Phytoplankton blooms are strongly impacted by microzooplankton grazing in coastal North Pacific waters

Phytoplankton growth and microzooplankton grazing were measured in two productive coastal regions of the North Pacific: northern Puget Sound and the coastal Gulf of Alaska. Rates of phytoplankton growth (range: 0.09–2.69 day⁻¹) and microzooplankton grazing (range: 0.00–2.10 day⁻¹) varied seasonally, with lowest values in late fall and winter, and highest values in spring and summer. Chlorophyll concentrations also varied widely (0.19–13.65 μg l⁻¹). Large (>8 μm) phytoplankton cells consistently dominated phytoplankton communities under bloom conditions, contributing on average 65% of total chlorophyll biomass when chlorophyll exceeded 2 μg l⁻¹. Microzooplankton grazing was an important loss process affecting phytoplankton, with grazing rates equivalent to nearly two-thirds (64%) of growth rates on average. Both small and large phytoplankton cells were consumed, with the ratio of grazing to growth (g:μ) for the two size classes averaging 0.80 and 0.42, respectively. Perhaps surprisingly, the coupling between microzooplankton grazing and phytoplankton growth was tighter during phytoplankton blooms than during low biomass periods, with g:μ averaging 0.78 during blooms and 0.49 at other times. This tight coupling may be a result of the high potential growth and ingestion rates of protist grazers, some of which feed on bloom-forming diatoms and other large phytoplankton. Large ciliates and Gyrodinium-like dinoflagellates contributed substantially to microzooplankton biomass at diatom bloom stations in the Gulf of Alaska, and microzooplankton biomass overall was strongly correlated with >8 μm chlorophyll concentrations. Because grazing tended to be proportionally greater when phytoplankton biomass was high, the absolute amount of chlorophyll consumed by microzooplankton was often substantial. In nearly two-thirds of the experiments (14/23), more chlorophyll was ingested by microzooplankton than was available for all other biological and physical loss processes combined. Microzooplankton were important intermediaries in the transfer of primary production to higher trophic levels in these coastal marine food webs. Received: 12 November 1999 / Accepted: 4 October 2000     

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.     

Excretion of ammonia by zooplankton and its potential contribution to nitrogen requirements for primary production in the Catalan Sea (NW Mediterranean)

Excretion of ammonia by mesozooplankton (>200 m zooplankton) and its potential contribution to the nitrogen requirement for phytoplankton growth has been estimated for different hydrographical situations along a transect across the Catalan Sea (Northwestern Mediterranean). The nitrogen excreted as ammonia was estimated from mesozooplankton biomass and specific excretion rates. Nitrogen requirements of phytoplankton were estimated by means of carbon fixation rates and C:N ratios of <200 m particulate organic matter. Minimum C:N ratios and maximum primary production, zooplankton biomass, phytoplankton nitrogen requirements, and nitrogen excretion of zooplankton occurred near the Catalan density front. On average, the nitrogen regenerated by the mesozooplankton accounted for 43% of the nitrogen requirements of the phytoplankton. The specific excretion rates of ammonia and the percentage of phytoplanktonnitrogen requirements supplied by excreted nitrogen were higher at coastal stations. In some coastal and frontal stations, the ammonia excreted exceeded the phytoplanktonnitrogen demand. Bacteria competing for nutrient supply and the possible uncoupling between rate processes and standing stocks of phyto- and zooplankton could explain the apparent excess of regenerated ammonia.     

A spatial model of phytoplankton patchiness

The one-dimensional theory of critical-length scales of phytoplankton patchiness is developed to include phytoplankton growth and herbivore grazing as functions of time and space. The critical-length scale L _c for the pathch is then determined by the initial spatial distribution and concentration of the limiting nutrient and herbivores in addition to the daily averaged values of the growth and loss processes. The response of an initial phytoplankton patch to the stresses of turbulent diffusion, nutrient depletion, light periodicity, and nocturnal or continuous herbivore grazing is investigated numerically for several oceanic conditions. Nocturnal grazing, while less stressful on primary production than continous grazing, results in lower phytoplankton standing stocks. Increase in biomass of vertically migrating zooplankton results in a net loss of nutrient which might otherwise be egested, recycled, and utilized in the euphotic zone under continuous grazing conditions. The Ivlev constant is shown via sensitivity analysis to be a significant parameter ultimately influencing phytoplankton production. It is demonstrated numerically that diffusion of phytoplankton cells from areas of high concentration to low concentration prevents the local extinction of the standing stock, thereby rendering a positive herbivore grazing-threshold unnecessary for ecosystem stability.     

Zooplankton abundance and grazing at Davies Reef,Great Barrier Reef,Australia

Zooplankton abundance and grazing on autotrophic and heterotrophic particulate matter were measured along a transect across Davis Reef (18°5S; 147°39E) and in the back-reef lagoon over tidal and diel cycles during austral winter (August 1984). Zooplankton entering the reef from the surrounding shelf waters decreased in abundance over the reef flat, presumably because of predation. Within the reef lagoon, maximum daytime densities of pelagic copepods occurred during high water, suggesting an external input. At night, water-column zooplankton biomass increased by a factor of 2 to 3 due to the emergence of demersal reef zooplankton. Zooplankton grazing rates on heterotrophic particulate matter (bacteria + detritus and Protozoa) compared to phytoplankton were higher on the reef flat than on the fore-reef or lagoon. Within the lagoon, zooplankton grazing rates on heterotrophic material were maximum during high water, coincident with maximum tidal concentrations of particulate organic carbon. The combined demersal and pelagic zooplankton community were often able to crop 30% of the daily primary production by >2µm phytoplankton. However, >50% of phytoplankton biomass was in cells <2µm, presumably unavailable to these zooplankton. Our particulate production and ingestion measurements, together with zooplankton carbon demand extrapolated from respiration estimates, suggest that the zooplankton community of Davies Reef derives much of its nutrition from detritus.Joint contribution from the University of Maryland, Center for Environmental and Estuarine Studies (No. 2015), and the Microbial Ecology on a Coral Reef Workshop (MECOR No. 19)     

Influence des conditions hivernales sur les productions phyto-et zooplanctoniques en Méditerranée Nord-Occidentale. V. Biomasse et production zooplanctonique — relations phyto-zooplancton

The cruise “Mediprod I” of the R.V. “Jean Charcot” covered an area of the Mediterranean Sea situated North of the 40th parallel and West of Corsica; during two 15-day legs, the first when Winter conditions ended, in March, the second in Spring conditions, in April, a 48-station network was surveyed as to primary and secondary production, as well as hydrological conditions. The first survey revealed a higher concentration of zooplankton in hydrologically stable areas, especially in the South-East, although zooplankton values were rather low throughout the whole area surveyed. The total zooplankton per unit surface was lower at the edges of the surveyed area, where phytoplankton was scarce. An increase in zoo-plankton biomass was observed between the two surveys, mostly in the central areas and near the surface. An important difference between both legs is in the proportion of organisms of different sizes collected by the Clarke-Bumpus sampler: the-200 μ:+200 μ organisms ratio, which is around 1 during the first leg, is much lower during the second leg. Two methods were used in estimating the biomass: the +200 μ fraction of a Clarke-Bumpus sample (Cl-B towed with a 50-μ net fitted on the sampler) was collected by sieving the sample through a 200-μ mesh nylon; standard vertical WP 2 hauls were performed (200 μ mesh). Both roughly show the same zooplankton (weight per unit surface) distribution pattern. However, higher estimations of the total biomass of the volume investigated were sometimes provided by the Clarke-Bumpus method during the first leg, probably due to the distribution of the animals according to their size-classes. A graph of chlorophyll versus zooplankton for surface waters suggests that zooplankton has a limiting effect on the development of phytoplankton in April only. Apparent growth rates of cooplankton are less than those for populations in the laboratory or enclosed environments. Values of the mean secondary production vary from 18 mg C.m^-2.day^-1 for the first leg, to 230 mg C.m^-2.day^-1 for the second leg. Estimations of net efficiency for energetic transfer between phyto- and zooplankton lie between 7 and 26%. As far as our hypotheses as regards physiological coefficients are valid, we can assume that the effect of grazing relative to primary production is greater in the border areas than in the central area, thus increasing the contrast between both areas with time. Phosphorus excretion rate by zooplankton seems to be less than that measured in the Atlantic Ocean. We suggest that in Spring zooplankton excretion is not the main phosphorus recycling process. Primary productivity measurements, apparent growth coefficients, and estimated grazing rate have been used to calculate the expected mean biomass per unit area during the second leg. A 7% loss of phyto- and zooplankton from the upper 100-m layer must be assumed to explain the observed biomass variation. Vertical mixing and sinking of surface water on an isopycnal slope are responsible for such loss, which can also affect, to an undetermined degree, the phosphorus stock introduced into the surface layer by the Mediterranean deep-water formation mechanism. We suggest that nearly half the total loss of phytoplankton from March to April is attributable to animal grazing.     

Mass occurrence of Salpa fusiformis in the spring of 1984 off Ireland: implications for sedimentation processes

Zooplankton species composition and biomass were investigated during the spring of 1984 in three areas west of Ireland. In general, biomass of the gelatinous zooplankters [Salpa fusiformis (Cuvier) forma gregata and solitaria, Cymbulia sp., Euclio sp.; max. 360 mg Cm^-3] exceeded that of other zooplankton namely copepods (max. 70 mg C m^-3). Feeding by salps in the upper layers of all areas during the observed diatom spring bloom resulted in sedimentation of diatom-rich salp fecal pellets. This process ended the diatom spring bloom prior to nutrient depletion in surface waters and, thus, prior to mass sedimentation of algal cells.Publication No. 17 of the SFB 313 at Kiel University     

Dependence of phytoplankton production on rhythm and rate of elimination

This paper investigates the dependence of phytoplankton production upon rhythm and rate of zooplankton grazing and presents a mathematical model for calculating the most important parameters. Both uniform and non-uniform grazing are described mathematically. Non-uniform grazing, expressed by a sinusoidal curve, is usually found in bathyplanktonic ecosystems with migratory consumers. Phytoplankton production depends on the time of grazing; the nearer grazing occurs toward nightfall, the higher is the phytoplankton production. In order to calculate phytoplankton productivity and the amount of food consumed by the zooplankton, experimental data on generation time of phytoplankters, their mortality rates, initial and final standing stocks, and information on diurnal grazing rhythms must be available. If the distribution of grazing rates is sinusoidal and mortality rate constant, the equations presented allow the calculation of phytoplankton productivity with an error of about 6%.     

Zooplankton biomass and indices of grazing and metabolism during a late winter bloom in subtropical waters

The development of the so-called late winter bloom in subtropical water was studied in an oceanic area north of the Canary Islands from January to May 2000. Zooplankton was sampled at short-term intervals (1–4 days) during the bloom (January–March), and biomass, indices of grazing (gut fluorescence) and metabolism (electron transfer system activity, ETS) were measured in four different size fractions (100–200, 200–500, 500–1000 and >1000 µm). During the bloom, ETS activity and gut fluorescence increased before the development of zooplankton biomass. At the end of February, the presence of an impressive cloud of dust formed in the Sahara desert was related to an increase in chlorophyll and small zooplankton a week later. The increments in biomass were the consequence of consumption by zooplankton as inferred from the indices of grazing and metabolism. Estimated grazing from gut fluorescence and gut evacuation rates during the period of study accounted for 55% of the assessed total ingestion from respiration and normal values of assimilation, showing the importance of the non-pigmented food in the diet of zooplankton in these waters. In contrast, the sharp decreases in zooplankton biomass observed during the bloom appeared during the dark period of the moon, the days in which the diel vertical migrants reach the shallower layers, in agreement with previous works in the area. Thus, the development of the late winter bloom in this region is suggested to be driven by the interplay between resource and consumer controls.Communicated by S.A. Poulet, Roscoff     

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.     

Variability and its effect on the 24h chlorophyll budget of a small marine basin

In an intensive study (lasting 25 h) of the production, export and grazing of phytoplankton in a small marine basin, it was found that 58% of the production (11% of the total standing stock) was lost by exchange with the sea and 34% was consumed by grazing of zooplankton. The measured production of phytoplankton could be balanced, to within a few percent, against grazing, export, and a small, measured, net change in the total standing stock of the basin. Large variations were observed in concentrations of chlorophyll and zooplankton at the mouth of the basin over the 25 h period. These variations were associated with changes in the height of the tide, but were about 2^1/2 h out of phase with it. Strong negative correlations were observed between chlorophyll and transport, such that only 35% of the chlorophyll exported was exchanged via the mean flow, while 65% was exchanged via the fluctuations. The correlation was even more striking with zooplankton, for which virtually all the export was associated with the fluctuations in the transport. Time series observations in the centre of the basin revealed considerable short-term variability in both chlorophyll and zooplankton, but the variations were smaller than those observed at the mouth of the basin, and the phase lag with the tide was longer. The variability studies enable suggestions to be made about more economical design of sampling programs, but illustrate the difficulty of providing verification data for any continuous model of primary production in such a basin.Bedford Institute Contribution No. 231.Canadian Contribution to IBP No. 97.     

Distribution and abundance of choanoflagellates (Acanthoecidae) in the coastal cold ocean of Newfoundland, Canada

Water samples from six bays were taken over a 5-year period (1988 to 1992) to determine the distribution and abundance of loricate choanoflagellates in coastal Newfoundland, and to assess the impact that these organisms might have on this cold ocean food web. Scanning electron microscopy was used to study the morphology of these flagellates, allowing us to identify 11 species of loricate choanoflagellates. Parvicorbicula socialis (Meunier) Deflandre was the most abundant species (80 × 10³ cells l⁻¹), particularly during the spring diatom bloom. Single-cell species, such as Bicosta spini fera (Throndsen) Leadbeater and Calliacantha natans (Grontved) Leadbeater, were found more commonly after the spring diatom bloom in the summer months. Many of the single-cell choanoflagellates were attached to bacteria-rich microaggregates and debris in the water column and in unpoisoned sediment traps. The P. socialis cell flux was calculated to be 5.3 × 10⁶ cells m⁻² d⁻¹ in late May sediment traps. P. socialis in the upper 100 m of the water column was removing 0.3% of the standing crop of bacteria each day (April/May), and the equivalent of 7.4% of the daily bacterial production over the water column. Diel studies of P. socialis in Conception Bay suggest that the sharp decline in population numbers observed in midnight samples may be related to the high number of grazing zooplankton observed during the same period. Pelagic tunicate and zooplankton fecal pellets were found to contain large numbers of choanoflagellate costae, thus providing a direct link from the microbial loop to the macrozooplankton. Received: 17 March 1997 / Accepted: 9 May 1997     

Trophodynamic function of copepods,appendicularians and protozooplankton in the late summer zooplankton community in the Skagerrak

The study was carried out in the Skagerrak during late summer when population development in the pelagic cycle culminated in the yearly maximum in zooplankton biomass. The cyclonic circulation of surface water masses created the characteristic dome-shaped pycnocline across the Skagerrak. The large dinoflagellate Ceratium furca dominated the phytoplankton biomass. Ciliates and heterotrophic dinoflagellates were the major grazers and, potentially, consumed 43–166% of daily primary production. The grazing impact of copepods was estimated from specific egg production rates and grazing experiments. The degree of herbivory differed between species (14–85%), but coprophagy (e.g. feeding on fecal pellets) and ingestion of microzooplankton were also important. The appendicularian Oikopleura dioica was present in lower numbers than copepods, but cleared a large volume of water. The grazing impact of copepods and O. dioica was estimated to 57±24% and 12±12% of daily primary production, respectively. Sedimentation of organic material (30 m) varied between 169 and 708 mg C m^–2 day^–1, and the contribution from the mesozooplankton (copepod fecal pellets and mucus houses with attached phytodetritus of O. dioica) was 5–33% of this sedimentation. Recycling of fecal pellets and mucus houses in the euphotic zone was 59% and 36%, respectively. However, there was a high respiration of organic material by microorganisms in the mid-water column, and 34% of the sedimenting material actually reached the benthic community in the deep, central part of the Skagerrak.     

Production and respiration of overall plankton and ultraplankton communities at the entrance to the Gulf of Finland in the Baltic Sea

Primary productivity and respiration of the overall plankton community and of ultraplankton (organisms passing through a 3-m Nuclepore filter) were studied at the entrance to the Gulf of Finland during the growth season in 1982. Data of the respiration measurements from previous years are also presented. During the development of a diatom spring bloom, the algal component could be successfully separated from the bacterial component by size fractionation with a 3-m Nuclepore filter and thus the algal respiration could be approximated, being on the order of 10 to 20% of the gross production. After the phytoplankton spring maximum, bacteria played an important role in mediating the energy flow from phytoplankton exudates to higher trophic levels. Maximum values of 1 230 and 740 mg O₂ m^-2 d^-1 were recorded for overall and for ultraplankton respiration, respectively, during late July. High productivity values coupled with low phytoplankton biomass and low inorganic nutrient values were also recorded in late July, indicating effective nutrient regeneration and rapid turnover of the plankton community. During late summer, a considerable fraction (over 30%) of phytoplankton production was released as exudates, suggesting that much of the energy is channeled to higher trophic levels via bacterial pathways rather than by direct herbivorous grazing during this season. The summer development of phytoplankton community structure and functioning is strongly controlled by hydrographic conditions, i.e. by nutrient inputs via upwelling and by water temperature. A carbon budget for late summer indicated that bacteria may contribute only up to 50% of the overall respiration of the plankton community, which suggests that heterotrophs other than bacteria play an important role in nutrient regeneration. The present study stresses the importance of energy flow via the phytoplankton exudatebacteria-micrograzer pathway in relatively oligotrophic, brackish water ecosystems.     

Influence des conditions hivernales sur les productions phyto- et zooplanctoniques en Méditerranée Nord-Occidentale. II. Biomasse et production phytoplanctonique

Hydrological observations, and measurements of nutrient chemistry, plankton biomass, and production were carried out during the Médiprod I cruise of the R.V. “Jean-Charcot”. The March cruise was characterized by almost winter conditions, exhibiting strong vertical mixing of water masses in the offshore region and nutrient transport up to the photic zone. According to the working hypothesis, the strong vertically mixed area (e.g. Station 15) and the surrounding oligotrophic area are separated by an intermediate zone, where both nutrients and stability have produced phytoplankton-bloom conditions. During the April cruise, highest biomass and production rates were encountered everywhere in the offshore region, and especially in the previously mixed area of central divergence. Production was as high as 2 gC·m^-2 day^-1, and the standing crop of chlorophyll was 3 mg·m^-3; such values are rather important for the so called “poor” Mediterranean Sea. Salinity-phosphate and chlorophyll-phosphate diagrams are presented. Biomass and production rates are in agreement with the potential fertility based on the nutrient content of the waters. The disappearance of 1 μatg P·PO₄ by photosynthetic uptake corresponds to 7.7 mg chlorophyll a, which represents the autotrophic biomass remaining after grazing by the simultaneous zooplankton bloom. Biomass and production features are analyzed in regard to interaction of both nutrient availability and the stability of water masses. Stability conditions can be created either by intrusion of local mixing in a stratified oligotrophic area (“winter bloom”), or by thermal stratification of the upper layer (“spring bloom”). In the latter case, the highest biomasses are present in the zone where the nutrients were previously introduced by mixing. The oligotrophic situation remained constant during the two crunises in the surrounding coastal area, which is characterized by low-salinity water and, therefore, absence of vertical nutrient transport into the photic zone. Chlorophyll pigment concentration and photosynthetic rates in the “Cote d'Azur” region are similar to those in the “Provence” region; this situation may result more from upwelling of nutrient-rich intermediate water than from the mixing process which predominates in the latter region.     

Winter atmospheric circulation signature for the timing of the spring bloom of diatoms in the North Sea

Analysing long-term diatom data from the German Bight and observational climate data for the period 1962–2005, we found a close connection of the inter-annual variation of the timing of the spring bloom with the boreal winter atmospheric circulation. We examined the fact that high diatom counts of the spring bloom tended to occur later when the atmospheric circulation was characterized by winter blocking over Scandinavia. The associated pattern in the sea level pressure showed a pressure dipole with two centres located over the Azores and Norway and was tilted compared to the North Atlantic Oscillation. The bloom was earlier when the cyclonic circulation over Scandinavia allowed an increased inflow of Atlantic water into the North Sea which is associated with clearer, more marine water, and warmer conditions. The bloom was later when a more continental atmospheric flow from the east was detected. At Helgoland Roads, it seems that under turbid water conditions (=?low light) zooplankton grazing can affect the timing of the phytoplankton bloom negatively. Warmer water temperatures will facilitate this. Under clear water conditions, light will be the main governing factor with regard to the timing of the spring bloom. These different water conditions are shown here to be mainly related to large-scale weather patterns. We found that the mean diatom bloom could be predicted from the sea level pressure one to three months in advance. Using historical pressure data, we derived a proxy for the timing of the spring bloom over the last centuries, showing an increased number of late (proxy-) blooms during the eighteenth century when the climate was considerably colder than today. We argue that these variations are important for the interpretation of inter-annual to centennial variations of biological processes. This is of particular interest when considering future scenarios, as well to considerations on past and future effects on the primary production and food webs.     

Production of DMSP and DMS during a mesocosm study of an Emiliania huxleyi bloom: influence of bacteria and Calanus finmarchicus grazing

We investigated the influence of bacteria and metazooplankton on the production of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) during blooms of Emiliania huxleyi (Lohmann) Hay and Mohler in seawater mesocosms. The phytoplankton succession was marked by the rapid collapse of an initial Skeletonema costatum (Greville) Cleve bloom followed by a small E. huxleyi bloom. The collapse of the diatom bloom was accompanied by an increase in concentrations of dissolved DMSP (DMSP_d) and bacterial abundance and activity (as determined by the thymidine incorporation technique). The increase in bacterial activity was followed by a rapid decrease in DMSP_d concentrations which remained low for the rest of the experiment, even during the subsequent collapse of the E. huxleyi blooms. The absence of DMSP_d and DMS peaks during the declining phase of the E. huxleyi blooms was attributed to the high bacterial activity prevailing at that time. The influence of metazooplankton grazing on DMSP and DMS production was investigated by adding moderate (24 mg dry weight m^-3) and high (520 mg dry weight m^-3) concentrations of Copepodite Stage V and adults of Calanus finmarchicus to two of four filtered (200 m mesh net) enclosures during the E. huxleyi blooms. The addition of C. finmarchicus, even in high concentrations, had no apparent effect on the dynamics of E. huxleyi, suggesting that the copepods were not grazing significantly on nanophytoplankton. The addition of copepods in high concentrations favored an accumulation of chlorophyll a and particulate DMSP. These results suggest that copepods were preying on the herbivorous microzooplankton which, in turn, was controlling the biomass of nanophytoplankton. DMS production was also enhanced in the enclosure with maximum metazooplankton biomass, suggesting that the grazing of C. finmarchicus on microzooplankton containing DMSP may contribute to DMS production. These results provide strong support to the emerging idea that bacteria and metazooplankton grazing play a dominant role in determining the timing and magnitude of DMS pulses following phytoplankton blooms.     

Differences in productivities between the Great Australian Bight and the Gulf of Carpentaria,Australia, in summer

Phytoplankton standing crop (chlorophyll a) and primary productivity were recorded, and zooplankton biomass was estimated in the two large bays of Australia, the Great Australian Bight on the south coast (December, 1965) and the Gulf of Carpentaria on the north coast (December, 1968). In the Gulf of Carpentaria, the phytoplankton standing crop (average, 27.3 mg chlorophyll a m^-2) and primary productivity (average, 133.1 mg C m^-2 h^-1), as well as zooplankton biomass (average, 305.3 mg wet weight m^-3) are much higher than in the Great Australian Bight (12.1 mg chlorophyll a m^-2, 18.2 mg C m^-2 h^-1, 7.1 mg wet weight m^-3, respectively). The unexpectedly low productivity values in the Great Australian Bight are attributable to environmental conditions of this bay, which obtains neither replenishment of nutrients from the land nor receives upwelling of deep water.     

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.     

Study of the nutrient and plankton dynamics in Lake Tanganyika using a reduced-gravity model

An eco-hydrodynamic (ECOH) model is proposed for Lake Tanganyika to study the plankton productivity. The hydrodynamic sub-model solves the non-linear, reduced-gravity equations in which wind is the dominant forcing. The ecological sub-model for the epilimnion comprises nutrients, primary production, phytoplankton biomass and zooplankton biomass. In the absence of significant terrestrial input of nutrients, the nutrient loss is compensated for by seasonal, wind-driven, turbulent entrainment of nutrient-rich hypolimnion water into the epilimnion, which gives rise to high plankton productivity twice in the year, during the transition between two seasons. Model simulations predict well the seasonal contrasts of the measured physical and ecological parameters. Numerical tests indicate that the half saturation constant for grazing by zooplankton and the fish predation rate on zooplankton affect the zooplankton biomass measurably more than that of phytoplankton biomass. This work has implications for the application of this model to predict the climatological biological productivity of Lake Tanganyika.