Differential impacts of alternate primary producers on carbon cycling

The intertidal zone on the Northeastern Pacific Ocean is a gladiatorial arena where forests of kelp battle with meadows of surfgrasses for access to light and space on the limited rocky shores. The outcome of this battle is of major consequence to the rest of the nearshore ecosystem as kelp and surfgrass form the base of the local food web. This study looked at the effects of this competition on the rest of the ecosystem - who contributes more to storing carbon, who produces more nutritious food (higher nitrogen content), who releases more sugars into the water (microbial snacks)?

To conduct this study, we took a hammer and crowbar to the intertidal and removed 1m by 1m patches of surfgrass meadows (surfgrasses also have the tastiest seeds to snack on during the tides!). We came back a year later to see who colonised these open spaces; ENTER KELP. Kelp tend to colonise open spaces a lot quicker than surfgrasses as all a kelp zygote has to do is settle in an open spot and then start growing. Surfgrasses, on the other hand, will slowly extend the tendrils of their rhizome (like roots) towards newer horizons - you could sometimes be standing in a large meadow that is effectively just one individual! We then took tissue samples and found out how much carbon and nitrogen are present - C and N are biological $$$. We also compared the photosynthetic rates and how much sugars were released into the water column for each species. We could even use this fancy gizmo known as the Diving-PAM to compare the complex photosynthetic machinery in each primary producer.

As an aside, all sampling was done on the shores of Tatoosh Island, the ancestral summer harvesting grounds of the Makah Tribe. This island possessed an incredible legacy for this sea-faring tribe, for scientists fortunate enough to groundtruth foundational ecological theories and for the coast guard to safeguard American interests during WWII. The staggering colours of the intertidal, with a backing orchestra of wizened whale wheezes and oyster-catcher trills as salt and iodine tickled your nostrils - all made for a full bodied sensory experience. At times I wish that I could weasle into this research paper my uncontrollable giggles of wonder when the fog cleared to reveal the looming jagged cliffs of Cape Flattery, or the deep sense of place as I would visit my favourite spots on the island to check out what the nudibranchs were up to today. Alas, descriptive wonder has no place in a research paper - but wasn’t that the reason we got into ecology in the first place?…

The analyses and data revealed some interesting findings. Surfgrass colonies lived for a lot longer than most of the kelps that would usually pop off after a year or so. In these long lived meadows, the grass tissues contained a higher ratio of C:N than kelp indicating that surfgrass retained a lot more carbon but kelp produced a lot more nutritious food! Kelp, however, turn over carbon a lot quicker than surfgrass. They produce sugars much quicker through photosynthesis but also release sugars faster than their surfgrass competition. Why they’re releasing these sugars is still a hotly debated topic! Finally, surfgrass tends to do better at low light than kelp counterparts which might give them an advantage during the winter months when the sun is lower in the sky for shorter days.

As waves rip up surfgrass meadows, creating space for opportunistic kelp, the outcome of this wave disturbance is a splotchy mosaic of brown and green. All the comparative effects we found earlier is spread out over this tiled landscape and this creates for a marvellous 3D structure for fish and other weird and wonderful creatures. Ultimately, more severe storms might favour kelp and the broader food chain as kelps are a more nutritious food source. Given enough time in calm waters, however, surfgrass meadows would establish a monoculture, which would benefit invertebrates that live in its rhizome, that are good recyclers of organic matter.

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Recommended citation: Miranda, K.K., Weigel, B.L., McCoy, S.J. and Pfister, C.A. (2021), Differential impacts of alternate primary producers on carbon cycling. Ecology. https://doi-org.proxy.uchicago.edu/10.1002/ecy.3455