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Patterns of potential recruitment, survival, age-structure, density, biomass and primary production were studied in the kelp Laminaria hyperborea along a wave-exposure gradient in western Norway. The distribution of L. hyperborea is positively related to wave exposure, and the present work aimed to study how demography and population dynamics are affected by wave exposure. Populations at 9 sites representing 3 different levels of wave exposure were investigated. The biomass and production of L. hyperborea doubled along the gradient from low-to high-exposure sites. This increase was caused by an increase in plant density and individual plant size. Recruits and sub-canopy plants made up the majority of all individuals at all sites, but potential recruitment increased more than mortality rate among recruits and young sub-canopy plants as wave exposure increased, leading to a higher density of these small individuals at highexposure sites. Despite their large numbers, recruits and sub-canopy plants were not important for total biomass and production, and variations in those parameters with wave exposure were largely driven by variations in the density and size of adult canopy plants. Canopy plants suffered higher rates of mortality at low wave exposure, leading to shorter longevity and lower density than at high-exposure sites. The density and individual size of canopy plants both increased by ca. 50% with increasing wave exposure, explaining the higher biomass and productivity at highexposure sites. We suggest that the high mortality rate, low density and small size of canopy plants at low-exposure sites is caused by a combination of high epiphytic load and self-shading, which may impair light conditions, affect nutrient uptake and increase drag on the blades during extreme wind events. The reason why plants at high-exposure sites reach a larger individual size remains unknown.

The perennial red macroalga Gracilaria vermiculophylla (Ohmi) Papenfuss has recently been introduced to the Baltic Sea and is a potential competitor to Fucus vesiculosus, the most common native perennial alga in large parts of the Baltic Sea. Gracilaria might interfere with Fucus through direct competition for resources. In addition, Gracilaria is a favoured refuge for mesograzers, which prefer to feed on Fucus. Mesocosm-experiments were conducted over one year in the Kiel Fjord in order to test the direct and indirect effects of Gracilaria on Fucus. Fucus was incubated with Gracilaria at three different densities and grazers in high or low abundances. High densities of Gracilaria inhibited the growth of Fucus adults and also reduced the half-life-time of Fucus germlings. Associated grazers also had a negative effect on Fucus adults. Our results suggest that Gracilaria is able to influence Fucus in the Baltic Sea through direct competition for resources and by exposing it to higher grazer pressure.

The invasive red alga Gracilaria vermiculophylla was first recorded in Denmark and Sweden in 2003 and has since been reported from several sites in Denmark, Sweden and northern Germany. The abundance of G. vermiculophylla is typically high at more eutrophic sites, whereas it remains relatively low at more oligotrophic sites. We recorded seasonal variations in growth and biomass development at 2 sites with contrasting nutrient status (eutrophic versus oligotrophic) to investigate whether nutrient limitation of growth and loss of biomass due to grazing from invertebrate herbivores could explain observed variations in biomass. The biomass of G. vermiculophylla at the eutrophic site (Holckenhavn Fjord) was ca. 300-fold larger than at the oligotrophic site (Fyns Hoved). Growth rates ranged from almost 0 in early spring to ca. 0.08 d -1 in mid-summer and did not vary among sites. The seasonal pattern of growth was correlated to insolation and water temperature, suggesting that nutrient availability played a minor role in controlling growth. Experimental nutrient enrichment confirmed these findings; nutrient enrichment enhanced the level of tissue nutrients, but only had a marginal effect on growth. Grazing losses were insignificant throughout the entire study period in both systems. Hence, site-specific variations in biomass could not be explained by differences in grazing pressure. Given the lack of support for nutrient limitation or herbivory to explain the observed variations of G. vermiculophylla biomass, we suggest that physical exposure caused by wind-driven waves may be the factor that controls biomass of G. vermiculophylla in these shallow estuaries. KEY WORDS: Macroalgae · Invasive species · Nutrient limitation · Grazing Resale or republication not permitted without written consent of the publisherAquat Biol 10: [249][250][251][252][253][254][255][256][257][258][259] 2010 systems has stimulated research on its ecological performance. A number of studies have investigated how environmental factors such as salinity, light and water temperature affect growth under laboratory conditions (e.g. Yokoya et al. 1999, Raikar et al. 2001). Yet, in situ growth and biomass development as well as the regulation of these parameters have received less attention (but see Thomsen & McGlathery 2007, Weinberger et al. 2008 Gracilaria vermiculophylla grows fast under optimal culture conditions (i.e. 0.1 d -1 , Yokoya et al. 1999), but little is known about the seasonal variations in growth or the factors controlling that growth under field conditions. Weinberger et al. (2008) found that seasonal changes in the growth of G. vermiculophylla in Kiel Bight were closely correlated to insolation, water depth and temperature, but not to salinity, although the salinity in Kiel Bight is relatively low (<16). These authors did not study the potential effect of nutrient availability on seasonality in growth, but fast-growing algae are typically more affected by nutrient limitation than more slow-growing species (Pedersen 1...

To cite this article: Lars Brammer Nejrup , Peter Anton Staehr & Mads Solgaard Thomsen (2013) Temperature-and light-dependent growth and metabolism of the invasive red algae Gracilaria vermiculophylla -a comparison with two native macroalgae, European Journal of Phycology, 48:3, 295-308, DOI: 10.1080/09670262.2013 We conducted two temperature experiments to investigate the invasion success of the coarsely branched red algae, Gracilaria vermiculophylla. Our working hypothesis was that the coarsely branched G. vermiculophylla, with well-known broad environmental tolerances, would have physiological traits in-between the typical r and K strategies. A factorial experiment provided light-dependent models of growth rate at six temperatures, with maximum growth of 0.045 day -1 at 20°C. Light-saturated growth and maximum light utilization efficiency both displayed a bell-shaped temperature dependency, with optima at 21°C and 23°C respectively. The minimum light required to maintain growth was low (<1 µmol photons m -2 s -1) for lower temperatures (5-20°C) and increased exponentially to 7 µmol photons m -2 s -1 at 30°C, documenting that G. vermiculophylla has a wide tolerance to low light levels under temperature ranges occurring in the upper littoral zone in the Baltic Sea. A second experiment investigated the metabolic acclimation of G. vermiculophylla to four temperatures, while comparing its physiological responses to those of two native species, Fucus vesiculosus and Ulva lactuca. This experiment showed that the optimum temperature for light-saturated photosynthesis increased for all three species as they became long-term acclimated to higher temperatures. Short-term incubation at high temperature (30°C) was suboptimal for all three algal species when grown at low temperatures (10-15°C) but the algae were unaffected when cultured at higher temperatures (20-25°C). Finally we evaluated the capacity of each of the three species for metabolic homeostasis and found that F. vesiculosus and G. vermiculophylla had an almost identical metabolic performance regardless of acclimation temperature, whereas net photosynthesis of U. lactuca increased significantly with growth temperature. Our results show that G. vermiculophylla shares traits with both the slow-growing, leathery F. vesiculosus (K-strategy) and the fast-growing, sheet-like U. lactuca (r-strategy), by combining relatively high growth rates with a robust metabolic response to changing temperatures. In conclusion, we suggest that having both K and r metabolic traits explains, in part, the invasion success of G. vermiculophylla in temperate estuaries.