Inter- and intra-specific diurnal habitat selection of zooplankton during the spring bloom observed by Video Plankton Recorder

Diel vertical migration (DVM) is a common behavior adopted by zooplankton species. DVM is a prominent adaptation for avoiding visual predation during daylight hours and still being able to feed on surface phytoplankton blooms during night. Here, we report on a DVM study using a Video Plankton Recorder (VPR), a tool that allows mapping of vertical zooplankton distributions with a far greater spatial resolution than conventional zooplankton nets. The study took place over a full day–night cycle in Disko Bay, Greenland, during the peak of the phytoplankton spring bloom. The sampling revealed a large abundance of copepods performing DVM (up during night and down during day). Migration behavior was expressed differently among the abundant groups with either a strong DVM (euphausiids), an absence of DVM (i.e., permanently deep; ostracods) or a marked DVM, driven by strong surface avoidance during the day and more variable depth preferences at night (Calanus spp.). The precise individual depth position provided by the VPR allowed us to conclude that the escape from surface waters during daytime reduces feeding opportunities but also lowers the risk of predation (by reducing the light exposure) and thereby is likely to influence both state (hunger, weight and stage) and survival. The results suggest that the copepods select day and night time habitats with similar light levels (~10^?9 μmol photon s^?1 m^?2). Furthermore, Calanus spp. displayed state-dependent behavior, with DVM most apparent for smaller individuals, and a deeper residence depth for the larger individuals.     

Lipid and fatty acids in naturally occurring particulate matter during spring and summer in a high arctic fjord (Kongsfjorden, Svalbard)

The total lipids and fatty acid composition of natural particulate matter and nutritional quality for zooplankton grazers was studied on a seasonal basis in the Arctic fjord Kongsfjorden (Svalbard) during the spring, summer of 2007 and during the early summer of 2006. Both years were abnormally warm, and the study attempted to evaluate the potential impact of the intrusion of North Atlantic waters. Samples were collected in surface layers and at deep chlorophyll maximum (DCM when present). Both years, chlorophyll concentrations were low (<2 μg L^?1) even during bloom periods. Species determination indicated Phaeocystis spp. as main constituent of the May bloom while ciliates and flagellates dominated the rest of the survey period. Total lipids showed similar changes at both depths with maximum values in mid-summer of 2007, while it showed reverse patterns between surface and DCM in 2006. Total fatty acid composition was dominated by saturates and monoenoic acids at both depths with significant percentages of pentaenoic acids and 22:6n-3 (DHA) recorded at all times. The 2007 fatty acid dynamics identified four main successions in term of particulate assemblage related mainly to the succession of living cells versus detrital material and to a lesser extent to phytoplankton community changes (diatoms versus non-diatoms). Redundancy analysis confirmed that live phytoplankton is one of the main drivers in the fatty acid changes. Temperature and density of the surface water are also influential in relation to water mass dynamics. Concentrations of fatty acids available to consumers showed n-3 PUFA ranging from 2 to 15 μg L^?1 and n-6 PUFA ranging from 0.3 to 2 μg L^?1. Concentration of EPA (20:5) and DHA are potentially limiting, suggesting a negative impact of Phaeocystis pouchetti-type phytoplankton linked to advection of Atlantic waters in relation to global warming of Arctic waters.     

Spatial distribution patterns of the soft corals Alcyonium acaule and Alcyonium palmatum in coastal bottoms (Cap de Creus, northwestern Mediterranean Sea)

Current knowledge on the abundance and distribution patterns of different soft coral species is relatively limited when compared to other benthic suspension feeders such as gorgonians and hard coral species. To overcome this scarcity of information, the distribution patterns of the soft corals Alcyonium acaule and Alcyonium palmatum were investigated in northwestern Mediterranean benthic communities over a wide geographical (60 km of coastline) and bathymetrical (0–70 m depth) extent using a remotely operated vehicle. A. acaule was the most abundant species in the study area with highest recorded density of 18 col m^?2 found at depths of 35–45 m in areas that are directly exposed to strong near-bottom currents. Conversely, A. palmatum was only found as scattered solitary colonies at greater depths in soft bottoms, with maximum density of 2.4 col m^?2. Medium and large colonies of A. acaule were preferentially found on sloping and vertical rocky bottoms where they form dense patches. High-density patches of A. acaule were preferentially found on vertical rocky bottom, while isolated colonies were preferentially observed on coralligenous substrata as well as on flat soft and maërl substrates. A. acaule biomass distribution showed highest values between 40 and 45 m depth, and between 60 and 65 m depth. This suggests that deeper populations are formed by colonies that are bigger than the equivalent shallower ones. Although both species are almost genetically identical, ecologically they are very different. For this reason, conservation plans should consider the differential ecological traits shown by these two soft coral species.     

Optimization of exergy and implications of body sizes of phytoplankton and zooplankton in an aquatic ecosystem model

Size appears to be an important parameter in ecological processes. All physiological processes vary with body size ranging from small microorganisms to higher mammals. In this model, five state variables — phosphorus, detritus, phytoplankton, zooplankton and fish are considered. We study the implications of body sizes of phytoplankton and zooplankton for total system dynamics by optimizing exergy as a goal function for system performance indicator. The rates of different sub-processes of phytoplankton and zooplankton are calculated, by means of allometric relationships of their body sizes. We run the model with different combinations of body sizes of phytoplankton and zooplankton and observe the overall biomass of phytoplankton, zooplankton and fish. The highest exergy values in different combinations of phytoplankton and zooplankton size indicate the maximum biomass of fish with relative proportions of phytoplankton and zooplankton. We also test the effect of phosphorus input conditions corresponding to oligotrophic, mesotrophic, eutrophic system on its dynamics. The average exergy to be maximized over phytoplankton and zooplankton size was computed when the system reached a steady state. Since this state is often a limit cycle, and the exergy copies this behaviour, we averaged the exergy computed for 365 days (duration of 1 year) in the stable period of the run. In mesotrophic condition, maximum fish biomass with relative proportional ratio of phytoplankton, zooplankton is recorded for phytoplankton size class 3.12 (log V μm³ volume) and zooplankton size 4 (log V μm³ volume). In oligotrophic condition the highest average exergy is obtained in between phytoplankton size 1.48 (log V μm³ volume) and zooplankton size 4 (log V μm³ volume), whereas in eutrophic condition the result shows the highest exergy in the combination of phytoplankton size 5.25 (log V μm³ volume) and zooplankton size 4 (log V μm³ volume).     

Distribution of lipopolysaccharide,an indicator of bacterial biomass,in subtropical areas of the sea

Lipopolysaccharide (LPS) concentrations, an indicator of bacterial biomass, were determined in the South China Sea and the Pacific Ocean. The distribution patterns of LPS were compared with those of chlorophyll a (Chl a), zooplankton biomass and the concentrations of several nutrients. LPS and total bacterial numbers in seawater were correlated with each other with a correlation coefficient (r) of 0.84 at Stations 7, 8 and 10. Diurnal fluctuation of LPS was negligible, but Chl a varied slightly in the vertical water column. Zooplankton stayed at a depth of around 400 m during the daytime and ascended quickly to the surface (0–50 m) early in the evening. The profiles of LPS and Chl a were negatively correlated to each other in the water layers above the Chl a maximum peak (r=-0.74; excluding the samples from 75 m at Station 7 and 10 m at Station 11 due to inadequate data for the statistical analysis). LPS and zooplankton biomass during the night-time, in contrast, paralleled each other at 5 stations surveyed (r=0.71). The presence of zooplankton resulted in an increase in bacterial numbers in the seawater in vitro. Based on these results, the factors controlling the occurrence and abundance of bacteria and phyto- and zooplankton in the pelagic sea are discussed.     

Description and use of an improved method for determining estuarine zooplankton grazing rates on phytoplankton

A method of rapidly determining zooplankton grazing rates on natural mixed phytoplankton populations using ¹⁴C is described. The method simplifies the design of grazing experiments as the grazing time can be kept short enough to prevent recycling of the isotope, and growth of the phytoplankton substrate. Very high specific activity, ¹⁴C-labelled phytoplankton concentrated either by centrifugation or sieving, may be used either as the sole grazing substrate, or as a tracer in natural mixed phytoplankton. Zooplankton, confined in glass jars at either ambient, or higher than ambient concentrations, are permitted to feed on the phytoplankton for periods of 30 min and 2 h, and are then separated by sieving. The zooplankton community grazing rate, or, if the samples are sorted into species, the individual species grazing rates, can be determined after scintillation counting of the zooplankton. The rate of appearance of ¹⁴C-labelled phytoplankton in the zooplankton is an estimate of the grazing rate, and the slope of the line joining the grazing rates at various phytoplankton concentrations gives an estimate of the grazing rate constant for the zooplankton population. The method provides a quick way of obtaining both zooplankton population, and individual species grazing rates on natural mixed phytoplankton. In two experiments, labelled phytoplankton was used as the sole grazing substrate in concentrations ranging between 0.4 and 5 times ambient levels. Grazing rate constants, for net-caught zooplankton concentrated to 46 times (Experiment 1) and 28 times (Experiment, 2) ambient estuarine levels were-0.14and-0.12 of the phytoplankton standing stock per day, respectively. There was a linear increase in the amount of phytoplankton grazed with an increase in phytoplankton concentration up to four times ambient phytoplankton levels. When tracer amounts of labelled phytoplankton were added to samples containing both phytoplankton and zooplankton at ambient concentrations the grazing rate constants were-0.28 and-0.42 of the phytoplankton standing stock per day. We conclude that zooplankton grazing was the major control factor of phytoplankton population size during October–November 1975 in South West Arm, Port Hacking, near Sydney, Australia.     

Distribution of phytoplankton communities in relation to the large-scale hydrographical regime in the Fram Strait

In June and July 1984 phytoplankton distribution was investigated in the Fram Strait between Greenland and Svalbard. Chlorophylla, particulate organic carbon, nitrate and phytoplankton species composition were determined from six different depths in the upper 200 m of the water column. Multivariate analysis methods were applied to identify phytoplankton communities in relation to different hydrographic regimes. Three main domains could be distinguished in terms of both hydrography and biology: (1) the East Greenland shelf polynya with a high biomass mainly produced by chain-forming diatoms, (2) the ice-covered East Greenland Current with an extremely low standing stock dominated by flagellates and (3) the marginal ice zone with a biomass maximum in 20 to 40 m depth formed by diatoms, dinoflagellates andPhaeocystis pouchetii.     

Grazing rates of the Atlantic menhaden Brevoortia tyrannus as a function of particle size and concentration

The feeding behavior of adult Atlantic menhaden (Brevoortia tyrannus) upon 5 species of phytoplankton and 2 species of zooplankton has been studied. Four recognizable feeding stages which were a function of the concentration and size of the food particles were observed. During rapid feeding the fish swam at a constant speed for a prolonged period over a wide range of particle concentrations. Particle and food carbon-concentrations at the threshold for initiation and termination of feeding were inversely related to particle size. Carteria chuii (13.2 μ) was not grazed at a significant rate, while two-cell chains of Skeletonema costatum (16. 5 μ) were filtered from the water, indicating a minimum-size threshold for filtration of between 13 and 16 μ. The most rapid filtering rates were observed for the copepod Acartia tonsa ( \(\bar x\) volume swept clear = 24.8 l/fish/min). The maximum food-particle size acceptable to a menhaden appears to be between Acartia tonsa (1200 μ) and adult Artemia salina (10 mm). These results suggest that the large schools of menhaden found in Atlantic coastal waters could have a significant effect on the plankton, selectively grazing zooplankton, larger phytoplankton, and the longer chains of chain-forming diatoms.     

Ecology of the northern Red Sea crinoids and their Epi- and Endozoic fauna

Fourteen crinoid species are found along the coral reefs of the northern Red Sea that form aggregations of different species at various depths. The shallowest aggregation consists partly of Lamprometra klunzingeri and Capillaster multiradiatus, occurring subtidal to a depth of 2 to 5 m. Of this group, Heterometra savignii inhabits depths down to 12–15 m. Further down, another group of 10-armed crinoids occurs, dominated by Decametra chadwicki and Oligometra serripinna. The deepest aggregation of crinoids occurs at 45 m; its most common members are Colobometra arabica and Comaster distinctus. Feeding on micro- and nanoplankton, shallow-water crinoids show a circadian activity rhythm; in deeper aggregations (10 to 12 m), this behaviour changes with decreasing illumination, to a diurnal activity rhythm. The symbiotic animals living on crinoids comprise 27 taxons, among them Copepoda (6 species), Mollusca (2 species), Polychaeta (11 species, especially Myzostomida); Ophiurida (1 species), Crustacea, Decapoda (6 species); and the clingfish Lepadichthys lineatus. These symbiotic animals form food webs at various depths, according to the distribution of their crinoid host. The occurrence of Indo-West Pacific and Mediterranean commensals on the same crinoid in the Gulf of Aqaba is of special interest.     

Mapping atmospheric depositions of cadmium and lead in Germany based on EMEP deposition data and the European Moss Survey 2005

Background

The vertical migration of phytoplankton was investigated in natural waters using in situ fluorescence profiling, chlorophyll a concentrations and life counts at two study sites differing in coloured dissolved organic matter (cDOM) concentrations. The data from the corresponding water depths (50-cm intervals down to 10 m) and times (hourly, before dawn to sunset, several days) were related to the highly resolved (2 nm) underwater ultraviolet radiation (UVR)/photosynthetic active radiation (PAR) transparency (290 to 700 nm).

Results

Chlorophyll a maxima of mainly motile dinoflagellates were observed in situ at all days and at both study sites (open marine, brackish waters), independent on prevailing weather conditions or cDOM concentrations. Phytoplankton migration was triggered solely by irradiance in the 400- to 700-nm wavelength range (PAR) at the particular water depth, irrespective of PAR/UVR ratios and surface UVR (290 to 400 nm), after an illumination period of about 40 min. Interestingly, the PAR tolerance levels of the phytoplankton, which have been lower in cDOM-rich waters, matched their light acclimation values determined by parallel PAM measurements.

Conclusions

The response of the phytoplankton to PAR is not a sufficient protection strategy versus increasing UVR levels, which might have wide ecological implications beyond the level of primary producers to impact important ecosystem functions such as the delicate trophic interactions.     

Recherches sur la situation trophique d'un groupe d'organismes pélagiques (Euphausiacea). III. Potentiel alimentaire du groupe

The position of a group of pelagic organisms among the food webs depends upon (1) its abundance in the biomass; (2) the factors allowing its utilization by predators; among these, the size distribution of animals and their bathymetric repartition appear to prevail. In the equatorial and southtropical Pacific Ocean, euphausiids represent 8% of the total macroplanktonic and micronektonic biomass, i.e., approximately 13 g/1000 m³ wet weight (somewhat less in oligotrophic tropical zones, a little more in the richer equatorial belt); Euphausia diomedae accounts for more than 50% of this biomass. Individuals measuring 9 to 18 mm in total length (4 to 37 mg wet weight) constitute 84% of the whole biomass of the group; the importance of each species in terms of size groups is discussed. Depth distribution is examined at the specific level. By night, 75% of the biomass concentrates in the 0 to 160 m water layer (ca. 10 g/1000 m³ wet weight; main species Euphausia diomedae), 22% between 160 and 300 m (3 g/1000m³ main species Nematoscelis spp.), and 3% (0.3g/1000 m³) in deeper layers. During the day, the only species abundant at depths less than 400 m belong to the genera Stylocheiron and Nematoscelis; specimens smaller than 15 mm and 20 mg remain at depths shallower than 200 m, individuals between 15 and 22 mm (20 to 65 mg) appear around 200 to 400 m, and larger animals are restricted to depths below 400 m.     

Die vertikale verteilung und tägliche migration der hydromedusen (Hydrozoa: Coelenterata) in der Bucht von Eilat (Rotes Meer)

During the summer of 1970, plankton samples were made in the Bay of Eilat (Red Sea) to study the vertical distribution and diurnal migration of zooplankton. During this project, three programs of 24 h were carried out with a closing net. The hauls were taken every 2 h in different depths, together with measurements of temperature and salinity. The vertical distribution of hydromedusae, especially of Aglaura hemistoma, Péron and Lesueur, 1809 and Liriope tetraphylla (Chamisso and Eysenhardt, 1821), between the surface and 300 m is described. The diurnal migration for these two species was studied. Presumably temperature and salinity had no direct influence on vertical distribution and diurnal migration, as there was a homogeneous water body. The results are discussed.     

Effects of a decrease in downwelling irradiance on the daytime vertical distribution patterns of zooplankton and micronekton

The daytime vertical distribution of several species of crustaceans, gelatinous zooplankton, and fish were monitored in situ simultaneously with measurements of downwelling irradiance in Oceanographer Canyon in July 1999. During this submersible-based research cruise, an influx of turbid water significantly decreased downwelling irradiance and had a substantial impact on the depth distributions of a number of organisms. Several species of crustaceans, (Thysanoessa gregaria and Sergestes arcticus) and gelatinous zooplankton (Salpa aspera and Salpa fusiformis) ascended over 100 m in the water column during the influx and returned to their pre-influx depths once the influx had ceased. In situ light measurements demonstrated that each of these species was associated with the same irradiance levels during the influx as they were under pre- and post-influx conditions. By contrast, a statistically significant change in temperature, salinity, and oxygen concentrations measured post-influx had no apparent impact on the depth distributions. These results indicate that these species were adjusting their depth distributions to remain within a range of preferred irradiances. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s002270020788     

Depth-dependent changes in chlorophyll fluorescence number at a Sargasso Sea station

Chlorophyll a-specific in vivo fluorescence exhibited depth-dependent changes in a Sargasso Sea phytoplankton community, decreasing from a maximum value at the surface to a minimum at 90m, and then increasing again below 90 m. This distribution pattern was not explained by irradiance conditions, diurnal variability, senescence in the deep population, or changes in light-absorption efficiency of chlorophyll a. However, a significant positive correlation was found between mean phytoplankton cell size and fluorescence number in the upper euphotic zone, where nutrient concentrations were low. We hypothesize that the direct cause for this observed correlation was nutrient limitation. In this picoplankton-dominated community, packaging effect was minimal. Under nutrient-limiting conditions, as mean cell size increases photosynthetic efficiency decreases and therefore fluorescence number increases. In the lower euphotic zone where nutrients were not limiting, changes in fluorescence number exhibited weak size-dependence and appeared to be related to species compositional changes.     

Depth limits of Bermudan scleractinian corals: a submersible survey

Depth distribution, zonation pattern and growth morphology of 17 hermatypic and 4 ahermatypic coral species were investigated at eight different locations along the Bermuda platform with the research submersible GEO and by SCUBA diving in August–September 1983. Hermatypic coral growth occurs to a depth of 50 to 70 m, with a single Montastrea cavernosa growing at 78 m. Dominant forms in shallow-water coral communities are Diploria sp. and Porites astreoides, while M. cavernosa, Agaricia fragilis and Scolymia cubensis occur in deep-water associations below 60 m. Vertical visibilities (up to 178 m) and distribution of the photosynthetically active radiation revealed good light penetration values (1% level at about 100 m depth), which should favour hermatypic coral growth to a much greater depth than it actually occurs. Nor should the prevailing temperatures limit the depth of coral growth. Most deep-water hermatypes observed grow on remnants of Pleistocene reefs down to about 60 m. The vast areas of large massed rhodolith nodules below 50 to 60 m are unsuitable bottom for coral colonisation. Macroalgae growth seems to be the strongest factor controlling coral growth in deep water. Bermuda stony corals have a low growth form diversity. Various intraspecific morphs may occur at the same as well as at different depths, with a general trend towards flatter shapes with depth. Comparison with a similar study on Red Sea corals suggests that annual distribution of radiant energy on the most northern Atlantic reefs of Bermuda may be responsible for the occurrence of flat and cuplike growth forms in relatively shallow water, and for the shallower depth limits of hermatypic growth.     

Microplankton of the Gulf of Elat: Aspects of seasonal and bathymetric distribution

The components of 4 major groups of microplankton were identified and their numerical abundance determined in net samples collected at depth intervals down to 600 m at a permanent station off the H. Steinitz Marine Biological Laboratory, Elat. The samples analyzed were collected once a fortnight over a period of 1 year beginning in June 1974. The groups studied were the Cyanophyta, Bacillariophyta and Pyrrhophyta of the phytoplankton and the Tintinnina of the microzooplankton. The pattern of vertical distribution of the phytoplankton as a whole showed a general decrease in cell numbers with increasing depth. The blue-green algae, consisting mainly of Trichodesmium sp. trichomes, were confined primarily to the upper 100 m. The diatoms were unevenly distributed, with one species, a minute centric diatom, Thalassiosira subtilis, associated with a massive bloom during March 1975 between 300 and 400 m. The peridinians, the group with the largest number of species, included forms which were evenly distributed throughout the whole water column and forms limited either to the upper or deeper water strata in accordance with their light intensity preferences. The mass occurrence of newly-emerged dinoflagellate cysts of Pyrophacus horologicum, a weakly-armoured dinoflagellate, in the 200 to 300 m depth interval during April 1975, suggests that reproductive processes in dinoflagellates may also be light-controlled. The tintinnids, like the phytoplankton groups, were most abundant in the upper 100 m with a gradual decrease in numbers of individuals, though not in species, in the deeper water strata. The overall yearly pattern of microplankton distribution indicates 3 peaks: late fall and early summer peaks consisting primarily of blue-green algae and one in early spring consisting of several species of diatoms and peridinians and of species of tintinnids which thrive in the same niches as the phytoplankton. Both phytoplankton and tintinnid production was lowest during the summer months.     

Day/night vertical distribution of euphausiids in the eastern tropical Pacific

Between March 23 and April 4, 1981, samples were taken in the eastern tropical Pacific. The day/night vertical distribution of euphausiid species and biomass are described and contrasted in detail on two eastern tropical Pacific stations, the DOME station, in a region of continuous upwelling and the BIOSTAT station, in a nonupwelling area. The effects of various biological parameters, such as temperature, salinity and oxygen concentrations on the distributions of the species are examined. The numbers of euphausiids m^-2 on both stations were highest during the day, indicating that avoidance of the sampler was not a problem. During the day the largest concentration of adult euphausiids was between 300 and 350 m whereas the juveniles were concentrated between 170 and 80 m on both stations. Very few individuals were found within the oxygen minimum layer, but low concentrations of some species were found below the oxygen minimum down to 1 000 m. At night the euphausiid concentration migrated upward into the mixed layer (20 to 30 m) at BIOSTAT and to the base of the mixed layer at the DOME. Significant differences in the night depths of the species were found on both stations. The oxygen minimum layer appeared to act as a barrier to the vertical distribution of all species. Only two species were found in water with an oxygen concentration of <0.1 ml O₂ l^-1. Twentyone species of euphausiids were found on the two stations but the adult population was dominated by only two or three species on both stations. The reproductive state of the species suggested that some species reproduced earlier on the DOME than on BIOSTAT. Analysis of the depth distribution by cluster analysis showed that the most abundant species occupied different depths during the night and day at BIOSTAT but the two most abundant species were concentrated at the same depth at the DOME station although portions of each species population occupied different pelagic zones.     

On plankton production in Kungsbacka Fjord,an estuary on the Swedish west coast

Environmental conditions, primary production, and zooplankton populations were studied from May, 1969 to November, 1970 at one station in Kungsbacka Fjord, Sweden. The fjord, with an arca of 53 km², is a moderately polluted estuary, with a small tidal range. Data for primary production and environmental parameters were correlated using Spearman's rank correlation coefficient. The annual rate of primary production in 1970 was about 100 gC·m^-2. Carbon fixation was about 80 g·m^-2 in May–November in1969 and 1970. The average monthly rate was highest in June, 1970, with 25 gC·m^-2; about 15 gC·m^-2 was recorded in August–October of both years. Carbon fixation by the phytoplankton was estimated to be about 2,800 tons in the whole fjord in 1970. The average fresh-water inflow to the fjord, amounting to about 13 m³·sec^-1, added about 380 tons of organic carbon, 45 tons of nitrogen, and 4.5 tons of phosphorus per month. Primary production displayed strong correlation with temperature at different depths (P<0.05 to 0.001), indicating the sediments to be the most important nutrient source. A total of 19 holoplanktonic zooplankton species was identified, copepods being the dominant group. The highest zooplankton biomass, 800 to 900 mg·m^-3, was recorded in June of both years. The production of copepods in May–October was about 1 gC·m^-2 in both years. The total secondary production of the zooplankton was calculated as only 1.8 gC·m^-2 in 1970.     

Causative species of diarrhetic shellfish poisoning (DSP) in Norway

This study was performed at Vikane in the Sognefjord, Norway, from September 1987 to October 1988 on the blue musselMytilus edulis collected monthly at three different depths (3 to 6, 6 to 8 and 8 to 12 m). Cell numbers of three species ofDinophysis from mussel digestive glands and in seawater were counted for each specimen. Multivariable statistical methods were used to detect annual cycles of phytoplankton abundance in the fjord and to examine the contribution of each species to toxification of mussels. Cluster analysis and principal component analysis indicated the following results: (1) an annual cycle and an inter-specific association of threeDinophysis species populations were pointed out.D. acuta andD. norvegica were well associated over the entire period of this study, and exhibited an autumnal peak.D. acuminata was dissociated from these species and reached its maximum abundance between spring and early summer. (2) The threeDinophysis species induced toxicity in the blue mussels in different manners.D. acuta andD. norvegica were responsible for high autumnal toxicity, andD. acuminata for the spring peak. (3) A long persistence of diarrhetic shellfish poisoning (DSP) in blue mussels (from April to February), and a depth gradient of toxicity were observed — the toxicity value in the upper layer being double those of other depths.