Limnology and Oceanography最新文献

While pelagic tunicates are known to exert strong influences on ecosystem trophic and biogeochemical function, it has long been assumed that their shortcutting of traditional planktonic food webs results in a dead-end with respect to higher trophic levels given their poor nutritional quality. Growing reports of salps as prey in various fish diets challenge this notion. Here we combine insights into salp feeding with a variety of other in-situ rate measurements of New Zealand's Chatham Rise to reconstruct the most likely energy flows within the ecosystems using linear inverse ecosystem modeling. We then utilize model results to assess diets, production, and ecosystem efficiency for each of the key functional groups in the system, including economically important higher trophic levels and fisheries. We find that rather than acting as a trophic dead-end, the shorter energetic pathways introduced by salps increase overall ecosystem efficiency. Through their direct consumption by fish such as myctophids and oreo, these salp-driven energetic savings are efficiently passed on to higher trophic levels, increasing NPP-normalized secondary production by 130% relative to areas with background salp abundances. This finding challenges the hypothesis that an increased dominance of gelatinous zooplankton (brought on by climate change) will negatively impact global fisheries and instead suggests that species able to directly or indirectly access enhanced gelatinous production may instead benefit from the tighter coupling to lower trophic levels.

Dreissenid mussels represent two of the most problematic invasive species in the Laurentian Great Lakes yet little is known regarding the biochemical and nutritional composition of the veliger larval stage. Here, we quantified energy densities and fatty acids (FA) in veligers as metrics of nutritional quality relative to those determined for three size fractions of crustacean zooplankton collected from Lake Huron's Saginaw Bay. Veliger energy densities ranged from 1.14 to 1.72 kJ g⁻¹ (wet wt.) and were significantly (p < 0.001) lower than energy densities of 1.91 ± 0.23 kJ g⁻¹ (64–150 μm); 1.94 ± 0.10 kJ g⁻¹ (150–250 μm); and 1.85 ± 0.19 kJ g⁻¹ (≥ 250 μm) for size fractionated crustacean zooplankton. Nutritional quality indicators including the unsaturation index (UI) suggested veligers (238 ± 229) were similar to those for 64–150 (233 ± 21), 150–250 (206 ± 30) and ≥ 250 μm (195 ± 49) zooplankton size fractions. However, veliger UIs were primarily driven by high docosahexaenoic acid content relative to zooplankton size fractions. For other essential compounds including arachidonic, α-linolenic and eicosapentaenoic FAs, veligers consistently ranked the lowest. These results demonstrate veligers to be of lower caloric content relative to zooplankton, especially compared to similarly sized 64–150 μm planktonic biomass and of lower overall nutritional quality with respect to fatty acid composition. This study provides benchmarks regarding the overall nutritional quality of veligers and furthers our understanding energy and nutrient availability among the lower trophic levels of food webs in invaded ecosystems.

Diazotrophic cyanobacteria, including those responsible for harmful algal blooms, fix atmospheric dinitrogen (N₂) to sustain growth when they are limited by dissolved inorganic nitrogen (DIN) supply. Conventional models fail to accurately predict the timing and magnitude of N₂ fixation, partly because of oversimplified assumptions about diazotrophic species responses to N. We hypothesized that cellular carbon (C) and N would co-vary with DIN availability, and cellular N quota reduced below a critical threshold would trigger heterocyst differentiation and N₂ fixation. Conversely, we hypothesized that heterocysts would be discarded, and N₂ fixation would cease at another higher cellular N quota threshold. To test this hypothesis, we undertook experiments to provide data to calibrate a model of cellular C and N of N₂ fixation in the cyanobacterium Raphidiopsis raciborskii. The model incorporated internal N quotas, external DIN supply and a time lag for N₂ fixation as key regulators of heterocyst production and N₂ fixation, explaining > 70% of the variation in measured cellular N and C : N molar ratios. Heterocysts were predicted to form at a cellular N quota of 0.085 mg N mg⁻¹ C and were discarded from filaments at 0.275 mg N mg⁻¹ C. There was a 5-d time lag between DIN deprivation and initiation of N₂ fixation. Continuous DIN supply was predicted to enhance N₂ fixation more than an equivalent one-off pulse. Our study highlighted the need to consider cellular N and N supply regimes in driving N₂ fixation to provide more accurate predictions of the response of cyanobacteria to DIN availability.

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.

The trophic transfer efficiency at the pelagic phytoplankton-zooplankton interface is—among other factors—governed by the dietary availability of polyunsaturated fatty acids (PUFAs) to herbivorous zooplankton. As most zooplankton taxa cannot synthesize PUFAs de novo, the phytoplankton PUFA composition is crucial for the herbivores' fitness. As PUFA synthesis is taxon-specific in phytoplankton, alterations in phytoplankton biodiversity lead to changes in the phytoplankton trait “nutritional quality” and can thus be expected to affect the relative fitness and intraspecific competition of herbivore populations. To study this potentially important link between producer diversity and consumer competitive interactions, we performed a freshwater mesocosm experiment with diversity-manipulated natural phytoplankton communities and naturally coexisting Daphnia longispina genotypes. We found that phytoplankton diversity is correlated with its PUFA composition, suggesting that a biodiversity loss on the primary producer level leads to decreased PUFA diversity and ultimately reduced dietary quality for consumers. This change in dietary PUFA availability further affected competitive interactions between co-occurring D. longispina genotypes, which was most evident for the ω3-PUFA α-linolenic acid (18:3 n-3) and the ω6-PUFA arachidonic acid (20:4 n-6). Interestingly, these two PUFAs had opposite effects on competition by favoring different genotypes. Our study thus demonstrates that phytoplankton PUFA content is a trait that affects consumers' functional traits and therefore represents a potential link between biodiversity and competition in aquatic herbivores.

Coccolithophores are abundant marine microalgae that produce a cell cover (coccosphere) composed of calcium carbonate platelets (coccoliths), and that play a unique role in the global carbon cycle. Gephyrocapsa huxleyi is the most prevalent coccolithophore in the oceans. Experiments in vitro indicate that the coccosphere size in G. huxleyi increases or decreases during phosphorus (P) or nitrogen (N) deprivation, respectively. To test whether coccosphere size variation occurs in native populations and relates to specific macronutrient availability, we examined communities at the open sea “station A” in the Gulf of Aqaba, northern Red Sea, and at the open sea station THEMO-2 in the Eastern Mediterranean Sea. At station A, the average diameter of G. huxleyi coccospheres was larger during the stratified, oligotrophic season (6.98 ± 1.29 μm), and smaller during the mesotrophic winter (6.16 ± 1.04 μm), and along the deep-chlorophyll maximum during stratified periods (6.36 ± 1.09 μm). A similar seasonal variation in coccosphere diameter was observed in the sister species Gephyrocapsa ericsonii. Complementary bioassays indicate that G. huxleyi was primarily P-limited during the stratified period, while non-limited or weakly P-limited in winter. The seasonal pattern in G. huxleyi diameter was the opposite at the Mediterranean THEMO-2 station. Larger coccospheres were detected in winter and smaller ones during summer. However, this pattern coincided with the prevalent patterns of inorganic macronutrient availability in the Eastern Mediterranean. Therefore, inorganic phosphorus availability is a key driver of coccosphere size variations in marine ecosystems.

The metabolism of heterotrophic bacteria acts as a key control on the turnover of organic matter in the ocean. However, much remains unknown about how nutrient availability, particularly iron concentration, impacts bacterial growth. In the dimly lit waters of the lower photic and upper mesopelagic zones, the attenuation of sinking particulate flux is intense, due in part to remineralization by heterotrophic bacteria. In the North Pacific Subtropical Gyre, dissolved iron concentrations display a subsurface minimum near the base of the photic zone, and this is also a region where bacteria have elevated cellular iron demands. In a series of field experiments, we examined how the availability of iron, nitrogen, and organic carbon impacts bacterial metabolism in the lower photic zone. Results of these experiments suggest that low iron conditions limit the turnover of organic carbon by bacteria, potentially enhancing the efficiency of organic carbon export to the deep sea. Uptake of both dissolved iron and inorganic nitrogen by heterotrophic bacteria was greatest when bacteria metabolized carbon-rich organic substrates, typical of the lower photic zone, compared to particulate material collected in the near-surface ocean. Vertical changes in the composition of organic matter may be an important control on bacterial cellular iron requirements, potentially pushing this community toward iron limitation in the interior waters of the ocean.

Organotrophic denitrification is an important nitrogen (N) removal process in lakes, but alternative N reduction processes such as lithotrophic sulfur (S)-oxidizing denitrification may be greatly underappreciated. We studied the redox transition zone (RTZ) in the meromictic water column of the North Basin of Lake Lugano (Switzerland) to characterize N transformation pathways coupled to the S and carbon (C) cycles. Incubations with ¹⁵N-labeled and unlabeled nitrate (NO₃⁻) revealed low denitrification rates and a general limitation of organic electron donors. The most accessible fractions of exported primary production biomass may have been largely consumed in the oxic water column during sedimentation and did not reach the RTZ at ~ 100 m depth. Conversely, sulfide (H₂S) and methane (CH₄), major end products of anaerobic degradation of the more recalcitrant organic carbon fractions in the sediment, represent a continuous source of energy to the RTZ, fostering the establishment of a community of S- and CH₄-dependent NO₃⁻ reducers, dominated by Sulfuritalea and Candidatus Methylomirabilis over several years of observation. Anoxic incubation experiments with H₂S amendments revealed a strong stimulation of dissimilatory NO₃⁻ reduction to ammonium (NH₄⁺) (DNRA), but not denitrification. High relative abundances of the archaeal NH₄⁺ oxidizer Candidatus Nitrosopumilus and bacterial nitrifiers indicate intense NO₃⁻ regeneration by nitrification in the upper RTZ. The potential interaction between nitrification and S-driven DNRA is unclear. However, their co-occurrence suggests that, at least under conditions of carbon limitation, N recycling between the NO₃⁻ and ammonium pools predominates over N removal via complete denitrification.

Coastal wetlands are critical in global carbon sequestration, but their biogeochemical cycling is highly sensitive to sediment mixing. Here, we quantified the impacts of storm-induced mixing on organic carbon (OC) storage across China's coastal wetlands using multiple radionuclides and machine learning (SHAP model). Field observations and SHAP results identified the storm surge as a key driver of sediment mixing, whose impact weakens with increasing offshore distance and vegetation cover. The vegetated, supratidal site in the Yellow River estuary wetland was less affected by the storm surge with a sediment mixing depth of only 18 cm. Sediment mixing was observed at 80% of stations from the literature in China's coastal wetlands, mainly driven by river discharge and waves. Furthermore, a negative correlation between sediment mixing depth and soil organic carbon (SOC) density (r = −0.42) in the top meter was found. For regions with SOC density > 5 kg m⁻², the depths of sediment mixing are lower than 40 cm. Under the medium-forcing scenario SSP2-4.5 and the high-forcing scenario SSP5-8.5, the average sediment mixing depth in China's coastal wetlands is expected to increase by 19% and 28% by 2100, leading to corresponding declines in SOC density of 2.5% and 3.5%. The findings show that intensified sediment mixing caused by climate warming will weaken the carbon storage capacity of coastal wetlands, providing a crucial scientific basis for the sustainable management of coastal wetlands and the formulation of carbon sink enhancement strategies.

Nitrification is a key process in the aquatic nitrogen (N) cycle, but its products, nitrate (NO₃⁻) and nitrous oxide (N₂O), contribute to eutrophication and greenhouse gas emissions, particularly in eutrophic lakes. Variations in in-lake N cycling and N₂O 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 NO₃⁻ and N₂O concentrations and isotope ratios, and measuring ammonium oxidation rates via ¹⁵N tracer incubations over one year. An N isotope mass-balance model revealed that nitrification sustained only 5.3 ± 0.7% of total NO₃⁻ consumption in the epilimnion, where external N loadings were influential, and considerably more in the hypolimnion (81.6 ± 18.5%) during stratification. Dual NO₃⁻ isotope signatures (Δδ¹⁸O : Δδ¹⁵N ~ 1.5–1.73) and associated negative NO₃⁻ 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 NO₃⁻ reduction (with negligible effects on δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻), transient suboxic conditions enabled water column denitrification during stratification (24.1–30.2% of the total hypolimnetic denitrification). High N₂O isotope site-preference values (30–35‰) confirmed hypolimnetic ammonium oxidation as the main N₂O production pathway. During winter overturn, N₂O transport from the hypolimnion caused epilimnetic N₂O oversaturation and atmospheric emissions up to 3.52 μmol m⁻² d⁻¹. Comparison with other lakes suggests that artificial oxygenation enhances N turnover, manifesting in greater ambient N₂O backgrounds and fluxes to the atmosphere.