轮形动物门和节肢动物门共27种,其中轮虫类21种、枝角类和桡足类各3种;桃花潭浮游动物年均生物量为0.70 mg/L,且该水体中浮游动物种数、富营养型指示种种数及生物量均大于兴庆湖的对应值。冗余分析(RDA)和Pearson相关性分析结果表明,两水体中轮虫、枝角类的生物量与水质中的磷酸盐和TP相关性强,藻类的暴发性增长对枝角类的贡献比对轮虫的贡献更大,而桡足类受影响较小。总之,TP是影响再生水补给的水体中浮游动物生物量的主导因子。     

Comparisons of trophic state measurements

Twenty-nine methods commonly used to measure trophic state were compared with respect to their ability to rank 44 test lakes against two standards: total phosphorus, a nutrient or driving force standard and chlorophyll a, an eutrophication response standard. The phosphorus loading models had high rank correlations with phosphorus; however, they did not rank the lakes well against chlorophyll. The relative trophic ranking of many lakes differed depending upon whether primary nutrients or biological manifestations were used as the ranking mechanism. Secchi disk was very effective in ranking the lakes against phosphorus but was relatively ineffective in ranking them against chlorophyll. Total Kjeldahl nitrogen was effective in ranking the lakes against phosphorus and had the highest rank correlation of all the methods tested against chlorophyll. Methods based upon phytoplankton community structure were especially ineffective in ranking the test lakes. Algal assay control yield gave disappointing results when compared against the chlorophyll standard. Most methods were much more effective in ranking the lakes against the total phosphorus standard than the chlorophyll a standard.     

Harmful effects of un-ionised ammonia on the zooplankton community in a deep waste treatment pond

The harmful effects of NH(3) on the zooplankton community in a deep waste treatment pond were evaluated under natural conditions. The pond, supplied with secondary effluent from a conventional urban wastewater treatment plant, was designed to improve water quality for agricultural reuse.The aim of the study was to verify the hypothesis suggested in Arauzo et al. (Water Environ. Res. 34(14), 3666) that during phytoplankton blooms in the stratification periods high un-ionised ammonia content values, due to an intense photosynthetic activity and high related pH, lead to a decrease in zooplankton biomass and, thus, to a collapse of the treatment process efficiency.Empirical models were developed to determine relationships between phytoplankton biomass, pH and NH(3) levels. They provided an easy and quick method of detecting when the system was liable to collapse due to the NH(3) effect on the zooplankton community and offered the possibility of adopting measures to guarantee water quality at the effluent. A significant decrease in zooplankton community biomass was observed at un-ionised ammonia levels over 2.5 mgL(-1).     

Seasonal periodicity of plankton and benthic fauna community structure and diversity in a small North African reservoir

Aquatic biota in terms of phytoplankton, zooplankton and benthic fauna were qualitatively and quantitatively followed seasonally during 2007 in a North African small reservoir, Aswan Reservoir. Simultaneously, seasonal variations in water quality were investigated in order to determine how far the aquatic biota could be altered by physicochemical and biotic factors. In phytoplankton of the investigated reservoir, 75 species belonging to four systematic groups were recorded, comprising 11 Cyanoprokaryota, 24 Bacillariophyceae, 2 Dinophyceae and 38 Chlorophyceae. Most numerous zooplankton were Rotifera (23 species). Cladocera and Copepoda were less numerous, with only seven and three species, respectively. Early development stages of copepods, nauplii and copepodites were sometimes recorded in considerable numbers. Twenty‐two invertebrate species related to five animal groups were exclusively benthic fauna: three Oligochaeta, nine Mollusca, seven larvae of Chironomidae, one Platyhelminthes, one Hirudinea and one Decapoda. Generally, the estimated values of phytoplankton species richness were relatively higher than those of zooplankton or benthic fauna. In particular, phytoplankton was influenced by physicochemical factors – water transparency, salinity and soluble reactive silica contents – as well as biotic factors – zooplankton and benthic fauna. Levels of pH value, Mg⁺², NO₃‐N and chlorophyll‐a contents appeared to be the most influential parameters dictating the pattern of seasonal periodicity of benthic fauna. Numerically, the phytoplankton was somewhat more abundant in winter and autumn than in spring or summer. The peak of zooplankton population densities was observed in spring season. Benthic fauna tended to be abundant in summer and autumn seasons, showing a reverse seasonal trend from that of zooplankton. Cyanoprokaryota and Bacillariophyceae were the most important groups that determined the seasonal fluctuation pattern of phytoplankton. Rotifera and Copepoda were the most predominant groups influencing the seasonal periodicity of zooplankton. Oligochaeta and Mollusca alternated the dominancy of benthic fauna community in Aswan Reservoir.     

沉水植物苦草腐解对水体水质的影响

沉水植物可以有效控制湖泊富营养化,但不同水环境下其周期性衰亡对湖泊生态系统的影响不容忽视。以苦草为研究对象,采用分解网袋法进行为期66 d的室内模拟试验,研究高纯水和底泥悬浮液环境下不同生物量密度的苦草腐解过程及水体氮、磷等营养盐的浓度变化。结果表明:①不同生物量密度(0.1、0.2、0.4 g/L)的苦草腐解过程均可以分为快速淋溶和缓慢分解两个阶段,不同阶段内组间质量损失率存在差异,但总体趋势一致。②高纯水环境下,水体营养盐含量(COD、TP)与生物量呈正相关(相关系数r分别为0.478、0.697,P<0.05),0.4 g/L的苦草会明显提高水中各营养盐含量,破坏水体水质;苦草腐解时,磷的矿化速率比氮快。③底泥悬浮液环境下,苦草残体腐解对水体水质的影响有明显的时效性和阶段差异性,TN浓度与水体中DO浓度、pH值显著相关(r分别为0.839、0.806,P<0.05),苦草腐解释放的氮、磷等营养盐在底泥-水界面发生复杂的吸附和降解反应,适量的苦草残体腐解可促进各形态氮、磷的迁移转化。     

Spring bloom in a hypereutrophic lake, lake Kasumigaura, Japan—III Phytoplankton abundance in standing stock of organic matter in lake water

The rapid increase of phytoplankton biomass was the major process for the increase of particulate organic matter in Lake Kasumigaura in 1980. During the first phytoplankton pulse in the bloom relative abundance of each component of organic matter in the lake water was: dissolved organic matter 100, phytoplankton 28, bacteria 27, and detrital particle 27. This relative abundance was maintained as a steady-state oscillation throughout the spring bloom similar to that usually observed in moderately eutrophic waters. Thus phytoplankton abundance in the standing stock of organic matter of the hypereutrophic water during the spring bloom does not show any marked difference from that of moderately eutrophic waters.     

Zooplankton assemblages and biomass during a 4-period survey in a northern Mediterranean coastal lagoon

The authors proposed to examine zooplankton biomass at three stations inside (T and Z) and outside (M) a coastal lagoon of the north-western Mediterranean Sea. Station T represented the lagoon central area, and station Z was positioned in a shellfish farming sector, while the seaside station (M) served as a reference of marine environment. Analyses were designed to outline the net zooplankton assemblages (taxonomic structures and length distributions) in different environmental conditions, including the farming activity. A discriminant analysis of environmental variables determined that temperature, salinity and phytoplankton implied mainly in spatial pattern of the samples. An ordination of taxa biomasses showed two main factors which might contribute to the organisation of the zooplankton assemblages: the geographical position and the thermal period. The geographical position integrated the lagoon-sea water exchange under forcing parameters (habitat, tides and winds). The thermal period reflected both the populations development cycles and the environmental constraints (temperature, salinity, trophic resources). The resulting effects appeared in structured zooplankton assemblages in space and time. The number of 50 microns interval length classes and of taxa decreased from the seaside and the lagoon central area free of farming activity to the shallower farming zone. But the biomass-length distribution profiles did not closely follow such an expected opposition between opened and confined areas: more extended profiles were observed at station Z. Biomass dominant size classes concerned the range up to 300 microns. This size category appeared to collapse in terms of biomass from the seaside or central area of the lagoon towards the farming area, similarly to zooplankton global biomass fluctuations. Difference between biomass levels and between biomass structures suggested that net zooplankton partly acted as food competitors of macro-filtering organisms, and as preys for farming shellfish and associated epifauna. This impact mainly concerned microzooplankton populations.