Aim:To identify and characterize the distribution of invertebrate taxa in the Orinoco basin and how their distribution affects the metacommunity structure along the river network.Location: Meta and Guaviare sub-basins, Orinoco basin, Colombia, South America. Methods:We characterized the invertebrate communities and environmental characteristics of 25 streams from six ecoregions in the Orinoco basin. The ecoregions present different historical evolution, altitude, slope, climate and vegetation features.We used multiplicative diversity partitioning to compare the contributions of riffle (α and β1), stream (β2) and ecoregional (β3) scales to the overall gamma diversity of the basin. We also applied the Elements of Metacommunity Structure framework to delineate metacommunity types and a distance-based redundancy analysis to assess the relative relevance of environmental, spatial and ecoregional drivers in species compositions of invertebrate communities.Results: Streams showed significant differences in community composition among the ecoregions. Several discrete pools of invertebrate taxa occurred in the basin that largely matched the distribution of the ecoregions. Consequently, the metacommunity in the basin resembled a Clementsian idealized structure. The species composition of invertebrate communities was mostly explained by ecoregion type and its interaction with the local environment, particularly its physiographic features. Main conclusions:Historical and evolutionary processes have resulted in species pools differing between the ecoregions of the Orinoco basin. At the basin scale, the metacommunity structure seems to be constrained by ecoregional features rather than by spatial structure or the local environment. Hence, using the basin as a unit for biodiversity conservation and river management would not adequately reflect the diversity and distribution patterns in highly heterogeneous basins such as the Orinoco basin. K E Y W O R D Sassembly process, beta diversity, Colombia, diversity partitioning, ecoregion, historical events

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.

-The aim of this study was to examine the relationship between the emergence patterns of various groups of aquatic insects and different hydrological discharge periods. We hypothesize that hydrological fluctuations exert a strong influence on insect emergence, even though the orders show different responses to discharge frequency. The emergence of Diptera, Trichoptera and Ephemeroptera was measured in a tropical stream with a mesh emergence net. The sampling took place every 2 days for 14 months. Based on water flow records taken throughout the sampling period nine discharge categories were defined. A significant relationship between macroinvertebrate emergence and discharge was found, and a high percentage of emergences was fundamentally concentrated within two discharge categories namely < 0.352 m . We used a time series consisting of 60 weekly samples to describe how emergence varied in an intra-annual period. Autoregressive integrated moving average (ARIMA) models were applied to analyze the time series of data collected for macroinvertebrate order. Time-series models for Trichoptera and Ephemeroptera showed a lagging relationship between the moving average (MA) of the emergences and environmental and biological factors, which occurred one (MAI) and two (MAII) periods earlier, respectively. For Diptera, a relationship was found between emergence and discharge for the autoregressive (AR) order. The relationship was regulated by the autecology of the organisms and occurred AR two time periods earlier (AR2). This study showed that the response of macroinvertebrates to discharge can be differentiated and can produce relevant changes in biodiversity.

Guatavita Lake is a small, sheltered tropical high mountain lake located in the Colombian Andes, with a closed watershed and a maximum depth of 25 m. It is the freshwater source for human consumption in nearby small towns, as well as being a site of cultural value for the country, as it was a sacred place to indigenous peoples until about five centuries ago. As the structure and function of this aquatic ecosystem is poorly understood, this study provides initial knowledge on its phytoplankton biomass dynamics, which should be useful in designing efficient management plans with environmental baseline information for similar lakes elsewhere. Physical and chemical data, and photosynthetic pigment concentrations, were measured for the period November 1999-November 2000 at the central vertical axis of Guatavita Lake. The vertical profile of the chlorophyll-a concentration was closely related to the dissolved inorganic nitrogen concentrations and the thermal stratification characteristics. The maximum chlorophyll-a concentration in the metalimnion was recorded for the thermal stratification period (November 1999-June 2000. Deepening of the maximum chlorophyll-a concentration began in February, continuing to June, when it reached its lowest value at the 15 m depth. The phytoplankton biomass values showed an increment within the entire water column at the beginning of the mixing period (July). The relation between the metalimnetic peak of chlorophyll-a and the dissolved inorganic nitrogen concentration suggests the growth of the phytoplankton community is limited mainly by the availability of nitrogen.

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