Dispersal and metacommunity dynamics in a soft-sediment benthic system – how well is the seafloor connected?

Norkko, A. (Supervisor)

Activity: Examination typesSupervisor or co-supervisor of doctoral thesis

Description

Connectivity depends on rates of dispersal between communities. For marine soft- sediment communities dispersal involves more than just initial colonization (recruitment) of substrate by pelagic larvae. Continued small-scale dispersal as post-larvae and as adults can be equally important in maintaining community composition. Our knowledge of post- larval dispersal potential of marine organisms in general is, however, very limited. This is particularly true for non-tidal benthic systems, such as the Baltic Sea, where dispersal has not previously been quantified. Having an understanding of how and when individuals are dispersing relative to underlying environmental heterogeneity within a given region is key to interpreting scale-dependent patterns of diversity (!-, "-, #-diversity). Nevertheless, in nature a difficulty has been to actually measure dispersal directly, which has caused empirical work to fall far behind theoretical developments; in both metacommunity and metapopulation ecology.
In this thesis, a variety of direct and indirect measures of dispersal were used to investigate connectivity in marine soft-sediment communities. Post-larval (juveniles and adults) dispersal was quantified using a variety of trap types, along with ambient community composition, at different sampling intervals across sites that varied in local environmental conditions (e.g. sediment grain size, exposure to wind-waves). Taxa dispersed in relative proportion that was distinctly different from resident community composition and a significant proportion (40%) of taxa were found to lack a planktonic larval life-stage. Several system and species–specific dispersal-related strategies were demonstrated, as well as underlying mechanisms by which communities are connected. Local community composition was found to change predictably under varying rates of dispersal and physical connectivity (waves and currents). This response was, however, dependent on dispersal related traits of taxa. Actively dispersing taxa will be relatively better at maintaining their position, as they are not as dependent on hydrodynamic conditions for dispersal and will be less prone to be passively transported by currents and deposited back down onto the sediment.
Community assembly was also re-started in a large-scale manipulative field experiment across several sites, which revealed how patterns of community composition (!-, "- and #-diversity) change depending on rates of dispersal. Dispersal can become limiting for some species and/or life-stages (patch dynamic) at early assembly or if a newly created disturbance (empty patch) is large relative to the scale of underlying environmental heterogeneity. In response to small-scale disturbances, however, findings suggest that initial dispersal and recruitment will be by nearby-dominant species after which species will arrive from successively further away. If rates of dispersal remain high the number of coexisting species will increase beyond what would be expected purely by local niche requirements (species sorting), thus transferring regional differences in community composition ("-diversity) to the local scale (!-diversity, mass effect).
In contrast to initial larval recruitment, frequent small-scale dispersal as post- larvae can significantly extend the dispersal period and thus contribute to resilience of benthic communities when faced with disturbance. In situ findings of this thesis complement several theoretical and laboratory-based studies in demonstrating how both dispersal and environmental heterogeneity contribute to the assembly and maintenance of spatio-temporal patterns of community composition.
Period19 Dec 2012
ExamineeSebastian Valanko
Examination held atÅbo Akademi University
Degree of RecognitionInternational