A future 1.2 C increase in ocean temperature alters the quality of mangrove habitats for marine plants and animals

Study Number:

113

Author:

Georgia Walden, Christelle Noirot, Ivan Nagelkerken

Abstract:

Global climate stressors, like ocean warming and acidification, contribute to the erosion of structural complexity in marine foundation habitats by promoting the growth of low-relief turf, increasing grazing pressure on structurally complex marine vegetation, and by directly affecting the growth and survival of foundation species. Because mangrove roots are woody and their epibionts are used to ever-changing conditions in highly variable environments, mangrove habitats may be more resilient to global change stressors than other marine foundation species. Using a large-scale mesocosm experiment, we examined how ocean warming and acidification, under a reduced carbon emission scenario, affect the composition and structural complexity of mangrove epibiont communities and the use of mangrove habitat by juvenile fishes. We demonstrate that even a modest increase in seawater temperature of 1.2 °C leads to the homogenisation and flattening of mangrove root epibiont communities. Warming led to a 24% increase in the overall cover of algal epibionts on roots but the diversity of the epibiont species decreased by 33%. Epibiont structural complexity decreased owing to the shorter stature of weedy algal turfs which prospered under elevated temperature. Juvenile fishes showed alterations in mangrove habitat use with ocean warming and acidification, but these were independent of changes to the root epibiont community. We reveal that the quality of apparently resilient mangrove habitats and their perceived value as habitat for associated fauna are still vulnerable under a globally reduced carbon emission scenario.

Main Results and Conclusions:

• Global climate change, especially water temperature rising, negatively affects mangroves.
  • “Warming led to a 24% increase in the overall cover of algal epibionts on roots but the diversity of the epibiont species decreased by 33%. Epibiont structural complexity decreased owing to the shorter stature of weedy algal turfs which prospered under elevated temperature. Juvenile fishes showed alterations in mangrove habitat use with ocean warming and acidification, but these were independent of changes to the root epibiont community.” (p.596)
  •  “We demonstrate that even a modest increase in seawater temperature of 1.2 °C leads to the homogenisation and flattening of mangrove root epibiont communities.” (p. 596)
• Many anthropogenic factors contribute to the degradation of mangrove ecosystems.
  • “Anthropogenic activities have led to the loss of foundation species in a variety of environments, having broad consequences for associated biota, biodiversity, and ecosystem function and stability (Ellison et al. 2005).” (p. 597)
  • “In the marine environment, the importance of physical structure  is shown by the rapid colonisation of manmade additions like jetty posts and wave breaks (Bohnsack, 1989; Rilov and Benayahu, 1998). Mangroves, oysters, kelps, corals and seagrasses are key foundation species and often the only natural providers of physical structure in coastal habitats.” (p.597)
• In warmer waters more algae is produced which covers more of the mangrove roots and disrupts algae biodiversity.
  • “The total area of colonised space on the mangrove prop roots increased by 22.5% and 24.9% under the elevated temperature (T) and the combined elevated temperature and pCO2 (OAT) treatments (T and OAT; F = 21.70, p b 0.001), respectively (Fig. 1).” (p. 599)
  • “This was primarily due to an increase in the cover of algal turf from an average of 56% in controls to ~83% under elevated temperature (T and OAT; F = 13.10 p b 0.001, Fig. 1).” (p. 599)
  • “Acidification alone had no effect on total epibiont cover, nor did it alter the relative cover of turf or macroalgae (Fig. 1).” (p. 599)
  • “Under elevated temperature (T and OAT), the average height of the epibiont community decreased by 0.4 cm (F = 4.32, p = 0.037). Epibiont height range (maximum height minus minimum height) also decreased in T and OAT mesocosms to 2.7 cm ± 0.2 and 3.3 cm ± 0.4, respectively, compared with 4.3 cm ± 0.2 in controls (near significant T × OA interaction: F = 4.44, p = 0.052).” (p. 600)
• Due to the rising ocean temperatures we are loosing structural complexity in mangrove habitats.
  • “The loss of structural complexity in foundation habitats decreases refuge availability and ultimately reduces the richness of associated fauna (Graham et al. 2007; Gratwicke and Speight, 2005). We found that temperature reduced the average height of mangrove root algal epibionts, as well as the range of epibiont heights.” (p. 600)
  • “Elevated temperature not only flattened the epibiont habitat but also reduced the epibiont species diversity. Calcareous algae and some species of fleshy algae were lost from elevated temperature communities entirely, the outcome being a simplified, more homogenised community, dominated by a single algal taxon.” (p. 600)
  • “As such, a number of studies have found positive correlations between fish species richness and substratum species diversity (see Gratwicke and Speight, 2005).” (p. 600)
• Rising ocean temperatures and acidification can affect other marine animals and their likelihood to visit mangrove habitats.
  • “Finally, the climate stressors may have indirectly altered the number of visits the fish made to mangrove roots via a pathway not fully explored in this study.” (p. 601)
  • “It is still possible that hierarchies were established in treatment mesocosms prior to the observation period, and that these led to the observed patterns, however, we were not able to find evidence of this. Alternatively, the climate treatments may have altered the abundance of natural food sources like amphipods or certain species of algae and that this led to increased visitation by some species in search of these food sources. The origin of the altered habitat behaviours noted in this study requires further investigation.” (p. 601) 

Works Cited:

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Walden, G., Noirot, C., Nagelkerken, I. 2019. A future 1.2 C increase in ocean temperature alters
the quality of mangrove habitats for marine plants and animals. Science of the Total
 Environment. 690: 596-603.