Ocean alkalinity enhancement (OAE) through mineral dissolution is a promising marine carbon dioxide removal strategy because it increases the buffering capacity of seawater and thereby enhances passive storage of atmospheric CO2. However, the ecological consequences of OAE for zooplankton, particularly gelatinous species, remain poorly understood. Here, we assessed the response of a key gelatinous zooplankton species to OAE in a 53-day mesocosm experiment in a temperate Norwegian fjord. Oikopleura dioica is a globally distributed zooplankton member, known for its high secondary production capacity and key role in vertical carbon flux. O. dioica continuously produces mucous feeding structures ('houses'), which efficiently retain submicron particles. Once discarded, these houses can sink rapidly and contribute to vertical carbon exports. To test the impacts of OAE on O. dioica abundances and their house production capacity, we exposed natural plankton communities to non-CO2-equilibrated OAE scenarios spanning a ΔTA range from 0-600 μmol kg-1, using silicate-based (olivine) and calcium-based (slaked lime) minerals. Population dynamics of O. dioica were monitored alongside the plankton community, and targeted bottle incubations were used to quantify house production and feeding rates. We show that O. dioica abundances varied by an order of magnitude within and across treatments. No interaction between O. dioica abundance and alkalinity levels or mineral types could be detected. Instead, O. dioica abundance variations were primarily explained by prey availability (picoplankton). Additionally, house production and feeding rate experiments showed that O. dioica were unaffected by OAE across all treatments. These findings indicate that O. dioica, as a key gelatinous zooplankton member, is physiologically resilient to OAE within the tested range. Future studies should incorporate gelatinous zooplankton into OAE assessments and investigate higher alkalinity perturbations to evaluate potential ecosystem impacts and larvacean-mediated changes in carbon export during under OAE deployments above ΔTA 600 μmol kg-1.

Abstract Non‐indigenous species (NIS) are often disregarded in conservation planning despite being a major threat to global biodiversity. Thus, spatially explicit risk assessments jointly accounting for the multiple underlying processes determining NIS establishment are urgently needed. Current assessments mainly consider the likelihood of NIS arrival and environmental suitability of recipient areas, overlooking other key biotic variables determining establishment, like niche overlap and potential interactions with native species. Here, we explore the potential of niche overlap between NIS and native species as an additional component for assessing invasion risk, and propose its integration into a general framework to evaluate the likelihood of NIS establishment at a global scale. Using 121 non‐indigenous marine fishes, this framework combines three key components of the invasion process: (i) connectivity to source areas, (ii) environmental suitability of recipient areas, and (iii) niche overlap with native species. These components are merged into a single metric of potential establishment risk, which we then decompose to examine global patterns and evaluate the relative contribution of each component. Our results show considerable variability in global risk patterns, with the Caribbean Sea and Gulf of Mexico standing out as the most sensitive areas to potential fish invasions due to a naturally suitable environment and the availability of niches for other tropical species, mainly from the Indo‐Pacific. We also examine the potential role of cumulative human impacts and marine protected areas (MPA), capable of elevating or mitigating such risks. Finally, the obtained risk scores largely correspond to the ranges where NIS are currently established. Synthesis and applications . Niche overlap may be critical to consider, even in areas that appear environmentally unsuitable or poorly connected for NIS. Preserving the intrinsic biotic resistance posed by native species can improve the effectiveness of targeted conservation measures. By considering niche overlap alongside connectivity and environmental suitability, the proposed approach can provide an enhanced understanding of the mechanisms behind NIS establishment. Knowing which component may benefit NIS can directly inform management and guide preventive actions. Finally, the proposed framework is aimed at being general and applicable across spatial scales, regions, and invasion scenarios given sufficient data.

Gelatinous zooplankton are an important component of many ecosystems and important for ecosystem structure and carbon cycling. However, this group is generally not considered in biogeochemical models. Here we investigate the biomass-to-volume ratio as an underappreciated "master trait" that allows for the incorporation of a large diversity of zooplankton groups into modeling exercises. By considering the biomass-to-volume ratio as a continuum, we investigate the potential trade-offs between body composition and physiological (e.g., clearance, respiration, carbon mass-specific growth, assimilation) as well as ecological (e.g., predator-prey size ratio, feeding modes) traits. We find that a low carbon composition has a positive effect on the organism's fitness, as more prey could be captured for the same active mass. Thus, taking the biomass-to-volume ratio into account could improve the estimation of physiological rates. Additionally, we show that gelatinous feeding-current feeders (e.g., tunicata, Mnemiopsis spp., Rhizostoma spp.) have an ability to catch smaller prey over a wider size range than non-gelatinous feeding-current feeding organisms (gelatinous feeding-current feeders min-max: 102-106 mu mpredator mu mprey-1; non-gelatinous feeding-current feeders min-max: 5 x 100-8 x 101 mu mpredator mu mprey-1). However, results are only valid for the respective feeding mode, highlighting new trade-offs. This allows us to re-evaluate the functional role of certain organisms, such as larvaceans (appendicularians), which were previously considered to be super-filters, or pteropods, which remain understudied. This study contributes to a wider representation of the complexity of the zooplankton community in size-structured models. We highlight that the biomass-to-volume ratio, along with size, is the most important parameter required to represent the full diversity of zooplankton.

The biodiversity and distribution of gelatinous macrozooplankton was assessed in the Baltic Sea during September 2020. The dataset includes 40,601 species-specific gelatinous macrozooplankton records, representative of 236,329 gelatinous organisms caught across 73 stations in the south-western, central and northern Baltic Sea. Focus was devoted to changes in depth distributions in relation to salinity and an extended oxygen depletion event in the south-western Baltic Sea. In total, 56 Multinet-midi casts (5 depth-strata), 4 Multinet-maxi casts (9 depth-strata), as well as 17 bongo and 52 WP2 casts were performed from the surface to >3 m above the bottom. Data include depth resolved information on the abundance (m−3 and m−2) and size structure of (i) the non-indigenous ctenophore Mnemiopsis leidyi - including larvae (1–3 mm), transitional (4-5 mm), young adult (6–9 mm) and adult (≥10 mm) ctenophores, as well as the native scyphozoan jellyfish species (ii) Aurelia aurita and (iii) Cyanea capillata. Additionally, the zooplankton community is described from WP2 nets including species-specific size and biomass data. In total 40,601 individual gelatinous macrozooplankton specimens from samples and sub-samples were analyzed (raw-counts) with 39,771 Mnemiopsis leidyi (corrected-count 235,499), 744 Aurelia aurita and 86 Cyanea capillata records.We provide a detailed account of catchability of different life-stages and comparison of different net types. In general, at 89 % of the stations, adult M. leidyi were caught, at an average ( ± SD) density of 1.27 ± 0.97 ind m−3 station−1 (max 8.4 ind m−3, Flensburg Fjord at 10-7.5m). Young M. leidyi adults were caught at 78 % of the stations with 3 ± 2.4 ind m−3 station−1 (max 29.4 ind m−3, Eckernförde Bight, south of Flensburg Fjord at 0–2.5 m), but were absent from the northern Arkona Basin and east of Bornholm. At 71 % of the stations, transitional M. leidyi were caught at an average density of 19.2 ± 21.8 ind m−3 station−1 (max 129.2 ind m−3, WP2, Kiel Bight). Transitional M. leidyi were additionally absent from the central Arkona Basin. M. leidyi larvae were present at 68 % of all stations, with the maximum density observed in Kiel Bight with 642 ind m−3 (WP2). Generally, high larvae densities with >500 ind m−3 were found in the south-western Kiel Bight, with an overall average density of 257 ± 188 ind m−3. Scyphozoan jellyfish species were found at much lower densities. Maximum abundance of A. aurita was observed in the Arkona Basin (n = 79 at 4 to 6m), with abundances ranging from 0.003 to 1.7 ind m−3 station−1. A. aurita was primarily found in the upper 30 m, while C. capillata was primarily present in waters >30 m (average of 0.002 to 0.07 C. capillata m−3). The here presented data are essential to further investigate responses of jellyfish and ctenophores to climate change, especially considering salinity and low oxygen conditions, important global change pressure which are of special concern for the Baltic Sea.Note: The non-indigenous hydromedusae Blackfordia virginica was only found at one station in Kiel Bight, off the Kiel Kanal exit to the SW Baltic Sea (n = 1, 8mm, 0.05 ind m−3).

Aim To quantify the connectivity and degree of population differentiation of a planktonic model species along an extreme environmental gradient in order to understand source-sink dynamics and processes leading to local adaptation.Location Baltic Sea/North Sea.Taxon Scyphozoan Jellyfish Aurelia aurita.Methods Levels of genetic connectivity and population differentiation were analyzed by integrating molecular data with 40 years of Lagrangian stepping-stone drift-route simulations. Drifters represented different A. aurita life-stages that were traced over 40 consecutive life-cycles from four empirically confirmed start locations. Suitable habitat for polyp recruitment was parameterized from high-resolution bottom topography maps to allow for stepping-stone range expansion across the Baltic Sea, with additional physiological experiments.Results Molecular and drift analyses revealed a large degree of isolation, identifying two major clusters in the Baltic Sea: (1) the low saline central-eastern and (2) the intermediate saline south-western regions, respectively. Additionally, molecular analyses confirmed the southern North Sea as an independent cluster with limited connectivity to the Baltic Sea. The Skagerrak/central North Sea was further confirmed as a transition zone between both regions. Lagrangian simulations suggested only one seeding event from this transition zone (Skagerrak) into the Baltic Sea over the 40-year study period. Furthermore, a 2.5-fold lower genetic diversity in the low saline Baltic relative to the high saline North Sea indicated limited current gene flow. A significant correlation was found between haplotype frequency and salinity in Baltic Sea samples, together with successful settlement of northern Baltic individuals at low salinities.Main Conclusions Limited connectivity leads to population differentiation of A. aurita in the Baltic Sea. Furthermore, active recruitment at low salinities suggests local adaptation within the low saline Baltic Sea. Local adaptation is likely widespread among Baltic species of marine origin, raising concerns for biodiversity conservation and ecosystem management of the Baltic Sea under continued freshening due to climate change.

Non-indigenous species (NIS) are on a rise globally. They can pose strong impacts on ecosystems in their non-native range and can therefore be a serious threat to biodiversity. Here, we compile the existing information available regarding the extent to which commercially and recreationally used fish stocks (exploited fish species) are affected by NIS. To do so, we conducted a literature review to summarize the known and presumed impacts of four case study NIS with already known strong effects on the Baltic Sea ecosystem: round goby, Neogobius melanostomus, sea walnut, Mnemiopsis leidyi, mud crab, Rhithropanopeus harrisii, and fishhook water flea, Cercopagis pengoi. We found that round goby, mud crab and fishhook water flea are documented to serve as a new food source for native fish species, while sea walnut and fishhook water flea are supposedly affecting planktivorous fish through resource competition. Round goby is very likely a strong competitor for the benthivore fish community. There are also indications that it feeds on juvenile fish and fish eggs. Generally, our results show that large knowledge gaps exist, while the published impacts on exploited fish species are often solely based on correlations (e.g. decreasing abundances of native species with cooccurring increasing abundances of NIS), regionally restricted studies or expert judgements. In addition, many studies are older and the current population size of the NIS, which is obviously associated with their impact, is unknown. Thus, the majority of described impacts of NIS on commercially and recreationally used fish stocks seems to stem from assumptions. Therefore, more field observations and experimental studies are needed to be able to scientifically evaluate the impact of NIS. Nevertheless, in this review, the available information was summarized, even if they are speculative, and specific knowledge gaps were identified. Moreover, we outline further investigations that are needed to advance our mechanistic understanding of the interactions between NIS and exploited fish species in the Baltic Sea. This knowledge is essential for the sustainable management of aquatic resources and management of NIS of the Baltic Sea.

The biodiversity and distribution of gelatinous macrozooplankton in the North Sea and adjacent waters during winter (January/February) 2023 is presented both quantitatively and qualitatively. The data include species-specific jellyfish and comb jelly community data, encountered during the North Sea - Midwater Ring Net (MIK) survey [1]. The MIK survey targets ichthyoplankton and is conducted at night during the quarter 1 (Q1) International Bottom Trawl Surveys (IBTS). Presented data about the gelatinous macrozooplankton community stems from Danish (DTU Aqua), the Swedish (SLU), and German (TI) partners. A total of 158 stations were investigated using a MIK net (2 m diameter, 13 m long, 1.6 mm mesh size with 0.5 mm for net end and cod end) [2]. Samples were collected by double oblique hauls from the surface to 5 m above the seafloor with a maximum depth of ∼100 m [2]. Eighteen gelatinous macrozooplankton species were encountered during the Q1 2023 survey. Species encountered are hydrozoans (i) Aequorea vitrina, (ii) Agalma elegans (siphonophore), (iii) Aglantha digitale, (iv) Apolemia uvaria (siphonophore), (v) Clytia spp., (vi) Eutima spp., (vii) Leuckartiara octona, (viii) Melicertum octocostatum, (ix) Muggiaea atlantica (siphonophore), (x) Nanomia cara (siphonophore), (xi) Tima bairdii; scyphozoans (i) Cyanea capillata and (ii) Cyanea lamarckii as well as the ctenophores (i) Beroe spp., (ii) Bolinopsis infundibulum, (iii) Pleurobrachia bachei, (iv) Pleurobrachia pileus, and (v) the non-indigenous Mnemiopsis leidyi.In total 12,093 individual specimens from samples and sub-samples were analyzed and extrapolated to generate a database with 77,099 records of gelatinous macrozooplankton caught in the investigation area during Q1 2023. For rare species, the entire sample was processed, while abundances were estimated from sub-samples for abundant taxa. Flow-meter recordings and maximum net depths during each haul were used to convert raw counts to volume-specific densities (individuals m-3) and area-specific abundances (individuals m-2). Further, sizes for the different species were obtained from a total of 5,566 individual gelatinous macrozooplankton organisms. Sizes are presented in the accompanying database and were used to calculate species-specific wet weights, using published size-weight regressions [3] and regressions outlined in Table 1. In addition, we present spatial wet weight distribution patterns for (i) the total gelatinous macrozooplankton community, (ii) hydrozoans only, (iii) scyphozoans only and (iv) ctenophora only. The presented data contribute to a time series describing the gelatinous macrozooplankton diversity and distribution in the extended North Sea area during winter [3,4] and summer [5] and are an important baseline to understand response of jellyfish to climate change.

Abstract. Ocean alkalinity enhancement (OAE) stands as a promising carbon dioxide removal technology. Yet, this solution to climate change entails shifts in water chemistry with unknown consequences for marine fish that are critical to ecosystem health and food security. With a laboratory and mesocosm experiment, we show that early life stages of fish can be resistant to OAE. We examined metabolic rate, swimming behavior, growth and survival in Atlantic herring (Clupea harengus) and other temperate coastal fish species. Neither direct physiological nor indirect food web-mediated impacts of OAE were apparent. This was despite non-CO2-equilibrated OAE (ΔTA = +600 µmol kg-1) that induces strong perturbations (ΔpH = +0.7, pCO2 = 75 µatm) compared to alternative deployment scenarios. Whilst our results give cause for optimism regarding the large-scale application of OAE, other life history stages (embryos) and habitats (open ocean) may prove more vulnerable. Still, our study across ecological scales (organism to community) and exposure times (short- to long-term) suggests that some fish populations, including key fisheries species, may be resilient to the carbonate chemistry changes under OAE.

Jellyfish and gelatinous zooplankton (GZ) in general, fulfill important ecological roles with significant impacts, although they are often oversimplified or misunderstood. This paper reviews the impacts, pressures, monitoring methods and current management strategies for various GZ groups. It also introduces potentially applicable indicators for their assessment in ecosystem-based management approaches, such as the European Marine Strategy Framework Directive (MSFD). This multi-faceted review is primarily envisioned to serve as a state-of-the-art document for scientists and policymakers to foster a holistic assessment and management of GZ across European regional seas. The systematic review on global impacts of GZ shows a notable increase in the number of studies since the early 2000s. Stings were the main cause of human health impacts. Mechanisms that impact biodiversity included direct predation, modification of trophic flows or competition for resources. Several GZ taxa may be beneficial to biodiversity acting as biological regulators and provide societal ecosystem services such as food provision or medical applications. The systematic review on monitoring techniques outlined a variety of methods, such as nets (the most common technique), continuous plankton recorder (CPR), polyp and jelly-fall monitoring, acoustic methods, remote aerial and underwater imaging, molecular methods, and citizen science. Furthermore, several currently employed management strategies were enumerated, including the use of anti-jelly nets, bubble curtains, chemical compounds, or the introduction of GZ predators. This study highlights the pressing need for enhanced GZ-dedicated monitoring, assessment, and anticipatory management of GZ populations to address future GZ crises more effectively and cost-efficiently. Moreover, exploring GZ ecosystem services unveils opportunities to harness marine resources while mitigating adverse effects, thereby supporting sustainable blue economies.

Ocean alkalinity enhancement (OAE) is a nature-based technology for CO 2 removal and storage, but little is known about its environmental safety. We tested a CO 2 -equilibrated OAE deployment in a close-to-natural community using in situ mesocosms in the oligotrophic subtropical North Atlantic and assessed metazoan zooplankton to inform about food web stability, structure, and production. In addition, a literature review complemented experimental results by summarizing physiological responses of marine animals to decreasing proton concentrations, or increased pH. The food web studied proved resistant, and zooplankton physiologically tolerant, to the OAE tested. We observed short-term effects of OAE on zooplankton reproduction and productivity, which were likely trophically mediated. Yet, these did not affect zooplankton populations or their nutritional value as food for fish. Our study demonstrates an environmentally safe OAE application, but also stresses the risks of more intense OAE options, and the vulnerabilities of other marine ecosystems.

Relatively little attention has been paid to the underlying mechanisms determining the dominance of non-indigenous species (NIS) once established, despite being regarded as a proxy of invasion success and potential impacts in recipient communities. To bridge this knowledge gap, here we evaluate the potential direct and indirect effects of community filters on the dominance of two widespread NIS in the Baltic Sea: Marenzelleria spp. and the round goby (Neogobius melanostomus) within their corresponding communities. We applied a structural equation modelling approach to assess the direct and indirect effects amongst multiple abiotic and biotic variables on the relative biomass (as proxy of dominance) of NIS. The biotic variables represented the taxonomic- and functional diversity of the recipient communities, as well as the trait similarity between NIS and native species. We observed a comparable influence of abiotic and biotic drivers on the dominance of both NIS, with biotic variables having a somewhat stronger overall direct effect. Specifically, the dominance of both NIS was similarly affected negatively by the richness and positively by the evenness of the native communities. However, we also detected that both NIS might need different ecological strategies to become dominant in their recipient communities, which underwent similar assembly processes. Such strategies were partly highlighted by the different degrees of trait similarity between each NIS and their respective co-occurring native species. A better understanding of the underlying processes affecting NIS dominance is of high relevance to mitigate potential impacts of NIS once established. Furthermore, the provided approach could be further applied to unveil the potential strategies that NIS might follow in other regions and ecosystem types.

Ocean alkalinity enhancement (OAE) stands as a promising carbon dioxide removal technology. Yet, this solution to climate change entails shifts in environmental drivers with unknown consequences for marine fish that are critical to ecosystem health and food security. Fish and their supporting food webs may be stressed by the novel carbonate chemistry or the nutrients contained in the deployed minerals. With a mesocosm experiment on natural plankton communities, we studied early life stages of fish under alkalinity (+600 mu mol kg-1) and silicate (+75 mu mol L-1) addition. Larvae and young juveniles of temperate coastal species, including Atlantic herring (Clupea harengus) and cod (Gadus morhua), were exposed to direct physiological and indirect food-web-mediated effects of OAE for 49 d. Neither in the shorter nor in the longer term did we find an impairment of fish growth and survival. Alkalization even led to an increase in fish biomass. This resistance to OAE was despite using non-CO2-equilibrated deployment that induces more severe perturbations in carbonate chemistry (Delta pH =+0.7, pCO2=75 mu atm) compared to alternative scenarios. Overall, our community-level study suggests that some fish populations, including key fisheries' species, may be resilient to the water chemistry changes under OAE. Whilst these results give cause for optimism regarding the large-scale application of OAE, other life history stages (embryos) and habitats (open ocean) may prove more vulnerable.

The diversity and distribution of gelatinous macrozooplankton is described by presenting qualitative and quantitative data of the jellyfish and comb jelly community encountered in the North Sea and Skagerrak/Kattegat during January/February 2022. Data were generated as part of the North Sea Midwater Ring Net (MIK) survey [1], an ichthyoplankton survey conducted at night-time during the quarter 1 (Q1) International Bottom Trawl Survey (IBTS), aboard the Danish R/V DANA (DTU Aqua) and the Swedish R/V Svea (SLU). A total of 100 stations were investigated using a 13 m long Midwater Ring Net (MIK net) with an opening diameter of 2 m and a mesh size of 1.6 mm, which is 0.5 mm meshed for the last meter of the net and the cod end [2]. Samples were collected by double oblique hauls from the surface to 5 m above the seafloor [2]. Twelve gelatinous macrozooplankton species were encountered during the Q1 2022 survey. Species encountered included the hydrozoan jellyfish i) Aequorea vitrina, ii) Aglantha digitale, iii) Clytia spp., iv) Leuckartiara octona, v) Tima bairdii, vi) Muggiaea atlantica; the two scyphozoan jellyfish i) Cyanea capillata and ii) Cyanea lamarckii as well as the comb jelly (ctenophora) species i) Beroe spp., ii) Bolinopsis infundibulum, iii) Pleurobrachia pileus and iv) the non-indigenous Mnemiopsis leidyi. In total 4882 individual specimens from samples and sub-samples were analyzed and extrapolated to 71,888 records of gelatinous macrozooplankton in the investigation area. For rare species, the entire sample was analyzed, while for abundant taxa, sub-samples were used to assess abundances. The raw counts were converted to volume-specific densities (individuals m-3) and area-specific abundances (individuals m-2), based on calibrated flow meter recordings and recorded maximum depth of the MIK net during each haul. Further, size data for the different species were obtained from a total of 4775 individual gelatinous macrozooplankton organisms. Size data are presented in the accompanying database and was used to calculate species-specific wet weights, using published size-weight regressions [3]. In addition, we present spatial distribution patterns of the weight specific biomass for the total gelatinous macrozooplankton community as well as the sub-groups i) hydrozoa, ii) scyphozoa and iii) ctenophora across the investigation area. The presented data contribute to a baseline describing the gelatinous macrozooplankton diversity and distribution in the extended North Sea area during winter [3,4] and summer [5]. The data can contribute to address the question if gelatinous macrozooplankton densities increase due to global change pressures and will help to understand their interaction with commercially important fish species, which are assessed during the same surveys. As such, this data paper presents a valuable resource on biodiversity and non-indigenous species records and highlights the importance of monitoring gelatinous macrozooplankton to facilitate an ecosystem approach to assess if the ecosystem state meets a ‘good environmental status (GES)’, as demanded by the EU Marine Strategy Framework Directive (MSFD).

Non-indigenous species (NIS) are of concern for biodiversity conservation and ecosystem functioning. We present an updated list of NIS, including cryptogenic species, from Danish marine waters containing 123 species. Benthic invertebrates (36%) and phytoplankton (28%) dominate the list, but fish (15%) and macroalgae (13%) are also important. The Limfjord in Northern Jutland emerges as a hotspot for the introduction of NIS. Data from multiple sources were included, i.e., the National Monitoring Program (NOVANA), the National Fish Atlas project, the citizen science project Arter.dk, research articles, and annual national reports of the ICES working group ITMO. Forty-six NIS species were subject to expert judging using a modified Harmonia protocol; 19 were found to fulfil the four selected criteria identifying a species as being ‘invasive’. Additionally, 38 species, not yet recorded in Danish waters, were evaluated using the same method, and 31 were found to fulfil the ‘invasive’ criteria. For nine selected species, introduction history, distribution maps, and time-series diagrams are presented. Our data document that the national monitoring efforts should be expanded to record macrozooplankton, coastal fish, and mobile epibenthic species. Furthermore, the national data repository, Arter.dk, should be expanded to enable more detailed documentation of new NIS records.

Zooplankton communities vary in space and time. Their composition is strongly influenced by lower trophic levels that are dependent on the availability of light and nutrients. As all marine ecosystems are relying on zooplankton as intermediate trophic step between primary production and higher trophic levels, changes in the zooplankton community composition and biomass can cascade through the food web with important impacts on fish communities and through that on fisheries yields. An intense fisheries exist around the Falkland Islands in the SW Atlantic Ocean, around 51° S, but to the best of our knowledge, no previous study has to date investigated the seasonal variation in zooplankton community composition in these waters. We show that copepods (39.2%), the larvae of the anomurid Grimothea gregaria (33.1%) and euphausiids (10.9%) dominate the local mesozooplankton community by biomass. All species showed seasonal patterns, including ontogenetic behaviour of G. gregaria migrating to deeper waters with development, which were significantly explained by temperature (p < 0.001). While overall biomass significantly decreased with distance from shore (p < 0.001), mesozooplankton diversity was highest at 30 km from shore. The presented study is the first assessment of the mesozooplankton biomass off the Falkland Islands and provides a first baseline to aid future ecosystem studies in the context of ecosystem based fisheries management in the region.

Larvaceans are gelatinous zooplankton abundant throughout the ocean. Larvaceans have been overlooked in research because they are difficult to collect and are perceived as being unimportant in biogeochemical cycles and food-webs. We synthesise evidence that their unique biology enables larvaceans to transfer more carbon to higher trophic levels and deeper into the ocean than is commonly appreciated. Larvaceans could become even more important in the Anthropocene because they eat small phytoplankton that are predicted to become more prevalent under climate change, thus moderating projected future declines in ocean productivity and fisheries. We identify critical knowledge gaps and argue that larvaceans should be incorporated into ecosystem assessments and biogeochemical models to improve predictions of the future ocean.

Hybridization of distinct populations or species is an important evolutionary driving force. For invasive species, hybridization can enhance their competitive advantage as a source of adaptive novelty by introgression of selectively favored alleles. Using single-nucleotide polymorphism (SNP) microarrays we assess genetic diversity and population structure in the invasive ctenophore Mnemiopsis leidyi in native habitats. Hybrids are present at the distribution border of two lineages, especially in highly fluctuating environments including very low salinities, while hybrids occur at lower frequency in stable high-saline habitats. Analyses of hybridization status suggest that hybrids thriving in variable environments are selected for, while they are selected against in stable habitats. Translocation of hybrids might accelerate invasion success in non-native habitats. This could be especially relevant for M. leidyi as low salinity limits its invasion range in western Eurasia. Although hybridization status is currently disregarded, it could determine high-risk areas where ballast water exchange should be prevented.

Microbial communities are found throughout the biosphere, from human guts to glaciers, from soil to activated sludge. Understanding the statistical properties of such diverse communities can pave the way to elucidate the common mechanisms ...Multiple ecological forces act together to shape the composition of microbial communities. Phyloecology approaches—which combine phylogenetic relationships between species with community ecology—have the potential to disentangle such forces but are often ...

Physical and topographic characteristics can structure pelagic habitats and affect the plankton community composition. For example, oxygen minimum zones (OMZs) are expected to lead to a habitat compression for species with a high oxygen demand, while upwelling of nutrient-rich deep water at seamounts can locally in -crease productivity, especially in oligotrophic oceanic waters. Here we investigate the response of the gelatinous zooplankton (GZ) assemblage and biomass to differing oxygen conditions and to a seamount in the Eastern Tropical North Atlantic (ETNA) around the Cape Verde archipelago. A total of 16 GZ taxa (>1100 specimens) were found in the upper 1000 m with distinct species-specific differences, such as the absence of deep-living species Atolla wyvillei and Periphylla periphylla above the shallow seamount summit. Statistical analyses consid-ering the most prominent groups, present at all stations, namely Beroe spp., hydromedusae (including Zygocanna vagans, Halicreas minimum, Colobonema sericeum, Solmissus spp.) and total GZ, showed a strong positive corre-lation of abundance with temperature for all groups, whereas oxygen had a weak negative correlation only with abundances of Beroe spp. and hydromedusae. To account for size differences between species, we established length-weight regressions and investigated total GZ biomass changes in relation to physical (OMZ) and topo-graphic characteristics. The highest GZ biomass was observed at depths of lowest oxygen concentrations and deepest depth strata at the southeastern flank of the seamount and at two stations south of the Cape Verde ar-chipelago. Our data suggest that, irrespective of their patchy distribution, GZ organisms are ubiquitous food web members of the ETNA, and their habitat includes waters of low oxygen content.