Decadal losses of canopy‐forming algae along the warm temperate coastline of Brazil

Abstract

The loss of canopy‐forming seaweeds from urbanized coasts has intensified in response to warming seas and non‐climatic pressures such as population growth and declining water quality. Surprisingly, there has been little information on the extent of historical losses in the South‐western Atlantic, which limits our ability to place this large marine ecosystem in a global context. Here, we use meta‐analysis to examine long‐term (1969–2017) changes to the cover and biomass of Sargassum spp. and structurally simple algal turfs along more than 1,000 kilometres of Brazil's warm temperate coastline. Analysis revealed major declines in canopy cover that were independent of season (i.e., displaying similar trends for both summer and winter) but varied with coastal environmental setting, whereby sheltered bays experienced greater losses than coastal locations. On average, covers of Sargassum spp. declined by 2.6% per year, to show overall losses of 52% since records began (ranging from 20% to 89%). This contrasted with increases in the cover of filamentous turfs (24% over the last 27 years) which are known to proliferate along human‐impacted coasts. To test the relative influence of climatic versus non‐climatic factors as drivers of this apparent canopy‐to‐turf shift, we examined how well regional warming trends (decadal changes to sea surface temperature) and local proxies of coastal urbanization (population density, thermal pollution, turbidity and nutrient inputs) were able to predict the changes in seaweed communities. Our results revealed that the most pronounced canopy losses over the past 50 years were at sites exhibiting the greatest degree of coastal warming, the highest population growth and those located in semi‐enclosed sheltered bays. These findings contribute knowledge on the drivers of canopy loss in the South‐western Atlantic and join with global efforts to understand and mitigate declines of marine keystone species.

Sargassum stenophyllum (Fucales, Ochrophyta) responses to temperature short-term exposure: photosynthesis and chemical composition

“…Sargassum that is found in tropical and subtropical regions makes up dense natural forest beds (De Wreede 1976;Casas-Valdez et al 2016), and in Brazil, they have representability in terms of abundance (Paula and Oliveira Filho 1980;Paula 1988;Széchy and Paula 2000). Although, in the last years, changes on Sargassum' coverage and biomass in Brazil have become to decline 2.6% per year by combined effects of urbanization and warming seas (Gorman et al 2020). Experimental short-term approaches with Sargassum species at + 3.5 °C treatment compared to ambient temperature resulted in 17-49% growth decline relative to controls and susceptibility to herbivory (Graba-Landry et al 2020).…”

Integrating Oceanographic Data and Benthic Community Structure Temporal Series to Assess the Dynamics of a Marginal Reef

“…Coral reefs all around the globe have experienced structural changes from coral-dominance to algae-dominance. Yet, some studies have also discussed the loss of canopy-forming macroalgae and the expansion of turf-forming algae on marine ecosystems across the globe (Benedetti-Cecchi et al, 2001;Gorgula and Connell, 2004;Connell et al, 2008;Filbee-Dexter and Wernberg, 2018;Gorman et al, 2019;, in most cases linked to increasing anthropic impacts and rising sea surface temperature. These dynamics, together with the temporal stability of turf algae (Gordon et al, 2016) and its high tolerance to disturbances (Littler and Littler, 1980;Airoldi, 1998;Cheroske et al, 2000), suggest a possible rearrangement of reef benthic communities around less complex stable states (i.e., reef flattening) (e.g., , including other possible alternative states afterward, such as cyanobacterial mats (e.g., de Bakker et al, 2017).…”

“…There are even records of structural changes on benthic communities, with replacement of coral cover by zoanthid and macroalgae (Cruz et al, 2014b(Cruz et al, , 2018, as well as decrease on Sargassum spp. coverage and increase of turf algae (Gorman et al, 2019) at some localities along the Brazilian coast. Additionally, an increase in frequency, duration, intensity and extension of marine heatwaves has been recently reported in the South Atlantic (Rodrigues et al, 2019), and this is very likely to intensify ecosystem damages (Fordyce et al, 2019).…”

A blueprint for securing Brazil's marine biodiversity and supporting the achievement of global conservation goals

“…In particular, the ecological functions performed by nearshore habitats are deteriorating rapidly due to the cumulative impacts of multiple human disturbances (e.g. Copertino et al., 2016; Cruz et al., 2018; Duarte et al., 2020; Giglio et al., 2015; Gorman et al., 2020; Magris & Giarrizzo, 2020; Magris et al., 2019). Our study therefore demonstrates the utility of the cumulative impact maps in developing integrated conservation planning and offers critical insights for the strategic implementation of conservation actions in Brazil.…”