Buoyancy of natural populations of marine phytoplankton (original) (raw)
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Diatom flotation at the onset of the spring phytoplankton bloom: an in situ experiment
Marine Ecology-progress Series, 2010
We used a new type of sediment trap to conduct an in situ test on the buoyancy properties of diatoms before and during the growth phase of the spring phytoplankton bloom (SPB). Diatoms shifted from a sinking pattern before the bloom, while their populations were not growing, to a neutrally buoyant pattern during bloom development, when calm conditions prevailed, light was abundant and phytoplankton were actively growing. This shift was mainly due to the upward motion of centric diatoms during the growth phase. Our field experiment confirms laboratory experiments and field observations showing that diatoms, the paradigm of sinking phytoplankton, approach neutral buoyancy when conditions are adequate for growth, a fact that is not taken into account in most SPB and phytoplankton dynamics models.
Sedimentation of phytoplankton during a diatom bloom: Rates and mechanisms
Journal of Marine Research, 1996
Phytoplankton blooms are uncoupled from grazing and are normally terminated by sedimentation. There are several potential mechanisms by which phytoplankton cells may settle out of the photic zone: sinking of individual cells or chains, coagulation of cells into aggregates with high settling velocities, settling of cells attached to marine snow aggregates formed from discarded larvacean houses or pteropod feeding webs, and packaging of cells into rapidly falling zooplankton fecal pellets. We quantified the relative significance of these different mechanisms during a diatom bloom in a temperate fjord, and evaluated their potential to control phytoplankton population dynamics. Overall specific sedimentation rates of intact phytoplankton cells were low during the 11 -day study period, averaging ca. 0.1 d-l, and mass sedimentation and bloom termination did not occur. Most cells settled attached to marine snow aggregates formed from discarded larvacean houses, whereas settling of unaggregated cells was insignificant. Formation rates of phytoplankton aggregates by physical coagulation was very low, and losses by this mechanism were co.07 d-t; phytoplankton aggregates were neither recorded in the water column (by divers) nor in sediment traps. The low coagulation rates were due to a very low 'stickiness' of suspended particles. The dominant diatom, Thalassiosira mendiolana, that accounted for up to 75% of the phytoplankton biomass, was not sticky at all, and did not turn sticky upon nutrient depletion in culture experiments. The low particle stickiness recorded may be related to low formation rates by diatoms of transparent exopolymeric particles (TEP), that occurred in low concentrations throughout the study period. Zooplankton grazing rate did not respond to the development of the bloom and accounted for a loss term to the phytoplankton populations comparable to sinking of intact cells; fecal pellets accounted for 3&50% of settled phytoplankton and phytodetritus. While coagulation may give rise to density-dependent losses to phytoplankton populations and, hence, control blooms, neither of the other mechanisms 1.
Journal of Marine Systems, 2004
The phytoplankton species succession and sedimentation characteristics were studied on a sheltered and an open coastal station during a spring bloom in the northern Baltic Sea. Biomass (phytoplankton carbon and chlorophyll a), inorganic nutrients and particulate organic carbon and nitrogen (POC and PON) were determined in the suspended material. Sediment traps moored at different depths were used to determine the vertical flux of total particulate material (TPM), POC and PON, chlorophyll a, phaeopigments and phytoplankton carbon. The spring phytoplankton biomass was dominated by dinoflagellates that formed a dense bloom of short duration, whereas diatoms were present in the water column throughout the spring period in more moderate biomasses. The vertical flux of phytoplankton carbon was however dominated by diatoms at all times. The formation of resting stages increased the sedimentation of dinoflagellates, but compared to the suspended biomass of vegetative cells, their sinking rates were low. The ambient silicate concentration did probably not limit diatom growth; these were rather removed from the surface layer through sinking, a situation beneficial for dinoflagellates capable to exploit deep nutrient reserves through vertical migration. Due to rapid sinking soon after bloom formation and high specific loss rates, diatoms can be considered important contributors to the vertical flux of autochtonous material. Dinoflagellates mostly disintegrate in the water column and may settle as phytodetritus, except for the fraction of the populations that form rapidly sinking cysts. In addition to vertical export, advection of water from the stations seems to have been an important loss factor in the phytoplankton community. The two stations differed in that resuspension and input from littoral sources to the vertical flux were more important in the inner and shallower archipelago zone. This was also reflected in the C/N ratio of the settling material and in the bottom surface layer. In our study area, both the hydrographical regime and the species composition of the phytoplankton community were found to affect sedimentation characteristics and the composition of the settling material during the spring period.
Aquatic Microbial Ecology, 2000
The objective of the study was to determine the role of microzooplankton in the fate of primary production and progression of the spring phytoplankton bloom in high latitude fjords. The 3 fjords, Balsfjord, Malangen and Ullsfjord (Norway), varied in oceanic influence and in the rate of development of the spring bloom. The abundance of Phaeocystis pouchetii was relatively low in the spring of 1997 compared to previous years, and diatoms dominated the biomass of the phytoplankton assemblage in all 3 fjords. The mean biomass of microzooplankton in the top 20 m averaged 72, 66 and 80 mg C m-3 and values integrated to 170 m averaged 4560, 4450 and 6820 mg C m-2 in Balsfjord, Malangen and Ullsfjord, respectively. The composition of the microzooplankton was consistent among the fjords and over time, with the proportion of biomass split evenly between nanoflagellates, dinoflagellates and ciliates. Grazing rates of the microzooplankton community were measured with the dilution technique. The impact of microzooplankton grazing was similar among the fjords, accounting for on average 68, 63 and 55% of the production of the < 200 µm phytoplankton in Balsfjord, Malangen and Ullsfjord, respectively. When integrated to 20 m, based on a microzooplankton biomass-specific ingestion rate, microzooplankton grazing accounted for an estimated 12 to 26% of the gross primary production. However, this does not account for the carbon requirements of a substantial proportion of the microzooplankton that occurred below 20 m. Clearance rates by ciliates of nanophytoplankton cells of a similar size to the single cells of P. pouchetii were determined from the uptake rates of fluorescently labelled algae (FLA). Generally, the taxa of ciliates that were found to ingest FLA accounted for ≥ 50% of the abundance of the ciliate population in each fjord. Taxon-specific ciliate clearance rates of FLA in surface waters varied with ciliate size from 5.6 to 1.3 µl cell-1 h-1. The FLA-consuming ciliate population cleared a total of 27 × 10 3 to 141 × 10 3 µl l-1 d-1 in surface waters. The ingestion rates of the FLA-consuming ciliates were equivalent to between 11 and 29% of the total microzooplankton consumption. It is possible that the grazing pressure exerted by the microzooplankton on single cells and small colonies was high enough to decrease the overall competitiveness of the P. pouchetii populations and contribute to their low abundance.
Simple mixing criteria for the growth of negatively buoyant phytoplankton
Limnology and Oceanography, 2003
Phytoplankton population dynamics are controlled by the relative rather than absolute timescales of mixing, growth, and loss processes such as sedimentation, grazing, and so on. Here, the vertical distribution and biomass of phytoplankton populations are quantified by two timescale ratios: the Peclet number Pe-the ratio of mixing and sedimentation timescales-and the growth number G-the ratio of sedimentation and net growth timescales. Three mixing regimes are defined for phytoplankton and other particles. For Pe Ն 100, the population is translated linearly down the water column over time and will leave the surface mixing layer completely after sedimentation time s . For 0.1 Ͻ Pe Ͻ 100, the population distribution depends on the relative magnitude of Pe and G. Finally, for Pe Յ 0.1, the population will be vertically uniform, and biomass changes exponentially over time with characteristic timescale c ϭ s /(G Ϫ 1). This analysis is valid for negatively buoyant phytoplankton, except when mixing time is much longer than growth time and Pe Յ 0.1, which can occur for very slow sinking species. These regimes can be used for assessing the effect of changes in the mixing, growth, or sedimentation conditions on population dynamics. Published data from a lake and diurnally stratified river weir pool are used here to verify a minimum thermocline depth hypothesis proposed by others. Mixing and growth regimes are used to calculate minimum mixing depth h min and to determine phytoplankton sinking rates from published sediment trap data.
Vertical distribution and settling of spring phytoplankton in the offshore NW Baltic Sea proper
Marine Ecology-progress Series, 2004
We studied the vertical distribution and settling of dominant diatoms and dinoflagellates during the 1996 spring phytoplankton bloom in the offshore NW Baltic Sea proper. We sampled phytoplankton at 11 depths (to 80 m) and collected settling cells in sediment traps at 25, 50 and 100 m depth, every week from March 26 to May 7. Phytoplankton were counted and sized from both water and trap samples, to estimate the share of phytoplankton in bulk settling carbon. Diatoms, mainly Chaetoceros spp. and Achnanthes taeniata, dominated the early bloom, but were replaced by the dinoflagellates cf. Scrippsiella hangoei and Peridiniella catenata when inorganic nitrogen was depleted above the seasonal pycnocline at ca. 10 m depth. By late April, vertically migrating dinoflagellates had depleted inorganic nitrogen down to 30 m, well below the seasonal pycnocline. We found clear species-specific sedimentation patterns. Scrippsiella hangoei and Chaetoceros spp., which dominated in the water column, were clearly underrepresented in the traps, while Thalassiosira baltica and T. levanderi, which were sparse in the water column, were overrepresented in sediment traps. Only 4, 3 and 0.5 g C m -2 (or 16, 12 and 2% of phytoplankton primary production) settled as intact phytoplankton cells at 25, 50 and 100 m, respectively, although these numbers may be overestimated due to migrating P. catenata. The settling bulk carbon was ~3 g C m -2 or 12% of the primary production at all depths. This is low compared to other estimates from coastal waters and suggests additional loss mechanisms, e.g. disintegration in the water column and grazing by zooplankton overwintering in the permanent halocline area.
Sinking movements of phytoplankton indicated by a simple trapping method : I. A population
1976
Small traps designed to intercept movements of freshwater phytoplankton in situ were suspended at different levels in a stratified lake. Recoveries of a single non-motile species, Fragilaria crotonensis, are analysed to assess trap-performance. Significant correlations are demonstrated between the numbers of alga trapped and the size of the standing population, and between the fraction of the population trapped and estimates of epilimnetic eddy diffusion coefficients, from which it is deduced that the traps performed most efficiently under conditions of stable stratification. It is also shown that Fragilaria probably sinks throughout most of the season at rates not dissimilar from those obtained in vitro.
Marine Biology, 1978
From February 24 to April 24, weekly samples were collected at fixed depths at one station in Lind&spollene, a landlocked Norwegian fjord. Adenosine triphosphate (ATP), chlorophyll a, phaeophytin, 14C assimilation, and respiratory activity [electron transport system (ETS) activity] were measured in the net-(>30 bm) and nanoplankton. Netplankton contained on the average 48% of the total chlorophyll a and 56% of the ATP, but contributed only 7% to the total carbon assimilation and 11% to the ETS activity. The assimilation numbers for net-and nanoplankton ranged from 0 to 1.2 and from 1.5 to 13.2, respectively. At the oxygen/hydrogen sulphide interface, high concentrations of ATP, but not of chlorophyll a, were found in the nanoplankton fraction. Netplankton algae grew actively only in the first phase of the bloom, and nanoplankton predominated later, apparently due to low nutrient concentrations. During the bloom, Skeletonema costatum made up the main part of the biomass. The number of cells in the chains decreased throughout the bloom, possibly reflecting the lowered silicate content. It appeared that only nanoplankton were grazed by zooplankton, while netplankton sank to the bottom and represented input to the benthos.
Depth distribution of photosynthetic pigments and diatoms in the sediments of a microtidal fjord
Hydrobiologia, 2005
The depth distribution of photosynthetic pigments and benthic marine diatoms was investigated in late spring at three different sites on the Swedish west coast. At each site, sediment cores were taken at six depths (7-35 m) by scuba divers. It was hypothesized that (1) living benthic diatoms constitute a substantial part of the benthic microflora even at depths where the light levels are <1% of the surface irradiance, and (2) the changing light environment along the depth gradient will be reflected in (a) the composition of diatom assemblages, and (b) different pigment ratios. Sediment microalgal communities were analysed using epifluorescence microscopy (to study live cells), light microscopy and scanning electron microscopy (diatom preparations), and HPLC (photosynthetic pigments). Pigments were calculated as concentrations (mg m )2 ) and as ratios relative to chlorophyll a. Hypothesis (1) was accepted. At 20 m, the irradiance was 0.2% of surface irradiance and at 7 m, 1%. Living (epifluorescent) benthic diatoms were found down to 20 m at all sites. The cell counts corroborated the diatom pigment concentrations, decreasing with depth from 7 to 25 m, levelling out between 25 and 35 m. There were significant positive correlations between chlorophyll a and living (epifluorescent) benthic diatoms and between the diatom pigment fucoxanthin and chlorophyll a. Hypothesis (2) was only partly accepted because it could not be shown that light was the main environmental factor. A principal component analysis on diatom species showed that pelagic forms characterized the deeper locations (25-35 m), and epipelic-epipsammic taxa the shallower sites (7-20 m). Redundancy analyses showed a significant relationship between diatom taxa and environmental factorstemperature, salinity, and light intensities explained 57% of diatom taxa variations. Hydrobiologia (2005) 534: 117-130 Ó Springer 2005