3 BIO - EOC - Ecosystems: Interactions, Energy, Dynamics

Biologists are monitoring the biotic and abiotic factors in a lake. This includes tracking the population sizes and overall health of aquatic organisms and measuring water temperature, pH, salinity, dissolved oxygen, nitrogen, and phosphorus levels. The biologists compare lake water samples to the values shown in Table 1 to determine overall water quality. Some abiotic factors directly affect one another. For example, high nitrate levels lower the pH level of lake water.

Table 1: Optimal Conditions for Aquatic Ecosystems

Most aquatic organisms require factors to be within the ranges of values shown in Table 1 to survive. However, some organisms can still survive when factors are outside the ranges of optimal conditions. Figure 1 shows some animals that can survive in lower pH levels in aquatic ecosystems.

Energy transformations occur among organisms occupying different trophic levels in the lake, as shown in Figure 2.

At times, photosynthetic algae can grow out of control at the lake’s surface, forming large algal blooms that disrupt water quality and the health of aquatic organisms. Algal blooms can occur when the temperature of lake water increases and excess nutrients like nitrogen and phosphorus enter the lake. Large algal blooms can reduce available sunlight for other producers and release toxins into the water. When algae die, the process of decomposition uses much of the dissolved oxygen in the lake, reducing oxygen availability for aquatic organisms.

Biologists have noticed changes in total fish counts in various lake species over the last ten years. The biologists have determined that the lake is not being overfished and are instead focusing on the effects of algal blooms on lake organisms. Figure 3 shows the fish count for the lake over a ten-year period.