MEVE 011: Unit 10 – Ocean Ecosystem

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 UNIT 10: OCEAN ECOSYSTEM

10.1 Introduction

The ocean ecosystem, covering over 70% of the Earth’s surface, plays a crucial role in regulating the climate, absorbing carbon dioxide, cycling nutrients, and supporting a vast array of marine biodiversity. Oceans act as a climate buffer, absorbing over 90% of excess heat generated by greenhouse gas emissions and nearly 25–30% of anthropogenic CO₂. However, these buffering capabilities are being tested due to accelerating climate change, leading to widespread and profound effects on ocean temperature, chemistry, sea level, currents, and ultimately, the biological organisms that inhabit marine environments.

10.2 Objectives

By the end of this unit, learners will be able to:

·         Understand how oceans respond to climate change.

·         Describe changes in physical, chemical, and biological properties of oceans.

·         Identify the impacts of these changes on marine biodiversity and ecosystems.

·         Recognize species vulnerability and shifts in geographic distribution and migration.

·         Understand emerging trends such as species extinction and emergence.

10.3 Ocean Ecosystem Responses to Climate Change

Oceans respond dynamically to climate change. As global temperatures rise, the oceans warm up, polar ice melts, and ocean circulation patterns alter. These changes disrupt marine habitats and food webs, leading to shifting species distributions, coral bleaching, and loss of biodiversity. Some regions become more productive while others suffer major ecological losses. Marine organisms must either adapt, migrate, or face extinction.

10.4 Climate Change Effects on Ocean Properties

10.4.1 Changes in Physical Properties

10.4.1.1 Changes in Water Temperature
Global sea surface temperatures have risen significantly since the 20th century. This warming leads to coral bleaching, reduced oxygen content, and shifts in species distribution toward cooler waters or deeper zones.

10.4.1.2 Melting of Polar Ice
The Arctic and Antarctic are experiencing unprecedented ice melt, contributing to habitat loss for species such as polar bears, walruses, and penguins, and increasing the freshwater influx into oceans, which alters salinity and circulation.

10.4.1.3 Rising Sea Levels
Thermal expansion and glacial melt contribute to rising sea levels. Coastal marine ecosystems like mangroves, estuaries, and coral reefs are highly vulnerable. Rising waters can submerge breeding grounds and change salinity levels in estuarine zones.

10.4.1.4 Changes to the Ocean’s Major Current Systems
Currents like the Gulf Stream and the Thermohaline Circulation are slowing down due to melting ice and warming. This affects regional climates and the movement of nutrients and species in the oceans.

10.4.2 Changes in Chemical Properties

10.4.2.1 Ocean Acidification
As CO₂ dissolves in seawater, it forms carbonic acid, lowering the ocean's pH. Ocean acidity has increased by about 30% since pre-industrial times, affecting shell-forming organisms like corals, mollusks, and plankton.

10.4.2.2 Hypoxia (Oxygen Depletion)
Warming reduces oxygen solubility, and nutrient runoff from land leads to dead zones where oxygen is too low for most marine life to survive. Hypoxic zones are expanding, especially in coastal waters.

10.4.3 Changes in Biological Properties

10.4.3.1 Ocean Warming & Biodiversity
Warm waters reduce coral health and cause bleaching. Fish stocks migrate toward poles, disrupting local fisheries and food security.

10.4.3.2 Melting Polar Ice & Biodiversity
Ice-dependent species like krill, essential to many Arctic and Antarctic food webs, are declining due to habitat loss.

10.4.3.3 Sea Level Rise & Biodiversity
Shallow marine ecosystems like seagrass beds and salt marshes can drown or be eroded, affecting species that rely on these areas for breeding and nursery grounds.

10.4.3.4 Ocean Current Changes & Biodiversity
Changes in current systems can disrupt plankton blooms and nutrient cycling, leading to food scarcity for higher trophic levels.

10.4.3.5 Increasing CO₂ & Biodiversity
High CO₂ levels and related acidification affect calcifiers, zooplankton, and other foundational species, disrupting food webs and reducing fishery yields.

10.5 Geographic Distributions

Marine species are shifting toward the poles or deeper depths to escape warming waters. Tropical species may move to temperate regions, while cold-adapted species find their habitats shrinking. Coral reefs are expanding into subtropical zones, though often with lower diversity.

10.6 Vulnerability of Marine Organisms

Species most at risk include:

·         Coral reefs (bleaching and acidification)

·         Shell-forming species (mollusks, plankton)

·         Cold-water species (e.g., cod, krill, polar bears)

·         Coastal-dependent species (mangrove fish, estuarine species)
Organisms with narrow thermal tolerance, low mobility, or specific habitat requirements are especially vulnerable.

10.7 Migration Patterns

Changes in temperature and ocean currents are influencing:

·         Spawning grounds (e.g., tuna and cod spawning earlier or in different regions)

·         Feeding migrations (e.g., whales following prey to cooler waters)

·         Seasonal movements (e.g., earlier arrival of migratory fish to breeding zones)

These shifts may cause mismatches between predator and prey or lead to conflicts among fishing communities due to transboundary species movement.

10.8 Species Emergence and Extinction

Some species thrive in warmer, low-oxygen, or more acidic environments (e.g., jellyfish, cyanobacteria), while others disappear. Coral reefs are among the most threatened ecosystems, with up to 90% of reefs projected to die off under 2 °C warming. Entire food chains may collapse in certain regions, particularly in the Arctic and tropics.

10.9 Let Us Sum Up

Climate change is profoundly transforming the ocean ecosystem. Rising temperatures, acidification, hypoxia, sea level rise, and altered currents collectively affect the physical, chemical, and biological dynamics of marine life. These changes threaten marine biodiversity, disrupt food webs, alter migration and geographic distribution, and may lead to the emergence of new species and extinction of others. Understanding these effects is critical for developing strategies to mitigate damage, adapt fisheries and coastal livelihoods, and protect global ocean health in a changing climate.

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