Our initial objective was strictly to use DNA barcode data to acquire a better understanding of seaweed diversity in Churchill. In this regard, we were largely successful and recorded 57 genetic groups where only ~50 morpho-species were identified in the field (with barcode data not generated for six of the latter). The discrepancy in these values was attributable to phenotypic plasticity, overlooked diversity, range extensions for some species and recognition that some genera are in need of substantial taxonomic revision in the North American flora. Whereas we were able to resolve some of these taxonomic issues and apply names to records from Churchill (e.g., Chordaria chordaeformis), we have generated far more taxonomic problems than we have resolved. These will serve as the foundation for a number of future studies dealing specifically with taxonomic issues for the various species and genera found in Canadian waters. On the positive, we have also recorded eight new distributional records for the Churchill region, as well as some for the Canadian Atlantic and Pacific regions through our efforts, while 17 genetic species/complexes that are from a variety of overlooked species groups will most certainly increase these values following detailed taxonomic study.
Perhaps more interesting to the general community were the inferences that we were able to make on the Likely Source Regions for the Churchill populations of the various species that have re-colonized this area following the last glacial retreat.
We must start by acknowledging that this aspect of the current manuscript has several weaknesses. Foremost, we have genetic data for a limited number of samples, which is exacerbated by a strategy to get as many species as we could (DNA barcode objective) at the expense of replication within species (necessary for biogeographical studies). A particular aspect in need of improvement would be the addition of samples for the pertinent species (Additional file 1) from the European Atlantic and the northern most reaches of the ‘Pacific region’ as defined here (e.g., Bering Sea, Chukchi Sea, etc.). Second, our goal was strictly to identify the Likely Source Region for species present in Churchill, all of which would have had to migrate to that region following the last glacial retreat (~10,000 years ago ). As such, longer-term phylogeographic patterns of Pacific contributions to the Canadian Arctic and Atlantic floras were not considered. For example, whereas the current population of P. washingtoniensis in Churchill has mitotypes consistent with contemporary Atlantic rather than Pacific populations (hence an Atlantic LSR in our summary), it is parsimonious (based on the ‘phylogeny’ in Figure 4) to conclude that this species had previously migrated from the Pacific into the Atlantic during an earlier warming event. Finally, the genetic markers that we used, although widely established as species-level barcode markers in the groups under study here and thus suitable for a floristic survey of a region , are not necessarily optimal markers for population level studies such that resolution was surely lost.
Nonetheless, we feel that our efforts have merit. First, for most of the genetic species considered here (Additional file 1) the issue of an LSR into the Churchill region was a species-level rather than population level question. In many instances these involved overlooked, but nonetheless distinct, genetic species that were masked under a single morpho-species (e.g., Desmarestia aculeata complex, Figure 3) with the molecules providing a clear indication of an LSR. Although more sampling from all regions is necessary to ensure that the novel species are not more widespread than currently interpreted based on the collections available, this study does set a foundation for future research on this question going forward. Second, in some cases we had fairly substantial number of samples coupled with reasonable within genetic group COI-5P variation such that population inferences were not unreasonable (e.g., Pylaiella washingtoniensis; Figure 4). Allowing for these caveats, we can note a number of significant trends in our data.
Based on our data we hypothesized that ~21% of the flora in the Canadian Arctic has recolonized through recent migrations from the Pacific region. This finding agrees with the predictions of the Adey et al.  Thermogeographic Model and the notion of trans-Arctic migrations having a strong Pacific to Atlantic migrational bias. This result also lessens the biogeographical ‘paradox’ regarding the Canadian Arctic benthic flora relative to fauna discussed in the literature , also see , which was also challenged by Lindstrom . In fact, it is highly likely that our current value represents an underestimate as a number of key species had to be excluded from our analyses owing to a lack of collections from the most northerly Pacific region. Many of the so-called Arctic species  are now considered iconic examples of Pacific contributors to the Arctic and Atlantic floras . If we include even a few of these, such as the brown alga Laminaria solidungula and the red algae Devaleraea ramentacea, Odonthalia dentata, Polysiphonia arctica and Coccotylus truncatus as being of putative Pacific origin, then the percent contribution from that source region climbs to 33%. Dilsea socialis, which was recorded as an uncertain LSR based on the molecular data at hand, and Turnerella pennyi, for which we were unable to generate barcode data here, were also noted as exemplar species in Adey et al.  and if treated as Pacific LSR’s would raise the percentage contribution from that region to 37%. Interestingly, the seven previous species are largely distributed only in the more northerly reaches of the Pacific region as defined here (e.g., Bering Sea), as are three of the six species for which we have argued a clear Pacific LSR (viz., Alaria esculenta, Chorda sp._1filum, Halosiphon sp._2tomentosus), and are not associated with the British Columbia flora. This biogeographical pattern is consistent with results from a DNA barcode survey of marine polychaetes suggesting a clear distinction between the biota in British Columbia relative to the northern reaches of the Pacific region and the Canadian Arctic . We additionally uncovered a number of taxa for which the molecular LSR was recorded as ‘uncertain’ (Figure 2b), including Agarum clathratum, Petalonia fascia, Punctaria sp._2GWS and Scytosiphon canaliculatus, that had virtually identical within species mitotypes for representative collections from all regions under study here suggesting recent migrations for these species between the Atlantic and Pacific through the Arctic although the direction remains uncertain (also true for Dilsea socialis, which was considered previously). If even half of these species had a Pacific LSR, the percentage contribution to the Canadian Arctic flora from that region would rise to 38.5%, while if all four derived from that region a Pacific region contribution of 44% would be realized. These last-mentioned species point to the pivotal stage of a trans-Arctic migration, i.e., evidence for the Pacific flora reaching into the Atlantic region as documented for Saccharina latissima. Inevitably some of these species may have had their mitotype distributions impacted by human-mediated introductions (e.g., Ulva spp. ), which will have to be considered as these data and analyses continue to develop. Additionally, only with enhanced sampling from the Pacific region, especially the northern most areas that are the least sampled among those under discussion here , will these hypotheses be further tested and a more complete picture of the recent source populations for the Canadian Arctic flora realized.
Another flora in need of intense sampling relative to the Canadian Arctic flora is that of the European Atlantic. We only had limited comparative COI-5P data from Europe for a few representatives of Alaria marginata, Palmaria palmata, Saccharina latissima and Rhodomela confervoides. However, in all four cases the COI-5P data collected to date indicated that the European specimens were distinct from the North American Atlantic and Churchill populations. This indicates that for these species the Churchill region was repopulated from the ‘relict’ North American Atlantic flora and not the European flora. In contrast, barcode data for Ptilota gunneri are consistent with a European source for both the Canadian Atlantic and Arctic populations. Further, our putative discovery of the European species Petalonia filiformis in Churchill, but not the Canadian Atlantic, is consistent with a European source for that species into our Arctic waters. Again, such hypotheses await the generation of comparative data for European conspecifics of all of the species identified in our Churchill survey. We certainly look forward to these new data and further exploration of the nascent hypotheses that we have framed here.