Limnology and Oceanography Letters最新文献

Environmental change, particularly warming and eutrophication, can alter phenology in aquatic systems. Understanding controls on zooplankton phenology is important due to their central role in food webs. While patterns in zooplankton phenology have been well documented, we lack an understanding of how abiotic and biotic drivers influence lake zooplankton phenology during the summer. We examined the phenology of four common zooplankton taxa (Daphnia retrocurva, Skistodiaptomus oregonensis, Mesocyclops spp., Dreissenid veligers) in western Lake Erie during 1995–2022, a period with increasing eutrophication and Bythotrephes longimanus biomass. Many phenology metrics varied by 3 weeks or more from 1995 to 2022. The dominant controls of phenology were temperature and biotic factors, especially grazer-defended phytoplankton (cyanobacteria and other colonial or filamentous taxa) and the invasive predator B. longimanus, which frequently interacted. Our results show that aspects of environmental change interact to shape zooplankton phenology, which can influence phytoplankton biomass and energy flow to higher trophic levels.

Headwater streams play a large role in aquatic greenhouse gas emissions. Carbon dioxide (CO₂) and dissolved oxygen in streams often undergo changes through diel cycles. However, methane (CH₄) and nitrous oxide (N₂O) have unknown diel dynamics. Here, we reveal consistent patterns in CO₂, CH₄, and N₂O over diel cycles and during flood events using high-frequency continuous observations in a subtropical headwater stream. Diel cycles were most pronounced during baseflow. Increased nighttime discharge due to higher groundwater inputs enhanced gas transfer velocities and concentrations. Overall nocturnal emissions were 31%, 68%, and 32% greater than daytime for CO₂, CH₄, and N₂O, respectively. Floods dampened diel signals. If both flood events and diel patterns are neglected, estimates of greenhouse gas emissions from headwaters may be greatly underestimated. Overall, CH₄ and N₂O emissions from headwater streams may be underestimated by ~ 20–40% due to a lack of observations during nighttime, floods, and in warmer climates.

The fish-derived bile salt 5α-cyprinol sulfate (CPS) has been identified as a kairomone inducing the predator avoidance behavior “diel vertical migration” (DVM) in Daphnia magna in response to fish. However, conclusions about the ecological significance of CPS have been derived from laboratory experiments only. Using a mesocosm approach, we investigate whether the role of CPS as a kairomone can be confirmed in the field. We demonstrate that CPS induces downward migration during daytime in a field-derived Daphnia community mainly consisting of Daphnia longispina and Daphnia cucullata within the experiment. In the study lake, the actual population of D. longispina shows a similar pattern of DVM, while concomitant quantification of CPS by HPLC-MS confirms that CPS in situ concentrations are sufficiently high for induction of daytime downward migration of D. longispina in this oligo-mesotrophic lake. Together, these observations infer that CPS is a significant kairomone-inducing DVM-like behavior in Daphnia.

Fluvial silicon (Si) plays a critical role in controlling primary production, water quality, and carbon sequestration through supporting freshwater and marine diatom communities. Geological, biogeochemical, and hydrological processes, as well as climate and land use, dictate the amount of Si exported by streams. Understanding Si regimes—the seasonal patterns of Si concentrations—can help identify processes driving Si export. We analyzed Si concentrations from over 200 stream sites across the Northern Hemisphere to establish distinct Si regimes and evaluated how often sites moved among regimes over their period of record. We observed five distinct regimes across diverse stream sites, with nearly 60% of sites exhibiting multiple regime types over time. Our results indicate greater spatial and interannual variability in Si seasonality than previously recognized and highlight the need to characterize the watershed and climate variables that affect Si cycling across diverse ecosystems.

Southern Ocean phytoplankton growth is limited by low iron (Fe) supply and irradiance, impacting the strength of the biological carbon pump. Unfavorable upper ocean conditions, such as low nutrient concentrations, can lead to the formation of deep chlorophyll or biomass maxima (DCM/DBM). While common in the Southern Ocean, these features remain understudied due to their subsurface location. To increase our understanding of their occurrence, we studied the responses of phytoplankton communities from a Southern Ocean DCM to increasing light, Fe, and manganese (Mn) levels. The DCM communities were light- and Fe-limited, but light limitation did not increase phytoplankton Fe requirements. The greatest physiological responses were observed under combined Fe/light additions, which stimulated macronutrient drawdown, biomass production and the growth of large diatoms. Combined Mn/light additions induced subtle changes in Fe uptake rates and community composition, suggesting species-specific Mn requirements. These results provide valuable information on Southern Ocean DCM phytoplankton physiology.

Current estimates of carbon dioxide (CO₂) evasion from Arctic lakes are highly uncertain because few studies integrate seasonal variability, specifically evasion during spring ice-melt. We quantified annual CO₂ evasion for 14 clear-water Arctic lakes in Northern Sweden through mass balance (ice-melt period) and high-frequency loggers (open-water period). On average, 80% (SD: ± 18) of annual CO₂ evasion occurred within 10 d following ice-melt. The contribution of the ice-melt period to annual CO₂ evasion was high compared to earlier studies of Arctic lakes (47% ± 32%). Across all lakes, the proportion of ice-melt : annual CO₂ evasion was negatively related to the dissolved organic carbon concentration and positively related to the mean depth of the lakes. The results emphasize the need for measurements of CO₂ exchange at ice-melt to accurately quantify CO₂ evasion from Arctic lakes.

Cobalamin, vitamin B₁₂, is an important micronutrient that has been investigated for decades in the marine context because it is required for phytoplankton growth. The biologically active forms (Me-B₁₂, Ado-B₁₂) and the synthetic form (CN-B₁₂) quickly convert to OH-B₁₂ after light exposure in various aqueous solutions, but puzzlingly have been frequently reported to dominate dissolved cobalamin pools in the sunlit ocean. Here, we document photodegradation timescales for these cobalamin forms in natural seawater using targeted mass spectrometry, providing quantitative evidence that OH-B₁₂ is expected to be the dominant dissolved form in irradiated seawater. Then, through high-resolution mass spectrometry, we identify four photodegradation products of OH-B₁₂ which represent potential building blocks microbes could salvage and remodel to satisfy cellular cobalamin requirements. Taken together, these results clarify the impact of light on marine cobalamin dynamics, laying a foundation for a more quantitative understanding of the role of cobalamin in microbial communities and biogeochemical cycles.

Nearshore environments are typically supersaturated with the potent greenhouse gases methane and carbon dioxide, due to intense remineralization of the elevated supply of organic carbon in these systems. These environments are characterized by overlapping biogeochemical gradients and heterogeneous morphology, and the overall spatial variability in nearshore greenhouse gas concentrations remains unclear. We measured surface water partial pressures of carbon dioxide and methane synoptically with water quality parameters in the coastal Baltic Sea, covering two ice-free seasons. The high-frequency flow-through data revealed sites with recurring very high partial pressures of carbon dioxide and methane (i.e., hot spots) scattered around the 50 km × 40 km study area, exceeding overall partial pressure averages by 455 μatm (CH₄) and 2396 μatm (CO₂). High partial pressures were linked with elevated inputs of allochthonous and autochthonous organic matter, underpinning the major role of organic enrichment of coastal environments in global carbon cycling.

Vertical motion is an important driver of sunlight exposure in aquatic environments, shaping the growth and fate of materials and organisms. We derive a simple model accounting for turbulent depth fluctuations of particles to predict the depth that contributes the most sunlight exposure (effective depth) as well as the single depth that, if measured at one place over time, produces the same total sunlight exposure as a moving particle (functional depth). Field measurements of light and depth in rivers using neutrally buoyant drifters and buoys validate our model. Effective depth varied from 0.1 to 1.5 m below the water surface and was ~ 30% of the overall water depth on average. Functional depth varied from 0.67 to 2.3 m and was ~ 50% of the overall water depth on average. Functional and effective depth are physically based concepts incorporating turbulent motion, spatial variability, and water clarity offering new approaches to characterize light exposure in aquatic environments.