Limnology and Oceanography: Methods最新文献

Global climate change has impacted ocean biogeochemistry and physical dynamics, causing increases in acidity and temperature, among other phenomena. These changes can lead to deleterious effects on marine ecosystems and communities that rely on these ecosystems for their livelihoods. To better quantify these changes, an array of floats fitted with biogeochemical sensors (BGC-Argo) is being deployed throughout the ocean. This paper presents an algorithm for deriving a deployment strategy that maximizes the information captured by each float. The process involves using a model solution as a proxy for the true ocean state and carrying out an iterative process to identify optimal float deployment locations for constraining the model variance. As an example, we use the algorithm to optimize the array for observing ocean surface dissolved carbon dioxide concentrations (pCO₂) in a region of strong air–sea gas exchange currently being targeted for BGC-Argo float deployment. We conclude that 54% of the pCO₂ variability in the analysis region could be sampled by an array of 50 Argo floats deployed in specified locations. This implies a relatively coarse average spacing, though we find the optimal spacing is nonuniform, with a denser sampling being required in the eastern equatorial Pacific. We also show that this method could be applied to determine the optimal float deployment along ship tracks, matching the logistics of real float deployment. We envision this software package to be a helpful resource in ocean observational design anywhere in the global oceans.

Dissolved inorganic carbon (DIC) and its stable carbon isotope (δ¹³C-DIC) are valuable parameters for studying the aquatic carbon cycle and quantifying ocean anthropogenic carbon accumulation rates. However, the potential of this coupled pair is underexploited as only 15% or less of cruise samples have been analyzed for δ¹³C-DIC because the traditional isotope analysis is labor-intensive and restricted to onshore laboratories. Here, we improved the analytical precision and reported the protocol of an automated, efficient, and high-precision method for ship-based DIC and δ¹³C-DIC analysis based on cavity ring-down spectroscopy (CRDS). We also introduced a set of stable in-house standards to ensure accurate and consistent DIC and δ¹³C-DIC measurements, especially on prolonged cruises. With this method, we analyzed over 1600 discrete seawater samples over a 40-d cruise along the North American eastern ocean margin in summer 2022, representing the first effort to collect a large dataset of δ¹³C-DIC onboard of any oceanographic expedition. We evaluated the method's uncertainty, which was 1.2 μmol kg⁻¹ for the DIC concentration and 0.03‰ for the δ¹³C-DIC value (1σ). An interlaboratory comparison of onboard DIC concentration analysis revealed an average offset of 2.0 ± 3.8 μmol kg⁻¹ between CRDS and the coulometry-based results. The cross-validation of δ¹³C-DIC in the deep-ocean data exhibited a mean difference of only −0.03‰ ± 0.07‰, emphasizing the consistency with historical data. Potential applications in aquatic biogeochemistry are discussed.

Nitrification rate measurements provide critical information on the performance of an environmental process central to the N cycle and are best studied using isotope labeling techniques. However, combining the high sensitivity of isotope labeling techniques with selected inhibition of nitrifiers as a whole or of specific nitrifier guilds has not been established in limnology. This can be achieved with different concentrations of the commonly used nitrification inhibitor allylthiourea (ATU). In the ¹⁵N-ammonium oxidation technique, the converted isotope label is typically captured in an excess pool of ¹⁴N-nitrite. Here, we assessed how different storage conditions affect the stability of the nitrite pool in freshwater samples treated with ATU. When stored frozen, the nitrite pool was rapidly destabilized to 25–31% after 7 d of storage and even to less than 5% after storage exceeding 90 d for samples treated with ATU, thus making them unusable for rate determinations in these cost and labor-intensive experiments. In comparison, this was not the case in marine samples or freshwater samples not treated with ATU, where the nitrite pool remained stable. Building on these results, we tested two options to stabilize nitrite during the storage of freshwater samples. The nitrite pool was stable if samples were stored at 4°C instead of freezing. We recommend this option for short-term storage. For long-term storage, samples should be supplemented with 0.5 mmol L⁻¹ NaCl to increase salinity before freezing. As in marine samples, this stabilized the nitrite pool. Our results provide important guidance for the storage of non-saline samples used for nitrification rate measurements in freshwater environments.

Accurate taxonomic classification and developmental stage determination of fish eggs are crucial for ecological monitoring, conservation efforts, and stock assessments. Traditional methods for fish and fisheries rely on visual examination of morphological traits, but they face challenges due to species overlap especially for early stages. Molecular tools, such as DNA barcoding, offer higher resolution in taxonomic identification but may not provide developmental stage information. This study explores the effectiveness of different formaldehyde fixation concentrations and storage procedures on fish eggs collected from Lofoten, Norway, for both visual and molecular analysis. Visual analysis successfully identified developmental stage for all fixation solutions. Molecular barcoding using the 16S rRNA gene identified up to 100% of eggs at the species level, with decreasing success rates over time when stored in formaldehyde fixation. The highest DNA barcoding success rates were accomplished using 4% formaldehyde fixation for 12- or 24-h following transfer to ethanol. Using 0.5% and 1% formaldehyde fixation up to 8 weeks also resulted in high DNA success rates, but results deteriorated with increasing storage time. This study provides valuable insights for integrating visual and molecular methods for fish egg analysis, with practical implications for sample preservation during marine surveys.

Diffusive equilibrium in thin films (DET) probes are passive samplers that are designed to reflect in situ porewater concentrations. In this study, we show that the gel and the plastic housing of DET probes store a substantial amount of oxygen (O₂) that affects the chemical composition of porewater. To ensure complete deoxygenation, DET probes need to be treated for 7 d with continuous nitrogen flow. Such fully deoxygenated probes can be handled in the air (exposure time: < 90 s) and deployed to sediments through oxic water (exposure time: < 2 s) without significant ad(b)sorption of O₂. Furthermore, we deployed a set of untreated (i.e., in equilibrium with atmospheric O₂) and a set of fully deoxygenated DET probes to lake sediments. The O₂ present in untreated DET probes altered iron (Fe) and phosphate (P) porewater profiles significantly. This is caused by the oxidation, immobilization, and accumulation of redox-sensitive Fe (oxyhydr)oxides in the probe over time. Since P has a high binding affinity to Fe (oxyhydr)oxides, it is not in equilibrium with the porewater and is overestimated as well. Our results highlight the importance of thorough deoxygenation of DET probes before deployment in sediments, especially when addressing redox-sensitive porewater species.

Human activities over the past 150 yr have led to significant carbon dioxide (CO₂) emissions, causing global warming and ocean acidification. Surface ocean temperature has risen by 0.93°C since 1850, with projections of an additional +1.42°C to 3.47°C by 2080–2099. Ocean acidification, driven by CO₂ absorption, has already lowered seawater pH by 0.1 units, affecting calcifying organisms, including shelled mollusks. Long-term multigenerational studies on mollusk responses to both ocean acidification and warming, under realistic environmental conditions, are scarce. To address this knowledge gap, two mobile experimental units that can be deployed at the vicinity of shellfish farming areas were developed within the framework of the CocoriCO₂ project. The experimental systems were designed to manipulate temperature and pH as offsets from ambient conditions. The experimental units have shown their effectiveness in terms of controlling and maintaining pH and temperature to assess the multigenerational effects of ocean warming and acidification on benthic invertebrates. Finally, the developed experimental systems can be modified easily to provide an educated assessment of the impact of other relevant environmental changes such as deoxygenation and changes in salinity.

Singe-turnover active chlorophyll a fluorometry (STAF) can be used to assess phytoplankton photosynthetic rates in terms of the photosystem II photochemical flux (JV_PII, μmol e⁻ m⁻³ s⁻¹) instantaneously, autonomously, and at high resolution. While JV_PII provides an upper limit to rates of phytoplankton primary productivity in units of carbon fixation, the conversion between these two rates is variable, limiting our ability to utilize high-resolution JV_PII data to monitor phytoplankton primary productivity. Simultaneous measurements of JV_PII and ¹⁴C-fixation help in understanding the factors controlling the variable ratio between the two rates. However, to date, methodological inconsistencies, including differences in incubation lengths and light quality, have greatly inhibited practical assessment of such electron to carbon ratios (Φ_e,C, mol e⁻ mol C⁻¹). We here present data from a range of dual-incubation experiments in northeast Atlantic waters during which JV_PII and ¹⁴C-fixation were measured simultaneously on the same sample. Time-course experiments show how Φ_e,C increases with incubation length, likely reflecting the transition from gross to net ¹⁴C-fixation. Dual-incubation experiments conducted under different light levels show a tendency for increased Φ_e,C under (super-)saturating light. Finally, data from a diurnal experiment demonstrate how Φ_e,C increases over the course of a day, due to downregulation of ¹⁴C-fixation. We provide a detailed description of our methodological approach, including a critical discussion of improvements to the calculation of JV_PII implemented in the LabSTAF instrument used for active fluorescence measurements and the limitations of the well-established ¹⁴C-fixation approach.

Planktonic Foraminifera have been collected from the water column with different plankton sampling devices equipped with nets of various mesh sizes, which impedes direct comparison of observed quantifications. Here, we use data on the community size structure of planktonic Foraminifera to assess the impact of mesh size on the measured abundance (ind m⁻³) of planktonic Foraminifera. We use data from the FORCIS database (Chaabane et al., 2023, Scientific Data 10: 354) on the global ocean at different sampling depths over the past century. We find a global cumulative increase in abundance with size, which is best described using a Michaelis–Menten function. This function yields multiplication factors by which one size fraction can be normalized to any other size fraction equal to or larger than 100 μm. The resulting size normalization model is calibrated over a range of different depth intervals, and validated with an independent dataset from various depth ranges. The comparison to Berger's (1969, Deep. Res. Oceanogr. Abstr. 16: 1–24) equivalent catch approach shows a significant increase in the predictive skill of the model. The new size normalization scheme enables comparison of Foraminifera abundance data sampled with plankton nets of different mesh sizes, such as compiled in the FORCIS database. The correction methodology may be effectively employed for various other plankton groups such as diatoms and dinoflagellates.

To predict the ecological consequences of expected global change, we need to understand the independent and combined effects of multiple stressors. Multiple experimental treatments are required to simultaneously test for effects of multiple stressors at different levels of intensity, independently and combined, and at different levels of biological organization. Most marine multiple stressors studies to date are conducted on assembled communities in mesocosms with a low number of treatments or low replication of treatments or both. These limitations prevent (1) robust data analyses, (2) characterization of single and combined effects of multiple stressors, and (3) identification of mechanisms underpinning biological responses. We present a new mesocosm-based experimental platform for benthic communities: Quantifying the Impacts of Multiple Stressors (QIMS). Here, 96 independent mesocosms facilitate multifactorial and multilevel experimental designs with the high replication required for robust tests of multiple stressors and biological interactions. For example, three distinct pH levels are achieved by manipulating CO₂ concentrations in the air supply, and three water temperature levels are provided by a cooling system in a fully crossed design that is required to identify all potential interactions, from which all combinations can be replicated 10 times (i.e., 90 experimental units). We demonstrate clearly how different levels of temperature and pCO₂/pH can be manipulated precisely and maintained for at least 7 weeks. QIMS complements the limited number of permanently installed marine mesocosm facilities worldwide that simulate ocean warming and/or acidification and expedites multiple stressor research by providing an unprecedented level of replication for statistical robustness.