Combining seascape connectivity with cumulative impact assessment in support of ecosystem-based marine spatial planning

1. Cumulative impact assessment (CIA) is a promising approach to guide
marine spatial planning (MSP) and management. One limitation of CIA is the
neglect of seascape connectivity, which may spread the impact of localised
pressures to ambient areas, e.g. through lost dispersal and recruitment of
organisms. 2. We here, for the first time, incorporate seascape
connectivity into a traditional CIA model using a connectivity matrix,
exemplified by dispersal of propagules estimated through biophysical
modelling. Two connectivity impactsare identified:
the source impact represents downstream areas losing
recruits because of reduced larval dispersal from sites affected by the
pressure, and the sink impact represents loss of
recruits originating from upstream areas prevented from settlement in the
site affected by the local pressure. 3. By including seascape connectivity
into the Swedish MSP-guiding CIA tool Symphony we demonstrate how to
practically account for remote effects of local environmental impact. Our
example on blue mussel shows how reducing mussel fitness in a given area
may have impacts on mussels far from the acting pressures. Overall,
results indicate that connectivity impact for blue mussels plays a minor
role in most areas, less than 10% of the ordinary cumulative impact.
However, in some smaller areas, e.g. on offshore banks and the Danish
Straits, seascape connectivity may increase ordinary cumulative impact
with 20-30%. In an example of scenario-based CIA analyses of MSP
projections, we demonstrate how impacts of particular management actions,
e.g. shipping rerouting and wind power developments, can be tracked far
from the original area of influence. 4. Depending on the dispersal ability
of ecosystem components, a local pressure may impact a considerable area
through seascape connectivity, transgressing management units and national
borders. Although the mean connectivity impact may be modest for a single
ecosystem component, the consideration of seascape connectivity across
multiple ecosystem components may significantly alter the mapping of
cumulative impact and the assessment of different MSP scenarios. 5.
Synthesis and applications. Our extension of Cumulative Impact Assessment
offers a new method for mapping and practically integrating seascape
connectivity with ecosystem-based MSP and other spatial instruments for
policy making, such as marine protected areas.