Climate change could drive marine food web collapse through altered trophic flows and cyanobacterial proliferation

Abstract

Global warming and ocean acidification are forecast to exert significant impacts on marine ecosystems worldwide. However, most of these projections are based on ecological proxies or experiments on single species or simplified food webs. How energy fluxes are likely to change in marine food webs in response to future climates remains unclear, hampering forecasts of ecosystem functioning. Using a sophisticated mesocosm experiment, we model energy flows through a species-rich multilevel food web, with live habitats, natural abiotic variability, and the potential for intra- and intergenerational adaptation. We show experimentally that the combined stress of acidification and warming reduced energy flows from the first trophic level (primary producers and detritus) to the second (herbivores), and from the second to the third trophic level (carnivores). Warming in isolation also reduced the energy flow from herbivores to carnivores, the efficiency of energy transfer from primary producers and detritus to herbivores and detritivores, and the living biomass of detritivores, herbivores, and carnivores. Whilst warming and acidification jointly boosted primary producer biomass through an expansion of cyanobacteria, this biomass was converted to detritus rather than to biomass at higher trophic levels—i.e., production was constrained to the base of the food web. In contrast, ocean acidification affected the food web positively by enhancing trophic flow from detritus and primary producers to herbivores, and by increasing the biomass of carnivores. Our results show how future climate change can potentially weaken marine food webs through reduced energy flow to higher trophic levels and a shift towards a more detritus-based system, leading to food web simplification and altered producer–consumer dynamics, both of which have important implications for the structuring of benthic communities.

A triple trophic boost: How carbon emissions indirectly change a marine food chain

“…The boosting effects of CO 2 have also been observed in marine mesocosm experiments (which are more complex than laboratory experiments, but less complex than natural communities) showing the one-way flow of resources from the bottom up (Sswat et al, 2018;Ullah, Nagelkerken, Goldenberg, & Fordham, 2018). Two key ecological factors, however, influence the abundance and biomass of species: resource availability, which triggers population expansion, and consumption, which reduces the expansion to some net level (Hunter & Price, 1992).…”

Tropicalisation of temperate reefs: Implications for ecosystem functions and management actions

“…A meta-analysis of more than 600 experiments showed that ocean warming typically increases metabolic rates as well as primary production and consumption (Nagelkerken & Connell, 2015). Mesocosm experiments suggest that this can lead to ecosystem collapse and simplified food webs if increases in primary production are converted to unpalatable detritus and not consumed (Ullah, Nagelkerken, Goldenberg, & Fordham, 2018). These mesocosm studies, however, typically do not incorporate the effects of "species on the move" that characterise tropicalisation and which can have important effects on altered food web dynamics.…”

Spatial insurance in multi‐trophic metacommunities

“…Environmental change and dispersal can both have indirect effects on communities by altering trophic interactions (Gilman et al 2010;Verreydt et al 2012), but this is an underappreciated complexity of the spatial insurance hypothesis. In both aquatic and terrestrial ecosystems, the effect of environmental change on primary producers can indirectly impact organisms at higher trophic levels (Wade et al 2017;Ullah et al 2018), and effects of environmental change on herbivores or predators can cascade down to lower trophic levels (Martin & Maron 2012;Amundrud & Srivastava 2016;Bell et al 2019). The interaction of environmental change and dispersal might entail additional indirect effects.…”