The impact of variable stoichiometry on predator-prey interactions: a multinutrient approach

A model for prey and predators is formulated in which three essential nutrients can limit growth of both populations. Prey take up dissolved nutrients, while predators ingest prey, assimilate a fraction of ingested nutrients that depends on their current nutrient status, and recycle the balance. Although individuals are modeled as identical within populations, amounts of nutrients within individuals vary over time in both populations, with reproductive rates increasing with these amounts. Equilibria and their stability depend on nutrient supply conditions. When nutrient supply increases, unusual results can occur, such as a decrease of prey density. This phenomenon occurs if, with increasing nutrient, predators sequester rather than recycle nutrients. Furthermore, despite use of a linear functional response for predators, high nutrient supply can destabilize equilibria. Responses to nutrient supply depend on the balance between assimilation and recycling of nutrients by predators, which differs depending on the identity of the limiting nutrient. Applied to microbial ecosystems, the model predicts that the efficiency of organic carbon mineralization is reduced when supply of mineral nutrients is low and when equilibria are unstable. The extent to which predators recycle or sequester limiting nutrients for their prey is of critical importance for the stability of predator-prey systems and their response to enrichment.     

Symbiotic soil fungi enhance ecosystem resilience to climate change

Substantial amounts of nutrients are lost from soils through leaching. These losses can be environmentally damaging, causing groundwater eutrophication and also comprise an economic burden in terms of lost agricultural production. More intense precipitation events caused by climate change will likely aggravate this problem. So far it is unresolved to which extent soil biota can make ecosystems more resilient to climate change and reduce nutrient leaching losses when rainfall intensity increases. In this study, we focused on arbuscular mycorrhizal (AM) fungi, common soil fungi that form symbiotic associations with most land plants and which increase plant nutrient uptake. We hypothesized that AM fungi mitigate nutrient losses following intensive precipitation events (higher amount of precipitation and rain events frequency). To test this, we manipulated the presence of AM fungi in model grassland communities subjected to two rainfall scenarios: moderate and high rainfall intensity. The total amount of nutrients lost through leaching increased substantially with higher rainfall intensity. The presence of AM fungi reduced phosphorus losses by 50% under both rainfall scenarios and nitrogen losses by 40% under high rainfall intensity. Thus, the presence of AM fungi enhanced the nutrient interception ability of soils, and AM fungi reduced the nutrient leaching risk when rainfall intensity increases. These findings are especially relevant in areas with high rainfall intensity (e.g., such as the tropics) and for ecosystems that will experience increased rainfall due to climate change. Overall, this work demonstrates that soil biota such as AM fungi can enhance ecosystem resilience and reduce the negative impact of increased precipitation on nutrient losses.     

Vernal nitrogen and phosphorus retention by forest understory vegetation and soil microbes

Northern hardwood forests experience annual maximal loss of nutrients during spring. The vernal dam hypothesis predicts that spring ephemeral herbs in northern hardwood forests serve as sinks for nutrients during this season and reduce the loss of nutrients from the terrestrial ecosystem. Soil microbes of northern hardwood forests also sequester nutrients during spring. We compared the vernal nutrient acquisition ability of a soil microbial community and an understory plant community with species of mixed leaf phenology. We monitored nitrogen and phosphorus pool sizes in understory vegetation and soil microbes during spring from 1999 through 2001 in a northern hardwood forest in the Catskill Mountains, New York. Vegetation nutrient content increased during two spring seasons by an average of 3.07 g N m^–2 and 0.19 g P m^–2 and decreased during one spring by 0.81 g N m^–2 and 0.10 g P m^–2. Evergreen, wintergreen, and deciduous plant species were able to sequester nutrients during spring. Soil microbial nutrient content decreased during one spring by 1.29 g N m^–2 and remained constant during the other two springs. Streamwater nitrogen losses were not correlated with biotic nutrient uptake suggesting a temporal disconnect between the two processes. We conclude that understory vegetation is a larger potential sink for vernal nutrients than are soil microbes in this northern hardwood forest and understory and species representing multiple phenologies are capable of vernal nutrient uptake.     

Effects of delayed hay removal on the nutrient balance of roadside plant communities

1. Mass losses and nutrient losses from fresh roadside cuttings were studied in the field during a 6-week period. Large amounts (over 50%) appear to be lost from the cuttings. The losses were positively related to initial nutrient concentrations during this short-term study. Mass and nitrogen losses were best explained by the initial C:N ratio, phosphorus and potassium losses by the initial phosphorus concentration.
2. For potassium the losses were particularly large (up to 90%). For this element only, the observed relationship between loss rate and initial chemical composition could not be established significantly. It is concluded that potassium is mainly lost by leaching whereas the major nitrogen, phosphorus and mass losses are most probably caused by rapid microbial decomposition of readily soluble substances.
3. Using existing data on chemical composition of other roadside cuttings, nutrient losses after different hay removal delay times were modelled for different plant communities. All or most of the losses were assumed to return to the soil system.
4. When soil impoverishment is aimed for, cuttings should be removed within 1 or 2 weeks in most plant communities. If removal is delayed longer, the amounts of nutrients removed will often fall below the annual atmospheric input. In plant communities where annual above-ground production of nitrogen and phosphorus are lower than the annual atmospheric deposition already, rapid removal of the cuttings may be the only way to maintain at least potassium at a limiting level.
5. The main effect of hay-making on the soil nutrient status most likely consists of a reduction of the potassium availability, at least on sandy soils with a low cation exchange capacity and provided there is little delay in hay removal.     

Nutrient changes in surface soils after an intense fire in jarrah (Eucalyptus marginata Donn ex Sm.) forest

The effects of an intense fire on the nutrient status of jarrah(Eucalyptus marginata Donn ex Sm.) forest soils were investigated. At seven experimental sites, representing a range of soil and vegetation types, surface soils were sampled to a maximum depth of 20 cm before, immediately after, and 1 year after the fire. Immediately after the fire, concentrations of total and extracted nutrients in surface (0–3 cm) soils increased markedly. One year later, concentrations of extracted nutrients in surface soils had returned to levels close to pre-burn concentrations. During this period, some properties of sub-surface (3–20 cm) soils also changed: Bray-extractable phosphorus and electrolytic conductivity decreased, whereas ex-tractable potassium and pH increased relative to pre-burn concentrations. Initial increases in concentrations of extractable forms of nutrients were determined largely by the amounts of nutrients deposited in fine ash, whereas the changes after 1 year depended more on the leaching of ions and their reactions with the mineral soil. Increases in total amounts of major nutrients in surface soils arising from additions of fine ash were generally well correlated with amounts in pre-burn litter and with amounts in the components of organic matter likely to be consumed in the fire. Significant quantities of all nutrients are redistributed from aboveground sources to the soil surface during intense fires, including nutrients which are most susceptible to losses through volatilization. This redistribution of nutrients is discussed in relation to short-term and long-term effects on plant growth and functioning of the jarrah ecosystem.     

Changes in carbon and nitrogen fractions of graminaceous straws after burning

Summary Changes in the carbon (C) and nitrogen (N) fractions of graminaceous straws were measured after their combustion under both laboratory and open-air conditions. Volatilization of C and N increased with increasing combustion, and losses exceeded 90% when combustion was nearly complete. The small proportion of total N already present in the inorganic form in straw was not increased during the ashing process.Burning of plant residues decreases the amount of organic microbial substrates returned to the soil, and hence directly affects soil biological processes. In addition, the volatile loss of nutrients occurring during the burning of vegetation must be considered in the long-term nutrient economy of ecosystems, and hence in the full evaluation of fire as a management tool.     

Nutrition and growth of birch and grey alder seedlings in low conductivity solutions and at varied relative rates of nutrient addition

Birch (Betula verrucosa Ehrh.) and grey alder (Alnus incana Moench) seedlings were grown with varied relative addition rates of all nutrients, up to optimum for vegetative growth. The root medium was basically distilled water to which the nutrients, contained in stock solutions in fixed proportions, were added every second hour and in exponentially increased amounts for consumption during the subsequent period. The nutrient weight proportions previously found to be required in birch (100 N:65 K:13 P) were used in all treatments. However, the nutrient proportions required in grey alder were found to be somewhat different (100 N:50 K:18 P). The use of the required proportions in the additions was important for maintenance of maximum growth, efficient nutrient utilization, and low concentrations in the root medium. Luxury consumption and inefficiency occurred at high concentrations. The results show that the nutrient requirements are sufficiently defined, for different relative growth rates, by the nutrient proportions and the relative addition rate. No clear relationships were found between conductivity or concentration in the root medium and the addition rate, net uptake rate, nutrient status, or relative growth rate. The results are in good agreement with data from low concentration and depletion experiments reported in the literature, showing that non-limited uptake rates occur down to very low concentrations. Thus, there is strong evidence that concentration has been incorrectly used when applied as the treatment variable for plant nutrition in plant science and cultivation practice. The dominant factors in sub-optimum and optimum nutrition are the amounts of nutrients available per unit of time, the growth rate, and the nutrient proportions. At low concentration levels, physical factors such as stirring and flow rate of nutrient solution and boundary layer effects are decisive for the rates with which the nutrients become available to the roots. Therefore, at low levels, concentration alone cannot be used as the factor determining nutrient uptake rate. At high levels, concentration is effective as a supra-optimum factor and increased internal percentage contents cause decreased uptake efficiency, thus counter-acting the concentration effect. Nitrogen effects dominated the stress indications when the internal nitrogen percentage content decreased from optimum to the level of the treatments in the beginning of the experiments. Leaf deficiency symptoms disappeared and the root/shoot ratio change ceased when nitrogen status stabilized. Strong linear regressions were found between any two of the variables: relative addition rate of nutrients, relative growth rate, and nutrient status.     

The effects of slash burning on ecosystem nutrients during the land preparation phase of shifting cultivation

The most commonly observed change in soil following slash-and-burn clearing of tropical forest is a short-term increase in nutrient availability. Studies of shifting cultivation commonly cite the incorporation of nutrient-rich ash from consumed aboveground biomass into soil as the reason for this change. The effects of soil heating on nutrient availability have been examined only rarely in field studies of slash-and-burn, and soil heating as a mechanism of nutrient release is most often assumed to be of minor importance in the field. Few budgets for above and belowground nutrient flux have been developed in the tropics, and a survey of results from field and laboratory studies indicates that soils are sufficiently heated during most slash-and-burn events, particularly in dry and monsoonal climates, to cause significant, even substantial release of nutrients from non-plant-available into plant-available forms in soil. Conversely, large aboveground losses of nutrients during and after burning often result in low quantities of nutrients that are released to soil. Assessing the biophysical sustainability of an agricultural practice requires detailed information about nutrient flux and loss incurred during management. To this end, current conceptual models of shifting cultivation should be revised to more accurately describe these fluxes and losses. This revised version was published online in June 2006 with corrections to the Cover Date.     

Biogeographic patterns of nutrient resorption from Quercus variabilis Blume leaves across China

The variation in nutrient resorption has been studied at different taxonomic levels and geographic ranges. However, the variable traits of nutrient resorption at the individual species level across its distribution are poorly understood. We examined the variability and environmental controls of leaf nutrient resorption of Quercus variabilis, a widely distributed species of important ecological and economic value in China. The mean resorption efficiency was highest for phosphorus (P), followed by potassium (K), nitrogen (N), sulphur (S), magnesium (Mg) and carbon (C). Resorption efficiencies and proficiencies were strongly affected by climate and respective nutrients concentrations in soils and green leaves, but had little association with leaf mass per area. Climate factors, especially growing season length, were dominant drivers of nutrient resorption efficiencies, except for C, which was strongly related to green leaf C status. In contrast, green leaf nutritional status was the primary controlling factor of leaf nutrient proficiencies, except for C. Resorption efficiencies of N, P, K and S increased significantly with latitude, and were negatively related to growing season length and mean annual temperature. In turn, N, P, K and S in senesced leaves decreased with latitude, likely due to their efficient resorption response to variation in climate, but increased for Mg and did not change for C. Our results indicate that the nutrient resorption efficiency and proficiency of Q. variabilis differed strongly among nutrients, as well as growing environments. Our findings provide important insights into understanding the nutrient conservation strategy at the individual species level and its possible influence on nutrient cycling.     

Parallel Exploitation of Diverse Host Nutrients Enhances Salmonella Virulence

Pathogen access to host nutrients in infected tissues is fundamental for pathogen growth and virulence, disease progression, and infection control. However, our understanding of this crucial process is still rather limited because of experimental and conceptual challenges. Here, we used proteomics, microbial genetics, competitive infections, and computational approaches to obtain a comprehensive overview of Salmonella nutrition and growth in a mouse typhoid fever model. The data revealed that Salmonella accessed an unexpectedly diverse set of at least 31 different host nutrients in infected tissues but the individual nutrients were available in only scarce amounts. Salmonella adapted to this situation by expressing versatile catabolic pathways to simultaneously exploit multiple host nutrients. A genome-scale computational model of Salmonella in vivo metabolism based on these data was fully consistent with independent large-scale experimental data on Salmonella enzyme quantities, and correctly predicted 92% of 738 reported experimental mutant virulence phenotypes, suggesting that our analysis provided a comprehensive overview of host nutrient supply, Salmonella metabolism, and Salmonella growth during infection. Comparison of metabolic networks of other pathogens suggested that complex host/pathogen nutritional interfaces are a common feature underlying many infectious diseases.     

Vertically transmitted fungal endophytes: different responses of host-parasite systems to environmental conditions

The relationship between vertically transmitted asexual fungal grass endophytes and their hosts is considered to be mutualistic. Results from agronomic field support this line of reasoning but recent studies have shown more variable results in natural systems. We investigated how high and low nutrient and water treatments affected biomass allocation patterns of endophyte‐infected and uninfected Festuca pratensis and F. rubra in greenhouse experiments over two growing seasons. Irrespective of infection status, both grass species showed improved performance on highly fertilized and watered soils. However, infected F. pratensis plants produced larger tillers than endophyte‐free plants on soil low in nutrients and water in the first growing season, although they (E+) otherwise showed decreased performance on nutrient‐poor soil. In low nutrient and water conditions, endophyte‐infected plants produced less tillers and had lower total biomass compared to uninfected plants, and displayed a negative phenotypic correlation between seed production and vegetative growth. The latter indicates costs of reproduction when the plant shares common resources with the fungal endophyte. However, endophyte infection status (E+, E?) interacted significantly with the soil fertilisation in terms of plant growth, having a stronger positive effect on growth in infected F. pratensis plants. In F. rubra, endophyte‐infected plants showed higher vegetative growth in fertilized and watered soils compared to uninfected plants. However, infected plants tended to produce fewer inflorescences. This had no effect on seed production, perhaps because seed production was partly replaced by asexual pseudovivipary. Contrary to the general assumption in the literature that fungal endophytes are plant mutualists, these findings suggest that the costs of endophytes may outweigh their benefits in resource limited conditions. However, the costs of endophyte infections appear to differ among the grass species studied; costs of endophytes were mainly detected in F. pratensis under low nutrient conditions. We propose that differences in response to endophyte infection in these species may depend on the differences in life‐history strategies and environmental requirements of these two fescue and fungal species and may change during the life span of the plant.     

Promotion of harmful algal blooms by zooplankton predatory activity

The relationship between algae and their zooplanktonic predators typically involves consumption of nutrients by algae, grazing of the algae by zooplankton which in turn enhances predator biomass, controls algal growth and regenerates nutrients. Eutrophication raises nutrient levels, but does not simply increase normal predator-prey activity; rather, harmful algal bloom (HAB) events develop often with serious ecological and aesthetic implications. Generally, HAB species are outwardly poor competitors for nutrients, while their development of grazing deterrents during nutrient stress ostensibly occurs too late, after the nutrients have largely been consumed already by fast-growing non-HAB species. A new mechanism is presented to explain HAB dynamics under these circumstances. Using a multi-nutrient predator-prey model, it is demonstrated that these blooms can develop through the self-propagating failure of normal predator-prey activity, resulting in the transfer of nutrients into HAB growth at the expense of competing algal species. Rate limitation of this transfer provides a continual level of nutrient stress that results in HAB species exhibiting grazing deterrents protecting them from top-down control. This process is self-stabilizing as long as nutrient demand exceeds supply, maintaining the unpalatable status of HABs; such events are most likely under eutrophic conditions with skewed nutrient ratios.     

EFFECTS OF APEX REMOVAL AND NUTRIENT SUPPLEMENTATION ON BRANCHING AND SEED PRODUCTION IN THLASPI ARVENSE (BRASSICACEAE)

The shoot apex of a plant may be damaged by herbivory, weather conditions, or other factors, leading to growth of some axillary buds into branches. This alteration of branching pattern can affect the location and extent of seed production. I examined the effects of removal of the shoot apex and of mineral nutrient addition on branching and seed production in Thlaspi arvense. Individual plants received no, early, or late apex removal and no, early, or late nutrient addition in a two-way factorial experiment. Apex removal led to greater production of secondary branches. Total seed weight per plant was higher for early- than late-removal plants under all nutrient treatments, but intact controls produced the highest total weight of seeds in two of three nutrient treatments, because their seeds were heavier. Addition of nutrients increased the numbers of secondary branches, fruits, and seeds, and the average and total weight of seeds produced. Apex removal resulted in increased seed production only when it occurred early and when nutrients were added at the same time. Resource availability and plant phenology can be influential in determining the effects of apical damage on seed production, and their effects in combination may differ from their individual effects.     

Competitive interactions between Nardus stricta L. and Calluna vulgaris (L.) Hull: the effect of fertilizer and defoliation on above- and below-ground performance

1 The role of nutrient supply and defoliation on the competitive interactions between pot-grown Calluna vulgaris and Nardus stricta plants was investigated.WP leading adjustment
2 Young plants were grown alone and together in pots under a combination of fertilizer and defoliation treatments. After 18 months, parameters reflecting both above- and below-ground performance were measured, namely: total above-ground biomass, shoot nitrogen and phosphorus content, root length and the extent of mycorrhizal infection of the roots.
3 In the pots that received fertilizer, the shoot nutrient content and above-ground biomass of Nardus plants increased to a greater extent than those of Calluna plants; this effect was more marked for Nardus plants growing with Calluna plants than for those growing with other Nardus plants. In contrast , Calluna plants growing in competition with Nardus failed to respond to the addition of nutrients. However, in unfertilized pots, Calluna gained more above-ground biomass during the experimental period than Nardus.
4 Calluna had greater root length than Nardus , but Nardus had a higher proportion of its root length infected by mycorrhizal fungi. In both plants, the addition of fertilizer reduced the mycorrhizal infection and increased the root length. Nardus root length was decreased when grown in competition with Calluna only in pots where no nutrients were added. Defoliation decreased the extent of mycorrhizal infection in Calluna roots but not in those of Nardus; defoliation decreased the shoot nutrient content in Calluna plants, but not in Nardus plants.
5 These results suggest that the competitive balance between Nardus and Calluna may be altered by the addition of nutrients, and by defoliation, which may have serious implications for the future dominance of Calluna in heathland ecosystems, particularly those where nutrient inputs are increasing significantly or where grazing pressures are high.     

Foliar demand and resource economy of nutrients in dry tropical forest species

Important phenological activities in seasonally dry tropical forest species occur within the hot‐dry period when soil water is limiting, while the subsequent wet period is utilized for carbon accumulation. Leaf emergence and leaf area expansion in most of these tree species precedes the rainy season when the weather is very dry and hot and the soil cannot support nutrient uptake by the plants. The nutrient requirement for leaf expansion during the dry summer period, however, is substantial in these species. We tested the hypothesis that the nutrients withdrawn from the senescing leaves support the emergence and expansion of leaves in dry tropical woody species to a significant extent. We examined the leaf traits (with parameters such as leaf life span, leaf nutrient content and retranslocation of nutrients during senescence) in eight selected tree species in northern India. The concentrations of N, P and K declined in the senescing foliage while those of Na and Ca increased. Time series observations on foliar nutrients indicated a substantial amount of nutrient resorption before senescence and a ‘tight nutrient budgeting’. The resorbed N‐mass could potentially support 50 to 100% and 46 to 80% of the leaf growth in terms of area and weight, respectively, across the eight species studied. Corresponding values for P were 29 to 100% and 20 to 91%, for K 29 to 100% and 20 to 57%, for Na 3 to 100% and 1 to 54%, and for Ca 0 to 32% and 0 to 30%. The species differed significantly with respect to their efficiency in nutrient resorption. Such interspecific differences in leaf nutrient economy enhance the conservative utilization of soil nutrients by the dry forest community. This reflects an adaptational strategy of the species growing on seasonally dry, nutrient‐poor soils as they tend to depend more or less on efficient internal cycling and, thus, utilize the retranslocated nutrients for the production of new foliage biomass in summer when the availability of soil moisture and nutrients is severely limited.     

Dietary diversity, child nutrition and health in contemporary African communities

Many infants in sub-Saharan Africa (SSA) begin to receive cereal-based supplemental feeds well before the age (6 months) recommended for the introduction of 'safe and nutritionally adequate' complementary foods, or in rarer instances, do not receive these until the second year. The diets offered are monotonous and bulky, and rarely cover the shortfall left by breast milk in providing the energy and nutrients required to support rapid growth, build nutrient stores and assure resistance to infection. The pattern of growth and prevalence of malnutrition observed from birth through the first 5 years in SSA are suggestive of the nutrient inadequacies of the diet and the experience of infection. However, it is difficult to link poor growth and specific nutrient deficiencies in epidemiological studies because multiple nutrients are required for growth and deficiencies usually involve several nutrients. Moreover, accurate measurement of nutrient intakes is no small challenge. In this regard, qualitative and easier-to-measure characteristics of diet which are associated with nutrient adequacy could serve as alternative determinant factors in studies looking at causes of malnutrition. Dietary diversity is proposed as a candidate indicator of food security and predictor of nutritional status, but there is need for further research to standardize definitions and methodology before it can be applied widely.     

A Definition of Optimum Nutrient Requirements in Birch Seedlings. I.

The aim of the investigation was to find a method of growing birch seedlings with a constant optimum nutrient status. The studies in this paper have indicated important factors in such a growth method which must be comprised in a definition of nutrient requirements. The optimum nutrient proportions in the seedlings and the optimum total concentration in the solution must be known. Special attention must be paid to the ratio between NH₄⁺ and NO₃^?in the solution and in the uptake. The amounts of nutrients taken up at optimum nutrition can be followed by measurement of pH and conductivity. Thus, the nutrients consumed may be replaced by titrations which may be automated by standard equipment. The nutrient proportions to be added are determined by the optimum internal proportions. It is not necessary to change the nutrient solution even if the volume is relatively small in relation to plant mass. There was no evidence of effects of accumulation of root exudates or infections. By using the system described, it is possible to attain considerably greater fresh and dry matter production in birch seedlings than when an optimum conventional solution is used.     

Cytoplasmic conductivity as a marker for bioprocess monitoring: Study of Chinese hamster ovary cells under nutrient deprivation and reintroduction

The ability to monitor the status of cells during nutrient limitation is important for optimizing bioprocess growth conditions in batch and fed-batch cultures. The activity level of Na⁺/K⁺ ATPase pumps and cytoplasm ionic concentrations are directly influenced by the nutrient level, and thus, cytoplasm conductivity can be used as a markerless indicator of cell status. In this work, we monitored the change in cytoplasm conductivity of Chinese hamster ovary (CHO) cells during nutrient deprivation and reintroduction. Employing single cell dielectrophoresis, the change in cytoplasm conductivity was measured over a 48-hr period. The conditions under which the cytoplasm conductivity would recover to a normal level after nutrient reintroduction was determined. In addition, numerical simulations of cell ion flux, for different levels of Na⁺/K⁺ ATPase pump inhibition, were used to predict the minimum conductivity expected for nutrient-deprived CHO cells. This predicted value is close to the minimum observed experimental cytoplasm conductivity for CHO cells that maintain the ability to restore the cytoplasm conductivity to the normal viable levels when nutrients are reintroduced. The recovery of starved cells was verified by reintroducing them to nutrient for 36 hr and measuring their proliferation using trypan blue exclusion assay. We conclude that cytoplasm conductivity can be used as a marker to indicate whether cells are in a recoverable state, such that the reintroduction of nutrients results in cells returning to a normal healthy state.     

Complexity in the eutrophication–harmful algal bloom relationship,with comment on the importance of grazing

This article seeks to guide the conceptual development and field application of the eutrophication–HAB hypothesis. After considering the evidence for this hypothesis, the importance of exogenous nutrients as a chemical habitat conditioner and the “family” of nutrient regulated effects that occur are discussed. The various definitions of eutrophication are applied; the conceptual ambiguity over how to perceive eutrophication, and the need to view eutrophication as a process and not as an ecological state are considered. The habitat irradiance-nutrient-flushing gradient regulates the bloom potential in response to exogenous nutrients. There is an apparent species-specific paradox within the eutrophication–HAB hypothesis related to the phycotoxin synthesis–nutrient relationship. Two nutrient–toxin relationships occur: toxin biosynthesis during nutrient sufficiency, and biosynthesis that requires a nutrient limitation. HAB events can be just as much nutrient depletion events as nutrient stimulated events, and whether the former develops depends upon the specific cellular toxicity–nutrient relationship of the bloom species. The importance of grazing in bloom regulation is highlighted. HABs and red tides generally should be viewed as blooms that are regulated by coupled nutrient-grazer processes – nutrient stimulation alone is inadequate, even when exogenous nutrients are not a factor. In assessing the eutrophication–HAB relationship, the collective grazing behavior of the micro-zooplankton, herbivorous copepods, filter feeding benthos, benthic larvae and, when present, omnivorous nekton must be considered. The importance of grazing in the bloom behavior of HAB species is illustrated using field and experimental data during a 5-month brown tide in Narragansett Bay. A cascade in grazing pressure regulated this bloom, with the initial collapse and then restoration of grazing pressure progressing through 7 stages of collective grazing pressure by micro-zooplankton, herbivorous copepods, benthic larvae, benthic filter feeders, and lytic virus infection.