ASLO Contributed Session.
Lead Organizer: Sandra Lage, smlage@ualg.pt (smlage@ualg.pt)
Co-organizers:
Presentations
05:00 PM
The effect of micrograzer cues on chain formation in marine diatoms (4971)
Primary Presenter: Fredrik Ryderheim, Technical University of Denmark (fred.ryderheim@gmail.com)
Many diatoms form long chains and the relative abundance of these compared to single-celled species depend on several environmental factors, among them grazing risk. Larger grazers, such as copepods, efficiently handle and ingest even very long chains, whereas some smaller grazers are unable to feed on chains exceeding a few cells in length. Thus, theory predicts that cues from small grazers should induce increased chain lengths, but this remains to be tested. How these smaller grazers influence colony formation has frequently been studied in prymnesiophytes and freshwater green algae, but there has been little focus on diatoms, despite their importance in marine food webs. We exposed three species of chain-forming diatoms to the indirect presence of micrograzers and recorded their response. We find that the effect of grazer presence on chain length varies depending on both the type of grazer and diatom. For example, Skeletonema marinoi increased its chain length when exposed to cues from the heterotrophic dinoflagellate Gyrodinium dominans and the ciliate Euplotes spp. by up to 40%, thus bringing the diatoms outside the prey size spectrum of the grazers. In contrast, Thalassiosira rotula did not respond to cues from G. dominans, but rather employed other defenses. Our results suggest that chain-forming diatoms are able to sense and appropriately respond to fast growing micrograzers, thus contributing to the success of diatoms in marine environments.
05:15 PM
Taxon-specific phytoplankton growth and microzooplankton grazing in the Ross Sea region: the race between diatoms and prymnesiophyceae (5477)
Primary Presenter: Andrés Gutiérrez Rodríguez, IEO - CSIC (andres.gutierrez@ieo.csic.es)
The Ross Sea region is one of the most productive areas of the Southern Ocean. Low microzooplankton grazing is considered a key feature underpinning the massive phytoplankton blooms observed during spring and summer. Growth and grazing measurements are, however, scarce and constrained to the surface mixed-layer. Here we present depth-resolved rates of phytoplankton growth and microzooplankton grazing in the euphotic zone (56 profiles) obtained from 2-point dilution experiments (n=336) carried out in the Ross Sea continental shelf, shelf-slope and off-shelf regions during 3 Antarctic summer voyages conducted as part of the New Zealand Ross Sea Research And Monitoring Programme (Ross-RAMP). Phytoplankton growth was higher than grazing in the upper euphotic zone, and decreased with depth, while grazing remained relatively constant vertically and led to net consumption in the lower euphotic zone. Depth-averaged growth and net growth rates (i.e., growth - grazing) tended to be positive and substantially higher in the shelf compared to off-shelf waters, where phytoplankton growth was lower and kept in check by microzooplankton grazing. Pigment-specific analysis of dilution experiments showed that diatoms growth and net growth rates were consistently higher than that of prymnesiophytes across all regions and depths. Understanding the factors that control phytoplankton growth, microzooplankton grazing variability is crucial to predict the effects that projected environmental changes will have on the trophic and biogeochemical functioning of the Ross Sea region pelagic ecosystem.
05:30 PM
Relationship between photorespiration and carbon concentrating mechanism in algae and its ecological role (7091)
Primary Presenter: Manoj Kamalanathan, Bigelow Laboratory for Ocean Sciences (manojkamalanathan711@gmail.com)
Photorespiration is a biochemical pathway, wherein light-dependent O2 consumption is coupled with CO2 release through catabolism of photosynthate at the expense of cellular energy. Therefore, photorespiration is largely viewed as unfavorable to primary productivity and widely considered to be a wasteful process. Algal species especially belonging to cyanobacteria, dinoflagellates and green algae would be CO2 limited under current levels, with nearly 50% of the carbon and energy being lost to photorespiration. Carbon concentrating mechanism (CCM) helps alleviate this CO2 limitation by concentrating CO2 near RUBISCO enzyme, thereby playing a significant role in net CO2 fixation. However, the processes activating CCM remains a mystery, with certain species repressing their CCM under high CO2 levels, while some maintain CCM activity. Photorespiration mutants depend on high CO2 to grow phototrophically, suggesting a link between photorespiration and CCM. Our data shows that metabolic inhibition of photorespiration results in reduced growth amongst species of cyanobacteria, dinoflagellates and green algae, but not diatoms. Furthermore, addition of hydroxypyruvate to photorespiration mutants restarted their growth and phototrophic viability. Further physiological and gene expression studies are underway to decipher the role of the photorespiration pathway in activating CCM and how taxa dependent variation in these processes will affect the phytoplankton community composition in a changing world.
05:45 PM
Productivity and photophysiology in a large, oligotrophic lake (7035)
Primary Presenter: Kate Evans, University of Montana (katherine1.evans@umontana.edu)
We used a combination of approaches to measure primary production and plankton photophysiology in oligotrophic Flathead Lake, Montana (USA). Radiocarbon (14C) assimilation incubations and photophysiological parameters revealed seasonal patterns in both 14C-primary production and photophysiology. Rates of 14C-primary production were highly seasonal, with peak rates in the summer and lower rates in the winter. Photophysiology was similarly seasonal, with phytoplankton demonstrating both increased adaptation to low light and increased susceptibility to photoinhibition during the winter. Throughout the water column, observed photophysiology diverged when the lake stratified, indicating functionally distinct communities of phytoplankton within the water column. Photosynthesis-irradiance curves indicated that planktonic productivity in the subsurface chlorophyll maximum was light-limited year-round, while planktonic production in the near-surface waters was light-saturated during the summer. However, diel variability in incoming radiation drove daily variation in light limitation, as organisms at a given depth can experience both photoinhibition and light limitation during a single day. Additionally, we found that, despite physiological evidence of photoinhibition during the summer, this process appears to play a minor role in constraining primary production in Flathead Lake. Overall, this work highlights the importance of quantifying the variability in photophysiology and primary production across a variety of time scales.
06:00 PM
Phytoplankton community and vertical nutrient fluxes during winter-spring in a deep temperate reservoir (7070)
Primary Presenter: Valeria Fárez-Román, Helmholtz - UFZ (claudiafarez@gmail.com)
Phytoplankton blooms in late winter or spring occur regularly in temperate lakes and transport significant amounts of nutrients to the sediment when they sink. However, information on the transfer of nutrients between the two seasons remains scarce. Here we studied the phytoplankton community composition and how it affects vertical nutrient fluxes in Rappbode Reservoir, a deep, dimictic, clear-water lake in central Germany. We conducted a sampling campaign from November to July spanning one winter circulation and one spring period and collected bi-weekly samples for phytoplankton, water chemistry, and physical variables, and monthly measurements of sedimentation fluxes using sediment traps. Phytoplankton, with special attention to diatoms, was quantitatively investigated. The reservoir mixed from mid-December until stratification onset in mid-April. Cyanobacteria were the most dominant group during mixing with peak biomass in March. The diatom bloom started at the onset of stratification in April and peaked in early June. The most abundant diatom species were Aulacoseira granulata, Tabellaria fenestrata, and Fragilaria spp. The particulate phosphorus content per unit of carbon in the sediment traps peaked in March and April, but the highest vertical phosphorus flux was in May during the diatom bloom. Our results show that winter-spring phytoplankton, particularly diatoms, and cyanobacteria can store substantial amounts of nutrients in the biomass. Moreover, the winter-spring conditions influence the phytoplankton composition, which in turn affects nutrient cycling in lakes.
06:15 PM
WINTER NITRATE AND SILICIC ACID CONTROL THE SUMMER PHYTOPLANKTON ASSEMBLAGE IN BAFFIN BAY (6016)
Primary Presenter: Maxime Benoît-Gagné, Université Laval (maxime.benoit.gagne@gmail.com)
Phytoplankton are at the basis of the Baffin Bay trophic network. Phytoplankton are a highly diverse set of organisms, with one group, diatoms, particularly important in the food web and in carbon export. Here we investigate what controls the relative abundance of diatoms in the Baffin Bay region. Using sensitivity analysis within a trait-based plankton model, we show how winter nitrate concentration and winter silicic acid concentration are controlling the relative importance of each biogeochemical group in the phytoplankton assemblage. We use a one-dimensional version of the MIT general circulation model coupled to its biogeochemical/ecosystem component. This model is particularly efficient at representing a diversity of phytoplankton functional traits and size classes, and its results are compared to observations at an ice camp for investigating the phytoplankton assemblage. Our study suggests that the absence of nutrient limitation before the bloom prevents a differentiation of the assemblage at this stage. However, numerical experiments varying winter nitrate and winter silicic acid modified the relative contribution of diatoms after the bloom. Hence, a possible departure from the average condition of nutrients following the impacts of climate change in the Arctic would change the relative contribution of diatoms to the phytoplankton assemblage after the bloom; this may have cascading effects on the function of Arctic marine ecosystems.
CS027 Phytoplankton ecology and physiology
Description
Time: 5:00 PM
Date: 6/6/2023
Room: Sala Palma