Abstract
Abstract. Coastal hypoxia and anoxia have become a global key stressor to marine ecosystems, with almost 500 dead zones recorded worldwide. By triggering cascading effects from the individual organism to the community- and ecosystem level, oxygen depletions threaten marine biodiversity and can alter ecosystem structure and function. By integrating both physiological function and ecological processes, animal behaviour is ideal for assessing the stress state of benthic macrofauna to low dissolved oxygen. The initial response of organisms can serve as an early warning signal, while the successive behavioural reactions of key species indicate hypoxia levels and help assess community degradation. Here we document the behavioural responses of a representative spectrum of benthic macrofauna in the natural setting in the Northern Adriatic Sea (Mediterranean). We experimentally induced small-scale anoxia with a benthic chamber in 24 m depth to overcome the difficulties in predicting the onset of hypoxia, which often hinders full documentation in the field. The behavioural reactions were documented with a time-lapse camera. Oxygen depletion elicited significant and repeatable changes in general (visibility, locomotion, body movement and posture, location) and species-specific reactions in virtually all organisms (302 individuals from 32 species and 2 species groups). Most atypical (stress) behaviours were associated with specific oxygen thresholds: arm-tipping in the ophiuroid Ophiothrix quinquemaculata, for example, with the onset of mild hypoxia (< 2 mL O2 L−1), the emergence of polychaetes on the sediment surface with moderate hypoxia (< 1 mL O2 L−1), the emergence of the infaunal sea urchin Schizaster canaliferus on the sediment with severe hypoxia (< 0.5 mL O2 L−1) and heavy body rotations in sea anemones with anoxia. Other species changed their activity patterns, for example the circadian rhythm in the hermit crab Paguristes eremita or the bioherm-associated crab Pisidia longimana. Intra- and interspecific reactions were weakened or changed: decapods ceased defensive and territorial behaviour, and predator–prey interactions and relationships shifted. This nuanced scale of resolution is a useful tool to interpret present benthic community status (behaviour) and past mortalities (community composition, e.g. survival of tolerant species). This information on the sensitivity (onset of stress response), tolerance (mortality, survival), and characteristics (i.e. life habit, functional role) of key species also helps predict potential future changes in benthic structure and ecosystem functioning. This integrated approach can transport complex ecological processes to the public and decision-makers and help define specific monitoring, assessment and conservation plans.
Highlights
The increasing intensity and diversity of anthropogenic stressors is triggering unprecedented habitat and biodiversity loss in marine ecosystems worldwide (Halpern et al, 2008)
Excessive nutrient inputs have the potential to tip systems into hypoxia (here defined as dissolved oxygen (DO) concentrations < 2 mL L−1; about 2.8 mg L−1 or 91.4 μM; Diaz and Rosenberg, 1995) and anoxia or exacerbate conditions in predisposed areas (Gooday et al, 2009; Howarth et al, 2011)
The in situ approach successfully mimicked the actual time course of oxygen depletion events documented earlier in the Northern Adriatic based on macrofauna behaviour (Stachowitsch, 1984): hypoxia was generated within ca. 1.5 days, anoxia within 3 days (Appendix A, Table A1)
Summary
The increasing intensity and diversity of anthropogenic stressors is triggering unprecedented habitat and biodiversity loss in marine ecosystems worldwide (Halpern et al, 2008). The effects, ranging from population depletion to species. B. Riedel et al.: Effect of hypoxia and anoxia on invertebrate behaviour extinction and community homogenisation, raise concerns whether ecosystem function and ecosystem goods and services can be maintained (UNEP, 2006; Worm et al, 2006; Cardinale et al, 2012). A key issue is understanding how species respond to such rapid human-induced environmental change (Sih et al, 2011; Tuomainen and Candolin, 2011). Excessive nutrient inputs have the potential to tip systems into hypoxia (here defined as dissolved oxygen (DO) concentrations < 2 mL L−1; about 2.8 mg L−1 or 91.4 μM; Diaz and Rosenberg, 1995) and anoxia (the complete absence of oxygen) or exacerbate conditions in predisposed areas (Gooday et al, 2009; Howarth et al, 2011)
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