Cambios en la estructura de la asociación de diatomeas epifitas de macrocystis pyrifera (L.) C. AG

To determine the structure of the epiphytic diatom assemblage and its potential time variations on apical blades of Macrocystis pyrifera from the coasts of the Baja California peninsula four fronds of M. pyrifera were collected monthly from February to August 2003 in El Sauzal, B.C.; the diatoms from 5 blades per frond were mounted on permanent slides and examined floristically and quantitatively. The structure of the assemblage followed the general pattern for most diatom associations, i. e., few abundant species and many rare or uncommon taxa. Species richness reached 171 diatom taxa, however S per sample was heterogeneous (6 - 51 taxa) and in general low. Also, low values of diversity and equitability were estimated: H' = 2.1 (max = 3.8, min = 0.3), and J' = 0.45. The most abundant taxa were Rhoicosphenia genuflexa (Kützing) Medlin, Gomphonemopsis pseudexigua (Simonsen) Medlin, Tabularia investiens (W.Smith) Williams & Round, and Cocconeis costata var. pacifica (Grunow) Grunow. All are considered permanent constituents of the diatom assemblage on M. pyrifera, inasmuch in five dates R. genuflexa dominated, whilst in June G. pseudexigua was the most abundant taxon, and C. costata var. pacifica dominated in February. Species composition and structure of the diatom assemblage in blades with different position did not change significantly down to the length of 2.5 m of Macrocystis fronds. Differences in structure and species composition between fronds from a same date suggest that mature blades export species to the new blades. Temporal changes in species composition and structure of the diatom assemblage showed to be significant in February and June. This may be associated to transitional periods.