USA Biology Olympiad (USABO) Practice Exam

Why might two sampling observations of diatom species in a stream differ significantly?

1. Higher temperatures at the second site

2. Organic wastes high in nitrates and phosphates entered the stream

3. Natural selection favored fewer species downstream

4. Increased predation in the second observation

The correct answer is: Organic wastes high in nitrates and phosphates entered the stream

The answer regarding organic wastes high in nitrates and phosphates entering the stream explains a significant reason for the observed differences in diatom species between two sampling sites. In aquatic ecosystems, the availability of nutrients such as nitrates and phosphates can greatly influence the composition and diversity of algal and diatom communities. When organic waste, especially in the form of agricultural runoff or sewage, enters a stream, it often raises nutrient levels in the water. This nutrient enrichment typically leads to increased primary productivity, which can favor certain species of diatoms that are better equipped to exploit these conditions. Some diatoms are more resilient to nutrient-rich environments and can outcompete others, leading to a shift in species composition between the two sampling observations. Additionally, the introduction of organic waste can result in eutrophication, which alters the physical and chemical properties of the water and can have cascading effects on the community structure present in the stream. Environmental changes induced by increased nutrients can lead to changes in habitat, competition, and potentially even the presence of pollutants, all of which can impact the diatom species observed. In contrast, while temperature, natural selection, and predation might influence diatom populations, their direct effects are less impactful in explaining the change in species observed