The impact of extreme flooding events and anthropogenic stressors on the macrobenthic communities’ dynamics

Cardoso, P. G., et al., 2008. Estuarine, Coastal and Shelf Science

Original research (primary data)
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Marine and coastal environments are among the most ecologically and socio-economically important habitats on Earth. However, climate change associated with a variety of anthropogenic stressors (e.g. eutrophication) may interact to produce combined impacts on biodiversity and ecosystem functioning, which in turn will have profound implications for marine ecosystems and the economic and social systems that depend upon them. Over period 1980-2000, the environment of the Mondego estuary, Portugal, has deteriorated through eutrophication, manifested in the replacement of seagrasses by opportunistic macroalgae, degradation of water quality and increased turbidity, and the system has also experienced extreme flood events. A restoration plan was implemented in 1998 which aimed to reverse the eutrophication effects, especially to restore the original natural seagrass (Zostera noltii) community. This paper explores the interactions between extreme weather events (e.g. intense floods) and anthropogenic stressors (e.g. eutrophication) on the dynamics of the macrobenthic assemblages and the socio-economic implications that follow. We found that during the previous decade, the intensification of extreme flooding events had significant effects on the structure and functioning of macrobenthic communities, specifically a decline in total biomass, a decline in species richness and a decline in suspension feeders. However, the earlier eutrophication process also strongly modified the macrobenthic community, seen as a decline in species richness, increase in detritivores and a decline in herbivores together with a significant increase in small deposit-feeding polychaetes. After the implementation of the management plan, macrobenthic assemblages seemed to be recovering from eutrophication, but it is argued here that those earlier impacts reduced system stability and the resilience of the macrobenthic assemblages, so that its ability to cope with other stressors was compromised. Thus, heavy flooding in the Mondego region during the recovery process had more severe effects on these assemblages than expected, effectively re-setting the recovery clock, with significant socio-economic impacts (e.g. high mortality of fish in fish farms, and a large decline of economically important species, such as the bivalves Scrobicularia plana and Cerastoderma edule). The frequency and magnitude of these extreme events is predicted to increase in future years [IPCC WGI, 2001. Climate change 2001: the scientific basis, contribution of working group I to the third assessment report of the Intergovernmental Panel on Climate Change (IPCC). In: Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Xiaosu, D. (Eds.), Cambridge University Press, UK, 944 p.] and there is a risk that impacted ecosystems will never recover fully, with far-reaching consequences for human well being.

Case studies

Basic information

  • Case ID: INT-121-1
  • Intervention type: Restoration
  • Intervention description:

    Seagrass bed restoration: Due to the decline of seagrass beds and progressive impover- ishment of the habitat, a management programme was imple- mented in 1998 in order to restore the original seagrass community. This programme included measures to decrease nutrient loading, physical protection of the seagrass bed and improvement of the hydraulic regime, by enlarging the con- nection between the two arms. The Pranto sluice-opening regime was changed so that most of the freshwater from the Pranto River was diverted to the northern arm, reducing the nutrient loading in the southern arm. In addition, the remain- ing seagrass patches were protected with wooden stakes to prevent further disturbance by fishermen digging in the sedi- ment for bait and cockles

  • Landscape/sea scape ecosystem management: Yes
  • Climate change impacts Effect of Nbs on CCI Effect measures
    Loss of food production  Mixed results biomass and density of commercially important species such as Scrobicularia plana and Cerastoderma edule
  • Approach implemented in the field: Yes
  • Specific location:

    The Mondego estuary, located on the Atlantic coast of Portugal (40 08 N, 8 50 W) consists of two distinct arms, northern and southern, separated by Murraceira Island

  • Country: Portugal
  • Habitat/Biome type: Seagrass |
  • Issue specific term: Not applicable


  • Notes on intervention effectivness: Not sure how to code for outcome effectiveness: the seagrass bed was stated to have degraded because of climate change and eutrophication and showed initial recovery post-restoration (so would be positive, assessed from changes over time) but then flooding reduced the fish biomass so authors say it wasn't good enough but there is no control (un restored) so don't know what the actual effect would have been of restoration in protecting the fished species (maybe if there had been no restoration, the drop in biomass would have been even worse). So we have one positive and one unclear, how to code, mixed? same question for biodiversity outcomes
  • Is the assessment original?: Yes
  • Broadtype of intervention considered: Not applicable
  • Compare effectivness?: No
  • Compared to the non-NBS approach: Not applicable
  • Report greenhouse gas mitigation?: No
  • Impacts on GHG: Not applicable
  • Assess outcomes of the intervention on natural ecosystems: Yes
  • Impacts for the ecosystem: Mixed
  • Ecosystem measures: Macrobenthic fauna biomass and density, trophic structure, community composition, species richness and evenness
  • Assess outcomes of the intervention on people: Yes
  • Impacts for people: No-effect
  • People measures: Anectodal observation – ‘fish farms were directly affected’, ‘local fishermen that exploit the mudflats’ were affected they report results that show that despite the intervention (seagrass restoration), the system was not resilient enough to withstand the severe floods that occurred. They directly link these impacts to local fishermen – see text below "the 2000/2001 floods had a major impact on this community, especially on the subsurface-deposit feeders and suspension feeders (e.g. Scrobicularia plana and Cerastoderma edule), probably through the clogging of the feeding structures of these suspension feeders by the high turbidity (Norkko et al., 2002). Both these bivalves are economically important for the region, especially for the local fishermen who depend on estuarine resources directly heavy flooding in the Mondego region during the process of recovery eutrophication had severe effects on these assemblages, effectively re-setting the recovery clock and slowing the overall return to the undisturbed state. This not only has implications for biodiversity conservation on the Mondego, but for the livelihoods of the people who depend on the estuary. Thus, fish farms were directly affected due to the low salinities recorded over several consecutive months, which led to high fish mortality. Also, local fishermen that exploit the estuary mudflats directly were also affected because commercially important species such as Scrobicularia plana, Cerastoderma edule declined dramatically after the floods "
  • Considers economic costs: No
  • Economic appraisal conducted: No
  • Economic appraisal described:
  • Economic costs of alternative considered: No
  • Compared to an alternative: Not reported

Evaluation methodology

  • Type of data: Quantitative
  • Is it experimental: No
  • Experimental evalution done: Not applicable
  • Non-experimental evalution done: Empirical case study
  • Study is systematic: