Question

Tropical environments on land are well known for their abundant life; rain forests are an example. By contrast, biological productivity in tropical oceans is meager. Why is this the case?

Short answer

Tropical oceans have low biological productivity due to low nutrient availability caused by water stratification, limiting phytoplankton growth.

Step-by-step solution

Understand Differences in Habitat

Begin by recognizing that tropical environments on land, such as rainforests, differ significantly from tropical oceans. On land, the environment provides abundant nutrients which support a wide diversity of plant and animal life.

Identify Nutrient Availability in Oceans

In tropical oceans, nutrient levels are lower compared to other oceanic regions. This is primarily due to the stable, warm water layers which do not easily mix with deeper, nutrient-rich layers.

Explain Ocean Stratification

The ocean's surface layer in tropical regions becomes warm and buoyant, leading to stratification. In stratified waters, there is little vertical mixing, preventing nutrients from reaching the surface where sunlight supports photosynthesis.

Assess Phototrophic Opportunities

Despite the abundant sunlight, the lack of nutrients in the upper ocean layers limits the growth of phytoplankton, which are the primary producers in ocean ecosystems.

Connect to Biological Productivity

Low phytoplankton populations lead to reduced biological productivity in tropical oceans. This impacts the entire food chain, resulting in the meager presence of life compared to nutrient-rich regions.

Key concepts

Tropical Environments

Tropical environments are characterized by their warm temperature and high levels of rainfall. On land, these conditions promote lush vegetation, such as that found in rainforests. Trees, plants, and a variety of microorganisms thrive due to the availability of sunlight and moisture.
These environments often have rich soil, abundant with nutrients essential for plant growth. The dense plant life supports a diverse array of animal species, forming complex ecosystems that are teeming with life.
In stark contrast, tropical ocean environments don't share the same nutrient-rich characteristics. The lack of nutrient cycling in tropical oceans limits the growth of marine life, creating ecosystems that appear deceptively tranquil.

Nutrient Availability

Nutrient availability is key to understanding the disparity in biological productivity between terrestrial and marine tropical environments. While land in tropical regions often has soil rich with essential nutrients like nitrogen and phosphorus, tropical oceans lack these.
This scarcity happens because of ocean stratification, where warm water lies on top of colder, denser water. Such stratification prevents the nutrients from deeper waters from reaching the surface where they would be accessible to marine life.
Without these nutrients, particularly nitrates and phosphates that are crucial for phytoplankton growth, the base of the marine food web suffers, limiting the ocean's overall productivity.

Biological Productivity

Biological productivity refers to the production of organic compounds from carbon dioxide through photosynthesis or chemosynthesis, primarily by plants and phytoplankton. It's a measure of how much life a particular environment can support.
Terrestrial tropical environments show high biological productivity due to their nutrient-rich soil and favorable climate, promoting growth across multiple trophic levels.
However, in tropical oceans, biological productivity is constrained. Due to low nutrient levels, there's a limited supply of essential energy at the base of the food chain. Phytoplankton, being the primary producers in marine ecosystems, are unable to flourish, which causes a ripple effect, reducing the abundance and diversity of higher marine species like fish and marine mammals.

Phytoplankton

Phytoplankton are microscopic organisms that, despite their small size, have a massive impact on marine ecosystems and global carbon cycling. They perform photosynthesis, converting sunlight into chemical energy that supports a large portion of marine food webs.
In nutrient-rich waters, phytoplankton populations can rapidly grow and multiply, providing food for zooplankton, which in turn feed larger marine animals.
In tropical oceans, however, the scarcity of nutrients like nitrates and phosphates severely limits phytoplankton growth. Even with abundant sunlight, the lack of essential nutrients prevents these organisms from thriving.
Phytoplankton's limited presence in tropical oceans is a primary reason why these regions exhibit lower biological productivity compared to more nutrient-rich parts of the ocean.