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Nutrients and Respiration » Nutrient Cycling in Marine Ecosystems

What you'll learn this session

Study time: 30 minutes

  • How nutrients cycle through marine ecosystems
  • The role of producers, consumers and decomposers in nutrient cycling
  • Key nutrient cycles: carbon, nitrogen and phosphorus
  • How human activities affect marine nutrient cycles
  • The importance of nutrient cycling for marine food webs
  • Real-world examples of nutrient cycling disruption

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Introduction to Nutrient Cycling in Marine Ecosystems

Imagine the ocean as a giant recycling centre that never stops working. Every day, nutrients flow through marine ecosystems in endless cycles, supporting all life from tiny plankton to massive whales. These nutrient cycles are like the ocean's circulatory system, moving essential chemicals around to keep everything alive and healthy.

Marine ecosystems depend on the constant recycling of nutrients. Without these cycles, the ocean would become a lifeless desert. Understanding how nutrients move through marine food webs helps us protect these vital ecosystems.

Key Definitions:

  • Nutrient cycling: The movement and recycling of chemical elements through living organisms and the environment.
  • Biogeochemical cycle: The pathway by which chemical elements move through both living (biotic) and non-living (abiotic) parts of an ecosystem.
  • Primary productivity: The rate at which producers (like phytoplankton) convert inorganic nutrients into organic matter through photosynthesis.
  • Decomposition: The breakdown of dead organic matter by bacteria and other decomposers, releasing nutrients back into the ecosystem.

🌊 The Ocean's Recycling System

Marine nutrient cycling works like a massive recycling plant. Dead organisms sink to the ocean floor where bacteria break them down, releasing nutrients back into the water. Ocean currents then carry these nutrients to surface waters where phytoplankton use them to grow, starting the cycle again.

Key Players in Marine Nutrient Cycling

Three main groups of organisms drive nutrient cycling in marine ecosystems. Each plays a crucial role in keeping nutrients moving through the system.

Producers: The Nutrient Converters

Phytoplankton are the ocean's primary producers. These microscopic plants float near the surface, using sunlight and dissolved nutrients to create organic matter through photosynthesis. They're like tiny factories converting raw materials (nutrients) into food that supports the entire marine food web.

🌱 Phytoplankton

Microscopic marine plants that form the base of most marine food chains. They absorb dissolved nutrients from seawater and convert them into organic compounds.

🌿 Seaweed & Kelp

Large marine algae that grow in coastal waters. They absorb nutrients directly from seawater through their surfaces and provide habitat for many marine animals.

🌳 Seagrasses

True plants that grow underwater in shallow coastal areas. They have roots that can absorb nutrients from sediments as well as from the water column.

Consumers: The Nutrient Movers

Marine consumers transfer nutrients through the food web by eating other organisms. When a fish eats phytoplankton, it's moving nutrients from the water column into its body. When larger fish eat smaller ones, nutrients move up the food chain.

Consumers also move nutrients horizontally and vertically through the ocean. Many marine animals migrate daily between surface and deep waters, transporting nutrients in their bodies. Some fish migrate thousands of kilometres, carrying nutrients across ocean basins.

Decomposers: The Nutrient Recyclers

Bacteria and other decomposers are the ocean's cleanup crew. They break down dead organisms and waste products, releasing nutrients back into the water where they can be used again. Without decomposers, nutrients would remain locked up in dead matter and unavailable to living organisms.

Amazing Fact: The Biological Pump

The ocean has a natural "biological pump" that moves carbon from surface waters to the deep sea. Phytoplankton absorb CO₂ from the atmosphere, then when they die, they sink to the ocean floor, taking carbon with them. This process removes billions of tonnes of carbon from the atmosphere each year!

Major Nutrient Cycles in Marine Ecosystems

Three key nutrients drive most marine life: carbon, nitrogen and phosphorus. Each follows its own cycling pattern, but they're all interconnected.

The Marine Carbon Cycle

Carbon is the building block of all life. In marine ecosystems, carbon cycles between the atmosphere, surface waters, deep ocean and living organisms. The ocean absorbs about 25% of all CO₂ released into the atmosphere.

Phytoplankton absorb dissolved CO₂ from seawater during photosynthesis. When marine organisms die, some carbon sinks to the deep ocean where it can remain for hundreds of years. Ocean currents eventually bring this carbon-rich water back to the surface, completing the cycle.

The Marine Nitrogen Cycle

Nitrogen is essential for making proteins and DNA. Most nitrogen in the ocean exists as dissolved nitrogen gas (N₂), which most organisms can't use directly. Special bacteria convert this nitrogen into forms that phytoplankton can absorb, such as nitrates and ammonia.

The nitrogen cycle includes several steps: nitrogen fixation (converting N₂ to usable forms), nitrification (converting ammonia to nitrates) and denitrification (converting nitrates back to N₂). Each step is carried out by different types of bacteria.

Nitrogen Limitation

In many parts of the ocean, nitrogen is the limiting nutrient - meaning there's not enough available nitrogen to support maximum phytoplankton growth. This makes the nitrogen cycle crucial for marine productivity.

The Marine Phosphorus Cycle

Phosphorus is needed for energy storage (ATP) and genetic material (DNA). Unlike carbon and nitrogen, phosphorus doesn't have a gaseous phase, so it cycles more slowly through marine ecosystems.

Phosphorus enters the ocean mainly through rivers carrying eroded rock and soil. It's often the limiting nutrient in freshwater systems but less commonly limiting in marine environments. However, in some ocean regions, phosphorus availability controls phytoplankton growth.

Human Impacts on Marine Nutrient Cycles

Human activities are dramatically altering marine nutrient cycles, often with serious consequences for marine ecosystems.

Nutrient Pollution and Eutrophication

When excess nutrients from fertilisers, sewage and industrial waste enter coastal waters, they can cause explosive phytoplankton growth called algal blooms. These blooms can create "dead zones" where oxygen levels drop so low that fish and other marine life cannot survive.

The Gulf of Mexico has one of the world's largest dead zones, caused by nitrogen and phosphorus pollution from the Mississippi River. Every summer, an area the size of Wales becomes virtually lifeless due to oxygen depletion.

Case Study Focus: The Baltic Sea Dead Zone

The Baltic Sea suffers from severe eutrophication due to nutrient pollution from surrounding countries. Agricultural runoff and sewage discharge have created a dead zone covering 70,000 square kilometres - larger than the entire Republic of Ireland. Countries around the Baltic are working together to reduce nutrient inputs and restore the ecosystem.

Climate Change and Nutrient Cycling

Rising ocean temperatures and changing currents are altering nutrient distribution patterns. Warmer water holds less dissolved gases, including oxygen, which affects decomposition rates and nutrient cycling. Changes in ocean circulation can disrupt the transport of nutrients from deep waters to the surface.

Ocean acidification, caused by increased CO₂ absorption, is changing the chemistry of seawater and affecting how organisms process nutrients. Some phytoplankton species are struggling to build their shells in more acidic conditions.

The Importance of Healthy Nutrient Cycles

Properly functioning nutrient cycles are essential for marine ecosystem health. They support primary productivity, maintain biodiversity and help regulate global climate patterns.

Supporting Marine Food Webs

Efficient nutrient cycling ensures that primary producers have the resources they need to support complex food webs. When nutrient cycles are disrupted, it can cause ripple effects throughout the entire ecosystem, from microscopic plankton to top predators like sharks and whales.

Climate Regulation

Marine nutrient cycles, especially the carbon cycle, help regulate Earth's climate by controlling atmospheric CO₂ levels. The ocean acts as a massive carbon sink, absorbing excess CO₂ from human activities. Healthy marine ecosystems are crucial for maintaining this climate regulation service.

🌎 Global Impact

Marine nutrient cycles don't just affect ocean life - they influence weather patterns, atmospheric composition and even the productivity of terrestrial ecosystems through their role in global climate regulation.

Conservation and Management

Protecting marine nutrient cycles requires both local and global action. Reducing nutrient pollution, managing fisheries sustainably and addressing climate change are all essential for maintaining healthy marine ecosystems.

Marine protected areas can help preserve natural nutrient cycling processes by protecting key habitats and species. International cooperation is needed to address large-scale issues like ocean acidification and climate change that affect nutrient cycles across entire ocean basins.

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