Physical Niche Research Articles

MT1-MMP Regulates Hematopoiesis Through HIF-Mediated Chemo-/Cytokine Release From the Bone Marrow Niche,

Abstract 3409Stem cells reside in a physical niche, a particular microenvironment. The organization of cellular niches has been shown to play a key role in regulating normal stem cell differentiation, stem cell maintenance and regeneration. Various stem cell niches have been shown to be hypoxic, thereby maintaining the stem cell phenotype, e.g. for hematopoietic stem cells (HSCs) or cancer stem cells. The bone marrow (BM) niche is a rich reservoir for tissue-specific pluripotent HSCs. Proteases, such as matrix metalloproteinases (MMPs) can modulate stem cell fate due to their proteolytic or non-proteolytic functions (abilities). We have investigated the role of membrane-type1 matrix metalloproteinase (MT1-MMP), known for its role in pericellular matrix remodeling and cell migration, in hematopoiesis. MT1-MMP is highly expressed in HSCs and stromal cells. In MT1-MMP−/− mice, release of kit ligand (KitL), stromal cell derived factor-1 (SDF-1/CXCL12), erythropoietin (Epo) and interleukin-7 were impaired resulting in erythroid, myeloid and T and B lymphoid differentiation. Addition of exogenous rec. KitL and rec. SDF-1 restored hematopoiesis in vivo and in vitro. Further mechanistic studies revealed that MT1-MMP in a non-proteolytic manner activates the HIF-1 pathway, thereby inducing the transcription of the HIF-responsive genes KitL, SDF-1 and Epo.These results suggested MT1-MMP as a critical regulator of postnatal hematopoiesis, which as a modulator of the HIF pathway alters critical hematopoietic niche factors necessary for terminal differentiation and or migration. Thus, our results indicate that MT1-MMP as a key molecular link between hypoxia and the regulation of vital HSC niche factors. Disclosures:No relevant conflicts of interest to declare.

Community Resources for Learning: How Capuchin Monkeys Construct Technical Traditions

The developmental importance to humans of the human-constructed physical environment, including myriad modified natural objects or manufactured objects, is well recognized. The importance of the physical dimension of the constructed niche has also been recognized in nonhuman animals with respect to dwellings (e.g., beavers’ dams, birds’ nests, and bees’ hives), but has not previously been applied to technical traditions, despite the fact that enduring alterations of the physical environment left by social partners are part of the constructed niche that supports the learning of technical skills through the phenomenon of delayed social facilitation. These alterations aid learning over a longer time scale than the actions themselves. Thus, technical skills that result in enduring physical artifacts, which themselves aid learning the skills, should be both more persistent in a population and more widespread than technical skills that do not share this feature. This perspective gives us a new lens through which to understand the origins of technical traditions in nonhuman animals, and by extension, in human ancestors. Understanding the process by which traditional technical skills are acquired in nonhuman species gives us insight into the ways that the combination of social and physical niche construction can support the evolution of technical aspects of culture from modest beginnings.

Bioturbating space enhances the effects of non‐additive interactions among benthic ecosystem engineers on cross‐habitat nutrient regeneration

Understanding the mechanisms that drive complementary interactions among species is key to the progress of the debate over the role of biodiversity in ecosystem functioning. In addition, interspecific interactions among physical ecosystem engineers have rarely been framed in the context of biodiversity experiments. Here, we provide an empirical test of how the physical niche space of species influences the effects of the biodiversity of bioturbators on cross‐habitat nutrient fluxes in benthic sediments. In the laboratory, we orthogonally manipulated the number and composition of three benthic invertebrate bioturbator species that differ in the dimensions of their bioturbating space niche; i.e. their vertical distribution in the sediment over a gradient of sediment depth and volume. The ammonium (NH4‐N) flux from the sediment to the water was positively related to bioturbator species richness only in the sediments with the deepest depth and greatest volume. The non‐additive effects of bioturbator species richness on the benthic–pelagic NH4‐N flux increased linearly with sediment depth and volume, but only in the three‐species mixtures. Furthermore, no individual species dominated the rates of H4‐N fluxes, indicating that biodiversity effects were mainly driven by complementarity. These results suggest that sediment bioturbating space mediates the magnitude of non‐additive effects among the three invertebrate species and sheds light on the importance of physical niche space in modulating the positive effects of biodiversity on ecosystem functioning.

Zooplankton partitioning in a tropical alkaline–saline endorheic Lake Nakuru, Kenya: Spatial and temporal trends in relation to the environment

Spatial and temporal zooplankton variations were studied for 1 year in tropical alkaline–saline Lake Nakuru to determine how they partition in the habitat, relative to environmental variables. Monthly samples were collected at 10 sampling sites, with subsurface tows, using 33.5-μm mesh plankton nets. Physicochemical parameters displayed clear seasonal variations associated with precipitation patterns. Nine species, belonging to two main zooplankton taxonomic groups (ciliates; rotifers), were identified in the samples. Brachionus dimidiatus dominated the samples, accounting for 80% of the total zooplankton abundance. Kruskall–Wallis tests indicated significant (P < 0.05) temporal and spatial variations among all taxonomic groups. Different zooplankton species displayed a clear succession throughout the year. The total abundance of the rotifers and ciliates peaked at sampling sites near inlets during the long rainy seasons, while those in the inshore sites displayed variable succession patterns. Spatiotemporal structure of the zooplankton assemblages, and its correlation with environmental variables, indicated each species displayed distinct niche-based partitioning. The ciliates niche was associated with increasing soluble reactive phosphorus, total phosphorus and nitrite–nitrogen (NO2–N) concentrations. Niche partitioning in rotifers was associated with nitrate–nitrogen (NO3–N), conductivity and pH. These results indicate physical niche separation, even in a small, relatively homogenous lake among species of rotifers and ciliates, providing information from which future changes in their abundance and spatial distributions can be predicted, given continuous water quality changes.

The Grand Challenges to Cellular and Molecular Oncology

In the last decade, our molecular and cellular understanding of cancer has been tremendously ameliorated. It has been demonstrated that, in some cellular settings, microRNAs (miRNAs) can exert bona fide oncogenic or oncosuppressive functions (Croce, 2009) and that cancer cells often, if not always, exhibit an extensive metabolic rewiring (Kroemer and Pouyssegur, 2008); the compartment of cancer stem cells has been intensely studied (Visvader and Lindeman, 2008); the concepts of oncogene/non-oncogene addiction (Luo et al., 2009) and of immunogenic cell death (Zitvogel et al., 2010) have been founded; and novel modalities of programmed cell death have been characterized (Vandenabeele et al., 2010), just to mention a few examples. These advances in our fundament knowledge of molecular and cellular cancer biology are rapidly being translated into ever more reliable diagnostic and prognostic biomarkers as well as into a large armamentarium of novel therapeutic tools. Personalized anticancer strategies, which at the end of the 1990s held great expectations, have now turned into a clinically exploitable reality. Moreover, there is now an emerging tendency to conceive tumors as micro-ecosystems that are composed by a heterogeneous population of cancer (stem) cells as well as a plethora of distinct stromal cells, including fibroblasts and endothelial cells, which either promote or limit tumor growth at the metabolic, architectonic, trophic, and immunological levels. New knowledge generates new questions, and there also remain several old unresolved issues. For instance, do cancer stem cells occupy a physical niche within solid tumors of do they represent a functional compartment only? How can we exploit cancer stem cell biology to increase their sensitivity to therapy? How can we induce the immunogenic demise of tumor cells and hence circumvent their resistance to conventional anticancer regimens? What components of the molecular machinery for necroptosis might be exploited for the generation of novel cytotoxic agents that do not operate via apoptosis? How can we target the tumor-stroma for developing safe and efficient anticancer strategies? Will nanoparticles and other molecular targeting devices progress until clinical applications? These are only some of the great questions that will drive the work of us, as cellular and molecular oncologists, for the next few years. We must concentrate our efforts to reach an ever more precise characterization of the mechanisms that underlie the origin, survival, and therapeutic response of cancer. It can be anticipated that this will lead to the development of new, efficient approaches for the treatment of this dreadful disease.

Potential mechanisms for achieving agricultural benefits from biochar application to temperate soils: a review

Natural organic biomass burning creates black carbon which forms a considerable proportion of the soil’s organic carbon. Due to black carbon’s aromatic structure it is recalcitrant and has the potential for long-term carbon sequestration in soil. Soils within the Amazon-basin contain numerous sites where the ‘dark earth of the Indians’ (Terra preta de Indio, or Amazonian Dark Earths (ADE)) exist and are composed of variable quantities of highly stable organic black carbon waste (‘biochar’). The apparent high agronomic fertility of these sites, relative to tropical soils in general, has attracted interest. Biochars can be produced by ‘baking’ organic matter under low oxygen (‘pyrolysis’). The quantities of key mineral elements within these biochars can be directly related to the levels of these components in the feedstock prior to burning. Their incorporation in soils influences soil structure, texture, porosity, particle size distribution and density. The molecular structure of biochars shows a high degree of chemical and microbial stability. A key physical feature of most biochars is their highly porous structure and large surface area. This structure can provide refugia for beneficial soil micro-organisms such as mycorrhizae and bacteria, and influences the binding of important nutritive cations and anions. This binding can enhance the availability of macro-nutrients such as N and P. Other biochar soil changes include alkalisation of soil pH and increases in electrical conductivity (EC) and cation exchange capacity (CEC). Ammonium leaching has been shown to be reduced, along with N2O soil emissions. There may also be reductions in soil mechanical impedance. Terra preta soils contain a higher number of ‘operational taxonomic units’ and have highly distinctive microbial communities relative to neighbouring soils. The potential importance of biochar soil incorporation on mycorrhizal fungi has also been noted with biochar providing a physical niche devoid of fungal grazers. Improvements in soil field capacity have been recorded upon biochar additions. Evidence shows that bioavailability and plant uptake of key nutrients increases in response to biochar application, particularly when in the presence of added nutrients. Depending on the quantity of biochar added to soil significant improvements in plant productivity have been achieved, but these reports derive predominantly from studies in the tropics. As yet there is limited critical analysis of possible agricultural impacts of biochar application in temperate regions, nor on the likelihood of utilising such soils as long-term sites for carbon sequestration. This review aims to determine the extent to which inferences of experience mostly from tropical regions could be extrapolated to temperate soils and to suggest areas requiring study.

A Eucrustacean Metanauplius from the Lower Cambrian

A new eucrustacean arthropod, Wujicaris muelleri gen. et sp. nov, is represented by a Lower Cambrian early metanauplius of strikingly modern morphology despite being the oldest known fossil of such an early immature crustacean larva. The morphology of the metanauplius closely mirrors that of corresponding developmental stages of living barnacles and copepods, and it is likely that its appendages had a similar function for feeding and locomotion. The metanauplius larva demonstrates remarkable stasis in morphology, life history, and lifestyle of (small) eucrustaceans over 525 million years, probably as a result of adaptation to a long-lasting physical niche and regime involving low Reynolds numbers and laminar current flow.

Microbial functional structure of Montastraea faveolata, an important Caribbean reef‐building coral, differs between healthy and yellow‐band diseased colonies

A functional gene array (FGA), GeoChip 2.0, was used to assess the biogeochemical cycling potential of microbial communities associated with healthy and Caribbean yellow band diseased (YBD) Montastraea faveolata. Over 6700 genes were detected, providing evidence that the coral microbiome contains a diverse community of archaea, bacteria and fungi capable of fulfilling numerous functional niches. These included carbon, nitrogen and sulfur cycling, metal homeostasis and resistance, and xenobiotic contaminant degradation. A significant difference in functional structure was found between healthy and YBD M. faveolata colonies and those differences were specific to the physical niche examined. In the surface mucopolysaccharide layer (SML), only two of 31 functional categories investigated, cellulose degradation and nitrification, revealed significant differences, implying a very specific change in microbial functional potential. Coral tissue slurry, on the other hand, revealed significant changes in 10 of the 31 categories, suggesting a more generalized shift in functional potential involving various aspects of nutrient cycling, metal transformations and contaminant degradation. This study is the first broad screening of functional genes in coral-associated microbial communities and provides insights regarding their biogeochemical cycling capacity in healthy and diseased states.

Towards cellular therapy with a spring in your step!

Surprisingly little is known regarding the effects of the physical microenvironmental niche on haemopoietic stem cells. We have explored the effects of matrix elasticity on stem cell properties using a unique synthetic substrate, tropoelastin, which we used to show that both murine and human primitive haemopoietic cells are more efficiently maintained on an elastically extensible substrate than controls. We have shown that culturing primitive murine and human cord blood haemopoietic cells on tropoelastin in vitro led to increases in cell number, clonogenicity, proliferation and engraftment following transplantation. Atomic force measurements of truncations and crosslinked tropoelastin strengthen the hypothesis that stem cells sense the elasticity of their microenvironment. A consequence of this idea is that alterations to niche elasticity in disease states may contribute to abnormal haemopoiesis. Further, elastic substrates such as tropoelastin may offer a new approach to biomaterial design aimed to achieve optimal ex vivo culture conditions for primitive haemopoietic cells. In order to take advantage of this type of observation arising from the basic research laboratory (and many other opportunities in cellular therapies), a basic introduction in the principles and regulatory aspects of good manufacturing practice of cellular therapeutics will be provided.

Temporal and vertical distributions of three appendicularian species (Tunicata) in Conception Bay, Newfoundland

Temporal and vertical distributions of the boreal appendicularian species Oikopleura vanhoeffeni, Fritillaria borealis and O. labradoriensis were observed in Conception Bay, Newfoundland, over a 1-year period in order to determine whether they share the same environmental niche. Total abun- dance of the three species together peaked after the spring maximum of chlorophyll a concentration, indicating that appendicularian abundance was generally related to the biomass of phytoplankton. The seasonal peak abundance of the three species did not overlap in time. The maximum abun- dance of O. vanhoeffeni occurred in May and June during the spring phytoplankton bloom, that of F. borealis occurred in July and August as upper mixed layer temperature increased and salinity decreased and that of O. labradoriensis occurred in October when salinity was at its annual minimum. Vertical distribution also differed among the species. Oikopleura vanhoeffeni occurred primarily below the surface layer and remained below the thermocline in summer. The vertical distribution of F. borealis was not correlated with the observed environmental variables, whereas O. labradoriensis occurred primarily at depths of minimum salinity. Each species displayed a distinct niche based on temperature and salinity. Oikopleura vanhoeffeni was a cryophile with a stenothermal and stenohaline niche, F. borealis was eurythermic and euryhaline and O. labradoriensis inhabited a mesothermal and mesohaline niche. These results indicate physical niche separation among these three sympatric, arcto-boreal appendicularian species and provide information from which future changes in their abundance and spatial distribution can be predicted given likely climatic variation.

A role for niches in hematopoietic cell development

Stem cells reside in a physical niche, a particular microenvironment. The organization of cellular niches has been shown to play a key role in regulating normal stem cell differentiation, maintenance and regeneration. Hematopoietic stem cells (HSC) emerge at distinct allocation territories during ontogenesis, notably the aorto-gonadal region, the fetal liver. Adult HSC expand and differentiate exclusively in the bone marrow (BM). They can be mobilized into the blood stream. This implies that stem cells are not autonomous units of development; rather, tissue specific niches control their destiny. Interaction of HSCs with their stem cell niches is critical for adult hematopoiesis in the BM. A niche is composed of stromal cells, which either through direct cell-to-cell contact or via release of soluble factors maintain the typical features of stem cells, mainly stem cell quiescence, maintenance or expansion. HSCs are keeping the balance of the quiescence and the self-renewal in the stem cell niche, and are maintaining long-term hematopoiesis. Therefore, an understanding of cellular and chemical architecture of the stem cell niche is vital in understanding stem cell behavior. This review summarizes the recent developments in our understanding of the stem cell niche with particular focus on the HSC niche.

Matrix Glycoprotein Osteopontin Is a Stem Cell Niche Constituent That Constrains the Hematopoietic Stem Cell Pool Size.

Stem cells reside in a physical niche where a balance of signals controls their growth, differentiation and death. Niche components have generally been defined in terms of cells and positive effects on stem cell maintenance or expansion. Here we define a role for a matrix glycoprotein that provides a constraining function in the hematopoietic stem cell niche. Osteopontin (OPN) is an abundant glycoprotein in bone that can function as either cytokine or cell adhesion mediator. It is known to be produced by multiple cells types including osteoblasts, cells recently defined to be a regulatory component of the hematopoietic stem cell niche. Using studies combining OPN deficient mice and exogenous OPN, we demonstrate that OPN modifies primitive hematopoietic cell numbers and function. In OPN deficient mice, increased primitive cell numbers were observed in vivo associated with reduced progenitors and reduced primitive cell apoptotic fraction. To determine whether the effect of OPN deficiency was stroma dependent, we performed in vitro stem cell assays on OPN−/− stroma and observed greater LTC-IC supportive capacity compared with wild type stroma. Furthermore, OPN−/− recipients showed a significantly higher proportion of hematopoietic stem cells after transplantation of OPN+/+ bone marrow in comparison to wild-type recipients, indicating that the OPN null microenvironment was sufficient to increase stem cell number. A reduction in apoptotic fraction was seen in primitive cells in the OPN−/− recipient marrows. A role for OPN in apoptosis was confirmed by exogenous OPN in in-vitro studies. Hypothesizing that OPN may serve as a physiologic constraint on stem cell pool size, we compared OPN−/− with wild type animals following parathyroid hormone activation of the stem cell niche. The expansion of stem cells by PTH was superphysiologic in the absence of OPN. Therefore, OPN is a restricting element of the stem cell niche, limiting the number of stem cells produced by niche activation. Extracellular matrix components such as OPN may serve as modulable, regulatory participants in the stem cell niche.

Hydrologic and hydraulic control of macrophyte establishment and performance in streams

Macrophytes play a key role in many unshaded lotic ecosystems, but little is known of the factors controlling their presence, abundance, and composition. Macrophyte abundance, diversity, and composition were studied in 15 New Zealand streams to test the hypotheses that the presence and development of macrophytes in lotic systems is primarily controlled by the hydrologic regime (frequency of high‐velocity flood events) and that the interflood spatial distribution and performance of taxa in more stable systems is strongly influenced by local hydraulic conditions (depth/velocity/sediments). Both hypotheses were supported by our results. We found that the abundance and diversity of macrophytes decreased as flood disturbance frequency increased (r2 = 0.52, P = 0.002 for abundance; r2 = 0.53, P = 0.022 for diversity) and that vegetation was absent in streams with more than ~13 high‐flow disturbances per year. An experiment in an ecohydraulics flume identified that the main mechanism causing these effects was not stem breakage at high water velocity but probably uprooting associated with bed sediment erosion. We found that plants with high propagule production constituted a greater proportion of the vegetation in more flood disturbed streams than in stable streams, suggesting that this species trait is important for the maintenance of macrophyte communities in flood prone streams. Distinct velocity, depth, and substrate particle size habitat preferences were displayed by four common species in the study streams. None of the macrophytes showed overlapping preferences for all three habitat variables, suggesting coexisting of the species in streams by physical niche separation. These results significantly expand our understanding of the role of flow regimes in determining lotic ecosystem structure and functioning.

Regulation of Biocalcification of Bovine Pericardial Tissue b Grafting Poly(Glycidyl Methacrylate-Butylacrylate) Copolymers

Calcification is the principal cause of failure of glutaraldehyde cross-linked bovine pericardium used as cardiac valve substitutes. Young age, glutaraldehyde pretreatment, and mechanical factors are identified as the main causes of biocalcification. Glutaraldehyde pretreatment is believed to create void spaces in the fiber matrix increasing the availability of physical niches for calcification. In the present paper, glutaraldehyde cross-linked pericardial tissue (GCPC) was filled by grafting polymers with different properties which impart flexibility to the pericardium. These polymers included glycidyl methacrylate (GMA) and butylacrylate (BA) initiated by ceric ion technique. Tensile strength, shrinkage temperature, and differential scanning thermogram were recorded for the glutaraldehyde cross-linked tissues before and after polymer grafting. Calcification studies were done subdermally in rat models. The best anticalcification properties were observed in tissues grafted with GMA: BA monomers of mole ratio of 15:7.2 and initiator concentration of 7.5 x 10-3 mol L-1.

Habitat Partitioning among Three Species of Ephemerelloidea

ABSTRACT Niche volumes according to several physical and water quality parameters are shown for Drunella grandis, Drunella doddsi and Tricorythodes minutus. Generalist strategies have been adopted by D. grandis for physical habitat characteristics and by T. minutus for water quality characteristics. Drunella doddsi, a specialist, has the smallest physical habitat and water quality niche dimensions of the three mayfly species. Even though each has its own unique niche, there is some overlap between them for most niche dimensions studied. These overlaps are probably present because density independent seasonal limiting factors in western United States streams are as strong or stronger selectors than many density dependent factors. Also, each species has its own unique niche “hypervolume” that changes from station to station and from time to time.

Environmental Profile ofDrunella grandisEaton (Ephemeroptera: Ephemerellidae) in the Western United States

ABSTRACT In 898 stream stations in 11 western states, Drunella grandis Eaton exhibited broad physical habitat niche dimensions but moderate to narrow water quality niche dimensions. Nymphs were found over a wide range of channel gradients and substrate types, with tolerance to fine substrates as long as there were some rocky substrates. Occurrence was mostly random in relation to riparian vegetation. Nymphs were found in waters with a wide range of alkalinities, but frequency was low when alkalinity exceeded 250 mg/l. Drunella grandis commonly inhabited streams with conductivity over 400 μmhos/cm but was rarely found when it exceeded 800 μmhos/cm. Nymphs were found in higher than expected frequencies when sulfates were less than 50 mg/l, lower than expected when sulfates are 75–250 mg/l, and were not found in waters with sulfates over 250 mg/l. Distribution was near random over all states sampled with occurrence higher than expected at elevations between 6,000 and 8,000 feet.

Microbial activity and survival in soils dried at different rates

The changes in microbial biomass C, soil respiration, microbial activity (respiration/microbial C) and the content of oxidizable organic C extracted by 0-5 M K2SO4, were measured in four soils of contrasting characteristics (a sand, two silt loam soils and a peat) which were air-dried at 22�C at three different rates in the laboratory. Respiration was also measured on samples of the drying soils rewetted with water. The rates of drying were: &lt;10 h (fast), &lt;33 h (medium) and &lt;62 h (slow); drying was carried out for 6 h on consecutive days, with overnight storage. Measurements were also made on soils stored at field-moisture content over the 15 day duration of the experiment. Respiration and activity declined continuously and in a generally linear manner as the volumetric water content (W,) decreased. The decline in respiration in relation to water content W, was similar for all four soils and for the three rates of drying. Microbial biomass C also declined but generally only after a considerable initial period of drying (after the soils had reached Wv of 0-1-0.3). Extractable C values increased, but only after an initial drying period (Wv below 0.06-0.12). The increases in extractable C were approximately coincident with the decreases in microbial C, but only part of the increase in extractable C could be accounted for by the decrease in microbial C. Rewetting of dried soils caused a marked increase in respiration, particularly when the rewetted soils had reached Wv values where extractable C had begun to increase. The relationship between microbial activity and extractable C was similar for all four soils and was not affected by the rate of drying. The similarity of the microbial responses in these contrasting soils, and the absence of any detectable differences between rates of drying suggest that the microbial communities had similar survival strategies to resist desiccation, and occupied comparable physical niches in the soils, despite these soils having widely differing textures, organic matter content, and soil moisture characteristics.

The Search for Brown Dwarfs

We know a lot about stars and planets. We know about stars because they are easy to detect and because we can observe them in the various stages of their life cycles. We know about planets because we live on a planet, because there are nearby planets that can be visited by spacecraft and observed in detail, and because the known planets and other solar system materials preserve some evidence of their history. However, the planets and stars we know about are separated in mass by about two orders of magnitude or more, and the history of astronomy suggests that we should adopt an Ecological Principle: All imaginable physical niches are probably occupied. The objects of intermediate mass, known by various names but here called brown dwarfs (Tarter 1975, 1986) fill this niche. The evidence for their existence is currently rather weak and confusing (Fienberg 1990): New discoveries seem scarcely more frequent than retractions or reinterpretations of old discoveries! From that point of view, this review might be premature by a few years. However, our theoretical understanding is rather well developed and there are enough tantalizing observations that an interim review and perspective seems worthwhile. Certainly, the observational situation is extremely dynamic and the interest level is very high in several parts of the astronomical and planetary communities.

AN ASSESSMENT OF ECOLOGICAL CHANGES DURING THE RECENT HISTORY OF LAKE ONTARIO BASED ON SILICEOUS ALGAL MICROFOSSILS PRESERVED IN THE SEDIMENTS1,2

ABSTRACTQuantitative analysis of siliceous algal microfossils (diatoms and chrysophytes) in a radiometrically dated core of Lake Ontario sediments shows striking modifications of the flora produced and preserved, apparently as a result of anthropogenic modification of nutrient loadings to the lake, Qualitatively similar microfossil assemblages were deposited during the period ca. 1700–1815. A single major event is recorded in assemblages deposited ca. 1769. Between this event horizon and ca. 1815 there was an apparent increase in algal production, but minimal change in species composition. A major disturbance occurred at the level of the Ambrosia horizon, ca. 1831–1847. Assemblages deposited between ca. 1855 and 1900 indicate increasing eutrophication. We infer that silica limitation became important by about 1860 and that the system reached its greatest effective production ca. 1900. Major modification of the flora occurred in the period ca. 1900–1945, with most local extinctions of major indigenous populations occurring ca. 1920–1945. Assemblages deposited in the period ca. 1945–1975 are composed of some stenothermal populations also present in pre‐disturbance assemblages, a large component of benthic and nearshore planktonic species, and a number of apparently planktonic populations rarely recorded from large temperate lakes, particularly species of the genus Nitzschia. Our results indicate that the major factor driving species succession in Lake Ontario was phosphorus enrichment and consequent depletion of available silica supplies. Our results also show the importance of a species' physical niche in determining its survival under these conditions.