Reproductive Allocation of the Habitat-Forming Intertidal Macroalga Ascophyllum nodosum Decreases at Its Northern Distribution Edge

The habitat-forming intertidal brown alga Ascophyllum nodosum has its colder northern distribution limit at 69°N in Disko Bay, Greenland. Its reproductive effort has never been assessed there despite expected northward expansion with climate change. We analyzed reproductive allocation and phenology at the northern distribution edge and across the geographical distribution range through field studies at three Greenland sites and one Danish site, supplemented with a literature survey. Because Ascophyllum is long-lived and forms annual segments through apical growth, old shoots sampled in the reproductive season revealed receptacle formation with segment age, from the tip to the base of the shoots. We confirmed the fertility of the northernmost populations, as zygotes formed from gametes. We found a consistent pattern of receptacle formation with larger receptacles closer to the canopy top and receptacle abundance following a quadratic relationship with segment age. Ascophyllum's reproductive allocation constitutes 33%–39% of its annual production in the Disko Bay and increases towards southern, warmer latitudes. Reproductive phenology also varies significantly with latitude and temperature, showing a 4.5-day delay in the reproduction peak for every degree northward and a 14-day delay with every 1°C decrease in temperature. The carbon flux released from the reproductive structures to the surrounding Arctic ecosystem at the end of the reproductive season was significant, amounting to 212–827 g C m⁻² year⁻¹ in Greenland, which should be considered in future productivity assessments. Synthesis: Our results indicate different life-history strategies at the opposing distribution edges and stress the importance of temperature as a regulator of Ascophyllum reproduction. Arctic warming will likely enhance reproductive output and stimulate an earlier onset of reproduction. Furthermore, the significant contribution from sexual reproduction to the annual carbon production (26%–41% in Greenland) highlighted its importance to the Arctic detrital community.