We assessed the relative influence of ecoregional features in explaining diatom distribution in the Orinoco river basin. Ecoregions in the Colombian Orinoco can be seen as imprints of the evolutionary history of the basin, for their current biodiversity and physiographic features are the result of the geological and climatic shifts that have occurred since the Tertiary. Thus, they represent an ideal testing ground for studying the interplay between ecological and evolutionary processes shaping diversity patterns of microorganisms, such as diatoms, in the present day. To study this interplay, we compared diatom community composition variance within and among seven ecoregions and assessed the explanatory power of environmental, spatial and historical drivers. This was done by a combination of correlation analyses, multivariate methods and constrained ordinations. We also deconstructed the whole community data set into ecological guilds (low‐ and high‐profile, and motile) to explore their individual response to the contemporary and historical drivers. Taken together, these analyses indicated that contemporary constraints to species occurrence and dispersal, as well as the legacies of historical events, can provide an explanation for the contemporary distribution of diatoms in the Colombian Orinoco. Specifically, we provided evidence showing that both historical legacies and contemporary environmental conditions (temperature, pH, and phosphorus concentration) are interacting to determine diatoms’ distribution. Our results suggest the need to consider ecoregional gradients for unraveling the mechanisms shaping tropical diversity as well as for designing conservation plans.
An in situ experiment determined that the addition of ammonia N-NH4+, nitrate N-NO3– and phosphate P-PO43– to an oligotrophic Andean stream significantly affected diatom composition and diversity. Over a nine-month period, fertilized and non-fertilized sections of the stream were studied for their environmental characteristics and diatom composition. Nutrient concentrations were increased three times in a downstream section with respect to those occurring naturally upstream. Alpha diversity (Shannon-Wiener [H”] and Simpson [S] indices) were determined monthly. In the non-fertilized control section, diatom diversity was significantly (p ≤ 0.05) related to water flow and P and N concentrations. In the fertilized section diversity was associated to the enhanced ammonia and phosphorus. A redundancy analysis (RDA) showed that conductivity, water flow and nutrient concentrations contributed to the diatom assemblage composition (83% along the first two axes). Achnanthidium minutissimum, Cocconeis placentula, and Epithemia sorex occurrence was related to low phosphorus concentration. Epithemia adnata var. minor, Nitzschia dissipata, and Reimeria sinuata were associated with high P concentrations, high water flow and low conductivity Navicula rynchocephala, Gomphonema parvulum, Navicula capitatoradiata, and Melosira varians were associated with high P.
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