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Identifying Saharan dust driven export of biogenic material in the ultraoligotrophic eastern Mediterranean Sea
No full textTo assess the effects of dust deposition on the strength of the biological pump in the Mediterranean Sea by acting as fertilizer and/or ballasting agent, we analyzed fluxes of mineral dust, particulate organic carbon (POC) and inorganic carbon (PIC), and source-specific lipid biomarkers (i.e., higher plant-derived long-chain fatty acids and phytoplankton-derived alkenones, C30 1,15 diols, and sterols) in sinking particles. Sinking particles were collected at ten-day intervals by a sediment-trap mooring deployed in the Ionian Basin from April 2017 to May 2018 at 2340 m water depth. High POC fluxes occur during summer, when surface ocean primary production is lowest due to thermal stratification. Notably, these high POC fluxes coincide with pulses of substantial dust deposition, suggesting that POC export is primarily driven by dust deposition and subsequent ballasting. However, the lipid composition, and thereby that of the phytoplankton community, differs between dust events. (Seasonal) variations in the properties of the deposited dust, presumably associated with its provenance, likely control the effect of dust deposition on phytoplankton response and export in the Ionian Basin. Although POC export is associated with dust deposition, the net effect of dust deposition on the biological pump is more ambiguous as not all dust events are associated with an increase in POC export, and most dust events are also associated with PIC export that has a counteracting effect on the biological pump. Multi-year time series of dust deposition and biogenic export are required to validate the seasonal variations in dust-driven export of biogenic material observed here, and to account for effects of interannual variations in dust fluxes and phytoplankton production on the strength of the biological pump
The nutritional quality of zooplankton in the sub-antarctic Southern Ocean during late austral summer
No full textKnowledge of the trophic ecology of zooplankton is essential for evaluating their functional roles in marine food webs and nutrient cycling since they represent the link between primary producers and higher trophic levels. Here we investigated the fatty acid (FA) composition of different zooplankton size classes and selected species collected in the vicinity of the sub-Antarctic Kerguelen Islands in late austral summer 2018 as part of the MOBYDICK research project. The analysis revealed that zooplankton FA composition varied significantly across size classes and species but not among stations. Larger zooplankton (>1000 μm) generally had higher total FA (TFA) amounts per dry weight than smaller classes (22.1 ± 3.0 vs. 61.9 ± 11.8 mg g−1). Essential FAs (EFA) accounted for 40.5 ± 0.8 % of TFA, with 22:6n-3 (DHA) and 20:5n-3 (EPA) being the most prominent. Diatom trophic markers (TM) were abundant in larger zooplankton size classes, while non-diatom TM were more prevalent in smaller size classes. The FA-based nutritional quality index (NQI) of zooplankton was positively correlated with EFA and DHA, and it was higher than the NQI of phytoplankton concurrently collected, indicating that zooplankton has a better nutritional quality than primary producers. This study highlights the importance of size and species-specific dietary preferences in determining zooplankton FA profiles and the high nutritional quality of this group collected during late austral summer, which significantly contributes to our understanding of zooplankton\u27s ecological role in sub-Antarctic pelagic food webs.</span
The effect of a climatic compound drought and heatwave event on the dune-building grass Elytrigia juncea
No full textBackground and aimsCoastal dunes provide vital ecosystem services, including flood protection and freshwater storage. These ecosystems are shaped by clonally-growing dune grasses that trap sediment as patch size increases, enabling the grasses to avoid stress from seawater flooding and freshwater scarcity. However, it remains poorly understood how increasing climate extremes will impact the establishment and survival dynamics of this vegetation. This study investigated the effects of an experimental climatic compound drought and heatwave (CDHW) on Elytrigia juncea in an embryonic dune field.MethodsOver a four-week field experiment, we examined two plant-patch sizes (0.014 m2 vs. 1.953 m2) and two climatic treatments (ambient vs. CDHW), monitoring plant response and soil moisture. We hypothesized that larger patches would better resist the CDHW due to their enhanced freshwater storage within larger dune bodies.ResultsContrary to our expectations, E. juncea exhibited a positive response to CDHW, with longer shoots in both patch sizes. Initial soil moisture profiles remained similar across patch sizes throughout the experiment. Moreover, soil moisture profiles indicated a substantial freshwater source within reach of roots in all plots, explaining the absence of drought stress.ConclusionOur findings suggest that pioneer dune grasses can be highly resistant to climatic CDHW when roots can reach fresh groundwater, a condition which can occur when a thin freshwater lens is present in proximity to larger dune complexes. For establishing dune grasses, it is not merely dune formation on a local scale but rather on a landscape scale that is crucial for coping with extreme climatic events
Nature-based mitigation of shoreline erosion risks in tidal marshes created by managed realignment vs. sediment nourishment
No full textTidal marshes provide many valuable ecosystem services and can play an important role in nature-based flood risk mitigation along low-lying coasts and estuaries, by attenuating waves and increasing erosion resistance. There is an effort around the world to restore or create tidal marshes, but it remains unknown how different marsh restoration and creation techniques affect the development of erosion resistant sediment beds, which is essential for their contribution to long-term erosion and flood risk mitigation. Here, we compared sediment shear strength and erosion resistance under very high flow velocity (i.e. as may occur during the breach of a dike or seawall behind the marsh) of a managed realignment site versus a sediment nourishment site, restored and created respectively, and assessed the effects of tidal inundation, sediment characteristics, and vegetation. Managed realignment consists of the landward relocation of flood defence structures like seawalls and dikes and tidal flooding of low-lying land, creating a sheltered environment for tidal deposition of fine-grained sediments, while sediment nourishment is the seaward placement of mostly more coarse-grained dredged sediment. This study showed that at both sites sediment shear strength and erosion resistance were higher at vegetated locations than at unvegetated locations. In addition, at the managed realignment site, tidal inundation duration affected shear strength negatively, while bulk density affected shear strength positively. At the sediment nourishment site, sediment grain size was the most important driving factor of shear strength and erosion resistance, besides vegetation presence: a decreasing shear strength and erosion resistance were observed with increasing sediment grain size. Managed realignment and sediment nourishment both have advantages and disadvantages for the fast development of an erosion resistant sediment bed. Managed realignment will likely lead to fine-grained, cohesive sediments, which are in this case colonised by dense, but slowly establishing, mud-loving Spartina vegetation. In contrast, sediment nourishments are typically done with more coarse-grained, non-cohesive sediments, which are in this case colonised by initially sparse, fast establishing Salicornia vegetation. Dense Spartina increased erosion resistance more than sparse Salicornia. If we plan well ahead of time and temporarily support their development, restored or created marshes have time to become erosion resistant, thereby enabling sustainable use of marsh ecosystem services for long-term nature-based flood risk mitigation.</span
The Octocoral Trait Database: a global database of trait information for octocoral species
No full textTrait-based approaches are revolutionizing our understanding of high-diversity ecosystems by providing insights into the principles underlying key ecological processes, such as community assembly, species distribution, resilience, and the relationship between biodiversity and ecosystem functioning. In 2016, the Coral Trait Database advanced coral reef science by centralizing trait information for stony corals (i.e., Subphylum Anthozoa, Class Hexacorallia, Order Scleractinia). However, the absence of trait data for soft corals, gorgonians, and sea pens (i.e., Class Octocorallia) limits our understanding of ecosystems where these organisms are significant members and play pivotal roles. To address this gap, we introduce the Octocoral Trait Database, a global, open-source database of curated trait data for octocorals. This database houses species- and individual-level data, complemented by contextual information that provides a relevant framework for analyses. The inaugural dataset, OctocoralTraits v2.2, contains over 97,500 global trait observations across 98 traits and over 3,500 species. The database aims to evolve into a steadily growing, community-led resource that advances future marine science, with a particular emphasis on coral reef research.</span
A sub-fossil coral Sr∕Ca record documents northward shifts of the Tropical Convergence Zone in the eastern Indian Ocean
No full textSea surface temperature (SST) variability in the south-eastern tropical Indian Ocean is crucial for rainfall variability in Indian Ocean rim countries. A large body of literature has focused on zonal variability associated with the Indian Ocean Dipole (IOD) which peaks in austral spring. In today\u27s climate, northward shifts of the Tropical Convergence Zone (TCZ) co-vary with the IOD, and it is unclear whether these shifts may also occur independently. We have developed a new monthly resolved record from a sub-fossil coral cored at Enggano Island (Sumatra, Indonesia). Core sections containing diagenetic phases are omitted from the SST reconstruction. UTh dating shows that the -based SST record extends from 1869–1918 and from 1824–1862 with a relative age uncertainty of ±3 years (2σ). At Enggano Island, coastal upwelling and cooling in austral spring impact SST seasonality and are coupled to the latitudinal position of the TCZ. The sub-fossil coral indicates an increase in SST seasonality between 1856 and 1918 relative to the 1930–2008 period. We attribute this to enhanced cooling due to stronger south-easterly (SE) winds driven by a northward shift in the TCZ in austral spring. A nearby sediment core indicates colder SSTs and a shallower thermocline prior to ∼1930. These results are consistent with an increase in the north–south SST gradient in the eastern Indian Ocean, calculated from historical temperature data, that is not seen in the zonal SST gradient. We conclude that the relationship between meridional and zonal variability in the eastern Indian Ocean is non-stationary and modulated by the long-term evolution of temperature gradients.</span
Emergence and evolution of heterocyte glycolipid biosynthesis enabled specialized nitrogen fixation in cyanobacteria
No full textHeterocytes, specialized cells for nitrogen fixation in cyanobacteria, are surrounded by heterocyte glycolipids (HGs), which contribute to protection of the nitrogenase enzyme from oxygen. Diverse HGs preserve in the sediment and have been widely used as evidence of past nitrogen fixation, and structural variation has been suggested to preserve taxonomic information and reflect paleoenvironmental conditions. Here, by comprehensive HG identification and screening of HG biosynthetic gene clusters throughout cyanobacteria, we reconstruct the convergent evolutionary history of HG structure, in which different clades produce the same HGs. We find that rudimentary HG biosynthetic machinery was already present in cyanobacteria before the emergence of heterocytes for functions unrelated to nitrogen fixation and identify HG analogs produced by specific and distantly related nonheterocytous cyanobacteria. These structurally less complex molecules represent precursors of HGs, suggesting that HGs arose after a genomic reorganization and expansion of ancestral biosynthetic machinery, enabling the rise of cyanobacterial heterocytes in an increasingly oxygenated atmosphere. Our results open a chapter in the potential use of diagenetic products of HGs and HG analogs as fossils for reconstructing the evolution of multicellularity and division of labor in cyanobacteria
The key to bubble-net feeding: how humpback whale morphology functionally differs from other baleen whales
No full textManeuverability in cetaceans is facilitated by pectoral flippers, flukes and spinal flexibility, features that are pronounced in humpback whales (Megaptera novaeangliae). Humpback whales exhibit several foraging tactics requiring high maneuverability not seen in other baleen whales, including bubble-net feeding. We hypothesized that the significant lift force produced by the humpback whale\u27s uniquely large pectoral flippers will result in them being the only species observed executing the tight, high-speed, sustained turns characteristic of solitary bubble-net feeding. To test this hypothesis, we used a combination of inertial sensor tag data and unoccupied aerial systems (UAS; drone) photogrammetry to quantify the turning performance of solitary bubble-net feeding humpback whales, and compared this to similar data from six other mysticete species. We found that solitary bubble-net feeding humpback whales exhibited centripetal accelerations (0.46 m s−2) that exceeded the upper limit quantified in comparable turns by all six other mysticetes. This enhanced turning performance can be attributed to a substantial lift force generated by the humpback whale\u27s pectoral flippers (7800±85 N), which contributes to centripetal acceleration and facilitates faster roll rates, allowing humpback whales to more quickly bank inwards and utilize their spinal flexibility to decrease their turning radius. Our findings demonstrate how humpback whales are uniquely adapted to exploit prey patches that might otherwise be insufficient for capture by animals of such a large size.</span
Coastal shorebirds delay maturity more than inland ones
No full textDelaying the age of first breeding will lower lifetime reproductive output unless compensated for by increased fecundity or survival. Yet, in many migratory shorebird species (Charadriiformes) individuals delay their first return migration to the breeding grounds until they are several years old. Using data from non-breeding and breeding season counts of shorebirds in the non-breeding areas, recaptures, and long-term banding studies, we assess the age of first return migration (as a measure of maturity) for 37 shorebird species that have migrated to Australian non-breeding grounds. We provide a comparative analysis of the association between the measure of maturity and habitat use during the non-breeding period, contrasting coastal and inland wetland habitats. After controlling for latitudinal and phylogenetic covariates, we found a positive relationship between body size and the age of first return migration. However, there was still a stronger relationship with the type of non-breeding habitat used. Coastal shorebird species delayed maturity more than species that spend the non-breeding season in non-tidal inland wetlands. This finding expands on previously identified ecological and physiological differences between coastal and inland shorebirds and leads to questions on the environmental characteristics embodied in the habitat contrast. We propose that the complicated tidal dynamics and differences in prey make it more difficult to become an individually competent coastal (rather than inland freshwater) forager.</span
Small fish biomass in the North Sea is far greater than previously estimated
No full textSmall fish, including species with small adult sizes and juveniles of larger species, play a central role in marine food webs as prey for top predators, such as seabirds, marine mammals, and piscivorous fish. However, reliable data on small fish are lacking as conventional fisheries surveys primarily focus on larger, commercially important species and underestimate small fish. Consequently, little is known about absolute biomasses, and fine-scale distribution patterns of this important trophic group. Based on 1307 quantitative Triple-D samples from the Dutch Exclusive Economic Zone and the UK sector of the Dogger Bank, biomass densities were estimated for pooled small demersal fish and for the most abundant species individually. Our estimates suggest that small demersal fish biomass is at least twice as high as reported in trawl-based studies. Uniformly distributed species such as dab (Limanda limanda) and plaice (Pleuronectes platessa) contributed most to the overall small fish biomass, while sandeels (Ammodytidae) showed particularly high local densities. These new prey biomass estimates might change our understanding of the North Sea ecosystem\u27s carrying capacity, and establishes a baseline for monitoring changes in small fish communities driven by climate change and human impacts