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

Abstract

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 1980e2000, 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.

Restricted ranges in physical factors may constitute subtle stressors for estuarine biota

“…Similarly, nearshore salinity appeared to affect the biota even though the range at our sites was only on the order of 22-32 psu, a far more subtle gradient than the 5-28 or even 0-40 psu differences noted as important to infauna in other estuaries (Rutger and Wing 2006;Rakocinski et al 2000, respectively). However, mechanisms behind the roles of salinity or temperature stressors in Puget Sound may well involve extreme events not captured by our sampling; and with predictions that extreme weather events may become more common in the future (Cardoso et al, 2008), such mechanisms will become increasingly important. As in most estuaries (Dame, 1996), combinations of temperature and salinity extremes may affect organisms more than just the sum of their individual stress effects.…”

Seasonal variability in the juvenile growth rate of an infaunal polychaete is related to major rain events

“…and other infauna, with high abundances in years following large floods, but little predictable seasonal variation. Understanding the effects of salinity stress on animals in this system requires methods that can assess the relative performance of infauna on a time scale of days to complement approaches that are less sensitive to episodic events, such as measuring the abundances of infauna or community composition (Balthis et al 2006;Cardoso et al 2008).…”

“…Salinity has long been considered a critical factor affecting estuarine benthos (Sanders et al 1965), and pollution associated with stormwater runoff is well documented in urban estuaries (Gersberg et al 2004). Although patterns of species' abundances and the composition of infaunal communities have been studied in relation to rainfall and salinity (Ritter et al 2005;Balthis et al 2006;Cardoso et al 2008), we are not aware of any previous in situ measurements of infaunal growth or performance in response to major rain events.…”

Efeito de um evento extremo de enchente sobre os decápodos infralitorais do estuário do Itajaí-Açu, SC, Brasil

“…Em novembro de 2008, no entanto, uma enchente de proporções catastróficas acometeu este estuário, causando severos impactos econômicos e sociais na região. Estudos sobre enchentes indicam que esses eventos extremos podem afetar significativamente a fauna estuarina, causando diminuição na biomassa e riqueza das espécies (Cardoso et al, 2008).…”