Calcium concentration regulates interspecific interaction of algae along the nutrient gradient

Although many studies have focused on the influence of abiotic stress
gradients on biological interactions, few studies explained the effects of
both resource and non-resource stress gradients on the facilitation
between species. We conducted a controlled experiment to study the effects
of nutrient stress gradient on the interspecific relationship between
Phormidium corium and Scenedesmus quadricauda under different calcium
concentration. The results showed that when the calcium concentration was
high, the change of interspecific relationship between the two algae along
the nutrient stress gradient accorded with the SGH theory, that is, the
competitive interaction changed into positive interaction. The reason for
this phenomenon may be that in the stress environment, the population
density is low and the encounter rate is low; secondly, facilitation may
improve the harsh environment and make it more beneficial to the
beneficiary species, which may enable the beneficiary species to survive
in the environment that they could not survive before. When the calcium
concentration was low, the change of interspecific relationship along
resource stress was in line with the "Hump hypothesis", that is,
facilitation reached the peak under medium resource stress. This
phenomenon may be caused by the interaction between salt stress and
nutrient. When calcium stress was high, it may not be conducive to the
absorption of nutrients by algae, resulting in the fierce competition for
limited resources in the harsh environment. While the facilitation effect
may be strongest in the medium stress environment due to the relatively
sufficient resource. These results emphasized the joint effects of various
stress factors on interspecific relationships, especially the importance
of non-resource stress in the study of community composition, pointed out
a new direction for the study of facilitation, and provided theoretical
guidance for the prevention and control of freshwater blooms.