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Researchers testing the effects of global warming on a 2000 litre fish tank have warned that the world faces a major collapse of coastal fisheries, because some of their fish died.
Climate change could drive coastal food webs to collapse

Authors

Ivan Nagelkerken
Professor, Marine Biology, University of Adelaide

Sean Connell
Professor, Ecology, University of Adelaide

Silvan Goldenberg
University of Adelaide

May 1, 2017 6.01am AEST

Coastal marine food webs could be in danger of collapse as a result of rising carbon dioxide levels, according to our new research. The study shows that although species such as algae will receive a boost, the positive effects are likely to be cancelled out by the increased stress to species further up the food chain such as predatory fish. [...]

Test tank

We used a self-contained ecosystem in a 2,000-litre tank to study the effects of warming and ocean acidification on a coastal food web. This approach can give us a good idea of what might happen to genuine coastal food webs, because the tank (called a "mesocosm") contains natural habitats and a range of species that interact with one another, just as they do in the wild.

Our food web had three levels: primary producers (algae), herbivores (invertebrates), and predators (fish).

The results show that carbon dioxide enrichment can actually boost food webs from the bottom up through increased algal growth. This benefited herbivores because of the higher abundance of food, and in turn boosted the very top of the food web, where fish grew faster.

But while this effect of ocean acidification may be seen as positive for marine ecosystems, it mainly benefits "weedy" species - a definition that can be applied to some species of algae, invertebrates, and even fish.

In contrast, habitat-forming species such as kelp forests and coral reefs are more likely to disappear with rising CO₂ emissions, and with them many associated species that are deprived of their habitats and food.

Detrimental effect

Our results therefore showed that warming had a detrimental overall effect on the coastal food web we studied. Although higher temperatures boosted algal growth, herbivorous populations did not expand. Because herbivore abundances remained similar and elevated temperatures result in a higher metabolic demand, predatory fish consumed more herbivorous prey,resulting in a collapse of these prey populations.
Read more here.

The abstract of the study;
Boosted food web productivity through ocean acidification collapses under warming

Authors
Silvan U. Goldenberg,
Ivan Nagelkerken,
Camilo M. Ferreira,
Hadayet Ullah,
Sean D. Connell
First published: 27 April 2017

Future climate is forecast to drive bottom-up (resource driven) and top-down (consumer driven) change to food web dynamics and community structure. Yet, our predictive understanding of these changes is hampered by an over-reliance on simplified laboratory systems centred on single trophic levels. Using a large mesocosm experiment, we reveal how future ocean acidification and warming modify trophic linkages across a three-level food web: that is, primary (algae), secondary (herbivorous invertebrates) and tertiary (predatory fish) producers. Both elevated CO2 and elevated temperature boosted primary production. Under elevated CO2, the enhanced bottom-up forcing propagated through all trophic levels. Elevated temperature, however, negated the benefits of elevated CO2 by stalling secondary production. This imbalance caused secondary producer populations to decline as elevated temperature drove predators to consume their prey more rapidly in the face of higher metabolic demand. Our findings demonstrate how anthropogenic CO2 can function as a resource that boosts productivity throughout food webs, and how warming can reverse this effect by acting as a stressor to trophic interactions. Understanding the shifting balance between the propagation of resource enrichment and its consumption across trophic levels provides a predictive understanding of future dynamics of stability and collapse in food webs and fisheries production.
Read more here (paywalled):

Note: the link to the study does not work in some web browsers, I had to view it using Google Chrome

Unfortunately the full study is paywalled, but attempting to infer global consequences of increased CO2 from a toy eco-system in a 2000 litre fish tank is absurd.

On the positive side, the researchers performed an actual experiment, rather than just running a computer model.

But anyone who has ever kept fish knows how difficult it can be to keep a fish tank eco-system stable. Fish in a tank are subject to numerous stresses, even a small mistake with feeding, water contamination or filtering waste can lead to disease and death.

If the researchers had instead studied regions of the ocean with elevated CO2 levels, they would have discovered plenty of places in the ocean where CO2 levels are naturally elevated well beyond anything anthropogenic CO2 will achieve, due to natural outgassing from volcanic sources.

Many of these reefs are ridiculously healthy, despite corals and fish growing in water which is continuously totally saturated with CO2.

The existence of healthy natural reefs with populations of fish growing in regions of the ocean which are full of CO2, strongly suggests whatever killed the fish in that 2000 litre research tank had nothing to do with CO2.