Late summer harvest of cultivated Saccharina latissima, prior to seasonally determined negative length growth, is considered advantageous in North Atlantic waters to optimize biomass yields. We hypothesized that seasonal increase in tissue protein and pigments over autumn and early winter would counterbalance the loss of biomass, and increase the absolute harvestable amount of protein and pigments. The hypothesis was tested in a land-based, factorial-designed, pilot-scale experiment using whole algae individuals exposed to naturally relevant high or low availability of nutrients and light. The experiment was conducted during fall/early winter in Grenaa, Denmark, in outdoor tanks, exposed to ambient light and temperature variations. With high nutrient availability, the absolute harvestable amounts of nitrogen, fucoxanthin, and chlorophyll a increased by 50.1-60.1, 21.7-53.7, and 47.0-73.5 %, respectively, despite a loss of biomass of 16.2-18.7 %. Under low nutrient availability, there was a net loss of biomass (8.1-9.5 %), tissue nitrogen (10.7-44.1 %), and fucoxanthin (7.1-17.2 %), and a minor increase in chlorophyll a (2.5-22.8 %). Nutrient availability had a significant negative impact on the biomass growth, but a positive control on the tissue concentration of nitrogen, chlorophyll a, and fucoxanthin. Our results, from a land-based experiment, indicate that early winter harvest of S. latissima biomass grown under high nutrient availability in Denmark, fulfills a higher degree of nutrient bioremediation, and has an improved biomass quality in regards of increased concentrations of pigments and nitrogen rich compounds.

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1. The future increase in the atmospheric CO 2 concentration is likely to affect the growth and performance of submerged freshwater macrophytes because of higher concentrations of free CO 2 in the water at air equilibrium. We measured the plastic responses to free CO 2 and light for several traits of four invasive aquatic plants (Elodea canadensis, Egeria densa, Hydrilla verticillata and Ceratophyllum demersum) that use bicarbonate. 2. The plants were grown in standard culture media at either high (HiC, c. 50 lmol L À1 ) or low concentrations of free CO 2 (LowC, c. 19 lmol L À1 ) combined with high (HiL, c. 150 lmol m À2 s À1 ) or low (LowL, c. 21 lmol m À2 s À1 ) light intensity in a 2 9 2 factorial experiment. We compared the relative growth rate (RGR), several morphological traits, the photosynthetic response to light intensity and the bicarbonate uptake capacity of the four species. For every trait measured, we established plasticity indices for light and CO 2 availability. 3. Light intensity had a greater effect than CO 2 concentration on all species. The RGR of all four species was higher at high light intensity, and photosynthetic light responses acclimated to low light with a lower light compensation point, and with higher concentrations of photosynthetic pigments and quantum yield. The bicarbonate uptake capacity was generally highest at the high light intensity and high concentrations of free CO 2 . Plasticity indices for light intensity were consistently higher than for CO 2 availability in all four species. 4. As the growth and photosynthesis of the four invasive bicarbonate users were only slightly affected by the CO 2 availability in air-equilibrated water, the future rise in atmospheric CO 2 is unlikely to exacerbate their invasive behaviour and may even reduce their competitiveness against species unable to use bicarbonate, especially at limiting light.

When cultivating the kelp Saccharina latissima, knowledge on the availability of seeding material for the production is essential. Applying a spore seeding approach requires spores from the reproductive organs of the fertile sporophytes (sori). As sori are generally not present during the time of seeding in late summer, the production of spores (sporogenesis) can be artificially induced by removing the meristematic part of the sporophyte and keeping the sporophyte under short day, temperate, and nutrient-replete conditions. Only limited information is available on the effect of light intensities including darkness on the sporogenesis of S. latissima. This study examined the natural pattern of sporogenesis in S. latissima from Middelfart, Denmark, and the effect of four different light regimes (0, 20, 60, or 120 μmol photons m−2 s−1) on the artificial induction of sporogenesis in S. latissima. Natural reproductivity and availability of spores in Denmark peaked in early winter, with 86% of the population being reproductive in November. Reproductive material was available from October until late spring, but with a variable spore release from 11 × 103 to 1.2 × 106 spores cm−2 sori. The artificial induction of sporogenesis was optimal in darkness with > 90% of sporophytes developing sori after 49 days, with an average spore release density of 1.15 ± 0.38 × 106 spores cm−2 sori. The results confirmed that S. latissima in Denmark follows the general pattern of reproduction of S. latissima in North Atlantic regions and demonstrated for the first time that sporogenesis in S. latissima can be efficiently induced in darkness.