Interactive Effects Of Nutrients Research Articles

Effects of multiple stressors on recruitment of long‐lived endangered freshwater mussels

Abstract Multiple stressors can affect populations interactively. However, the process by which they affect recruitment remains unclear, especially in long‐lived organisms with complex life cycles. This study explored the bottleneck stages of juvenile recruitment in the endangered freshwater pearl mussel Margaritifera togakushiensis and the interactive effects of nutrients and fine sediments on these stages. The proportion of M. togakushiensis juveniles aged ≤10 years was investigated in 24 rivers from north‐eastern Japan. Margaritifera togakushiensis has a complex life cycle with an obligate parasitic larval stage. Gravid female density, glochidia density (the number of glochidial infections per stream area), and juvenile survival rate, used as indices for adult, parasitic, and post‐parasitic juvenile stages, respectively, were obtained from 11 of these rivers. Twenty‐four population status assessments showed that the proportion of juveniles aged ≤10 years ranged from 0.00 to 0.23, and that they were absent in 12 rivers. Parasitic and post‐parasitic juvenile stages were bottlenecks for recruitment; juvenile survival rates had a stronger positive effect on recruitment in rivers with high glochidia density. Juvenile survival rate was negatively associated with the synergistic effects of nutrients and fine sediments, although factors limiting glochidia density were not identified. These findings suggest that the interaction between nutrients and fine sediments can synergistically reduce the recruitment of M. togakushiensis, indicating an underestimated impact of stressors on mussel recruitment if treated individually. Future mitigation measures should consider both nutrient levels and fine sediments to restore mussel recruitment. These results also emphasize the importance of considering both parasitic and post‐parasitic juvenile stages of mussels to maximize the success of future restoration actions.

Interactive effects of nutrients and salinity on zooplankton in subtropical plateau lakes with contrasting water depth

Both eutrophication and salinization are growing global environmental problems in freshwater ecosystems, threatening the water quality and various aquatic organisms. However, little is known about their interactive effects on theses stressors and the role of lake depth on these interactions. We used field surveys to compared zooplankton assemblages over four seasons in eight Yunnan Plateau lakes with different trophic states, salinization levels, and water depths. The results showed that: 1) the species number (S), density (DZoop), and biomass (BZoop) of zooplankton exhibited strong seasonal dynamics, being overall higher in the warm seasons. 2) Data collected over four seasons and summer data both revealed highly significant positive relationships of S, DZoop, and BZoop with total nitrogen (TN), total phosphorus (TP), and phytoplankton chlorophyll a (Chl a). 3) S, DZoop, and BZoop displayed a unimodal relationship with salinity, peaking at 400–1000 μS/cm (conductivity, to reflect salinity). 4) The two large-sized taxa (cladocerans and copepods) generally increased at low-moderate levels of TN, TP, Chl a, and Cond and was constant or decreased at high levels. The average body mass (biomass/density) of crustaceans decreased with increasing TN, TP, Chl a, and conductivity. Our findings indicate that zooplankton may be more vulnerable in deep lakes than in shallow lakes when exposed to conductivity stress even under mesotrophic conditions, and the overall decrease in size in zooplankton assemblages under the combined stress of eutrophication and salinization may result in a lowered grazing effect on phytoplankton.

Interactive effects of nutrients and temperature on herbivorous predation in a coastal plankton community

AbstractMarine microbial communities in coastal environments are subject to both seasonal fluctuations and anthropogenic alterations of environmental conditions. The separate influences of temperature and resource‐dependency on phytoplankton growth, community, and ecosystem metabolism are relatively well understood. However, winners and losers in the ocean are determined based on the interplay among often rapidly changing biological, chemical and physical drivers. The direct, indirect, and interactive effects of these conditions on planktonic food web structure and function are poorly constrained. Here, we investigated how simultaneous manipulation of temperature and nutrient availability affects trophic transfer from phytoplankton to herbivorous protists, and their resulting implications at the ecosystem level. Temperature directly affected herbivorous protist composition; ciliates dominated (66%) in colder treatment and dinoflagellates (60%) at warmer temperatures. Throughout the experiments, grazing rates were < 0.1 d−1, with higher rates at subzero temperatures. Overall, the nutrient–temperature interplay affected trophic transfer rates antagonistically when nutrients were amended, and synergistically, when nutrients were not added. This interaction resulted in higher percentages of primary production consumed under nutrient unamended compared to nutrient amended conditions. At the ecosystem level, these changes may determine the fate of primary production, with most of the production likely exported out of the pelagic zone in high‐temperature and nutrient conditions, while high‐temperature and low‐nutrient availability strengthened food web coupling and enhanced trophic transfer. These results imply that in warming oceans, management of coastal nutrient loading will be a critical determinant of the degree of primary production removal by microzooplankton and dependent ecosystem production.

Top-Down and Bottom-Up Control in the Galápagos Upwelling System

Increased standing macroalgal biomass in upwelling zones is generally assumed to be the result of higher nutrient flux due to upwelled waters. However, other factors can strongly impact macroalgal communities. For example, herbivory and temperature, via their effects on primary producers and the metabolic demands of consumers, can also influence macroalgal biomass and productivity, respectively. We assessed the effects of nutrient availability, temperature, and herbivory on macroalgal biomass on a subtidal nearshore rocky reef in the Galápagos Islands. We manipulated nutrient availability and herbivory in field experiments performed in two seasons: the first during a cool, upwelling season, and the second during a warm, non-upwelling season. Excluding macro-herbivores had a clear effect on standing macroalgal biomass, independent of season or nutrient availability. However, we found different interactive effects of nutrients and macro-herbivores between the two seasons. During the cool season, macroalgal biomass was significantly higher in herbivore exclusions than in open areas under ambient nutrient conditions. However, when nutrients were added, macroalgal biomass was not significantly different across all herbivore treatments, which suggests reduced top-down control of herbivores (hence a greater standing algal biomass) in open areas. In the warm season, macroalgal biomass was significantly higher in herbivore exclusions compared to open treatments, both with and without nutrient addition. Furthermore, biomass reached 11X in herbivore exclusions with nutrient additions, which hints nutrient limitation only during warm, low-upwelling conditions. Overall, our results support the hypothesis that macro-herbivores reduce macroalgal biomass in this system and suggest that nutrient availability, but not temperature, modulate herbivory.

Nutrient History Affects the Response and Resilience of the Tropical Seagrass Halophila stipulacea to Further Enrichment in Its Native Habitat.

Eutrophication is one of the main threats to seagrass meadows, but there is limited knowledge on the interactive effects of nutrients under a changing climate, particularly for tropical seagrass species. This study aimed to detect the onset of stress in the tropical seagrass, Halophila stipulacea, by investigating the effect of in situ nutrient addition during an unusually warm summer over a 6-month period. We measured a suite of different morphological and biochemical community metrics and individual plant traits from two different sites with contrasting levels of eutrophication history before and after in situ fertilization in the Gulf of Aqaba. Nutrient stress combined with summer temperatures that surpassed the threshold for optimal growth negatively affected seagrass plants from South Beach (SB), an oligotrophic marine protected area, while H. stipulacea populations from North Beach (NB), a eutrophic and anthropogenically impacted area, benefited from the additional nutrient input. Lower aboveground (AG) and belowground (BG) biomass, reduced Leaf Area Index (LAI), smaller internodal distances, high sexual reproductive effort and the increasing occurrence of apical shoots in seagrasses from SB sites indicated that the plants were under stress and not growing under optimal conditions. Moreover, AG and BG biomass and internodal distances decreased further with the addition of fertilizer in SB sites. Results presented here highlight the fact that H. stipulacea is one of the most tolerant and plastic seagrass species. Our study further demonstrates that the effects of fertilization differ significantly between meadows that are growing exposed to different levels of anthropogenic pressures. Thus, the meadow’s “history” affects it resilience and response to further stress. Our results suggest that monitoring efforts on H. stipulacea populations in its native range should focus especially on carbohydrate reserves in leaves and rhizomes, LAI, internodal length and percentage of apical shoots as suitable warning indicators for nutrient stress in this seagrass species to minimize future impacts on these valuable ecosystems.

Coupling of biostimulation and bioaugmentation for enhanced bioremoval of chloroethylenes and BTEX from clayey soil.

The bioremoval potential of Pseudomonas plecoglossicida toward mixed contaminants was explored through the coupled biostimulation and bioaugmentation in soil microcosm. Response surface methodology was employed to optimize nutrients and innoculum size for the cometabolic removal of two representative chloroethylenes, trichloroethylene (TCE) and cis-1,2-dichloroethylene (cis-DCE), mixed with benzene, toluene, ethylbenzene, and xylenes (BTEX). The interactive effects of nutrients [nitrogen (N) and phosphorus (P)] and inoculum size toward the bioremoval of mixture of BTEX (600 mg kg-1), cis-DCE (10 mg kg-1), and TCE (10 mg kg-1) were estimated using principal component analysis and two-dimensional hierarchical cluster analysis. The optimal condition was confirmed with C:N:P ratio of 100:26.7:1.8-4.8 and higher inoculum size (≥25%), where 97.7% of benzene, 98.3% of toluene, 91.2% of ethylbenzene, 45.6% of m,p-xylene, 31.2% of o-xylene, 26.9% of cis-DCE, and 33.5% of TCE were bioremoved.

Environmental context shapes the long‐term role of nutrients in driving producer community trajectories in a top–down dominated marine ecosystem

Abstract Two predominant anthropogenic impacts on ecosystems, nutrient enrichment and the removal of consumers, are predicted to interact in their effects on producer diversity. Yet, measures of diversity alone may not capture changes occurring in the underlying mechanisms structuring communities. Furthermore, evidence for these interactions in rocky intertidal systems is mixed and may be confounded by variable baseline productivity or short experimental durations that do not capture seasonality, environmental heterogeneity or successional processes. We conducted a 2‐year experiment examining the main and interactive effects of nutrients and herbivores in a low‐productivity environment on the northern shore of Cook Strait near Wellington, New Zealand. We explicitly accounted for the small‐scale spatial environmental heterogeneity characteristic of rocky shores. Rapid and dramatic shifts in diversity metrics and cover of algal communities in the first year were driven by removal of herbivores. Within 2 months of herbivore exclusion, open space decreased from >90% to <15%, regardless of nutrient addition, and both algal diversity and evenness plummeted while species richness rose. In contrast to theoretical predictions, strong interactions between top–down and bottom–up forces on diversity were rare; rather, over time, the addition of nutrients negatively impacted diversity. Further, where herbivores were removed, nutrients inhibited succession beyond ephemeral forms, contrasting with the tolerance model of succession observed under ambient nutrients. The effects of nutrients were only elucidated by accounting for variation in habitat complexity and seasonality, demonstrating that environmental heterogeneity can mask bottom–up processes. Moreover, shifts in relative species abundances revealed that after 10 months, nutrient addition caused increased community turnover regardless of herbivore presence or absence, indicating reduced stability not apparent in simple measures of diversity. Synthesis. Our results suggest that nutrient addition and herbivore reduction exert strong but mostly independent control on community biodiversity, where context dependency in habitat complexity, seasonality and successional stage played key roles in determining community outcomes. These findings demonstrate the importance of longer‐term field studies incorporating environmental context and multiple metrics of diversity to reveal the mechanisms underlying effects of nutrient enrichment and herbivore loss on producer community structure.

Phytoplankton community responses to temperature fluctuations under different nutrient concentrations and stoichiometry

Nutrient availability and temperature are important drivers of phytoplankton growth and stoichiometry. However, the interactive effects of nutrients and temperature on phytoplankton have been analyzed mostly by addressing changes in average temperature, whereas recent evidence suggests an important role of temperature fluctuations. In a laboratory experiment, we grew a natural phytoplankton community under fluctuating and constant temperature regimes across 25 combinations of nitrogen (N) and phosphorus (P) supply. Temperature fluctuations decreased phytoplankton growth rate (rmax ), as predicted by nonlinear averaging along the temperature-growth relationship. rmax increased with increasing P supply, and a significant temperature×P×N interaction reflected that the shape of the thermal reaction norm depended on nutrients. By contrast, phytoplankton carrying capacity increased with N supply and in fluctuating rather than constant temperature. Higher phytoplankton N:P ratios under constant temperature showed that temperature regimes affected cellular nutrient incorporation. Minor differences in species diversity and composition existed. Our results suggest that temperature variability interacts with nutrient supply to affect phytoplankton physiology and stoichiometry at the community level.

Nutrients and sediment modify the impacts of a neonicotinoid insecticide on freshwater community structure and ecosystemfunctioning.

Pesticides are important contributors to the global freshwater biodiversity crisis. Among pesticides, neonicotinoids are the best-selling class of agricultural insecticides and are suspected to represent significant risks to freshwater and terrestrial ecosystems worldwide. Despite growing recognition that neonicotinoid impacts may be modified by the presence of additional stressors, there is limited information about their interactions with other agricultural stressors in freshwater ecosystems. We conducted an outdoor pond-mesocosm experiment to investigate the individual and interactive effects of nutrients, fine sediment, and imidacloprid (a neonicotinoid insecticide) inputs on freshwater community structure (density, diversity, and composition of zooplankton and benthic invertebrates) and ecosystem functioning (ecosystem metabolism, primary production, and organic matter decomposition). We hypothesized antagonistic nutrient-imidacloprid, and synergistic sediment-imidacloprid interactions, affecting aquatic invertebrate communities. The three stressors had significant individual and interactive effects on pond ecosystems. The insecticide neutralized the positive effects of nutrient additions on benthic invertebrate richness and mitigated the negative effects of sediment on zooplankton communities (antagonistic interactions). Moreover, we observed compensatory responses of tolerant benthic invertebrates, which resulted in reversal interactions between sediment and imidacloprid. Furthermore, our observations suggest that imidacloprid has the potential to increase net ecosystem production at environmentally relevant concentrations. Our findings support the hypothesis that the impacts of imidacloprid may be modified by other agricultural stressors. This has important implications on a global scale, given the widespread use of these pesticides in intensive agricultural landscapes and the growing body of literature suggesting that traditional pesticide assessment frameworks, based on laboratory toxicity tests alone, may be insufficient to adequately predict effects to complex freshwater ecosystems.

Complex interactions between nutrient enrichment and zooplankton in regulating estuarine phytoplankton assemblages: Microcosm experiments informed by an environmental dataset

The interactive effects of nutrients and zooplankton grazing on phytoplankton assemblage composition and bloom formation are not well understood, especially in coastal ecosystems. Therefore, the objective of this study was to evaluate phytoplankton assemblage responses to changing environmental conditions in Raritan Bay, an estuary between the states of New York and New Jersey with a long history of cultural eutrophication and harmful algal blooms (HABs). Environmental monitoring of water quality and plankton species composition (monthly data collected from April 2010–April 2013), multivariate ordination techniques, and microcosm experiments were integrated to achieve this objective. Multivariate analysis of the monitoring dataset led to the generation of a number of hypotheses regarding phytoplankton composition, individual nuisance species, environmental factors, and zooplankton composition. The field observations were then supplemented with a series of controlled microcosm experiments designed to test those hypotheses. In particular, the effects and interaction of varying mesozooplankton abundance (sieved, unsieved and enriched) and enrichment with two nutrients (nitrate and dissolved iron) on spring and summer phytoplankton assemblages were evaluated. The environmental monitoring data and the results of the microcosm experiments both indicate that Si:N ratios are important factors governing phytoplankton dynamics in Raritan Bay and that effects of nutrient enrichment on phytoplankton species composition are magnified when mesozooplankton abundance is low. The microcosm experiments, however, led to unexpected results regarding the influence of iron on phytoplankton assemblages. Iron did not emerge as one of the environmental parameters related to spring and summer phytoplankton species composition in the multivariate analysis of field data. Yet, enrichment with iron in microcosms resulted in significant increases in diatoms, and enrichment with both iron and nitrate resulted in significant increases in dinoflagellates and HAB taxa, including Heterocapsa triquetra (Ehrenberg) Stein and Dinophysis spp. This finding suggests that small pulses of iron, in combination with low Si:N ratios (<1) and low mesozooplankton abundance, would shift phytoplankton assemblages in Raritan Bay toward greater dinoflagellate dominance.

The effects of herbivory and nutrients on plant biomass and carbon storage in Vertisols of an East African savanna

Herbivory and nutrients are major ecosystem drivers in African tropical savanna. Although previous studies have determined the influence of herbivory on carbon storage in savanna ecosystems, little is known about the interactive effects of nutrients and herbivory. We determined the effects of long term grazing and short-term factorial nitrogen (N) and phosphorus (P) additions on aboveground biomass, soil organic matter (SOM) content, and plant nutrient storage. Grazing reduced aboveground biomass, foliar P and N stocks by 45%, 38% and 45%, respectively, compared to ungrazed plots, although the foliar P concentration was 20% greater in grazed plots. There was no significant increase in the aboveground biomass after nutrient addition despite increases in foliar N and P concentrations, suggesting that productivity was limited by a different resource (e.g., moisture). There were no significant interactions between nutrient enrichment and grazing. We conclude that grazing reduced aboveground biomass, but improved grass quality through increased foliar P concentration.

Effects of nutrients, temperature and their interactions on spring phytoplankton community succession in Lake Taihu, China.

We examined the potential effects of environmental variables, and their interaction, on phytoplankton community succession in spring using long-term data from 1992 to 2012 in Lake Taihu, China. Laboratory experiments were additionally performed to test the sensitivity of the phytoplankton community to nutrient concentrations and temperature. A phytoplankton community structure analysis from 1992 to 2012 showed that Cryptomonas (Cryptophyta) was the dominant genus in spring during the early 1990s. Dominance then shifted to Ulothrix (Chlorophyta) in 1996 and 1997. However, Cryptomonas again dominated in 1999, 2000, and 2002, with Ulothrix regaining dominance from 2003 to 2006. The bloom-forming cyanobacterial genus Microcystis dominated in 1995, 2001 and 2007–2012. The results of ordinations indicated that the nutrient concentration (as indicated by the trophic state index) was the most important factor affecting phytoplankton community succession during the past two decades. In the laboratory experiments, shifts in dominance among phytoplankton taxa occurred in all nutrient addition treatments. Results of both long term monitoring and experiment indicated that nutrients exert a stronger control than water temperature on phytoplankton communities during spring. Interactive effect of nutrients and water temperature was the next principal factor. Overall, phytoplankton community composition was mediated by nutrients concentrations, but this effect was strongly enhanced by elevated water temperatures.

Impacts of alien plant invasion on native plant communities are mediated by functional identity of resident species, not resource availability

Alien plant invasion and nutrient enrichment as a result of anthropogenic landscape modification seriously threaten native plant community diversity. It is poorly understood, however, whether these two disturbances interact with the functional identity of recipient native plants to drive community change. We performed a mesocosm experiment to examine whether the interactive effects of invasion by a stoloniferous turf‐grass Stenotaphrum secundatum and nutrient enrichment vary across different plant growth forms of an endangered coastal plant community. Communities contained 18 species (drawn without replacement from a pool of 31 species) with either runner, tufted or woody growth forms. Species were well‐established and reproductively mature prior to S. secundatum introduction. Species growth (% cover), reproductive output, soil temperature and light availability were monitored for two growing seasons. Invasion and nutrient enrichment (two levels: ‘natural control’ and ‘enriched’) had no effect on species richness, community composition, reproductive output, soil temperature or light penetration. There was no interactive effect of nutrients and invasion on community productivity (i.e. final biomass), such that invasion caused a reduction in community biomass at both natural and enriched nutrient levels. This was driven only by reduced biomass of functionally‐similar native runner species, which share similar root morphologies and nutrient‐acquisition strategies with S. secundatum. Our study indicates that impacts of invasion are dependent upon the functional identity of species within recipient communities, not the availability of resources. This shows that management cannot buffer invader effects by manipulating resource availability. Revegetation strategies should target functionally‐similar natives for replacement following invader control.

Effects of fertilization and drought stress on tannin biosynthesis of Casuarina equisetifolia seedlings branchlets

Nutrient, water, and their interactions influence the allocation of investment by plants to resistance and tolerance traits. We used a completely crossed randomized-block design experiment to examine the independent and interactive effects of nutrients and water availability on tannin production of C. equisetifolia seedlings. The results showed that nitrogen and phosphorus fertilizer have significant effects on total phenolics (TP) and extractable condensed tannins (ECT) concentrations of branchlets. TP and ECT concentrations decreased with fertilizer addition and increased in arid condition. This pattern lends to support source-sink hypothesis such as the carbon-nutrient balance (CNB) hypothesis and the growth-differentiation balance (GDB) hypothesis. Soluble sugars or starch concentrations were both inversely related to TP concentrations. However, there was no significant correlation between them and ECT concentrations. In addition, chlorophyll concentration had a positive linear correlation with TP and no significant correlation with ECT. On the contrary, chlorophyll a/b ratios were negatively correlated with TP and positively correlated with ECT. The discrepancy of relationship between carbohydrates and TP or ECT showed that the biosynthetic routes of different tannins were different. In this study, no significant correlation between TP and N, or ECT and N, did not support protein competition model (PCM). TP:N and ECT:N ratios were higher in nutrient deficiency and arid conditions, which were one of the important nutrient conservation strategies for C. equisetifolia.

Joint control by rodent herbivory and nutrient availability of plant diversity in a salt marsh-salty steppe transition zone

Questions: Do current models that predict shifting effects of herbivores on plant diversity with varying nutrient conditions apply to stressful systems like salt marshes? Do herbivores affect different components of the diversity as nutrient availability varies? Location: Salt marsh–salty steppe transition zone at the SW Atlantic Mar Chiquita coastal lagoon (37°44′52″S, 57°26′6″W), Argentina. Methods: We experimentally evaluated the separate and interactive effect of nutrients and rodent (Cavia aperea) herbivory, using exclosures and applying fertilizer (mostly nitrogen), following a factorial design in 50 cm × 50 cm plots. Results: We found a negative effect of herbivory on diversity in the resource-poor scenario (due to a reduction in species richness), but a positive effect when nutrients were added, by reducing the abundance of the dominant plant (and hence increasing evenness). Conclusions: Our experimental results contribute to the limited factorial evidence evaluating the role of nutrients and herbivory on the diversity of terrestrial plant communities, even in highly stressful environments like salt marsh–salty steppe transition zones. Our results also support the model that predicts negative effects of herbivores on plant diversity in low-nutrient conditions and positive effects in nutrient-enriched scenarios, and also support the mechanism assumed to act in these situations.

Disturbance mediates the effects of nutrients on developing assemblages of epibiota

Local dynamics such as resource enhancement (e.g. nutrient supply) and stochastic events of destruction (disturbances that provide new space) are hypothesized to counteractively affect species diversity and composition.We tested the independent and interactive effects of nutrients and disturbance on the development of assemblages of epibiota attached to vertical surfaces in an oligotrophic system. Nutrient concentrations were manipulated at three levels (ambient, medium and high) while disturbance was manipulated by removing biomass at seven frequencies (0x, 2x, 3x, 4x, 5x, 7x, 12x). Nutrient and disturbance regimes had opposing effects on diversity such that species richness increased with resource enhancement (nutrients) and declined with disturbance. These results support the model that increased heterogeneity of distribution of limiting resources allows the coexistence of species with low and high resource requirements.

Evaluation of the differential osmotic adjustments between roots and leaves of maize seedlings with single or combined NPK-nutrient supply.

Many physiological mechanisms associated with nutrient supply have been implicated as improving plant growth under drought conditions. However, benefits to plant growth under drought might derive from an increased recovery of soil water through osmotic adjustment in the shoots and especially in the roots. Thus, experiments were carried out to investigate the effects of the nutrients N, P and K applied singly or in combination, on the osmotic adjustment and turgor maintenance in the roots and leaves of maize seedlings. The seedlings were harvested between 18 and 37 days after sowing according to the soil matric threshold potentials. Soil matric potentials and shoot and root biomass were determined at harvest. Turgor pressure and osmotic adjustment of the leaves and roots were estimated by measurements of their water and osmotic potentials. Results showed that plants with either of the combined fertilisation treatments NPK or NP grew faster at a given level of drought stress than those with no fertilisation, N, P or K applied individually or the combined nutrient treatments PK and NK. Among the fertiliser applications with either a single or two combined nutrients, plants treated with any of N, P or NP grew faster than those with either K or NK. The association between the interactive effects of nutrients and drought stress on the osmotic adjustment and turgor maintenance in roots may partially explain the role of nutrients in drought tolerance of maize seedlings. In particular, the roots exhibited a higher osmotic adjustment than the leaves for all nutrient treatments, suggesting that shoot growth shows a higher sensitivity to water deficit compared to root growth. We conclude that the maintained turgor of roots under drought stress obtained with an optimal nutrient supply results in better root growth and apparently promotes overall plant growth, suggesting that osmotic adjustment is an adaptation not only for surviving stress, but also for growth under such conditions.

A novel interaction between nutrients and grazers alters relative dominance of marine habitats

Strong top-down control by grazers is considered a moderating influence on the nega- tive effects of elevated nutrients on marine algae. Little experimental work has been done in a sys- tem that has weak grazing pressure (weak top-down control), which is also subjected to elevated nutrients. We experimentally elevated nutrient concentration to test (1) the effects of nutrient enrich- ment on algal assemblages in the presence and absence of canopies (Ecklonia radiata) and (2) the interactive effects of nutrients and molluscan grazers (meso-grazers) on algal assemblages in the absence of canopies. We established that the loss of canopy-forming algae is likely to be a key pre- cursor to nutrient-driven changes of assemblages of benthic algae, because nutrients had no effects on algal assemblages in the presence of canopy-forming algae. In the absence of canopy-forming algae, space was monopolised by filamentous, turf-forming algae, and it was only in the presence of grazers that nutrients caused a change to the relative covers of algal habitat that monopolise canopy- free space. When grazers were present at natural densities, elevated nutrients reduced the monopoly of turf-forming algae in favour of foliose algae. These results demonstrate a novel interaction between nutrients (bottom-up control) and grazing pressure (top-down control), which are funda- mental to predictions about management of human activities that continue to reduce densities of herbivores and increase nutrient availability on temperate coasts.

Recent Trends in Inter-Relationship of Nutrients with Various Agronomic Practices of Field Crops in India

ABSTRACT The spectacular performance of Indian agriculture witnessed during the second half of the 20th century against the onslaught of rising demographic pressure is a vivid demonstration of the growing effectiveness of our agricultural research and development system. However, the issue of sustainability in agriculture still remains a question. Management of natural resources, especially soil and its fertility status and vegetation following suitable agronomic management practices for sustainable agricultural production, represents a most daunting task and is certainly going to receive much attention in the years to come. Many workers established functional relationships between nutrients and other agronomic factors at different locations in India but systematic compilation of those relationships is a rare effort. The present paper reviews the latest information on inter-relationships between nutrients and other agronomic factors (seed rate, planting density, age of the seedling, planting method, irrigation, weed control, tillage, bacterial inoculation and cultivar etc.) on field crops. Major findings of first and second order interactions are highlighted. None of the second order interaction was found significant; thus only first order interactions were discussed. The interactive effect of nutrients and seed rate varied with soil type. In sandy loam soil when 125 kg seed ha−1 was used, wheat responded up to 60:30:40 kg N, P2O5 and K2O ha−1, whereas in clay loam soil it was up to 80:40:30 kg N, P2O5 and K2O ha−1 with the use of 25 kg higher seed rate. When chickpea was grown with a medium seed rate of 75 kg ha−1 it responded up to 30 kg N and 60 kg P2O5 ha−1 saving 10 kg N and 20 kg P2O5 in comparison to when a higher seed rate of 100 kg was used. Younger seedlings (20–25 days) performed better than old seedlings (35–40 days) in timely sown rice with the recommended dose of N, P2O5 and K2O (120:60:40). However, the adverse effect of late transplanted rice with aged seedling (35 days) on yield was nullified by using higher number of seedling/hill (4–5) and using higher dose of fertilizer (150:75:75 kg N, P2O5 and K2O ha−1). French bean, a negligible nodule-bearing legume responded to higher dose of N (160 kg) even under medium plant density (30 cm × 10 cm) in the tarai belt of North India. In general, interaction between cultivar and nitrogen level was non-significant in cereal but significant in oilseeds. The optimum dose of N for rapeseed cv. TH-68-8 and cv. TCH-2 was calculated to be66.0 kg ha−1, whereas it was 90kg N ha−1 for cv. RH-30. Soil moisture influenced the response of crops to fertilizer and vice-versa. Mustard responded up to 40 kg N ha−1 under residual soil moisture and up to 60 kg N ha−1 under limited irrigation; sunflower up to 120 kg N ha−1 when irrigation was applied at 0.4 irrigation water/cumulative pan evaporation (IW/CPE) ratio. However, at 0.6 IW/CPE ratio the crop responded up to 80 kg N. Significant interaction of nitrogen and weed control on grain yield of wheat was observed. Inter estingly, isoproturon, an urea group herbicide, along with 75 kg N ha−1 gave significantly higher grain yield than with 100 and 125 kg N ha−1 indicating a saving of 25–50 kg fertilizer N. Mustard inoculated with Azotobacter or Azospirillum, and receiving moderate level of fertilizer N (30 kg ha−1) gave similar grain yield to the uninoculated crop receiving higher dose of N (60 kg ha−1).

Interactive effects of nutrients and contaminants from pulp mill effluents on riverine benthos

Bleached kraft mill effluents (BKME) contain both nutrients(nitrogen and phosphorus) and contaminants that can have a stimulatoryor inhibitory effect on riverine food webs. For the Northern RiverBasins Study (NRBS), separation of the effects of nutrients andcontaminants in BKME was an important issue because field biomonitoringwas unable to isolate these impacts. We addressed this problem by usingriverside mesocosm experiments, along with field observations, todetermine the effects of BKME on benthic food webs of the upperAthabasca River near Hinton, Alberta, Canada. Response variablesincluded periphyton biomass, benthic insect density and composition, andthe biomass of abundant invertebrates (i.e., caddisfly, mayfly andstonefly taxa). Experiments were conducted during autumn when nutrientenhancement was hypothesized to increase autochthonous primaryproduction as a result of low river discharge and high water clarity.Treatments included reference river water, 1% BKMEconcentrations, and nutrients (nitrogen plus phosphorus) equivalent tothe 1% BKME treatment. Although pulp mill effluents can inducesublethal toxicity in benthic biota, BKME concentrations equivalent tolevels in the Athabasca River did not cause measurable toxicity.Relative to reference water treatments, BKME-associated increases inavailable phosphorus produced enrichment effects including increases inperiphyton and insect biomass, and invertebrate abundance. Insect familyrichness was not affected by the treatments. The development andapplication of a field-based artificial stream system for the NRBSimproved our mechanistic understanding of the effects of BKME on benthicbiota of nutrient and contaminant stressors. By combining artificialstream results with field observations, we were also able to link thismechanistic understanding of stressor effects directly to impacts in theriver. We conclude that the response to BKME in the Peace-AthabascaBasin is largely one of nutrient enrichment rather than that oftoxicity.