Kelps are important foundation species in many cold-temperate coastal systems, and the loss of these organisms is a threat to ecosystem structure and function. The abundance of sugar kelp Saccharina latissima has recently declined in Northern Europe, which has been explained by increasing water temperature. We tested heat tolerance of sugar kelp exposed to simulated heatwave scenarios of 15, 18, 21 and 24°C for 3 wk, followed by a 2 wk recovery period at 15°C. Growth rate and photosynthetic performance decreased significantly with increasing temperature, while mortality remained low among treatments except at 24°C, where > 90% of the algae died within a few days. Although exposure to 18 and 21°C had limited effect on mortality, kelps exposed to these temperatures had negative growth and continued to show impaired photosynthesis during the subsequent recovery period. Reductions in growth were strongly correlated to reduced carbon acquisition and, hence, photosynthetic performance, which was strongly correlated to heat-related changes in pigmentation. We suggest that reduced performance after exposure to elevated but non-lethal temperatures was caused by oxidative stress resulting from a discrepancy between light absorption and photosynthesis. Our results show that exposure to high but sub-lethal temperatures can have significant long-term effects, which may cause loss of biomass and leave sugar kelp susceptible to other stressors.

The fast expansion of the global seaweed aquaculture industry has created an interest in translocating seedlings cultivated from wild type brood stock. However, such translocations must be applied with caution as introduced cultivars can reduce genetic structure and diversity of wild populations. An understanding of the genetic structure and connectivity of target species is required to guide decision making around aquaculture translocation activities. In this study we used 14 microsatellite loci in a three-level hierarchical sampling design to analyze the genetic structure and connectivity of the native kelp Ecklonia radiata across 12 sites among four geographic regions (Northland, Bay of Plenty, Gisborne, and Wellington) in the North Island of New Zealand. Our aim was to provide guidance for translocation of cultivars to prevent the introduction of locally absent genotypes of E. radiata. Strong genetic structure and low geneflow were observed at all hierarchical levels, indicating the presence of multiple genetically distinct sub-populations. On a regional scale, high genetic differentiation was found between the Wellington region and the other three regions (FST = 0.407–0.545), and within regions most sites were significantly different (measured by pairwise FST) with high relatedness found between individuals within sites (mean 28.2% ± 0.7 SE). Bayesian modeling and redundancy analysis showed a high degree of genetic clustering and indicate that ocean currents and other factors that have resulted in biogeographical breaks along the coast are likely to be the main factors shaping genetic structure and connectivity of E. radiata on the North Island, rather than isolation by distance. Based on these findings, we recommend that that cultivars of E. radiata should not be translocated outside their area of origin to avoid introducing locally absent genotypes to local sub-populations.

The biomass composition of kelp varies within species both spatially and temporally. However, this variation in biomass quality has not yet been investigated for the native kelp Ecklonia radiata within New Zealand, where the kelp is a target for the emerging seaweed aquaculture industry. In this study we quantified spatial and temporal variation in the composition of E. radiata biomass, collected from 12 sites around the North Island of New Zealand and from 12 months across a full year at a single site (n = 138). High spatial variation was detected for most components, including alginate (range: 16.6 – 22.7% DW, n = 12), fucoidan (range: 1.2 – 1.6% DW, n = 12), phlorotannins (range: 4.8 – 9.3% DW, n = 72), and glucose (range: 9.3 – 22.6% DW, n = 12). The biomass composition of E. radiata varied significantly among sites but with no clear patterns among regions, indicating that geographic differences were mostly local rather than regional, possibly due to site-specific environmental conditions. Significant temporal variation (measured by positive autocorrelation between months) was detected in the content of lipids, proteins, glucose, guluronic acid, nitrogen, phosphorous, iodine, arsenic, and mercury, and for the mannuronic to guluronic acid (M:G) ratio. Overall, E. radiata had comparable biomass composition to that of commercially grown northern hemisphere species but with substantially higher phlorotannin content. These results demonstrate that E. radiata could be a viable southern hemisphere alternative for a broad range of commercial applications.