High Nutrient Addition Research Articles

Experimental evaluation of temperature, nutrients, and initial concentration on medium-chain carboxylic acids production from winery wastes.

In the wine industry, grape processing is accompanied by waste generation, such as grape stalks, winery wastewater, and grape pomace (GP). GP can be used to produce value-added compounds such as medium-chain carboxylic acids (MCCA). This work aimed to determine the operational conditions (temperature, addition of nutrients, and initial waste concentration) to improve MCCA production using waste GP from the winery industry as a substrate. The electron donor (ethanol) and electron acceptor (acetate) were directly generated from the GP and consecutively used to produce MCCA. The treatment with high concentration, temperature, and nutrient addition promotes caproic acid's maximal yield and concentration (0.11 ± 0.02 g MCCA/g TS). Nutrients' presence and temperature significantly affected electron acceptor production. The addition of nutrients and 30 °C leads to elevated acetate production. However, at 37 °C, butyrate and MCCA were mainly produced without adding nutrients, and high ethanol consumption was observed. A higher metabolic diversification was observed at 37 °C than at 30 °C. Temperature and nutrient availability significantly affected the metabolic pathway and the type of carboxylic acid produced.

Mechanism for the restoration of degraded typical steppe by nitrogen and phosphorus co-addition.

The reduction of soil nutrient content is one of the major reasons caused grassland degradation in China. Nutrient addition is thus considered as an effective measure for the restoration of degraded grasslands. However, over-fertilization can lead to decrease in plant diversity. To clarify the appropriate amount of nutrient addition and the underlying mechanism that promotes grassland restoration, we set up a nitrogen and phosphorus co-addition experiment in a degraded typical steppe of Inner Mongolia, and examined the responses at community, functional group and species levels to nutrient addition. The results showed that nutrient addition enhanced biomass while did not reduce species richness at the community level. The biomass showed a saturation response with the increases of nutrient addition, which approached saturation under the 12.0 g N·m-2, 3.8 g P·m-2 treatment. Species richness increased significantly under the lower nutrient treatments (N <9.6 g·m-2, P < 3.0 g·m-2) compared with the control, while the two high nutrient treatments did not alter species richness. At the functional group level, biomass and abundance of perennial rhizome grasses increased significantly with the increases of nutrient addition levels. Biomass and density of annuals increased significantly under high nutrient addition levels. However, the abundance and biomass of perennial bunchgrasses and perennial forbs were rarely affected. At the species level, six target species responded differently to nutrient addition. Biomass of Leymus chinensis was significantly increased due to the increase of population density and individual biomass. Biomass of Stipa grandis, Agropyron cristatum and Cleistogenes squarrosa change little. Biomass of Potentilla acaulis and Carex korshinskyi were reduced due to the decreases in individual biomass and population density, respectively. As a measure of restoring degraded grassland, nutrient addition could significantly increase biomass and species diversity, decrease biomass of the degradation indicator species, and increase biomass of perennial rhizomes grasses.

Response of Microphytobenthos and Benthic Bacteria Viability to Eutrophication in a Benthic–Pelagic Coupling Mesocosm Experiment

Excessive primary productivity due to nutrient inputs is a potential problem in coastal areas when resulting in high organic matter sedimentation rates. Microphytobenthos and heterotrophic bacteria are two components of the benthic ecosystem that contribute to nutrient cycling and decomposition of organic matter. In this context, the effects of nutrient addition and the associated in situ produced organic matter on microphytobenthos community composition and benthic bacterial viability were assessed in a mesocosm experiment for 58 days. The experimental setup included triplicate mesocosms filled with sediment and water under three levels of nutrient addition (‘control’, ‘low’ and ‘high’). Benthic algal community composition was assessed using chemotaxonomy and bacterial viability was estimated using flow cytometry and a double-staining protocol. Multivariate analysis detected a significant effect of treatment and time on microphytobenthic community composition indicating a difference between control and low mesocosms and also between low and high treatments at Day 12 and Day 24 of the experiment. Nonetheless, microphytobenthos implied high resistance and redundancy of benthic algae to disturbance as all three treatments showed no significant difference in community structure between Day 0 and Day 58. Bacterial viability responded quickly to the high nutrient addition and was significantly lower than in the ‘control’ and ‘low’ treatments at Days 6 and 12. Both pelagic and benthic environmental variables were correlated to these changes in benthic community.

A Case Study on the Dairy Project “Full Bucket” in Brazil: Socioeconomic and Environmental Characteristics in Areas Under Its Guidelines

Socioeconomic and environmental characteristics in areas under Brazilian Balde Cheio Project (BCP) were evaluated. Questionnaires were applied to identify productive and socioeconomic characteristics of small farmers, and farmers&amp;rsquo; perceptions on the implementation and operation of BCP. Soil and water samples were also taken. It seems that farmer&amp;rsquo;s perception about the BCP is associated to their socioeconomic reality; while environmental issue does not have the same relevance. According to the farmers, it was verified increase of productivity, and financial income. All farmers agree that they have not followed carefully the NGO guidelines, which is a matter of concern, because the differences among reality farms. In studied soils were observed that organic carbon levels decreased in areas under the BCP management. The NO32-, Cl- and SO42- concentrations in the waters samples were below the maximum stipulated by Brazilian environmental legislation. However, it should be highlighted that high nutrient addition and low C input in the system, can result in high losses of nutrients and also CO2 into the atmosphere.

Responses in bacterial community structure to waste nutrients from aquaculture: an in situ microcosm experiment in a Chilean fjord

AEI Aquaculture Environment Interactions Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AEI 9:21-32 (2017) - DOI: https://doi.org/10.3354/aei00212 Responses in bacterial community structure to waste nutrients from aquaculture: an in situ microcosm experiment in a Chilean fjord Lasse Mork Olsen1,*, Klaudia L. Hernández2, Murat Van Ardelan3, Jose Luis Iriarte4,5, K. Can Bizsel6, Yngvar Olsen7 1Norwegian Polar Institute, Fram Centre, 9296 Tromsø, Norway 2Facultad de Ecologia y Recursos Naturales, Universidad Andres Bello, 2520000 Viña del Mar, Chile 3Norwegian University of Science and Technology, Dept. of Chemistry, 7491 Trondheim, Norway 4Instituto de Acuicultura and Centro FONDAP de Investigación Dinámica de Ecosistemas Marinos de Altas Latitudes (IDEAL), Universidad Austral de Chile, 5480000 Puerto Montt, Chile 5COPAS-Sur Austral, Centro de Investigación Oceanográfica en el Pacífico Sur-Oriental (COPAS), Universidad de Concepción, 4030000 Concepción, Chile 6Dokuz Eylül University, Institute of Marine Sciences and Technology, Bakü Bulv. No. 100, 35340 Inciralt, Izmir, Turkey 7Norwegian University of Science and Technology, Dept. of Biology, 7491 Trondheim, Norway *Corresponding author: lasse.mork.olsen@npolar.no ABSTRACT: Chilean salmon farms release inorganic nutrients excreted by the fish into the surrounding water in Patagonian fjords. The objective of this experiment from the Comau Fjord (42.2° S) in southern Chile was to study how increased input of ammonium (NH4) and phosphate (PO4) from salmon farms might affect the community structure of bacteria in surface waters where fish farms are located. We used microcosms (35 l) with NH4-N and PO4-P added to the natural seawater in a gradient of nutrient-loading rates, with the same N:P ratio as in salmon aquaculture effluents. Additionally, we measured bacterial community structure at different depths in the Comau Fjord to assess the natural variation to compare with our experiment. We used denaturing gradient gel electrophoresis (DGGE) to create 16S rDNA fingerprints of the bacterial communities and monitored biological and environmental variables (chlorophyll a, inorganic nutrients, pH, microbial abundance). The nutrient-loading rate had a significant impact on the bacterial community structure, and the community dissimilarity between low and high nutrient additions was up to 78%. Of the measured environmental variables, phytoplankton abundance and increased pH from photosynthesis had a significant effect. We observed no significant changes in bacterial diversity, which remained at the same level as in the unmanipulated community. Thus, the bacterial community of the fjord was not resistant, but resilient within the time frame and nutrient gradient of our experiment. KEY WORDS: Aquaculture · Eutrophication · Microbial community · Bacterial diversity · Chile · Patagonia Full text in pdf format PreviousNextCite this article as: Olsen LM, Hernández KL, Ardelan MV, Iriarte JL, Bizsel KC, Olsen Y (2017) Responses in bacterial community structure to waste nutrients from aquaculture: an in situ microcosm experiment in a Chilean fjord. Aquacult Environ Interact 9:21-32. https://doi.org/10.3354/aei00212 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AEI Vol. 9. Online publication date: January 31, 2017 Print ISSN: 1869-215X; Online ISSN: 1869-7534 Copyright © 2017 Inter-Research.

Ecological impacts of an exotic benthivorous fish, the common carp (Cyprinus carpio L.), on water quality, sedimentation, and submerged macrophyte biomass in wetland mesocosms

We examined the interactions of the common carp (Cyprinus carpio L.) and nutrient additions on water quality, sedimentation rates, and submerged macrophyte biomass in mesocosms in Delta Marsh, Manitoba, Canada. We wanted to determine if carp and nutrients interacted synergistically to increase phytoplankton biomass. A two-by-three duplicated, factorial design had the following treatments: (1) control mesocosms with no carp or nutrient additions; (2) low carp density and no nutrient additions; (3) high carp density and no nutrient additions; (4) no carp and nutrient additions; (5) low carp density and nutrient additions; and (6) high carp density and nutrient additions. The presence of carp increased ammonia concentrations, turbidity, and phytoplankton biomass as expected but did not increase total reactive phosphorus concentrations. The presence of carp did not appear to interact synergistically with nutrient additions to increase phytoplankton as has been suggested by others. In mesocosms with high carp density and receiving nutrient enrichment, phytoplankton appeared to be suppressed relative to mesocosms receiving nutrient enrichment only, and nutrient enrichment and low carp density. Overall, the presence of carp appears to mimic the effects of eutrophication. Our results demonstrate that carp can cause a shift from a clear, macrophyte-dominated state to a turbid phytoplankton-dominated state at a biomass of less than 600 kg ha−1.

Effects of water and nutrient additions on the timing and duration of phenological stages of resprouting Terminalia sericea

There is limited information about the factors that affect phenology of coppicing trees. This study examined the effects of water and nutrient additions on the phenology of coppicing Terminalia sericea trees in a semi-arid savanna in South Africa. Cut trees were exposed to different levels of water and nutrient (nitrogen and phosphorus) supplementations over a period of two years. Phenological stages monitored fortnightly included leaf bud presence, leaf presence and fruit production. Leaf presence followed a similar distribution to bud presence, peaking during the wet season (November–April), with the low water and high nutrient additions having the highest significant effect. Change in leaf color from the typical silvery green to yellow was delayed to later in the wet season, while higher leaf numbers were recorded during the wet seasons for trees supplemented with water and nutrients. Results indicate that the increase in water and nutrient availability has direct consequences for extending the growing season of resprouting trees. This extension has positive effects on increasing the photosynthetically active period, thus potentially increasing the carbon gain and growth of trees. Although generally low throughout the study in all treatments, fruit presence was mainly in supplemented trees.

Interaction of a pesticide and larval competition on life history traits of Culex pipiens

Mosquito larval development occurs in aquatic habitats that are directly or indirectly exposed to chemical contaminants. Little is known about how interaction of these chemicals with other biotic and abiotic stressors impact mosquito populations. We used two levels of nutrient (low and high) and four larval densities (10, 20, 30, 40) to examine the effects of low concentrations of insecticide malathion, on Culex pipiens L. mosquitoes experiencing stress from larval competition. Addition of malathion at the high nutrient condition enhanced survival with increasing larval densities, but this effect was not observed at low nutrient condition. Males exposed to malathion were significantly larger than those from control treatments while the effect of malathion on size of females varied with larval density and the level of nutrients. Larval exposure to malathion and low nutrient resulted in significantly larger females with increases in larval densities compared with other treatments. The effect of malathion on male longevity varied with larval density and amount of nutrients. At higher densities, male longevity was consistently higher in low nutrient than in high nutrient conditions and addition of malathion in high nutrient treatment increased male longevity at the highest density. These effects are most likely attributable to release from competition among survivors after mortality from malathion and density-dependent effects. We conclude that biotic conditions of the larval habitat can alter the impact of low concentrations of chemical contaminants on mosquito populations in ways that may influence the pattern of disease transmission and the outcome of vector control efforts.

Light, nutrients and grazing interact to determine diatom species richness via changes to productivity, nutrient state and grazer activity

Summary Productivity and grazing pressure interact in determining autotroph diversity, because high productivity increases the capability of a plant community to compensate for grazing losses. However, further factors may play a role in shaping diversity, including primary producer nutrient stoichiometry and grazer activity. Our study focuses on the interactions between light, nutrients and grazing in determining species richness and evenness of stream diatoms. By measuring primary producer productivity and nutrient content as well as grazer activity, we attempt to disentangle the different pathways by which the three factors affect diatom species richness and evenness. We hypothesized that high light intensities and nutrient addition would increase species richness by increasing primary productivity and that higher levels of light and nutrients would compensate for negative grazer effects on species richness of primary producers. We also hypothesized that high light intensities would decrease the nutrient content of primary producers, especially when nutrients are limiting, whereas nutrient addition would increase primary producer nutrient content. Last, in addition to changing primary producer nutrient content, light and nutrients would also change grazer activity, thus modifying the interactions between light, nutrients and grazing. We used periphyton and gastropod grazers in an experiment with circular stream channels with four nutrient, two light and four grazing levels to determine individual and combined effects on benthic diatom richness and evenness. After 3 weeks, we determined algal biomass, periphyton nutrient content, diatom species richness and evenness as well as grazer activity. Our results showed that light and nutrients increased species richness and primary producer productivity and nutrient content. Grazing decreased species richness but only at low light levels, possibly because high light levels reduced grazer activity. Evenness was not affected by any single factor alone, but was influenced by nutrient–light and grazing–light interactions. Synthesis. Light, nutrients and grazing interacted in determining primary producer species richness. Their effects were mainly mediated through changes in productivity but primary producer nutrient content and grazer activity also played important roles.

Does morphological plasticity of the Phalaris arundinacea canopy increase invasiveness?

Morphological plasticity could facilitate invasions of wetland plants into areas that experience increased durations of flooding and eutrophication. We explored canopy plasticity of Phalaris arundinacea, an aggressive invader of wetlands, as it differentially invaded wet prairie mesocosms under 3 flooding durations and 3 levels of nutrient addition. Phalaris grew as a sward with intermittent and early-season flooding but shifted to tussocks under constant flooding. These two growth forms differed by >20% in several canopy ratios. Clones that formed tussocks produced 45% more shoots per unit biomass (P = 0.007) and a 25% higher ratio of total shoot length to biomass (P = 0.04). Lighter-weight shoots supported 33% fewer leaves and, consequently, had 35% less leaf area per shoot height (P < 0.002). Tussocks developed a continuous mat of adventitious roots, with root mats reaching 20.9 ± 0.6 cm in diameter and 4.7 ± 0.3 cm in height over two growing seasons. While forming tussocks, Phalaris tolerated longer durations of flooding and more than doubled its aboveground biomass. Invasions occurred rapidly, with Phalaris exceeding 75% canopy cover and accounting for 66% of the total aboveground biomass under constant flooding. Early-season flooding increased the lateral spread of individual shoots. High nutrient addition produced shoots that were 27% taller and 50% heavier (P < 0.02), with 81% more leaf area (P < 0.0003) than shoots that received no nutrients. Consequently, under early-season flooding with high nutrient additions, Phalaris was primed to invade, nearly doubling its proportion of the total aboveground biomass and exceeding 50% canopy cover during year two.

Nutrient Addition to Experimental Rocky Shore Communities Revisited: Delayed Responses, Rapid Recovery

Coastal eutrophication may alter the dominance patterns of marine macroalgae, with potential consequences for the associated fauna and the entire ecosystem. Benthic macroalgae and animals in control and nutrient-enriched mesocosms were monitored to investigate eutrophication-induced changes in rocky shore communities. During a 3-year project, nutrient addition had only minor effects on the community structure, such as increased cover and biomass of green Ulva spp. and increased abundance of certain animal species at high nutrient levels. This study is a 4-year extension of a previously reported project, with 2 extra years of effect studies (altogether 5 years) and a subsequent 2 years for recovery. During the 4th year of nutrient enrichment, the cover of Fucus vesiculosus and Fucus serratus started to decline. In the 5th year, these canopy species crashed and there was an evident take-over by green algae at high nutrient addition levels. The previously observed abundance stimulation for fauna disappeared later in the time series, probably due to the loss of the macroalgal canopy. After less than 2 years on regular seawater, the algal and animal communities had returned to within the range of normal variability. The results indicate that established rocky shore communities of perennial algae with associated fauna are able to persist for several years, even at very high nutrient levels, but that community shifts may suddenly occur if eutrophication continues. They also indicate that rocky shore communities have the ability to return rapidly to natural undisturbed conditions after the termination of nutrient enhancement.

Threshold of gross primary production for planktonic metabolic balance in the Southern Ocean: An experimental test

The proposed threshold planktonic gross primary production (GPP) value for O2 of 2.05 mmol m−3 d−1 separating net heterotrophic from net autotrophic communities in the Southern Ocean was tested experimentally using large mesocosms (20 m3). A set of eight mesocosms was moored in Johnson's Dock (62°39.576'S, 60°22.408'W, Livingston Island, Antarctica) and a gradient of GPP was experimentally generated by imposing four light levels (100%, 50%, 25%, and 10%) in the presence or absence of nutrient additions (0.1 mol NH4Cl, 0.1 mol of F6Na2Si, and 0.01 mol KH2PO4 per mesocosm per day). The experimental treatments resulted in a broad range of chlorophyll a (Chl a) (0.31–93.5 mg m−3) and GPP (O2, 0.17–16.7 mmol m−3 d−1). Community respiration (R) increased with increasing GPP, but not proportionately, resulting in a range of P :R ratios ranging from 0.12 in intensely shaded communities to 1.3 in those receiving high irradiance and nutrient additions, with the compensation irradiance for community metabolism (i.e., percentage irradiance at P:R = 1) being reached at 83% of the ambient irradiance. The experimental estimate of the threshold GPP for metabolic balance of the community investigated (i.e., P :R = 1) was 2.2 (SE ± 0.016) mmol O2 m−3 d−1, thereby validating the estimate of 2.05 mmol O2 m−3 d−1 derived in the past from comparative analyses of planktonic metabolism across the Southern Ocean. The validation of this threshold suggests that net heterotrophic planktonic communities may be more prevalent in the Southern Ocean than hitherto believed.

Use of a simulation model to examine effects of nutrient loading and grazing on Potamogeton perfoliatus L. communities in microcosms

We constructed a numerical simulation model of an enclosed, submersed macrophyte ecosystem to examine the effects of nutrient supply rates, grazer densities and initial conditions on macrophytes and epiphytic algae. The model included an internal, non-structural plant nutrient pool that controlled root and leaf nutrient uptake and was calibrated to literature values and mesocosm experiments. Simulations were run to examine how initial conditions, nutrient supply rates and the presence of grazers may affect macrophyte–algal competition experiments. Simulations revealed that the outcome of an experiment could be largely controlled by the initial conditions or by biomass changes during the experiment. For example, high initial macrophyte biomass reduced light and nutrients available for algae, which prevented the overgrowth of algae, even at high nutrient addition rates. Epiphytic algae biomass increased with water exchange rate regardless of inflow nutrient concentration. Submersed plants grew best at lower exchange rates that allowed nutrient concentrations to be drawn down, slowing algal growth. Simulations showed that the effect of grazers on epiphyte biomass was greatest at intermediate nutrient addition rates. When grazers were absent, macrophyte biomass could be highest with low or high inflow nutrient concentrations, depending on the exchange rate. Model analysis also revealed that it is essential to consider nutrient loading rate per unit macrophyte biomass, not just nutrient concentration, when quantifying the effects of eutrophication on submersed macrophytes in shallow water. These results show the utility of using a simulation model together with ecosystem experiments. They also show how water exchange rates, which can depend on bed size, could affect eutrophication responses of submersed macrophyte ecosystems.

Daily bacterioplankton dynamics in a sub-Saharan estuary (Senegal River, West Africa): a mesocosm study

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 40:13-24 (2005) - doi:10.3354/ame040013 Daily bacterioplankton dynamics in a sub-Saharan estuary (Senegal River, West Africa):a mesocosm study Marc Troussellier1, Patrice Got1, Maimouna Mboup2, Daniel Corbin2,Laura Giuliano3, Simone Cappello3, Marc Bouvy1,2,* 1Laboratoire Ecosystèmes Lagunaires UMR-CNRS 5119, Equipe EMMA, Université Montpellier II, 34095 Montpellier, France2Centre IRD Bel Air, BP 1386, UR 098 Dakar, Senegal3Istituto Sperimentale Talassografico—Consiglio Nazionale delle Ricerche, Spianata San Raineri 84, 98122 Messina, Italy *Corresponding author. Email: bouvy@dakar.ird.sn ABSTRACT: Temporal variability in bacterial communities was studied on a daily scale in the estuarine part of the largest river on the West African coast, the Senegal River. Duplicate mesocosms (3 m3 volume) were placed in the upper part of the estuary at the end of the dry season (May 2002) and treated with low and high inorganic nutrient (N and P) enrichment. High nutrient additions were followed by a 7-fold increase in phytoplankton biomass and a 6-fold increase in bacterial abundance after 4 and 9 d, respectively. Heterotrophic nanoflagellates (HNF) showed their maximal abundance (1 × 106 ml–1) 2 d after the bacterial peak. The low bacteria to flagellate ratios recorded on Day 10 may suggest enhanced bacterivory from HNF. Simultaneous measurements of growth and grazing rates on bacteria during the bacterial growth phase were performed with the dilution method and seemed to indicate that the HNF community was capable of quickly controlling bacterial development. However, estimates of the carbon demand of HNF during their growth phase (915 µg C l–1 d–1) appeared to be more elevated than the bacterial carbon production (63% of the HNF carbon demand). To cover HNF carbon requirements, an alternative/complementary prey might be the picophytoplanktonic cells, which were very numerous during the study (i.e. 85% of the total phytoplankton count). During the period of high grazing pressure, bacterial populations were characterized by higher specific activity (from tritiated thymidine incorporation) and culturability (from plate counts). The presence of very large bacteria, as detected by epifluorescence microscopy and flow cytometry measurements, may be an escape response from flagellate grazing. 16S rDNA sequences from bacterial isolates showed the presence of 2 types of taxonomic units (Vibrio natriegens– and Flexibacter maritimus–like bacteria), which can be considered by their forms and growth rates to be strains that have developed strategies to protect against grazing. Thus, as demonstrated in various temperate systems, predation by HNF in estuarine tropical ecosystems may also be of importance in shaping the structure and functions of the bacterial community. KEY WORDS: Bacteria · Heterotrophic nanoflagellates · Mesocosms · Growth and grazing rates · Senegal estuary · West Africa Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 40, No. 1. Online publication date: July 18, 2005 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2005 Inter-Research.

Non‐lethal effects of predators on prey growth rates depend on prey density and nutrient additions

A number of studies show that predators can depress prey growth rates by inducing reductions in foraging activity, but the size of this non‐lethal effect is quite variable. Here I investigate how prey density and resource productivity may alter the extent to which predators depress the growth rates of their prey. Theory predicts that when resources are overgrazed, an increase in predation risk will have little net effect on individual food intake because the decline in foraging effort will be offset by an increase in resource level. Thus, the non‐lethal effects of predators on prey growth rates should depend upon prey density and resource productivity in a predictable manner, with the growth penalty imposed by predators being strongest when resources are undergrazed and weakest when resources are overgrazed. I tested this hypothesis by manipulating predation risk, prey density, and nutrient additions in a mesocosm experiment with the pulmonate snail Helisoma trivolvis. Refuge use by snails was 45% higher in the presence of caged crayfish than in their absence. Snail growth rates were reduced, on average, by 24% in the presence of caged crayfish. However, the magnitude of the growth penalty exacted by crayfish depended on snail density and nutrient additions. When snails were stocked at high density and nutrient additions were low, growth suppression was just 2.6%. At the other extreme, when snails were at low density and nutrient additions were high, growth suppression was 44.6%. Thus, the non‐lethal effects of predators on prey growth depend on environmental context, illustrating an important link between individual traits and system‐level properties.

Recruitment of resting stages may induce blooms of Microcystis at low N:P ratios

Some species of cyanobacteria form resting stages at the sediment surface when environmental conditions become unfavourable. As conditions turn more favourable, these resting stages hatch to the water phase, where the cells grow, reproduce, and sometimes form blooms. Since blooms of cyanobacteria have become an increasing threat to inland and brackish waters, it is important to assess the mechanisms and processes involved in the initiation of such blooms. One such mechanism is recruitment from the sediment surface. Potential factors regulating the recruitment of resting stages include variations in nutrient concentrations and ratios, as well as variations in grazing. To investigate how the recruitment of Microcystis responds to different levels of these factors, we performed an enclosure experiment (zooplankton abundances were regulated by predation from fish). We found that recruitment and growth were most pronounced at the second highest nutrient concentration (average concentrations were 498 mug l(-1) of dissolved nitrogen and 134 mug l(-1) of total phosphorus), while no direct response to different grazing levels was detected. We also found that resting stages can be important for initiating and sustaining blooms. The environmental conditions most important in regulating the recruitment rate from resting stages corresponded to the requirements of the plankton cells, namely high nutrient addition and low N:P ratio. (Less)

HIGHER MOSQUITO-BORNE DISEASE RISK IN ANTHROPOGENICALLY MODIFIED ENVIRONMENTS IN THE KAPITI COAST REGION, NEW ZEALAND.

The recent worldwide spread of disease-vector mosquitoes is a serious threat to public health. The ‘emergence’ and ‘re-emergence’ of some major arboviral diseases (eg. Dengue Fever virus and Ross River virus) in recent decades have been linked to the flow-on effects of anthropogenic environmental change on the physical structure and biodiversity of mosquito larval habitats. Although New Zealand has so far remained free from arboviral human diseases an eventual outbreak is very probable in the long term. Four exotic disease vector species have already become well established in New Zealand. Escalating international travel and commerce may introduce more exotic disease vectors as well as disease-infected people, which both exotic and native species may ingest from. To predict how anthropogenic environmental change alters the risk of exposure to disease-vector mosquitoes we need an intimate knowledge of the ecology and biology of species currently in New Zealand. The overarching aim of this paper is to compare the effects of different land uses on the productivity of container breeding species in the Kapiti Coast region, New Zealand. In 2002–03, disease-vector mosquito productivity was examined in artificial containers and other resident water bodies in three land use types, farmland, suburban area and native bush, at three sites in the Kapiti Coast region. Additionally, nutrient and shade levels were experimentally manipulated in mosquito container habitats placed in native bush to approximate the effects of deforestation and agriculture on larval populations. In the field survey, suburban and farmland sites had lower resident biodiversity and substantially higher numbers of disease-vector mosquitoes than native bush sites. Disease-vector mosquitoes also colonised experimental containers in an open, non-forested environment with medium to high nutrient additions more quickly and in greater numbers than in containers under a forest canopy with low or ultra high nutrient levels. These results suggest that modified environments have increased mosquito-borne disease risk from invading species, and that ecosystem integrity in the form of aquatic macro-invertebrate biodiversity may play a fundamental role in protecting against arboviral introduction and spread.

Effects of Enrichment on Simple Aquatic Food Webs

Simple models, based on Lotka-Volterra types of interactions between predator and prey, predict that enrichment will have a destabilizing effect on populations and that equilibrium population densities will change at the top trophic level and every second level below. We experimentally tested these predictions in three aquatic food web configurations subjected to either high or low nutrient additions. The results were structured by viewing the systems as either food chains or webs and showed that trophic level biomass increased with enrichment, which contradicts food chain theory. However, within each trophic level, food web configuration affected the extent to which different functional groups responded to enrichment. By dividing trophic levels into functional groups, based on vulnerability to consumption, we were able to identify significant effects that were obscured when systems were viewed as food chains. The results support the prediction that invulnerable prey may stabilize trophic-level dynamics by replacing other, more vulnerable prey. Furthermore, the vulnerable prey, such as Daphnia and edible algae, responded as predicted by the paradox of enrichment hypothesis; that is, variability in population density increased with enrichment. Hence, by describing ecosystems as a matrix of food web interactions, and by recognizing the interplay between interspecific competition and predation, a more complete description of the ecosystem function was obtained compared to when species were placed into distinct trophic levels.

Effects of Enrichment on Simple Aquatic Food Webs

Simple models, based on Lotka‐Volterra types of interactions between predator and prey, predict that enrichment will have a destabilizing effect on populations and that equilibrium population densities will change at the top trophic level and every second level below. We experimentally tested these predictions in three aquatic food web configurations subjected to either high or low nutrient additions. The results were structured by viewing the systems as either food chains or webs and showed that trophic level biomass increased with enrichment, which contradicts food chain theory. However, within each trophic level, food web configuration affected the extent to which different functional groups responded to enrichment. By dividing trophic levels into functional groups, based on vulnerability to consumption, we were able to identify significant effects that were obscured when systems were viewed as food chains. The results support the prediction that invulnerable prey may stabilize trophic‐level dynamics by replacing other, more vulnerable prey. Furthermore, the vulnerable prey, such as Daphnia and edible algae, responded as predicted by the paradox of enrichment hypothesis; that is, variability in population density increased with enrichment. Hence, by describing ecosystems as a matrix of food web interactions, and by recognizing the interplay between interspecific competition and predation, a more complete description of the ecosystem function was obtained compared to when species were placed into distinct trophic levels.

No‐tillage corn response to placement of fertilizer nitrogen, phosphorus, and potassium

Fertilizer placement for corn (Zea mays L.) has been a major concern for no‐tillage production systems. This 3‐yr study (1994 to 1996) evaluated fertilizer phosphorus (P) or potassium (K) rates and placement for no‐tillage corn on farmers’ fields. There were two sites for each experiment involving fertilizer P or K. Treatments consisted ofthe following fertilizer rates: 0,19,and 39 kg P ha‐1 or 0, 51, and 102 kg K ha‐I. The fertilizer was broadcast or added as a subsurface band 5 cm beside and 5 cm below the seed at planting. Early plant growth, nutrient concentrations, and grain yields were measured. At the initiation of the study, soil test levels for P and K at the 0–1 5 cm depths ranged from optimum (medium) to very high across sites. Effects of added fertilizer and placement on early plant growth and nutrient concentrations were inconsistent. Added fertilizer had a significant effect on grain yields in two of twelve site‐years. Therefore, on no‐tillage soils with high fertility, nutrient addition, and placement affected early plant growth and nutrient utilization, but had limited effect on grain yield. Consequently, crop responses to the additions of single element P or K fertilizers under no‐tillage practices and high testing soils may not result in grain yield advantages for corn producers in the Northern cornbelt regardless of placement method.