SEAWEEDS UNDER STRESS: MANIPULATED STRESS AND HERBIVORY AFFECT CRITICAL LIFE‐HISTORY FUNCTIONS

Terrestrial plants respond in complex ways to the interaction of stress and herbivory, but much less is known about responses of seaweeds. We studied the common intertidal rockweed Fucus gardneri to determine how stress and herbivory interact to influence its survival, growth, reproduction, and the concentration of phlorotannins, which can deter herbivory and constitute substantial mass. We manipulated stress and herbivory both in the field and in outdoor mesocosms over 2–4 months during spring and summer. In the field, disruptive stress was reduced in the upper range of the seaweed's distribution with awnings made of screen. Temperatures and relative humidities were more extreme in the no‐awnings locations. This treatment was crossed with a limpet‐removal treatment. In mesocosms, the tidal period was controlled so that half of the experimental thalli were exposed to air daily for 3–4 hours longer (greater limitation stress) than the other half, and three levels of isopod grazing were maintained. Grazing intensity was effectively reduced in the field and increased in the mesocosms. Net rates of photosynthesis were used as independent measures of seaweed stress. Fucus gardneri in the locations without awnings exhibited significant reductions in net photosynthesis, survival, and final size compared to thalli under awnings. The mesocosm environment was more benign; all individuals survived longer and grew larger. However, results from both experiments were generally consistent. Stress resulted in slightly lower growth and substantially reduced phlorotannin mass. Severe stress had a negative effect on reproductive output in the field experiment, but mild stress had a small positive effect in the mesocosms. Herbivory resulted in reduced thallus mass, slightly reduced growth, and reduced reproduction, but it had no effect on phlorotannin levels in either experiment. Herbivory and stress thus influenced different performance indicators in Fucus . Stress had more dramatic effects on phlorotannins than it did on growth, whereas herbivory more strongly influenced growth and reproduction. Few interactive effects were found, unlike in many terrestrial studies. If stress and herbivory act largely independently, as they appear to do here and in the limited number of other seaweed studies, predicting the effects of changes in the intertidal physical environment and in herbivory might be simpler than anticipated. The physiological basis for the observed responses to stress and herbivory is suspected to involve trade‐offs in resource allocation among these key life‐history parameters.