Michael A. Sleigh

Picoplanktonic community structure on an Atlantic transect from 50°N to 50°S

Plankton samples were collected from 10 depths at 25 stations spaced at intervals of about 4° of latitude along a transect from the British Isles to the Falkland Islands. Four categories of picoplankton were discriminated: Synechococcus spp., Prochlorococcus spp., eukaryotic picophytoplankton and heterotrophic bacteria. The populations in each category in the samples were counted by flow cytometry and the mean size of bacterial cells was determined by fractionation through filters. Categories of phototrophic cells were discriminated by size and by the fluorescence of photosynthetic pigments; samples stained with the fluorochrome TOTO were used to enumerate heterotrophic bacteria (and Prochlorococcus in surface waters where their chlorophyll content was very small). The carbon biomass concentration of each category in each sample was calculated. Prochlorococcus was present at all stations between 47°N and 38°S, and reached peak population densities above 200,000 cells ml-1 in equatorial waters; the depth occupied by these cells increased in oligotrophic waters, where they dominated picophytoplankton biomass. Synechococcus reached high concentrations in the Mauritanian upwelling region and in the frontal region near the southern end of the transect, where they represented the largest single component of picophytoplankton biomass, but was almost absent in oligotrophic regions. Picoeukaryotes were present in low numbers at all latitudes, but they are larger cells and constituted a substantial part of the total picophytoplankton biomass at most latitudes. The depth-integrated (200 m) biomass of heterotrophic bacteria was nearly as great as that of the picophytoplankton at all latitudes, because substantial numbers of cells occurred at all depths. Numbers and biomass of these bacteria were maximal in the upwelling region and high at both ends of the transect. There was a clear contrast in the composition of the picoplankton community in both the North and South Atlantic between mesotrophic waters where Synechococcus and picoeukaryotes dominated the biomass, and oligotrophic waters where the smaller total biomass was dominated by Prochlorococcus.

Picoplankton community structure on the Atlantic Meridional Transect: a comparison between seasons

Abstract Samples collected from 10 depths at 25 stations in September–October 1996 and 12 depths at 28 stations in April–May 1997 on an Atlantic Meridional Transect between the British Isles and the Falkland Islands were analysed by flow cytometry to determine the numbers and biomass of four categories of picoplankton: Prochlorococcus spp, Synechococcus spp, picoeukaryotic phytoplankton and heterotrophic bacteria. The composition of the picoplankton communities confirmed earlier findings ( Zubkov, Sleigh, Tarran, Burkill & Leakey, 1998 ) about distinctive regions along the transect and indicated that the stations should be grouped into five provinces: northern temperate, northern Atlantic gyre, equatorial, southern Atlantic gyre and southern temperate, with an intrusion of upwelling water off the coast of Mauritania between the northern Atlantic gyre and equatorial waters. Prochlorococcus was the most numerous phototrophic organism in waters of both northern and southern gyres and in the equatorial region, at concentrations in excess of 0.1×106ml−1; it also dominated plant biomass in the gyres, but the biomass of the larger picoeukaryotic algae equalled that of Prochlorococcus in the equatorial region; higher standing stocks of both Prochlorococcus and picoeukaryotes were present in spring than in autumn in waters of both gyres. In temperate waters at both ends of the transect the numbers and biomass of picoeukaryotes and, more locally, of Synechococcus increased, and the Synechococcus, particularly, were more numerous in spring than in autumn. There was a pronounced southward shift of the main populations of both Synechococcus and Prochlorococcus in April–May in comparison to those of September–October, associated with seasonal changes in solar radiation, the abundance of Prochlorococcus dropping sharply near the 17°C contour, while Synechococcus was still present at temperatures below 10°C. Picoeukaryotes were more tolerant of low temperatures and lower light levels, often being more abundant in samples from greater depths, where they contributed to the deep chlorophyll maximum. Heterotrophic bacterial numbers and biomass tended to be highest in those samples where phototrophic biomass was greatest, with peaks in temperate and equatorial waters, which were shifted southwards in April–May compared with September–October.

Microzooplankton And Their Herbivorous Activity In The Northeastern Atlantic-Ocean

Microzooplankton and their herbivorous activity were studied at four stations between 60 and 47°N along the 20°W meridian in mid-summer, as part of the 1989 U.K. Biogeochemical Ocean Flux Study. Microzooplankton were abundant in the surface mixed layer at all stations, with concentrations ranging between 2000 and 14,000 organisms l−1. The community was dominated by Protozoa, which contributed > 98% of the numerical abundance of microzooplankton and > 80% of their standing stocks. Three families of Protozoa—the Strombidiidae (Ciliophora: Oligotrichida), Gymnodiniidae and Peridiniidae (both Sarcomastigophora: Dinoflagellida)—dominated the microzooplankton, contributing > 70% of the standing stocks. This varied between 0.6 and 20.3 μC l−1 and was positively correlated to ambient phytoplankton concentrations. Microzooplankton standing stocks within the mixed layer showed a distinct latitudinal gradient that varied between 6.5 and 9.8 μgC l−1 at 60 and 47°N, respectively. When integrated over the mixed layer depth, microzooplankton standing stocks ranged between 300 and 428 mgC m−2 at 47 and 60°N, respectively. Microzooplankton standing stocks were approximately one-quarter to one-third that of the phytoplankton. Microzooplankton herbivory, investigated using dilution experiments, was vigorous. Microzooplankton turned over between 2 and 45% of the phytoplankton standing stock daily, thereby consuming between 0.5 and 17 μg phytoplankton carbon l−1 day−1. The magnitude of this activity was positively related to seawater temperature and to phytoplankton concentration. Microzooplankton consumed the equivalent of 100–800% their body carbon each day. Analysis of phytogenic carbon flux in the surface mixed layer showed that microzooplankton herbivory was more than an order of magnitude higher than copepod herbivory. Microzooplankton in the mixed layer grazed between 288 and 589 mgC m−2 day−1 and accounted for between 39 and 115% of the phytoplankton production at 60 and 52°N, respectively. Microzooplankton are clearly important in determining the fate of phytogenic carbon in the northeastern Atlantic Ocean during the mid-summer period.

Microbial community structure and standing stocks in the NE Atlantic in June and July of 1996

The standing stocks of nanophytoplankton and picoplankton in the northeast Atlantic Ocean in June and July 1996 were quantified using flow cytometry and microscopy. Diatoms and dinoflagellates were analysed by microscopy and coccolithophores, other nanophytoplankton, picoeukaryotic phytoplankton, cyanobacteria (Synechococcus spp.), prochlorophytes (Prochlorococcus spp.) and heterotrophic bacteria by flow cytometry. The research was divided into three components: a lagrangian study of a nutrient replete cold-core eddy centred around 59° 12′N 20° 12′W; a transect close to the 20°W meridian from 59° 18′N to 37°N, which passed through contrasting water masses; and a lagrangian study in oligotrophic waters, centred around 36° 42′N 19° 12′W. The eddy was characterised by a bloom of the coccolithophore Coccolithus pelagicus whose standing stocks averaged 4.26 g C m−2 over the upper 50 m. C. pelagicus and other nanophytoplankton (excluding diatoms and dinoflagellates) dominated the standing stocks of the microbial community, averaging approx. 70% of the total microbial standing stocks of the groups quantified. The majority of the remaining biomass was accounted for by the picoeukaryotic phytoplankton and heterotrophic bacteria. The microbial community immediately outside the eddy was significantly different in both composition and standing stocks. There were no C. pelagicus outside the eddy and fewer nanophytoplankton, resulting in microbial standing stocks of approx. one-third that found in the eddy. The transect was characterised by a frontal region at approx. 52° 30′N. There was a general decrease in the standing stocks of all components of the microbial community from the start of the transect to the front. Just to the south of the front, nanophytoplankton, Synechococcus spp. and heterotrophic bacteria showed marked increases in standing stocks, especially the nanophytoplankton, which increased from 3.43 to 7.90 g C m−2. The nanophytoplankton dominated the microbial standing stocks throughout the transect, even in the oligotrophic waters where the integrated carbon biomass was 4.58 g C m−2, representing 69% of the total microbial standing stocks. During the lagrangian study around 37°N the picoplanktonic community was dominated by heterotrophic bacteria. However, heterotrophic bacteria standing stocks decreased with time, along with Synechococcus spp. and picoeukaryotic phytoplankton. Peak biomass for these three groups shifted deeper down in the water column with time. Prochlorococcus spp. were only present towards the end of the transect and at the oligotrophic site. At the oligotrophic site their standing stocks increased, unlike other groups, so that they became the dominant picophytoplanktonic group.

Hydrodynamic aspects of particle capture by Mytilus

Available data on the size distribution of particles captured by Mytilus indicates that a somewhat leaky, but non-sticky, filter with a minimum mesh size of a little over 1.0 μm is present. The side branches of the latero-frontal cirri have such a mesh and are positioned so as to filter water passing between the gill filaments. Information on the pressure distribution within the water circulation, and calculations of the pressure required to pass watet ihrough the latero-frontal filter at observed speeds, suggest that a little more than half the pressure generated by the lateral ciliary pump is required to overcome resistance in the latero-frontal filter and the remainder is required to circulate water in through the inhalant opening and out in an exhalant jet. The lateral cilia appear theoretically capable of generating sufficient power to produce the required pressures. Calculations indicate that although hydro-mechanical shear forces are unlikely to provide a significant contribution to particle capture, they may provide a means of keeping particles within the frontal current without a need for mucus, after filtration and transfer of particles to the frontal surface by the latero-frontal cirri.

Planktonic ciliates in Southampton Water: abundance, biomass, production, and role in pelagic carbon flow

The abundance and biomass of marine planktonic ciliates were determined at monthly intervals at two stations in Southampton Water between June 1986 and June 1987. The two stations, an outer one at Calshot and an inner one at N. W. Netley, were subject to differing marine and terrestrial influences. The potential ciliate production at cach station on each visit was estimated from these data. Enumeration of ciliates and measurements of biovolume were performed on Lugols iodinepreserved samples and potential production was calculated using a predictive relationship based on temperature and cell volume. Heterotrophic ciliate abundance and biomass were greatest at both stations during spring and summer months, with respective maxima of 16x103 organisms 1-1 and 219 μg Cl-1 recorded at N. W. Netley. Estimates of the potential production of the ciliate community ranged from <1 to 18 μg Cl-1 d-1 at Calshot and <1 to 141 μg Cl-1 at N. W. Netley, with annual values of 2 and 9 mg Cl-1 yr-1, respectively. Abundances, biomass and potential production estimates were generally greater at N. W. Netley than at Calshot. Carbon flow through the ciliate community was assessed using annual production values from both this study and the literature. The annual ciliate carbon requirement was equivalent to 9 and 11% of annual primary production at Calshot and N. W. Netley, and potential annual ciliate production was equivalent to 34% and >100% of the energy requirements of metazoan zooplankton at these locations, although comprising only 8 and 10% of their available food.

Adaptations of ciliary systems for the propulsion of water and mucus.

1. The characteristics of ciliary systems are determined by the dominance of viscous effects over inertial effects. 2. The velocity of water propulsion depends on ciliary length, beat frequency, pattern of beating, the arrangement of the cilia and their co-ordination. Beating cilia influence a layer of water only two or three cilium lengths deep, with maximal velocity near the ciliary tip. 3. Mucus is propelled by the tips of short cilia that penetrate the mucus; these cilia are closely spaced on epithelia, and achieve slow propulsion that is relatively independent of load and does not require strong ciliary co-ordination.

Protozoan communities in chalk streams

The population densities of the main groups of protozoa in the principal protozoan habitats in a chalk stream were surveyed over period of a year or more. These data were used to estimate the mean biomass and annual production of flagellates, ciliates and amoebae in the different habitats, and to make comparisons with the estimated production of macrophytes, algae and bacteria in the stream. Protozoa epiphytic on macrophytes accounted for about 30% and protozoa in soft sediments about 45% of the overall protozoan production. Lesser amounts were attributed to protozoa in stony sediments, those epizoic on insects and crustaceans, and least to protozoa in the water column. Ciliates contributed about 75% of the protozoan production, flagellates 15% and amoebae 10%. The total annual production by protozoa was estimated to be approximately 16 g dry wt m−2 stream floor, which was equal to about 20% of the bacterial and about 1% of the algal production. However, protozoan production was of the same order of magnitude as that by Gammarus, and more than the production by fish.

Ingestion and Assimilation by Marine Protists Fed on Bacteria Labeled With Radioactive Thymidine and Leucine Estimated Without Separating Predator and Prey

A procedure has been developed for preparing living bacteria, quantitatively labeled with 3H-thymidine and 14C-leucine, for short-term grazing experiments. The negligible rate of accumulation in protozoan macromolecules of moieties of bacterial macromolecules labeled with 3H compared with moieties labeled with 14C permits estimation of the consumption, digestion, and assimilation of prey biomass in protists without separating them from bacteria. The principles of this method are described, and the results of its application in examples of grazing by the ciliates Euplotes and Uronema and the flagellate Pteridomonas on the bacterium Vibrio are outlined.

Growth of Amoebae and Flagellates on Bacteria Deposited on Filters

A bstractArtificial bacterial biofilms were formed by making microwave-irradiated, dual-radioisotope-labelled Vibrio bacteria adhere to 0.4 μm pore size filters with albumin. The rate of release of 3H from thymidine label in these bacteria into the surrounding seawater when protozoa were incubated with the biofilm indicated the predators grazing rate, and the rate of accumulation of 14C in the predators from leucine label in the bacteria indicated the assimilation rate of the protozoa. The amoeba Vanella septentrionalis consumed about 60% of the available bacteria between the 5th and 15th days of incubation with a gross growth efficiency of 22 ± 6%, compared with about 75% consumption at 29 ± 8% efficiency for the surface-feeding flagellate Caecitellus parvulus, and about 55% consumption at 16 ± 5% efficiency for the suspension-feeding flagellate Pteridomonas danica. As a result of their grazing and metabolism these protozoa regenerated about 70–85% of the nutrients present in their food and released these nutrients in the immediate vicinity of the bacterial biofilm. The biomass of the amoeba Vanella was calculated to be 166 pg protein cell−1 during maximum growth and 93 pg protein cell−1 in the stationary phase.