Relationship between growth promoting factors and resilience traits in Fucus vesiculosus – is there a trade-off?

Abstract

Fucus vesiculosus is an important foundation species that provides habitat and food for other species. Its abundance depends both on tissue toughness and chemical defenses, traits that allow the seaweed to be resilient to tearing during storms or to grazing predators. A recent study found that elevated CO2 could contribute to reduced tissue toughness and chemicaldefense levels, which could result in decreased distribution range of the species. However, whether the reduction was due to lower pH or the fertilizing effect of CO2 has not been clarified. In the present study we aim to investigate whether other fertilizing factors such as light and/or nutrients could contribute to reduced tissue toughness and chemical defense levels of F. vesiculosus. Thirty-six individuals of F. vesiculosus were collected and divided into four groups (9 samples/group). Each group was exposed to one of the following treatments i) light and no nutrient, ii) shadow and no nutrient, iii) light and nutrient added and iv) shadow and nutrient added. Changes in wet weight (ww) and area were used to estimate growth and measured at baseline and re-measured after the treatment. Furthermore, water content (dry weight (dw) to wet weight ratio), tissue toughness (pressure to cut through the seaweed tissue), and chemical defenses (phlorotannin content) were measured to test if there was a trade-off with growth. The seaweeds in the light exposed treatment had approximately 67% higher wet weight change, and 6% lower dw:ww ratio, compared to those in the shadow treatment (p < 0.01 for both). Change in area, mean pressure to cut, and phlorotannin content were not significantly different in seaweeds exposed to different treatments. These results indicate, that during winter, F. vesiculosus growth is light- rather than nutrient-limited and that increased growth leads to a higher tissue water content but not to reduced resilience traits.