Pelagic Processes Research Articles

Mesocosm approaches to the examination of benthic–pelagic coupling, with emphasis on turbulence

AbstractBenthic and pelagic processes are closely coupled in shallow‐water estuarine and marine environments and in lakes yet benthic–pelagic coupling is difficult to simulate in mesocosms. Realistic water column (WC) and bottom turbulence often are not simultaneously mimicked in mesocosm experiments, leading to artifacts and omitting direct and indirect linkages and feedbacks between the sediment and the WC. The information on benthic–pelagic coupling mesocosm studies assembled illustrates the importance of realistic physics in mesocosms. Mesocosm approaches including both WC turbulence and bottom turbulence that are appropriately designed to resemble natural conditions can provide a deeper mechanistic understanding of feedback processes associated with biogeochemical, food web, and the habitat shifts necessary for models, and will help identify cause‐and‐effect relationships. Benthic–pelagic mesocosm experiments with realistic WC and bottom turbulence may fill the large gaps in our understanding of the responses of inorganic nutrient and organic matter fluxes between benthic habitats and the WC. Future applications of such benthic–pelagic coupling mesocosm studies are outlined.

Weak seasonality in benthic food web structure within an Arctic inflow shelf region

The Arctic Ocean is characterized by pronounced seasonality in the quantity and quality of organic matter exported from the surface ocean. While it is well established that changes in food availability can alter the abundance, biomass and function of benthic organisms, the impact on food web structure is not well studied. We used bulk carbon and nitrogen stable isotope analysis to assess the quantity and quality of sediment organic matter and structure of the benthic food web in four seasons within the Northern Barents Sea (76°N − 82 °C). Despite a highly seasonal vertical flux, we found that the organic carbon and chlorophyll-a content of surface sediments was seasonally stable, suggesting a lack of seasonality in food availability at the seafloor. However, organic biomarkers indicate that the quality of sediment organic matter increased to a maximum in August and December, up to 6 months after the spring bloom. The seasonal stability of food quantity was mirrored in food-web structure (e.g., total isotopic range, number of trophic levels) which did not change significantly across sampling periods. We expected that suspension and deposit feeders would respond more readily to seasonal changes in food quality compared to predators. However, we observed no significant seasonal changes in the trophic levels or isotopic niche areas of benthic functional groups. The centroids of isotopic niches of all benthic functional groups shifted seasonally by <2 ‰ along the δ13C-axis, suggesting minimal shifts in carbon resource use. Because the northern Barents Sea experiences significant changes in seasonal sea ice cover, we expected that stable-isotope ratios of benthic organisms would show an increased consumption of sympagic-derived organic matter through less negative δ13C values in early spring and summer. However, only two taxa (the soft coral Gersemia spp. and bivalves in the family Yoldiidae) showed 13C-enrichment in spring or summer consistent with the assimilation of sympagic-derived organic matter, despite previous evidence suggesting widespread use of this carbon source. Overall, our results show that there is an apparent de-coupling in time between pelagic processes and benthic food-webs in which the accumulation and assimilation of high-quality organic matter occurs for benthos during the fall and early winter months when there is little to no fresh organic matter generated at the surface. This temporal mismatch highlights the importance of considering the timescales over which components of the marine ecosystem respond to short-term environmental changes and the methods employed to assess seasonality.

Uncovering hidden structures: previously undescribed pseudopodia and ectoplasmic structures in planktonic foraminifera.

The trophic strategies of cold-water planktonic foraminifera are not well understood due to the challenge of culturing them in polar conditions. Here, we identify previously unknown ectoplasmic and cytoplasmic projections in three species of planktonic foraminifera thriving in polar and subpolar marine environments: Globigerina bulloides, Neogloboquadrina incompta and Neogloboquadrina pachyderma. These structures were observed during routine monitoring of cultured specimens sampled from the Norwegian coast, Greenland Sea and Baffin Bay. Two types of projections were discovered, including permanent and non-permanent structures such as ectoplasmic roots, twigs and twig-like projections, similar to those observed in benthic taxa Cibicides and Cibicidoides. Additionally, a previously undescribed filopodia-like projection was observed in N. pachyderma. We discuss the function, the ecological significance and the potential impact on pelagic processes of the presence of these structures in foraminifera species that occupy diverse niches in the water column. Our findings suggest that these structures may play an important role in the trophic strategies of cold-water planktonic foraminifera, and further research and observations are necessary to fully comprehend their significance in the carbon cycle.

Basin circulation affecting sediment partitioning in a fine-grained carbonate–siliciclastic, subaqueous clinoform: the Upper Jurassic–Lower Cretaceous Vaca Muerta Formation, Neuquén Basin, Argentina

Fine-grained sediments deposited under oxygen-deficient conditions are enriched in organic matter and hold considerable economic interest as unconventional reservoirs. The present analysis integrates sedimentological, ichnological and geochemical datasets to understand depositional processes in the Upper Jurassic–Lower Cretaceous Vaca Muerta Formation (Argentina), the most important economic unconventional play outside North America. For this purpose, cores from eight wells and one outcrop were examined. The succession comprises 13 facies associations corresponding to marginal-marine, basin, drift and slope environments, deposited in a mixed carbonate–siliciclastic, shelf-margin, subaqueous clinoform system. Estuarine circulation during warm and humid climates produced a stable stratification. Sediment delivery was dominated by pelagic processes in the bottomset, and hemipelagic processes and fluid mud flows in the foreset. However, the typical estuarine circulation changed to weakened estuarine or anti-estuarine during cool and arid climates, triggering wind- and thermohaline-driven contour (bottom) currents that reworked and oxygenated the complete clinoform system. The analysis suggests that distal–proximal trends are of lesser importance for the delineation of sedimentary environments in settings with contrasting basin circulation regimes and provides several insights to understand sediment partitioning in fine-grained clinoform systems. Supplementary material : The stratigraphic context of the Vaca Muerta Formation, an example of facies distribution in a well, trace-fossil descriptions, and X-ray diffraction and total organic carbon geochemical data are available at https://doi.org/10.6084/m9.figshare.c.6294914

Vertical Transport Timescale of Surface‐Produced Particulate Material in the Chesapeake Bay

AbstractAccumulation and remineralization of surface‐produced particulate organic matter (POM) in the water column and seabed link closely to hypoxia and the health of aquatic ecosystems. The POM retention time provides a key timescale to interpret biochemical reaction processes. In this study, we investigated the spatiotemporal variations in the vertical particulate age (VPA) of surface‐produced POM, which is the mean time elapsed since the particulates last contact the surface, by incorporating major physical processes including sinking, resuspension, and deposition in the Chesapeake Bay. It was found that the vertical transport time for the particulates (i.e., VPA) is much longer than the dissolved counterparts as the former consists of new material from the surface and the resuspended aged material that has elongated resting on the seabed after deposition. The VPA is sensitive to settling velocity, especially in low‐frequent resuspension environments, and varies over 2 orders of magnitude with settling velocity from 0 to 10 m/day. Slow‐sinking material can remain in suspension and seldom settle to the seabed, thus mainly contribute to pelagic processes, while the fast‐sinking material connects closely with benthic processes. The seasonality of VPA decreases as the settling velocity increases. No significant difference in VPA was found between wet and dry years, yet the episodic strong flood events entrain old materials from the depositional lateral shoals to increase VPA in the channel. The transport age bridges cross disciplinaries by providing the fourth‐dimensional age information as a common currency to compare the physical transport timescale with the timescales for biochemical reactions.

Impact of shallow‐water hydrothermal seepage on benthic biogeochemical cycling, nutrient availability, and meiobenthic communities in a tropical coral reef

AbstractWe investigated the influence of high‐CO2 hydrothermal seepage on element cycling, early diagenetic processes, and meiobenthic communities in sediments of a coral reef in Papua New Guinea. Based on fluid flow velocities, determined from temperature gradients, and element concentrations, the solute fluxes from the seeps were estimated, showing that seepage through sediments can be a source of nutrients but also of potentially toxic elements to the reef ecosystem. The sediment pore waters consisted of up to 36% hydrothermal fluids, enriched in As, Si, Li, Mn, Fe, Rb, and Cs relative to ambient seawater. During their ascent to the seabed, the acidic fluids reacted with the sediments, leading to increases in total alkalinity, nutrients, and alkali elements in the fluids. Mixing of hydrothermal fluids with seawater within the sediments lead to precipitation of redox‐reactive species, including Fe‐oxides, but the sediment pore waters were still a source of trace metals to the water column. Presence of the low‐pH fluids in the sediments resulted in dissolution of sedimentary carbonates and left behind finer‐grained volcanoclastic sands containing As, Cr, and Ni in concentrations toxic to biota. These finer‐grained sediments had a reduced permeability, reducing the rate of remineralization of organic matter. Benthic meiofauna and nematode abundance and functional diversity were relatively lower at sites with hydrothermal seepage through the sediment. As benthic and pelagic processes are tightly coupled, it is likely that the changes in benthic biogeochemical processes due to sediment acidification will also affect epibenthic and pelagic communities.

Factors regulating lake periphyton biomass and nutrient limitation status across a large trophic gradient

Abstract Because of the historical focus of limnology on pelagic processes, the factors controlling lake periphyton growth and nutrient limitation are understudied compared to the phytoplankton. We deployed nutrient‐diffusing substrata at 28 sites spanning a wide trophic status gradient in Lakes Superior and Michigan to assess periphyton biomass accrual on control substrata and the response of periphyton to single and combined phosphorus (P) and nitrogen (N) additions. Periphyton growth was unimodally related to a composite metric of site trophic status, with highest biomass at mesotrophic sites and lower growth at oligotrophic and highly eutrophic sites. Contrary to expectations, P limitation was rare. Instead, several lines of evidence pointed to primary N or N + P co‐limitation of periphyton. Limitation extent was negatively related to site trophic status, with stronger nutrient limitation at oligotrophic sites. Our results support the hypothesis that phytoplankton and periphyton biomass respond differently to nutrient enrichment and suggest that different nutrients may limit pelagic and benthic primary production, even in the same system. Our findings also support the use of periphyton as an early warning indicator of nutrient pollution and help explain why large, oligotrophic lakes may be especially susceptible to localised benthic algal blooms.

Assessment of multiple ecosystem metabolism methods in an estuary

AbstractEcosystem metabolism is a key ecological attribute and easy to describe, but quantifying metabolism in estuaries is challenging. Properly scaling measurements through time and space requires consideration of hydrodynamics and mixing water from heterogeneous sources, making any estimation uncertain. Here, we compared three methods for modeling ecosystem metabolism in a portion of the Sacramento‐San Joaquin Delta. Metabolism estimates based on laboratory incubations, continuous in situ buoys, and an oxygen isotope approach all indicated the system was net heterotrophic, and calculated rates were comparable in magnitude when averaged over the 2‐month study. Daily metabolic rates based on in situ buoys were the most variable, likely due to horizontal and vertical advection and poor portrayal of the dissolved oxygen budget. After temporally averaging in situ buoy estimates or smoothing the dissolved oxygen time series for tidal effects, rates were more comparable to the other methods, which may be necessary to account for tidal advection and unbalanced contributions from subhabitats within the metabolic footprint. Incubation‐based rates represent the finest temporal and spatial scale and only account for pelagic processes, which may explain why incubation‐based rates were lower than the other two methods. The oxygen isotope method provided temporally and spatially integrated rates that were bracketed by the other two methods and may be a valuable tool in systems matching the model requirements. Because uncertainty arises in each method from a number of assumptions and scaling calculations, the resolution of metabolic rates in estuaries is likely coarser and more variable than in other aquatic ecosystems.

Contribution of boulder reef habitats to oxygen dynamics of a shallow estuary

We assessed the importance of boulder reefs to the oxygen dynamics of a shallow estuary during two growing seasons in 2017 and 2018. Using open-system diel oxygen measurements and benthic and pelagic incubations, we evaluated the relative contribution of pelagic and benthic habitats to the ecosystem metabolism along a depth gradient in two areas, with (Reef) and without (Bare) boulder reefs in the Limfjorden, Denmark. System integrated areal rates of gross primary production (GPP) and ecosystem respiration (ER) both increased with depth in both areas. Benthic contribution to system GPP and ER was highest at shallow depth where it represented 47 and 53% respectively. However, with increasing depth pelagic processes dominated GPP and ER (98 and 94%) even in the Reef area. Although the Reef area had higher biomass of auto- and heterotrophic organisms (macroalgae and macrofauna), benthic GPP was at similar level in both areas, due to a significant contribution from micro-phytobenthic organisms. The Reef area had lower sediment pools of organic matter, nitrogen and phosphorous and was slightly more oxygenated compared to the nearby Bare area. Extreme temperatures and higher levels of nutrients in 2018 caused a marked increase in benthic ER rates resulting in net heterotrophy (NEM = GPP − ER < 0) in 2018 compared to net autotrophy (NEM > 0) in 2017. Under current eutrophic conditions, boulder reefs do not contribute positively to the oxygen dynamics in the estuary. Reoccurring blooms of phytoplankton with high organic matter decomposition combined with high temperatures and dominance of fauna stimulate depletion of oxygen around the reefs. Significant improvements in water clarity are needed to regrow perennial macroalgae and induce net autotrophy. Under current turbid conditions, it is only recommended to establish boulder reefs in shallow (<3 m) parts of the estuary.

Microbial eukaryote assemblages and potential novel diversity in four tropical East African Great Lakes.

East African Great Lakes are old and unique natural resources heavily utilized by their bordering countries. In those lakes, ecosystem functioning is dominated by pelagic processes, where microorganisms are key components; however, protistan diversity is barely known. We investigated the community composition of small eukaryotes (<10µm) in surface waters of four African Lakes (Kivu, Edward, Albert and Victoria) by sequencing the 18S rRNA gene. Moreover, in the meromictic Lake Kivu, two stations were vertically studied. We found high protistan diversity distributed in 779 operational taxonomic units (OTUs), spanning in 11 high-rank lineages, being Alveolata (31%), Opisthokonta (20%) and Stramenopiles (17%) the most represented supergroups. Surface protistan assemblages were associated with conductivity and productivity gradients, whereas depth had a strong effect on protistan community in Kivu, with higher contribution of heterotrophic organisms. Approximately 40% of OTUs had low similarity (<90%) with reported sequences in public databases; these were mostly coming from deep anoxic waters of Kivu, suggesting a high extent of novel diversity. We also detected several taxa so far considered exclusive of marine ecosystems. Our results unveiled a complex and largely undescribed protistan community, in which several lineages have adapted to different niches after crossing the salinity boundary.

Climate warming and heat waves alter harmful cyanobacterial blooms along the benthic-pelagic interface.

In addition to a rise in mean air and water temperatures, more frequent and intense extreme climate events (such as heat waves) have been recorded around the globe during the past decades. These environmental changes are projected to intensify further in the future, and we still know little about how they will affect ecological processes driving harmful cyanobacterial bloom formation. Therefore, we conducted a long-term experiment in 400-L shallow freshwater mesocosms, where we evaluated the effects of a constant +4°C increase in mean water temperatures and compared it with a fluctuating warming scenario ranging from 0 to +8°C (i.e., including heat waves) but with the same +4°C long-term elevation in mean water temperatures. We focused on investigating not only warming effects on cyanobacterial pelagic dynamics (phenology and biomass levels), but also on their recruitment from sediments-which are a fundamental part of their life history for which the response to warming remains largely unexplored. Our results demonstrate that (1) a warmer environment not only induces a seasonal advancement and boosts biomass levels of specific cyanobacterial species in the pelagic environment, but also increases their recruitment rates from the sediments, and (2) these species-specific benthic and pelagic processes respond differently depending on whether climate warming is expressed only as an increase in mean water temperatures or, in addition, through an increased warming variability (including heat waves). These results are important because they show, for the first time, that climate warming can affect cyanobacterial dynamics at different life-history stages, all the way from benthic recruitment up to their establishment in the pelagic community. Furthermore, it also highlights that both cyanobacterial benthic recruitment and pelagic biomass dynamics may be different as a result of changes in the variability of warming conditions. We argue that these findings are a critical first step to further our understanding of the relative importance of increased recruitment rates for harmful cyanobacterial bloom formation under different climate change scenarios.

The formation of a metalimnetic oxygen minimum exemplifies how ecosystem dynamics shape biogeochemical processes: A modelling study

Metalimnetic oxygen minima are observed in many lakes and reservoirs, but the mechanisms behind this phenomena are not well understood. Thus, we simulated the metalimnetic oxygen minimum (MOM) in the Rappbode Reservoir with a well-established two-dimensional water quality model (CE-QUAL-W2) to systematically quantify the chain of events leading to its formation. We used high-resolution measured data to calibrate the model, which accurately reproduced the physical (e.g. water level and water temperature), biogeochemical (e.g. nutrient and oxygen dynamics) and ecological (e.g. algal community dynamics) features of the reservoir, particularly the spatial and temporal extent of the MOM. The results indicated that around 60% of the total oxygen consumption rate in the MOM layer originated from benthic processes whereas the remainder originated from pelagic processes. The occurrence of the cyanobacterium Planktothrix rubescens in the metalimnion delayed and slightly weakened the MOM through photosynthesis, although its decaying biomass ultimately induced the MOM. Our research also confirmed the decisive role of water temperature in the formation of the MOM since the water temperatures, and thus benthic and pelagic oxygen consumption rates, were higher in the metalimnion than in the hypolimnion. Our model is not only providing novel conclusions about the drivers of MOM development and their quantitative contributions, it is also a new tool for understanding and predicting ecological and biogeochemical water quality dynamics.

Distinguishing nutrient-depleting effects of scallop farming from natural variabilities in an offshore sea ranch

While bivalve farming is well recognized in terms of modifying the biogeochemical cycle through filter-feeding and biodeposition, its impacts on water column nutrient concentrations in various ecosystems may vary from depletion to addition. Hypothesizing that the variability arises from overlap of farming-associated impacts and pelagic processes, nutrient concentrations and phytoplankton biomass were investigated monthly in an offshore sea ranch, to detect the real impacts of bottom-seeding scallop farming. Nutrient concentrations in water column average increased from July to December following successive surface freshening and disappearance of thermal stratification. Abrupt decrease was observed in March, along with increase in chlorophyll a concentration. The silicate concentration decreased below 2 μM in all depth layers in the mariculture area in March and April, whereas silicate limitation in open waters was commonly observed in April. Compared to open waters, inorganic nitrogen and phosphate concentrations were significantly lower in the mariculture area on an annual cycle, whereas silicate concentration was lower in spring and higher in summer. Our results indicate that scallop farming in frequently refreshed waters can act as nutrient sink through top-down effects. Without exogenous supply, it can also introduce the limitation of nutrient (silicate) with low regeneration rate and stimulate shift in the phytoplankton community structure.

Mismatch between microalgae and herbivorous copepods due to the record sea ice minimum extent of 2012 and the late sea ice break-up of 2013 in the Beaufort Sea

The unprecedented pace of warming of the Arctic Ocean affects a wide range of pelagic processes, from primary production to fish recruitment. Sequential sediment traps deployed on oceanographic moorings at the Beaufort Sea shelf break were used to investigate the impact of variations in physical conditions on the phenology of microalgae and herbivorous copepods. Water temperature, salinity, microalgal fluxes, and zooplankton fluxes were monitored over 6 of the 7 annual cycles from September 2009 to September 2016 with a hiatus from September 2013 to August 2014. Satellite-derived sea ice concentration and snow depth were retrieved for the same period. For 5 of the 6 cycles, a consistent interval between the vernal abundance of the Calanus species (C. glacialis and C. hyperboreus), the export of the ice alga Nitzschia frigida, and peak in nauplii abundance and diatom export, reflected a match between microalgae and herbivorous copepods. Drastic changes in water temperatures and ice conditions during the 2012–2013 cycle affected this synchronization. In 2012, the record sea ice minimum extent led to anomalously high upper water temperatures during autumn and winter that led to a shoaling of the vertical distribution of C. hyperboreus females and to the production of their nauplii at shallower depths several weeks before diatom export. Due to several oceanic and atmospheric forcing, the record sea ice minimum extent was followed in 2013 by a rebound in ice cover that delayed snowmelt, sea ice melt, and ice algae export. This delay resulted in a mismatch between C. glacialis females and N. frigida fluxes and in fewer C. glacialis nauplii in June and July 2013. These mismatch events likely had a negative impact on Calanus recruitment and on the subsequent transfer of energy to carnivorous zooplankton and fish. It is crucial to maintain mooring deployments to accurately assess the wide range of effects of climate warming on the marine ecosystems.

Metalimnetic oxygen minimum and the presence of Planktothrix rubescens in a low-nutrient drinking water reservoir

Dissolved oxygen is a key player in water quality. Stratified water bodies show distinct vertical patterns of oxygen concentration, which can originate from physical, chemical or biological processes. We observed a pronounced metalimnetic oxygen minimum in the low-nutrient Rappbode Reservoir, Germany. Contrary to the situation in the hypolimnion, measurements of lateral gradients excluded the sediment contact zone from the major sources of oxygen depletion for the metalimnetic oxygen minimum. Instead, the minimum was the result of locally enhanced oxygen consumption in the open water body. A follow-up monitoring included multiple chlorophyll a fluorescence sensors with high temporal and vertical resolution to detect and document the evolution of phytoplankton. While chlorophyll fluorescence sensors with multiple channels detected a mass development of the phycoerythrin-rich cyanobacterium Planktothrix rubescens in the metalimnion, this species was overlooked by the commonly used single-channel chlorophyll sensor. The survey indicated that the waning P. rubescens fluorescence was responsible for the oxygen minimum in the metalimnion. We hypothesize that pelagic processes, i.e., either oxygen use through decomposition of dead organic material originating from P. rubescens or P. rubescens extending its respiration beyond its photosynthetic activity, induced the metalimnetic oxygen minimum. The deeper understanding of the oxygen dynamics is mandatory for optimizing reservoir management.

Numerical Modeling of Long-Term Biogeochemical Processes and Its Application to Sedimentary Bed Formation in Tokyo Bay

Even though models of water quality have become increasingly detailed over time, their applicability to analyze long-term effect on sedimentary bed formation is yet to be clarified. Hence, an integrated, layer-resolved, process-based, sediment–water coupled, long-term robust, three-dimensional (3D) ecosystem model, including realistic sedimentary and pelagic processes, was developed. The constituents of the integrated model included a multi layered ecosystem model, a quasi-three dimensional hydrodynamic model, a wave hindcasting model, and a sediment deposition and erosion model. Because numerical modeling difficulties arise in setting the initial conditions, especially for a sedimentary bed, this novel modeling approach suggests to initiate the model with the simplest initial conditions of no particulate organic carbon content (POCC) and uniform inorganic sediment distribution. The model was applied to Tokyo Bay and quasi-steady state POCC in the sediment was obtained through long-term computation with realistic sedimentary and pelagic processes. Wave induced bed shear stress (WBSS) is critical for sediment erosion and spatial movement of sediment pollution, while a stable morphology is determined by the balance between the current induced bed shear stress (CBSS) and WBSS in Tokyo Bay. This novel modeling approach, with the simplest sedimentary initial conditions and realistic sedimentary and pelagic processes, provides a great tool for long-term ecosystem modeling in future studies.

Novel crab predator causes marine ecosystem regime shift

The escalating spread of invasive species increases the risk of disrupting the pathways of energy flow through native ecosystems, modify the relative importance of resource (‘bottom-up’) and consumer (‘top-down’) control in food webs and thereby govern biomass production at different trophic levels. The current lack of understanding of interaction cascades triggered by non-indigenous species underscores the need for more basic exploratory research to assess the degree to which novel species regulate bottom-up and/or top down control. Novel predators are expected to produce the strongest effects by decimating consumers, and leading to the blooms of primary producers. Here we show how the arrival of the invasive crab Rhithropanopeus harrisii into the Baltic Sea – a bottom-up controlled ecosystem where no equivalent predators ever existed – appeared to trigger not only strong top-down control resulting in a decline in richness and biomass of benthic invertebrates, but also an increase in pelagic nutrients and phytoplankton biomass. Thus, the addition of a novel interaction – crab predation – to an ecosystem has a potential to reduce the relative importance of bottom-up regulation, relax benthic-pelagic coupling and reallocate large amounts of nutrients from benthic to pelagic processes, resulting in a regime shift to a degraded ecosystem state.

Light or tide? Effects on the emergence and recolonization of harpacticoid copepods from sand flats of the Wadden Sea (southern North Sea)

The Wadden Sea is a highly productive coastal ecosystem and an important nursery ground for various demersal fishes of the southern North Sea. Copepods are one of the major food sources of the ecosystem, yet little is known about the movement patterns of this benthic meiofaunal group within both sediment and water column in the intertidal environment, where they would be more vulnerable for predation. The diversity and abundance of harpacticoids emerging and recolonizing intertidal sand flats were investigated during two diurnal and two nocturnal high tides, using specifically designed traps. Our study revealed no significant difference in the amount of copepods emerging from the sediment into the water column during the day and the night. Furthermore, the proportion of emerging adults and copepodids did not differ from each other and most individuals persisted in the sediment during high tide. Interestingly, the most abundant species Arenosetella tenuissima and Tachidius discipes showed an opposite emergence behavior, with the former migrating in higher numbers during the night and the latter during the day.Although tidal inundation was the proximate factor initiating emergence, we suggest that sediment reworking by tidal currents, waves and bioturbation as well as water and food availability above and within the sediment may be additional factors controlling emergence of harpacticoids in the Wadden Sea, with light intensity playing a minor role.Therefore, emergence and recolonization patterns of harpacticoids from subtidal ecosystems cannot directly be transferred to intertidal areas as the time for benthic pelagic processes is limited to high tides.

Plio–Quaternary seismic stratigraphy and depositional history of the Ulleung Basin, East Sea: Association with debris-flow activity

We analyzed multi-channel seismic reflection profiles to decipher the Plio–Quaternary stratigraphic architecture and resolve the depositional history of the Ulleung Basin. A detailed seismic stratigraphic interpretation revealed nine seismic units (Units 1–9) separated by unit boundaries in the Plio–Quaternary deposits of the Ulleung Basin. Unit 1 (Pliocene) is dominated by debris-flow deposits (DFDs) throughout the basin. Units 2–9 (Quaternary) are characterized by stacked DFDs interbedded with thin hemipelagic sediments along the southern slope, whereas the northern basin floor is filled with hemipelagic and pelagic sediments. The areal distribution and dimensional characteristics of the DFDs indicate that sedimentation in the Ulleung Basin has been controlled by a combination of tectonic movement, eustatic changes in sea level, and gas hydrates dissolution and dissociation. During the Pliocene (Unit 1), sedimentation was mainly controlled by tectonic movements, causing margin-wide slope failures under the compressional regime. Large sediment deposits were supplied to the basin through mass-transport processes, resulting in an abundance of DFDs throughout the basin. During the Quaternary (Units 2–9), compressive stress decreased sharply, causing the DFDs to retreat landward. At that time, sedimentation was mainly influenced by eustatic changes in sea level and gas hydrates dissolution and dissociation. As the sea fell to the lowstand, slope failures were activated by the generation of high pore-water overpressure and the generation of weak layers in the sediments, resulting in stacked DFDs. However, as the sea level rose to the highstand, sediment was primarily deposited via hemipelagic and pelagic processes due to increased hydrostatic pressure and slope stability.

Scale-dependent natural variation in larval nutritional reserves in a marine invertebrate: implications for recruitment and cross-ecosystem coupling

In species with complex life cycles, laboratory studies have shown that variations in the traits of settling larvae can affect post-settlement survival and influence recruitment and benthic− pelagic coupling. However, we still know little about the magnitude and spatial scale of natural trait variation. We studied spatial variation in body size and nutritional reserves (carbon, nitrogen and lipids) of settled cyprids of the barnacle Semibalanus balanoides along the coast of West Scotland. We quantified variation among regions (north vs. south: range ~700 km), locations (~50 km), shores (~10 km) and within shores (~10 m). We also evaluated trait responses to gradients in chlorophyll and shore openness and compared swimming vs. settled cyprids in order to infer the likely influence of costs of substratum search on trait variation. Variability between regions was large, with higher trait values (e.g. carbon cyprid−1: 35 to 50% higher) in the north. Most traits correlated negatively with pelagic chlorophyll a (a proxy for larval/juvenile food availability); this counter-gradient pattern suggests an adaptive role of increased reserves, buffering benthic juveniles from low food availability during the critical early post-settlement period. Body size and nitrogen content correlated positively with shore openness; lower than expected carbon content suggest increased costs of substratum search on open shorelines. Higher nitrogen content but lower percent carbon was found in settled vs. swimming larvae, suggesting costs of sub - stratum search at the time of settlement. Overall, we uncovered the spatial scales at which trait variation, shaped by pelagic processes, can affect post-metamorphic survival, recruitment and benthic−pelagic coupling.