Trophic state resilience to hurricane disturbance of Lake Yojoa, Honduras

Impact of extreme coastal events on a brackish lake on the Burin Peninsula, Newfoundland, Canada

The impacts of extreme coastal events (ECEs), such as marine inundations or extreme wind events, on lake ecosystems vary widely from minimal to catastrophic. Accurately predicting the response of a specific system remains challenging due to a limited understanding of the attributes that drive the resilience of lakes. In an attempt to better understand the possible impacts of ECEs on shallow brackish lakes, we employed a paleolimnological approach to reconstruct the responses of Broad Pond to four ECEs identifiable from clear sedimentary markers and dated to ca. 1740, 1790, 1862, and 1993 CE. We aimed to evaluate the overall impacts of ECEs on Broad Pond and their specific effects on various hydrobiont groups. We reinvestigated a previously studied sediment core covering the last ca. 400 years by examining terrestrial and aquatic pollen, spores and non-pollen palynomorphs, cladocerans, and chironomids. Additionally, available diatom data were reexamined. Overall, Broad Pond exhibited resilient responses to ECEs, as indicated by mostly limited compositional turnovers in our proxy records. Statistically significant relationships between species composition and ECEs were observed only for diatoms. The only prolonged change identified is the spread of previously near-absent chironomids in the aftermath of a slight lake refreshment likely associated with the ca. 1740 CE event. This event was also followed by a short-lived (ca. 15 years) five-fold increase in the abundance of Scenedesmus that could have been triggered by the effects of the ECE on nutrient availability. The impact of the remaining three ECEs was discernible only in the diatom record, consistently showing a decline in two calcifobic and oligotrophic species, Achnanthidium petersenii and Platessa oblongella, also evident for the ca. 1740 CE event. The relatively minor ecosystem alteration induced by ECEs in Broad Pond lacks a single explanation and requires consideration of multiple site- and event-specific factors.

Dominant nitrogen metabolisms of a warm, seasonally anoxic freshwater ecosystem revealed using genome resolved metatranscriptomics

Nitrogen (N) availability is one of the principal drivers of primary productivity across aquatic ecosystems. However, the microbial communities and emergent metabolisms that govern N cycling in tropical lakes are both distinct from and poorly understood relative to those found in temperate lakes. This latitudinal difference is largely due to the warm (>20°C) temperatures of tropical lake anoxic hypolimnions (deepest portion of a stratified water column), which result in unique anaerobic metabolisms operating without the temperature constraints found in lakes at temperate latitudes. As such, tropical hypolimnions provide a platform for exploring microbial membership and functional diversity. To better understand N metabolism in warm anoxic waters, we combined measurements of geochemistry and water column thermophysical structure with genome-resolved metatranscriptomic analyses of the water column microbiome in Lake Yojoa, Honduras. We sampled above and below the oxycline in June 2021, when the water column was stratified, and again at the same depths and locations in January 2022, when the water column was mixed. We identified 335 different lineages and significantly different microbiome membership between seasons and, when stratified, between depths. Notably, nrfA (indicative of dissimilatory nitrate reduction to ammonium) was upregulated relative to other N metabolism genes in the June hypolimnion. This work highlights the taxonomic and functional diversity of microbial communities in warm and anoxic inland waters, providing insight into the contemporary microbial ecology of tropical ecosystems as well as inland waters at higher latitudes as water columns continue to warm in the face of global change.IMPORTANCEIn aquatic ecosystems where primary productivity is limited by nitrogen (N), whether continuously, seasonally, or in concert with additional nutrient limitations, increased inorganic N availability can reshape ecosystem structure and function, potentially resulting in eutrophication and even harmful algal blooms. Whereas microbial metabolic processes such as mineralization and dissimilatory nitrate reduction to ammonium increase inorganic N availability, denitrification removes bioavailable N from the ecosystem. Therefore, understanding these key microbial mechanisms is critical to the sustainable management and environmental stewardship of inland freshwater resources. This study identifies and characterizes these crucial metabolisms in a warm, seasonally anoxic ecosystem. Results are contextualized by an ecological understanding of the study system derived from a multi-year continuous monitoring effort. This unique data set is the first of its kind in this largely understudied ecosystem (tropical lakes) and also provides insight into microbiome function and associated taxa in warm, anoxic freshwaters.