Distribution and trophic links of gelatinous zooplankton on Dogger Bank,North Sea

The ecology of small, gelatinous zooplankton is not integrated into management of Dogger Bank (54° 00′ N, 3° 25′ E to 55° 35′ N, 2° 20′ E). In pursuit of this goal, gelatinous zooplankton and their potential prey were sampled along a transect across the bank on June 10–16, 2007. Eleven species of small medusae and ctenophores were collected, with six abundant taxa occurring in greater numbers below the thermocline and in the shallower, southeastern portion of the bank. There were no statistically significant diel changes in distribution. In contrast, potential prey were distributed more evenly across the bank and throughout the water column. Isotopic analyses revealed that gelatinous zooplankton fed on both smaller (100–300 μm) and larger (>300 μm) mesozooplankton, but also potentially on each other. These ecological insights suggest that small medusae and ctenophores should be integrated into sustainable management of Dogger Bank.     

Supply of organic matter to the sediment in the northern North Sea during a spring phytoplankton bloom

Strings of moored sediment traps were deployed in a 150 m water column over a period covering the growth and collapse of the spring bloom (4 April–3 June 1976) in an area of the northern North Sea. The efficiency of collection of material in the moored traps was compared to collections in free-drifting traps in the same area of deployment. The ways in which the data from the trap collections may be interpreted was considered at some length and a best estimate of the flux of organic carbon and nitrogen to the sediment was made. For the period prior to the spring bloom (4–23 April) this flux was 50 mg C m^–2 d^–1 (about 20% of primary production). During the bloom (24 April–19 May) it was about 185 mg C m^–2 d^–1 (35% of production) and during early summer (20 May–3 June) it was 115 mg C m^–2 d^–1, about 25% of the overlying production. The organic carbon and nitrogen content of the material collected was measured and the material was examined microscopically. There was evidence of a large settlement of diatoms immediately after the spring bloom which was reflected in changes in the C:N and C:chlorophyll ratios of the material collected. This change in biochemical composition of the material may affect its nutritional quality and have a stimulatory effect on the growth and reproduction of the animals living in the sediment.     

Quantitative relationships among phytoplankton body size classes and production processes in the North Adriatic frontal region

The variation patterns of phytoplankton standing crop and productivity in the North Adriatic frontal region and the relative importance of pico-, nano-, micro-phytoplankton are shown. Data of standing crop (chlorophyll a - Chl a ) and productivity ( 14 C assimilation) and PAR radiation ( w Em m 2 r s m 1 ) were collected during the four oceanographic cruises of the PRISMA II project. Average standing crop and productivity in the study area were 1.41 - 0.42 r mg r m m 3 Chl a and 1.23 - 0.37 r mg r C r m m 3 r h m 1 ; average assimilation number (P/B) was 0.872 - 0.589 r mg r C (mg-Chl a ) m 1 h m 1 and average photosynthetic efficiency (PE) was 0.020 - 0.054 (mg r C(mg-Chl a ) m 1 r h m 1 ) [ w Em m 2 r s m 1 ] m 1 . Phytoplankton biomass and productivity showed significant patterns of variation with the distance from the coast, with increasing depth and decreasing light intensity. The same patterns were shown by the three phytoplankton size classes. The spatio-temporal variations were significantly larger within the microplankton than within the pico- and nano-plankton size classes. Planktonic guilds were dominated by picoplankton, both as standing crop and productivity, in the northern stations (0.539 - 0.21 r mg r m m 3 Chl a and 0.572 - 0.25 r mg r C r m m 3 r h m 1 ) and in those more offshore, while microplankton was more important in the coastal and southern stations (0.727 - 0.58 r mg r m m 3 Chl a and 0.63 - 0.28 r mg r C r m m 3 r h m 1 ). In relative terms, picoplankton accounts for the 53% and 46% of biomass and primary production, while the microplankton account for the 43.6% and 48%. Assimilation number and photosynthetic efficiency did not show spatio-temporal variations but PE was inversely related with PAR radiation for all the size classes. Data suggest that the spatio-temporal patterns observed in this study are affected by the competitive relationships among body size classes in the phytoplankton guilds.     

Copepod grazing during a declining spring phytoplankton bloom in the Northern North Sea

Grazing rates of larger (Calanus finmarchicus) and smaller (Acartia clausii Pseudocalanus elongatus etc.) copepods on naturally occurring phytoplankton populations were measured during a declining spring phytoplankton bloom. During the initial period, dominated by Chaetoceros spp. diatoms, constant ingestion rates were observed in Calanus finmarchicus at suspended particulate concentrations above 300 g carbon l^-1. Average daily intake during this time amounted to 35 to 40% of body carbon and reached a maximum of 50%. The feeding response of the smaller copepods was not so well defined, although a maximum daily intake of 56% body carbon was recorded. In both groups, feeding thresholds were at particulate concentrations around 50 g C l^-1. The feeding response of C. finmarchicus was correlated with both a change in their own population and in the food cell type. Linear regressions describing the concentration-dependent feeding response were: ingestion rate (IR)=1.16 total particulate volume (TPV)-36.15 during the initial part of the period compared with IR=0.41 TPV-12.18 for the latter period. C. finmarchicus filtered out slightly larger (x 1.2 diameter) particles than the small copepods and, in both groups, some filtering adjustment was made to accomodate to modal changes in the phytoplankton population from 20–30 m to 10 m diameter cells. Particle production during feeding was frequently evident in the smallest size ranges of particles and the ratio of particle production to ingestion rate was greater at low feeding rates.     

Growth and development of Calanus spp. (Copepoda) during spring phytoplankton succession in the North Sea

Experiments were carried out to determine growth and development rates of the herbivorous copepod Calanus finmarchicus (Gunnerus) under natural conditions during the phytoplankton spring bloom in the northern North Sea. From 28 April to 25 May 1983 copepodite stages I, IV and V were incubated for a 3-d period on board a ship in vessels with naturally occurring phytoplankton or cultured algae as food. Highest rates of growth and development were achieved while the diatom Chaetoceros sp. was the dominant phytoplankton organism. These rates decreased considerably when this chain-forming diatom was succeeded after one week by the small-celled diatom Thalassiosira conferta. Again one week later, during the bloom of the succeeding colonial microflagellate Corymbellus aureus, copepodite stage IV still managed to maintain moderate rates of growth and development, but these rates dropped to almost zero in CV, suggesting the start of a resting stage. Nevertheless, brood collected from this generation and from Calanus helgolandicus (Claus) was raised in the laboratory to the adult stage at high speed. Since temperature and the total phytoplankton concentration in the sea remained almost constant it seems that the retardation and arrestment of growth and development were an immediate response to a qualitative change of the food composition related to the successive blooms of different algal species.     

Variability of epifauna and temperature in the northern North Sea

Benthic epifauna in three areas of the northern North Sea was studied from 1999 to 2007 to investigate the effect of temperature changes on community structure and species abundance and biomass. Abundance and/or biomass of 16 epifauna species was significantly correlated with temperature anomalies of the mean sea surface temperature (SST) from 1971 to 2000. The response of species to SST changes was different in the study areas depending on species life history and, most likely on food supply, which in turn is strongly influenced by the timing and duration of primary production and regional hydrographical conditions (e.g. stratification). Also, changes in community structure were obvious in the three areas between 2002 and 2003 coinciding with high temperature anomalies and SST. On the other hand, these changes were mainly caused by the variability in abundance of dominant species and altogether no clear trends in community structure were found. In contrast to epifauna communities in the shallow southern North Sea temperature changes in the northern North Sea affected only single epifauna species until now.     

Comparative biometry of Eunicella singularis (Gorgonian) sclerites at East Mediterranean Sea (North Aegean Sea, Greece)

The two forms of Eunicella singularis sclerites, spindles and clubs, were examined in two sites in East Mediterranean Sea, Greece. The comparison of the sclerites size was based on the hydrodynamic profile of the sites (the site of Arethoussa was exposed and the Phidonissi site was sheltered), on the sites bathymetry (AR 5–8 m, AR 9–13 m, PH 5–10 m, PH 11–13 m) and on the position occupied by the sclerites on the gorgonian colonies (top, middle and base). With the usage of image system analysis the following microfeatures were measured or calculated: projection, length, width, elongation factor, compactness factor, length × width and length/width. The present study confirms the hypothesis that spindles are involved in the recovering of the gorgonian colonies and clubs contribute to the flexion capacity of the colonies. The spindles of the shallow E. singularis colonies (AR 5–8 m) were significantly bigger (length, width) compared with the others, in order to assure the best recovery of the colonies after their flexion, induced by higher water current velocity in shallow water. The clubs occupy the intern part of the mesoglea and their size (length, width) increased in colonies from the shallow exposed site in order to facilitate the gorgonian flexion. The present study demonstrates the gorgonian adaptation (in a microscopic scale) to the environmental pressure of hydrodynamic conditions.     

Changes in the phytoplankton community at Helgoland, North Sea: lessons from single spot time series analyses

In recent regime shift analyses, the phytoplankton compartment of the marine food web was essentially represented by phytoplankton color or chlorophyll concentration. A detection of changes directly at the species level is highly desirable. The Helgoland Roads data series, a collection of high frequency long-term time series comprising biological and physico-chemical components of the southern North Sea, allow such an investigation at the level of single species. Aiming at a detection and characterization of habitat and community changes in the observation period (1962 until the end of 2008), we selected six species as representatives of certain classes, for example, benthic or neritic species, and applied a combination of novel analysis methods—a fitness-based analysis of the realized niche, a bloom-triggered averaging and a Markovian analysis of co-occurrence and succession patterns—to related abundance time series and concurrent environmental parameter time series. We found a general trend toward enlargement of niche size and shifts of the niche position, interesting salinity patterns around bloom events of two species, and statistically highly significant changes of a phytoplankton community segment after 1965 and after 1998. Interpreting our observations in ecological terms leads to the formulation of testable hypotheses.     

Continuous Plankton Records: Seasonal cycles of phytoplankton and copepods in the North Atlantic ocean and the North Sea

Data from the Continuous Plankton Recorder survey of the North Atlantic Ocean and the North Sea are used to study geographical variations in the amplitude, duration and timing of the seasonal cycles of total phytoplankton and total copepods. It is shown that the distribution of overwintering stocks influences the distributions throughout the year. There is a relationship between the timing of the spring increase of phytoplankton and the amplitude of the seasonal variation in sea surface temperature. In the open ocean, the timing of the spring increase of phytoplankton corresponds with the spring warming of the surface waters. In the North Sea the spring increase occurs earlier, associated, perhaps, with transient periods of vertical stability, resulting in a relatively slower rate of increase. It is suggested that in the open ocean the higher rate of increase is under-exploited by copepods due to low overwintering stocks and longer generation times. Exceptionally early spring increases of phytoplankton off the west coast of Greenland and over the Norwegian shelf are probably associated with permanent haloclines. A high and late autumn peak of phytoplankton off the coast of Portugal may be associated with coastal upwelling.     

Demands of the herbivore community on phytoplankton production in the Celtic Sea in August

The apodous detritivore holothurian Leptosynapta galliennei (Herapath, 1865) was collected from the intertidal zone (Brittany, France) in spring 1983, and the effect of short-term starvation on the biochemical composition of different body parts was studied. Variations in the levels of the main organic components of the somatic tissues did not correspond to the observed ponderal growth of the gonad in either males or females. Part of the increase in the energetic value of the testis was due to carbohydrates. Accumulation of carbohydrates, proteins, and especially lipids occurred in the ovary. Glycogen, often considered as the most readily mobilized energy source, varied little in concentration or in quantity in any body part. Lipids were not mobilized. No organ of this thin-skinned holothurian functions as a site of rapidly usable food reserves. The organic nitrogen and carbon necessary to compensate for metabolic loss and to permit gonadal growth during shortterm starvation may be provided by dissolved organic matter. An efficient transintegumentary uptake of dissolved organic matter, perhaps mediated by associated subcuticular bacteria, and/or intestinal uptake are hypothesized.     

Microphytobenthos of the Dogger Bank: a comparison between shallow and deep areas using phytopigment composition of the sediment

The surface sediment characteristics related to benthic microalgae primary production were studied at the Dogger Bank, North Sea, in order to evaluate the potential role of microphytobenthos as a food source for the macrobenthic fauna. Twenty-one stations were sampled in July 2001 and May 2002, with water depth ranging from 16.3 to 68.5 m. High-performance liquid chromatography pigment analyses revealed that concentrations of chlorophyll a, chlorophyll c and fucoxanthin are mainly associated with benthic diatom flora at most parts of the Dogger Bank. High percentage of phytopigments (>50%) was firmly attached to sand grains at the stations shallower than 40 m water depth. The deeper stations were characterized by a phytopigment composition originating from pelagic phytoplankton settled on the sea floor. Qualitative microscopy showed that the benthic microflora on top of the Bank mainly consists of small diatoms (5–10 μm), such as e.g., Diploneis spp., living attached to the sand grains. The results are discussed concerning possible implications for ecology and biogeochemistry of the Dogger Bank area.     

Succession and growth limitation of phytoplankton in the Gulf of Bothnia (Baltic Sea)

A one year field study of four stations in the Gulf of Bothnia during 1991 showed that the biomass was ca. two times, and primary productivity ca. four times, lower in the north (Bothnian Bay) than in the south (Bothnian Sea) during the summer. Nutrient addition experiments indicated phosphorus limitation of phytoplankton in the Bothanian Bay and the coastal areas in the northern Bothnian Sea, but nitrogen limitation in the open Bothanian Sea. A positive correlation between the phosphate concentration and the production/biomass ratio of phytoplankton was demonstrated, which partly explained the differences in the specific growth rate of the phytoplankton during the summer. Differences in photosynthetic active radiation between the stations also showed a covariation with the primary productivity. The relative importance of nutrient or light limitation for photosynthetic carbon fixation could not, however, the conclusively determined from this study. Marked differences in phytoplankton species composition from north to south were also observed. The number of dominating species was higher in the Bothnian Sea than in the Bothnian Bay. The distribution of some species could be explained as due to nutrient availability (e.g. Nodularia spumigena, Aphanizomenon sp.), while salinity probably limits the distribution of some limnic as well as marine species. The potentially toxic phytoplankton N. spumigena, Dinophysis acuminata and Chrysochromulina spp. were common in the Bothnian Sea but not in the Bothnian Bay. The pico- and nanoplankton biomass during late summer was higher than previously reported due to a revised carbon/volume ratio.     

Homing of eels (Anguilla anguilla) in the southern North Sea

Tagging and transplantation experiments on a total of 1538 cels (Anguilla anguilla L.) caught in the German Bight near the islands Helgoland and Föhr, as well as in the area northwest of the Elbe estuary near Cuxhaven and in the port of Hamburg, resulted in recapture of 77 eels (5.0%). Of the recaptured eels 64% were reported from the area of their original capture. Some homed from distances of 180 km. Eels tagged and released within the area of original capture (control experiments) were recaptured only in these areas. A maximum migration speed of at least 1 km/h was found in one case, and of 3 km/h in another. Average speed of migration may, however, have been much lower, since in most other cases recaptures were reported at least 2 weeks after release. The homing abilities demonstrated cannot in all cases be accounted for by olfactory sensitivity alone; home areas were in some cases located in such a way that the water circulation in the southern North Sea could not have provided any lead for the migrating eels.     

黄海春季表层叶绿素和初级生产力及其粒径结构研究

根据2006年4月对黄海浮游植物分级叶绿素及初级生产力的调查,研究了黄海叶绿素及初级生产力的水平分布及粒级结构特征,并分析了其主要影响因素。黄海海域调查站位表层叶绿素a质量浓度变化范围为0.20～4.94μg·L-1,平均值为0.96μg·L-1。叶绿素最大值出现在临近长江口的站位。叶绿素分级结果表明黄海春季以粒径〉5μm的浮游植物占优势。黄海表层初级生产力的变化范围为2.03～15.64mg·m-3·h-1,平均值为6.08mg·m-3·h-1。其中南黄海海域初级生产力平均为6.58mg·m-3·h-1,北黄海海域初级生产力平均为4.92mg·m-3·h-1。高值区分布在南黄海中部。受水体透明度的影响,低值区出现在临近长江口的站位。断面站位分析表明浮游植物初级生产力由北向南逐步升高,温度随纬度的变化是南北海域初级生产力水平差异的主要原因。由于粒径较小（〈5μm）的浮游植物单位叶绿素具有较高的碳固定能力,调查期间整个海区初级生产力以粒径〈5μm的浮游植物贡献为主。     

Mechanisms of production and fate of organic phosphorus in the northern Adriatic Sea

In the period from 1980 to 1984 organic phosphorus, nutrients, primary production rates (¹⁴C), chlorophyll a (chl a) standing crops, and basic oceanographic parameters were measured during 23 cruises at six stations in the open waters of the northern Adriatic Sea. These waters are significantly influenced by polluted Po River discharge. Organic phosphorus was correlated with several parameters which characterize phytoplankton activity and organic matter decomposition processes. In the late winter-spring period, organic phosphorus is produced during phytoplankton blooms. It is hypothesized that microzooplankton grazing is the main factor increasing the organic phosphorus concentrations in summer (up to 1.1 mol 1^-1). Fall and winter had much lower values (below 0.3 mol 1^-1) due to remineralization processes and an increased water mass exchange between the northern and central Adriatic regions. The direct contribution of organic phosphorus by freshwater discharge was not found to be significant. The higher organic phosphorus concentrations that can occur in low salinity waters are most likely due to their increased capability to support primary production.     

Analysis of phytoplankton pigments by HPLC before,during and after mass occurrence of the microflagellate Corymbellus aureus during the spring bloom in the open northern North Sea in 1983

Daily observations in April and May 1983 near a subsurface drifter launched in the Fladen Grounds area of the North Sea revealed that a large crop of the microflagellate Corymbellus aureus Green, producing up to 3 g C m^-2 d^-1, succeeded the diatoms dominating during the earlier phase of the spring bloom. The concentration of fucoxanthin was highest during the first half of May, while pigment fingerprints of high-performance liquid chromatograms of suspended matter sampled during the second half of May were dominated by 19-hexanoyloxyfucoxanthin, the main carotenoid of C. aureus (0.8x10^-9 mg cell^-1). This colonial Prymnesiophycean was described for the first time less than 10 years ago and has nerver been reported to be present in the North Sea. After 16 May the C. aureus population rapidly lost its growth potential, but the peak of the bloom in terms of cell number was found several days later (up to 9x10⁶ cells per dm³ on 19 May). The population then started to decline; senescence was associated with decreasing pigment contents of cells. The low growth rate of copepods registered during the second half of May was probably related to the poor quality of C. aureus as copepod food: the concentration of a phaeophorbide a typically found in copepod fecal pellets was highest during the diatom phase of the spring bloom preceding the C. aureus bloom.     

Dissolved organic substances during a Phaeocystis pouchetii bloom in the German Bight (North Sea)

Dissolved organic substances (carbohydrates, organic nitrogen, free amino acids) were measured in the German Bight (North Sea) in June, 1981. During and before this survey, sea foam was observed in the east Frisian coastal water and it accumulated on the nearby beaches to an unusually high extent. In this coastal water area, a large Phaeocystis pouchetii Lagerheim bloom and very high concentrations of dissolved organic matter were found. The above dissolved organic substances were all positively correlated to a highly significant degree with P. pouchetii cell numbers in the bloom area. An influence of salinity (or river water) on this correlation could be excluded. Thus, exudation or decomposition products of P. pouchetii were most likely the cause of the unusually high concentrations of dissolved organic matter in the bloom area off the east Frisian coast, where P. pouchetii blooms have been reported for many years. Ammonia concentrations were very low in the P. pouchetii bloom area; this and the accumulation of dissolved organic substances might lead to speculation that decomposition of dissolved organic matter by bacteria could have been reduced due to antibiotic activity of P. pouchetii.     

Genetic differentiation in Hediste diversicolor (Polychaeta: Nereididae) for the North Sea and the Baltic Sea

Genetic differentiation between North Sea and Baltic Sea Hediste diversicolor (O.F. Müller, 1776) (Polychaeta: Nereididae) populations was studied by allozyme electrophoresis on starch gel. Thirteen loci were analyzed in eight populations. The level of genetic variation was very low (mean H _o = 0.000 to 0.015). Differentiation between H. diversicolor populations is quite high (F _ST = 0.892) and reflected by three enzyme loci (MDH-I*, MDH-II*, IDH-I*). The reduced gene flow (N _m<1) may be explained by the limited dispersal capacity of the species. Regardless of whether found in the North Sea or Baltic Sea, there appear to be two different genetic types which are parapatric or sympatric in some places. The two types hybridize at three localities, but no signs of hybridization have been found at one (Tallinn). Received: 26 June 1997 / Accepted: 10 September 1997