Development of habitat models for Nephtys species (Polychaeta: Nephtyidae) in the German Bight (North Sea)

The aim of the study was the development of habitat models for Nephtys species (Polychaeta: Nephtyidae). The investigation area was the German Bight, the southeastern part of the North Sea. Models were developed based on field data collected between 2000 and 2006. In addition, data on environmental variables were retrieved from long-term monitoring data sets and from the sediment map by Figge [Figge, K., 1981. Nordsee. Sedimentverteilung in der Deutschen Bucht. Map No. 2900. Publisher: Deutsches Hydrographisches Institut, Hamburg]. The statistical modelling technique used was multivariate adaptive regression splines (MARS). Models were fitted individually for each species. Evaluation of predictive discrimination and predictive accuracy of the developed models was by calculation of the area under the receiver operating curve (AUC) or sensitivity and specificity, respectively. Habitat models with best predictive fit were selected for the presentation of habitat suitability maps.Six Nepthys species were found: Nephtys assimilis, N. caeca, N. cirrosa, N. hombergii, N. incisa and N. longosetosa. N. hombergii was most common whereas N. incisa and N. longosetosa were rare. Habitat preferences varied considerably among the species. For all investigated Nephtys species except N. longosetosa a habitat model could be developed based on four predictor variables. The habitat models with best predictive fit were those for N. cirrosa and N. hombergii. The N. caeca habitat model was of limited predictive accuracy and only accept predictive discrimination. The number of predictors as well as the relative importance of the respective predictors in the model varied among the different species. Direct comparison of most suitable habitats for the different species based on modelling revealed that in the mostly sandy regions parallel to the German coast in water depths up to 20 m an overlap between N. caeca, N. hombergii and N. cirrosa exists. In the deeper central German Bight with mostly fine sands with increased mud contents N. hombergii, N. assimilis and, at least partially and rare in numbers, N. incisa co-occur. It can be concluded that important sediment characteristics like grain size median and mud content as well as water depth and mean salinity are useful parameters to describe the habitat requirements of most Nepthys species in the German Bight. However, additional variables need to be incorporated into such analyses.     

Validation of the three-dimensional ECOHAM model in the German Bight for 2004 including population dynamics of Pseudocalanus elongatus

A three-dimensional ecosystem model for the North Sea which includes competition between Pseudocalanus elongatus and the rest of the zooplankton biomass was applied to describe the seasonal cycle of zooplankton in 2003–2004. The paper presents the comparison of simulated stage-resolved abundances with copepod counts at several stations in the German Bight during the GLOBEC-Germany project from February to October 2004. A validation of influential state variables gives confidence that the model is able to calculate reliably the stage development and abundances of P. elongatus as well as the range of bulk zooplankton biomass, and thus the ratio of population biomass to total biomass. In the German Bight, the population is below 20% in spring. The ratio increases up to 50% during summer. The number of generations was estimated from peaks in egg abundance to about 4–8 generations of P. elongatus in the southern North Sea. A mean of four generations per year were estimated in the central North Sea, six to eight generations northwest of the Dogger Bank (tails end) and five generations in the German Bight.     

Trans-Atlantic responses of Calanus finmarchicus populations to basin-scale forcing associated with the North Atlantic Oscillation

Populations of the copepod species Calanus finmarchicus often dominate the springtime biomass and secondary production of shelf ecosystems throughout the North Atlantic Ocean. Recently, it has been hypothesised that interannual to interdecadal fluctuations observed in such populations are driven primarily by climate-associated changes in ocean circulation. Here, we compare evidence from the North Sea and Gulf of Maine/Western Scotian Shelf (GoM/WSS) linking fluctuations in C. finmarchicus abundance to changes in ocean circulation associated with the North Atlantic Oscillation (NAO). A particularly striking contrast emerges from this Trans-Atlantic comparison: whereas the North Sea C. finmarchicus population exhibits a negative correlation with the NAO index, the GoM/WSS population exhibits a more complex, positive association with the index. The physical processes underlying these contrasting population responses are discussed in the context of regional- to basin-scale circulation changes associated with the NAO.     

Environmental controls on phytoplankton community composition in the Thames plume, U.K.

The aim of this study was to investigate controls on the phytoplankton community composition and biogeochemistry of the estuarine plume zone of the River Thames, U.K. using an instrumented moored buoy for in situ measurements and preserved sample collection, and laboratory-based measurements from samples collected at the same site. Instrumentation on the moored buoy enabled high frequency measurements of a suite of environmental variables including in situ chlorophyll, water-column integrated irradiance, macronutrients throughout an annual cycle for 2001 e.g. nitrate and silicate, and phytoplankton biomass and species composition. The Thames plume region acts as a conduit for fluvial nutrients into the wider southern North Sea with typical winter concentrations of 45 μM nitrate, 17 μM silicate and 2 μM phosphate measured. The spring bloom resulted from water-column integrated irradiance increasing above 60 W h m^− 2 d^− 1 and was initially dominated by a diatom bloom mainly composed of Nitzschia sp. and Odontella sinesis. The spring bloom then switched after  30 days to become dominated by the flagellate Phaeocystis reaching a maximum chlorophyll concentration of 37.8 μg L^− 1. During the spring bloom there were high numbers of the heterotrophic dinoflagellates Gyrodinium spirale and Katodinium glaucum that potentially grazed the phytoplankton bloom. This diatom–flagellate switch was predicted to be due to a combination of further increasing water-column integrated irradiance > 100 W h m^− 2 d^− 1 and/or silicate reaching potentially limiting concentrations (< 1 μM). Post spring bloom, diatom dominance of the lower continuous summer phytoplankton biomass occurred despite the low silicate concentrations (Av. 0.7 μM from June–August). Summer diatom dominance, generally due to Guinardia delicatula, was expected to be as a result of microzooplankton grazing, dominated by the heterotrophic dinoflagellate Noctiluca scintillans, controlling 0.7–5.0 μm ‘flagellate’ fraction of the phytoplankton community with grazing rates up to 178% of ‘flagellate’ growth rate. The Thames plume region was therefore shown to be an active region of nutrient and phytoplankton processing and transport to the southern North Sea. The use of a combination of moorings and ship-based sampling was essential in understanding the factors influencing nutrient transport, phytoplankton biomass and species composition in this shelf sea plume region.     

Rapid invasion of Crassostrea gigas into the German Wadden Sea dominated by larval supply

Invasions of non-indigenous species into coastal habitats have been a frequent phenomenon in the last decades, sometimes with significant impact on the receiving ecosystem. However, the understanding of the entire process and especially the relative importance of larval supply and local recruitment remains unclear. In this study, we simulate the invasion of a benthic invertebrate into a previously uncolonised habitat over several years and validate the results with field data. Therefore, we present field data from a monitoring programme revealing the rapid invasion of the oyster Crassostrea gigas into the East Frisian Wadden Sea, North Sea, between 2003 and 2005. The applied model combines a simple, spatially-explicit population dynamics model for the adult stage with a particle tracking model for the larval stage of the life cycle. Simulation results are able to reproduce the large-scale pattern of the field data and indicate a domination of larval supply on the population dynamics in the early stage of the invasion. Though monitoring and simulations suggest a single larval source outside the study area in the west, the population dynamics in the eastern part is only explainable with an additional source within the study area attributed to an unintentional input of juveniles by mussel fishery. High sensitivities to uncertain parameters result in distinct deviations between monitoring and simulations at particular sites. Especially the impact of site-specific variations of the post-settlement mortality underlines the variability of local recruitment conditions and indicates the need for spatially resolved information for exact predictions.     

Seasonal and interannual variability of the air–sea CO2 flux in the Atlantic sector of the Barents Sea

The seasonal and interannual variability of the air–sea CO₂ flux (F) in the Atlantic sector of the Barents Sea have been investigated. Data for seawater fugacity of CO₂ (fCO₂^sw) acquired during five cruises in the region were used to identify and validate an empirical procedure to compute fCO₂^sw from phosphate (PO₄), seawater temperature (T), and salinity (S). This procedure was then applied to time series data of T, S, and PO₄ collected in the Barents Sea Opening during the period 1990–1999, and the resulting fCO₂^sw estimates were combined with data for the atmospheric mole fraction of CO₂, sea level pressure, and wind speed to evaluate F.The results show that the Atlantic sector of the Barents Sea is an annual sink of atmospheric CO₂. The monthly mean uptake increases nearly monotonically from 0.101 mol C m^− 2 in midwinter to 0.656 mol C m^− 2 in midfall before it gradually decreases to the winter value. Interannual variability in the monthly mean flux was evaluated for the winter, summer, and fall seasons and was found to be ± 0.071 mol C m^− 2 month^− 1. The variability is controlled mainly through combined variation of fCO₂^sw and wind speed. The annual mean uptake of atmospheric CO₂ in the region was estimated to 4.27 ± 0.68 mol C m^− 2.     

Spatial variability of epifaunal communities from artificial habitat: Shipwrecks in the Southern Bight of the North Sea

We investigated the cover, community structure and abiotic environment of nine shipwrecks lying at increasing distance from the Belgian coast. Results indicated that all shipwrecks were strongly dominated by cnidarians in terms of biomass and by amphipods in terms of abundances. Based on their epifaunal composition, three groups of shipwrecks could be determined. Metridium senile dominated a species poor community of the coastal sites. On the same sites, a Tubularia larynx community with a more species-rich assemblage was also developing. The T. larynx community had a lower biomass value (102 g AFDW m⁻²) and significantly lower species richness compared to the other sites. The coastal sites were characterized by periodic salinity decreases, large seasonal temperature fluctuation, high total suspended matter load and reduced current velocity. Channel water masses influence the offshore sites causing a more stable temperature and salinity environment, less turbid waters and high current speed. Tubularia indivisa dominated this community, with an average biomass of 229 g AFDW m⁻². Intermediate sites were also dominated by T. indivisa, but a higher biomass (424 g AFDW m⁻²) was observed. They showed intermediate results for the abiotic parameters and fast current velocities. Hypotheses for the observed variation in community structures are discussed in the light of the abiotic characterization of the shipwrecks.     

Contrasting shell morphology, ingestion and grazing preferences in the neritid gastropod Smaragdia viridis (L.) on two seagrass species

The neritid Smaragdia viridis represents the only known native marine mollusc that feeds on seagrass tissues in the European coasts, displaying a strong association with the seagrasses Cymodocea nodosa and Zostera marina in southern Spain. Seasonal dynamics, shell and radular morphology, growth and feeding of this gastropod have been studied in relation to each seagrass species for contrasting trends resulting from a different type of substrate and food source. In both seagrass species, stable populations of this gastropod occur at similar densities and displaying similar growth rates. Nevertheless shells of individuals from C. nodosa are narrower than those from Z. marina and some differences, possibly a consequence of increased wearing on C. nodosa, were noted amongst the radulae. In C. nodosa, a pre-ingestive selection for young epidermal tissues occurs as it was previously observed in Z. marina. The ingestion rate is higher in C. nodosa than in Z. marina but the absorption of ingested tissues is lower in the former. If both seagrasses are present, most individuals ingested preferentially Z. marina rather than C. nodosa, probably due to the lower digestibility of the epidermal tissues in the latter. Seagrass beds, especially those of Z. marina, are suffering a strong regression in southern Spain and the presence of stable populations of this neritid may be restricted to other declining seagrass species in the area.     

Mobile sand deposits and shoreface sediment dynamics in the inner German Bight (North Sea)

This paper presents a conceptual model for the net bedload transport regime on the shoreface of the German Bight. The model is based on the spatial distribution of the surficial sediment cover (North Sea sands) which is identical to the uppermost layer in the seismic recordings. Sediment thickness was measured using very high resolution seismic profiling (chirp sonar) and vibrocoring. The three-dimensional sediment distribution was estimated using geostatistical methods (cokriging). The results demonstrate a longshore sand distribution with three distinct zones. In Zone 1 (0–10 m water depth) the sediments attain their maximum thickness of 10±2.5 m. Between 10 and 15 m water depth a relatively thin sand layer of 0.4–1.5 m is observed within Zone 2. The seaward adjacent Zone 3 (15–20 m water depth) is characterized by an averaged sand thickness of 2–3 m with local maxima of 5–6 m. Further offshore, the sand layer decreases to about 1–2 m thickness. The net bedload transport directions inferred from this sediment zonation comprise a longshore sediment bypassing in Zone 1 which results in a substantial sediment supply to the innermost part of the German Bight due to bedload convergence. Shore-normal bedload transport shifts sand to and fro across the coastal profile although the net directional transport is seawards. This results in sediment depletion between the 10 and 15 m-isobaths (Zone 2) and an adjacent sediment accumulation in deeper waters (Zone 3).     

浙江洞头岛自由生活线虫两个优势种的形态描述及其系统发育分析

为了了解浙江洞头岛自由生活线虫(Nematoda)的种类和多样性, 作者对该海域线虫进行分类研究, 发现了2个优势种并进行了重新描述。其中, 丑萨巴线虫(Sabatieria foetida Gagarin & Nguyen Vu Thanh, 2008)为一新纪录种, 主要特征为: 表皮具环状排列的小点, 有较大的点组成的侧装饰, 4根头刚毛较短(3~4 μm), 化感器呈螺旋状(2.5~3.0圈), 交接刺细长, 近端略膨大, 中间开始至远端变细(65~69 μm), 引带具直的引带突(20~25 μm), 具13~15个小的乳突状肛前辅器; 厦门后菱光线虫(Metachromadora xiamenensis Zhou, Zeng, Cai, Fu & Tan, 2020)特征为: 表皮具明显环纹, 4根头刚毛短且粗, 脊线由食道中部延伸至尾中部, 双环状化感器位于身体最前端, 交接刺近端具头状膨大(58~65 μm), 肛前辅器细管状(15个), 尾为短圆锥形。同时, 获得了两种线虫核糖体大亚基(LSU)的D2-D3片段序列数据, 构建了两种线虫基于贝叶斯推论法(Bayesian inference, BI)和最大似然法(maximum likelihood, ML)的系统发育树, 结果表明萨巴线虫属和后菱光线虫属表现出了多系进化的趋势。     

Oceanic distribution and life cycle of Calanus species in the Norwegian Sea and adjacent waters

The distribution and demography of Calanus finmarchicus, C. glacialis and C. hyperboreus were studied throughout their growth season on a basin scale in the Norwegian Sea using ordination techniques and generalized additive models. The distribution and demographic data were related to the seasonal development of the phytoplankton bloom and physical characteristics of water masses. The resulting quantified relationships were related to knowledge on life cycle and adaptations of Calanus species. C. finmarchicus was the numerically dominant Calanus species in Coastal, Atlantic and Arctic waters, showing strong association with both Atlantic and Arctic waters. C. hyperboreus and C. glacialis were associated with Arctic water; however, C. glacialis was occasionally observed in the Norwegian Sea and is probably an expatriate advected into the area from various origins. Demography indicated one generation per year of C. finmarchicus, a two-year life cycle of C. hyperboreus, and both one- and two-year life cycles for C. glacialis in the water masses where they were most abundant. For the examined Calanus species, young copepodites of the new generation seemed to be tuned to the phytoplankton bloom in their main water mass. The development of C. finmarchicus was delayed in Arctic water, and mis-match between feeding stages and the phytoplankton bloom may reduce survival and reproductive success of C. finmarchicus in Arctic water. Based on low abundances of C. hyperboreus CI–III in Atlantic water and main recruitment to CI prior to the phytoplankton bloom, we suggest that reproduction of C. hyperboreus in Atlantic water is not successful.     

Etropus crossotus, an annual flatfish species; age and growth of the fringed flounder in South Carolina

Fringed flounder, Etropus crossotus, a small flatfish common along the southeastern coast of the US, were collected along the South Carolina coast in the fall of 1992, and in the North Inlet estuary from August 1992 until July 1994. Standard length was 82% of total length and the relationship between standard length and wet weight showed near-isometric growth with no significant differences between males and females. Dry weight was 21% of wet weight and ash-free dry weight 14% of wet weight with no significant differences between sexes. Daily increments in the sagittal otoliths were used for age determination. No annual structures could be identified. The maximum age was about 14.5 months. Age-at-length was not different between the sexes. The relationship between age and length was exponential, with age (in days) equalling an e power of 4.242 + 0.135 × standard length (in cm). Back-calculated hatch dates and catches of early juveniles indicated a spawning season that lasted from March to October. Females started to be sexually mature at 7 to 7.5 cm standard length, but 50% maturity was reached at 8 to 8.5 cm. Females could grow to that size within one spawning season. Individuals hatched early in the spawning season could produce eggs in the latter portion of that season's spawning period.     

The hydrography and biology of a bloom of the coccolithophorid Emiliania huxleyi in the northern North Sea

In June/July 1994 a study was made of a small bloom of the coccolithophorid Emiliania huxleyi in an area of the North Sea to the east of the Shetland Islands. Observations on the hydrography of the study area indicated the bloom was associated with Atlantic water and was confined to an area in which a stable shallow mixed layer had formed. There was no evidence to suggest association of horizontal physical structure with the bloom development. High cell densities of >1– cells dm⁻³, together with low concentrations of PIC (<50 μg dm⁻³) and detached liths (2– liths cm⁻³) indicated that the bloom was studied at an early stage of development. Biochemical and physiological observations indicated active growth was taking place. The results presented are discussed in comparison with previous studies carried out in both oceanic and shelf seas.     

Resource partitioning by two large planktivorous fishes Micromesistius australis and Macruronus magellanicus in the Southwest Atlantic

The ontogenetic, seasonal, bathymetric and regional variations in the feeding spectrum of 922 specimens of southern blue whiting Micromesistius australis and 512 specimens of hoki Macruronus magellanicus were studied on the Falkland Islands’ shelf (Southwest Atlantic) between November 1999 and April 2003. A total of 49 different prey items were found in the stomach contents of the two species, with the hyperiid Themisto gaudichaudii and Euphausiacea being amongst the most important prey. Although the species composition did not change over fish size, the proportions of individual prey items in their diets did, with an increase in T. gaudichaudii and Euphausiacea with increasing fish size in southern blue whiting. The opposite occurred in hoki. Seasonal variations in the diet were found to mirror the seasonal abundance of prey around the Falkland Islands for the two species. Intra-specific differences in the diet of both predators reflected the distribution of prey, which in turn was determined by the water structure in the two regions sampled, leading to very different diets. In the limited time that the two species occupied the same space there was little or no competition resulting in almost total segregation of their trophic niches in space and time.     

夏季南黄海主要环境因子对微微型浮游生物分布影响

利用流式细胞技术, 获取南黄海夏季微微型浮游生物丰度数据, 分析了其组成和分布规律, 并探讨了主要的影响因子。2011年夏季, 聚球藻、微微型真核藻、异养细菌在整个调查海区的平均丰度分别在1×104、1×103、1×106 cells/mL数量级上。在全调查海区, 聚球藻和微微型真核藻受温度和光照的限制明显, 主要集中分布在温跃层及其以上水层;而营养盐的限制较小, 它们的影响只有在沿岸流影响明显的西部海区才能较为明显的体现出来。结果表明在该海域浓度较高的营养盐能够促进微微型浮游生物的生长, 但不是其限制因素;异养细菌受环境因子限制较小, 即使在深海也保持着较高的丰度。     

东海春季赤潮过程中优势种的演替:生物-物理耦合数值模拟

In the East China Sea(ECS), the succession of causative species responsible for blooms is a recurrent phenomenon during the spring, which changes from diatoms to dinoflagellates. Observations from space and in situ cruises captured this pattern of succession during spring of 2005. In this study, we coupled two biological models, which were developed previously for Skeletonema costatum and Prorocentrum donghaiense,into a circulation model tailored for the ECS. The coupled biophysical model was used to hindcast the blooms and to test the hypothesis proposed in earlier studies that phosphate(PO4 3–) is the first-order decider of the succession. The coupled model successfully reproduced the hydrodynamics(as described in a companion paper by Sun et al.(1), the spatiotemporal distribution of the chlorophyll a(Chl a) concentration, and the species succession reasonably well. By analyzing the effects of different factors on the surface Chl a distribution, we confirmed that the offshore boundaries of the blooms were confined by PO4 3–. In addition, we suggest that surface wind fields may modulate the horizontal distribution of blooms. Thus, during the dispersal of blooms, surface winds coupled with PO4 3– may control the succession of blooms in the ECS. The proposed coupled model provides a benchmark to facilitate future improvements by including more size classes for organisms, multiple nutrient schemes, and additional processes.     

Geographical distribution and age composition of Euphausia superba larvae (Crustacea：Euphausiacea) in the South Shetland Islands region and southern Scotia Sea in relation to environmental conditions

Spatial distribution patterns of the different life stages of Euphausia superba in the region of the South Shet- land Islands and southern Scotia Sea （Antarctica） were assessed based on scientific survey data collected in lanuary and February of 2010. Adults, eggs, nauplii, metanauplii, calyptopis I-Ill, and furcilia I-II were found in the investigation. The abundance of larvae averaged 1 172.8 ind./mz, with calyptopis I and II as the dominant stages. Habitat occupancy patterns varied among Euphausia superba at different stages, and three sub-regions were identified by cluster analysis. The degree of larval development increased from west to east. Larvae were not observed north of the South Shetland Islands. Calyptopis I was predominant in the water between Elephant Island and the South Orkney Islands, which featured no thermocline. Older stages, including calyptopis II and III and furcilia I and II, were common in north and northeast of the South Orkney Islands, which were characterized by high temperature and high chlorophyll concentration. Distri- bution and abundance of the early life stages of E. superba were associated with specific environmental con- ditions. According to Biota-Environment matching （BIOENV）, the distributions of E. superba larvae were correlated with a combination of temperature at the surface and 200 m, and 0-100 m integrated chlorophyll a concentration.     

Feeding ecology of five commercial shark species of the Celtic Sea through stable isotope and trace metal analysis

In order to trace their feeding habits, stable carbon and nitrogen isotope ratios (delta15N and delta13C), as well as trace metal concentrations (Zn, Cd, Fe, Cu, Se and Hg) were analysed in the tissues of five commercial shark species from the Celtic Sea: the tope shark Galeorhinus galeus, the black-mouthed catshark Galeus melastomus, the starry smooth hound Mustelus asterias, the spiny dogfish Squalus acanthias and the lesser-spotted dogfish Scyliorhinus canicula. Our results were compared to previously described stomach contents and isotopic composition of potential preys. Isotopic ratio delta15N suggested that tope sharks fed at a higher trophic level (16.7 per thousand in the muscle) than the other species, reflecting its piscivorous diet. The lower values of spiny dogfish (11.6 per thousand in the muscle) might be explained, amongst other things, by either its migratory behaviour or its preference for preys from lower trophic levels. Cd and Hg were correlated with isotopic ratios delta13C and delta15N, and were shown to be diet-related whereas Zn, Fe and Cu seemed much more linked to species-specific metabolism. Although this multidisciplinary approach is revealed as a useful tool for the study of shark ecology, the lack of known trophic fractionation suggests that isotopic data be compared to traditional diet analyses.