Sea surface temperature, wave energy and freshwater discharge from rivers influence the abundance and size of the marine organisms that inhabit rocky shores along the coast of Southeast Brazil more than ecological processes such as competition and predation. In areas where the water is colder, such as the Lakes Region (Região dos Lagos) in Rio de Janeiro state, marine organisms are 25%-100% larger than in areas where it is warmer, such as the coast of São Paulo state.
These are the main conclusions of a study conducted by researchers affiliated with the Federal University of São Paulo (UNIFESP) in partnership with colleagues at the State University of Northern Rio de Janeiro (UENF) and the University of São Paulo's Center for Marine Biology (CEBIMAR-USP) in Brazil, as well as research institutions in other countries. The study was supported by FAPESP. The results are reported in an article published in the journal Marine Environmental Research.
According to the researchers, the findings can also be used to help estimate the potential effects of climate change on marine biodiversity. "Understanding the impact of climate change on biodiversity is a challenge because of the vast spatial scale on which the events in question play out, the difficulty of including all these factors in the design of controlled experiments, and the fact that they may vary significantly over time," Ronaldo Christofoletti, a researcher at UNIFESP's Institute of Marine Sciences (IMAR) and last author of the article, told Agência FAPESP.
The researchers assessed the influence of environmental factors, such as substrate topography, wave exposure and sea surface and air temperatures, as well as ecological variables, such as predation, on the population structure of the main species of marine organisms inhabiting rocky shores in an area of more than 800 km between Itanhaém in São Paulo state and Armação dos Búzios in Rio de Janeiro state. The area corresponds to more than 50% of Brazil's rocky (as opposed to sandy) coastline.
The first stage of the study consisted of field campaigns to collect data from 62 rocky shores with the aim of ensuring that all the indicators were under the influence of the same seasonal climate regime. Other data collection campaigns were conducted as a basis for analyzing predation among the main species, as well as experiments at 18 rocky shore sites to find out how climate change could influence predatory relations among the animals concerned.
In parallel with the fieldwork and analysis of organisms in the laboratory, the researchers conducted satellite-based remote sensing surveys to collect data with which to model sea surface temperature, coastal freshwater discharge and wave impact in order to understand how each of these factors varied on a scale of less than 10 km along the coast.
The results of the analysis showed that most of the species studied were smaller in areas of warmer water – from the Baixada Santista in São Paulo state (a metropolitan area named after the port city of Santos) to the south coast of Rio de Janeiro state, for example – than in areas of colder water, such the Lakes Region. Filter feeders such as barnacles and mussels were 25%-35% larger, carnivores up to 50% larger, and herbivores 100%-130% larger in colder water.
One explanation for the link between water temperatures and size, according to the researchers, may be that animals tend to reach sexual maturity more quickly in warmer water so that they spend less time investing energy in growth and reach a smaller size, but then invest more energy in reproduction. In addition, the Lakes Region is influenced by an upwelling that sweeps nutrients from the seabed to the surface and may offer the food chain more energy.
"The results didn't show that food supply was a limitation in any area, as there was always prey available, but the fact that there were more nutrients in some areas may exert an influence in terms of contributing to faster growth. This hypothesis was to have been tested during the project but it was interrupted by the COVID-19 pandemic and couldn't be resumed later," Christofoletti explained.
Effects of climate change
The researchers also set out to discover how the populations of marine organisms diversify in a natural environment with a sea surface temperature gradient of about 3 °C between the warm-water area from Baixada Santista in São Paulo to Ilha Grande in Rio de Janeiro and cold-water areas such as the Lakes Region. Understanding how this temperature gradient influences the natural environment, they felt, could help predict the potential effects of ocean warming, which reached 1 °C-2 °C above average in 2023 in the South Atlantic.
The researchers sought rocky shores with this temperature gradient and differing degrees of wave impact in Brazil's Southeast region, ranging from sheltered areas with weak wave energy to areas exposed to strong wave energy. This enabled them to gauge the local effect of wave exposure as well as the influence of sea surface temperature. "We had a chance to see how storm surges and rip tides affect biodiversity, for example" Christofoletti said.
In areas where wave energy was a stronger influence, barnacles were 50% more abundant, while mussels and Stramonita brasiliensis (a species of sea snail or whelk) were three times as abundant. One explanation for this is that the stronger waves boosted the supply of nutrients via larvae and other food sources.
An experiment conducted during the study of rocky shores showed that barnacles preyed less on S. brasiliensis in areas where wave energy was a stronger influence because the waves frequently drove predators off the rocks. Predation was more significant in areas where wave energy was lower, prey was less abundant, and predators fed more efficiently because the waves were not a hindrance.
"In a scenario characterized by rising sea levels and more frequent extreme events, the results show that these organisms may become more abundant but will face harder challenges to feed efficiently, potentially leading to an ecological imbalance. This is even more likely in the context of sea surface warming, where they will also tend to become smaller. The outcome could be a comprehensive change in natural populations in this environment," said André Pardal, a professor at UNIFESP and second author of the article.
They also selected rocky shore sites near to and far from river mouths to find out how freshwater discharge influenced marine biodiversity. Climate change has included periods of extreme rainfall resulting in higher levels of freshwater discharge into the ocean from rivers, the researchers note.
"In the context of climate change, a rocky shore near a river mouth from which freshwater discharge increases owing to heavy rain, and which is exposed to stronger waves and warmer water, will see an increase in mussels and a decrease in predation of them, so that they become more abundant, dominate the space, and alter the natural biodiversity in this environment. That's an example focusing on a single species," Christofoletti said.
About São Paulo Research Foundation (FAPESP)
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration. You can learn more about FAPESP at www.fapesp.br/en and visit FAPESP news agency at www.agencia.fapesp.br/en to keep updated with the latest scientific breakthroughs FAPESP helps achieve through its many programs, awards and research centers. You may also subscribe to FAPESP news agency at http://agencia.fapesp.br/subscribe.
