Lake Champlain offshore benthic invertebrate community before and after zebra mussel invasion

Benthic invertebrates are important bio-indicators of water quality and play a significant role in aquatic systems. Lake Champlain has limited benthic invertebrate data which hinders development of food web models, assessment of invasive species impacts, and evaluation of management actions. In June 2016, we assessed benthic invertebrates along three transects in the main basin of Lake Champlain ranging from 5 to 100?m, and then compared results to densities from a limited survey in 1991 prior to the zebra mussel (Dreissena polymorpha) invasion. In 2016, total biomass and density were 1–2 orders of magnitude greater at 5?m than at 20–100?m. Zebra mussels, chironomids, oligochaetes, and gastropods were dominant at 5?m, and oligochaetes and sphaeriids were dominant at 20–100?m. Total density at the 5-m site was 94% lower in 2016 compared to 1991, but similar at the 100-m site. Diporeia, while abundant in many freshwater bodies, is historically rare in Lake Champlain and was not detected in our sampling. Because Lake Champlain benthic invertebrate densities are low and display dissimilar distributions to the Great Lakes, we hypothesize the offshore fish community is likely much more reliant on pelagic rather than benthic production. Although the current composition and biomass suggest the benthic community in Lake Champlain may not be greatly impacted by an invasion of quagga mussel (D. rostriformis bugensis), the potential for quaggas to re-route energy from pelagic to benthic habitats, as it has in the Great Lakes, could limit the Lake Champlain offshore fish community.     

Study of the chemical composition of suspended particles in Lake Baikal

Samples of suspended material (solid particles) from Lake Baikal were analysed by using X‐ray fluorescence elemental analysis with the excitation of monochromatized synchrotron radiation (SRXFA, Novosibirsk, Russia) and electron probe X‐ray microanalysis (EPXMA, Antwerp, Belgium). The chemical composition of particles in different depths and basins of the lake is discussed. The chemical composition of the central parts of the lake are stable, but concentrations are higher in the southern basin, which is liable to pollution.     

Influence of water motion on the spatial distribution of Spirogyra in Lake Baikal

We investigated the diversity of benthic algal communities as well as phytoplankton in Lake Baikal. The structure of benthic algal communities changed in comparison to the period before 2000 due to intense development of filamentous algae, particularly Spirogyra. Percent cover of filamentous algae in different areas of the coastal zone varied from 0 to 100%. The lowest Spirogyra biomass was recorded in the surf and wave-breaking zones, whereas the highest biomass was observed in the area of the bottom less affected by waves. Fragments of Spirogyra thallomes were also recorded in the phytoplankton community of Lake Baikal's southern basin which is a new phenomena not previously recorded in the lake. Hydraulic characteristics of Spirogyra were similar to those of planktonic diatoms. Currents and wave effects on the bottom favored transfer and distribution of Spirogyra from locations with intense development to the coastal area of Lake Baikal. Spirogyra is now found throughout Lake Baikal.     

Lake Baikal: Model for sustainable development of the territory

Created 20–30 million years ago, Lake Baikal is one of approximately 13 ancient lakes in the world. Lake Baikal is also the deepest lake in the world with 365 rivers flowing into it and holding a full 20% of the liquid fresh water on the surface of the Earth. The lake stretches from the south‐eastern section of Siberia to portions of northern Mongolia. Administratively, it is divided between three members of the Russian Federation: the Republic of Buryatia, Irkutsk Oblast and Chita Oblast. Lake Baikal and its catchment area are characterized by considerable biodiversity and by a vast richness of natural resources. Well over half of the 2615 plant and animal species found in the Baikal region are endemic. Lake Baikal’s unusual biota includes the Baikal seal (Phoca sibirica) which lives nowhere else on Earth and is separated by 3220 km from its nearest relative. These freshwater seals give birth to their young in snow dens on the lake’s ice. The Baikal region is a spectacular example of coexistence among different cultures and faiths over centuries. Over 1000 archaeological remains of past cultures have been found near Lake Baikal shores. Preserving the unique lake culture, values and ecological, ethnic and cultural traditions of the local people is one of the key elements in developing a model for sustainable development in the Baikal region. Lake Baikal is currently still considered clear and healthy overall, however the number and type of pollution sources are growing, and local pollution can be severe. The main sources of pollution in Lake Baikal are industrial wastes and atmospheric emissions from the Baikalsk pulp and paper mill, atmospheric emissions from nearby industrial facilities, excessive nutrient loads from farming and sediment and wastes that flow into Lake Baikal through the Selenga River. In May 1999, protection of Lake Baikal was codified in federal law for the first time. Progress is being made on a number of fronts despite the turbulent political transformations in Russia in the last 10 years. For instance, in the Irkutsk region, stakeholders have worked together to reduce the flow of unpurified drainage into the lake by 10 million m³ year^–1.     

Distribution and carbon isotope composition of lipid biomarkers in Lake Erhai and Lake Gahai sediments on the Tibetan Plateau

Lipid biomarkers and carbon isotopes of n-alkanes, n-fatty acids, and n-alkan-2-ones were used to determine organic carbon sources to surface sediments in lakes Erhai and Gahai on the Tibetan Plateau. All sediment samples contained n-fatty acids with lower concentrations of n-alkanes and n-alkan-2-ones. Long chain n-alkanes in lake sediments were characteristic of a source mixture of epicuticular waxes of higher plants and submerged littoral zone plants while n-fatty acids sources were bacteria and floating and/or submergent macrophytes; the n-alkan-2-ones had a possible source in epicuticular waxes of higher plants and/or grass from catchment which entered the lake and were reworked by the microorganisms. Sediment samples examined from Lake Erhai had heavier δ¹³C values of bulk organic carbon and specific carbon compounds than samples from Lake Gahai and meadow soil. This heavier isotopic composition can be best interpreted by the enhanced productivity occurring in Lake Erhai due to its lower salinity and possibly to limited CO₂ concentrations; for the latter, this may have enriched ¹³C in the dissolved inorganic carbon pool of the lake water.     

Nearshore benthic blooms of filamentous green algae in Lake Baikal

For the first time, species of the genus Spirogyra, non-typical of the open nearshore waters of Lake Baikal, formed algal mats with Ulothrix zonata, Ulothrix tenerrima, and Ulothrix tenuissima near the village of Listvyanka, Russia. Normally widely distributed in the 0- to 1.5-m depth range, the growth of U. zonata was now evident and dominant (63% of the biomass) in the 2- to 5-m depth range. The overgrowth of the lake bottom by filamentous green algae, changes in distributional boundaries, the emergence and mass development of species of the genus Spirogyra, the presence of the eutrophic diatom indicator Fragilaria capucina var. vaucheriae, elevated abundances of coliform bacteria, and elevated levels of nutrients suggest an early stage of cultural eutrophication in the nearshore of Lake Baikal near Listvyanka, a popular tourist destination. The unusual abundance of Fragilaria associated with the filamentous green algae consisted of long-ribbon colonies of F. capucina var. vaucheriae, a eutrophic species, wound around the filamentous green algae, enhancing the dense algae mats. Historically dominant species, such as Didymosphenia geminata, Tetraspora cylindrica var. bullosa, and Draparnaldioides baicalensis typically observed at deeper depths of Lake Baikal, are now subdominants or minor species in the nearshore along the shoreline near Listvyanka.     

Evidence of a remnant self-sustaining strain of lake trout in the Lake Michigan basin

Managers have long embraced the need to maintain diversity as a requisite condition for population and community sustainability. In the case of Great Lakes lake trout, diversity has been severely compromised. The identification of new gamete sources may be beneficial to lake trout reintroduction efforts, particularly in situations where native stocks have been completely extirpated such as in Lake Michigan. Lake trout from Elk Lake, Michigan, are genetically distinct from domestic hatchery strains and historical forms of lake trout from Lake Michigan. Importantly, Elk Lake fish were genetically distinct from Marquette strain lake trout which were previously stocked into Elk Lake. Elk Lake fish were most similar to Lake Michigan basin-derived Lewis Lake (LLW) and Green Lake (GLW) hatchery strains and to historical Lake Michigan populations from the Charlevoix, Michigan area. While all individuals exhibited characteristics of lean form lake trout, the body shape of lake trout from Elk Lake, stocked lean fish from Lake Michigan and Lake Superior wild lean strains from near Isle Royale differed. Elk Lake fish were more fusiform, elongate, and streamlined with a narrower caudal peduncle compared to hatchery lean strains and wild lean forms from the Isle Royale region of Lake Superior. The lake trout population in Elk Lake is a remnant of a now extirpated native Lake Michigan population that was established either by natural colonization or stocking from historical Lake Michigan populations. Elk Lake lake trout is as genetically diverse as other strains used in Great Lakes reintroduction efforts and likely represent a viable gamete source representing genetic diversity lost from Lake Michigan.     

Methane and nitrous oxide measured throughout Lake Erie over all seasons indicate highest emissions from the eutrophic Western Basin

Eutrophication has been linked to increased greenhouse gas emissions from inland waters. Phytoplankton blooms in Lake Erie have increased since the 1990s, although its greenhouse gas emissions are not well characterized. We measured CH₄ and N₂O concentrations and diffusive fluxes in four seasons around the entire lake, and CO₂ fluxes in one summer season. Lake Erie is a source of CH₄ all year across the lake, concentrated in spring and summer in the Western Basin. Methane emissions ranged from 0.03 to 14.87 mg C m⁻² d^-1. Methane is predominantly biogenic, and natural gas leaks are an insignificant source. While Lake Erie is an overall N₂O source, it is an N₂O sink in winter and occasionally during summer. Emissions of N₂O ranged from −0.08 to 1.22 mg N m⁻² d^-1. We also measured CO₂ fluxes in summer only, when Lake Erie is a small atmospheric CO₂ sink. While areal fluxes of CH₄ and N₂O are similar to those observed elsewhere, total fluxes from Lake Erie are higher due to its surface area. Lake Erie emits ~ 6300 (±19%) metric tons of CH₄-C yr⁻¹ and ~600 (±37%) metric tons N₂O-N yr⁻¹: almost 500,000 metric tons CO₂-eq yr⁻¹ total. This is the first comprehensive dataset of CH₄ and N₂O concentrations and diffusive emissions in a very large lake. More measurements and monitoring are needed to determine whether increased eutrophication in the Great Lakes is tied to increased emissions of these powerful climate forcers in a possible positive feedback to climate warming.     

Fractioning of petroleum hydrocarbons from seeped oil as a factor of purity preservation of water in Lake Baikal (Russia)

Polycyclic aromatic hydrocarbons (PAHs) and n-alkanes were found in five different types of oil-containing samples emanating from deep oil seeps in Lake Baikal, the deepest and oldest lake in the world containing 20% of the world’s surface freshwater. The n-alkanes were the dominant component of the seeped oil on the water surface. PAHs were minor compounds of the oil, but this group is included in a priority series of organic contaminants that are subject to environmental regulation. The set of studied samples from the seep included: (i) freshly-emitted oil on the lake surface, (ii) oil at the lake bottom, (iii) samples of asphalt towers at the lake bottom, (iv) oil from voids within the sediment core and (v) samples of water from different depths. High variability in the concentrations of n-alkanes and PAHs suggested the presence of oil fractionation at the sediment-water interface. A light fraction of oil enriched by n-alkanes migrates to the surface of the lake’s water column, where n-alkanes are biodegraded by the Baikal microbial community while heavy oil fractions remain at the lake bottom and form asphalt towers in which PAHs with maximum toxicity accumulate. In the lake’s photic water layer, PAHs from light oil can be accumulated by phytoplankton. In the lake’s water column, benzo[a]pyrene was not detected over their detection limit (0.1 ng/L). The Toxic Equivalent (TEQ) value of PAHs identified in the Baikal water ranged from 0.001 to 0.012 ng/L.     

Current state of the sponge fauna (Porifera: Lubomirskiidae) of Lake Baikal: Sponge disease and the problem of conservation of diversity

The spatial distribution of healthy and diseased sponges in Lake Baikal was investigated. The endemic sponges were studied in May ? June 2015 at 11 transects within the depth range of 0 to 40 m. More than 500 sponge specimens were collected, and 29 taxa including 12 species and 17 morphospecies (sp. taxa) were identified. Among the 29 taxa, only B. martinsoni specimens and 14 morphospecies were healthy, whereas specimens of the other 11 species and 3 morphospecies were diseased, making up 35% of the total number of specimens (512). The different body forms of sponge found among the specimens were encrusting (66%), globulous (21%), and branched (13%). Sponges with necrosis signs, discolored specimens or covered with violet film or brown spots, were found in all transects under study (37 stations of 68). The percent cover of the bottom (per m²) by various body forms of sponges was evaluated with a sponge faunal state assessment test. Changes in the diversity and abundance of sponges due to their disease may jeopardize the functioning of the entire ecosystem of Lake Baikal which is a World Heritage Site. This baseline data on patterns of sponges, including their spatial distribution and percent cover, will be utilized to support future management and monitoring of Lake Baikal. Data on the current state of Baikal endemic sponge fauna are essential for understanding the causes and prognosis of disease processes of freshwater sponges.     

Distribution of seston and nutrient concentrations in the eastern basin of Lake Erie pre- and post-dreissenid mussel invasion

Increased human population growth, reduction of phosphorus (P) loading, and the invasion of dreissenid mussels may have changed the spatial pattern and relationships between the nearshore and the offshore seston and nutrient concentrations in the eastern basin of Lake Erie over the past 30 years. We compared seston characteristics, nutrient concentrations, and phytoplankton nutrient status between nearshore and offshore zones in years before (1973–1985) and after (1990–2003) the dreissenid invasion. In 1973 (the only pre-dreissenid year nearshore data was collected), chlorophyll a (chla) and nutrient concentrations were higher nearshore than offshore. In post-dreissenid years, nearshore chla concentrations became significantly lower than the offshore, while carbon (C):chla ratios became higher, which was related to mussel grazing and possibly photoacclimation. Phosphorus deficiency in the phytoplankton increased over the 30-year period, and in the post-dreissenid years was less acute in the nearshore than offshore. Mean water column irradiance became higher in the nearshore relative to the offshore in the post-dreissenid years. The nutrient changes and phytoplankton physiology were consistent with the expected effects of nutrient cycling by mussels and diminished demand by phytoplankton despite increased demand from benthic algae in the nearshore. This basin-scale study suggests that dreissenid mussel invasion can be associated with alterations in the spatial pattern of water column properties in large lakes even on open coasts with vigorous circulation and exchange.     

Comparative analysis of free-living and particle-associated bacterial communities of Lake Baikal during the ice-covered period

Free-living (FLC) and particle-associated (PAC) bacterial communities contribute differentially to the cycling of matter in the environment. In this work we have for the first time studied the taxonomic composition and diversity of FLC and PAC in the oligotrophic freshwater Lake Baikal during the under-ice plankton peak. The communities on the bottom ice surface and in the water column under the ice were studied. Analysis by light microscopy showed that the bottom surface of the ice was dominated by green algae Chlorella spp. A complex of microalgae developed in the water column: green algae Monoraphidium griffithii, Monoraphidium contortum, Chlorella spp., chrysophyte Dinobryon cylindricum, haptophyte Chrysochromulina parva, and diatom Synedra acus. Algal abundance on the ice sub-surface was several orders of magnitude higher than that in the water column. According to pyrosequencing data of the bacterial 16S rRNA V3-V4 region, diversity of FLC (as measured by Shannon, Simpson, Chao1 and ACE indices) was higher than that of PAC. Proteobacteria (Burkholderiaceae) dominated the FLC associated with ice; FLC of the water column under ice were dominated by Actinobacteria (Sporichthyaceae and Ilumatobacteraceae) and Verrucomicrobia (Methylacidiphilaceae). PAC associated with ice were dominated by Bacteroidetes (Flavobacteriaceae), while water column-associated PAC were dominated by Verrucomicrobia (FukuN18_freshwater_group, Methylacidiphilaceae) and cyanobacteria (Cyanobiaceae). The difference between free-living and particle-associated, as well as between water column and ice sub-surface bacterial communities, were explored on operational taxonomic unit (OTU) and phylotype levels.     

Ecological assessment of the Selenga River basin,the main tributary of Lake Baikal,using aquatic macroinvertebrate communities as bioindicators

The Selenga River is the main tributary of Lake Baikal (Siberian, Russia). In 2015/2016, the water quality at previously identified contaminated hotspot regions in the lower Selenga River basin was evaluated using resident aquatic macroinvertebrate communities as bioindicators. Benthic macroinvertebrate communities within the Selenga River were found to be relatively sensitive to water pollution as was highlighted by three evaluated biotic indices:Average Score per Taxon (ASPT); Ephemeroptera-Plecoptera-Trichoptera density index (EPT); and Trent Biological Index (TBI). The human impact on the Selenga River basin water quality was evident due to the significant decrease of the biotic indices at several sample locations including downstream of the wastewater discharge point of Ulan-Ude city, in the Dzhida River downstream of the confluence of the Modonkul River, and especially in the Modonkul River near to the mining operations at Zakamensk. At the same time, our study revealed a high self-regeneration ability of the aquatic ecosystem throughout the basin; with resident benthic macroinvertebrate communities appearing to recover in both the Selenga River and the Dzhida River within two to five km downstream of the contamination source. The changes in the benthic communities at the Selenga delta sampling sites were shown to occur under the influence of natural factors such as hydrological conditions and benthic sediment type, which significantly changed from the upper to the lower regions of the delta. For the Selenga delta, a typology of benthic macroinvertebrate communities including a map of their spatial distribution is presented.     

Small eddies observed in Lake Superior using SAR and sea surface temperature imagery

Making use of the fine resolution of satellite SAR imagery, we observe small eddies during the spring and summer months in several locations in Lake Superior. During these months there is a thermal gradient between warmer nearshore waters and colder offshore waters which enhances cyclonic coastal currents. Using spaceborne SAR imagery from the European Space Agency's ERS-1 and ERS-2 missions from 1992 to 1998, we observe small eddies, identifying and mapping basic eddy characteristics including diameter, location, and rotational sense. In total, 45 eddies were located, of which 41 were cyclonic and 4 anticyclonic. Average diameter was 9.8 km and average distance to shore was 8.1 km. Based on sea surface temperature data from AVHRR, the eddies are located within the region of sharp thermal gradients of order 3–5 °C per 3 km. Spatial and temporal coverage was uneven, however, more eddies were seen in SAR images taken in late summer along the southern and eastern shores as well as areas where the boundary current interacts with topographic features including islands and promontories.     

Accumulation rates,focusing factors,and chronologies from depth profiles of 210Pb and 137Cs in sediments of the Laurentian Great Lakes

Sediment cores from 41 sites were collected from the Laurentian Great Lakes during 2010–2014, sectioned into 0.5–2.0?cm intervals, and the activities of ²¹⁰Pb, ¹³⁷Cs, and ²²⁶Ra were measured in the upper 25 to 40?cm of the sediment column by gamma spectrometry. Sediment mass accumulation rates (dry mass) calculated from ²¹⁰Pb profiles range from 0.006?±?0.001 to 0.59?±?0.06?g?cm^?2?yr ^?1 and are similar to those reported in previous Great Lakes sediment studies. Sediment mass accumulation rates decreased with increasing water depth. ²¹⁰Pb-based models in cores exhibiting favorable characteristics (i.e., those having the highest unsupported-²¹⁰Pb activity at the sediment-water interface, exponential decrease of unsupported-²¹⁰Pb with increasing depth in sediment cores, and a clear peak in ¹³⁷Cs activity at some depth below the sediment-water interface) give calendar date profiles that are largely concordant with the maximum ¹³⁷Cs peak activity at 1963. Sediment focusing factors derived from unsupported-²¹⁰Pb inventories range from 0.09 to >5.34, and are well correlated with those derived from ¹³⁷Cs inventories that range from 0.07 to 4.04, demonstrating the ubiquitous occurrence of horizontal sediment transport processes within the lakes. This more recent survey provides a Great Lakes-wide chronological framework for comparing the depositional histories and inventories of a wide variety of persistent, bioaccumulative and toxic pollutants that have been measured in the same sediment cores. This information will be useful for resolving scientific and practical issues pertaining to the environmental quality and management of contaminated sediments in the Laurentian Great Lakes ecosystem.     

Dolichospermum blooms in Lake Superior: DNA-based approach provides insight to the past,present and future of blooms

Cyanobacterial blooms are increasing in frequency, duration, and severity globally in freshwater ecosystems. The Laurentian Great Lakes are prone to toxin-producing cyanobacterial blooms and have experienced annually recurring blooms. Because of its oligotrophic nature, Lake Superior has been relatively free of bloom occurrences. However, in recent years, Dolichospermum blooms have occurred with increasing frequency, especially in the western arm. During a Dolichospermum bloom in 2018, opportunistic samples were collected from the offshore bloom and investigated with shotgun metagenomics. We identified a near-complete Dolichospermum genome that is highly similar to genomes from cultures recovered in Lakes Erie and Ontario. The genomes from the Laurentian Great Lakes are typified by their putative ability to produce a suite of secondary metabolites like anabaenopeptin, but not toxins like microcystin. Additionally, we recovered a Dolichospermum lemmermannii 16S rRNA gene from the bloom and using datasets collected from the epilimnion and sediments in Lake Superior show this organism is ubiquitous and that several strains may exist. While there is much to learn about Lake Superior cyanobacterial bloom development and triggers, understanding this organism is endemic to the region, what its genome is capable of and that specific strains may have provenance within the lake provides a distinct ecological basis for understanding and working towards a predictive framework for future blooms.     

Genetic estimates of jurisdictional and strain contributions to the northeastern Lake Michigan brown trout sportfishing harvest

The Lake Michigan brown trout (Salmo trutta) fishery is sustained by the stocking of five hatchery strains by four state natural resource agencies. In the absence of exhaustive marking programs, strain-specific measures of stocking success are lacking for brown trout in Lake Michigan. We used microsatellite-based genetic assignment testing and genetic stock identification (GSI) to determine the strain of 122 angler-caught brown trout from four northeastern Lake Michigan ports. We compared strain composition estimates for sportfishing harvest to expected proportions of each brown trout strain in Lake Michigan at the time of harvest using stocking records corrected for age-specific mortality rates. Reassignment rates of individuals from baseline strains averaged 92.1% (range: 84.1–98.0%). Assignment testing and GSI analyses consistently found Wild Rose strain brown trout represented approximately 89% of the northeastern Lake Michigan sportfishing harvest, while only comprising 43.8% of the expected stock. Of the Michigan angler harvest of Wild Rose strain brown trout, approximately half were estimated to have originated from Wisconsin hatcheries, demonstrating a propensity for lake-wide movements. Continued assessments will improve understanding of strain relative contributions to angler harvests that can direct future stocking efforts.     

Microplastic pollutants in the coastal dunes of Lake Erie and Lake Ontario

Microplastic particles, often studied as aquatic pollutants, have been recovered from coastal dunes along the shores of Lake Ontario and Lake Erie in New York and Pennsylvania. Surface and shallow sub-surface sand samples were collected from coastal dunes in 1 m² areas from 5 locations along Lake Erie: Sunset Bay, Dunkirk Harbor, Point Gratiot, and Canadaway Creek in New York and Presque Isle State Park Beach #11 in Pennsylvania. Samples were also collected from coastal dunes on Lake Ontario at Sandy Island Beach State Park, NY. Abundances, shapes, sizes, textures, and degradation of microplastics were characterized. Twenty-one of 26 samples yielded a variety of microplastics: pellets, fragments, and fibers. Larger microplastics (5.0–1.0 mm) were dominated by spheroidal and disk-shaped pellets with fewer fragments. Smaller microplastics (≤1.0 mm) were predominantly fibers and small fragments. Some microplastic particles exhibited evidence of degradation and weathering as a consequence of transport and exposure to the elements. The presence of microplastics in coastal dunes is attributable to aeolian transport from the adjacent beach.     

Spatial and temporal variations of dissolved CO2, CH4 and N2O in Lakes Edward and George (East Africa)

We report dissolved CO₂, CH₄ and N₂O concentrations in two large East African lakes, Edward (surface area 2,325 km², average depth of 37 m) and George (surface area 273 km², average depth of 2 m). Lake George showed modest seasonal and spatial variations, and lower partial pressure of CO₂ (pCO₂) (26 ± 16 ppm, mean ± standard deviation), CH₄ (234 ± 208 nmol/L) and N₂O saturation levels (%N₂O) (80 ± 9 %) than Lake Edward (404 ± 145 ppm, 357 ± 483 nmol/L, 139 ± 222 %). Surface waters in both lakes were over-saturated in CH₄, and Lake George was under-saturated in CO₂ while Lake Edward was slightly over-saturated in CO₂. This difference was related to higher phytoplankton biomass in Lake George than Lake Edward, with average chlorophyll-a concentrations of 177 ± 125 and 18 ± 25 µg/L, respectively. Permanent high cyanobacterial biomass in Lake George led to uniform dissolved CO₂, CH₄ and N₂O concentrations. In surface waters of Lake Edward, spatial variations of pCO₂, CH₄ and N₂O were related to bottom depth, and locally (in particular in Katwe Bay) also related to the inputs of water from Lake George via the Kazinga Channel, a 40-km natural channel connecting the lakes. Short-term mixing events related to storms increased CO₂, CH₄ and N₂O content in surface waters, in particular for CH₄ and N₂O. This indicates that mixing events in response to storms can create ‘hot moments’ for CH₄ and N₂O emissions to the atmosphere in tropical lakes, given the weaker vertical density gradients compared to higher latitude systems.     

Revisiting the Great Lakes Water Quality Agreement phosphorus targets and predicting the trophic status of Lake Michigan

LM3-Eutro is a high-resolution eutrophication model with several improved features lacking in historical Great Lakes models. We calibrated LM3-Eutro using a 2-year (1994-1995) dataset and performed a hindcast simulation from 1976 to 1995 to evaluate the model's ability to make predictions over an extended period of time. Results show a reasonable agreement between model output and field data over this time period. The model predicted that an annual loading of 5600 metric tons (MT) would result in a lake-wide annual total phosphorus (TP) concentration of 7.5 μg L^− 1. Using best estimates of future TP loadings, LM3-Eutro forecasts suggest that Lake Michigan will remain oligotrophic and will continue to meet the 7 μg L^− 1 spring TP concentration Great Lakes Water Quality Agreement objective.