Funded Research

Quantifying plankton predation rates, and effects on primary production, phytoplankton community composition, size spectra and potential for export

Menden-Deuer, Susanne: University of Rhode Island (Project Lead)

NRA: 2016 NASA: Ocean Biology and Biogeochemistry   

Predation by microzooplankton (MZ) is the single largest loss factor of marine primary production (PP) and alters the abundance and size spectrum of particles in the water column. MZ consume living cells and sinking aggregates and serve as prey to vertically migrating metazoans. Our hypotheses are 1) MZ predation of PP impacts rates of export production through all 5 pathways predictably and 2) high rates of predation yield low carbon export rates. We propose to quantify predation effects on PP, particle abundance and size distribution as a function of co-occurring environmental and biological conditions (i.e. Ecosystem/Carbon Cycling States (ECC)) to gain a mechanistic and predictive understanding of the carbon flow in the euphotic zone and below. We will provide essential data described in the implementation plan and address elements of science questions 1&2. We will identify linkages among predator mediated carbon flows, environmental drivers and export production, which are crucial for developing a) the diagnostic and prognostic relationships necessary to address SQ3 and b) the predictive modeling framework to achieve key goals of the EXPORTS mission. We propose to parameterize plankton population dynamics in the surface ocean and extend measurements of grazing potential to depth using traditional dilution experiments paired with novel genetic markers by studying: 1. Surface plankton population dynamics: We will concurrently measure phytoplankton growth and protist grazing rates in the surface ocean using dilution experiments. Daily experiments will be conducted at 3 depths incubated deck-board at in situ conditions to assess predation induced shifts in biomass, particle size spectra and species composition. Particle size spectra and composition will be examined across a broad size range (1-100μm) using microscopy and flow cytometry. Repeat measurements across multiple ECC states will reveal associations among key biotic (e.g. abundance and taxonomic composition) and environmental (e.g. light, hydrography) correlates of biologically mediated carbon flow. By comparing predation rate estimates to the optical properties measured by collaborators, we will determine remotely or autonomously measurable proxies for grazing rates. 2. Feeding capacity in the deep ocean: We will measure feeding potential of MZ over vertical profiles (0-1500m) to determine the potential for grazer mediated modifications of carbon flow below the euphotic zone. Fluorescent stains that illuminate the feeding process will be used to measure feeding capacity, as in situ concentrations of predator and prey are likely too low to deliver a measurable signal from incubation experiments. Comparison of the feeding potential over depth with rate measurements of plankton population dynamics made at the surface will enable us to extend carbon flow estimates from the surface to the difficult to assess grazing dynamics in twilight zone. 3. Novel genetic grazing marker: We will assess new genetic markers indicative of feeding in MZ to achieve high-resolution grazing rate measurements, on time scales similar to the capacity of autonomous or remote sensors. Biomass for downstream analysis will be collected daily throughout the water column and concurrently from the dilution experiments. This first field application of grazing markers will provide gene expression across ECC states to expand the resolution of grazing rate measurements. 4. Contributions to EXPORTS: Predation rates are crucially missing from global biogeochemical models but necessary to parameterize carbon flows at the base of the marine food web. The data proposed here provides key algorithms to relate plankton growth and mortality to export through all 5 pathways and across ECC states. Numerous EXPORTS studies require these rate estimates, which will be essential for building a diagnostic modeling framework to predict export of global PP, now and in future climate scenarios.