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Biotic community temperature index

The Biotic community temperature index (CTI) expresses the temperature preferences of the community of species in an area. A change in the CTI over time, consistent with sea warming, indicates that the community of species is reorganising itself towards warmer water species. This process is known as tropicalisation or Mediterraneanisation and is detrimental to species that have adapted to the colder waters of the Basque coast (deborealisation).

  • Sea warming along the Basque coast in recent decades is causing changes in the relative abundance of species, favouring colder waters species (a process known as tropicalisation or Mediterraneanisation) and being detrimental to species that have adapted to warmer waters (deborealisation).

Relationship of the indicator to climate change

Sea warming is already shifting (and is expected to shift in the future) the distribution of populations of species towards the poles (Poloczanska et al. 2013). As a consequence of this latitudinal displacement of populations, which is a response to climate warming, the abundance in each latitudinal range (more specifically, in each temperature range) may vary. However, the displacement of a particular species may be the result of other factors unrelated to climate change. Therefore, studying the response of a set of species (community) to warming is the most complete and reliable analysis strategy for determining the causes.

An analysis of the trends shows that sea warming may have had an impact on the redistribution of the benthic community along the Basque coast, favouring warm-water species over cold-water species, while there is no trend among phytoplankton (Chust et al. 2022) (Figure 2).

Figure 2. Trends over time of the CTI for the communities of the Basque coast: phytoplankton, soft substrate benthos, and rocky substrate benthos.

The community temperature index (CTI) is a methodology for testing whether changes in the composition of community species reflect their thermal preferences. Inter-annual changes in CTI are compared against local temperature trends, as has already been done for zooplankton species. (Villarino et al. 2020).

Three types of communities were analysed: phytoplankton, soft substrate benthic communities and rocky substrate benthic communities.

The abundance of the soft substrate and rocky substrate benthic community, gathered for the Ecological Status Quality Network in the Basque Country, has been carried out since 1995. An annual sample is taken for the soft substrate community, at each station, during the winter season, consisting of 3 replicates, and the macroinvertebrates are analysed. 26 transects are sampled every 3 years in the intertidal zone along the Basque coast for the rocky substrate community, made up of macroalgae, lichens and macroinvertebrates.

Data has been compiled from 6 stations in the open sea to study the phytoplankton. (Muñiz et al. 2020).

Figure 1. (Left) Rocky substrate benthic community. (Right) Sampling in a soft substrate benthic community).

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AZTI compiled the information for this indicator and analysed the trends related to it. We would like to thank the Basque Water Agency (URA), which funded the collection of data on the benthos along the Basque coast through AZTI.

Chust, G., M. González, A. Fontán, M. Revilla, P. Alvarez, M. Santos, U. Cotano, M. Chifflet, A. Borja, I. Muxika, Y. Sagarminaga, A. Caballero, I. de Santiago, I. Epelde, P. Liria, L. Ibaibarriaga, R. Garnier, J. Franco, E. Villarino, X. Irigoien, J. A. Fernandes-Salvador, A. Uriarte, X. Esteban, D. Orue-Echevarria, T. Figueira, and A. Uriarte. 2022. Climate regime shifts and biodiversity redistribution in the Bay of Biscay. Science of the Total Environment 803:149622.

Muñiz, O., J. G. Rodríguez, M. Revilla, A. Laza-Martínez, S. Seoane, and J. Franco. 2020. Inhomogeneity detection in phytoplankton time series using multivariate analyses. Oceanologia 62:243-254.

Poloczanska, E. S., C. J. Brown, W. J. Sydeman, W. Kiessling, D. S. Schoeman, P. J. Moore, K. Brander, J. F. Bruno, L. B. Buckley, M. T. Burrows, C. M. Duarte, B. S. Halpern, J. Holding, C. V. Kappel, M. I. O’Connor, J. M. Pandolfi, C. Parmesan, F. Schwing, S. A. Thompson, and A. J. Richardson. 2013. Global imprint of climate change on marine life. Nature Climate Change 3:919-925.

Villarino, E., X. Irigoien, F. Villate, A. Iriarte, I. Uriarte, S. Zervoudaki, J. Carstensen, T. O’Brien, and G. Chust. 2020. Response of copepod communities to ocean warming in three time-series across the North Atlantic and Mediterranean Sea. Marine Ecology Progress Series 636:47-61.