Question

A storm near the coast produces sediment-rich runoff that causes water in the euphotic zone to become cloudy. Describe how this would affect the (a) plankton, (b) nekton, and (c) benthos.

Short answer

The cloudiness negatively affects plankton's light-dependent activities, limits nekton visibility and food sources, and alters benthic habitats.

Step-by-step solution

Understanding the Scenario

A storm near the coast introduces a significant amount of sediment into the euphotic zone, the upper layer of the ocean where sunlight penetrates and photosynthesis commonly occurs. This causes the water to become cloudy, reducing the amount of light that can pass through.

Analyzing Effects on Plankton

Plankton, especially phytoplankton, rely heavily on sunlight for photosynthesis to produce energy. The cloudiness caused by sediment reduces light availability, which can decrease photosynthesis rates, limiting their growth and reproduction.

Analyzing Effects on Nekton

Nekton, or actively swimming aquatic animals like fish, may be affected by the reduced visibility for hunting and evading predators. Additionally, changes in plankton populations, a primary food source for many nekton, can indirectly affect their nutrition and survival.

Analyzing Effects on Benthos

Benthos are organisms living on the ocean floor. An influx of sediments may settle on the benthic environment, potentially smothering benthic organisms and altering habitats. This can affect their respiration, feeding, and overall survival.

Key concepts

Euphotic Zone

The euphotic zone is the sunlit upper portion of the ocean where sunlight penetrates, allowing photosynthesis to occur. This zone is crucial for marine ecosystems, serving as the primary area for plant-like organisms to produce energy through photosynthesis. Typically extending to about 200 meters below the surface, the euphotic zone is essential for the survival of many marine species, as it forms the base of the food chain.
When the euphotic zone becomes cloudy due to sediment runoff, the amount of sunlight decreases significantly. This reduction in light can impact photosynthetic organisms, disrupting energy production and potentially affecting the entire marine food web that depends on these organisms.

Plankton

Plankton are small plants and animals that drift with ocean currents. They are divided into two main types: phytoplankton, which are plant-like and perform photosynthesis, and zooplankton, which are animal-like and often feed on phytoplankton.
Phytoplankton are particularly sensitive to changes in light availability. When sediment clouds the water, the amount of light that reaches them is reduced, inhibiting their ability to photosynthesize. This decrease in photosynthesis can limit their growth and reproduction, impacting both planktonic communities and the larger animals that depend on them for food. Phytoplankton are a critical food source for many marine organisms, including the zooplankton, nekton, and benthos.

Nekton

Nekton refers to actively swimming aquatic animals, such as fish, whales, and squid. Unlike plankton, nekton can move independently of water currents, allowing them to hunt or escape predators efficiently. However, their ability to see and locate food is highly dependent on water clarity.
In situations where sediment clouds the euphotic zone, visibility is reduced. This makes it harder for nekton to find prey and avoid predators. Moreover, because many nekton feed on plankton, any reduction in plankton populations due to decreased sunlight can also impact their food supply. Consequently, this can affect their growth, reproduction, and survival.

Benthos

Benthos are organisms that reside on the ocean floor, ranging from sea stars and sponges to various types of shellfish and coral. Unlike plankton and nekton, benthos mostly live in more stable environments, but they too are affected by sedimentation.
When sediments settle on the seabed, they can smother benthic organisms, disrupting their living conditions. This sediment can cover feeding and respiratory structures, causing stress, and possibly leading to the death of benthic communities. Such an impact can change the biodiversity of the benthic environment and disrupt local ecosystems, as these organisms play critical roles in nutrient cycling and habitat structure.