Limnology and Oceanography最新文献

The arrival order of species can strongly influence the early assembly of ecological communities. Such events, known as priority effects, are documented in various ecological settings, but remain understudied within the context of the gut microbiome. Given the fundamental link between the gut microbiome and host health, exploring the potential role of priority effects in shaping the assembly and development of microbial communities within the gut becomes imperative. Using the freshwater planktonic crustacean Daphnia magna as a model system, we manipulated the immigration order of three bacterial strain pairs in two germ-free genotypes and quantified gut microbiome composition and host fitness at two time points, namely day 8 and day 12. Priority effects were observed; however, their presence, amplitude, and direction (suppressive or facilitative) were found to be contingent on the identity of bacterial strain and host genotype. These findings were accompanied by notable differences in Daphnia life history traits across inoculation order treatments, shedding light on the tangible consequences of priority effects triggered by the sequence of bacterial strain arrival in the gut environment, for host fitness. Our results thus highlight the complex nature of priority effects in gut community assembly, their strain/genotype specificity, and their potential impact on the host.

Past changes in the input/output, and internal cycling, of bioavailable nitrogen (N) in marine and lacustrine environments can be reconstructed by analyzing the N isotopic composition (δ¹⁵N) of organic matter in the sedimentary record. To verify, and eliminate, potential biases of bulk sedimentary δ¹⁵N (δ¹⁵N_bulk) signatures by diagenetic alteration and external N inputs, we applied, for the first time, the diatom-bound N isotope (δ¹⁵N_db) paleo-proxy to lake sediments. By comparing δ¹⁵N_bulk and δ¹⁵N_db in a sedimentary record from eutrophic Lake Lugano (Switzerland), we demonstrate that changing redox conditions influence the degree of N-isotopic alteration of the bulk sediment, emphasizing the need for caution when interpreting δ¹⁵N_bulk in paleolimnological studies. Furthermore, in combining δ¹⁵N_db measurements with X-ray fluorescence scanning and state-of-the-art molecular biomarker analyses, we reconstruct nutrient cycling and paleoenvironmental conditions in the lake over the past ~ 125 yr. Coeval with the period of severe eutrophication in Lake Lugano in the 1960s, our proxy data indicate that export production, δ¹⁵N_db, and the concentration of heterocyst glycolipids (a biomarker for N₂-fixing cyanobacteria) increased simultaneously. Together, these data suggest that the rise in δ¹⁵N_db is likely the result of enhanced water-column denitrification in response to increased phytoplankton productivity. We hypothesize that greater export production during eutrophication led to anoxic conditions in the hypolimnion as a result of enhanced organic matter remineralization, raising water-column denitrification. Enhanced N loss and remobilization of phosphorous (P) from the sediments under anoxic conditions lowered the N : P ratio in the lake, fostering cyanobacterial N₂ fixation in surface waters.

In winter, the Kuroshio Current intrudes onto the continental shelf in the East China Sea with a cyclonic branch occasionally veering northeast of the Taiwan Strait. The dynamic process and mechanism by which the southward Kuroshio branch enters the Taiwan Strait is investigated using a numerical model. Decomposition of the flow field and sea surface height (SSH) using the empirical orthogonal function (EOF) isolates the first mode of the southward flow and the east-westward SSH slope, showing that the time series is closely related to wind strength. The southward Kuroshio branch is mainly controlled by geostrophic and Ekman effects. When northeasterly winds are weak, the elevated SSH on the eastern side, caused by the Kuroshio, generates a northward geostrophic current. This northward current overwhelms the Ekman current and triggers the Taiwan Warm Current. Conversely, when northeasterly winds are strong, an elevated SSH on the western side generates a southward geostrophic current. A theoretical equation is devised, which accounts for Ekman layer depth related to wind strength and geostrophic adjustments by the Ekman effect. This equation explains why the Kuroshio branch flows into the northern Taiwan Strait and why there is a counter-wind current (i.e., Taiwan Warm Current) during winter. Furthermore, we propose a novel perspective that suggests the alongshore current and cross-shore sea level in the Taiwan Strait are determined by the wind-driven current and geostrophic adjustment, which are generated by the competition or cooperation between the wind and the alongshore pressure gradient caused by the Kuroshio.

Coloniality may grant colony members an energetic advantage in the form of lower individual respiration rates as colony size increases. If this occurs it should be apparent as negative allometric scaling of respiration with colony size, and colonial organisms should have scaling factors < 1. However, colonial members from phylum Rotifera have yet to be examined. To test if colonial rotifers possess allometric scaling relationships between respiration rate and colony size, we measured respiration rates for four solitary and three colonial rotifer species; from these respiration rates we estimated scaling factors. We found mixed evidence for allometric scaling of respiration rate in colonial rotifers. Both rotifers with allometric scaling of respiration rate, Conochilus hippocrepis and Lacinularia flosculosa, have extensive mucilaginous coverings. These coverings may represent an investment of colony members into a shared structure, lowering individual metabolic costs and thus respiratory needs. Additionally, we determined which traits are associated with allometric scaling of respiration. We compiled known scaling factors for animal phyla from a wide phylogenetic spectrum with colonial representatives and conducted a hierarchical mixed regression that included attributes of colonies. Traits associated with allometric scaling in colonial animals included colony shape, the presence of shared extrazooidal structures, and planktonic lifestyle. There are many other colonial rotifers and animal taxa for which allometric scaling factors have yet to be estimated, knowing these may enhance our understanding of the benefits of coloniality in animals.

Recent studies have challenged the validity of the Redfield ratio. It is proposed that physical and biogeochemical processes govern the geographical variations in carbon : nitrogen : phosphorus (C : N : P) ratios. However, this proposal remains to be examined through concurrent observations of C : N : P ratios with physical and biogeochemical processes in various marine reservoirs. Here, we sampled the Arabian Sea for its C, N, and P content in organic and inorganic pools during the winter monsoon. We analyzed the role of convective mixing, eddies, and N₂ fixation to explain the variation in observed elemental ratios. Convective mixing injected the cold water and enhanced the supply of N and P nutrients in the top layer (surface to 50–75 m) of the northern Arabian Sea. This led to a decrease in the N : P and C : P ratios in the particulate organic matter in the northern region, but C : N : P increased equatorward, averaging 245 : 32 : 1 in the top layer of the Arabian Sea. The variation in the elemental ratios in the top layer is best explained by the changes in water temperature. N₂ fixation contributed negligibly to the N : P ratio of the export flux. The substantial decrease in N : P ratios of nutrients in the subsurface waters is most likely caused by the denitrifying conditions in the Arabian Sea. As the processes of convective mixing and eddies are are prevalent oceanic processes, our observations underpin that the interplay of these processes leads to changes in the elemental ratios globally.

Nitrogen (N) bioavailability affects phytoplankton growth and primary production in the aquatic environment. N bioavailability is partly determined by biological N cycling processes that either transform N species or remove fixed N. Reliable estimates of their kinetic parameters can help understand the distribution of N cycling processes. However, available estimates of kinetic parameters are often derived from microbial isolates and may not be representative of the natural environment. Observations are particularly lacking in estuarine and coastal waters. We conducted isotope tracer addition incubations to evaluate substrate affinities of nitrification, denitrification and anammox in the Chesapeake Bay water column. The half-saturation constant for ammonia oxidation ranged from 0.38 to 0.75 μM ammonium, substantially higher than observed in the open oceans. Half-saturation constants for denitrification—0.92–1.86 μM nitrite or 1.15 μM nitrate—were within the lower end or less than those reported for other aquatic environments and for denitrifier isolates. Interestingly, water column denitrification potential was comparable to that of sedimentary denitrification, highlighting the contribution of the water column to N removal during anoxia. Mostly undetectable anammox rates prevented us from deriving the half-saturation constants, suggesting a low affinity of anammox. Using these substrate kinetics, we were able to predict in situ N cycling rates and explain the vertical distribution of N nutrient concentrations. Our newly derived substrate kinetics parameters can be useful for improving model representation of N nutrient dynamics in estuarine and coastal waters, which is critical for assessing the ecosystem productivity and function.

Human pressures are changing ocean environments, such as a shift from fish- to squid-dominated ecosystems in overfished, poorly oxygenated environments. After death, carcasses of upper ocean fauna sink to the seafloor where they provide food for demersal scavengers. It is unclear how shifts in carcass type impact abyssal scavengers. We performed baited benthic camera lander deployments in the Cabo Verde Abyssal Basin to test how a shift from fish- to squid-dominated carrion could modify abyssal scavenger ecology. At the fish bait, peak scavenger abundance was greater and occurred later for the majority of observed fauna. However, removal rates of squid bait were up to 10-fold greater, and a significantly different community composition developed, favoring faster organisms with lower chemosensory thresholds. At the fish bait, slower organisms were less disadvantaged as the bait persisted for longer periods allowing the development of a more complex community and dense amphipod aggregations. The rapid squid consumption indicates that the accumulation of this type of food fall at the seafloor may not occur, preventing scientific observations necessary to estimate the importance of squid carrion to the biological C pump and deep-sea food webs. As such, the flux of squid carrion to the seafloor is likely greater than currently recognized in this part of the Atlantic. The differences observed between bait types indicate how future changes in upper ocean ecosystems may impact abyssal scavengers and their ecosystem functions, including controlling seafloor biomass, regulating the behavior of benthic fauna, and contributing to nutrient cycling and energy transfer.

Niche theory has been widely used in ecology; however, few studies have attempted to combine information on functional and ecological niches (i.e., variation in traits and environmental requirements), especially for freshwater macrophytes. In this study, we aim to describe the functional and ecological niches of four key nymphaeid species (Nuphar lutea, Nymphaea alba, Nelumbo nucifera, and Nymphoides peltata) to investigate their environmental tolerance and functional adaptability. Twelve Italian populations per species were sampled. Functional and ecological niches were determined using hypervolumes based on eight functional traits and environmental variables, related to leaf structure and economics spectrum, and to water and sediment quality. Among the three Italian native species, N. lutea and N. alba showed intermediate niche size and position in the functional and ecological space, although N. alba appears to be less competitive due to a small functional niche and lower trait performance, which could explain its general tendency to decline. N. peltata appeared more specialized in its environmental requirements and characterized by highly acquisitive leaves, while the invasive alien N. nucifera exerted its competitive success by distinguishing its functional niche and expanding its ecological niche, through high investment of resources in leaves. Overall, all four target species share similar ecological niches, colonizing eutrophic ecosystems typical of intensive agricultural landscapes, but show different patterns in their functional niche. We demonstrate the applicability of an approach based on both functional and ecological niches to unravel species' adaptation and strategies.

The Southern California Borderland hosts a variety of geologic and oceanographic features that allow for diverse habitats to occur in a restricted region with a strong oxygen minimum zone (OMZ) and hard substrates. These include ferromanganese (FeMn) crusts and phosphorites targeted for deep-seabed mining in other regions. Baseline studies regarding hardground macro- (> 0.3 mm) and megafaunal (> 2 cm) invertebrates are lacking, although they contribute to understanding nutrient cycling and resilience of deep-sea communities to ocean deoxygenation, fishing, or mineral extraction. With the goal of understanding how substrate type, depth, and dissolved oxygen concentration influence invertebrate trophic structure, we surveyed δ¹³C and δ¹⁵N values of invertebrates on hard substrates on the Southern California Borderland margin along a depth gradient (120–2400 m) through the OMZ at inshore (< 100 km from shore) and offshore (100–250 km from shore) sites, using generalized additive models and community-level metrics. Macrofaunal isotopic values correlate with substrate type, exhibiting higher trophic diversity on FeMn crusts and specialized communities on phosphorites. Megafaunal isotopic values correlate with proximity to shore; animals offshore seem to depend more on phytoplanktonic production than animals inshore. In general, δ¹⁵N increased with decreasing dissolved oxygen and increasing depth, possibly due to remineralization processes within the OMZ and with depth. We discuss how feeding modes and community composition might influence the observed patterns. This study elucidates the importance of the environmental context in shaping invertebrate trophic structure on continental margins and provides baseline knowledge that may be useful in regions where these minerals are targeted for extraction.

The 2014–2015 warm anomaly (aka “the Blob”), the largest of periodic and intensifying marine heat wave (MHW) perturbations in the northeast Pacific, may provide some insight about the future warmer ocean. Here, we use mixed-layer carbon estimates for total phytoplankton, major size classes and functional groups from 45 CalCOFI cruises to: (1) compare 2014–2015 MHW impacts in the southern California Current System to baseline estimates from 2004 to 2013 and (2) to test a space-for-time exchange hypothesis that links biomass structure to variability of nitracline depth (NCD). Seasonal and inshore-offshore analyses from nine stations revealed almost uniform 2°C MHW warming extending 700 km seaward, fourfold to sixfold declines in nitrate concentration and 18-m deeper NCDs. Phytoplankton C decreased 16–21% compared to 45–65% for Chl a, with the threefold difference due to altered C : Chl a. Among size classes, percent composition of nanoplankton decreased and picophytoplankton increased, driven by higher Prochlorococcus biomass, while Synechococcus and picoeukaryotes generally declined. Diatom and dinoflagellate C decreased in both onshore and offshore waters. Seasonally, the MHW delayed the normal winter refresh of surface nitrate, resulting in depressed stocks of total phytoplankton and nanoplankton, Synechococcus and picoeukaryotes during winter. Consistent with the space-for-time hypothesis, biomass variations for baseline and MHW cruises followed similar (not significantly different) slope relationships to NCD. All biomass components, except Prochlorococcus, were negatively related to NCD, and community biomass structure realigned according to regression slopes differences with NCD variability. Empirically derived biomass-NCD relationships could be useful for calibrating models that explore future food-web impacts in this coastal upwelling system.