Seasonal variations of Fucus vesiculosus fertility under ocean acidification and warming in the western Baltic Sea

Abstract

Ocean warming and acidification may substantially affect the reproduction of keystone species such as Fucus vesiculosus (Phaeophyceae). In four consecutive benthic mesocosm experiments, we compared the reproductive biology and quantified the temporal development of Baltic Sea Fucus fertility under the single and combined impact of elevated seawater temperature and pCO 2 (1100 ppm). In an additional experiment, we investigated the impact of temperature (0-25°C) on the maturation of North Sea F. vesiculosus receptacles. A marked seasonal reproductive cycle of F. vesiculosus became apparent in the course of 1 year. The first appearance of receptacles on vegetative apices and the further development of immature receptacles of F. vesiculosus in autumn were unaffected by warming or elevated pCO 2 . During winter, elevated pCO 2 in both ambient and warmed temperatures increased the proportion of mature receptacles significantly. In spring, warming and, to a lesser extent, elevated pCO 2 accelerated the maturation of receptacles and advanced the release of gametes by up to 2 weeks. Likewise, in the laboratory, maturation and gamete release were accelerated at 15-25°C relative to colder temperatures. In summary, elevated pCO 2 and/or warming do not influence receptacle appearance in autumn, but do accelerate the maturation process during spring, resulting in earlier gamete release. Temperature and, to a much lesser extent, pCO 2 affect the temporal development of Fucus fertility. Thus, rising temperatures will mainly shift or disturb the phenology of F. vesiculosus in spring and summer, which may alter and/or hamper its ecological functions in shallow coastal ecosystems of the Baltic Sea.

Model simulation of seasonal growth of Fucus vesiculosus in its benthic community

“…Overall, temperature effects are more pronounced than CO 2 effects when simulating growth of Fucus in the KOBs under different temperature and p CO 2 treatments. This result was previously obtained in the KOB studies (see Graiff et al b , ; Werner et al ). However, now, having established the model, it is possible to better highlight the actual threat of increasing temperatures to the entire Fucus –grazer–epiphyte system in the shallow nearshore waters.…”

“…4e,f). Biomass loss due to reproduction plays a larger role only in spring, when the reproductive parts of Fucus are shed after release of gametes, which comprise a considerable proportion of the biomass (Graiff et al ).…”

“…We have explicitly included other aspects of seasonality, such as reproduction. Fucus loses considerable amounts of biomass due to reproduction in spring, when the reproductive parts are shed after gamete release (Graiff et al ). The model might be useful in studying such phenomena in more detail.…”

“…However, most likely this underestimation results from the inclusion of reproductive biomass loss in spring in our modeling approach, but not in measurements of growth in the length of vegetative Fucus apices in the KOB experiments (Graiff et al b ). During the other seasons, Fucus individuals do not lose considerable amounts of biomass due to reproduction (Graiff et al ), which means that the modeled biomass growth rates better resemble measured length growth rates in the KOBs.…”

Season affects strength and direction of the interactive impacts of ocean warming and biotic stress in a coastal seaweed ecosystem

“…All abiotic variables assessed in the KOB experiments are available at http://doi.pangaea.de/10.1594/PANGAEA.842739. Per seasonal study, a wide variety of response variables (Table ) were sampled at various temporal intervals (for details, see Werner and Matthiessen ; Graiff et al , b ; Al‐Janabi et al , b ; Werner et al , b ; Graiff et al ; Raddatz et al ). The results were published in the cited references with the raw data being accessible on Pangaea (DOIs in the cited articles).…”