Why is Ulva intestinalis the only macroalga inhabiting isolated rockpools along the Swedish Atlantic coast?

Ulva intestinalis (previously called Enteromorpha intestinalis) is the principal marine macroalga growing in isolated rockpools along the Swedish west coast. Given that this alga can take up HCO3- under the conditions of high pH and low concentrations of inorganic carbon (Ci) that it creates in those pools, we hypothesized that those conditions would be unfavourable for the photosynthetic performance of other algae such that the), would not be able to grow in the same pools. This was tested by transferring 2 of the most common macroalgae, Fucus vesicalosus and Chondrus crispus, from the shallow waters of a bay adjacent to some rockpools, to 2 rockpools containing U. intestinalis. Both photosynthetic electron transport rates (ETR) and maximal quantum yields (F-v/F-m) were measured 3 d later. Similar measurements were also performed throughout the day in simulated outdoor rockpools where the 3 species were kept either alone or together for 24 h. It was found that midday ETRs were repressed for all species in the (upper) rockpool as compared to the open bay, but more so for the transferred F. vesiculosus and C. crispus (33 and 0.6%, respectively, of the bay values) than for U. intestinalis (58%). Also, C. crisp us showed exceptionally repressed midday F-v/F-m values in the rockpools (some 22% of that in the bay), and had largely photobleached alter 3 d. Similarly, ETRs declined in the simulated rockpools during the day, especially for C. crispus when the other algae were present (to 7% of morning values, compared to 36% when alone). Also, F-v/F-m. declined in the simulated rockpools during the day, indeed much more so for F. vesiculosus and C. crispus when the other algae were present (to ca. 12 and 10% of morning values as compared to 76 and 54%, respectively, when alone). As in the rockpools, the latter alga photobleached when kept together with the other species, but not when alone. Based on these results. and considering their different Ci utilisation mechanisms, it was concluded that the absence in rockpools of algae such as those tested here may be largely due to the adverse conditions of high pH and low Ci concentrations caused by the photosynthetic traits of U. intestinalis.