Range Of Nutrient Concentrations Research Articles

Submerged Rootless Macrophytes Sustain a Stable State Against Free-Floating Plants

Both non-rooted submerged vegetation dominated by coontail (Ceratophyllum demersum) and non-rooted floating duckweed vegetation (Lemna gibba) can maintain their stable dominance in small ponds and channels. We examined the competitive interactions between them and how Ceratophyllum can sustain its stable state against floating plants in a range of nutrient concentrations. Coontail and duckweed were co-cultured in static and semi-static microcosm experiments, and their impact on the nutrients (N, P, Fe, Mn) in the water column was analysed. Coontail strongly reduced the growth of duckweed under a low nitrogen level (0.2–2 mg N L−1). This reduction seems to be due to the low availability of nutrients in the water as derived from the lower nutrient concentrations in duckweed tissue or high pH in water. High nitrogen levels in semi-static media (5–10 mg N L−1) resulted in an increasingly higher chance to overgrow C. demersum by L. gibba. Field observations revealed that C. demersum dominated over L. gibba in water bodies with total N below 3 mg L−1, while L. gibba dominance over C. demersum occurred above 5 mg L−1 total N. Ceratophyllum occurrence correlated negatively with total N in the water, while Lemna showed a positive correlation. Furthermore, the occurrence of L. gibba was negatively correlated with the frequency of C. demersum. All findings together support the theory that under a certain nutrient range, rootless submerged macrophytes have a strong potential to inhibit the dominance of floating plants in ponds, ditches and channels, and thus, they stabilize the submerged vegetation state.

Competition for nitrogen between the seaweed Caulerpa prolifera and the seagrass Cymodocea nodosa

The rhizophytic seaweed Caulerpa prolifera has been expanding rapidly in the Ria Formosa lagoon, southern Portugal, taking over deeper unvegetated areas and mixing with the native seagrass Cymodocea nodosa in shallower areas. In the Ria Formosa lagoon, belowground ammonium uptake from the sediment represents the main source of nitrogen for the 2 macrophytes, except during the ammonium pulses from the sediment to the water column that are incorporated through aboveground plant parts. We examined the competition for inorganic and organic nitrogen between C. prolifera and C. nodosa through a series of 15N-ammonium and 15N-amino acid surge uptake experiments combining single-species and mixed incubations at a range of nutrient concentrations. Our results showed that C. prolifera is generally faster than C. nodosa in the acquisition of ammonium and amino acids by both above- and belowground parts, and that the uptake rates of ammonium and amino acids of one species were not affected by the presence of the other species. The exception was the amino acid uptake through the rhizoids of C. prolifera, which was slightly enhanced in the presence of C. nodosa. In this situation, the aboveground ammonium uptake becomes the main contributor to the nitrogen budget of C. nodosa but not to that of C. prolifera. When ammonium pulses are considered, C. nodosa is more competitive for nitrogen than C. prolifera. In this case, the leaf uptake of ammonium is the largest contributor to the total nitrogen (ammonium plus amino acids) budget of the seagrass. Our results showed that the different nutritional strategies of the 2 macrophytes allow their coexistence in the Ria Formosa lagoon.

Assessment of multi-stressors on compositional turnover of diatom, invertebrate and fish assemblages along an urban gradient in Pacific Northwest streams (USA)

• Gradient forest (GF) models evaluate taxon specific responses to stressors. • GF diatom, invertebrate, fish models showed several consistent variables selected. • Variables included substrate size, dissolved oxygen, and various contaminant measures. • Nitrogen and phosphorus important in both diatom and invertebrate models but not fish. • Taxa responded to multiple stressors following patterns of sensitive versus tolerance. This study is part of the regional stream-quality assessment (RSQA) conducted by the U.S. Geological Survey (USGS) National Water Quality Assessment (NAWQA) project. The purpose of this study is to examine small streams along land-use and stressor gradients at the regional scale and to evaluate the relative importance of instream stressors on diatom, macroinvertebrate, and fish assemblages. In 2015, the RSQA project assessed stream quality in 82 wadeable streams that were selected along an urban land-use gradient in the Pacific Northwest Region (PNW) of the United States. This study evaluates the effects of four major categories of measured instream stressors – flow (i.e. alteration), water quality, habitat, and contaminants (in water and sediment) – on stream biota. We used gradient forest (GF) models to evaluate taxon specific responses to the various stressors for the three biotic assemblages. Results for diatom, invertebrate and fish assemblages showed that several environmental variables including substrate size, dissolved oxygen, and two or more different contaminants were selected in each of the GF models. In general, all three assemblages were negatively associated with any contaminant measures above zero, except the more tolerant taxa in each assemblage, which responded positively to contaminants. Total nitrogen (TN) and total phosphorus (TP) were important in both the diatom and invertebrate GF models but not in the fish models, which were related to temperature and stream flow. TP and TN were the top two variables for diatom GF models and various taxa responded at a range of nutrient concentrations; however, some taxa responded at low concentrations, for example around 0.02 for TP and 0.5 mg/L for TN. In general, the three biotic assemblages responded to multiple stressors following general patterns of known sensitive versus tolerant taxa for each of the biotic groups studied, yet the GF models allow us to explore taxon specific responses. For example, most of the sensitive Ephemeroptera, Plecoptera, Trichoptera invertebrate taxa (EPT) responded negatively when any contaminant increased above zero; yet some taxa such as the tolerant Trichoptera Cheumatopsyche responded positively to contaminants and many of the other stressors. The findings of this study demonstrate the value of using multiple assemblages to monitoring stressor gradients associated with urban stream systems and the importance of evaluating the responses of individual taxa to stressors.

Decoupling of Nuclear Division Cycles and Cell Size during the Coenocytic Growth of the Ichthyosporean Sphaeroforma arctica

SummaryCoordination of the cell division cycle with the growth of the cell is critical to achieve cell size homeostasis [1]. Mechanisms coupling the cell division cycle with cell growth have been described across diverse eukaryotic taxa [2, 3, 4], but little is known about how these processes are coordinated in organisms that undergo more complex life cycles, such as coenocytic growth. Coenocytes (multinucleate cells formed by sequential nuclear divisions without cytokinesis) are commonly found across the eukaryotic kingdom, including in animal and plant tissues and several lineages of unicellular eukaryotes [5]. Among the organisms that form coenocytes are ichthyosporeans, a lineage of unicellular holozoans that are of significant interest due to their phylogenetic placement as one of the closest relatives of animals [6]. Here, we characterize the coenocytic cell division cycle in the ichthyosporean Sphaeroforma arctica. We observe that, in laboratory conditions, S. arctica cells undergo a uniform and easily synchronizable coenocytic cell cycle, reaching up to 128 nuclei per cell before cellularization and release of daughter cells. Cycles of nuclear division occur synchronously within the coenocyte and in regular time intervals (11–12 hr). We find that the growth of cell volume is dependent on concentration of nutrients in the media; in contrast, the rate of nuclear division cycles is constant over a range of nutrient concentrations. Together, the results suggest that nuclear division cycles in the coenocytic growth of S. arctica are driven by a timer, which ensures periodic and synchronous nuclear cycles independent of the cell size and growth.

SRC willow as a bioremediation medium for a dairy farm effluent with high pollution potential

Dairy farm dirty water (FDW), consisting of parlour washings contaminated with cattle faecal matter, urine and agri-nutrients was used to irrigate a 5 ha short rotation willow coppice planted with six genotypes of commercially available, high yielding energy crop willows in an experiment conducted over 5 years. The willow genotypes selected were chosen primarily for their disease and pest resistance properties though they also differed in yield potential, dry matter, energy and nutrient content; factors critically important for growers and end users. Throughout the duration of the trial the pH, conductivity, nitrogen, phosphorus and potassium concentrations of the FDW were consistent and without significant differences, although biochemical oxygen demand (BOD) levels did differ significantly between years (2753 ± 259 mg L−1). Soil, ground and drainage waters were generally not seen to be adversely affected by application of the FDW, with nutrients found to remain at pre-irrigation concentrations or similar to waters from non-irrigated catchments. Soil phosphorus content remained relatively stable at the lower treatment rates but did rise significantly at the highest irrigation rate. Biomass dry matter yields, harvested after a 3-year growth rotation, ranged from 26.7 to 42.9 Mg ha−1 3yr −1 and showed significant differences (P < 0.05) in yields between genotypes but not treatments. Three year old harvested stem tissues and leaves had significant differences in nutrient concentrations that were related more to genotype than treatment. Off-takes of nitrogen, phosphorus and potassium (average 0.22 ± 0.04, 0.04 ± 0.01 and 0.09 ± 0.01 Mg ha−1 3y−1 respectively) found in harvested biomass showed significant differences between genotypes but not treatments. The results indicated that this particular type of farm effluent, with a particular range of nutrient concentrations, was applied at specific rates to SRC willow without detriment to soil, ground and drain waters, site soils and biomass yields. Genotype was the major influence on crop yield and nutrient off-take.

Colimitation and the coupling of N and P uptake kinetics in oligotrophic mountain streams

Ecosystem element cycles can be tightly linked by both abiotic and biotic processes. Evidence for multi-element limitation (i.e., colimitation) of a variety of ecosystem processes is growing rapidly, yet our ability to quantify patterns of coupled nutrient dynamics at the ecosystem level has been hindered by logistical and methodological constraints. Here we quantify coupled nitrogen and phosphorus uptake kinetics in three oligotrophic mountain streams by using novel experimental techniques that quantify colimitation dynamics across a range of nutrient concentrations and stoichiometries. We show that relative demand for NO3-N and PO4-P varied across streams, but that short term availability of one nutrient consistently resulted in elevated, but variable, uptake of the other nutrient at all sites. We used temporally offset, pulsed nutrient additions to parameterize dual-nutrient Michaelis–Menten uptake surface models that represent NO3-N and PO4-P uptake at any given concentration or dissolved NO3-N:PO4-P stoichiometry. Our results indicated that the uptake of N and P were strongly enhanced in the presence of the other nutrient. Surface models quantitatively reflect patterns of colimitation and multi-element demand in streams, and should allow for parameterization of more realistic stream network models that explicitly account for interactions among element cycles.

Validation of a Macroinvertebrate-Based Index of Nutrient Status in Streams Using Macroinvertebrate, Water-Chemistry, and Diatom Data

Abstract Determination of the causes of water-impairment is a critical part of bioassessment, and it is useful to be able to infer causes from the same sampling data used to assess the impairment. Determination of excess nutrient inputs to a waterbody as a cause of impairment is especially important because of the severity and ubiquity of nutrient-related water-quality problems nationwide. To that end, we tested and validated in New Jersey waters a macroinvertebrate-based nutrient biotic index (NBI) for phosphorus and nitrogen developed by the New York State Department of Environmental Conservation (NYSDEC). We used macroinvertebrate, water-chemistry, and diatom data from New Jersey streams collected in the state biomonitoring program and a study of diatom—nutrient relationships. We calculated tolerance values for widespread taxa based on frequency of occurrence in samples from sites with a range of nutrient concentrations. The NBI of a sample was calculated as a sum of the tolerance values of taxa in a s...

Multiple Metazoan Life-span Interventions Exhibit a Sex-specific Strehler-Mildvan Inverse Relationship Between Initial Mortality Rate and Age-dependent Mortality Rate Acceleration.

The Gompertz equation describes survival in terms of initial mortality rate (parameter a), indicative of health, and age-dependent acceleration in mortality rate (parameter b), indicative of aging. Gompertz parameters were analyzed for several published studies. In Drosophila females, mating increases egg production and decreases median life span, consistent with a trade-off between reproduction and longevity. Mating increased parameter a, causing decreased median life span, whereas time parameter b was decreased. The inverse correlation between parameters indicates the Strehler-Mildvan (S-M) relationship, where loss of low-vitality individuals yields a cohort with slower age-dependent mortality acceleration. The steroid hormone antagonist mifepristone/RU486 reversed these effects. Mating and mifepristone showed robust S-M relationships across genotypes, and dietary restriction showed robust S-M relationship across diets. Because nutrient optima differed between females and males, the same manipulation caused opposite effects on mortality rates in females versus males across a range of nutrient concentrations. Similarly, p53 mutation in Drosophila and mTOR mutation in mice caused increased median life span associated with opposite direction changes in mortality rate parameters in females versus males. The data demonstrate that dietary and genetic interventions have sex-specific and sometimes sexually opposite effects on mortality rates consistent with sexual antagonistic pleiotropy.

Predictable communities of soil bacteria in relation to nutrient concentration and successional stage in a laboratory culture experiment.

It is difficult to understand the processes that structure immensely complex bacterial communities in the soil environment, necessitating a simplifying experimental approach. Here, we set up a microcosm culturing experiment with soil bacteria, at a range of nutrient concentrations, and compared these over time to understand the relationship between soil bacterial community structure and time/nutrient concentration. DNA from each replicate was analysed using HiSeq2000 Illumina sequencing of the 16S rRNA gene. We found that each nutrient treatment, and each time point during the experiment, produces characteristic bacterial communities that occur predictably between replicates. It is clear that within the context of this experiment, many soil bacteria have distinct niches from one another, in terms of both nutrient concentration, and successional time point since a resource first became available. This fine niche differentiation may in part help to explain the coexistence of a diversity of bacteria in soils. In this experiment, we show that the unimodal relationship between nutrient concentration/time and species diversity often reported in communities of larger organisms is also evident in microbial communities.

섬진강 하구를 통한 용존무기영양염 유출량 변동

We investigated the spatiotemporal variations of dissolved inorganic nutrients along a saline gradient to estimate nutrient fluxes in the Seomjin River estuary during dry (March 2005, March 2006, March 2007, and March 2008) and rainy seasons (August 2005, July 2006, July 2007, and July 2008). Dissolved inorganic nitrogen concentrations were similar in the endmembers of freshwater for the rainy and dry seasons. In contrast, the concentrations of dissolved inorganic phosphate and silicate in the rainy season were approximately 2-3 times higher than those in the dry season. River discharge was approximately 10 times higher in the rainy season (<TEX>$212m^3sec^{-1}$</TEX>) than in the dry season (<TEX>$21m^3sec^{-1}$</TEX>). The fluxes of dissolved inorganic nitrogen, phosphate, and silicate were 2.91, 0.004, and 2.51 tons <TEX>$day^{-1}$</TEX> in the dry season and 7.45, 0.421, and 30.5 tons <TEX>$day^{-1}$</TEX> in the rainy season, respectively. Although the range of nutrient concentrations were similar to previous results from investigations in the Seomjin River estuary, the nutrient fluxes were differed according to river discharge for different survey periods.

Effectiveness of Biochar for Sorption of Ammonium and Phosphate from Dairy Effluent

The use of biochar for recovery of excess nutrients in dairy manure effluent and the use of nutrient-enriched biochar as soil amendment can offer a robust solution for multiple environmental issues. In this study we determined the capacity of biochar, produced by pyrolyzing mixed hardwood feedstock at 300°C, to adsorb and retain or release two major nutrient ions: ammonium (NH) and phosphate (PO). We conducted the experiment using a range of nutrient concentrations that represent those commonly observed in dairy manure effluent (0-50 mg L for PO and 0-1000 mg L for NH). Up to 5.3 mg g NH and 0.24 mg g PO was adsorbed from manure by biochar (18 and 50% of total amount in the manure slurry, respectively). During the desorption phase of the experiment, biochar retained 78 to 91% of the sorbed NH and 60% of the sorbed PO at reaction times <24 h. Our findings confirm that biochar can be used for recovering excess nitrogen and phosphorus from agricultural water, such as dairy manure effluent.

Comparison of nitrogen and phosphorus removal efficiencies by storm runoffs for the ponds in the upper and lower reaches of a typical sub-catchment in Lake Chaohu drainage basin

从流域上下游环境条件及氮磷输出强度差异出发,探讨上下游水塘对径流氮磷去除的特征及效率,选取巢湖小柘皋河源头流域上下游水塘开展水塘去除暴雨径流氮磷的对比试验,研究暴雨及暴雨间期上下游水塘氮磷去除效率差异及原因,为流域上下游设计不同类型净化塘去除氮磷提供科学依据.结果表明:暴雨期,上游径流氮磷浓度高于下游,且颗粒态所占比例上游大于下游,流域上游应作为防治暴雨径流氮磷流失的重点区域;暴雨期,上游塘对暴雨径流中的氮磷去除效果明显,氮、磷去除率分别为74%和52%,且对颗粒态去除效果好于溶解态,下游塘没有表现出明显的去除效果;暴雨间期,上游塘塘内氮磷浓度平均下降50%和20%,下游塘则分别为72%和16%,且均以溶解态去除为主;水塘去除暴雨径流氮磷有一定的浓度适用范围,浓度过低,去除效果不明显;流域部位不同引起入塘径流氮磷浓度和形态的差异是上下游水塘对暴雨径流去除效果差异的主要外部原因.流域上游出山口,可以在渗透性好的山前洪积扇上构建深水宽塘,通过增加暴雨径流拦截量和降低流速增强物理沉降作用,实现暴雨径流氮磷的高效去除;流域下游农田区,宜构建水面较大的浅滩湿地,通过延长滞留时间和促进生物活动增强去除暴雨径流氮磷的效果.;In this paper, we analyze the removal efficiencies and characteristics of nitrogen and phosphorus in storm runoff in two ponds located in the upper and lower reaches of Xiaozhegao Stream, Lake Chaohu drainage basin. Results indicate that in the storm period, the runoff nitrogen and phosphorus levels in the upper reaches of stream are higher than those in the lower reaches. There is a good removal performance for the upper pond on both nitrogen and phosphorus (mainly in particulate), with a removal efficiency of 74% and 52%, respectively, while no evident removal performance has been found in the lower pond. During non-storm period, both ponds have good removal performances for dissolved nutrients. In the upper pond, the removal rates are 50% and 20% for nitrogen and phosphorus, respectively; in the lower pond, the rates are 72% and 16% for nitrogen and phosphorus, respectively. There is a range of nutrient concentration that ponds could play a good removal performance:if the nutrient concentration is too low, there will be no evident nutrient removal. The two ponds have different removal efficiencies on runoff nutrients due to their different locations in the catchment. The study concludes that in the upper reaches of a catchment, which is the most important area to prevent and control the loss of nutrients from storm runoff, ponds should be constructed deep and wide in the front of alluvial fan. It will enhance the physical settlement of nutrients by intercepting the runoff and lowering the flow velocity. While in the lower reaches of a catchment, it is appropriate to construct big and shallow wetlands which will remove much nitrogen and phosphorus in storm runoff by delaying nutrient retention time and promoting biological activity. The research could benefit better design of the ponds which aims to control water eutrophication.

Biomechanical and Nutrient Controls in the Growth of Mammalian Cell Populations

Growth kinetics and morphologies of growing mammalian cell populations are part of the growth dynamics of tumors. A biophysical agent-based simulation model describes a biological cell by a homogeneous elastic adhesive object able to migrate, grow and divide, and die. A comparison of simulation results with experimental data shows that the growth kinetics of growing multicellular spheroids (MCS) over a wide range of nutrient concentrations is explained by a bio-mechanical form of contact inhibition between cells. This inhibition mechanism explains the growth kinetics of the tumor diameter and the cell population size, the size of the necrotic core, the median cell volume, which decreases when the tumor diameter increases, and the spatial distribution of cell volumes in the tumor. The same model is used to predict how cell populations survive in low nutrient concentrations. Spatial patterns are different for changes of the cell phenotype by regulation or mutation. The cells appearing in the simulations decrease cell-cell adhesion, display chemotaxis movement, increase micro-motility and decrease cell cycle time. Each of these have been observed in invasive cancers.

Efficiency of top-down control depends on nutrient concentration in a Mediterranean shallow lake: A mesocosm study

In shallow lakes, the influence of top-down control is considered to be less important under a Mediterranean bioclimate, although little experimental data support this hypothesis. The present study aimed to experimentally assess the importance of top-down forces in the trophic status of Mediterranean shallow lakes. A bifactorial mesocosm experiment was carried out in a shallow lake using a range of nutrient concentrations and fish population densities. The results demonstrated an expected increase in turbidity levels as nutrients and fish density increased, as a result of both sediment resuspension and chlorophyll a concentrations. Nevertheless, at higher nutrient additions, turbidity decreased in the mesocosms in response to cladoceran proliferation because the fishes were unable to control zooplankton growth under such conditions. Fish diet content analysis showed a generalist diet with an important contribution of non-planktonic components; the proportion of zooplankton in the diet contents increased with nutrient additions. Fishes preferred cladocerans over copepods and rotifers, these latter taxa dominated the resulting zooplankton community. Some fish-added mesocosms retained high transparency levels, most likely because of physiological changes in response to high ammonium concentrations. In summary, the dietary spectrum of Chondrostoma arcasii [Steindachner], despite some electivity patterns, depends largely on available resources. The trophic ecology of cyprinids, acting both through direct and indirect effects on food webs, is a determinant factor explaining, in part, stable high turbidity levels in many shallow lakes.

Photosynthetic characteristics and diversity of freshwater Synechococcus at two depths during different mixing conditions in a deep oligotrophic lake

We studied the photosynthetic characteristics and genetic diversity of Synechococcus assemblages at two different light and mixing conditions in a deep oligotrophic lake (Lake Maggiore, Northern Italy). Sampling was performed at the beginning of summer stratification, when the base of the photic zone was isolated by the presence of a shallower thermocline, and during late summer, when the mixing layer extended below the photic zone. Two depths were sampled (15 % and 1 % of surface PAR). To study the diversity of Synechococcus, we used a classical molecular fingerprinting technique (Denaturing Gradient Gel Electrophoresis, DGGE) with ambient samples and sequencing of the prominent bands. The ecotypes selected within the samples from different depths were different in photosynthetic characteristics and grew in a range of nutrient concentrations. The OTUs colonizing different niches in the water column did not show any significant clustering. However, the OTU richness was significantly different at the two depths in spring, reaching the highest values at 15 % of surface PAR. Cluster analysis of DGGE lanes provided evidence of different community compositions between spring and late summer. Sequencing of the most prominent bands showed one spring OTU affiliated to the Synechococcus subalpine cluster I, with 100% similarity to LM94, MW76B2 and MW15#2 (sensu Crosbie et al. 2003a), not present in late summer. Another OTU present both in spring and late summer showed 100% similarity to MH301, a strain near to subalpine cluster II. Our results support the coexistence of ecotypes in the vertical gradients, rapidly acclimating and performing differently in the microhabitats.

A filamentous green algae-dominated temperate shallow lake: Variations on the theme of clear-water stable states?

Shallow lakes can have two alternative stable states over wide range of nutrient concentrations: a macrophyte-dominated clear-water state and a phytoplankton-dominated turbid-water state. Little Budworth Pool, Cheshire, UK is a small (surface area 3 ha), shallow (max. depth < 1m), clear-water lake with very high nitrate concentrations (annual mean 9.2 mg NO 3 -N 1 -1 ), relatively low phytoplankton growth (annual mean 13.5 μg chlorophyll-a 1 -1 ) and high metaphytic filamentous green algal abundance (FGA, mainly Spirogyra), but without any submerged plant-beds. To understand the mechanism limiting phytoplankton biomass in summer, the lake was sampled every two weeks for two years (November 2001-October 2003) for water chemistry, phytoplankton, zooplankton, benthic macroinvertebrates, FGA and benthic diatoms. Phytoplankton growth was mainly dominated by small species like Cyclotella spp., Chroomonas acuta and Cryptomonas spp., whereas among the zooplankton Cladocera showed high abundance of both small and large species (Bosmina longirostris and Daphnia longispina) especially in summer and autumn. FGA attained highest biomass in the warmer months and showed negative correlations with nitrate-N and total nitrogen concentrations suggesting nutrient removal ability of these plants, but also possible increased denitrification in summer. While phytoplankton growth was negatively correlated with FGA biomass, cladoceran densities were positively related to the latter. A comparison among macrophyte-dominated lakes and FGA-dominated lakes showed similarities among them regarding water clarity, nutrient concentrations, phytoplankton growth and species composition, zooplankton assemblage and abundance, but not for abundance of benthic macroinvertebrates. We suggest that in a lake lacking submerged plants, abundance of FGA may act as a buffer against a forward switch to phytoplankton dominance, thus maintaining a clear-water state. The control of phytoplankton growth was primarily not through FGA removing nutrients from the water because sufficient available nutrients were recorded in water during summer. Rather it was by intense grazing by cladocerans ostensibly supported in refuges provided by FGA.

Effect of soil-use on deep groundwater quality in Southern Chile

This study evaluated the effects of farming activities on the quality of deep well water in Southern Chile. In this area, the use of fertilisers is becoming more widespread and the accumulation and poor management of cattle slurry and silage sub-products increases the risk of contamination. The study was carried out in the province of Osorno, where rainfall occurs all year round (maximum in winter); total annual rainfall exceeds 1300 mm. The soil is of volcanic origin and this province includes areas used for intensive farming and cattle raising activities, residential and recreational purposes and extensive farming practices. Water quality was analysed over a 15 month period in deep wells in either intensive farming areas and residential/recreational zones or extensive farming land. Although levels of nitrate were higher in deep wells located in intensive farming areas, levels of phosphor and pH were lower than those of wells located in extensive farming areas. Nevertheless, the range of nutrient concentration (nitrite, iron, manganese and phosphor) revealed significant statistical differences between both types of farming land; nitrite dominated in well samples located in intensive farming areas, while iron and manganese dominated in extensive farming areas. Seasonal differences were registered in temperature, nitrate, total coliforms and Escherichia coli. Maximum levels of nitrate and bacteria were reached in spring and autumn, respectively. No significant relationships were established between nutrient and bacterial levels with meteorological factors such as temperature, rainfall and number of days when rainfall occurred. Variations in nutrients and bacteria, both temporal and between locations (spatial), are associated to soil use and proximity to septic well wastewaters. Iron and manganese concentrations exceeded permitted drinking-water standards in a considerable percentage of samples, which may be due to soil composition. Bacterial pollution occurred infrequently in autumn and winter. The quality of deep-well waters in Southern Chile is generally good. Nevertheless, evidence indicates that intensive farming practices, slurry, silage and poor management of rural residential areas, constitute a significant potential risk.

Clear water associated with biomass and nutrient variation during the growth of a Charophyte stand after a drawdown, in a tropical coastal lagoon

The community of Charophytes in the Imboassica coastal lagoon in Brazil (22° 24′ S and 42° 42′ W) sometimes occupies almost the entire benthic region, and presents a large variation in C:N:P ratio. The effect of drawdown on the regeneration and buildup of biomass and on the nutrient concentration of these macroalgae was studied at three different sampling sites. Drawdown results in a high level of mortality in the macroalgae stands and after the water level later rises, the process of oospore germination begins. The drawdowns occurred in November 96 and January 97, and after March 97 we took samples in order to determine biomass values and the concentration of carbon, nitrogen and phosphorus. The results indicate that the fast growth of Charophytes may absorb a great amount of the nutrients entering the lagoon. The biomass reached maximum values of between 400 and 600 g DW m−2, and the C:N:P ratio varied from 51:7:1 to 1603:87:1, indicating that this macroalgae may grow in a wide range of nutrient concentration. The presence of this community in the Imboassica lagoon may act as one of the limiting factors controlling phytoplanktonic primary production, decreasing nutrient availability in the water column (`bottom-up' control) and keeping the water clear after drawdowns. Probably through the habitat structure produced by the great biomass reached, they provide substrate and shelter for the structuring of a community with grazing zooplankton, which acts as a `top-down' controlling mechanism on the phytoplankton.

Alternative stable states in eutrophic, shallow freshwater systems: A minimal model

A simple mathematical model was constructed, describing the relationships between pike, bream, aquatic macrophytes and the nutrient loading of shallow lakes. The model is analyzed with the use of zero-isoclines. It is concluded that, over a certain range of nutrient concentrations, the ecological relations incorporated in the model can give rise to the existence of two alternative stable equilibria;viz. a turbid bream-dominated one, and a clear state in which pike and aquatic vegetation are abundant. Under oligotrophic conditions, the clear-water state represents the only stable equilibrium; however, at high nutrient levels, the clear state is absent, or only locally stable. The response of the model to both increase and decrease of the nutrient level is characterised by hysteresis. The results indicate that manipulation of fish densities as a measure to improve water quality is only likely to produce long-term results when the nutrient level is below a certain threshold.

Nutrient Relations of Groundsel (Senecio vulgaris) Infected by Rust (Puccinia lagenophorae) at a Range of Nutrient Concentrations II. Uptake of N, P and K and Shoot-Root Interactions

Groundsel (Senecio vulgaris), healthy or infected with rust, Puccinia lagenophorae, was grown at a range of nutrient concentrations in sand culture. Specific absorption rates calculated on the basis of root dry weight (SARW) were greater in rusted than control groundsel for nitrogen, potassium and phosphorus. While the magnitudes of these stimulations varied, they occurred across the whole range of nutrient concentrations. By contrast, specific absorption rate on the basis of root length (SARL) were little changed by rust at any external nutrient concentration; SARL for phosphate and potassium were slightly reduced when nutrients were freely available. Water flux per unit dry root weight and length was stimulated by rust because transpiration per unit leaf area was more rapid in infected plants after fungal sporulation. However, water-flux and the rate of uptake of nutrients were correlated only when expressed on the basis of root weight and increased transpiration did not appear to be the mechanism underlying increased root activity. Rather, increased SARW for N, P and K could very largely be attributed to increased shoot demand per unit root, which resulted from the higher shoot: root (S: R) ratios of infected individuals. Changes in S: R accounted for 92, 81 and 57% of total variation in SARW for K, P and N respectively. Greater values for SARW were possible because specific root length (SRL) increased, producing more functional root per unit root weight. The lack of stimulation in SARL in response to rust could be explained since the higher SRL of infected plants resulted in stable values of shoot weight per unit root length, i.e. shoot demand was not increased by infection on this basis.