Changes In Nutrient Limitation Research Articles

Vegetation restoration of abandoned cropland improves soil ecosystem multifunctionality through alleviating nitrogen-limitation in the China Danxia.

The microbial requirement for nutrient resources can be estimated by soil extracellular enzyme stoichiometry (EES) and their stoichiometries. Implementing the Grain for Green Program has significantly impacted land use and soil nutrient management in the China Danxia. However, drivers of soil microbial nutrient limitation changes in abandoned cropland (AC) remained unclear after vegetation restoration. Here, according to vector analysis, we evaluated microbial nutrient limitation by studying soil EES across vegetation restoration types (naturally restored secondary forests (NF) and artificially planted forests (AF)) with AC as a control. Results showed both NF and AF soils averaged higher C- and P- acquiring enzyme, indicating rapid C and P turnover rates after vegetation restoration. However, vegetation restoration resulted in higher C requirement for microorganisms with higher enzyme C:N and vector length. In addition, microorganisms shifted from N- (< 45°) to P-limited (> 45°) conditions with enzyme N:P less than 1 after vegetation restoration, and NF exacerbated microbial P limitation compared to AF. Decreased N limitation following vegetation restoration could be contributed to improving soil ecosystem multifunctionality. The greater variation of EES was explained by the interaction of pH, soil nutrient, and microbial biomass than by any one of these factors alone, suggesting that both abiotic and biotic factors regulate microbial nutrient limitation and microbial process. Overall, our results revealed vegetation restoration could alleviate N limitation in the China Danxia, and thus enhance soil ecosystem by regulating lower microbial N limitation, which provide insight into nutrient management strategies under ecological restoration of degraded areas.

A New Ocean State After Nuclear War

AbstractNuclear war would produce dire global consequences for humans and our environment. We simulated climate impacts of US‐Russia and India‐Pakistan nuclear wars in an Earth System Model, here, we report on the ocean impacts. Like volcanic eruptions and large forest fires, firestorms from nuclear war would transport light‐blocking aerosols to the stratosphere, resulting in global cooling. The ocean responds over two timescales: a rapid cooling event and a long recovery, indicating a hysteresis response of the ocean to global cooling. Surface cooling drives sea ice expansion, enhanced meridional overturning, and intensified ocean vertical mixing that is expanded, deeper, and longer lasting. Phytoplankton production and community structure are highly modified by perturbations to light, temperature, and nutrients, resulting in initial decimation of production, especially at high latitudes. A new physical and biogeochemical ocean state results, characterized by shallower pycnoclines, thermoclines, and nutriclines, ventilated deep water masses, and thicker Arctic sea ice. Persistent changes in nutrient limitation drive a shift in phytoplankton community structure, resulting in increased diatom populations, which in turn increase iron scavenging and iron limitation, especially at high latitudes. In the largest US‐Russia scenario (150 Tg), ocean recovery is likely on the order of decades at the surface and hundreds of years at depth, while changes to Arctic sea‐ice will likely last thousands of years, effectively a “Nuclear Little Ice Age.” Marine ecosystems would be highly disrupted by both the initial perturbation and in the new ocean state, resulting in long‐term, global impacts to ecosystem services such as fisheries.

Seasonal nutrient dynamics in the P depleted Eastern Mediterranean Sea

The Eastern Mediterranean Sea (EMS), is ultra-oligotrophic with unusual anti-estuarine circulation. It is P depleted and the limited enrichment studies which have been carried out, suggest seasonal changes in nutrient limitation. In this study high sensitivity dissolved nutrients (and associated parameters) were determined monthly over an annual cycle at a pelagic location in the SE Levantine basin. Nitrate & Nitrite (Nox) concentrations were high (300–500 nM) during the winter mixing period and enabled a concurrent phytoplankton increase in which larger picoeukaryotes and eukaryotes were dominant. After the thermal stratification of the water column commenced, Nox decreased through early summer transition period to low values (generally 50 nM or less) in the late summer. DIP remained at low nM concentrations the entire year while DOP decreased from 40 nM in winter to 30 nM in summer.. Prochlorococcus, the smallest picocyanobacteria, that does not typically utilize nitrate, dominated during the summer when both Nox and DIP concentrations were lowest. Ammonium concentrations were low (10–100 nM) with no systematic changes with season or depth. As a result, the DIN:DIP ratios were high (20–825) in winter and low (2–66) in summer, showing that these ratios vary seasonally. The dynamics of nutrient availability combined with the temporal changes in total chlorophyll and the altered dominance of the predominant phytoplankton species (i.e. nitrate-metabolizing Synechococcus during winter versus the smaller Prochlorococcus abundant in summer), leads to our hypothesis that the seasonal change in DIN:DIP indicates a switch from P limitation in winter during the annual phytoplankton increase to N&P or even N limitation in summer. Export Production (172 mmol N m−2 y−1) determined from the calculated loss of Nox from the photic zone, was similar to previous estimates in the EMS. Our results in terms of seasonally changing nutrient dynamics and resulting productivity confirm that the EMS has many of the characteristics found in P starved ocean gyre systems.

Spatial and Long-Term Temporal Changes in Water Quality Dynamics of the Tonle Sap Ecosystem

Tonle Sap lake-river floodplain ecosystem (TSE) is one of the world’s most productive freshwater systems. Changes in hydrology, climate, population density, and land use influence water quality in this system. We investigated long term water quality dynamics (22 years) in space and time and identified potential changes in nutrient limitation based on nutrient ratios of inorganic nitrogen and phosphorus. Water quality was assessed at five sites highlighting the dynamics in wet and dry seasons. Predictors of water quality included watershed land use, climate, population, and water level. Most water quality parameters varied across TSE, except pH and nitrate that remained constant at all sites. In the last decade, there is a change in the chemical nutrient ratio suggesting that nitrogen may be the primary limiting nutrient across the system. Water quality was strongly affected by development in the watershed i.e., flooded forest loss, climatic variation, population growth, and change in water level. Seasonal variations of water quality constituents were driven by precipitation and hydrology, notably the Mekong’s distinct seasonal flood pulse.

Nutrient limitations for overstory and understory plants during Robinia pseudoacacia afforestation in the Loess Plateau, China

AbstractOverstory trees and understory herbs are in different niches in the community; however, how their nutrient limitations change following afforestation remains unclear. Therefore, we used plant nutrient resorption and ecological stoichiometry to illustrate the nutrient relationship between plants and soil while exploring the nutrient limitations of different plants during afforestation. Soil and plant samples were collected from farmland and three Robinia pseudoacacia L. (RP) forests that were 17, 26, and 42 yr old. The physical characteristics of the soil and the content and stoichiometric ratios of C, N, and P in the plants, litter, soil microbes, and soil were measured. Additionally, the resorption of N and P in RP and understory herbaceous biomass (UHB) was assessed. Results showed that, at all sites, N resorption (0.45–0.52) and P resorption (0.39–0.49) were approximately equal in RP, while P resorption (0.48–0.65) was significantly higher than N resorption (0.06–0.32) in UHB. With afforestation age, the N/P resorption ratios (1.06–1.25) were not significantly different in RP but declined significantly from 0.50 to 0.14 in UHB. Results suggest that RP is co‐limited by N and P while understory herbs are limited by P. Although the nutrient limitation of RP and UHB may be different, their nutrient relationship with the soil is consistent after afforestation. There was a significant positive relationship for C and N content but a negative relationship of the C/N ratios between soil and plants. These findings deepen the understanding of a linkage between the aboveground and belowground systems and provide evidence that nutrient limitation changes occur in plants of different ecological niches after afforestation in the Loess Plateau.

Observed variation in soil properties can drive large variation in modelled forest functioning and composition during tropical forest secondary succession.

Censuses of tropical forest plots reveal large variation in biomass and plant composition. This paper evaluates whether such variation can emerge solely from realistic variation in a set of commonly measured soil chemical and physical properties. Controlled simulations were performed using a mechanistic model that includes forest dynamics, microbe-mediated biogeochemistry, and competition for nitrogen and phosphorus. Observations from 18 forest inventory plots in Guanacaste, Costa Rica were used to determine realistic variation in soil properties. In simulations of secondary succession, the across-plot range in plant biomass reached 30% of the mean and was attributable primarily to nutrient limitation and secondarily to soil texture differences that affected water availability. The contributions of different plant functional types to total biomass varied widely across plots and depended on soil nutrient status. In Central America, soil-induced variation in plant biomass increased with mean annual precipitation because of changes in nutrient limitation. In Central America, large variation in plant biomass and ecosystem composition arises mechanistically from realistic variation in soil properties. The degree of biomass and compositional variation is climate sensitive. In general, model predictions can be improved through better representation of soil nutrient processes, including their spatial variation.

The impact of development on patterns of nutrient limitation.

Development is often accompanied by major changes in an organism's functioning and in the way it interacts with its environment. We consider how developmental events such as allocation changes at maturity, ontogenetic diet shift or metamorphosis may affect the likelihood and nature of nutrient limitation and explore the consequences of these changes in nutrient limitation for individual life history and patterns of biomass production.To this purpose, we develop a general model for individual growth and reproduction that is based on the assumption that biomass production and metabolism require several nutrients and that individuals may require them in different proportion at different stages of their lives.We parameterize this model for Daphnia based on its physiological requirements for carbon (C) and phosphorus (P). Growth and reproduction have different nutrient requirements, and this affects the likelihood of C vs. P limitation of differently sized individuals. This translates into a size‐dependent threshold elemental ratio (TER), with a difference of up to twofold between juveniles and adults, a difference comparable to measured interspecific differences.The main implications of these findings are that, at the population level, co‐limitation of biomass production by several nutrients is likely to occur under a wide range of food qualities. In addition, different regimes of nutrient limitation strongly influence the relative difference in biomass production of differently sized individuals, which has been shown to be a major driver of population and community dynamics. Our results point to development as a key determinant of a population's response to food quality. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13101/suppinfo is available for this article.

Simulated nitrogen deposition affects stoichiometry of multiple elements in resource-acquiring plant organs in a seasonally dry subtropical forest

Increase of anthropogenic atmospheric nitrogen (N) deposition markedly influences biogeochemical cycles of elements in a wide range of ecosystems. However, our knowledge of how N deposition affects stoichiometry of plants in forests experiencing regular seasonal droughts remains limited. Using a 3-year (2013–2015) N-manipulative experiment, we examined the stoichiometric responses of ten mineral elements, including the most limiting elements (N and P) to plant growth, base cations (K, Ca, and Mg), and trace metal cations (Mn, Zn, Cu, Al, and Fe) in resource-acquiring organs (foliage vs. absorptive roots) of Pinus elliottii to N additions in both wet and dry seasons in a seasonally dry subtropical forest. Stoichiometric responses of both organs depended on rate of N addition (generally stronger under high rate) and season. N additions increased foliar [N] and decreased foliar [P] only in dry season and the relative changes in foliar N:P ratio were twice higher in dry than wet seasons, suggesting an aggravated P limitation in dry season. The stoichiometry of absorptive roots was more responsive to N additions than that of foliage, especially for the base cations. N additions increased [Mn] and decreased Fe:Mn ratio in both organs, indicating increased risk of Mn2+ toxicity to this tree species. Our results have implications for understanding the N-induced changes in nutrient limitation of forests influenced by seasonal drought, and highlight the contrasting stoichiometric responses of above- and below-ground resource-acquiring plant organs to N loading.

Environmental variability drives rapid and dramatic changes in nutrient limitation of tropical macroalgae with different ecological strategies

Nitrogen (N) or phosphorus (P) limits primary productivity in nearly every ecosystem worldwide, yet how limitation changes over time, particularly in connection to variation in environmental drivers, remains understudied. We evaluated temporal and species-specific variability in the relative importance of N and P limitation among tropical macroalgae in two-factor experiments conducted twice after rains and twice after dry conditions to explore potential linkages to environmental drivers. We studied three common macroalgal species with varying ecological strategies: a fast-growing opportunist, Dictyota bartayresiana; and two calcifying species likely to be slower growing, Galaxaura fasciculata and Padina boryana. On the scale of days to weeks, nutrient responses ranged among and within species from no limitation to increases in growth by 20 and 40 % over controls in 3 d with N and P addition, respectively. After light rain or dry conditions, Dictyota grew rapidly (up to ~60 % in 3 d) with little indication of nutrient limitation, while Padina and Galaxaura shifted between N, P, or no limitation. All species grew slowly or lost mass after a large storm, presumably due to unfavorable conditions on the reef prior to the experiment that limited nutrient uptake. Padina and Galaxaura both became nutrient limited 3 d post-storm, while Dictyota did not. These results suggest that differing capabilities for nutrient uptake and storage dictate the influence of nutrient history and thus drive nutrient responses and, in doing so, may allow species with differing ecological strategies to coexist in a fluctuating environment. Moreover, the great variability in species’ responses indicates that patterns of nutrient limitation are more complex than previously recognized, and generalizations about N versus P limitation of a given system may not convey the inherent complexity in governing conditions and processes.

Above‐ground and below‐ground responses to long‐term nutrient addition across a retrogressive chronosequence

Summary There is much interest in understanding ecosystem responses to local‐scale soil fertility variation, which has often been studied using retrogressive chronosequences that span thousands of years and show declining fertility and plant productivity over time. There have been few attempts to experimentally test how plant nutrient limitation changes during retrogression. We studied a well‐characterized system of 30 forested lake islands in northern Sweden that collectively represent a 5350‐year post‐fire retrogressive chronosequence, with fertility and productivity decreasing as time since fire increases. For each island, we set up four plots on understorey vegetation, each subjected to a different fertilizer treatment over 6 years: no additions, nitrogen (N) only, phosphorus (P) only and N + P. We found that both N and P additions reduced feather moss and thus total plant biomass. Meanwhile, the three dominant vascular plant species showed contrasting biomass responses, but similar responses of foliar nutrient concentrations to nutrient additions. Fertilization reduced most microbial groups and altered CO2 fluxes, most likely through feather moss reduction. Against expectations, the majority of interactive effects of N and P were antagonistic. Changes in effects of nutrient additions during retrogression were usually modest. Empetrum hermaphroditum biomass was increasingly promoted by P and N + P addition, while vascular plant N‐to‐P ratios were increasingly reduced by P addition, indicating increasing plant limitation by nutrients (notably P) during retrogression. Below‐ground, positive effects of N addition on soil mineral N increased, while negative effects of N addition on soil fungi decreased during retrogression; no other below‐ground effects of fertilization changed along the gradient. Synthesis. Our results show that forest understorey communities on islands of different fire history and thus stages of retrogression show relatively modest differences in how they respond to nutrient addition despite large changes in ecosystem productivity and soil fertility, probably because of high species turnover and adaptation of communities to infertile conditions. While increased nutrient availability (as expected through global change) may have important ecological consequences, these effects are likely, especially below‐ground, to be rather similar across ecosystems that differ greatly in nutrient availability and productivity.

Seasonal changes in nutrient limitation and nitrate sources in the green macroalga Ulva lactuca at sites with and without green tides in a northeastern Pacific embayment

In Penn Cove, ulvoid green algal mats occur annually. To examine seasonal variation in their causes, nitrogen and carbon were measured in Ulva lactuca in May, July, and September and stable nitrogen and oxygen isotope ratios were quantified in U. lactuca, Penn Cove seawater, upwelled water from Saratoga Passage, water near the Skagit River outflow, and effluents from wastewater treatment facilities. Ulvoid growth was nitrogen limited and the sources of nitrogen used by the algae changed during the growing season. Algal nitrogen concentrations were 0.85–4.55% and were highest in September and at sites where algae were abundant. Upwelled waters were the primary nitrogen source for the algae, but anthropogenic sources also contributed to algal growth towards the end of the growing season. This study suggests that small nitrogen inputs can result in crossing a “tipping point”, causing the release of nutrient limitation and localized increases in algal growth.

Long‐Term Trends of Nutrients and Sediment from the Nontidal Chesapeake Watershed: An Assessment of Progress by River and Season

AbstractTo assess historical loads of nitrogen (N), phosphorus (P), and suspended sediment (SS) from the nontidal Chesapeake Bay watershed (NTCBW), we analyzed decadal seasonal trends of flow‐normalized loads at the fall‐line of nine major rivers that account for &gt;90% of NTCBW flow. Evaluations of loads by season revealed N, P, and SS load magnitudes have been highest in January‐March and lowest in July‐September, but the temporal trends have followed similar decadal‐scale patterns in all seasons, with notable exceptions. Generally, total N (TN) load has dropped since the late 1980s, but particulate nutrients and SS have risen since the mid‐1990s. The majority of these rises were from Susquehanna River and relate to diminished net trapping at the Conowingo Reservoir. Substantial rises in SS were also observed, however, in other rivers. Moreover, the summed rise in particulate P load from other rivers is of similar magnitude as from Susquehanna. Dissolved nutrient loads have dropped in the upland (Piedmont and above) rivers, but risen in two small rivers in the Coastal Plain affected by lagged groundwater input. In addition, analysis of fractional contributions revealed consistent N trends across the upland watersheds. Finally, total N:total P ratios have declined in most rivers, suggesting the potential for changes in nutrient limitation. Overall, this integrated study of historical data highlights the value of maintaining long‐term monitoring at multiple watershed locations.

Effects of nutrient enrichment on mangrove leaf litter decomposition

Nutrient enrichment of mangroves, a common phenomenon along densely populated coastlines, may negatively affect mangrove ecosystems by modifying internal carbon and nutrient cycling. The decomposition of litter exerts a strong influence on these processes and is potentially modified by eutrophication. This study describes effects of N and P enrichment on litter decomposition rate and mineralisation/immobilisation patterns.By making use of reciprocal litter transplantation experiments among fertiliser treatments, it was tested if nutrient addition primarily acts on the primary producers (i.e. changes in litter quantity and quality) or on the microbial decomposers (i.e. changes in nutrient limitation for decomposition). Measurements were done in two mangrove forests where primary production was either limited by N or by P, which had been subject to at least 5years of experimental N and P fertilisation.Results of this study indicated that decomposers were always N-limited regardless of the limitation of the primary producers. This leads to a differential nutrient limitation between decomposers and primary producers in sites where mangrove production was P-limited. In these sites, fertilisation with P caused litter quality to change, resulting in a higher decomposition rate. This study shows that direct effects of fertilisation on decomposition through an effect on decomposer nutrient availability might be non-significant, while the indirect effects through modifying litter quality might be quite substantial in mangroves. Our results show no indication that eutrophication increases decomposition without stimulating primary production. Therefore we do not expect a decline in carbon sequestration as a result of eutrophication of mangrove ecosystems.

Nutrient limitation along the Jurien Bay dune chronosequence: response to Uren &amp; Parsons ()

Summary Uren &amp; Parsons () criticize our nutrient addition experiment and question the integrity of the Jurien Bay dune chronosequence. Their primary criticisms relate to (i) inconsistencies in parent material along the chronosequence and (ii) the lack of consideration of micronutrients in our glasshouse experiment. We reaffirm that the Quindalup–Spearwood succession is a consistent Holocene–Middle (possibly Early) Pleistocene soil chronosequence in which parent material, topography, and present‐day climate and vegetation type (i.e. Mediterranean shrubland) are held relatively constant. The older (Early Pleistocene–Late Pliocene) Bassendean Sand probably contained less carbonate initially, but nevertheless represents the endpoint of an exceptionally strong gradient in soil nutrient availability. The claim that we did not consider the potential importance of micronutrients is unfounded. We included a micronutrient treatment in our experiment and discussed the importance of micronutrient availability in young calcareous substrates. Synthesis. We restate that our experimental results support predictions about changes in nutrient limitation during long‐term pedogenesis, and affirm that the Jurien Bay chronosequence is a valuable model system for addressing ecological questions related to pedogenesis, plant diversity and plant–soil interactions.

Are patterns in nutrient limitation belowground consistent with those aboveground: results from a 4 million year chronosequence

Accurately predicting the effects of global change on net carbon (C) exchange between terrestrial ecosystems and the atmosphere requires a more complete understanding of how nutrient availability regulates both plant growth and heterotrophic soil respiration. Models of soil development suggest that the nature of nutrient limitation changes over the course of ecosystem development, transitioning from nitrogen (N) limitation in ‘young’ sites to phosphorus (P) limitation in ‘old’ sites. However, previous research has focused primarily on plant responses to added nutrients, and the applicability of nutrient limitation-soil development models to belowground processes has not been thoroughly investigated. Here, we assessed the effects of nutrients on soil C cycling in three different forests that occupy a 4 million year substrate age chronosequence where tree growth is N limited at the youngest site, co-limited by N and P at the intermediate-aged site, and P limited at the oldest site. Our goal was to use short-term laboratory soil C manipulations (using 14C-labeled substrates) and longer-term intact soil core incubations to compare belowground responses to fertilization with aboveground patterns. When nutrients were applied with labile C (sucrose), patterns of microbial nutrient limitation were similar to plant patterns: microbial activity was limited more by N than by P in the young site, and P was more limiting than N in the old site. However, in the absence of C additions, increased respiration of native soil organic matter only occurred with simultaneous additions of N and P. Taken together, these data suggest that altered nutrient inputs into ecosystems could have dissimilar effects on C cycling above- and belowground, that nutrients may differentially affect of the fate of different soil C pools, and that future changes to the net C balance of terrestrial ecosystems will be partially regulated by soil nutrient status.

Exploring lake ecosystems: hierarchy responses to long‐term change?

AbstractShifts in climate regime have provoked substantial trophic‐ and species‐dependent changes within ecosystems. With growing concerns of present global warming, we examined potential lake ecosystem responses, natural hierarchy responses (i.e. immediate responses at lower system levels as opposed to delayed responses at higher system levels), and possible shifts among abiotic (physics, nutrients) and biotic (phytoplankton, zooplankton) system components. Specifically, we analyzed decadal data collected from Müggelsee, a lake in Berlin, Germany, for climate‐induced abiotic and biotic changes, their timing and type, and classified them as abrupt permanent, gradual permanent, abrupt temporary, or monotonic. We further categorized variable changes as a function of system hierarchy, including lake physics (ice, temperature, stratification), nutrients (phosphorus, nitrogen, silicate), plankton, and levels of integration (i.e. species, taxonomic groups, and total plankton). Contrary to current theory, data suggest abrupt responses did not occur in a hierarchy‐dependent manner, nor was a clear pattern observed among functional system‐based categories. Abrupt permanent changes were the most prominent response pattern observed, suggesting they may be driven by large‐scale climatic oscillations and by surpassed thresholds, as noted in previous case studies. Gradual changes coincided with affected abiotic parameters spanning an expansive time range; for example, climatic effects in spring preceded changes in nutrient limitation. Variables displaying no long‐term changes pointed to compensation processes caused by, e.g., simultaneously acting forces of warming trends and climate‐independent changes in trophic state. Nevertheless, the complexity of response patterns at the single system level manifested clear chronological regime shifts in abiotic and biotic parameters in spring and, to a lesser extent, in summer. With regard to projected global warming, the majority of currently unaffected system levels may face impending thermal thresholds, achievement of which would result in an accelerated shift in ecosystem state.

Changes in nutrient limitation of spring fen vegetation along environmental gradients in the West Carpathians

AbstractQuestion:How does nutrient limitation vary along major environmental gradients in fens and wet meadows?Location:West Carpathians (Czech Republic, Slovakia).Methods:We recorded total plant species composition in 83 plots of 9 m2. Living above‐ground biomass of total vegetation was sampled. Concentrations of Ca, Fe, N, P, K were determined. Obtained data were analysed by various univariate and multivariate statistical techniques.Results:Major environmental gradients correspond to the poor‐rich vegetation gradient, from poor Sphagnum‐fens to calcareous fens, and the fen‐meadow gradient from sedge‐moss fens to forb‐rich wet meadows. Ca‐concentration in above‐ground biomass was strongly positively correlated with the poor‐rich gradient, while Fe‐ and N‐concentrations had an opposite trend. Poor‐fen vegetation contained little calcium and much iron. The lowest P‐concentrations were found in calcareous tufa‐forming fens. Variation in N:K ratio indicated a slight decrease of K‐availability towards poor fens. Along the fen‐meadow gradient, vegetation uptake of P, K and Ca increases and the uptake of Fe decreases.Conclusions:Our results suggest that variation in the type of nutrient limitation is not a dominant cause of the poor‐rich gradient in fens due to the important role of calcium and iron. Nevertheless, species distribution along the poor‐rich gradient is influenced by phosphorus shortage in tufa‐forming fens and relatively high N‐uptake and low K‐availability in poor fens. Additional P‐ and K‐supply to Ca‐rich fens can alter species composition towards forb‐rich meadows.

Atrazine catabolism by a combined bacterial association (KRA30) under carbon- and nitrogen-limitations in a retentostat.

Nutrient-limited atrazine catabolism study in continuous cultures with biomass retention to mimic in situ environmental conditions and thus gain insight of the efficacy of biosupplementation/biostimulation to eliminate reduced herbicide bioavailability. Carbon- and nitrogen-limited retentostat (1 and 5 l) cultivation of a combined atrazine (100 mg l-1)-catabolizing association KRA30 was made. As a nitrogen source, through citrate supplementation, increased herbicide catabolism resulted and was complete in the absence of NH4-N. Co-metabolism of the molecule in the presence of succinate was identified. Population characterization by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) indicated component species numerical dominance shifts in response to changes in nutrient limitation, mineral salts composition and biofilm formation, although the total species complement and catabolic potential were retained. Biomass and catabolic capacity maintenance, through cost-effective biosupplementation/biostimulation, should promote atrazine bioavailability and so ensure successful amelioration. All planning, implementation and monitoring of bioremediation programmes should be underpinned by a combination of molecular and (continuous) culture-based methods.

Nutrient limitation of phytoplankton and periphyton growth in upland lakes

SUMMARY 1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were visited three times between April and August 2000. On each occasion water chemistry was measured and phytoplankton bioassays were performed in the laboratory to assess growth‐rate and yield limitation by phosphorus and nitrogen. In addition, yield limitation of periphyton growth was investigated twice, in situ, using nutrient‐diffusing substrata.2. Over the whole season the percentage frequency of P, N and co‐limitation was 24, 13 and 63%, respectively, for phytoplankton rate limitation and 20, 22 and 58%, respectively, for phytoplankton yield limitation.3. A clear response of periphyton yield to nutrient additions was found in 75% of all cases and of these, co‐limitation was most common (54%). Average percentage frequency for P and N limitation was 26 and 20%, respectively.4. Phytoplankton and periphyton showed seasonal changes in nutrient limitation within sites. In particular, co‐limitation became progressively more common as the season progressed.5. The response of phytoplankton growth rate to ammonium and nitrate addition was identical, but ammonium was a slightly better source of nitrogen than nitrate for phytoplankton yield on 7% and for periphyton yield on 10% of the occasions. However, the magnitude of the effect was small.6. The concentration of dissolved inorganic nitrogen (DIN) and the molar ratio of DIN to total dissolved phosphorus (TDP), appeared to be the main environmental factors controlling the extent of nitrogen or phosphorus limitation at a given site. Nitrogen limitation was more likely than phosphorus limitation where the DIN was &lt;6.5 mmol m−3 and the ratio of DIN : TDP was &lt;53. Co‐limitation was the most likely outcome at a DIN concentration &lt;13 mmol m−3 and at a DIN : TDP molar ratio &lt;250. Above these values phosphorus limitation was most likely.7. The relatively high frequency of nitrogen limitation and co‐limitation at higher N : P ratios than previously reported, may result from the inability of nitrogen‐fixing cyanobacteria to thrive in these upland lakes where pH and the concentration of phosphorus tended to be low and where flushing rates tended to be high.

OPTIMUM N:P RATIOS AND COEXISTENCE OF PLANKTONIC ALGAE1

ABSTRACTThe optimum atomic ratio of N to P, the ratio at which one nutrient limitation changes over to the other, was determined in seven species of freshwater planktonic algae. The ratio varied over a wide range among species; the average for these species was 17. If the cellular nutrient ratios in marine species are comparable with those in freshwater organisms, Redfield's ratio of 15 is remarkably close to the average. Cellular N:P ratios varied over a 24‐h period under a light:dark cycle. The variation of the optimum ratio between species and diel change in cellular N:P ratios within a species could play an important role in population dynamics by enhancing the probability of coexistence of species.