象山港国华宁海电厂附近海域小型底栖动物的群落结构

于2008年夏季(6月)和冬季(12月),在象山港国华宁海电厂沿排水口右侧、从近到远相隔500m设立三个断面A、B、C,并对小型底栖动物丰度及其群落结构变化进行调查研究,结果表明:共鉴定出10个小型底栖动物类群,平均丰度达9407.9inds/10cm².从类群上看,自由生活海洋线虫占总丰度的62.3%,介形类占第二位,为19.3%.ANOSIM分析结果显示小型底栖动物季节间的丰度存在明显差异(p<0.05),夏季平均丰度(8055.3±1282.9inds/10cm²)比冬季(2141.1±614.2inds/10cm²)约高出3倍.冬季不同断面间的丰度无显著差异,但夏季断面间差异显著(p<0.05),尤其在排水口附近的A断面丰度最低,为1002.8inds/10cm².SIMPER分析结果显示海洋线虫、介形类、腹足类、涡虫、桡足类是各断面之间非相似的关键类群,这些类群的迁移指示电厂温排水已导致排水口附近海域小型底栖群落结构不稳定,不仅水平分布有差异,垂直分布也明显不同.     

象山港小型底栖动物群落结构及其与环境因子的相关性

为阐明象山港小型底栖动物群落的结构和组成,于2011年10月(秋)、2012年2月(冬)、5月(春)和8月(夏),对小型底栖动物进行12个站位的取样分析,结果表明:共鉴定出12个类群,包括线虫、介形类、双壳类、腹足类、寡毛类、多毛类、涡虫类、桡足类、端足类、涟虫类、水螅幼体和其他类。春、夏、秋、冬各季节的小型底栖动物平均丰度分别为(22.3±34.4)个/10 cm~2、(74.8±140.8)个/10 cm2、(31.4±64.5)个/10 cm~2和(97.4±206.5)个/10 cm~2,且以线虫和介形类为主要优势类群;相应季节的平均生物量分别为(73.0±144.4)μg/10 cm~2、(1261.7±2244.1)μg/10 cm~2、(440.7±1003.7)μg/10 cm~2和(1010.5±2365.6)μg/10cm~2,介形类为主要贡献类群。相似性分析(ANOSIM)显示,各季节间小型底栖动物群落结构差异显著(R=0.085,P=0.001)。据小型底栖动物全年平均丰度的MDS分析,在空间上将12个站位分成3组,其中,港底区组分别与其他2组群落结构差异显著,这些组间的主要差异类群是介形类、线虫、腹足类和多毛类。同时,对相应样品的14个沉积环境因子(水深、温度、盐度、水份、有机碳、叶绿素a、pH值、氧化还原电位、电导率、溶解固体,以及沉积物的砂、粉砂、粘土含量和中值粒径)进行了监测,结果发现,象山港沉积物以粘土和粉砂为主,中值粒径在港口区较高。沉积物中,有机碳季节间差异明显,春、夏有机碳含量高于秋、冬季节;而叶绿素a含量在春季和港底区较高。据生物-环境(BIOENV)分析,小型底栖动物与环境因子间相关系数仅为0.270,筛选的变量子集为砂含量、含水率、水深和中值粒径。进一步的spearman相关分析显示,小型底栖动物丰度与叶绿素a和砂含量呈显著正相关,与水深、盐度呈显著负相关;小型底栖动物生物量与砂含量呈显著正相关,分别与盐度、粉砂含量呈显著负相关。主要类群线虫丰度与叶绿素a呈显著正相关,而介形类丰度与砂含量呈显著正相关,分别与氧化还原电位、盐度呈显著负相关。寡毛类、桡足类与环境因子间均未检出显著相关性。     

2005年夏季东太平洋中国多金属结核区小型底栖生物研究

“大洋一号”调查船于2005年7月在太平洋结核区中国合同区东、西小区调查中共进行了12个站位的小型底栖生物的取样.研究表明:调查区域小型底栖生物共有13个类群.东、西两小区小型底栖生物平均密度分别为(104.4±20.48)个/10 cm2,(40.26±25.84)个/10 cm2,线虫平均生物量分别为(5.25±0.99)(μg·干重)/10 cm2,(1.68±0.77)(μg·干重)/10 cm2,东小区小型底栖生物平均密度、线虫生物量明显高于西小区.线虫是绝对优势类群,分别占东、西小区小型底栖生物总密度的93.13％和91.36％,其它优势类群有桡足类,多毛类,介形类等.小型底栖生物密度随着沉积物深度的增加而减少,其数量的75％以上分布在0-2 cm层,东小区深层的小型底栖生物所占比例比西小区要大.与环境因子相关分析表明:调查区域的小型底栖生物密度,线虫密度,线虫生物量,桡足类密度,多毛类密度与经、纬度呈显著负相关,与脱镁叶绿素呈显著正相关,与沉积物微型生物生物量相关性不显著.小型底栖生物密度,线虫密度,线虫生物量,多毛类密度与叶绿素a呈显著正相关,桡足类与叶绿素a相关性不显著.脱镁叶绿素与沉积物微型生物生物量呈显著正相关.     

Sexual dimorphism in the energetics of reproduction and growth of North Sea plaice, Pleuronectes platessa L.

The chemical composition and energy content of North Sea plaice during the spawning period were examined in mature males and females and in immature fish, to study differences in the allocation of energy over reproduction and somatic growth between the sexes. At the beginning of the spawning period mature males and females had equal dry weights of lipid that were 70% higher than in immatures. Protein content in mature males was equal to that in immatures but was 23 % higher in mature females. Immature males and females did not differ in chemical composition. At the end of the spawning period, spent and immature fish had equal lipid contents, but protein content in spent females was 10% lower than in spent males, and 17% lower than in immatures. Gross energy content of the body decreased by 44% (65·2 to 36·3 J cm^-3) in mature females, 27% (55·0 to 40·OJ cm^-3) in mature males, and 9% (48·7 to 44·2J cm^-3) in immatures. Energy content of plaice eggs was estimated at 6·60 kJ per 1000 eggs. Reproductive investment was estimated from the energy loss during the spawning period and included the energy of sex products and spawning metabolism. Somatic growth comprised the annual increase in energy content of fish. The pattern of energy allocation over reproduction and somatic growth differed between males and females. Males started their reproduction at a smaller length and a younger age and allocated a higher proportion of the available energy into reproduction than females. Available energy resources for somatic growth and reproduction (surplus production) were equal between the sexes up to a length of about 30 cm. Beyond this length male surplus production levelled off whereas female surplus production continued to increase. The differences in surplus production and the allocation patterns are discussed. For female plaice the energy allocated into egg production was estimated as between 48 and 64% of the total amount of energy lost during spawning. The remaining energy is used for metabolism during the spawning period, yielding an estimate of the metabolic rate of mature females of between 6·4 and 9·1 kJ day^-1. A maximum estimate of the metabolic rate of mature males was 7·4 kJ day^-1.     

Horizontal and vertical distribution of meiofauna on sandy beaches of the North Sea (The Netherlands, Belgium, France)

Sandy intertidal zones were analysed for the presence of meiofauna. The material was collected on six macro-tidal sandy beaches along the North Sea (The Netherlands, France, Belgium), in order to analyse the vertical and horizontal meiofaunal distribution patterns. Eleven higher meiofauna taxa (one represented by larval stage—Copepoda nauplii) were recorded. The maximum total meiofauna abundance was observed on the Dutch beach (4,295±911 ind. 10 cm⁻²) in the Westerschelde estuary, while the lowest values (361±128 ind. 10 cm⁻²) were recorded in France at the Audresselles beach. Meiofauna of the different localities consisted mainly of nematodes, harpacticoids and turbellarians. Nematodes numerically dominated all sampled stations, comprising more than 45% of the total meiofauna density. Meiofauna was mainly concentrated at the sand surface with about 70% present in the uppermost 5 cm. Meiofauna occurred across the entire intertidal zone. A clear zonation pattern in the distribution of meiofauna taxa across the beaches was observed. The present work suggests that designation of exposed sandy beaches as physically controlled (McLachlan 1988) does not explain their biological variability.     

Community structure of the deep sea fishes in the northern Gulf of Aqaba,Red Sea (Osteichthyes and Chondrichthyes)

The aim of this study was to investigate the community structure of deep sea fishes in the northern Gulf of Aqaba. Deep fish traps, short lines and long lines were deployed at depth ranges from 60 to 700 m between June 2014 and May 2015. A total of 369 fish individuals belonging to 37 species in 21 families were collected. The most abundant family observed in deep fish traps and short line was the commercially important family Sparidae, whereas the most abundant family in long line catch was the commercially unimportant fish family Muraenidae. The most abundant fish species sampled by deep fish traps and shortline was Blueskin Seabream, Polysteganus coeruleopunctatus. The most abundant species in long line catch White-spotted Moray, Gymnothorax johnsoni. In fish traps and with short line, the most commonly caught species was Blueskin Seabream. White-spotted Moray was the most common long line catch. Depth distribution for 37 deep fish species and GIS maps for the two main commercial fish species Blueskin Seabream and Bigeye Hound Shark, Iago omanensis were documented.     

Effects of the severe winter 1995/96 on the benthic macrofauna of the Wadden Sea and the coastal North Sea near the island of Sylt

The development of benthic macrofauna in the Wadden Sea and in the coastal North Sea after the severe winter of 1995/96 is compared with the preceding years with mild to moderate winters. In the intertidal of the Wadden Sea, ice-drift and low temperature caused the expected changes in species composition by increasing winter mortality in sensitive species, and by exceptionally high recruitment of some species during the succeeding summer. In the shallow subtidal (10–20 m depth), similar winter effects were observed. However, recovery of many subtidal populations was still incomplete until the summer of 1997. It is suggested that this was due to hydrographic conditions that carried many larvae or drifting juveniles into more distant offshore areas. This may have limited larval supply and may have delayed recovery at the onshore sites. Since in the eastern North Sea severe winters are accompanied by frequent easterly winds, it is not clear whether decreasing winter abundances in some species were due to increased mortality, or to a seaward dislocation of organisms. Received in revised form: 7 May 2001 Electronic Publication     

渤海中、南部大型底栖动物的群落结构

在对渤海中、南部大部分海区 3个航次的大型底栖动物现存量、生产力和物种多样性研究的基础上 ,对群落结构及其与环境因子的关系进行了分析。结果表明 ,种群的相对重要性存在着年份和季节的差异 ,与大型底栖动物群落关系最为密切的环境因子为水深和底层水中的硝酸盐浓度。底层水中的 NO3-N和 PO4-P含量、N∶ P值及大型底栖动物的类群组成显示 ,渤海已成为典型的富营养化海域 ,有些群落已有受到污染或富营养化扰动的趋势 ,但总体上污染尚未给研究海域的大型底栖动物群落结构带来明显影响。与 2 0世纪 80年代初相比 ,渤海大型底栖动物群落组成发生了一些变化 ,高速的富营养化进程、渔业活动的加剧以及底栖动物捕食者的改变 ,可能是造成变化的原因     

渤海小型底栖动物丰度的分布格局

在渤海的 2 2个站位 ,分 3个航次采集未受扰动的沉积物样品 ,进行了小型底栖动物类群的丰度、分布格局及其与沉积环境因子间相互关系的研究。结果表明 ,1 997年航次 ,5个站位小型底栖动物的平均丰度为 2 2 74± 1 0 3 9ind./1 0 cm2 ;1 998年和 1 999年航次 ,小型底栖动物的丰度分别为 869± 5 0 9ind./1 0 cm2和 63 2± 3 99ind./1 0 cm2 ,其中 ,小型底栖动物和自由生活海洋线虫丰度的高值主要出现在渤海中东部和海峡口的站位 ,底栖桡足类的丰度在海峡口的 A4、E5、D5站和辽东湾湾口的几个站位较高。在小型底栖动物中 ,线虫是数量上占绝对优势的类群 ,桡足类位于第 2位 ,处在第 3位的类群在两个航次中有所不同 ,在 1 998年航次 ,双壳类幼体的数量位于第 3位 ;1 999年航次 ,多毛类的数量位于第 3位。对小型底栖动物丰度与其沉积环境因子的分析表明 ,水深与小型底栖动物丰度、自由生活海洋线虫丰度和桡足类丰度的相关性为极显著 ;沉积物的中值粒径与桡足类的丰度和小型底栖动物总丰度呈负相关 ,前者为极显著 ,后者为显著 ;砂、粉砂和粘土含量影响三者的丰度变动 ,其中与桡足类丰度的相关性为极显著或显著。沉积物中的叶绿素 a、脱镁叶绿酸 a、含水量和有机质含量与三者丰度的关系不很明显。     

Benthic meiofauna assemblage structure of headwater streams: density and distribution of taxa relative to substrate size

Permanent meiofauna taxa and portions of the population of other invertebrates that are temporarily in the meiofauna size class are often precluded from stream studies and assessments. This study was designed to determine the identity, density, and distribution of major meiofauna taxa relative to substrate size in a set of similar headwater streams. Using a coring technique, meiofauna (80 μm–1 mm) and substrate samples were collected from 11 Ozark headwater streams in the Boston Mountain ecoregion of Arkansas, USA. Mean meiofauna density among streams was 1739 ± 436 organisms per l. Permanent meiofauna taxa (Copepoda, Cladocera, Ostracoda, Rotifera, Nematoda, Hydrachnida, and Tardigrada) comprised 22.5% of the organisms collected with a mean density of 394 ± 233 organisms per l; temporary meiofauna taxa (Oligochaeta, Turbellaria, Hydroidea, Chironomidae, Ephemeroptera, and other insects) comprised the remainder with a density of 1346 ± 308 organisms per l. Chironomidae was the most numerous temporary meiofauna taxon, and Hydrachnida was the most numerous permanent taxon. Streams were found to differ significantly in substrate composition and densities of major taxonomic categories. Substrate size was found to predict densities for most of these taxonomic categories. Meiofauna patchiness was reflected in high variability within streams. Canonical correspondence analysis revealed positive associations between Copepoda and Nematoda and silt, and between Copepoda, Nematoda, and Rotifera and fine sand. Hydrachnida and Rotifera were negatively associated with silt and coarse sand, respectively. The potential value of inclusion of meiofauna in stream environmental assessments is discussed.     

Marine molluscs and fish as biomarkers of pollution stress in littoral regions of the Red Sea, Mediterranean Sea and North Sea

The intensive development of industry and urban structures along the seashores of the world, as well as the immense increase in marine transportation and other activities, has resulted in the deposition of thousands of new chemicals and organic compounds, endangering the existence of organisms and ecosystems. The conventional single biomarker methods used in ecological assessment studies cannot provide an adequate base for environmental health assessment, management and sustainability planning. The present study uses a set of novel biochemical, physiological, cytogenetic and morphological methods to characterize the state of health of selected molluscs and fish along the shores of the German North Sea, as well as the Israeli Mediterranean and Red Sea. The methods include measurement of activity of multixenobiotic resistance-mediated transporter (MXRtr) and the system of active transport of organic anions (SATOA) as indicators of antixenobiotic defence; glutathione S-transferase (GST) activity as an indicator of biotransformation of xenobiotics; DNA unwinding as a marker of genotoxicity; micronucleus test for clastogenicity; levels of phagocytosis for immunotoxicity; cholinesterase (ChE) activity and level of catecholamines as indicators of neurotoxicity; permeability of external epithelia to anionic hydrophilic probe, intralysosomal accumulation of cationic amphiphilic probe and activity of non-specific esterases as indicators of cell/tissue viability. Complete histopathological examination was used for diagnostics of environmental pathology. The obtained data show that the activity of the defensive pumps, MXRtr and SATOA in the studied organisms was significantly higher in the surface epithelia of molluscs from a polluted site than that of the same species from control, unpolluted stations, providing clear evidence of response to stress. Enhanced frequency of DNA lesions (alkaline and acidic DNA unwinding) and micronucleus-containing cells was significantly higher in samples from polluted sites in comparison to those from the clean sites that exhibited genotoxic and clastogenic activity of the pollutants. In all the studied molluscs a negative correlation was found between the MXRtr levels of activity and the frequency of micronucleus-containing hemocytes. The expression of this was in accordance with the level of pollution. The complete histopathological examination demonstrates significantly higher frequencies of pathological alterations in organs of animals from polluted sites. A strong negative correlation was found between the frequency of these alterations and MXRtr activity in the same specimens. In addition to these parameters, a decrease in the viability was noted in molluscs from the polluted sites, but ChE activities remained similar at most sites. The methods applied in our study unmasked numerous early cryptic responses and negative alterations of health in populations of marine biota sampled from the polluted sites. This demonstrates that genotoxic, clastogenic and pathogenic xenobiotics are present and act in the studied sites and this knowledge can provide a reliable base for consideration for sustainable development. Received: 2 March 1999 / Received in revised form: 2 August 1999 / Accepted: 3 August 1999     

Mammalian turnover and community structure in the Paleocene of North America

The effects of faunal turnover on mammalian community structure are evaluated for 17 faunal zones of the North American Paleocene through early Eocene land mammal ages (Puercan through early Wasatchian). Generic disappearances were significantly high at the end of the Puercan, Torrejonian, and Tiffanian land mammal ages, but appearances were significantly high only during the early Puercan. Generic richness rose rapidly in the early Puercan, remained stable throughout most of the Paleocene, and increased from the late Paleocene into the early Eocene. The null hypothesis that generic turnover clustered preferentially according to dentally defined trophic or body size categories could be rejected or attributed to sampling problems for all but the early (Pu0) and late Paleocene (Ti5‐Cf2). Early Paleocene change in community structure most probably represented endemic radiation of mammals into previously unoccupied niches. Community restructuring in the late Paleocene reflected a complex of causes, including climatic wanning, intercontinental dispersal, and competition.     

Macrofauna communities of the Dogger Bank (central North Sea) in the late 1990s: spatial distribution,species composition and trophic structure

Macrofauna samples were taken in May 1996–1998 at 28 stations on the Dogger Bank (central North Sea) to describe species composition, spatial distribution and trophic structure of the macrofaunal communities. In general, five communities could be distinguished depending on the influence of different water masses, depth, type of sediment and food availability. Except for those species which feed in the sediment, macrofauna communities did not show any correlation to the measured sediment variables. In fact, macrofauna communities of the Dogger Bank were mainly structured by lateral food supply via frontal systems. This was indicated by the dominance of interface feeders foraging in the benthic boundary layer, which are able to switch from surface deposit to suspension feeding depending on flow conditions. The importance of sandlickers in the shallowest parts of the Dogger Bank hints at an important contribution of benthic primary production to the nutritional and energetic needs of the benthic community. Electronic Publication     

Reinterpreting two regime shifts in North Sea plankton communities through the lens of functional traits

Aim

The so-called regime shifts in North Sea plankton communities provide an important historical case study to understand marine regime shifts. Previous studies characterized regime shifts using a variety of community metrics (e.g., indicator species abundances, taxonomic composition and chlorophyll biomass) but left the functional traits of plankton unassessed. Here, we explicitly re-assess the historically recognized North Sea regime shifts through the lens of plankton functional traits to gain a better understanding of these events.

Location

The North Sea (NW European shelf).

Time period

1958–2018, focusing on the 1980s and 1996–2003 regime shifts.

Major taxa studied

Marine phyto- and zooplankton.

Methods

We compute trait spaces for both phyto- and zooplankton of the North Sea using traits from the literature and a Gower's distance-based method. Using abundance data from the Continuous Plankton Recorder Survey, we then compute monthly time series of the centroids of the communities, an indicator of functional composition. We then use principal component analysis on the centroids to assess the main temporal changes in plankton functional composition associated with the 1980s and 1996–2003 regime shifts.

Results

Little change in plankton functional composition was associated with the 1980s regime shift. In contrast, the functional composition of plankton communities changed markedly after the 1996–2003 regime shift, with an increase in the summer relative abundance of non-motile autotrophs (i.e., diatoms) and the spring relative abundance of meroplankton.

Main conclusions

The North Sea regime shifts were not associated systematically with changes in functional composition, calling into question the definition of regime shifts and illustrating the importance of taking different metrics into account to interpret ecological events accurately. Taking into account functional composition, we interpret the 1980s so-called regime shift as a latitudinal shift in communities that was insufficient to impact functional composition and the 1996–2003 so-called regime shift as a period of change in bentho-pelagic coupling.     

春季东海海域虾类群落结构及其多样性

依据1998年5月(春季)在东海26°00′N～33°00′N、127°00′E以西海域的虾类资源调查资料,分析了虾类的种类组成、优势度、丰度、生物量的空间分布,多样性、相似性等群落结构特征.结果表明,东海大陆架春季共有虾类52种,已鉴定的43种,隶属于13科27属,其中优势种有戴氏赤虾(Metapenaepsis dalei)、假长缝拟对虾(Parapenaeus fissuroides)、须赤虾(Metapenaepsis barbata)、葛氏长臂虾(Palaemon gravieri)、中华管鞭虾(Solenocera crassicornis)、东海红虾(Plesionika izumiae)、鹰爪虾(Trachypenaeus curvirostris)、方板赤虾(Metapenaepsis tenella)、长角赤虾 Metapenaepsis longirostris、高脊管鞭虾(Solenocera alticarinata)、细巧仿对虾(Parapenaeopsis tenella)、周氏新对虾(Metapenaeus joyneri)、脊腹褐虾(Crangon affinis)和凹管鞭虾(Solenocera koellbeli).东海大陆架春季虾类可划分为3个生态群落,即广温低盐生态群落、广温广盐生态群落、高温高盐生态群落.丰度和生物量的平面分布趋势相似,以30°00′N为界限,30°00′N以北站位普遍较低,以南站位较高,主要集中在舟山渔场南部和鱼山渔场.多样性分析表明:Margalef的种类丰富度指数(D)和Shannon-Wiener多样性指数(H′)较高的站位主要集中在舟山渔场以北,整个海区的均匀性指数(J′)较为稳定.运用聚类和多维标度对群落结构的相似性进行分析,结合水深等环境因子对东海大陆架虾类的群聚类型进行了讨论.     

Evidence for density-dependent growth in North Sea gadoids

The classical view on density-dependent growth is that an increase in population density heightens intraspecific competition for food, leading to reduced food intake and depressed growth. This paper summarizes the analysis of English Groundfish Survey, International Young Fish Survey and ICES Roundfish Working Group data for evidence of density-dependent growth of I- and II-group cod, haddock and whiting in the North Sea.
Within Roundfish areas there was a negative relationship between growth and density in some instances, but it was small. However, there were persistent differences in both growth rate and density between Roundfish areas. In all cases, except for I-group cod, there appeared to be a tendency for fish in low-density areas to be larger than fish in high-density areas; this might be due to feeding competition but other explanations are possible. The results urge caution in interpreting commercial catch data for evidence of density-dependent growth, since changes in fishing effort in particular areas could significantly bias the conclusions.
There was evidence of enhanced survival of small fish in good year-classes; this would depress the mean size at age, thereby mimicking the effects of density-dependent growth.     

Community structure and biogeography of shore fishes in the Gulf of Aqaba,Red Sea

Shore fish community structure off the Jordanian Red Sea coast was determined on fringing coral reefs and in a seagrass-dominated bay at 6 m and 12 m depths. A total of 198 fish species belonging to 121 genera and 43 families was recorded. Labridae and Pomacentridae dominated the ichthyofauna in terms of species richness and Pomacentridae were most abundant. Neither diversity nor species richness was correlated to depth. The abundance of fishes was higher at the deep reef slope, due to schooling planktivorous fishes. At 12 m depth abundance of fishes at the seagrass-dominated site was higher than on the coral reefs. Multivariate analysis demonstrated a strong influence on the fish assemblages by depth and benthic habitat. Fish species richness was positively correlated with hard substrate cover and habitat diversity. Abundance of corallivores was positively linked with live hard coral cover. The assemblages of fishes were different on the shallow reef slope, deep reef slope and seagrass meadows. An analysis of the fish fauna showed that the Gulf of Aqaba harbours a higher species richness than previously reported. The comparison with fish communities on other reefs around the Arabian Peninsula and Indian Ocean supported the recognition of an Arabian subprovince within the Indian Ocean. The affinity of the Arabian Gulf ichthyofauna to the Red Sea is not clear. Received in revised form: 2 November 2001 Electronic Publication     

Spatial variability of epifaunal communities in the North Sea in relation to sampling effort

Benthic epifauna was sampled in six areas from the German Bight towards the Norwegian Sea using a 2-m standard beam trawl. Nine replicates were taken in each area and year from 1999 to 2006. This data set (60–67 replicates per area) was used to describe the spatial variability in local species composition and to assess the effect of increasing sampling effort on species richness and community structure. Our results confirmed the importance of the 50-m depth contour for the separation of benthic fauna in the North Sea. Low species richness, sparse sessile fauna and high abundances of scavenging species such as Asterias rubens, Liocarcinus holsatus, Astropecten irregularis and Ophiura albida were characteristic of the low-water area south of the 50-m contour. Differences in community structure were less conspicuous in deeper waters north of the 50-m contour, but distribution patterns and abundances of single species such as Echinus elegans, Hyalinoecia tubicola, Ophiothrix fragilis, Scaphander lignarius as well as several hermit crabs resulted in well-defined epifaunal communities. One replicate caught 17–28% of the species found in 60–67 samples and was sufficient to separate the community in the German Bight from those in the central and northern North Sea by using multidimensional scaling. Nine replicates sampled a proportion of 53–60% and provided additional information on the spatial variability of community structure in the central and northern North Sea. Our study indicates that appropriate replication enhances the quality of the data and can partly overcome the constraints of sampling with a 2-m beam trawl. This might be helpful for future monitoring programmes.     

Seasonal variability of inorganic and organic nitrogen in the North Sea

This study considers the cycling of nitrogen in the waters of the North Sea, particularly focussing on organic nitrogen. Dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and particulate organic nitrogen (PON) were measured in the North Sea over a one-year period (autumn 2004–summer 2005). The surface water concentrations of nitrate, ammonium, DON and PON during the present study ranged from <0.1–7.2 μM, <0.1–2.0 μM, 1.9–11.2 μM and 0.3–5.6 μM, respectively, with DON the dominant fraction of total nitrogen at all times. These nutrients concentrations were significantly lower compared to previous studies in the southern North Sea. The seasonal variations showed high mean surface concentrations of nitrate (4.7 ± 0.6 μM) and DON (8.9 ± 0.9 μM), low ammonium (<0.1 μM) and PON (0.8 ± 0.1 μM) in winter, shifting to low nitrate (0.3 ± 0.3 μM) and DON (4.2 ± 1.2 μM) in summer, with high ammonium (0.8 ± 0.4 μM) in autumn and PON (2.5 ± 1.2 μM) in spring. Highest mean surface DON concentration was measured in winter and may be due to resuspension of the organic matter from the bottom sediments. For autumn and spring, phytoplankton DON release was likely to be the most significant source of DON as shown by high concentrations of low molecular weight (LMW) DON and its positive correlation to chlorophyll a. Low total and LMW DON concentrations during summer were likely to be due to the uptake of the LMW DON fraction by phytoplankton and bacteria and the stratification of the water column. DON is therefore shown to be a potentially important source of nitrogen in shelf seas especially after the spring bloom has depleted nitrate to limiting concentrations. Handling editor: L. Naselli-Flores