The role of ciliates,heterotrophic dinoflagellates and copepods in structuring spring plankton communities at Helgoland Roads,North Sea

Mesocosm experiments coupled with dilution grazing experiments were carried out during the phytoplankton spring bloom 2009. The interactions between phytoplankton, microzooplankton and copepods were investigated using natural plankton communities obtained from Helgoland Roads (54°11.3′N; 7°54.0′E), North Sea. In the absence of mesozooplankton grazers, the microzooplankton rapidly responded to different prey availabilities; this was most pronounced for ciliates such as strombidiids and strobilids. The occurrence of ciliates was strongly dependent on specific prey and abrupt losses in their relative importance with the disappearance of their prey were observed. Thecate and athecate dinoflagellates had a broader food spectrum and slower reaction times compared with ciliates. In general, high microzooplankton potential grazing impacts with an average consumption of 120% of the phytoplankton production (P _p ) were measured. Thus, the decline in phytoplankton biomass could be mainly attributed to an intense grazing by microzooplankton. Copepods were less important phytoplankton grazers consuming on average only 47% of P _p . Microzooplankton in turn contributed a substantial part to the copepods’ diets especially with decreasing quality of phytoplankton food due to nutrient limitation over the course of the bloom. Copepod grazing rates exceeded microzooplankton growth, suggesting their strong top-down control potential on microzooplankton in the field. Selective grazing by microzooplankton was an important factor for stabilising a bloom of less-preferred diatom species in our mesocosms with specific species (Thalassiosira spp., Rhizosolenia spp. and Chaetoceros spp.) dominating the bloom. This study demonstrates the importance of microzooplankton grazers for structuring and controlling phytoplankton spring blooms in temperate waters and the important role of copepods as top-down regulators of microzooplankton.     

Long-term trends in the trophic structure of the North Sea fish community: evidence from stable-isotope analysis, size-spectra and community metrics

Fishing has wide-ranging impacts on marine ecosystems. One of the most pervasive signs of intensive fishing is "fishing down the food web", with landings increasingly dominated by smaller species from lower trophic levels. Decreases in the trophic level of landings are assumed to reflect those in fish communities, because size-selective mortality causes decreases in the relative abundance of larger species and in mean body size within species. However, existing analyses of fishing impacts on the trophic level of fish communities have focused on the role of changes in species composition rather than size composition. This will provide a biased assessment of the magnitude of fishing impacts, because fishes feed at different trophic levels as they grow. Here, we combine body size versus trophic level relationships for North Sea fishes (trophic level assessed using nitrogen stable-isotope analysis) with species-size-abundance data from two time-series of trawl-survey data (whole North Sea 1982-2000, central and northern North Sea 1925-1996) to predict long-term trends in the trophic structure of the North Sea fish community. Analyses of the 1982-2000 time-series showed that there was a slow but progressive decline in the trophic level of the demersal community, while there was no trend in the trophic level of the combined pelagic and demersal community. Analyses of the longer time-series suggested that there was no trend in the trophic level of the demersal community. We related temporal changes in trophic level to temporal changes in the slopes of normalised biomass size-spectra (which theoretically represent the trophic structure of the community), mean log₂ body mass and mean log₂ maximum body mass. While the size-based metrics of community structure showed long-term trends that were consistent with the effects of increased fishery exploitation, these trends were only correlated with trophic level for the demersal community. Our analysis suggests that the effects of fishing on the trophic structure of fish communities can be much more complex than previously assumed. This is a consequence of sampled communities not reflecting all the pathways of energy transfer in a marine ecosystem and of the absence of historical data on temporal and spatial changes in the trophic level of individuals. For the North Sea fish community, changes in size structure due to the differential effects of fishing on species and populations with different life histories are a stronger and more universal indicator of fishing effects than changes in mean trophic level.     

Continuous plankton records: Changes in the composition and abundance of the phytoplankton of the north-eastern Atlantic ocean and North Sea, 1958–1974

In most areas of the north-eastern Atlantic Ocean, diatoms have declined drastically in abundance in the last decade. Additionally, in areas to the north of 59°N Ceratium species and an index of total phytoplankton have also declined. South of 59°N the phytoplankton index has increased, diatoms have declined and Ceratium species have remained at a constant level of abundance. A possible explanation of the increase in the phytoplankton index at a time when the diatoms were declinig south of 59°N is the development of unidentified phytoplankton organisms such as microflagellates. As many of the variables influencing phytoplankton standing crop are governed in turn by the prevailing weather, the phytoplankton changes may well be a consequence of the general deterioration, since 1940, of North Atlantic weather. Changes in phytoplankton which may be attributed to an amelioration of climate since 1971 are evident as yet only in the southern North Sea.     

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.     

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.     

Potential impact of climate warming on the recruitment of an economically and ecologically important species, the European lobster (Homarus gammarus) at Helgoland, North Sea

A laboratory-based study was performed to assess the impact of climate warming on the recruitment of the endangered population of the European lobster (Homarus gammarus) at Helgoland (North Sea, German Bight). Egg-bearing females collected in situ just after spawning in late summer were subjected to various seasonal temperature regimes. Regimes with elevated temperatures (mild winters) resulted in a strong seasonal forward shift of larval hatching. Hatching took place at significantly lower temperatures than under regimes with normal winters. Experiments on larval development across a range of constant temperatures showed that no successful larval development occurred at temperatures below 14°C. Larval survival increased from 9% at 14°C to 80% at 22°C, while duration of larval development decreased correspondingly from 26 to 13 days. We hypothesize that an ongoing warming of the North Sea will strongly affect the recruitment success of the Helgoland lobster, mainly resulting from a decoupling of the seasonal peak appearance of larvae from optimal external conditions (temperature, food availability) for larval development.     

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.     

Primary production of phytoplankton at a frontal zone located at the northern slope of the Dogger Bank (North Sea)

In July 1988 a survey was made in the Dogger Bank area of the North Sea. As a result of wind stress the area was found to be frequently well mixed. At the northerly slope a transition zone was observed between the stratified central North Sea and the well-mixed Dogger Bank area. Low nutrient concentrations were observed in surface waters; especially for nitrate (<0,1µM). High concentrations of phosphate (>0,5µM), nitrate (>1µM), ammonium (>2µM) and silicate (>2µM) only prevailed below the thermocline. Chlorophylla values were below 1µg l^–1 near the surface. Enhanced values (up to 4µg l^–1) were observed in the deeper layer at the transition zone and just below the thermocline at well-stratified locations. At the transition zone high specific C-fixation rates (up to 100 mg C mg^–1 chla d^–1) at the surface indicated the presence of enhanced productivity. The compensation depth for primary production was found to coincide with a specific C-fixation rate of 5 mg C mg^–1 chla d^–1. At greater depths, phytoplankton was only found where tidally induced vertical mixing allowed a regular exposure to higher light intensities. Storms resulted in a rapid redistribution of chlorophylla and enhancement of the C-fixation rate in the upper layer of the water column.Publication No. 10 of the project Applied Scientific Research Netherlands Institute for Sea Research (BEWON)     

大亚湾浮游植物群落结构变化及其对水温上升的响应

大亚湾核电站1994年建成运行。文章研究大亚湾长时期的生态和环境变化,分析大亚湾浮游植物群落结构变化与环境变化,特别是水温上升的相关性。研究显示,网采浮游植物种类数目在1994年后减少了16种;1994—2004年间,网采浮游植物细胞相对密度以2.13×107m^-3·a^-1的速率下降,而在1985—2004年间核电站建站的下降速率为2.85×106m^-3·a^-1;与1982年和1994年相比,1998年的浮游植物群落优势种中细胞个体相对较大的种的比重下降,细胞个体相对较小的种的比重上升;从1985年到2003年,叶绿素a（Chl-a）质量浓度的上升速率为0.069mg·m^-3·a^-1,而1994年到2003年的上升速率加快,为0.076mg·m^3·a^-1。与此同时,大亚湾的水温在1994年后明显上升,海水盐度、溶解氧（DO）、化学耗氧量（COD）、和营养盐在1994年后变化幅度也增大。研究结果揭示,核电站热排放导致的水温上升是影响大亚湾浮游植物群落结构的主要因素。     

Changes in nutrient ratios and phytoplankton community structure caused by hydropower development in the Maotiao River,China

Reservoirs created for hydropower production have become an important feature impacting a river. Understanding the effects of river impoundment on the downstream environment is critical to decision-making for water resource protection. The changes caused by impoundment are changes in water quality and the resulting effect on the phytoplankton community structure. The impacts caused by a series of reservoirs along a river are still not well understood. In this study, we conducted an investigation of five reservoirs along the Maotiao River, China. We found that a series of impoundments plays a role in decreasing the phytoplankton biomass in downstream reservoirs. Within the studied area, nitrogen is not a limiting factor for phytoplankton growth. The ratio of silicon to phosphorus (Si:P) can become a major factor in the regulation of phytoplankton community structure. The Si:P ratio increased from upstream to downstream reservoirs, causing a concurrent increase in the percentage of Bacillariophyta, particularly during the winter. In addition, our results indicate that the creation of dams eliminates Si limitation downstream.     

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.     

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.     

Growth and DNA damage in young Laminaria sporophytes exposed to ultraviolet radiation: implication for depth zonation of kelps on Helgoland (North Sea)

Growth as an integrative parameter of all physiological processes was measured in young sporophytes of temperate Laminaria digitata, Laminaria saccharina and Laminaria hyperborea exposed in the laboratory to irradiance consisting of either only photosynthetically active radiation (PAR) or to a spectrum including ultraviolet radiation (UVR) (PAR+UVA+UVB) by use of cut-off glass filters. Size increment was measured every 10 min over a period of 18–21 days using growth chambers with on-line video measuring technique. In the chamber, plants were grown at 10±2°C and 16:8 h light–dark cycles with 6 h additional UVR exposure in the middle of the light period. Tissue morphology and absorption spectra were measured in untreated young sporophytes while chlorophyll a content and DNA damage were measured in treated thalli at the end of the experiment. Sensitivity of growth under UVR was found to be related to the observed upper depth distribution limit of the upper sublittoral L. digitata, upper to mid sublittoral L. saccharina and lower sublittoral L. hyperborea. Tissue DNA damage is, however, dependent on thallus thickness which minimizes UVR effect where outer cell layers shade inner cells and provide longer pathlength for UVR. Exposure to UVR causes cellular, enzymatic and molecular damage. Presence of UV-absorbing compounds further reduces effective UVR from reaching physiological targets. The cost of producing higher amount of UV-absorbing compounds and effective DNA repair mechanism can, however, divert photosynthate at the expense of growth. Tissue chlorophyll a content was not significantly different between treatments suggesting a capacity for acclimation to moderate UVR fluence. Growth acclimation to repeated UVR exposure was observed within a period of 12 days while growth inhibition was observed after a longer UVR exposure period of 21 days. The results give further insight into the effects of UVR on the cellular level and show how ecological parameters such as the upper depth distribution limit are dependent on cellular processes.     

Zooplankton: pre-southwest and northeast monsoons of 1993 to 1994, from the North Arabian Sea

Zooplankton samples from the North Arabian Sea Environment and Ecosystem Research (NASEER) cruises were analyzed to determine the basic taxonomic composition, biomass (standing stock) and the total and copepod numeric abundance; these characteristics are discussed with reference to the different monsoon periods. Cruises carried out during March 1993 and May 1994, categorized as pre-southwest monsoon periods, and a cruise in December 1994, categorized as a northeast monsoon period, are discussed in detail. The biomass of January 1992 versus August 1992 and August 1992 versus March 1993 differed significantly (F = 6.44, P≤ 0.05). Ranges of highest and lowest biomass from each cruise are also given. Distinct “high” and “low” production areas of statistically significant difference (F = 12.67, P≤ 0.05) were observed. The “high” and “low” production areas were mobile and followed the surface wind circulation patterns (wind reversal pattern) during the northeast and southwest monsoon periods. Overall zooplankton showed a patchy distribution. The overall zooplankton abundance and total copepod counts differed significantly between the Cruises 3 versus 4 and 4 versus 5 (F = 15.67, P≤ 0.05 and F = 34.39, P≤ 0.05, respectively). There was no significant difference (P≥ 0.05) in biomass, between eutrophic and oligotrophic stations, suggesting no difference between near shore and offshore waters. Thirty-eight taxonomic groups were identified from the samples, with copepods being the most dominant group, followed by chaetognaths and siphonophores. Copepods constitute an average of 52.50 to 74.93% of the total zooplankton count and reach maxima of 92.14% of the total zooplankton count at the outset of the southwest monsoon (March 1993) and 91.39% at the outset of the active northeast monsoon (December 1994). Received: 27 February 1998 / Accepted: 8 October 1999     

Nutrient kinetics modeled from time series of substrate depletion and growth: dissolved silicate uptake of Baltic Sea spring diatoms

We estimated silicate uptake kinetics for 8 spring diatom species using a model based on time series measurements of the depletion of dissolved silicate (DSi) and increases in biomass. Furthermore, the carbon: nitrogen: silicate stoichiometric relationships and maximum growth rates were determined. Differences in DSi uptake kinetics and maximum growth rate were distinct among the species. All the most common diatom species (Chaetoceros wighamii, Pauliella taeniata, Skeletonema costatum and Thalassiosira baltica) were relatively lightly silicified and had variable but relatively low half-saturation constants (K _s ), indicating that they are well adapted to low DSi concentrations. The less common Diatoma tenuis and Nitzschia frigida had higher K _s values, suggesting that they are more vulnerable to DSi limitation. The much used nitrogen:silicate ratio of 1 for marine diatom biomass was too low for most of the examined species, while a ratio of 2–3 seems to be more appropriate for these Baltic Sea species.     

Population dynamics of Eudendrium racemosum (Cnidaria, Hydrozoa) from the North Adriatic Sea

Benthic suspension feeders in shallow waters develop in relation to the food availability and the variation of physical parameters giving rise to complex communities that act as a control factor on the plankton biomass. The aim of the work is to establish the role of the hydrozoan Eudendrium racemosum in the energy transfer from the plankton to the benthos in marine food chains of the North Adriatic Sea. This study highlighted that the hydroid biomass changed over time in relation to temperature and irradiance, and the highest abundance was observed during summer with about 400,000 polyps m⁻² (about 19 g C m⁻²). The population suffered an evident summer decrease in relation to a peak of abundance of its predator, the nudibranch Cratena peregrina, whose adult specimens were able to eat up to 500 polyps day⁻¹ and reached an abundance of 10 individuals m⁻². The gut content analysis revealed that the hydroid diet was based on larvae of other benthic animals, especially bivalves and that the amount of ingested preys changed during the year with a peak in summer when it was estimated an average predation rate of 13.7 mg C m⁻² day⁻¹. In July, bivalves represented over 60 % of the captured items and about 18 mg C m⁻² day⁻¹. Values of biomass of E. racemosum are the highest ever recorded in the Mediterranean Sea, probably supported by the eutrophic conditions of the North Adriatic Sea. Moreover, our data suggest that settling bivalves provide the greater part of the energetic demand of E. racemosum.     

Relationship between the zooplankton,phytoplankton, particulate matter and dissolved free amino acids in the Celtic Sea

Estimates of the biomass of zooplankton, phytoplankton and particulate matter collected in the Celtic Sea during mixed-water conditions (on 8 and 9 April 1983) were compared to the concentration and diversity of sixteen dissolved free amino acids (DFAA) measured in seawater and in particles. During a day profile, variations of dissolved amino acids with depth reflected the feeding activity of copepods. The relationship was not apparent in a night profile and other processes, such as heterotrophic utilization of dissolved nitrogen by microorganisms, were thought to be involved. The ratios of total DFAA concentration (nM litre^-1) in the particulate phase over the concentration in seawater ranged from 1 to 200 within the water column. Of the sixteen amino acids measured, ornithine, a decomposition product of arginine, was responsible for more than 70% of the total concentration of DFAA in seawater. In the particles, phenylalanine ranged from 30 to 88% of total DFAA. In seawater this amino acid occurred in the 20 to 40 m depths (1.3 to 9.9% of total DFAA) in the day profile and at 5 m (12.4%) and 80 m (6.4%) in the night profile. Previously it has only been found in very low concentrations (<5%) in seawater, and its presence is considered to be the result of zooplankton feeding.     

Temperature and metabolic rate in sedentary fish from the Antarctic,North Sea and Indo-West Pacific Ocean

Resting metabolic rate was measured in demersal stages of the teleostNotothenia neglecta Nybelin from the South Orkney Islands, Antarctica, from 1985 to 1987. The relationship between and body mass (Mb) conformed to the general relationship , wherea is a proportionality constant andb is the scaling exponent. (mg O₂ h^–1) was found to scale toMb ^{(0.82±0.011)} in the summer (November to April, 1.6 to 1 850 g,n=56) and toMb ^{(0.76±0.013)} in the winter (May to October, 0.9 to 1 850 g,n=57) (values ofb are means ± SD). Although the scaling exponents were significantly different (P<0.01), was similar in the juvenile stages of summer- and winter-caught fish matched for body mass. The effects of activity on oxygen consumption was studied using a Brett respirometer. Adult stages had a factorial aerobic scope for activity of 5.7, which is similar to that reported for demersal fish from temperate latitudes. The effects of temperature on resting metabolism was investigated in fish with similar sedentary lifestyles from the North Sea (Agonus cataphractus andMyoxocephalus scorpius) and the Indo-West Pacific (Paracirrhites forsteri, P. arcatus, Neocirrhites armatus andExallias brevis). Extrapolated values of for the tropical species approached zero at 5 to 10°C. For a standard 50 g fish, for the tropical species at 25°C was in the range 3.4 to 4.4 mg O₂ h^–1, compared with 1.3 mg O₂ h^–1 forNotothenia neglecta at its acclimation temperature. Thus, the maximum metabolic rate of sedentary tropical species at 24°C is likely to be 2 to 4 times higher than inN. neglecta at 0°C. This suggests that the energy available for sustained activity is significantly lower in cold- than in warm-water fish.     

Detecting the order of population dynamics from time series: nonlinearity causes spurious diagnosis

Partial autocorrelation and partial rate correlation functions are frequently used to detect the order of the endogenous process generating an observed population time series. Here we uncover a problem with this approach: the diagnosis of spurious second order autocorrelation due to strong nonlinearity in a first order endogenous process, as exemplified by time series data from a population of Soay sheep. Causes and a possible solution are discussed.