A consideration of small marine gulfs as biogeographic habitat islands using a case study of the Normano-Breton Gulf

The study of marine habitat islands is mostly limited to distinct, offshore spaces that fit the
geographic assumptions of the terrestrial island model. This study considered the validity of
coastal gulfs as marine habitat islands within the context of the biogeography discipline. The
concept was explored via the Normano-Breton Gulf on the south side of the English Channel.
The value of gulfs as marine habitat islands was considered in terms of speciation,
adaptation, and biodiversity, as well as in their potential use as natural laboratories for marine
system modelling.
The study reviewed historical and modern oceanographic data from the Normano-Breton Gulf
to understand the currents and tides that create the isolation of the inner gulf. Highlighted
from this data was the six to eight week retention time for water in that zone, and the unique
thermal pattern. Within this context, biological examples were examined to test biological
solation within the gulf. eDNA was used to investigate biodiversity and genetic studies were
considered to understand larval transport.
The study identified biological isolation within the gulf and found the area to be a good fit for
the biogeographic island rules for small to medium size islands, with some applicability of
offshore island rules for distant-origin / introduced non-native species. For stock and
conservation system modelling the Normano-Breton Gulf showed value as a simplified,
scalable system that could be used to consider and predict patterns before they impact the
wider English Channel. Together these factors highlight the merit of consideration of small
gulfs as biogeographic habitat islands.