In an aquatic ecosystem, the organism present at the trophic level equivalent to cows in grasslands is

Options

A. phytoplanktons
B. large fishes
C. sea gulls
D. zooplanktons.

Explanation

To solve the question, "In an aquatic ecosystem, the organism present at the trophic level equivalent to cows in grasslands is," we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Trophic Levels**:
- Trophic levels represent the position of an organism in a food chain. They are categorized into producers, primary consumers, secondary consumers, and tertiary consumers.

2. **Identify the Trophic Level of Cows**:
- Cows are herbivores that feed on grass (producers) in grasslands. Therefore, they occupy the primary consumer level in the terrestrial ecosystem.

3. **Determine the Equivalent in Aquatic Ecosystems**:
- In aquatic ecosystems, the primary consumers are organisms that feed on producers (like phytoplankton).

4. **Identify Primary Consumers in Aquatic Ecosystems**:
- The primary consumers in aquatic ecosystems include zooplankton, which feed on phytoplankton (the producers in aquatic environments).

5. **Compare Options**:
- The options given are phytoplankton, large fishes, seagulls, and zooplankton.
- Phytoplankton are producers, large fishes are typically secondary or tertiary consumers, and seagulls are omnivores that can occupy various levels.

6. **Select the Correct Answer**:
- Since zooplankton are the primary consumers in aquatic ecosystems and are equivalent to cows in grasslands, the correct answer is **zooplankton**.

### Final Answer:
The organism present at the trophic level equivalent to cows in grasslands in an aquatic ecosystem is **zooplankton**.