Eddy Exploration and Ecosystem Dynamics

An Ocean’s Breath

Apr. 10 2018

There are only a few places in the ocean where the dynamics of the microbial organisms are so tightly coupled that they give rise to perfect synchronicity. The North Pacific Subtropical Gyre (NPSG) that we are sailing through for this research cruise is one of these special places. In this ecosystem, light is one of the main drivers of photosynthetic carbon fixation by phytoplankton. Consequently, light also affects the supply of organic matter (i.e. carbon) to larger organisms in the food web. The interactions between light and marine microbes are complex, and they vary over depth, time, space, and different environmental conditions. Fortunately, the inherent properties of light itself provides us with some tools to work with.

When light encounters a particle in the ocean (e.g phytoplankton, bacteria, zooplankton, minerals, detritus) two things can happen: this light can get absorbed by the particle, or scattered in different directions. The amount of light that gets absorbed or scattered (we can measure both!) is a complex function of the concentration of particles in the water, their size, and their composition. By knowing particle concentration, we can infer the balance between cellular growth and organic matter transfer and characterize the dynamics of the food web. By knowing size, we can calculate how much of the fixed CO2 is incorporated into phytoplankton biomass. By knowing their composition, we can distinguish living and dead particles and diagnose which particles are more likely to be exported from the surface-lit layer down to the seafloor, removing that carbon from the system for thousands of year. The field of bio-optical oceanography thus aims to use the optical properties of the water as tools to help make sense of particle dynamics in the ocean (Figure 1).

Figure 1: Representation of the marine food web as sampled by an Imaging FlowCytoBot (IFCB) during the Falkor cruise. Phytoplankton forms the base of the marine food web by fixing the atmospheric CO2. Zooplankton ingests phytoplankton and transfer the carbon into higher trophic levels like fish.

The interactions and carbon transfer between the microorganisms that make up the marine food web are so synchronized to the diel (light/dark) cycle that their individual dynamics are directly linked to bulk changes in particle concentration. This is clearly observed by the particle beam attenuation coefficient, an optical property that is often used as a surrogate for changes in the overall concentration of particles sized 0.5-100 micrometers (μm) in the water. This range encompasses most of the phytoplankton we are expecting to see in this region of the NPSG. We have been measuring beam attenuation during the Falkor cruise, and the clear diel cycles we observe help inform the balance between daytime production and nighttime loss of particles (Figure 2).

Figure 2: Time series of particle beam attenuation coefficients from a c-star instrument (black) and number of particles per liter as observed by the IFCB instrument (teal) from surface waters sampled during the Falkor cruise. Yellow shading denote daylight periods. Beautiful diurnal cycles are observed, with minimum beam attenuation (i.e. lowest particle concentration) and lowest numbers of particles observed at around local sunrise, and maximum values (i.e. highest particle concentrations) at around sunset. This pattern is consistent with the idea that, as a whole, the phytoplankton community grow, divide and increase their concentration during the day when light is available, whereas at night the decline in concentration is believed to be linked to respiration, grazing, and other processes that remove phytoplankton (and carbon!) from the surface layer. Note the great coherence between the c-star and IFCB estimates, which are obtained independently. One can think of these diurnal cycles as capturing the ocean’s breathing patterns.

The difference between daytime production and nighttime loss of particles is an important term called “net community production” (NCP). NCP depicts how much carbon may be exported to the deep ocean and, yet, it remains poorly characterized. Describing how particle concentration, size, composition and diversity change over time and space has huge implications. Ultimately, it may help us understand the pathways and fates of carbon in the ocean and how those may change with a changing climate. We are all hoping these rhythms won’t fade as our oceans continue to be stressed by human activities. Either way, we will be out here, listening to the ocean breathe.

Mathilde Dugenne, Fernanda Henderikx, and Kaarel Kaspar Rais recovering the optics package at midnight. The package comes up loaded with fresh data from multiple sensors.SOI / Thom Hoffman


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