Giant kelp is a highly productive foundation species that forms dense forests on temperate reefs around the world. These ecosystems have remarkably high rates of carbon fixation—giant kelp can grow half a meter a day—and the physical structure formed by kelp’s tall fronds profoundly alters the benthic habitat. However, giant kelp is regularly ripped out by storms. These periodic losses followed by potentially rapid recovery mean giant kelp abundance can fluctuate greatly over relatively short timescales. In addition to influencing patterns of net primary productivity, these fluctuations may have cascading effects on the benthic community. When kelp is abundant, its canopy shades and inhibits the growth of other macroalgae, which opens benthic space for invertebrates like bryozoans, sponges, and tunicates. When kelp is absent, more light reaches the seafloor, allowing understory macroalgal species to proliferate. Climate change is predicted to increase the frequency and intensity of storms across much of giant kelp’s range. Thus, an important question is how will changing disturbance regimes impact the structure and functions of kelp forest ecosystems?

To address this question, a paper recently published in Ecology explored the effects of variable storm regimes on giant kelp dynamics and benthic community structure. The authors built a novel mathematical model describing this system and validated their model with long-term data collected by the Santa Barbara Coastal LTER. This model predicts that increases in storm frequency and intensity could lead to persistent shifts in benthic community composition by facilitating the dominance of understory macroalgae. The model, together with the SBC LTER data used to validate it, provides a predictive framework for understanding how climate-driven shifts in storm regimes may affect kelp forest community dynamics. These dynamics, which influence key ecosystem functions like primary production, could have wide-reaching implications for how the ecosystem services provided by kelp forests will be altered in a changing climate.

Figure 1: The top two images depict reefs following a winter with severe and mild storms, respectively. (a) Giant kelp frond density (green) and mean percent cover of sessile invertebrates (blue) and understory macroalgae (orange) on Mohawk Reef from 2008 to 2019. Black vertical lines indicate the occurrence of major wave events (severe storms). (b) Model output from a simulation of a similar storm regime.

Authors:
A. Raine Detmer, Robert J. Miller, Daniel C. Reed, Tom W. Bell, Adrian C. Stier, Holly V. Moeller (all University of California, Santa Barbara)

The world is in need of effective methods to reduce CO₂ in the atmosphere, and the ocean could potentially help us to meet this objective. One idea, explored since the 1980s, is to grow seaweeds on platforms in the open ocean to fix carbon and sequester it at great depth (ocean afforestation). But would that work at scale?

To address this question, we utilized the Great Atlantic Sargassum Belt as a natural analogue for seaweed farms distributed across the (sub)tropical Atlantic. We found that the CO₂ removal potential of ocean afforestation is substantially modified by biogeochemical and physical processes associated with seaweed growth and ecology in the open ocean. Most noticeably, seaweeds provide habitat for calcifying organisms, which form CO₂ through changes to seawater carbonate chemistry occurring during the calcification process. Furthermore, seaweed carbon fixation is fuelled by limiting nutrients, which once taken up by seaweeds are not available any longer to support CO₂ removal by the natural inhabitants of the open ocean – phytoplankton.

After accounting for offsets to carbon fixation we also considered that seaweeds fix CO₂ dissolved in seawater, not atmospheric CO₂. The atmospheric CO₂ has to be subsequently absorbed by the oceans in a second step, which takes much longer (weeks to years) and is more difficult to quantify than one may think.

We also investigated how ocean afforestation influences the albedo (i.e., reflectance) of the Earth. The sign and magnitude of afforestation-related albedo changes on radiative forcing depend on multiple indirect effects linked to the release of substances that influence cloud formation. We didn’t dare to go down this rabbit hole. However, we were surprised to see that even just the brightening of the sea surface through seaweed growth (and thus the direct reflection of light) may have a stronger influence on radiative forcing than the CO₂ removal associated with ocean afforestation.

How did we interpret all this? Most importantly, our study does not suggest that CO₂ removal with seaweed farming is impossible. However, it shows that successful CO₂ removal and its verification requires much more than simply generating seaweed biomass. The complexity of this method could be its Achilles heel and hence become its knock-out criterion, because a large number of interacting processes must be quantified to determine the sign and magnitude of the net climatic impact of ocean afforestation.

Figure from Bach et al https://doi.org/10.1038/s41467-021-22837-2

Authors:
Lennart Bach, Catriona Hurd, Philip Boyd (Institute for Marine and Antarctic Studies, University of Tasmania)
Veronica Tamsitt (University of New South Wales and CSIRO Hobart)
Jim Gower (Fisheries and Oceans Canada)
John Raven (University of Dundee at the James Hutton Institute and University of Technology, Sydney and University of Western Australia)

Articles referenced above:
Bach, L.T., Tamsitt, V., Gower, J. et al. Testing the climate intervention potential of ocean afforestation using the Great Atlantic Sargassum Belt. Nat Commun 12, 2556 (2021). https://doi.org/10.1038/s41467-021-22837-2

Jones, D. C., T. Ito, Y. Takano, and W.-C. Hsu (2014), Spatial and seasonal variability of the air-sea equilibration timescale of carbon dioxide, Global Biogeochem. Cycles, 28, 1163–1178, doi:10.1002/2014GB004813.

Orr, J.C., Sarmiento, J.L. Potential of marine macroalgae as a sink for CO₂: Constraints from a 3-D general circulation model of the global ocean. Water Air Soil Pollut 64, 405–421 (1992). https://doi.org/10.1007/BF00477113

Wang, Mengqiu, Chuanmin Hu, Brian B. Barnes, Gary Mitchum, Brian Lapointe, Joseph P. Montoya. The great Atlantic Sargassum belt. Science. 05 Jul 2019. Vol. 365, Issue 6448, pp. 83-87. DOI:10.1126/science.aaw7912

Learn more in the recorded talks and information on the OCB2021 Plenary Session: Ocean-based negative emissions technologies

The C:N:P stoichiometry of global ocean biological production is the gear by which marine biology turns the wheels of the global ocean carbon cycle. From the days of Alfred Redfield until fairly recently, this stoichiometry was assumed a constant in ocean biogeochemistry (e.g., the Redfield ratio). Traditional stoichiometry would predict a carbon export production decline in the future, as warming is generally expected to enhance water column stratification and diminish the vertical supply of nutrients. However, recent observations clearly show that the C:N:P ratio is quite variable: the ratio is higher in oligotrophic waters than in eutrophic waters, and the ratio is higher in cyanobacteria than eukaryotes. How can flexible C:N:P ratio in phytoplankton affect future carbon export?

A recent publication in Environmental Research Letters discusses three drivers of global ocean production stoichiometry in the future: physiological response of phytoplankton, changes in phytoplankton community composition, and shifts in the regional production, which the authors focused most on as it is the least explored. The authors carried out global warming experiments using a numerical model of global ocean biogeochemistry that represents flexible C:N:P ratios in multiple phytoplankton types with dependence on ambient N and P concentrations, temperature, and light. These model results indicate the importance of the physiological response of cyanobacteria to warming and of eukaryotes to P depletion in driving a higher C:N:P ratio (Figure 1).

Figure caption: Diagnosed effect of regional production changes on the future global mean C:P ratio. The diagnosis (c) is given by the product of the sign of the effect (a) and the production changes (b). The sign of the effect of the regional production changes is positive (+1) for locations with local C:P > global mean (red) and negative (−1) for locations with C:P < global mean (blue). The diagnosis shows a N-S dipole response in both the Southern Ocean and the North Atlantic, where changes in production both increase and decrease the global C:N:P ratio.

Global production C:N:P ratio can vary by shifting the regional production, even in the absence of any change in phytoplankton stoichiometry or taxonomy. For example, simply increasing the production in polar waters—which typically have low C:N:P ratios (eukaryote-dominant, P-rich, and cold)—will lower the global C:N:P ratio, because the polar production then makes a relatively larger contribution to the global production. In this model, future Antarctic sea ice retreat has this very effect, but is offset by production changes downstream and elsewhere. Current literature indicates substantial uncertainty in the future projection of regional production changes. This study illuminates regional production as a new driver of global export C:N:P ratio and an important area of future investigation.

Authors:
Katsumi Matsumoto (University of Minnesota)
Tatsuro Tanioka (University of Minnesota, now University of California, Irvine)

It is critical to quantify the amount of phytoplankton that microzooplankton consume to better understand the flow of carbon toward higher trophic levels in the ocean. The Northeast US Shelf (NES) sustains intense fisheries with huge economical importance, but the links between the planktonic food web and the fish stocks are poorly constrained.

A recent study, which is part of the NES-LTER site (NES Long-Term Ecological Research, est. 2017), quantified phytoplankton growth rates and microzooplankton grazing rates across the NES during one summer and two winter cruises. The strong seasonal differences of physical and biogeochemical properties, associated with their intense spatial heterogeneity along the shelf, provided a unique opportunity to investigate the planktonic food web dynamics across contrasting environmental conditions.

Seasonal (winter vs. summer) and spatial (coastal vs. offshore) variability of the phytoplankton community structure, phytoplankton growth rates and microzooplankton grazing rates in the Northeast US Shelf.

During both winter and summer, coastal waters, which originate in the Arctic, were colder and fresher, and hosted greater phytoplankton biomass compared to the offshore warm slope-sea waters of the Gulf Stream (Figure 1). In winter the phytoplankton community was dominated by large cells (>10 µm), and most of the primary production was directly consumed by microzooplankton, allowing direct and efficient transfer of carbon to higher trophic levels (Figure 1). In contrast, small phytoplankton cells dominated in summer, when fast growing phytoplankton were not preyed upon by microzooplankton grazers, limiting their role conducting organic matter, yet the lack of phytoplankton accumulation during summer suggests some unidentified loss processes and more complex trophic interactions (e.g., trophic cascades, mixotrophy).

The ongoing NES-LTER project is an outstanding opportunity to investigate the flow of carbon amongst the planktonic food web for a predictive understanding of ecosystem productivity in a changing coastal ocean.

Authors:
Pierre Marrec, Heather McNair, Gayantonia Franzè (present address: Institute of Marine Research, Norway), Françoise Morison, Jacob P. Strock and Susanne Menden-Deuer
(All authors: Graduate School of Oceanography, University of Rhode Island)