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People who live on tropical deltas are making local adaptations to tackle myriad challenges, but should we be looking upstream? Rapid expansion of dams for hydropower is damaging the rivers that supply deltas, increasing the uncertainty about how delta communities should adapt in the future.

Anthropogenic and climatic stressors on freshwater ecosystems are of global concern. However, the interactions and effects of multiple stressors (e.g., nutrient enrichment, climate warming, altered wind and precipitation) acting over different spatial and temporal scales are often complex and remain controversial. Here, we reconstructed one‐century dynamics of eutrophication and primary producer communities in algal‐dominated and macrophyte‐dominated zones of a large shallow lake (Taihu, China), by integrating sedimentary photosynthetic pigments and geochemical records with water monitoring and historical archives. We aimed to explore the long‐term underlying mechanisms of the responses of water quality and lake biota to multiple environmental perturbations. We found that water quality degradation and algal community modification showed similar trends but distinct timings and trajectories in contrasting ecological zones. Onset and intensity of eutrophication in north Meiliang Bay (since the 1950s) exceeded far beyond that of macrophyte‐dominated Eastern Taihu (~1990s). Anthropogenic nutrients overtook past climatic control on production and composition of phototrophic assemblages. More importantly, lake phytoplankton responded markedly to climate warming, decreasing wind speed, and extreme weathers after cultural eutrophication. Synergistic interactions of nutrients and climate on lake ecosystems became increasingly significant in promoting harmful algal blooms (HABs) dominated by Microcystis, close to the hyper‐eutrophic north lake zones. The asynchronous limnological and ecological responses also indicated the modulating roles of lake ecological regime and catchment hydrogeomorphic characteristic. Collectively, our findings suggest that mitigation of eutrophication and HABs calls for a triple management strategy integrating anthropogenic nutrients, climate change, and lake‐catchment setting.

Tropical river deltas, and the social-ecological systems they sustain, are changing rapidly due to anthropogenic activity and climatic change. Baseline data to inform sustainable management options for resilient deltas is urgently needed and palaeolimnology (reconstructing past conditions from lake or wetland deposits) can provide crucial long-term perspectives needed to identify drivers and rates of change. We review how palaeolimnology can be a valuable tool for resource managers using three current issues facing tropical delta regions: hydrology and sediment supply, salinisation and nutrient pollution. The unique ability of palaeolimnological methods to untangle multiple stressors is also discussed. We demonstrate how palaeolimnology has been used to understand each of these issues, in other aquatic environments, to be incorporated into policy. Palaeolimnology is a key tool to understanding how anthropogenic influences interact with other environmental stressors, providing policymakers and resource managers with a ‘big picture’ view and possible holistic solutions that can be implemented.

ABSTRACTThe effects of salinity (as chloride [Cl] at 600, 1000, 1600, and 2500 mg L−1) and nitrate (as nitrogen [N] loading rates using concentrations of 1, 2, 5, and 10 mg L−1) additions on phytoplankton communities (as chlorophyll and carotenoid pigments) were determined using a fully factorial 3 m3mesocosm pond experiment. Redundancy analysis followed by variance partitioning analysis (VPA) statistically compared phytoplankton with water chemistry, zooplankton, phytobenthos (aquatic plants and periphyton), and zoobenthos to understand relationships among benthic and pelagic components. Repeated measures analysis of variance (RM-ANOVA) indicated no interactive effects of the 2 treatments. In VPA, physicochemical variables explained the most variance (33.6%) in the phytoplankton pigment dataset relative to benthic primary producers (0.4%) and invertebrates (2.3%). Salinisation led to an increase in biomass of planktonic siliceous algae (Cl ≥1600 mg L−1) and chlorophytes and cyanobacteria (Cl ≥2500 mg L−1), which we infer was caused by increased phosphorus release from sediments while aquatic plants and periphyton declined. Nitrate additions increased the biomass of cryptophytes and chlorophytes at intermediate N loading rates using concentrations of 5 mg L−1(associated with greater ammonium [NH4-N] availability and shifts in aquatic plant composition). These findings support the hypothesis that the relative availability of reduced versus oxidised N forms is an important driver of phytoplankton composition. Together, these results suggest that pelagic biota are highly sensitive to salinity and nitrate increases, and that the phytoplankton compositional shifts are driven by indirect effects on water chemistry (bioavailable P mobilisation, changes in N forms), which are mediated by benthic processes.

Near‐surface wind speeds have changed over recent decades, raising questions about the extent to which these changes are altering the vertical thermal structure of lakes and affecting lake food webs. Neo‐ and paleolimnological techniques were used to assess wind‐driven changes in lake thermal habitat and resulting effects on primary producers in two lakes in Isle Royale National Park, an island archipelago located in Lake Superior, where wind speed has increased in recent decades. Responses in Siskiwit Lake, a large (16 km2surface area), deep (Zmax= 49 m), oligotrophic lake, were compared to those of Lake Desor, a moderately large (4.3 km2) but shallower (Zmax= 13 m), mesotrophic lake. High‐frequency sensor data suggested that changes in wind speed affected epilimnion thickness in both lakes synchronously (ρ= 0.7, p< 0.001). Diatom‐inferred mixing depths suggested a coherent shift in both lakes to deeper mixing (an increase of 3 and 6 m) since 1920 (ρ= 0.8), which was correlated with an increase in regional wind speed during the 20thcentury at the decadal‐scale in Lake Desor and Siskiwit Lake (ρ= 0.6 and 0.4, respectively). In Lake Desor, algal biomass declined as mixing deepened from 1920 to 1980, and then cyanobacteria and cryptophyte pigments increased from 1980 to present, a period of inferred stable and deep mixing. Algal pigment concentrations in Siskiwit Lake were unchanged as mixing depth deepened. Although changes in wind speed altered lake physical structure similarly, the ecological consequences of these changes differed between lakes and were most likely influenced by lake‐specific variability in nutrient and light availability.

Ancient lakes are among the best archivists of past environmental change, having experienced more than one full glacial cycle, a wide range of climatic conditions, tectonic events, and long association with human settlements. These lakes not only record long histories of environmental variation and human activity in their sediments, but also harbor very high levels of biodiversity and endemism. Yet, ancient lakes are faced with a familiar suite of anthropogenic threats, which may degrade the unusual properties that make them especially valuable to science and society. In all ancient lakes for which data exist, significant warming of surface waters has occurred, with a broad range of consequences. Eutrophication threatens both native species assemblages and regional economies reliant on clean surface water, fisheries, and tourism. Where sewage contributes nutrients and heavy metals, one can anticipate the occurrence of less understood emerging contaminants, such as pharmaceuticals, personal care products, and microplastics that negatively affect lake biota and water quality. Human populations continue to increase in most of the ancient lakes’ watersheds, which will exacerbate these concerns. Further, human alterations of hydrology, including those produced through climate change, have altered lake levels. Co‐occurring with these impacts have been intentional and unintentional species introductions, altering biodiversity. Given that the distinctive character of each ancient lake is strongly linked to age, there may be few options to remediate losses of species or other ecosystem damage associated with modern ecological change, heightening the imperative for understanding these systems.

Lake sediments constitute natural archives of past environmental changes. Historically, research has focused mainly on generating regional climate records, but records of human impacts caused by land use and exploitation of freshwater resources are now attracting scientific and management interests. Long-term environmental records are useful to establish ecosystem reference conditions, enabling comparisons with current environments and potentially allowing future trajectories to be more tightly constrained. Here we review the timing and onset of human disturbance in and around inland water ecosystems as revealed through sedimentary archives from around the world. Palaeolimnology provides access to a wealth of information reflecting early human activities and their corresponding aquatic ecological shifts. First human impacts on aquatic systems and their watersheds are highly variable in time and space. Landscape disturbance often constitutes the first anthropogenic signal in palaeolimnological records. While the effects of humans at the landscape level are relatively easily demonstrated, the earliest signals of human-induced changes in the structure and functioning of aquatic ecosystems need very careful investigation using multiple proxies. Additional studies will improve our understanding of linkages between human settlements, their exploitation of land and water resources, and the downstream effects on continental waters.

ABSTRACTAcross a small geographic area (<180 km), the region of South-West Greenland covers a natural climate gradient. Variation in temperature and precipitation result in marked differences in limnology at three discrete locations: ice sheet margin, inland, and the coast. Replicate lakes from each location were sampled for physical (temperature, light), chemical (dissolved oxygen, pH, conductivity, nutrients), and biological (chlorophyll a[Chl a], photosynthetic pigments) variables on three occasions within a 12-month period: July–August 2010, April–May 2011, and June–July 2011 spanning ice cover. Variation in ice phenology was linked to the climate gradient; however, phytoplankton production and community composition did not differ regionally. Large-scale seasonal fluctuations in temperature and nutrient availability were the strongest predictors of phytoplankton production, with a shift from nitrate to phosphorus controlled production between ice-cover and ice-free conditions. Underlying seasonal drivers, variables predicting production were unique to each location—ice sheet margin (soluble reactive phosphorus), inland (temperature), and coast (silicate)—and reflect local differences in nutrient availability. Results from the current study have important consequences when controls over phytoplankton production in Arctic lakes are inferred from a limited number of sites, but up-scaled to represent pan-Arctic trends.

We employ water-isotope tracers and multi-proxy paleolimnological records to characterize contemporary controls on water balances of floodplain lakes in the Athabasca Delta, Canada, within the context of its hydroecological evolution over the 20th century. The insight gained from these approaches is necessary to gauge the hydroecological resiliency of the Athabasca Delta to past and future changes in Athabasca River flow regime. Results obtained from three lakes located in different regions of the Athabasca Delta indicate that hydroecological conditions were strongly affected by an engineered meander cut-off on the Athabasca River in 1972, intended to maintain flow in the river main stem, and a natural bifurcation of one of the major distributaries (Embarras River) in 1982, in response to progressive overextension of the delta to the east. Climate warming and naturally declining river discharge have also contributed to directional change. Recent drying trends reconstructed from sediment cores at two of the three lakes are likely representative of rapidly evolving hydroecological conditions in the south-eastern sector, based on mapping of a recent high-magnitude ice-jam flood that failed to recharge this portion of the delta, while wetting in the region of the third lake due to increased frequency of river flooding reflects increasing diversion of Athabasca River flow northward. Our findings highlight the hydroecological sensitivity of the Athabasca Delta to changes in the magnitude and timing of discharge in the Athabasca River and heighten the need for informed management strategies to safeguard the integrity of this unique wetland ecosystem. Copyright © 2008 John Wiley & Sons, Ltd.

Relationships between phytoplankton growth and supplies of phosphorus and nitrogen have been variously investigated in three lakes in two contrasted English lake areas using physiological nutrient indices (alkaline phosphatase activity, phosphorus debt, surplus phosphorus, phosphorus uptake kinetics, dark ammonium uptake, ammonium uptake kinetics, stimulation of 14C uptake, and batch bioassay) and by inspection of water chemistry. None of the lakes was significantly affected by wastewater effluent but all lay in agricultural catchments. Physiological indices and water chemistry suggested limitation of phytoplankton biomass mostly by phosphorus, and to some extent by nitrogen in summer in North Ormesby Broad, by nitrogen in summer and to some extent by phosphorus in Lily Broad and by nitrogen in White Mere. Indices did not always accord with one another, though were not severely misleading. They added little to conclusions that could be drawn from water chemistry alone. Reasons for the differential relative importance of nitrogen and phosphorus in the two lake areas are discussed. The paradigm of phosphorus limitation, though unchallenged in upland waters and those on poorly weathered rocks, may be less relevant in lowland lakes.

Relationships between phytoplankton growth and supplies of phosphorus and nitrogen have been variously investigated in three lakes in two contrasted English lake areas using physiological nutrient indices (alkaline phosphatase activity, phosphorus debt, surplus phosphorus, phosphorus uptake kinetics, dark ammonium uptake, ammonium uptake kinetics, stimulation of 14C uptake, and batch bioassay) and by inspection of water chemistry. None of the lakes was significantly affected by wastewater effluent but all lay in agricultural catchments. Physiological indices and water chemistry suggested limitation of phytoplankton biomass mostly by phosphorus, and to some extent by nitrogen in summer in North Ormesby Broad, by nitrogen in summer and to some extent by phosphorus in Lily Broad and by nitrogen in White Mere. Indices did not always accord with one another, though were not severely misleading. They added little to conclusions that could be drawn from water chemistry alone. Reasons for the differential relative importance of nitrogen and phosphorus in the two lake areas are discussed. The paradigm of phosphorus limitation, though unchallenged in upland waters and those on poorly weathered rocks, may be less relevant in lowland lakes.