Increases in atmospheric temperature and nutrients from land are thought to be promoting the expansion of harmful cyanobacteria in lakes worldwide, yet to date there has been no quantitative synthesis of long-term trends. To test whether cyanobacteria have increased in abundance over the past ~ 200 years and evaluate the relative influence of potential causal mechanisms, we synthesised 108 highly resolved sedimentary time series and 18 decadal-scale monitoring records from north temperate-subarctic lakes. We demonstrate that: (1) cyanobacteria have increased significantly since c. 1800 ce, (2) they have increased disproportionately relative to other phytoplankton, and (3) cyanobacteria increased more rapidly post c. 1945 ce. Variation among lakes in the rates of increase was explained best by nutrient concentration (phosphorus and nitrogen), and temperature was of secondary importance. Although cyanobacterial biomass has declined in some managed lakes with reduced nutrient influx, the larger spatio-temporal scale of sedimentary records show continued increases in cyanobacteria throughout the north temperate-subarctic regions.
The premise of this article is that climate effects on lakes can be quantified most effectively by the integration of process-oriented limnological studies with paleolimnological research, particularly when both disciplines operate within a common conceptual framework. To this end, the energy (E)-mass (m) flux framework (Em flux) is developed and applied to selected retrospective studies to demonstrate that climate variability regulates lake structure and function over diverse temporal and spatial scales through four main pathways: rapid direct transfer of E to the lake surface by irradiance, heat, and wind; slow indirect effects of E via changes in terrestrial development and subsequent m subsidies to lakes; direct influx of m as precipitation, particles, and solutes from the atmosphere; and indirect influx of water, suspended particles, and dissolved substances from the catchment. Sedimentary analyses are used to illustrate the unique effects of each pathway on lakes but suggest that interactions among mechanisms are complex and depend on the landscape position of lakes, catchment characteristics, the range of temporal variation of individual pathways, ontogenetic changes in lake basins, and the selective effects of humans on m transfers. In particular, preliminary synthesis suggests that m influx can overwhelm the direct effects of E transfer to lakes, especially when anthropogenic activities alter m subsidies from catchments.The structure and function of lake ecosystems is regulated by complex interactions among climate, humans, ecosystem morphology, and catchment characteristics, each of which varies in time and space (Schindler 2001). For example, climatic controls range from daily meteorological variations in local irradiance, temperature, and water fluxes (Keller 2007) through to large-scale interactions of the atmosphere and oceans (Trenberth and Hurrell 1994;Hurrell 1995;Mantua et al. 1997) and millennium-long changes in energy (E) and mass (m) flux around the planet (Williams et al. 1997;Diffenbaugh et al. 2006). Within this framework, chemical, physical, and ecological processes combine to determine the production and composition of aquatic communities, both uniquely and in consort with other mechanisms (Carpenter 1999). In addition, lakes are affected both by human disturbance of biotic communities and biogeochemical cycles (e.g., land use, urbanization, fisheries management) and by creation of novel stressors (e.g., acidic precipitation, toxicants, ozone loss). However, because these factors interact over diverse spatial and temporal scales, it is difficult to determine the relative importance of regulatory processes using only traditional site-based observation and experimentation. Instead, it is the premise of this article that the combined use of limnology and paleoecology represents the best means to quantify and scale interactions between climate and other control mechanisms and to develop a hierarchical understanding of how these regulatory processes are likely to influence lakes pres...
Eutrophication can initiate sudden ecosystem state change either by slowly pushing lakes toward a catastrophic tipping point beyond which self-reinforcing mechanisms establish an alternate stable state, or through rapid but persistent changes in external forcing mechanisms. In principle, these processes can be distinguished by determining whether historical changes in focal parameters (phytoplankton) exhibit transient (rising then declining) or continuously-elevated variability characteristic of alternate stable states or a "paradox of enrichment," respectively. We tested this hypothesis in the south basin of Lake Winnipeg, Canada, a site with intense blooms of N 2 -fixing cyanobacteria since 1990, but for which little is known of earlier limnological conditions, causes of eutrophication, or whether modern conditions represent a alternate stable state. Paleolimnological analysis revealed that the basin was naturally mesotrophic (15-20 lg P L 21
Supply of anthropogenic nitrogen (N) to the biosphere has tripled since 1960; however, little is known of how in situ response to N fertilisation differs among phytoplankton, whether species response varies with the chemical form of N, or how interpretation of N effects is influenced by the method of analysis (microscopy, pigment biomarkers). To address these issues, we conducted two 21-day in situ mesocosm (3140 L) experiments to quantify the species- and genus-specific responses of phytoplankton to fertilisation of P-rich lake waters with ammonium (NH4 +), nitrate (NO3 −), and urea ([NH2]2CO). Phytoplankton abundance was estimated using both microscopic enumeration of cell densities and high performance liquid chromatographic (HPLC) analysis of algal pigments. We found that total algal biomass increased 200% and 350% following fertilisation with NO3 − and chemically-reduced N (NH4 +, urea), respectively, although 144 individual taxa exhibited distinctive responses to N, including compound-specific stimulation (Planktothrix agardhii and NH4 +), increased biomass with chemically-reduced N alone (Scenedesmus spp., Coelastrum astroideum) and no response (Aphanizomenon flos-aquae, Ceratium hirundinella). Principle components analyses (PCA) captured 53.2–69.9% of variation in experimental assemblages irrespective of the degree of taxonomic resolution of analysis. PCA of species-level data revealed that congeneric taxa exhibited common responses to fertilisation regimes (e.g., Microcystis aeruginosa, M. flos-aquae, M. botrys), whereas genera within the same division had widely divergent responses to added N (e.g., Anabaena, Planktothrix, Microcystis). Least-squares regression analysis demonstrated that changes in phytoplankton biomass determined by microscopy were correlated significantly (p<0.005) with variations in HPLC-derived concentrations of biomarker pigments (r 2 = 0.13–0.64) from all major algal groups, although HPLC tended to underestimate the relative abundance of cyanobacteria. Together, these findings show that while fertilisation of P-rich lakes with N can increase algal biomass, there is substantial variation in responses of genera and divisions to specific chemical forms of added N.
Annually resolved fossil records of nitrogen (N) inputs (as sedimentary d 15 N, N content), aquatic production (d 13 C, C content), and algal abundance and gross community composition (pigments, nonsiliceous microfossils) from Lough Neagh, Northern Ireland (NI), were compared with annual records of climatic variability, atmospheric and urban nutrient loading, whole-catchment nutrient budgets, and limnological monitoring data to identify the unique effects of N on the eutrophication of a phosphorus (P)-rich lake during ca. 1933-1995. Cluster analysis revealed two major biostratigraphic zones. Zone I (ca. 1933-1955) was characterized by moderate lake production, as inferred from low concentrations of most fossil pigments and reduced d 15 N signatures but elevated d 13 C values and chlorophyte microfossil concentrations. In contrast, Zone II (ca. 1955II (ca. -1995 exhibited greatly increased contents of 15 N, N, C, and algal pigments, combined with strongly reduced d 13 C ratios and chlorophyte fossil abundance, a pattern consistent with recent severe eutrophication. Overall, microfossils of diazotrophic cyanobacteria were most abundant during the transition period between zones (ca. 1955-1964). Regression analysis revealed that past N influx to the lake (as d 15 N; r 2 5 0.916, p , 0.0001), colonial cyanobacterial abundance (as myxoxanthophyll; r 2 5 0.837, p , 0.0001), and total algal standing crops (as b-carotene; r 2 5 0.388, p , 0.0001) were all strongly correlated to agricultural inputs of N to NI farmland, weakly correlated to P inputs to NI farmland (r 2 d15N 5 0.503, p , 0.0001; r 2 cyanobacteria 5 0.296, p , 0.0001; r 2 total algae 5 0.046, p . 0.05), and uncorrelated to most measures of climatic variability and atmospheric or urban nutrient inputs. Thus, degradation of water quality during the 20th century resulted from excessive loading of diffuse N to the lake from P-rich agricultural lands.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.