Influence of Aerial Exposure Length on Mortality and Assemblage Structure of Benthic Stream Diatoms

In Texas, where droughts are common and water levels fluctuate during the summer months, episodic desiccation becomes a potentially negative stressor. This study investigated under experimental conditions the impact of desiccation on the long-term structure and diversity of diatom assemblages on hard substrates in rivers. We hypothesized that moderate desiccation stress through aerial exposure under high temperature will increase diversity. In contrast, prolonged desiccation stress would reduce diversity, favoring taxa that are desiccation tolerant. Diatom assemblages were collected from the Bosque and the Colorado Rivers and were cultured in lab mesocosms to colonize glass slides as a substrate. The study consisted of two experiments applying desiccation stress to these cultured assemblages. First, continuous exposure until approximately 100% mortality of epilithic diatom taxa was attempted to identify taxon-specific mortality rates and desiccation tolerance ranges. Second, episodic aerial exposure over six weeks characterized the effects of varying lengths of repeated aerial exposure and submersion on the structure and richness of the diatom assemblage. At the conclusion of the experiment, most benthic diatoms tolerated short durations of desiccation in the lower-stress treatments, but extended periods of desiccation caused mass mortality in most species. Taxa in the experiments that demonstrated higher desiccation tolerance were Amphora sp., Planothidium sp., and Gomphonema spp., particularly Gomphonema truncatum. The assemblages exposed to the minimum desiccation exposure treatment had the highest abundance and the lowest mortality percentage. When exposed to the high-stress treatment, they showed a significant reduction in abundance and richness, leaving only a few live cells. Overall, this study demonstrated that benthic diatoms of the Colorado and Bosque Rivers may not survive weeks of desiccation in high air temperatures, though no assemblage reached 100% mortality in either experiment. Understanding the taxon-level impacts of desiccation stress on diatom assemblages is crucial for better identifying how drought and periods of desiccation influence the structure and function of benthic diatom assemblages. By examining species-specific responses, we can better predict how future droughts and high air temperatures may alter freshwater ecosystems.