Abstract This synthesis seeks to spark research on the metabolic regimes of lakes, ponds, and reservoirs. A metabolic regime is the generalized annual pattern of ecosystem metabolism—gross primary production and ecosystem respiration—that characterizes a set of ecosystems. Characterizing metabolic regimes has increased understanding of terrestrial, marine, and stream ecosystems, but has been underexplored for lakes, ponds, and reservoirs despite a growing abundance of metabolism data from these systems. We propose three hypotheses about lake metabolic regimes and how and why they vary around the world. We motivate these hypotheses by examining results from a literature review and a recent large compilation of lake metabolism data, and by using those results to inform a conceptual model built from existing theory. Finally, we introduce a new global repository of lake metabolism data to enable tests of these hypotheses and other advances in lake ecosystem science.
Abstract Refractory dissolved organic nitrogen is a major component of the ocean's nitrogen pool, yet the mechanisms producing and preserving it remain poorly understood. Here, we show that deaminated peptides (DAPs), derived from microbial processing of proteinaceous material, are ubiquitous across oxygenated marine environments—from productive coastal waters to the oligotrophic ocean—and persist from surface to bathypelagic depths. Using ultrahigh‐resolution mass spectrometry and a curated peptide database, we detected candidate DAP formulas with substantial contributions from hydrophobic amino‐acid residues and recurrent compositions containing diaminopimelic acid (Dapa), consistent with a contribution of bacterial cell‐wall material. Laboratory incubations demonstrate substrate control on DAP composition: simple‐substrate treatments yield narrow, source‐specific amino acid profiles, whereas complex substrates produce broader assemblages resembling field observations. Together, widespread DAP occurrence and substrate‐dependent production in microbial incubations support peptide deamination as a potential microbial nitrogen pump pathway, linking labile surface nitrogen to persistent storage in the ocean interior.
Abstract Escalating ultraviolet radiation (UVR) and ocean warming are expected to interactively shape phytoplankton performance, yet their combined effects on diatom thermal plasticity remain poorly resolved. We quantified how UVR alters growth‐based thermal performance curves in three model marine diatoms grown across species‐specific temperature gradients (5–28°C) under UV‐excluded (PAR) and UV‐present (PAB) solar radiation. UVR generally suppressed growth most strongly at thermal extremes and consistently lowered the critical thermal maximum (CTmax) in all three species, indicating reduced tolerance to high temperature. UVR also narrowed thermal breadth in two species and reduced thermal tolerance in two species, whereas effects on maximum growth rate (rmax), optimum temperature (Topt), and thermal safety margin (TSM) were species specific. These results indicate that UVR modifies growth‐based thermal performance rather than acting solely as an additive stressor, bringing populations closer to their upper thermal limits under the tested conditions. Incorporating UVR‐sensitive thermal traits into predictive frameworks may help constrain projections of phytoplankton productivity under warming and changing light climates.
Abstract Runoff and calved ice volumes from the Greenland Ice Sheet are well constrained, but quantifying the associated suspended sediment flux remains challenging. Here we present near‐terminus observations from three glacierized fjords in southeast Greenland, combining full‐depth turbidity profiles and current velocity from summer 2024 to evaluate sediment export. We identify extensive subsurface sediment plumes from marine‐terminating glaciers which are undetectable using remote sensing approaches and underrepresented in fjord sediment‐core reconstructions. Sediment cores document an exponential increase in sediment accumulation rates from glacier‐distal to glacier‐proximal coring sites. Therefore, the scarcity of sediment cores collected in the immediate vicinity of glacier termini suggests that sediment export from marine‐terminating glaciers has likely been systematically underestimated. Our findings underscore the importance of direct observations for quantifying sediment transport in glacierized fjords and suggest the Greenland Ice Sheet contributes more sediment to ocean margins than previously assumed.
Abstract The stratification of ice‐covered lakes is currently understood as either cryomictic (mixing near 0°C) or cryostratified (inverse stratification reaching 4°C). However, internal or external salt sources may alter these patterns, creating stable stratification with high under‐ice bottom water temperatures, a condition we term cryohalostratification, which may lead to anoxia and interference with spring mixing. We measured stratification and water quality over two winters in a rural Canadian Shield lake exposed to a highway. We found high under‐ice water temperatures (> 4°C) and low dissolved oxygen (< 2 mg L−1) because of increases in chloride and sodium. Following a cold winter, stability remained high after ice‐off, preventing mixing and prolonging seasonal hypoxia. Lake stability estimates showed that the stability of the lake was enhanced by wind sheltering from the surrounding forest, suggesting that even rural lakes with low road density are susceptible to salinization.
