Abstract This study examines the distribution of primary and secondary space holders integrated in morphological and feeder functional groups as well as the species richness over the seasons on northern Patagonian rocky shores. We also evaluated whether the temporal changes in species richness, assemblage structure and species turnover were related to seasonal changes. These patterns were studied by non-destructive samplings between December 2013 and March 2015. Thirty taxa were identified, including six algal morphological functional and three invertebrate feeder groups. The cover of seaweeds and invertebrates was positively correlated with changes in the environmental factors. The marked seasonality in this Patagonian zone changed between two contrasting periods. Species richness and assemblage structure were associated with seasonal changes and were influenced by the more abundant morphological and feeder functional groups. We suggest that habitat-formers, such as articulated calcareous and suspensor feeder groups, can expand and retract over time-scales of months to seasons, generating available space for the colonization and growth of other organisms. We conclude that seasonal changes and the dynamics of articulated calcareous and suspensor feeder groups drive changes in the algal and invertebrate abundances, in turn driving changes in species richness and assemblage structure.

Batrachospermum atrum var. puiggarianum was collected for the first time in Buenos Aires province. The identification of samples was based on microscopic morphology and chromosomic number analyses of each generations of the life cycle under culture conditions. The thalli were studied with optical microscopy, and karyology according to carmine acetic technique. Life cycle showed three generations: an haploid gamethophyte, a diploid carposporophyte and the last Chantransia diploid phase. Gametophytes exhibited separate whorls of densely compressed primary branches, some of those branches bearing spermatangia or carpogonium. Internodes were constituted by globose axial cells and cortical cells. The ovoidal carposporophytes were formed by gonimoblastic filaments bearing terminal carposporangia. The Chantransia stage was characterized for short filaments of few cells. The studied material showed a chromosome haploid number n = 4, and diploid number 2n = 8.

Se coleccionó Batrachospermum atrum var. puiggarianum por primera vez en la provincia de Buenos Aires. La identificación de las muestras se basó en el análisis de la morfología microscópica y en el número cromosómico de cada una de las generaciones de su ciclo de vida bajo cultivo. Los talos se estudiaron con microscopio óptico, y la cariología, por medio de la técnica de carmín acético. El ciclo de vida presentó tres generaciones: una gametofítica haploide, una carposporófitica diploide que originó la última fase Chantransia diploide. Los talos gametófitos estuvieron formados por verticilos separados por zonas internodales, cada uno de ellos constituidos por ramas primarias densamente comprimidas. Entre éstas se observaron ramas portadoras de espermatangios y de carpogonios. Las zonas internodales estuvieron constituidas por células corticales y axiales. Los carposporófitos ovoideos estuvieron forma-dos por filamentos gonimoblásticos portadores de carposporangios terminales. El estado Chantransia se caracterizó por presentar filamentos cortos con pocas células. El material estudiado presentó un número haploide n = 4 y diploide 2n = 8.