Climate Change

Appreciate seafood? Climate change isn’t your friend.

Paper: Ullah H, Nagelkerken I, Goldenberg SU, Fordham DA (2018) Climate change could drive marine food web collapse through altered trophic flows and cyanobacterial proliferation. PLoS Biol 16(1): e2003446.

Seafood…it’s what’s for dinner

Over half the world’s population depends on seafood for more than 15% of  their intake of animal protein. But, fish on your plate requires fishermen finding fish to harvest. Fish abundance depends on complex food webs of marine plants, animals, and environmental conditions working together to sustain fish populations around the globe.  If marine food webs get shaken up or disrupted due to changing environmental conditions, fish and other marine creatures stand to lose their sources of food or their habitat, decreasing the number of fish available to fishermen to harvest.

An ambitious experimental approach

One recent study from researchers based in Adelaide, Australia suggests food webs that support fisheries and other marine creatures will be hugely impacted by climate change effects in the oceans. Specifcally, they found that warming water temperatures in combination with ocean acidification change the flow of energy through a coastal food web, with the possibility to impact The researchers came to this conclusion by setting up an advanced mesocosm experiment. A mesocosm is a controlled experimental system that mimics real-world conditions; you can think of a mesocosm as a jumbo fish tank that has enough room for different species and physical habitat types (Fig. 1).

Fig. 1. Mesocosms are large tanks that better simulate real-world conditions. Pictured here are the mesocosm tanks found at MERL on the URI-GSO campus in Narragansett, Rhode Island. Image credit: MERL

The researchers set up four of these mesocosms, each holding 1800 L of water; this is equivalent to about 45 bathtubs full of water. The mesocosms were designed to be as similar to each other as possible while mimicking a coastal temperate food web. The tanks included rocky reef, seagrass, and open sand habitat. Fish and invertebrates were collected from Gulf St. Vincent in South Australia  and each mesocosm tank contained a similar distribution of representative temperate species from three different food web levels.

After collection and set-up, the communities were subjected to different environmental conditions to determine how increased temperature and acidity may impact community health and characteristics. At the end of the experiment, each organism from each mesocosm was counted and weighed to get an idea of biomass in each treatment.

New truths from the tanks

The researchers found that increased acidity and increased temperature, both outcomes of climate change, decreased the amount of biomass at higher tropic levels (Fig. 2).

Fig. 2. With ocean acidification and increased tempeture, the amount of consumer biomass, measured in grams per meter squared, decreases. Image credit: Ullah et al. 2018, PLoS Biology.

Warming and acidification caused an increase in cyanobacteria over more nutritious types of turf algae; cyanobacteria biomass became detritus, or decaying material, rather than being eaten by consumers and fueling higher levels in the food chain. This means that energy harnessed by primary producers flowed less efficiently to primary and secondary consumers, such as fish or other marine organisms, thereby decreasing the ability of the community to support fish populations or populations of other commercial important consumers.

The researchers also found that ocean acidification on its own increased the biomass primary and secondary consumers, shifting food web structure from a producer-dominated system to a carnivore-heavy system. It’s unclear why exactly acidification has this effect, but it may not translate realistically to real world situations, as acidification without an accompanying water temperature increase is less likely considering pervasive surface water warming seen around the globe.

Overall, this study improves upon previous experiments by more accurately representing a complex food web of short-lived organisms in a physical habitat akin to a real world setting. Previous mesocosm experiments typically contain only one or a few organisms in a simplified experimental system, and thus aren’t able to provide insightful results capturing complex and messy real world food web responses.

Most of all, this study serves as a pointed reminder to anyone who enjoys the fruits of the sea: climate change is likely to alter food webs that support shellfish and finfish we eat and rely on. If you care about seafood, fishermen, or healthy marine food webs overall, work to mitigate climate change in big and small ways around you.

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