Intermediate Depths Research Articles

Study of a monopile with pre-tensioned tethers for offshore wind turbines at deeper waters

Currently, monopiles are the industry's first choice to support offshore wind turbines (OWTs) due to the complexities associated with fabrication, transportation and installation of various foundation solutions. These are, however, considered suitable for water depths up to ∼30–35 m, beyond which they become massive and complex to handle. Increasing the application range of monopiles without making them too large by adopting some suitable means may significantly impact the rapid growth of OWTs at intermediate water depths (∼50 m), where existing solutions such as jackets or concrete gravity foundations involve large capital expenditure. In this work, a novel approach of utilising pre-tensioned tethers along with a 6.0 m wide monopile was explored for 50 m water depth. Under an extreme load case, the stress ratio was found to be reduced from 1.17 to 0.44 when pre-tensioned tethers are used. Similarly, it was found that by adjusting parameters associated with the proposed concept, fatigue damage can be brought down from a very high value to 0.63 or less. The results thus show that the proposed approach can open new avenues to bring competitiveness to the cost of offshore wind energy at intermediate water depths.

Deglacial temperature and carbonate saturation state variability in the tropical Atlantic at Antarctic Intermediate Water Depths

Deglacial temperature and carbonate saturation state variability in the tropical Atlantic at Antarctic Intermediate Water Depths

FLUIDOGENIC LANDFORMS WITHIN THE PERMAFROST ZONE ON THE SHELF OF THE PECHORA AND KARA SEAS

The study is based on the results of multibeam echo-sounding and high-frequency seismic profiling during the 2018-2022 cruises of the R/V “Akademik Nikolai Strakhov” and “Akademik Boris Petrov”. Regular changes of morphometric parameters and the internal structure of pingo-like formations of the shelves of the Pechora and Kara seas were revealed. A morphometric analysis of pingo-like formations was carried out, which made it possible to draw conclusions about their relative age, as well as the role of near-bottom currents and slope processes in their modern dynamics. It was found that the density and morphological variety of pingo-like formations depend on the geological and tectonic features of the bottom area, the presence and nature of permafrost, the intensity of degassing, and the time of shelf flooding during the Holocene transgression. Pingo-like formations on the shelf, where the depth exceeds 70-80 m, emerged at the early stages of the Holocene transgression, and by now the permafrost there has largely thawed out. At the same time, pingo-like formations are still prominent in the relief and actively transformed by bottom currents, slope and, possibly, pseudovolcanic processes associated with ongoing degassing. Pingo-like formations are rare within shallow (up to 20-30 m) shelf areas close to the shore, and, apparently, continue their evolution at present. At the same time, the large thickness and continuity of permafrost prevent active fluid flow, acting as a seal. Pingo-like formations in the shallow-water zones are mainly cone-shaped mounds without intensive degassing. The density of pingo-like formations is maximum at the intermediate depths (from 20-30 to 70-80 m), in the presence of insular or discontinuous permafrost, under high fluid flow intensity within the fault zones and oil- and gas-bearing structures. Near-surface sediments in such areas are characterized by a combination of localized processes of heaving and active degassing. It predetermines a wide variety of the morphological types of pingo-like formations.Keywords: Arctic, bottom topography, degassing, pingo-like features, acoustic anomalies, multibeam echo sounding, seismoacoustic profiling

Magnetotelluric investigations at Andean volcanoes: Partial melt or saline magmatic fluids?

Knowledge of the architecture of active magmatic systems is important for both volcanic hazard assessment and evaluating potential geothermal energy production and metals recovery from magmatic fluids. Increasingly, magmatic systems are imaged using the magnetotelluric method to detect electrically conductive partial melt and/or saline magmatic fluid reservoirs. We review recent magnetotelluric studies at eight Andean volcanoes, revealing electrical conductivity anomalies with variable magnitudes and locations. Six of the studied volcanoes exhibit three main electrical conductivity anomalies, located at shallow (<3 km), intermediate (≈5 km), and deep (>10 km) depths. The shallow anomalies are often thin and laterally extensive, consistent with clay cap alteration layers, while the deep anomalies are generally interpreted as partial melt reservoirs. The intermediate depth anomalies, although also often attributed to partial melt, have less clear origins. By analysing laboratory-derived electrical conductivity relationships, we show that the intermediate depth anomalies are generally most consistent with saline magmatic fluids stored in porous rock. However, other geophysical and petrological data suggest that localised partial melt also exists at intermediate depths. Therefore, the intermediate depth anomalies likely represent mixed melt and saline magmatic fluid systems, such as those responsible for forming magmatic-hydrothermal alteration zones and copper porphyry deposits. At individual volcanoes, refining the generalised three layer model proposed here by using additional geophysical or petrological data is key to constraining the resources and/or hazard potential of the magmatic system.

On the estimation of wave heights for periodic water waves from velocity and pressure data

In this paper, we present some wave height estimates for the two-dimensional steady periodic water waves subject to the effect of vorticity. The lower bounds of wave heights for the case of positive vorticity and negative vorticity are derived, respectively. These results are associated with the pressure data at the flat bed and velocity data on trough line, and to the pressure data at the flat bed and velocity data on crest line, respectively. Furthermore, we provide some upper bounds of wave heights, which rely on velocity and pressure data at an arbitrary intermediate depth. In addition, we show some applications of the present model.

Factors influencing the meridional width of the equatorial deep jets

Abstract. Equatorial deep jets (EDJs) are vertically alternating, stacked zonal currents that flow along the Equator in all three ocean basins at intermediate depth. Their structure can be described quite well by the sum of high-baroclinic-mode equatorial Kelvin and Rossby waves. However, the EDJ meridional width is larger by a factor of 1.5 than inviscid theory predicts for such waves. Here, we use a set of idealised model configurations representing the Atlantic Ocean to investigate the contributions of different processes to the enhanced EDJ width. Corroborated by the analysis of shipboard velocity sections, we show that widening of the EDJs by momentum loss due to irreversible mixing or other processes contributes more to their enhanced time mean width than averaging over meandering of the jets. Most of the widening due to meandering can be attributed to the strength of intraseasonal variability in the jets' depth range, suggesting that the jets are meridionally advected by intraseasonal waves. A slightly weaker connection to intraseasonal variability is found for the EDJ widening by momentum loss. These results enhance our understanding of the dynamics of the EDJs and, more generally, of equatorial waves in the deep ocean.

Pressure analysis in nonlinear waves by revisiting the breaking wave onset

Breaking waves are numerically simulated by solving the two dimensional nonlinear free surface boundary conditions in potential theory. Onset of breaking wave is determined for two configurations: A focused wave in intermediate depth and a dambreaking flow leading to a soliton. Two criteria are identified: first, a kinematic criterion follows from the ratio of the horizontal velocity at the crest to the phase velocity; second, a dynamic criterion is formulated in terms of the vertical pressure gradient. Thresholds of these criteria are proposed in the literature. For the two configurations studied here, it is shown that these thresholds are not universal but they are reasonable estimates of the onset of breaking wave. To facilitate this study, the Hessian matrix of the pressure is analyzed. It is shown that there exists a continuous curve that links the crest of the wave to the sea bottom. This line follows from the colinearity of the pressure gradient and one of the two eigenvectors of the Hessian matrix. The analysis of the pressure, its gradient and its second derivatives along this line gives more insights into the definition of the thresholds associated to the onset of breaking wave.

Diversity and seasonal development of large zooplankton along physical gradients in the Arctic Barents Sea

Due to ongoing climate change, a new Arctic Ocean ecosystem is emerging. Within the framework of the Nansen Legacy project, we investigated the community composition of the large zooplankton and its seasonal development along a latitudinal gradient in the northern Barents Sea. Total biomass was maximal in summer and early winter, and minimal in spring, with copepods contributing considerably in all seasons. Euphausiids represented a minor fraction of the biomass, whereas chaetognaths and other gelatinous zooplankton contributed substantially to the sampled zooplankton at all stations, particularly in winter. Amphipod biomass was high in early winter, but otherwise low. Temperature in the water column interior and bottom-depth had the highest explanatory power for the community composition of the large zooplankton, both revealing the same distinct Atlantic and Arctic domains along the studied section. The continental shelf of the northern Barents Sea had an Arctic signature and was in terms of biomass characterized by a dominance of cold-water species, such as Themisto libellula, and Calanus glacialis. The copepod Calanus hyperboreus was the dominant over the continental slope. Locations at the southern and northern end of the studied section were influenced by Atlantic Water (at intermediate depth at the northern stations), and contained a mixture of temperate species, deep-water species, and sympagic amphipods in northern ice-covered waters. In the northern Barents Sea, a seasonal change was observed in the biomass fractions of different zooplankton feeding guilds, with dominance of herbivores in summer and carnivores in winter. This suggests switching between bottom-up and top-down control through the year. On the continental slope, species that are typically considered omnivores seemed to increase in importance. The role of seasonally changing food preferences to bridge periods outside of the main primary production season is discussed in light of ecosystem resilience to the expected changes in the Arctic Ocean.

Differential deformation of a strike-slip fault in the Paleozoic carbonate reservoirs of the Tarim Basin, China

Ultra-deep carbonate reservoirs buried over 7000 m are distributed around strike-slip faults, which control hydrocarbon accumulation and enrichment in the Tarim Basin, China. This study proposes a fracture development index (FDI) method for analyzing the width of the strike-slip fault zone based on 3D seismic data. Based on the FDI method and geological and geophysical data, we analyze the structural characteristics of the SB5 strike-slip fault and the scaling of fault zone width and throw.In map view, at depth the strike-slip fault is characterized by single fault segments, extensional overlaps, and contractional overlaps, at intermediate depths by left-stepping en echelon normal faults, and in shallow layers by right-stepping en echelon normal faults. In cross section, we find composite flower structures and positive flower structures and medium-shallow layer negative flower structures or deep layer negative flower structures and medium-shallow layer negative flower structures. The formation time of deep, middle, and shallow layer faults is the middle Caledonian, Late Caledonian, and Late Hercynian. The heterogeneity of the strike-slip fault zone varies from strong to weak from contractional overlaps, to extensional overlaps, and single fault segments. Strike-slip fault-related fractures are dominated by three sets intersecting with strike-slip faults at small angles. Where throw is less than 100 m, the fault zone width and throw have a good power law relationship. Where the throw is over 100 m, the width tends to diminish with throw.

The Relationship of Oral Reading Fluency Endurance to Comprehension in an Intermediate Depth Orthography

One-minute oral reading fluency (ORF) tests are widely used, but concerns have been raised regarding whether readers are able to maintain their performance if asked to read for a larger period. The main goals of this study were to investigate whether students are able to maintain their ORF performance in a three-minute task and whether scores measured at one and at three minutes are equally good predictors of the performance in a standardized reading comprehension measure. The sample was composed of 159 Portuguese primary school students (grades 2-4). The results suggested that the number of words read correctly (WRC) declined across reading time and that three-minute mean score underestimated fluency in all grade levels compared to a one-minute reading score. The WRC measured either at one minute or using a three-minute average score was an equal predictor of reading comprehension in all grades. Implications for theory and practice are discussed.

Massive influence of Himalayan source on sedimentation along northern Indian peninsular basin

A 39 m long sediment core (MD161-19) spanning 320 ka, collected from the intermediate water depths of (1480 m) of Mahanadi offshore was subjected to provenance analyses using Sr and Nd isotope systematics. The Sr–Nd isotope ratios of MD161-19 fall on the mixing line defined by Ganga and Brahmaputra main stem averages. Strontium and Nd isotope ratios along with Fe/Al ratio clearly show the predominance of Himalayan sediments in the study area. The significant temporal variations in the Sr–Nd isotope ratios observed for the entire depositional age (320 ka) may be attributed to changes in the relative sediment contribution by Ganga and Brahmaputra river systems draining terrains with contrasting Sr–Nd isotope ratios. The temporal geochemical profiles coupled with precipitation profile show that wet-phases during stadials led to enhanced erosion of the trans-Himalayan batholiths (THB) rocks (characterized by high εNd values) possibly due to glacial expansion in the higher reaches of the Brahmaputra catchment. The warm interstadials coupled with enhanced precipitation are accompanied by a shift towards lower εNd values which may be attributed to extensive erosion of the Ganga catchment area comprising rocks characterized by lower εNd values and diminished THB contribution due to lack of glacial influence.

Re‐Examining Temporal Variations in Intermediate‐Depth Seismicity

AbstractChanges in the frequency of intermediate‐depth (60–300 km) earthquakes in response to static stress transfer can provide insights into the mechanisms of earthquake generation within subducting slabs. In this study, we use the most up‐to‐date global and regional earthquake catalogs to show that both the aftershock productivity of large earthquakes, and the changes in the frequency of intermediate‐depth earthquakes around the timing of major megathrust slip, support the view that faults within the slab are relatively insensitive to static stress transfer on the order of earthquake stress drops. We interpret these results to suggest the population of faults within the slab are much further from their failure stress than is typical for shallow fault systems. We also find that aftershock productivity varies within slabs over small spatial scales, indicating that the mechanism that enables faults to rupture at intermediate depths is likely to be spatially heterogeneous over length‐scales of a few tens of kilometres. We suggest dehydration‐related weakening mechanisms can best account for this heterogeneity.

Metamorphic Domes in Northern Tunisia: Exhuming the Roots of Nappe Belts by Widespread Post‐Subduction Delamination in the Western Mediterranean

AbstractCenozoic extension in the Western Mediterranean has been related to the dynamics of back‐arc domains. Although, in most of its orogenic belts extension propagated into the fore‐arc nappe domains. Here we revisit the structure, metamorphism and radiometric ages of the Tunisian Tell, where HP/LT rocks (350°C at 0.8 GPa), were exhumed by the sequential activity of extensional detachments after heating and decompression (410°C–440°C at 0.6–0.3 GPa) in a plate convergent setting. Normal faults thinning the Tunisian Tell detached at two different crustal levels. The shallower one cuts down into the Atlas Mesozoic sequence, involving Tellian Triassic evaporites in the hanging‐wall forming halokinetic structures in the Mejerda basin late Miocene. The deeper‐detachment bounds metamorphic domes formed by marbles and metapsammites from the Atlas domain. Illite crystallinity on Triassic rocks shows epizonal to anchizonal values, at deep and intermediate structural depths of the Tell‐Atlas nappe belt, respectively. New U‐Pb 49.78 ± 1.28 Ma rutile ages from Tellian metabasites, together with existing phlogopite 23–17 Ma K‐Ar ages in Atlas marbles from the footwall of the deepest detachment, indicate a polymetamorphic evolution. The Tell rocks underthrusted the Kabylian flysch in the early Eocene. Further, early Miocene shortening thrusted the metabasites over lower‐grade sediments, producing HP/LT metamorphism and ductile stretching at the base of the Atlas belt. The exhumation of midcrustal roots of Western Mediterranean nappe belts after tectonic shortening is a common feature related to tearing at the edges of the subduction systems and inboard delamination of their subcontinental lithospheric mantle.

Nitrogen Isotope Record From a Mid‐oceanic Paleo‐Atoll Limestone to Constrain the Redox State of the Panthalassa Ocean in the Capitanian (Late Guadalupian, Permian)

AbstractThe Capitanian stage is characterized by marine anoxia possibly related to the extinction, although the global redox structure of the Capitanian ocean has not been constrained. We newly report a nitrogen isotope (δ15N) record from a paleo‐atoll limestone at the top of a mid‐Panthalassan seamount to constrain the spatial extent and duration of the Capitanian marine anoxia. The δ15N value of limestone after acid treatment is substantially high for ∼5‐Myr up to +28‰, the highest through the Phanerozoic oceans, suggesting that the nitrogen source (nitrate) was substantially enriched in 15N via denitrification within subsurface oxygen‐deficient zones (ODZs; O2 &lt; 5 µM). Numerical modeling of nitrogen isotope dynamics in the upwelling system along the seamount suggests that the possible minimum δ15N value of a global deep‐oceanic nitrate reservoir was ca. +9‰ in the Capitanian (∼4‰ higher than at the present). Furthermore, a redox‐dependent nitrogen isotope mass balance model constrained the global redox structure of the Capitanian superocean. Substantially reducing conditions (O2 ≤ 20 µM) prevailed at intermediate water depths (100–1,000 m), in association with expanded ODZs with anoxic/euxinic cores along continental margins (≥ ∼0.4% of global ocean volume), while the deep‐ocean remained to be more oxidizing (O2 ≤ 60 µM). The enhanced open‐ocean productivity associated with the low sea‐level and high nutrient flux to the ocean resulted in the global ocean deoxygenation during the cooling stage. Our model is consistent with previous geologic observations and with a possible link between the long‐term (∼5‐Myr) development of marine dysoxia and the extinction.

Comparison of methodologies for creating spread-out Bragg peaks in proton therapy using TOPAS and MCNP codes

In proton beam treatments, the superposition of several weighted Bragg curves with different incident energies is required to homogeneously irradiate a large tumor volume, creating a spread-out Bragg peak (SOBP). This paper confirms on the suitability of two different methods to create SOBPs – Bortfeld/Jette's and MCMC (Monte Carlo calculations and Matrix Computations), using Monte Carlo simulations performed with TOPAS and MCNP6.1. To generate the SOBPs, algorithms were developed for implementation of the two methods, which enabled to find the weights for thirty variations of SOBPs, categorized according to their width and maximum depths. The MCMC method used weight optimization in designing SOBPs to avoid negative values. In contrast, the Bortfeld/Jette's method yielded the SOBPs according to the variation of a power-law parameter (p) introduced by the range-energy relationship. Optimal values of p, from MCNP and TOPAS, were selected in order to retrieve SOBPs with the best smoothness and then related to those obtained from the literature. In comparing both methods and codes, dose homogeneity parameters (HOM) were used to examine the SOBP flatness and gamma analyses were employed to assess the dose deposition along its full extension. The results showed that the SOBPs designed using the MCMC method had better HOM values and computational performance for both codes when compared to the Bortfeld/Jette's method. The gamma analyses highlighted significant differences between the entrance doses comparing the two different methods, for SOBPs with intermediate and high depths and small width. This evaluation was not possible with the HOM values alone, which stresses the relevance of a broad analysis to avoid unintended doses in healthy tissues.

Diel, seasonal and vertical changes in the pelagic amphipod communities in the subarctic Pacific: insights from imaging analysis

Abstract Vertically stratified day and night samples were collected from a depth of up to 1000 m during four seasons at a single station in the western subarctic Pacific. Subsequently, the abundance, biovolume, community structure and population structure of the pelagic amphipods were evaluated from the imaging analysis using ZooScan. The stable isotope ratio (δ15N) was also measured for each species. In total 10 amphipod species were identified in total, and their community structures were separated into four groups. The four communities were characterized by the upper-layer community dominated by Themisto pacifica, and the deep-water community that was characterized by the absence of T. pacifica. The other two groups were observed for the intermediate depths at night only, which was characterized by the low abundance of T. pacifica performing diel vertical migration from those depths. The deep-dwelling species displayed higher δ15N and positive relationships were detected with the habitat depth of each species. For the predominant T. pacifica, population structure (cohort) analysis was possible based on equivalent spherical diameter data from ZooScan. This study demonstrates that imaging analysis by ZooScan is applicable for analyses of minor taxonomic groups (amphipods) in zooplankton communities.

Nutriclines and nutrient stoichiometry in the eastern Arabian Sea: Intra-annual variations and controlling mechanisms

Repeat measurements (nine times) of nutrients at seven to ten coast-offshore transects along the eastern Arabian Sea (EAS) in 2018–19 revealed nitrogen as the major limiting nutrient controlling the primary production. The summer monsoon upwelling at varying extents along the EAS and the winter convection in the north EAS primarily controlled the depth of nitracline (DNO3 = depth of ≥ 3 µM NO3–). DNO3 was found positively aligned with the upper boundary of thermocline (depth of 26 °C isotherm or D26), with the strongest relationship in the south EAS (R2 = 0.96) due to its relatively stratified and stable conditions during most of the year compared to the north EAS (R2 = 0.71) which is unstable during winter convection. Mixed layer nutrient availability varied with DNO3/D26 and relatively faster nitrogen exhaustion than phosphorus during the nutrient enrichment events viz. upwelling and convective mixing yielded low N/P ratios. The N/P ratios of nutricline waters upwelled/entrained to the mixed layers were found significantly low (6–14), especially at the north EAS where perennial denitrifying zone persisted in the intermediate depths. With continued uptake of nutrients by the plankton following the Redfield ratio, the system during oligotrophic inter-monsoons became nitrogen-limited with surplus (excess) phosphate (P*), resulting in the lowest N/P ratios (1.5 ± 0.7) in the mixed layer. This stoichiometrically excess phosphate (P*) in the mixed layer waters was highest during nutrient enrichment events of both the summer and winter monsoons. The reduction in P* along with high cyanobacterial abundance (evident by increased zeaxanthin pigment concentrations) towards the late phase of summer monsoon and fall inter-monsoon indicates the dominance of diazotrophy. This diazotrophy after the nutrient enrichment seasons reduced the P* to almost half (0.22 ± 0.12 µM) which counter balance the N-limiting conditions to some extent in the EAS surface waters. The N-limiting condition can be severe in a global warming scenario, so more observations and extensive experiments are needed to ascertain the capability of diazotrophs to counteract the N-limiting situations by utilising the excess phosphate. The findings of this study are highly beneficial, as such an extensive year-long information on large spatio-temporal variability and their cause factors is available for the first time from the EAS basin, especially to improve the validation and forecasting of biogeochemical models under changing climate conditions. Comparison of present nutrient data with climatology data of World Ocean Atlas showed complete disagreement of silicate, which is an issue of major concern.

HyWaves: Hybrid downscaling of multimodal wave spectra to nearshore areas

Long-term and accurate wave hindcast databases are often required in different coastal engineering projects. The assessment of the nearshore wave climate is often accomplished by using downscaling techniques to translate offshore waves to coastal areas. However, dynamical downscaling approaches may incur huge computational cost. Additionally, the common use of bulk parameterizations are often not accurate for multidimensional waves. To overcome these limitations, we present a hybrid downscaling approach that combines mathematical algorithms (statistical downscaling) and numerical modeling (dynamical downscaling) over the individual spectral partitions. Every wave partition is downscaled and aggregated afterward by using principles of wave linear theory. By assuming linearity in the propagation of the wave celerity, the application of the method is limited from offshore to intermediate water depths. In addition, the method proposed uses a technique to simplify the spectral boundary conditions in complex domains. The methodology has been applied and validated in the island states of Samoa, American Samoa, Majuro, and Kwajalein, showing good skill at reproducing the spectral hourly time series of significant wave height, peak period, and peak direction. Moreover, an accurate representation of the observed energy spectrum was achieved. This study provides insight into the numerical approximation of the combined sea-swell states while improving the quality of fast spectral forecasting and early warning systems.

Oxygen and hydrogen isotopic evidence that Kamaʻehuakanaloa (Lōʻihi) Seamount hydrothermal systems are recharged by deep Pacific seawater

We observed negative and positive δ18O and δ2H deviations from fluids collected at Kamaʻehuakanaloa (previously known as Lōʻihi) seamount relative to Pacific seawater from the same depth. Hydrothermal vents on the crater floor of Pele's Pit at Kamaʻehuakanaloa, at a depth of 1320 m, had δ18O and δ2H values as low as −0.19‰ and −0.3‰, respectively. Seawater collected within the caldera, within a zone 45 m above the crater floor, had δ18O and δ2H values as high as 1.15‰ and 6.5‰, respectively. In comparison, Pacific seawater at 1200 m and 1400 m at nearby station ALOHA exhibit intermediate δ18O and δ2H values of 0.2‰ and 0.34‰. The high δ18O and δ2H values observed in the caldera water-column may be explained by isotopic modification processes, including water-rock reactions. However, we did not observe a vent source with similar isotopic composition; this suggests that substantial hydrothermal flow is entering the caldera through unidentified sources, potentially individually small but collectively important. Further, the low δ18O and δ2H values of the crater floor vents cannot be readily explained by isotopic modification processes alone. We conclude the crater floor vents predominantly reflect the isotopic composition of the Pacific seawater entrained into the hydrothermal system. Our observations suggest recharging seawater is entrained from below >4500 m, in the zone of Pacific Bottom Water. These findings illustrate the heterogeneity of hydrothermal transport processes at a volcanic seamount and how seamount hydrothermal convection can provide a mechanism that may contribute to vertical ocean mixing by transporting deep bottom waters to intermediate ocean depths.

Maximizing sampling efficiency to detect differences in fish community composition using environmental DNA metabarcoding in subarctic fjords

AbstractEnvironmental DNA (eDNA) has gained popularity as a tool for ecosystem biomonitoring and biodiversity assessment. Although much progress has been made regarding laboratory and fieldwork protocols, the issue of sampling efficiency requires further investigation, particularly in three‐dimensional marine systems. This study focuses on fish community composition in marine ecosystems and aims to analyze the efficiency of sampling design given the sampling effort for distinguishing between different communities. We sampled three fjords in Northern Norway, taking samples along fjord transects and at three different depths, and amplified a fragment of the mitochondrial 12S rRNA gene of bony fishes using the MiFish primers. We evaluated the effect of (i) the number of sampling stations, (ii) samples' spatial distribution, and (iii) the data treatment approach (presence/absence versus semiquantitative) for maximizing the efficiency of eDNA metabarcoding sampling when inferring differences of fish community compositions between fjords. We found that the manner of data treatment strongly affected the minimum number of sampling stations required to detect differences among communities; because the semiquantitative approach retained some information about abundance of the underlying reads, it was the most efficient. Furthermore, we found little‐to‐no difference of fish communities in samples from intermediate depths when comparing vertical fish communities. Lastly, we found that the differences between fish communities at the surface were the highest across the horizontal distance and overall, samples ~30 km apart showed the highest variation in the horizontal distribution. Boosting sampling efficiency (reducing sampling effort without compromising ecological inferences) can significantly contribute to enhanced biodiversity management and efficient biomonitoring plans.