Global lake research is skewed toward economically and socially developed regions, overlooking remote areas. Enhancing resilience and fostering synergistic approaches could help redress these inequities.

When a sediment laden river flows into a stratified water body, the water mass can either intrude as an overflow, interflow, or underflow depending upon the density contrast between the river and the lake. If the river is sufficiently warm or fresh to compensate for the additional mass of sediment, an overflow results, below which convective sedimentation occurs. If the sediment load is sufficiently high, then an underflow initially occurs, from which the warm/fresh interstitial material can subsequently loft as sedimentation reduces the initial density. Such convection can even potentially overturn the water column stratification if there is a very fresh, but very high sediment load turbidity current. For intermediate cases, an interflow can occur. Here it is possible for both lofting and sediment driven convection to occur above and below the pycnocline. All these different regimes can be described in terms of two dimensionless parameters: RS and RA, which are ratios that compare the density contrast due to sediment between the river and the upper layer with the density contrast between the upper and lower layers and the density contrast between the river and upper layer, respectively. We used laboratory experiments to describe the vigour of convection in terms of these dimensionless parameters, which then allows the behaviour in various rivers inflows into lakes to be predicted. We also apply our observations to predict how a turbidity current could lead to lofting and possible overturn of the stratification of meromictic Lake Kivu.

Lakes on volcanic soils of the North Island of New Zealand, compared to north temperate lakes, have low total nitrogen:total phosphorus (TN:TP) ratios, low dissolved inorganic nitrogen concentrations during summer stratification, and have positive chlorophyll a responses to N additions more frequently than to P in nutrient enrichment bioassays. This response has resulted, in some cases, to the imposition of N loading caps on some lake catchments, in contrast to nutrient management in north temperate lakes focused more on P management. To explore this possible difference in limiting nutrients, a suite of nutrient status indicators based on measurement of ambient seston stoichiometry and metabolic activity, which have been widely used in north temperate lakes, were applied to 8 lakes on volcanic terrain with previously reported N limitation. These nutrient status measurements were previously calibrated to inform whether in situ phytoplankton are growth-rate limited and, if so, which nutrient is limiting growth rate. In austral summer 2015, all 8 lakes showed indications of P limitation, and P limitation was more extreme, pervasive, and persistent (among samplings) than N limitation. Indications of N limitation were not observed without contemporaneous evidence of P limitation, but P limitation was frequently observed without evidence of N limitation. One lake, Rotorua, was not strongly nutrient limited, and phytoplankton were likely growing at or near optimum growth rates. In this study the commonly used TN:TP ratio was a poor predictor of which nutrient was potentially limiting in situ phytoplankton.

The quagga mussel (Dreissena rostriformis bugensis) is a filter-feeding invasive species that has re-engineered many freshwater ecosystems worldwide. High clearance rates (CRs) and dense populations underpin their ecological impacts. CRs, however, are highly variable, as are environmental factors that regulate them. Despite their widespread distribution in Europe and North America, knowledge of how multiple environmental factors regulate CRs of quagga mussels remains limited. We investigated quagga mussel CRs under varying conditions including water temperature, food availability, habitat depth, flow velocity, and duration of incubation in chambers with both static and flowing water. We found that CR was positively related to water temperature and initial food concentration in static chambers. When coupled with limited food concentration, coldwater (7.5 degrees C), due to a deep-water upwelling event, produced very low CR (similar to 10x lower) compared to warmer water (12-24 degrees C) (0.47 vs. 3.12-5.84 L g(-1) DW h(-1)). Mussels from deeper water (20 m) had CRs that were similar to 3.5x higher than from shallower depths (2-10 m) and CRs were inversely affected by total mussel dry weight. Flowrates from1 to 22 cms(-1) generated a unimodal pattern of CR with an optimal flow velocity of 6-12 cm s(-1) (similar to 2x higher than suboptimal CRs). Enhanced flow velocity (22 cm s(-1)), reflective of storm conditions in shallow waters, significantly increased the closing/reopening activity of mussel valves relative to lower velocities (1-12 cm s(-1)). Incubation time had a strong negative effect (similar to 2-4x reduction) on CRs likely reflecting refiltration in static chambers versus food saturation of mussels in flowing chambers, respectively. Our findings highlight how multiple factors can influence quagga mussel CRs by factors of 2-10. Givenwidespread habitat heterogeneity in large aquatic ecosystems, whole-lake estimates of mussel impacts should include multiple regulatory factors that affect mussel filtration. (C) 2020 Elsevier B.V. All rights reserved.

The African Great Lakes (AGL) have rich fisheries and are renowned “biodiversity hotspots”. Consequently the AGL and the ecosystem services they provide, underpin the welfare and livelihoods of over 50 million people across 10 countries. Despite the recognized importance of the AGL, these vital ecosystems and their livelihood support systems are threatened by numerous anthropogenic stressors at local, regional, and global scales. Past and continued efforts to address critical challenges on these lakes are often short-term, parochial, disparate, and uncoordinated resulting in a lack of comprehensive and comparable scientific data and inadequate resources to influence evidence-based policy. Over the past two decades, several international workshops, conferences and scientific publications have identified the need for collaboration, knowledge sharing, and harmonization of research and management as key elements to enhance conservation efforts in the AGL. In this commentary, we introduce the African Center for Aquatic Research and Education (ACARE), which aims to strengthen research and provide the scientific evidence needed to make informed decisions related to sustainable fisheries and aquatic resource management in the AGL. To do this, ACARE will administer a highly collaborative network of experts with three long-term goals: (1) strengthen global and regional research partnerships; (2) establish transboundary and inter-jurisdictional lake advisory groups; and (3) build capacity of freshwater scientists through experiential education and public engagement.

The occurrence and dynamics of Deep Chlorophyll Maxima (DCM) in stratified water bodies has implications for aquatic productivity, biogeochemistry, and energy flow. Attributes of the DCM related to structure and function have been well studied in temperate systems but substantial knowledge gaps exist in tropical lakes due to limited studies. We investigated the seasonal and annual patterns of vertical chlorophyll distribution and behaviour of DCM along the water column of a tropical lake by monitoring Chl-a vertical profile biweekly over a two-year period. The aim was to understand the factors controlling seasonal variability of DCM and the functional relationship between DCM and epilimnetic Chl-a and changes in the physicochemical environment influencing these dynamic relationships. We found remarkable persistence of the DCM all year round with high average Chl-a and was not dissipated by lake mixing despite precipitous seasonal decline in euphotic depth. The persistence of DCM was linked with prolonged thermal stratification and high nutrient enrichment below the thermocline. DCM and epilimnetic algal chlorophyll were controlled by differing regulating mechanisms affected by stratification, mixing and restratification cycles of the lake. The euphotic depth and TP were the strongest predictors of DCM dynamics. Doubling of euphotic depth during thermal stratification increased DCM chlorophyll by 85 % above epilimnetic chlorophyll. These limnological patterns could be predicted from year to year to a high degree of certainty.

Variability in physico-chemical water column structure along a gradient between Nyanza Gulf and Lake Victoria was characterized, and the exchange between the two basins estimated. Specific conductivity (SC) measurements between March 2005 and March 2006 identified three morphologically distinct zones, the inner-gulf, the connecting Rusinga Channel and the main lake basin. The relatively shallow, wide and river-influenced inner-gulf had higher turbidity and SC compared to the channel and the main lake. The deep, narrow and more wind-exposed mid-channel area was significantly cooler than the rest of the study areas. Observational data revealed "density-driven" underflow of the cooler channel water into the main lake and compensatory surface flow into the gulf as well as seasonal patterns of gulf-main lake exchange in relation to variation in tributary input. The wind field was dominated by a diurnal lake-land breeze which resulted in wind-driven diurnal reversal of water flow in and out of the gulf, where the constriction of flow within the channel induced strong currents. The exchange between the gulf and the main lake was estimated using a box model, with SC as a conservative tracer. A net annual residual water flow of 35 m(3) s(-1) occurred from the gulf into the main lake although during months with low tributary discharge the flow was towards the gulf. The daily flow in and out of the gulf resulted in relatively high exchange flux compared to residual flow across the Rusinga Channel leading to the development of the well-defined physicochemical gradient along the Rusinga Channel. (C) 2018 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

The primary productivity and seasonality of phytoplankton and its seasonality in Lake Bosomtwe (Ghana), West Africa were studied from September 2005 to August 2006 using the dissolved oxygen method. The wet weight biomass (0.41±0.37 gCm -2 d -1 , n=25), Chlorophyll a (52.11±19.51 mg m -2 , n=17), mixed layer depth (9.28±3.47 m, n=25), euphotic depth (4.43±1.76 m, n=25), ratio of mixed layer to euphotic depth (2.17±0.74, n=25), secchi disc depth (1.52±0.34 m, n=25), irradiance (1180±341.28 µE m -2 s -1 , n=25), extinction coefficient (0.97±0.46, n=25), and total phosphorus concentrations (1.84±0.49 µmol L -1 , n=20) were measured to assess their influence on the productivity of the phytoplankton. Our assessment revealed high areal gross productivity of the phytoplankton (4.72 ± 1.56 gC m -2 d -1 , n = 25) which is within the range for tropical African lakes, but concurrent high areal community respiration rates (4.34 ± 2.78 gC m -2 d -1 , n = 25) contributed to a low net productivity (0.37 ± 2.32 gC m -2 d -1 , n = 25) and growth rates (0.14 ± 0.33 d -1 , n = 21). Variabilities in the gross productivity of the phytoplankton were high (CV =33.10 %) and driven by similarly high variabilities in the physicochemical and biological parameters. Productivity also exhibited clear seasonality associated with the mixing and stratification of the lake. The physicochemical and biological parameters (mixed layer depth, euphotic depth, secchi disc depth, irradiance, chlorophyll a , and the community respiration) all had a positive relationship with the primary production) whiles ratio of mixed layer to euphotic depth, total phosphorus, wet weigh biomass had a negative relationship with the gross productivity. Of all the measured parameters, only areal community respiration (r 2 = 30.6 %) had a significant predictive value (r 2 < 0.05).

A whole-ecosystem experiment in Lake 227 (L227) at the Experimental Lakes Area, ongoing since 1969, examined the roles of carbon (C), nitrogen (N), and phosphorus (P) in controlling eutrophication. During 2011, we conducted a series of sub-experiments and more intensive monitoring to improve estimates of N fixation and its ability to meet algal growth demands in the decades following the cessation of artificial N loading, while maintaining long-term high artificial P loading. Stoichiometric nutrient ratios indicated both moderate N and P limitation of the phytoplankton during spring, preceding a shift in phytoplankton community structure toward dominance by N fixing cyanobacteria. During bloom development, and for the remainder of the stratified period, stoichiometric nutrient ratios indicated moderate to strong P limitation. N fixation rates, corrected using 15N2 methods, increased 2× after 1990, when N loading ceased. Ambient dissolved inorganic nitrogen prior to the bloom represented less than 3% of N demands of the phytoplankton. N fixation accounted for between 69–86% of total N loading to the epilimnion during the period of rapid bloom development, and 72–86% of total N loading during the May–October period. Phytoplankton biomass did not decline in L227 during the 40 years since artificial N loading was reduced, or the nearly 25 years since artificial N loads ceased entirely (1990–2013), and remained approximately 20× higher than four nearby reference lakes. These results suggest that despite constraints on biological N fixation, it retains a large capacity to offset potential N loading reductions in freshwaters.

Мore free environmental science resources аt: www.ScienceJournalForKids.org Scientists agree that some human activities, like burning fossil fuels, cause the Earth’s atmosphere to trap more heat, leading to climate change. We wanted to find out how climate change affected large freshwater lakes, so we took a series of sediment cores in Lake Superior (the biggest of the Great Lakes in North America) to study samples of the mud at the lake bottom. Back in the lab, we analyzed tiny bits of dead plants and algae trapped within the sediments. From these samples, we could see that the amount of algae in Lake Superior has slowly increased over time. However, in the past hundred years, the amount of algae and plants in the Lake has increased dramatically. We think this recent drastic change happened because human-caused climate change led to warmer temperatures and longer ice-free periods in the lake. This made it easier for algae and plants to grow in the water.

Stable isotopes are increasingly being used to infer past and present trophic interactions in light of environmental changes. The Lake Victoria haplochromine cichlids have experienced severe environmental changes in the past decades that, amongst others, resulted in a dietary shift towards larger prey. We investigated how the changed environment and diet of the haplochromines influenced stable isotope values of formalin-then-ethanol-preserved cichlid specimens, and then investigated how these values differed among species before (1977–1982) and after substantial environmental changes (2005–2007). We found a small preservation effect on both δ13C and δ15N values, and significant differences in isotope values among haplochromine species collected before the environmental changes. In contrast, there was a remarkable similarity in δ13C and δ15N values among species collected from the contemporary ecosystem and two out three species showed significantly different stable isotope values compared to species of the historic ecosystem. In addition, we found a putative isotopic gradient effect along our 5-km-long research transect indicating that the studied demersal species are more stenotopic than previously thought. The environmental changes have resulted in dietary change and overlap of the haplochromines which provides insight into the trophic plasticity of these species, which are often considered trophic specialists.

Eutrophication is an increasing global threat to freshwater ecosystems. East Africa's Lake Victoria has suffered from severe eutrophication in the past decades which is partly responsible for the dramatic decline in haplochromine cichlid species diversity. However, some zooplanktivorous and detritivorous haplochromine species recovered and shifted their diet towards macro invertebrates and fish. We used four formalin preserved cichlid species caught over the past 35 years to investigate whether stable isotopes of these fish are reflecting the dietary changes, habitat differences and if these isotopes can be used as indicators of eutrophication. We found that δ15N signatures mainly reflected dietary shifts to larger prey in all four haplochromine species. Shifts in δ13C signatures likely represented habitat differences and dietary changes. In addition, a shift to remarkably heavy δ13C signatures in 2011 was found for all four species which might infer increased primary production and thus eutrophication although more research is needed to confirm this hypothesis. The observed temporal changes confirm previous findings that preserved specimens can be used to trace historical changes in fish ecology and the aquatic environment. This highlights the need for continued sampling as this information could be of essence for reconstructing and predicting the effects of environmental changes.

ADVERTISEMENT RETURN TO ISSUEPREVLetter to the EditorNEXTResponse to the Letter, Nitrogen is Not a "House of Cards"D. W. Schindler*, S. R. Carpenter, S. C. Chapra, R. E. Hecky, and D. M. OrihelCite this: Environ. Sci. Technol. 2017, 51, 4, 1943Publication Date (Web):January 5, 2017Publication History Received9 December 2016Accepted11 December 2016Published online5 January 2017Published inissue 21 February 2017https://pubs.acs.org/doi/10.1021/acs.est.6b06106https://doi.org/10.1021/acs.est.6b06106article-commentaryACS PublicationsCopyright © 2017 American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views1010Altmetric-Citations6LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (125 KB) Get e-AlertscloseSUBJECTS:Environmental pollution,Nitrogen Get e-Alerts

Anthropogenic climate change has the potential to alter many facets of Earth's freshwater resources, especially lacustrine ecosystems. The effects of anthropogenic changes in Lake Superior, which is Earth's largest freshwater lake by area, are not well documented (spatially or temporally) and predicted future states in response to climate change vary. Here we show that Lake Superior experienced a slow, steady increase in production throughout the Holocene using (paleo)productivity proxies in lacustrine sediments to reconstruct past changes in primary production. Furthermore, data from the last century indicate a rapid increase in primary production, which we attribute to increasing surface water temperatures and longer seasonal stratification related to longer ice-free periods in Lake Superior due to anthropogenic climate warming. These observations demonstrate that anthropogenic effects have become a prominent influence on one of Earth's largest, most pristine lacustrine ecosystems.

Deep Chlorophyll Maxima (DCM) is an important feature of stratified lakes and oceans but very little is known about the phenomenon in tropical systems. Proximate factors accounting for DCM presence include light, thermal stratification and nutrients but biotic interactions such as zooplankton grazing can actively support DCM formation, structure and maintenance. We examined DCM characteristics in tropical Lake Bosumtwi at biweekly intervals between April and July of 2005 and 2006 and also assessed zooplankton herbivore interactions with the DCM. The onset and development of the seasonal DCM peak was based on stable water column, steep thermocline, availability of a nutricline and nutrient limitation in the epilimnion. Euphotic depth doubled during DCM formation and increasing transparency tripled DCM Chl a in the metalimnion just below the oxycline. DCM, however, disaggregated with seasonally induced deeper mixing. The range of variability in surface chlorophyll did not differ significantly between seasonal and aseasonal DCM period. There was about one-fourth difference between mean surface and metalimnetic chlorophyll concentrations but this shift in resource location did not trigger a descent to deeper waters by herbivores. Herbivore grazing could not be linked to surface variations in water clarity and deep-water irradiance needed for DCM formation and maintenance. DCM presence therefore was not enhanced by surface water grazing activities of zooplankton. This suppressed biotic influence indicated that DCM dynamics is driven by lake clarity, thermal and nutrient gradients which constitute key abiotic drivers. Direct consumption of the DCM by herbivorous zooplankton was not evident and the energy therein is possibly lost to the anoxic deep waters of the lake through sedimentation. The sinking organic matter could be returned to the pelagic food web via the microbial loop represented by bacteria, ciliates and heterotrophic/mixotrophic flagellates inhabiting the lake's deep layers.

Zooplankton vertical distribution was studied in Lake Bosumtwi to understand how spatial and temporal variations in the water column affected diurnal and nocturnal patterns. Biweekly samples were collected for 24 consecutive months at the deepest point from 2005 to 2006. Water column properties shaping habitat suitability, namely temperature, light intensity, dissolved oxygen and phytoplankton biomass, varied markedly with depth but less with season. Most zooplankton stayed in the epilimnion continuously and their distribution was unaltered by lake stratification, mixing and restratification. Species dwelt permanently in the epilimnion without any necessity to migrate. Reduced water column variability and resultant weak selective pressures suppressed greater dispersion of species. Chaoborus ceratopogones (Theobald) was the main invertebrate predator and performed upward and downward movements on a diel cycle. Competing prey species overlapped extensively in spatial and temporal degree of habitat use. Chaoborus , however, segregated in deep water during the day, thereby minimizing prey vulnerability and promoting coexistence among predators and prey. During night-time residence, prey species may be exposed to increased predation risk from Chaoborus but gains in reproductive potential in warm, food-rich surface waters may offset losses to mortality. Our results suggest that high degree of habitat constancy and consequently weak selective pressures contributed to the lowered behavioural responses of zooplankton. The persistently skewed distribution of zooplankton is adaptive to predation and competition and promotes species coexistence. This behaviour will optimize growth and reproductive benefits over the cost of descent to suboptimal conditions in deep water.