Abstract Lake eutrophication remains a significant challenge for water‐quality management across the world. Many management strategies focus on reducing nutrient inputs, but other environmental factors can substantially influence the yield of algal biomass for a given level of nutrients. While various large‐scale studies have explored how different lake characteristics impact eutrophication, there remains a need to integrate these factors into a comprehensive model capable of assessing nitrogen vs. phosphorus limitation. In this study, we refine chlorophyll–nutrient relationships across the conterminous United States by considering auxiliary variables (i.e., temperature, lake depth, and nutrient enrichment) within a Bayesian hierarchical framework. We leverage over 4000 sampling events of 2755 different lakes from the National Lakes Assessments (2007–2022) to inform model development. We first consider auxiliary variables independently, exploring how they influence the intercept, slope, and critical nutrient ratio (nitrogen : phosphorus) in a regression to predict chlorophyll based on the limiting nutrient. Next, we integrate significant auxiliary variables into a comprehensive model for chlorophyll prediction. Results indicate that the critical nutrient ratio increases in relation to increased lake depth, and the slope of the nutrient–chlorophyll relationship increases with increasing temperature. We apply the model to map mean summer conditions across US lakes and find that 23% and 16% of lakes are strongly limited by phosphorus and nitrogen, respectively (i.e., at > 90% probability). These proportions, however, vary substantially across different subregions. Overall, the probabilistic modeling approach and results can serve as an effective tool to inform water resources management, especially at large spatial scales.

Abstract Phytoplankton biomass is often limited by nitrogen (N) and/or phosphorus (P). Quantifying the relative availability of N and P in an ecosystem is useful in indicating nutrient limitation status. In subtropical lakes, nitrate availability generally follows a seasonal pattern with maxima in the late winter/early spring and minima in late summer/early autumn, also known as seasonal nitrate drawdown. Phytoplankton nutrient limitation in response to seasonal nitrate drawdown is largely understudied in lakes. We used a long‐term dataset of seasonal nitrate drawdown/accumulation alongside short‐term fertilization bioassay experiments and phytoplankton metatranscriptomes to quantify phytoplankton nutrient limitation and physiological response to seasonally changing nutrient availability in Beaver Lake, Arkansas. We hypothesized a seasonal shift in the phytoplankton community from P to N limitation. We found that the seasonal nitrate drawdown pattern in Beaver Lake has consistently occurred for at least 25 yr and is related to water temperature and season. We also found that Beaver Lake shifts from P limitation in spring to N and P co‐limitation in summer. A transition in phytoplankton physiological response from N assimilation and transport in spring to synthesis and fixation in summer/early autumn suggests that microbial activity and phytoplankton gene expression are responsive to seasonality and nutrient availability. We concluded that while N 2 fixation expression increases in response to N deficiency, it is not sufficient to offset N limitation. This study is the first to formally test the nitrate drawdown hypothesis , highlighting nitrate drawdown effects on phytoplankton and its implications for dual nutrient control in lakes.

Abstract Nitrogen fixation, the microbial conversion of dinitrogen gas to ammonia, is a key nitrogen source and critical for sustaining marine productivity. We investigated the diversity and activity of nitrogen‐fixers in surface waters of the North Sea and Norwegian coast during July 2023. We detected, for the first time, the presence of Braarudosphaera /UCYN‐A (up to 10 4 nifH copies L −1 ) and measurable nitrogen fixation rates (up to 110 nM d −1 ) throughout the North Sea. Nitrogen‐fixing organisms may be transported from the northeast Atlantic across the northern North Sea by southward current systems, via Baltic Sea inflow or be a persistent but overlooked component of the phytoplankton community. We estimate nitrogen fixation may support between 4% and 24% of summertime net primary production. Nitrogen fixation is currently neglected in numerical models of shelf seas, yet may play a critical role in sustaining shelf sea productivity in the contemporary and future ocean.

Abstract Permeable sands on continental shelves host microbial communities that drive organic carbon turnover, oxygen fluxes and nitrogen loss. Advective porewater flow associated with sedimentary bedforms maintains these rates and fluxes. Modeling and laboratory studies suggest that advective porewater flow is tightly coupled to bedform stability. However, the impacts of in situ bedform stability on biogeochemical fluxes in subtidal sands remain unconstrained. We deployed a benthic lander at six stations in the North Sea to measure in situ bedform geometries, oxygen fluxes and primary productivity accompanied with ex situ incubations, glycan extraction and microscopy. We observed bedform migration velocities ranging between 0 and 3.2 cm h −1 , which were only 9–16% of expected values from mechanistic models. Bedform stability may be enhanced by high interstitial colloidal and particulate algal glycans concentrations (2–52 mmol C L −1 ) that are a representative component of extracellular polymeric substances. Benthic primary productivity as a source of carbon was negligible (< 0.003 mmol C m −2 d −1 ). Pore space glycan accumulation was attributed to algal biomass filtered out of the overlying water column by advective flow through the permeable sediment. Benthic glycan concentrations correlated significantly with oxygen consumption rates per volume porewater (17–207 μ mol L −1 PW h −1 ), whereas sediment oxygen fluxes (6–17 mmol m −2 d −1 ) significantly correlated with modeled porewater velocities. Overall, the impact of glycans as a proxy for pelagically derived algal biomass was twofold: (1) they drove higher oxygen consumption rates in surface sediments and (2) contributed to stabilization of bedforms, which almost doubled oxygen fluxes into the sediment.

Abstract Previous studies of organic matter (OM) dynamics in frontal zones have emphasized biological and chemical controls, whereas the role of vertical physical processes remains insufficiently understood. Here, we integrated an isopycnic mixing model and principal component analysis with biogeochemical measurements to investigate OM cycling in the Luzon Strait frontal system. Four subregions were identified: cyclonic eddy core (CEC), cyclonic bend (CB), anticyclonic eddy core (AEC), and anticyclonic bend (AB), each characterized by distinct vertical regimes. Each subregion exhibited distinct OM transformation pathways governed by different mechanisms. (1) Upwelling vs. Downwelling : Upwelling (strong in CEC, weak in CB) enhanced photosynthesis by supplying nutrient‐rich waters to the euphotic zone, whereas downwelling (strong in AEC, weak in AB) suppressed it; AB showed concurrent strong upwelling and downwelling due to submesoscale fronts. (2) Stratification vs. Mixing : Stratification promoted photosynthesis and semi‐labile dissolved OM (DOM) accumulation by trapping chlorophyll‐ a and balancing light and nutrients, while mixing accelerated fresh particulate OM (POM) and semi‐labile DOM degradation by prolonging oxygen exposure. Stratification and mixing coexisted in CEC, CB, and AEC, but AB was dominated by intense mixing from submesoscale dynamics. (3) Total suspended matter ( TSM ) modulation : Elevated TSM facilitated DOM‐to‐POM conversion via mineral adsorption, with mesoscale eddies enhancing TSM in CEC and AEC by trapping terrigenous inputs. Our findings demonstrate that vertical hydrological processes are fundamental in controlling the transformation, degradation, and ultimate sequestration of organic carbon in frontal systems, providing a mechanistic basis for refining carbon cycle models and assessing ocean‐based carbon management strategies.

Abstract In the Southern Ocean, phytoplankton are critical drivers of biogeochemical cycling and food web dynamics, and show sensitivity to shifting climates. Along the West Antarctic Peninsula, climate‐driven variations in sea ice and hydrography have been linked to long‐term changes in summer phytoplankton productivity. Such changes are hypothesized to reflect decadal shifts in algal light environments, but diagnostic evidence of this light‐dependent response is limited. Using a 27‐yr timeseries of summer phytoplankton pigments and productivity collected along the West Antarctic Peninsula by the Palmer Long Term Ecological Research (LTER) program, we quantified trends in surface phytoplankton photophysiology, productivity, and composition in response to environmental change. Our results revealed a decadal doubling in proportions of phytoplankton photoprotective pigments correlated with long‐term shoaling and strengthening of mixed layer depths. This biophysical signature is consistent with photophysiological response to increased light supply, signifying a long‐term shift in light environments for surface phytoplankton. The long‐term change in community pigment signature could not be explained by trends in phytoplankton composition alone, indicating a key role of photoacclimation. Phytoplankton community biomass, productivity, and production efficiency (chlorophyll‐normalized productivity) were similarly correlated to upper ocean structure and increased decadally, demonstrating a functional benefit from enhanced light availability. These physiological findings support the hypothesis that decadal trends in summer production along the peninsula are light‐dependent. Such large‐scale shifts in community pigment signature, linked to oceanographic forcing, suggest that photophysiological indices provide useful insights into how shifting climates will influence phytoplankton communities.

Abstract The significant contribution of phytoplankton to global primary production is regulated by several abiotic and biotic factors, which are often difficult to account for in natural systems. To address these challenges, we perturbed a summer plankton community from Narragansett Bay, Rhode Island, USA, by manipulating the temperature and nutrient conditions in a controlled short‐term incubation experiment and tracked changes in phytoplankton community structure in response to fluctuations in phytoplankton physiology and microzooplankton grazing. Water was incubated at the in situ temperature (22°C) and at deviations from that temperature (± 4°C) with both macronutrient amendments (N, P, and Si addition) and unamended controls. We found nutrient availability and microzooplankton grazing to have pronounced and opposite impacts on phytoplankton size composition, with nutrient amendments shifting the phytoplankton community toward larger cells and grazing rates correlated with smaller phytoplankton communities and low‐nutrient conditions. Nutrient amendments also altered cellular elemental ratios by increasing chlorophyll : C. Conversely, temperature was not found to have a direct impact on size or elemental stoichiometry, but did influence community composition. These findings paralleled prior observations from lab and field studies in Narragansett Bay which together suggest that over shorter timescales, nutrient availability may have a greater impact on phytoplankton community composition than temperature or grazing, altering phytoplankton nutritional value for higher trophic levels and as a result, secondary production. Thus, understanding underlying nutrient dynamics will be necessary to decipher how short‐term changes in temperature or grazing may impact phytoplankton communities and the ecosystems they support.

Abstract Nitrification is a key process in the aquatic nitrogen (N) cycle, but its products, nitrate (NO 3 − ) and nitrous oxide (N 2 O), contribute to eutrophication and greenhouse gas emissions, particularly in eutrophic lakes. Variations in in‐lake N cycling and N 2 O production pathways, as a function of seasonality and artificial oxygenation, remain poorly understood. We investigated nitrification in the artificially oxygenated eutrophic Lake Baldegg, by analyzing NO 3 − and N 2 O concentrations and isotope ratios, and measuring ammonium oxidation rates via 15 N tracer incubations over one year. An N isotope mass‐balance model revealed that nitrification sustained only 5.3 ± 0.7% of total NO 3 − consumption in the epilimnion, where external N loadings were influential, and considerably more in the hypolimnion (81.6 ± 18.5%) during stratification. Dual NO 3 − isotope signatures (Δδ 18 O : Δδ 15 N ~ 1.5–1.73) and associated negative NO 3 − isotope anomalies confirmed epilimnetic nitrification, though external inputs partly obscured this signal. During stratification, relatively high hypolimnetic nitrification rates correlated with organic matter export, and seemed linked to sediment resuspension and artificial oxygenation. While sedimentary denitrification/DNRA dominated hypolimnetic NO 3 − reduction (with negligible effects on δ 15 N‐NO 3 − and δ 18 O‐NO 3 − ), transient suboxic conditions enabled water column denitrification during stratification (24.1–30.2% of the total hypolimnetic denitrification). High N 2 O isotope site‐preference values (30–35‰) confirmed hypolimnetic ammonium oxidation as the main N 2 O production pathway. During winter overturn, N 2 O transport from the hypolimnion caused epilimnetic N 2 O oversaturation and atmospheric emissions up to 3.52 μ mol m −2 d −1 . Comparison with other lakes suggests that artificial oxygenation enhances N turnover, manifesting in greater ambient N 2 O backgrounds and fluxes to the atmosphere.

Abstract In aquatic ecosystems, greater food web complexity is theorized to increase persistence and resilience of primary production to pulse disturbances, yet experimental evidence is limited. We simulated two storm‐induced pulse disturbances by adding nutrients (~ 3%–5% increase in ambient concentrations) to three ponds with low, intermediate, and high food web complexity and compared to reference ponds. We evaluated the ecological stability of primary production by quantifying persistence as the number of days it took chlorophyll‐ a or ecosystem metabolism to deviate significantly from reference conditions and resilience as the time to recover to reference conditions following each disturbance. We also evaluated if a critical transition occurred following the disturbance. The high complexity pond did not significantly deviate from reference conditions following either nutrient pulse, suggesting high ecological stability. The intermediate complexity pond had lower stability, with persistence relatively consistent at 18 and 24 d after each nutrient pulse, and resilience trending toward a substantial increase from 23 d to less than a week before the experiment concluded. Stability was lowest in the low complexity pond where persistence decreased from 24 d to just 8 d and resilience decreased from 5 to 22 d. There was also evidence of a critical transition after the first pulse in the low complexity pond, but not for higher complexity ponds. This experiment provides strong support that food web connectivity and food chain length can aid in buffering aquatic ecosystems against increasing and intensifying by influencing persistence and resilience to repeated nutrient pulses.