Sugar kelp spores settle too fast to mix Maine's four coastal populations

The boundaries here are oceanographic. Sugar kelp spends most of its life cycle fixed to the seafloor and is mobile only during a brief early spore stage ^[9]. Inside a population, the evidence suggests currents mix genes readily between sites ^[8]. Across populations, spores seem to settle short of the distance that would matter, even in strong currents ^[9].

Maine's kelp aquaculture is the largest of its kind in the country, and each year it begins with reproductive tissue cut from wild plants ^[3][2]. Rene Francolini, the lead author and a former University of Maine doctoral student in Doug Rasher's lab ^[23], tied the genetics to that practice. "That's important for restoration work or when we think about farmers who might collect reproductive tissue in one location and outplant seed in another," Francolini said ^[14].

Eleven sites divided among at least four populations averages fewer than three sampled sites per population, about 2.75 ^[21]. That is enough to establish that structure exists along the outer coast. Drawing a boundary on a chart takes more. The paper reports at least four populations, and that floor leaves room for structure 11 sites left unresolved ^[5].

The choice of the 11 sites was deliberate. The authors say they picked that band of the outer coast because it is far enough out to be subject to the oceanographic forces acting on the whole coastline, and close enough to shore to be a plausible industry sourcing area later ^[7].

What the study measured is genetic structure. Previous work has shown that diversity of this kind can bolster resilience to marine heat waves ^[10]. That is a general result about diversity, and it leaves these four populations unranked. The lead author's wording keeps the distinction. "Having distinct populations means there are different genetic signatures along our coast, and some populations could have individuals that are more or less suited to thrive in varied environments," Francolini said ^[13].

The scale is what separates this paper from its predecessors. Earlier studies asked the same questions across all of New England, and at the very fine scale of adjacent bays; this is the first to treat the Maine coast as a holistic unit ^[15]. "We brought it down to the scale that really matters from a management perspective," said Rasher, the senior author ^[16]. The Bigelow Laboratory team writes that the findings highlight the importance of regional management strategies for the resource ^[22].

None of the mapping was available until another team published the first complete sugar kelp genome last year, a resource that let this group place samples into populations and begin identifying genes unique to each one ^[18]. "We have yet to be able to connect these genes to a specific function, but when we do, we will be able to pinpoint kelp variants that are uniquely strong candidates for improving kelp conservation, restoration and aquaculture outcomes," Francolini said ^[20].