Plankton of the fladen ground during FLEX 76 III. Vertical distribution,population dynamics and production of Calanus finmarchicus (Crustacea: Copepoda)

Samples taken in the northern North Sea with the Continuous Plankton Recorder (CPR), the Undulating Oceanographic Recorder (UOR), the Longhurst Hardy Plankton Recorder (LHPR) and by our colleagues from other participating Institutes during the Fladen Ground Experiment (FLEX 76) were used to describe the vertical distribution and population dynamics of Calanus finmarchicus (Gunnerus) and to provide estimates of the production and carbon budget of the population from 19 March to 3 June, 1976. Total production of the 19 March to 3 June, 1976. Total production of the nauplii and copepodite stages (including adults), during the exponential growth phase in May, was estimated to be in the range of 0.49 to 0.91 g C m^-2 d^-1 or 29.0 to 55 g dry wt m^-2 (14.5 to 27.8 g C m^-2) for the three successive 10 d periods in May. Two gross growth efficiencies (K ₁) (20 and 34%), together with the lower value of C. finmarchicus production, were used to calculate the gross ingestion levels of algae as 2.45 and 1.44 g C m^-2 d^-1 (73.5 and 43.2 g C m^-2 over the May period). These ingestion levels, together with the algae ingested by other zooplankton species, are greater than the estimated total phytoplankton production of 45.9 g C m^-2 over the FLEX period. A number of factors are discussed which could explain the discrepancies between the production estimates. One suggestion is that the vertical distribution of the development stages of this herbivorous copepod and their diel and ontogenetic migration patterns enable it to efficiently exploit its food source. Data from the FLEX experiment indicated that the depletion of nutrients limited the size of the spring bloom, but that it was the grazing pressure exerted by C. finmarchicus which was responsible for the control and depletion of the phytoplankton in the spring of 1976 in the northern North Sea.JONSDAP Contribution No. 51     

Plankton of the fladen ground during FLEX 76 I. Spring development of the plankton community

Results from plankton sampling in the northern North Sea with the Continuous Plankton Recorder (CPR) and the Undulating Oceanographic Recorder (UOR) during the Fladen Ground Experiment in 1976 (FLEX 76) are summarised. The first evidence of the spring outbreak of phytoplankton was on 19 April, the day after the first signs of vertical stability of the water column were observed. This was followed by spawning of the euphausiid Thysanoessa inermis and rapid increase in the numbers of Calanus finmarchicus. C. finmarchicus was the most abundant species over the FLEX period (19 March to 3 June) and, together with T. inermis, accounted for over 80% of the dry weight of the zooplankton standing stock. By early June the standing crop of phytoplankton had been depleted and nutrients levels were reduced to very low concentrations in the upper 50 m.JONSDAP Contribution No. 49     

Population dynamics and production of euphausiids II. Thysanoessa inermis and T. raschi in the North Sea and American Coastal Waters

Results from the Continuous Plankton Recorder (CPR) survey for 1966 and 1967 are used to describe seasonal changes in abundance, size and aspects of the population structure of Thysanoessa inermis (Krøyer) and T. raschi (M. Sars) at a depth of 10 m in the North Sea and in American coastal waters from the Grand Banks to the Gulf of Maine. Production and dry weight were estimated from these data. Two year-groups were usually present in the breeding population, the proportion surviving into a second year being higher in American waters than in the North Sea. Annual production for each species was within the range 0.69 to 4.66 mg m^-3 and the ratio between production and biomass (P:B) was between 1.3 and 4.2; values outside these ranges were obtained only for American coastal waters in 1967, when the frequency of sampling was low.     

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.     

High reproduction of <Emphasis Type="Italic">Calanus finmarchicus</Emphasis> during a diatom-dominated spring bloom

Feeding, egg production, hatching success and early naupliar development of Calanus finmarchicus were measured in three north Norwegian fjords during a spring bloom dominated by diatoms and the haptophyte Phaeocystis pouchetii. Majority of the copepod diet consisted of diatoms, mainly Thalassiosira spp. and Chaetoceros spp., with clearance rates up to 10 ml ind⁻¹ h⁻¹ for individual algae species/groups. Egg production rates were high, ranging from ca 40 up to 90 eggs f⁻¹ d⁻¹, with a hatching success of 70–85%, and fast naupliar development through the first non-feeding stages. There was no correlation between the egg or nauplii production and diatom abundance, but the hatching success was slightly negatively correlated with diatom biomass. However, the overall high reproductive rates suggested that the main food items were not harmful for C. finmarchicus reproduction in the area, although direct chemical measurements were not conducted. The high population egg production (>1,20,000 eggs m⁻² d⁻¹) indicated that a large part of the annual reproduction took place during the investigation, which stresses the importance of diatom-dominated spring phytoplankton bloom for population recruitment of C. finmarchicus in these northern ecosystems.     

Does the spawning of Calanus finmarchicus in high latitudes follow a reproducible pattern?

In the spring of 1989, an experimental study of the spawning behaviour of Calanus finmarchicus was carried out in Malangen, northern Norway. Here, a single cohort of females reproduce from mid-March to May, approximately coinciding with the wax and wane of the spring phytoplankton bloom. An evaluation of population characteristics such as the proportion of adults, sex ratio, as well as gonad maturation and daily productivity of the females clearly reveals three phases within the population's reproductive period. In between incline and decline, the highest spawning rates (on average >20 eggs female^-1 d^-1, equivalent to 5.7% body C d^-1) occur after the males have disappeared from the population and almost all females have mature gonads. During this period, the ratio of adults to copepodid Stage Vs changes from dominance of adults to that of CVs. Although first egg production was observed prior to the phytoplankton increase, it is suggested that the onset of the phytoplankton spring bloom in the first few days of April enhances the final maturation of ovaries in the females and therefore triggers the onset of the main spawning period. The clutch sizes (max. 95 eggs clutch^-1) vary with the age of the females, while the spawning frequencies depend on the available food quantities. The overlap of an estimated minimal 4 wk spawning period for the individuals leads to a main reproductive phase for the population of ca. 3 wk, during which time mean clutch sizes and spawning frequencies are maximal (highest average clutch size: 70 eggs female^-1 clutch^-1, 100 to 60% of the females spawning). This period ends before the end of the phytoplankton bloom. Calculated by stepwise interpolation and summation of the mean daily egg production in the population, an average female produced ca. 600 eggs during the spring bloom in Malangen 1989. We suggest that reproduction and population development of C. finmarchicus in spring follows a reproducible pattern for a given temperature regime and non-limiting food conditions. In the case of clearly identifiable cohorts, it seems possible to trace the state of reproduction by evaluating population parameters.     

Vertical distribution,and seasonal and diurnal migration of Calanus helgolandicus in the Celtic Sea

The vertical distribution and migration (seasonal, diel and ontogenetic) of Calanus helgolandicus are described from the shallow (100 m) shelf-seas to the south-west of the British Isles. In 1978 and 1979, the overwintering population of C. helgolandicus consisted primarily of Stage V copepodites and adults. By late winter/early spring the copepodites had moulted to adult females (>90%), which matured and bred the first cohorts of the year, prior to onset of the spring phytoplankton bloom in April/May. C. helgolandicus reached a peak of numerical abundance in August of 20x10³ copepodites m^-2 (over the depth range sampled -0 to 70 m), which was 200 times the population in winter. The seasonal peak of abundance occurred 4 mo after the peak of the bloom of phytoplankton in spring. The yearly development of the copepod was not always out of phase with the diatom bloom, as seen when the data from 1978 was placed in the context of a longer time-series collected at 10 m over 22 yr (1960–1981, inclusive). Large vertical migrations were observed in the younger copepodites (CI and II) in May from below to above the thermocline. In the remainder of the year, the CI and CII stages behaved differently and were located above the thermocline within the euphotic zone. The largest vertical displacements of biomass were seen in the summer months due to the migrations of the CV stages and adults, which had developed from the spring cohorts. It was contended that the seasonal and vertical migrations of C. helgolandicus are part of a more complex pattern of inherent behavior than has been reported previously and that, however difficult this is to discern in the natural populations, it always expresses itself.     

Population biology of hyperbenthic crustaceans in a cold water environment (Conception Bay,Newfoundland). I.<Emphasis Type="Italic"> Mysis mixta</Emphasis> (Mysidacea)

Seasonal population dynamics of Mysis mixta Lilljeborg were studied from December 1998 to November 2000 at a 240 m deep site in Conception Bay, Newfoundland. At this depth, temperature was <0°C and salinity between 32.0 and 34.0 psu year-round. The spring phytoplankton bloom began in early or late March and reached a maximum in late April to mid-May. M. mixta exhibited a highly synchronised life cycle, with spawning and mating occurring in October to November, embryos brooded for ~5 months, and juveniles released during spring bloom sedimentation in April and May. Females were semelparous and died at age 2.5 years, following release of juveniles in spring, whereas the majority of mature males died at age 2 years, following mating in November. The biennial life cycle of this population resulted in the presence of two cohorts in the hyperbenthos at any given time. Variation in density and biomass was low among cohorts but high within cohorts, the latter probably due to the high motility of mysids. Densities in 1999 and 2000 were 242±379 and 544±987 ind. per 100 m³ (mean±SD), respectively. Although growth rates were similar between years, rates measured from changes in dry mass differed both seasonally and among life-history stages (range from –4 to 7 mg month^–1). Annual secondary production was estimated at 29–73 mg C m^–2 in 1999 and 53–205 mg C m^–2 in 2000. The annual P/B ratios were 1.62 and 1.19 in 1999 and 2000, respectively.Communicated by J.P. Grassle, New Brunswick     

Seasonal variations in the growth rates of euphausiids (<Emphasis Type="Italic">Thysanoessa inermis</Emphasis>, <Emphasis Type="Italic">T. spinifera</Emphasis>, and <Emphasis Type="Italic">Euphausia pacifica</Emphasis>) from the northern Gulf of Alaska

The euphausiids Thysanoessa inermis (Kroyer 1846), Thysanoessa spinifera (Holmes 1900), and Euphausia pacifica (Hansen 1911) are key pelagic grazers and also important prey for many commercial fish species in the Gulf of Alaska (GOA). To understand the role of the euphausiids in material flows in this ecosystem their growth rates were examined using the instantaneous growth rate (IGR) technique on the northern GOA shelf from March through October in 2001–2004. The highest mean molting increments (over 5% of uropod length increase per molt) were observed during the phytoplankton bloom on the inner shelf in late spring for coastal T. inermis, and on the outer shelf in summer for T. spinifera and more oceanic E. pacifica, suggesting tight coupling with food availability. The molting rates were higher in summer and lower in spring, for all species and were strongly influenced by temperature. Mean inter-molt periods calculated from the molting rates, ranged from 11 days at 5°C to 6 days at 8°C, and were in agreement with those measured directly during long-term laboratory incubations. Growth rate estimates depended on euphausiid size, and were close to 0 in early spring, reaching maximum values in May (0.123 mm day⁻¹ or 0.023 day⁻¹ for T. inermis) and July (0.091 mm day⁻¹ or 0.031 day⁻¹ for T. spinifera). The growth rates for E. pacifica remained below 0.07 mm day⁻¹ (0.016 day⁻¹) throughout the season. The relationship between T. inermis weight specific growth rate (adjusted to 5°C) and ambient chlorophyll-a concentration fit a Michaelis–Menten curve (r ² = 0.48) with food saturated growth rate of 0.032 day⁻¹ with half saturation occurring at 1.65 mg chl-a m⁻³, but such relationships were not significant for T. spinifera or E. pacifica.     

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.     

Ecology of the euphausiidsThysanoessa raschi,T. inermis andMeganyctiphanes norvegica in Ísafjord-deep,northwest-Iceland

The seasonal abundance, distribution, maturity, growth and population dynamics of the euphausiidsThysanoessa raschi (M. Sars, 1864),T. inermis (Krøyer, 1846) andMeganyctiphanes norvegica (M. Sars, 1857) were studied in Ísafjord-deep, a fjord in northwest Iceland, from February 1987 to February 1988. Sampling was made at nine stations along the length of the fjord at approximately monthly intervals, along with hydrographic measurements and water sampling for nutrient analysis and measurements of chlorophylla concentrations. Spring warming of the water began in late May and maximum temperatures (8° to 10°C) were observed in late July–September. The phytoplankton spring-bloom started in early April, and the highest chlorophylla levels were measured in early May (7.0 mg m^–3). A small increase was observed in the chlorophylla content in August. The greatest abundance of juveniles and males and females of all three species was observed during January and February 1988, during which period the euphausiids were concentrated in the middle and inner parts of the fjord. Euphausiid eggs were first recorded in the plankton in mid-May, and the greatest abundance ofThysanoessa spp. larvae occurred at the end of May. Larvae ofM. norvegica were not observed in Ísafjord-deep, indicating that recruitment of this species was occurring from outside the fjord.T. raschi andT. inermis had a life span of just over 2 yr; the life span ofM. norvegica was more difficult to determine. Almost all femaleT. raschi were mature at the age of 1 yr, while mostT. inermis females appeared not to mature until 2 yr of age. Most males of both species took part in breeding at 1 yr of age. The maximum carapace length ofT. raschi andT. inermis was 8 to 9 and 9 to 10 mm, respectively. The largestM. norvegica had a carapace length of 9 to 10 mm. The spawning of the euphausiids in Ísafjord-deep appeared to be closely related to the phytoplankton spring bloom; water temperature appeared to have no influence on spawning.     

Biology of euphausiids in the subarctic waters north of Iceland

The seasonal abundance, maturity, spawning, and population dynamics of Thysanoessa inermis (Krøyer, 1846), T. longicaudata (Krøyer, 1846), and Meganyctiphanes norvegica (M. Sars, 1857) were studied in the subarctic waters north of Iceland from February 1993 to February 1994. The material was sampled at approximately monthly intervals along a transect of eight stations extending from 66°16′ to 68°00′N at 18°50′W. Information on temperature and chlorophyll a concentrations is also presented. Spring warming of the water began in March to April and maximum temperatures were recorded in August (3.8?°C). The spring bloom of the phytoplankton started in late March and highest chlorophyll a concentrations were measured during middle to late April (7.0?mg chlorophyll a m^?3). T. inermis was the dominant species in the samples, constituting 77% of juvenile, male and female euphausiids present. The greatest abundance of juvenile, male and female T. inermis and M. norvegica was observed during autumn and winter, with lower abundance in spring and summer. T. longicaudata showed only limited changes in seasonal abundance. Male T. inermis had spermatophores in their ejaculatory ducts from February to May, while mature females had spermatophores attached during April and May. T. longicaudata males bore spermatophores from February to July, whereas females only bore spermatophores in April and May. M. norvegica males had spermatophores from February to April, while the single female with spermatophores was caught in February. Euphausiid eggs were first recorded during the latter part of April; the highest numbers of eggs were observed in the samples taken in late May. Maximum numbers of nauplii of both Thysanoessa spp. and M. norvegica were recorded in late May. The main spawning of the euphausiids coincided with the phytoplankton spring bloom. Most male T. inermis took part in breeding at 1 yr of age while most females appeared not to mature until 2 yr of age. T. inermis has a life span of just over 2 yr, T. longicaudata appears to live just over 1 yr. Limited data did not allow the life span of M. norvegica to be determined.     

Interannual variability in the spring phytoplankton bloom in Auke Bay,Alaska

Data on phytoplankton primary production, biomass, and species composition were collected during a 5 yr (1985–1989) study of Auke Bay, Alaska. The data were used to examine the interannual differences in the timing, duration, and magnitude of the spring phytoplankton blooms during each year and to relate these differences to interannual variations in weather patterns. Within any given year, a pre-bloom phase was characterized by low available light, low rates of primary production, low biomass, and predominantly small (<10µm) diatoms. During the primary bloom, integrated production rates rose to 4 to 4.5 g C m^–2 d^–1, and integrated biomass levels reached 415 to 972 mg chlorophyll m^–2. Primary blooms were usually dominated by large diatoms (Thalassiosira spp.), and in a single year (1989) byChaetoceros spp. The primary blooms terminated upon nutrient depletion in the euphotic zone. Secondary blooms, triggered by nutrient resupply from below, occurred sporadically after the primary bloom and accounted for 4 to 31% of total spring production. The date of initiation and the duration of the primary bloom varied little from year to year (standard deviation 3 and 5 d, respectively). Seasonal production rates and biomass levels varied interannually by a factor of 2 to 3. In contrast, intra-annual variations of more than an order of magnitude, especially in biomass, occurred over periods as short as 10 d. These large variations over short time periods indicate the importance of synchronous timing between spring blooms and the production of larval fish and shellfish, which depend on an appropriate and adequate food supply for growth and survival. Parameters describing primary production (e.g. peak daily production, mean daily production, and total production during the primary bloom and the entire season) exhibited little interannual variation (coefficient of variation, CV = 10 to 19%), but a large degree of intra-annual variation (CV = 77 to 116%). Similarly, interannual variations in biomass (peak chlorophyll, mean chlorophyll) were also lower (CV = 20 to 33%) than intra-annual variations (CV = 85 to 120%).     

Phytoplankton production patterns in Massachusetts Bay and the absence of the 1998 winter–spring bloom

The seasonal productivity cycle and factors controlling annual variation in the timing and magnitude of the winter–spring bloom were examined for several locations (range: 42°20.35′–42°26.63′N; 70°44.19′–70°56.52′W) in Boston Harbor and Massachusetts Bay, USA, from 1995 to 1999, and compared with earlier published data (1992–1994). Primary productivity (mg C m⁻² day⁻¹) in Massachusetts Bay from 1995 to 1999 was generally characterized by a well-developed winter–spring bloom of several weeks duration, high but variable production during the summer, and a prominent fall bloom. The bulk of production (mg C m⁻³ day⁻¹) typically occurred in the upper 15 m of the water column. At a nearby Boston Harbor station a gradual pattern of increasing areal production from winter through summer was more typical, with the bulk of production restricted to the upper 5 m. Annual productivity in Massachusetts Bay and Boston Harbor ranged from a low of 160 g C m⁻² year⁻¹ to a high of 787 g C m⁻² year⁻¹ from 1992 to 1999. Mean annual productivity was higher (mean=525 g C m⁻² year⁻¹) and more variable near the harbor entrance than in western Massachusetts Bay. At the harbor station productivity varied more than 3.5-fold (CV=40%) over an 8 year sampling period. Average annual productivity (305–419 g C m⁻² year⁻¹) and variability around the means (CV=25–27%) were lower at both the outer nearfield and central nearfield regions of Massachusetts Bay. Annual productivity in 1998 was unusually low at all three sites (<220 g C m⁻² year⁻¹) due to the absence of a winter–spring phytoplankton bloom. Potential factors influencing the occurrence of a spring bloom were investigated. Incident irradiance during the winter–spring period was not significantly different (P > 0.05) among years (1995–1999). The mean photic depth during the bloom period was significantly deeper (P < 0.05) in 1998, signifying greater light availability with depth. Nutrients were also in abundance during the winter–spring of 1998 with stratified conditions not observed until May. In general, the magnitude of the winter–spring bloom in Massachusetts Bay from 1995 to 1999 was significantly correlated with winter water temperature (r ²=0.78) and zooplankton abundance (r ²=0.74) over the bloom period (typically February–April). The absence of the 1998 bloom was associated with higher than average water temperature and elevated levels of zooplankton abundance just prior to, and during, the peak winter–spring bloom period. Received: 3 July 2000 / Accepted: 6 December 2000     

Benthic response to sedimentation of a spring phytoplankton bloom: Process and budget

The response of benthos to sedimentation of the spring phytoplankton bloom in the Kiel Bight (Western Baltic Sea) is described in terms of biomass (ATP) and activity (heat production and ETS-activity). Input of the bloom (11.5 g C m^-2) over a period from March 25 to April 19, 1980 to the sediment surface was in the form of cells and fresh phytodetritus as indicated by low C/N ratios (7) and high energy charge values (0.78). Benthic microbial activity was immediately stimulated by this input as heat production doubled and the activity of ETS tripled over winter values within 12 d in the absence of a significant increase in ambient temperature. A comparison of the two activity parameters suggests that anaerobic metabolism is more important during the winter (February and March) than after input of the bloom. Meiofauna was not able to take part in the first activity outburst. Benthic ATP-biomass (excluding macrofauna) doubled in late April due to microbial production, and doubled again in early May when meiofauna started reproductive activity. For macrofauna a general statement was not possible, although the sediment surface feeder Macoma baltica commenced a build up of glycogen and lipid resources immediately following bloom input whereas Nephtys ciliata, feeding on sediment and small macrofauna, showed a less pronounced and delayed effect from this input. An energy budget based on heat production measurements was calculated. A daily heat loss of the benthic community of 21.7 KJ m^-2 d^-1 (35.5 KJ m^-2 d^-1) was found, when a depth of 3 cm sediment (5 cm) was assumed. Heat production of macrofauna contributed less than 5% of this activity. The input of the bloom was burned within 21 (13) d. Preliminary estimations for an annual budget suggest that the vertical transport of particulate organic matter via sedimentation can only explain 25% (15%) of the benthic activity in the shallow water ecosystem of the Kiel Bight. This indicates the presence of other sources of organic carbon such as benthic primary production or other transport processes providing carbon to the sediments.Publication No. 384 of the Joint Research Program of Kiel University (Sonderforschungsbereich 95)     

Buoyancy of natural populations of marine phytoplankton

Buoyancy of natural populations of marine phytoplankton was studied in a fjord in western Norway during the diatom bloom and in autumn. The study was carried out under approximate in situ conditions by means of an apparatus described in the paper. During the spring bloom, positive buoyancy was observed only once. Sinking rates of individual fractions ranged from 0 to more than 9 m day^-1, and the mean sinking rates of the total chlorophyll content from 0 to at least 2. 2 m day^-1. The highest rates occurred in the post-bloom period, while sinking appeared negligible from the onset of the bloom up to its culmination. In autumn, the population was dominated by small, flagellated cells. Positive buoyancy, or upward migration, was then observed in two out of three experiments.     

Phytoplankton community structure and primary production in small intertidal estuarine-bay ecosystem (eastern English Channel,France)

From May 2002 to October 2003, a fortnightly sampling programme was conducted in a restricted macrotidal ecosystem in the English Channel, the Baie des Veys (France). Three sets of data were obtained: (1) physico-chemical parameters, (2) phytoplankton community structure illustrated by species composition, biovolume and diversity, and (3) primary production and photosynthetic parameters via P versus E curves. The aim of this study was to investigate the temporal variations of primary production and photosynthetic parameters in this bay and to highlight the potential links with phytoplankton community structure. The highest level of daily depth-integrated primary production Pz (0.02–1.43 g C m⁻² d⁻¹) and the highest maximum photosynthetic rate P ^B _max (0.39–8.48 mg C mg chl a ⁻¹ h⁻¹) and maximum light utilization coefficient α^B [0.002–0.119 mg C mg chl a ⁻¹ h⁻¹ (μmol photons m⁻² s⁻¹)] were measured from July to September. Species succession was determined based on biomass data obtained from cell density and biovolume measurements. The bay was dominated by 11 diatoms throughout the year. However, a Phaeocystis globosa bloom (up to 25 mg chl a m⁻³, 2.5 × 10⁶ cells l⁻¹) was observed each year during the spring diatom bloom, but timing and intensity varied interannually. Annual variation of primary production was due to nutrient limitation, light climate and water temperature. The seasonal pattern of microalgal succession, with regular changes in composition, biovolume and diversity, influenced the physico-chemical and biological characteristics of the environment (especially nutrient stocks in the bay) and thus primary production. Consequently, investigation of phytoplankton community structure is important for developing the understanding of ecosystem functioning, as it plays a major role in the dynamics of primary production.     

Trophic implications of cross-shelf copepod distributions in the Southeastern Bering Sea

Spring distributions of some numerically dominant copepods reflect associations with two distinct water masses separated along the 80- to 100-m isobaths. Seaward of this middle shelf front, the oceanic Bering Sea hosts populations of Calanus cristatus, C. plumchrus, and Eucalanus bungii bungii; Metridia pacifica, Oithona similis, and Pseudocalanus spp. are also present. The large oceanic species are much less abundant in waters shallower than 80 m where the community is seasonally dominated by smaller copepods, O. similis, Acartia longiremis, and Pseudocalanus spp. Experimental and field-derived estimates of carbon ingestion indicate that the oceanic/outer shelf copepods can occasionally graze the equivalent of the daily plant production and probably routinely remove 20–30% of the primary productivity. Conversely, stocks of middle shelf copepods rarely ingest more than 5% of the plant carbon productivity. During 45 d between mid April to late May, 1979, approximately three times more organic matter was ingested m^-2 by the outer shelf/oceanic copepod community than by middle shelf species. This imbalance in cross-shelf grazing permits middle shelf phytoplankton stocks to grow rapidly to bloom proportions, and to sink ungrazed to the seabed. Over the outer shelf and particularly along the shelf break, a much closer coupling to phytoplankton supports a large biomass of oceanic grazers. Here, copepod stocks approaching 45 g dry wt m^-2 occur in late spring as a narrow band at the shelf break.Supported by National Science Foundation Grant DPP 76-23340Contribution no. 485, Institute of Marine Science, University of Alaska, Fairbanks     

Mass encystment of a planktonic oligotrich ciliate

Sediment traps were deployed in a 60 m water column near the Eddystone Lighthouse on the south west coast of England for one year from March 1984. In April, the traps collected the cysts of a planktonic ciliate with a flux of almost 35 000 m^-2 d^-1. The cysts were attributed to a planktonic oligotrich [provisionally identified as Strombidium crassulum (Leegard) Kahl], which was common in the water at the time. Identification was achieved through the discovery of the incipient formation of the cysts in preserved water samples and by the similarity of the morphology of the cysts with that of other oligotrichid ciliates. Attempts to prove the relationship by incubation have so far failed. The production of the cysts followed the main spring bloom of diatoms and coincided with a small bloom of autotrophic oligotrichs. In the traps, the total number and percentage of cysts with contents decreased rapidly after the encystment event. However, potentially viable cysts were still recorded in the plankton eleven months later, with minimum fluxes of 200 cysts m^-2 d^-1. Resuspension of bottom sediments by tides and storms ensured that a large population of cysts was always present in the water column during the winter, awaiting the right conditions to stimulate excystment and the initiation of a new motile population.     

Life cycle, seasonal vertical distribution and feeding of Calanus finmarchicus in Skagerrak coastal water

 Day/night zooplankton sampling in Kosterfjorden, Sweden, gave information on population succession, vertical distribution and feeding of Calanus finmarchicus over 17 months. Copepodid Stage 1 and 2 (C-I, C-II) were present from December to August, indicating reproduction during most of the year. Mating and breeding for the overwintering generation mainly took place in February/March, resulting in peak abundance of C-I in March/April. Secondary breeding periods were in April and July/August, but the resulting recruitment from these were low. The relative recruitment success in the first spring was around 13 times higher than during the second spring. Low temperature in the surface water early in the year and depression of the spring phytoplankton bloom may have caused the failure in the second year. Population biomass peaked at >15 g dry weight m⁻² during the numeric peak of the youngest stages in March/April. A more sustainable level of high biomass of 8 to 10 g dw m⁻² was gradually built up during summer, mainly due to a continuous accumulation of C-Vs. Adults and C-Vs comprised the overwintering population, with 7 to 14% and 85 to 93%, respectively, for the 2 years, but only C-Vs staying in the deep water were in a resting state. Adult males showed a strong diurnal vertical migration (DVM) of the usual type from spring to early autumn and a reversed DVM during the cold season. They were in a feeding state throughout the year. Adult females showed the usual type of DVM during summer to autumn, but commonly a reversed DVM during winter to spring. They were usually in a feeding state, with no pronounced differences between surface and deep water or between day and night. C-Vs aggregated in the deep water from October to March and performed DVM in April to June. They were commonly feeding in the deep water between March and June but showed no or very low feeding activity there from July to February. C-Vs in the surface water were commonly feeding and showed the highest proportion of feeding in autumn, when the population in the deep water was inactive. C-V constituted up to nearly 100% of the population biomass, and therefore must be of profound ecological importance. Defined by this dominant role, the population of C. finmarchicus can be characterised as having an active period of feeding, reproduction and development from February to July with a following 6 to 7 months of resting in the deep water, when development is arrested and no feeding occurs. Received: 1 October 1999 / Accepted: 27 April 2000