We describe here the first SEM observations on the auxospores of two species of Epithemia, E. reicheltii and E. lacus-douglasi, from Douglas Lake, Michigan, USA. Both species were found in abundance with auxospores in October of 2024. The auxospores of both species were semicircular in shape, initially covered with an organic matrix, which split as incunabular caps. Internal to this matrix were transverse incunabular rows of quadrangular, imbricating scales overlapping towards the valve ends, with the longer abaxial edges fimbriate and the other edges straight. And, internal to the scales, was a single, siliceous covering - the primary longitudinal perizonial band - which had transversely-oriented fissures. Internal to the primary band were three (or four) dorsally positioned secondary longitudinal perizonial bands. The complex coverings of the auxospores of these Epithemia species are unique amongst the diatoms.

In the past 50 years, there has been an apparent increase in the frequency and magnitude of abrupt change in large freshwater ecosystems, including from altered nutrient fluxes, invasive species, and climate. However, often relatively little is known of historical changes in ecosystem function due to difficulties in representative sampling. We used paleoecological analysis of multiple biological (diatoms, chironomids, fossil pigments), geochemical (organics, carbonates, mineral, biogenic silica, C/N isotopes), and sedimentary DNA (sedDNA, specifically botulinum neurotoxin BONT/E gene copies) proxies in three sediment cores to quantify timing, extent, and causes of ecosystem change over the past 150 years in northeast Lake Michigan and evaluate the role of nearshore food-web variability in recent botulism outbreaks. Sediment cores preserved records of nearshore food web changes including shifting phytoplankton, historical eutrophication and recent oligotrophication, nearshore energy shunt, and restructuring of the nearshore food web and ecology. The deepest station sediments recorded significant changes in the early 1900s in response to Euro-American settlement, catchment development, and whole-lake eutrophication. Principal coordinates analysis of biological sediment proxies also identified unprecedented historical changes in primary and secondary producer communities in the late 1990s–early 2000s at all nearshore stations, with particularly marked changes in diatom community structure, primary producer biomass, and chironomid assemblages. Further, sedimentary analyses implicate changes in Cladophora abundance and the nearshore benthic food-web as important recent covariates of increased prevalence of Clostridium botulinum. Together, these findings suggest that large-scale changes in offshore ecosystems are amplified in coastal regions in large lakes and may be important regulators of aquatic disease transmission.

During recent investigations on the diatom and algal flora from the Aras Dam, northwestern Iran, an interesting epilithic diatom species belonging to the genus Nitzschia was collected and examined using both light and scanning electron microscopy. This diatom species shares morphological and taxonomic features with and N. potapovae; however, the taxon possesses narrow, elongate frustules that are weakly sigmoid in both valve and girdle views. The sigmoid character shows itself mostly at the apex of the valve and sometimes results in a rhomboid-like appearance; it is less clear in valve view and becomes weaker in the largest frustules, which are nearly symmetrical along the apical, transapical, and pervalvar axes. Valve ends are acutely rounded and blunt, but in shorter and more sigmoid valves, they become pointed to nearly triangular. Morphology and comparison with related taxa suggests the Aras Dam species is new and here described as Nitzschia arasensis Atazadeh, Ghavam et Edlund sp. nov. Water chemistry in the Aras Dam suggests that N. arasensis inhabits fresh to slightly brackish water.

P availability plays a fundamental role in regulating primary production in Arctic stream ecosystems, where baseline nutrient concentrations are typically low. As the Arctic undergoes rapid environmental changes-including permafrost thaw, altered hydrology, and increased nutrient flux-understanding the ecological responses of sustained nutrient enrichment is increasingly important. Diatoms, which are sensitive to nutrient availability and environmental conditions, are powerful indicators of change in Arctic freshwaters, where low baseline productivity and accelerating nutrient inputs underscore the need to understand ecosystem responses to enrichment. We evaluated the effects of long-term P enrichment on diatom assemblages of the Upper Kuparuk River, Alaska (USA), by comparing epilithic diatom samples collected annually in fertilized and reference reaches between 1997 and 2022. Previous research documented that the low-level P enrichment profoundly influenced community structure in the fertilized reach of the river, notably by stimulating luxuriant growth of 2 species of mosses that had never been previously observed in the reach. However, a 25-y-long set of epilithic diatom samples from the P-enrichment experiment had not been previously analyzed. Permutational multivariate analysis of variance and redundancy analysis indicated persistent shifts in diatom species abundance attributable to P enrichment. In particular, the species Nitzschia paleacea Grunow and Reimeria sinuata (W. Greg.) Kociolek & Stoermer were reliable indicators of P-enriched conditions. Diatom assemblages did not immediately revert to pre-fertilization compositions when the P-fertilization treatment was terminated in 2017. This lack of reversion was likely due to an interaction with the moss assemblage that colonized the fertilized reach during the enrichment phase of this experiment and persisted for several years after the enrichment ended. These findings highlight the importance of long-term studies in understanding and managing the effects of nutrient enrichment on aquatic ecosystems, particularly in regions facing rapid environmental changes.

Harmful blooms of cyanobacteria (cyanoHABs) are increasingly reported in remote and low-nutrient lakes, prompting a paradigm shift in our understanding of the drivers of cyanoHABs and a need to study factors beyond watershed nutrient inputs. In this study, we examined eight lakes within the Superior National Forest (Minnesota, USA) to characterize the role of lake stratification and oxygen depletion on nutrient and cyanobacteria abundance. Study lakes represented a gradient of mixing regimes, including well-mixed, dimictic, and polymictic. We found that mixing regime was a significant control on epilimnetic nutrient concentrations and cyanobacterial abundance, likely mediated through anoxic bottom waters and sediment phosphorus release. The frequent and intermittent lake turnover with anoxic bottom waters, which we term ‘anoximixis’, can help explain the rise of cyanoHABs in relatively pristine lakes with little watershed nutrient input. Climate change is predicted to alter stratification regimes which will promote more frequent anoximixis, sediment phosphorus release, and cyanoHAB formation; therefore, improving our understanding of the linkages between lake mixing and cyanoHABs will allow us to better characterize the effects of climate change in remote and low-nutrient input lakes.

Significant changes have been proposed for the order Rhopalodiales, including subsuming Rhopalodia O.Mull. into Epithemia Kutz. to maintain monophyly and recognition of the genus Tetralunata Hamsher et al. However, hesitancies about adopting these higher-level classifications remain. We report widespread and homologous internal valve and life history traits as evidence to guide higher-level classification within the Rhopalodiales. We provide the first reports of internal valve formations in five species and show examples from six species representing multiple branches in the Rhopalodiales: Epithemia (Rhopalodia) gibba (Ehrenb.) Epithemia catenata Schvarcz, Stancheva et Steward, and Epithemia reicheltii Fricke. This observation, along with shared, unique cell development and life history traits including endosymbiosis and sexual reproduction, should be considered in phylogenetic revisions.

Diatom fossils offer reliable means of evaluating biotic, climatic, and ecological conditions of past environments. Here we present new insights in reconstructing the mid-Miocene paleoenvironment of the Ashfall Fossil Beds in Nebraska, USA. Extensive excavation of megafauna and flora at the Ashfall site has provided a well-preserved glimpse into the Clarendonian Land Mammal age of the North American Plains. From sedimentary evidence the environment at the site has been described as a small waterhole in a seasonally wet depression located in a stream bed. Here we report a diatom fossil assemblage containing 38 species spanning 30 genera found in the ash and underlying sand formations at the Ashfall site. We also provide insights into environmental conditions prior to and during the eruption and deposition of the ash, including water chemistry. Diatom assemblages from the underlying sand and the ash deposits indicate drastically different depositional environments. We find that the diatom fossils robustly separate across the sand-ash transition and propose two possible explanations for the differences observed in the assemblages. Either the diatom assemblages across the sand-ash transition represent a dry to wet season preservation, or the ash from the eruption changed the water chemistry and the diatom assemblage changed in response to changing water conditions. It is also possible both scenarios have some influence on diatom composition.

Late summer, recurring cyanobacterial blooms in Lake of the Woods (LOW) are polycyanobacterial and typically dominated by Aphanizomenon flos-aquae. LOW waters are typically nitrogen limited, relative to phosphorus. As such, the dominance of Aphanizomenon flos-aquae, a putative nitrogen-fixing cyanobacterium, suggests that its ability to fix nitrogen may be advantageous and aid in its ability to bloom. This study sought to quantify nitrogen fixation rates and identify cyanotoxin-producing species during the blooms. Throughout the 2021 season, we quantified nutrients, N-fixation rates, microbial community composition, and gene expression to determine who is responsible for cyanotoxin synthesis and nitrogen fixation. We found nitrogen fixation rates increased throughout the season, coincided with the bloom, but likely cannot fully support the bloom's nitrogen demand. However, the transcription of nitrogenase genes was solely done by less abundant Dolichospermum spp. and not by A. flos-aquae. Genome analysis suggests this population of A. flos-aquae cannot create a functioning nitrogenase, but they do still express the genes to initiate heterocyst differentiation. Microcystin gene transcripts were primarily from Microcystis spp. and Planktothrix spp. and coincided with microcystin concentrations. Interestingly, Planktothrix highly expressed anabaenopeptin genes, suggesting the presence of additional bioactive compounds in LOW. This work suggests that rare cyanobacterial members drive nitrogen fixation, and may be necessary for the seasonal bloom's function, toxicity, and longevity.

Surface sediment analysis offers a cost-effective way to document the spread of exotic micro-organisms, potentially presaging the spread of other exotic and invasive species. An analysis of surface sediment samples from 11 inland lakes in Ohio revealed the presence of an exotic diatom species, Discostella asterocostata, in seven lakes. While the presence of D. asterocostata has been documented in the waterways of surrounding states, this is the first documentation of the species in the lakes of Ohio. Previous research suggests the exotic diatom may have crossed the Pacific in the digestive tracts of the invasive bighead (Hypophthalmichthys nobilis) and silver carp (Hypophthalmichthys molitrix), planktivorous fish that were originally imported from Asia to North America for aquaculture. However, these carp species have not been reported in the Ohio lakes in which the exotic diatom was found, making it likely that D. asterocostata has spread throughout the state’s inland waterways via a different vector, potentially stocked fish.All seven lakes containing D. asterocostata are in the Ohio River Watershed, while the remaining four without D. asterocostata are in the Lake Erie Watershed. Ordination of the diatom assemblage data from each lake revealed that the diatom communities form two distinct groups, separated by watershed, which were significantly different from each other. Exotic and invasive diatoms can outcompete native diatoms and can potentially affect the environmental availability of nutrients and silica in a lake, leading to the restructuring of algal communities; however, it is currently unclear whether the presence of D. asterocostata is causing a restructuring of the assemblages in this case. Future work should focus on retrieval and analysis of sediment cores to shed light on the timing and ecological consequences of D. asterocostata’s introduction into Ohio’s inland lakes.

A macroalgal colony collected on Thalassia blades from the El Palo Reef on Puerto Rico's southern coastline represents a new species of schizonematous symmetrical biraphid diatom, Berkeleya witkowskii sp. nov. Additional collections of the golden yellow-brown floral-shaped colonies (2-6 cm diameter) collected epiphytic on Thalassia blades at other reefs along Puerto Rico's southern coast confirm the broader distribution of the new taxon and provided cells with lanceolate valves ranging from 32-47 mu m long with acutely rounded to subrostrate ends that differ from known Berkeleya taxa in colony form, valve shape, size, separation of proximal raphe ends, and stria density.

Headwater streams draining northern hardwood forests are vulnerable to environmental pressures, including climate change, atmospheric deposition, land-use changes, and recreational impacts. These streams play a crucial role in the forest ecosystem by influencing nutrient dynamics, water quality, and serving as refugia for aquatic species. This study focuses on epiphytic diatom communities that colonize bryophytes within Hubbard Brook Experimental Forest (HBEF), where extensive ecological research exists but with limited data on diatom community composition. Bryophytes provide stable, nutrient-rich microhabitats that support diatom growth, offering refugia from flood scour and accumulating stream sediments, making them useful for studying diatom dynamics. We compared diatom communities colonizing artificial bryophytes across seven streams weir ponds in HBEF in New Hampshire and explored the influence of various environmental factors on species richness. Distinct diatom communities were observed, along with increased species richness under higher light exposure. Further analysis reveals significant variability in diatom communities driven by watershed-specific environmental conditions and temporal changes. A total of 86 diatom taxa spanning 43 genera were identified, with species presence significantly associated with environmental variables such as light intensity (lux), dissolved organic carbon, pH, and total dissolved nitrogen. These findings reinforce the value of Diatoms are sensitive environmental indicators in this study, provide useful insights into ecosystem health and resilience of streams flowing through northern hardwood forests in the face of environmental stressors.

Burge DRL, Edlund MB, Bowe S, Bowe K, Anderson JP, Bouchard RW, Heathcote AJ, Leavitt PR, Engstrom DR. 2023. Managing the Red Lake Nation's and Minnesota's largest lake: monitoring and paleolimnology support a site-specific standard for Upper and Lower Red Lakes (Red Lake Nation and Minnesota, United States). Lake Reserv Manage. XX:XXX-XXX.Water quality and the ability of lakes to provide benefits such as recreation and aquatic life are negatively impacted by climate change, aquatic invasive species, and nutrient enrichment. To address these lake-management challenges, regionally tailored water quality criteria need to assign achievable and protective goals while minimizing assessment errors. In Minnesota, when lakes exceed state water-quality standards, restoration plans are developed and implemented based on eco-regional standards, or in cases where it is determined that the regional standard is not appropriate, a site-specific standard may be developed. Upper Red Lake and Lower Red Lake, located within the boundaries of the Red Lake Indian Reservation (the Red Lake Nation) in northwestern Minnesota (United States), have been recently experiencing cyanobacteria blooms, as well as routinely exceeding the regional water quality criteria for total phosphorus and chlorophyll a. The last 20 yr of water quality monitoring suggest a stable recent history with no significant trends. To extend this time perspective, biogeochemical evidence from paleolimnology was used to reconstruct a 150 yr history for these 2 large, shallow lakes to determine whether site-specific water quality standards might be recommended. Biogeochemical evidence (e.g., sedimentation rates, phosphorus fractions, biogenic silica, diatom community composition, and fossil algal pigments) from 10 sediment cores revealed complex sedimentary dynamics together with a small gradual increase in limnological productivity over the last 2 centuries. Despite the implied productivity change, diatom-inferred phosphorus showed no significant increase among the sediment cores. Furthermore, the reconstructed total phosphorus values were within the range of modern monitored values. Because the paleolimnological evidence suggests little change to the aquatic ecology of the Red Lakes over the last 150 yr, and monitoring data show regular exceedance of regional TP standards with no trend in the last 20 yr, site-specific water quality standards are deemed appropriate for this large, shallow lake system.

Responses of Arctic lakes to climate change are complex and heterogeneous at regional to local scales, but there are few studies comparing multiple sites within the same landscape. We studied sediment cores recovered from seven lake sites located within the forest-tundra biome of northern Manitoba, Canada. We first used sediment core pollen from the seven sites to reconstruct July temperatures and total annual precipitation for the past 7500 years based on the modern analog technique. Lake responses and peatland expansion were analyzed based on changes in the concentration and quality of organic matter, biogenic silica (bSi), Sphagnum spores, and published peat core basal dates. Changes in sediment organic matter and bSi were concentrated prior to 4500 cal. Yr BP. From 7500 to 3100 cal. Yr BP, July temperatures rose by ~3°C, and total annual precipitation fell by 200 mm; there was little change in temperature or precipitation after 3100 cal. Yr BP. Sphagnum spore concentrations exceeded 0.5% at all of the sites at 7500 cal. Yr BP indicating the early presence of peatlands. Spores increased markedly after 4500 cal. Yr BP in agreement with timing of peat basal dates, but the magnitude and timing of changes varied widely among sites likely reflecting local differences in hydrology or topography. The use of hierarchical generalized additive models provided a unified regional pattern of Holocene climate for northern Manitoba, even though lake responses to climate and peatland expansion differed greatly.

Coastal rock pools of Lake Superior in North America are a mosaic of biologically rich ecosystems that vary in their physical morphology and chemistry. Occurring in bedrock depressions between the forest edge and the lake, the aquatic communities in these pools are susceptible to impacts from both international shipping lane traffic and changing climate patterns. Individual pools are small, but the number of pools across the coastal landscape provide an expansive network of variable habitats, with >70,000 pools found in the 48 km of shoreline in the study area. In this study, we aimed to understand the relationships between aquatic invertebrates and abiotic conditions in coastal pools. The Dipteran family Chironomidae were studied because of their ubiquity and diversity in freshwater systems. We used nonmetric multidimensional scaling to describe how Chironomidae diversity and abundance were related to seasonality, habitat, and environmental factors (water chemistry and trace metals). We observed 2 distinct chironomid assemblages associated with habitat type based on chemical characteristics and physical location on the shoreline, including numerous rheophilic species near the lake, where consistent wave wash mimics lotic conditions, and species preferring lentic-mesotrophic conditions near the forest edge. Conductivity and pH were higher in pools near the lake, whereas temperature and dissolved O-2 varied seasonally. Nutrients and metals differed between study sites and pools at each site. At 2 sites, naturally occurring Cu and Al were found at concentrations that exceeded estimated chronic toxicity for sensitive aquatic invertebrates. Pool separation of known tolerant or sensitive species indicated a potential response to elevated metal concentrations. Metals may be an important driver for sensitive species occupancy and survival in certain pools, raising more general concerns regarding community responses to natural conditions in pristine environments.

The Lut Desert is one of the world’s driest and hottest large deserts. This large basin has shallow groundwater pools that have been studied little for microbial flora, especially diatoms, and the present study is the second attempt. We report a new genus with one new species and the related morphotypes. The genus Lutophila and its type species L. iranica is found in hypersaline pools and a saline river in the Central Lut Desert depression, an area known as the hottest spot in the world. The morphology, systematic position, distribution, and ecological conditions of the new species and its morphotypes are discussed. The new genus Lutophila has a linear-lanceolate, symmetric outline with broadly rounded apices and two kinds of areolae, one on the valve face and one on the valve mantle; these characters resemble those shared by the diatom genera Krasskella, Biremis, Chamaepinnularia and Oestrupia. Lutophila iranica exhibits two morphotypes with similar outline; however, L. iranica morphotype 2 is wider than the L. iranica morphotype 1 and has a more distinctive central area.

Our understanding of drivers of cyanobacterial harmful algal blooms (cHABs) is evolving, but it is apparent that not all lakes are created equal. Nitrogen (N) is an important component of all cHABs and is crucial for cyanotoxin production. It is generally assumed that external nitrogen inputs are the primary N source for cHABs. However, in northern lakes, nitrogen inputs are typically low, and suggests that internal nitrogen cycling, through heterotrophic organic matter decomposition or nitrogen fixation, may play a significant role in cHAB development and sustainment. Using Lake of the Woods as a testbed, we quantified nutrients, cyanotoxins, nitrogen fixation, and the microbial community in the southern extent of the lake. During our temporal study, inorganic nitrogen species (NO 3 - +NO 2 - and NH 4 + ) were either at very low concentrations or below detection, while phosphorus was in excess. These conditions resulted in nitrogen-deficient growth and thereby favored nitrogen fixing cyanobacterial species. In response, nitrogen fixation rates increased exponentially throughout the summer and coincided with the Aphanizomenon sp. bloom. Despite nitrogen limitation, microcystin, anatoxin, saxitoxin, and cylindrospermopsin were all detected, with microcystin being the most abundant cyanotoxin detected. Microcystin concentrations were highest when free nitrogen was available and coincided with an increase in Microcystis. Together, our work suggests that internal nitrogen dynamics are responsible for the dominance of nitrogen fixing cyanobacteria and that additions of nitrogen may increase the likelihood of other cyanobacterial species, currently at low abundance, to increase growth and cyanotoxin production. Statement of Significance This study is the first assessment of nitrogen fixation rates and water column 16S rRNA gene amplicon sequencing in Lake of the Woods during a harmful algal bloom season. The aim of this study is to better understand nitrogen dynamics and the microbial ecology of cyanobacterial harmful algal blooms on Lake of the Woods. Result from this study reveal that internal nitrogen cycling via nitrogen fixation may alleviate nitrogen deficiencies, and structure and control the cyanobacterial community and cyanotoxin production. Molecular analysis reveals that cyanotoxins in Lake of the Woods are produced by less abundant cyanobacteria that are limited by nitrogen. This study has significant management implication as agencies continue to mitigate toxic blooms on Lake of the Woods, the largest shoreline lake in the United States. Our work is an important initial assessment and jumping off point for further research on Lake of the Woods when assessing how nitrogen plays a role in bloom formation and toxicity. Submitting to L&O, we believe would allow for the greatest outreach and access to an audience that will continue to build upon our findings. Additionally, submitting with L&O our work will reach beyond the scientific audience, but also reach other parties participating in the mitigation of harmful algal blooms.