13.2 Ocean ecosystem impacts and marine life

Ocean Ecosystem Impacts

Impacts of ocean acidification

The ocean acts as a massive carbon sink, absorbing roughly 30% of the $CO_2$ humans release into the atmosphere. That absorption comes at a cost: it triggers a chemical process that makes seawater more acidic, threatening marine life from tiny plankton to massive coral reefs.

Here's how the chemistry works:

1. Atmospheric $CO_2$ dissolves in seawater.
2. It reacts with water to form carbonic acid ($H_2CO_3$).
3. Carbonic acid dissociates into hydrogen ions ($H^+$) and bicarbonate ions ($HCO_3^-$).
4. The extra $H^+$ ions lower the ocean's pH, making it more acidic.

Since the Industrial Revolution, ocean pH has dropped by about 0.1 units (from ~8.2 to ~8.1). That might sound small, but pH is a logarithmic scale, so this represents roughly a 26% increase in acidity.

Impacts on calcifying organisms:

• The extra $H^+$ ions react with carbonate ions ($CO_3^{2-}$), reducing their availability in seawater.
• Organisms that build shells or skeletons out of calcium carbonate ($CaCO_3$) struggle to form and maintain those structures. This includes corals, mollusks, sea urchins, and certain plankton like pteropods and coccolithophores.
• In severely acidified water, existing $CaCO_3$ structures can actually begin to dissolve.

Physiological effects on marine life:

• Impaired growth, reproduction, and survival rates across many species
• Altered metabolic processes, meaning organisms spend more energy maintaining basic body functions and less on feeding or reproducing
• Increased vulnerability to diseases, parasites, and other stressors like pollution

Ecosystem-level consequences:

• Shifts in species composition as acid-sensitive species decline and more resilient ones take over
• Reduced biodiversity and altered community structure
• Cascading effects on ecosystem services like nutrient cycling, habitat provision, and fisheries productivity

Effects on coral reefs

Coral reefs cover less than 1% of the ocean floor but support roughly 25% of all marine species. That makes them one of the most important and most vulnerable ecosystems on the planet.

Coral bleaching happens when water temperatures rise above a coral's tolerance threshold, typically just 1–2°C above the normal summer maximum. Under heat stress, corals expel the symbiotic algae (called zooxanthellae) living in their tissues. These algae provide corals with up to 90% of their energy through photosynthesis, so losing them is a serious problem. Without zooxanthellae, the coral turns white (hence "bleaching") and begins to starve.

Bleaching doesn't always kill coral. If temperatures drop back to normal quickly enough, the algae can recolonize and the coral can recover. The problem is that bleaching events are becoming more frequent and more severe:

• Mass bleaching events now affect entire reef systems across hundreds of kilometers. The Great Barrier Reef experienced unprecedented back-to-back bleaching in 2016 and 2017.
• Recovery from a major bleaching event typically takes 10–15 years, but events are now occurring faster than that.
• Repeated bleaching dramatically increases coral mortality and weakens the reef's ability to bounce back.

Implications for marine biodiversity:

• Loss of living coral reduces the structural complexity of reefs, eliminating the nooks and crevices that fish and invertebrates depend on for shelter, feeding, and breeding.
• Declines in reef fish populations ripple through food webs and affect species far beyond the reef itself.
• Reef degradation also undermines ecosystem services that humans rely on, including coastal storm protection, tourism revenue, and fisheries that feed hundreds of millions of people.
• Some reef-dependent species face local extinction, while others attempt to shift to more suitable habitats.

Ocean Circulation and Marine Species

Climate change and ocean circulation

Ocean currents distribute heat, nutrients, and oxygen around the globe. Climate change is altering the forces that drive these currents, with consequences for marine productivity everywhere.

Changes in global wind patterns:

• The intensity and position of major wind systems (trade winds, westerlies) are shifting as the atmosphere warms unevenly.
• These winds drive surface ocean currents and power upwelling zones, where nutrient-rich deep water rises to the surface and fuels some of the ocean's most productive ecosystems.

Increased thermal stratification:

Warmer surface waters become lighter and sit more stably on top of cooler, denser deep water. This increased stratification acts like a lid on the ocean, reducing vertical mixing between layers. The result is less exchange of nutrients and oxygen between the deep ocean and the surface.

Impacts on nutrient distribution and productivity:

• Weakened upwelling means fewer nutrients reach the sunlit surface layer where phytoplankton grow.
• Reduced nutrient availability limits primary productivity, the base of the entire marine food web.
• Phytoplankton community composition may shift, favoring smaller species that thrive in low-nutrient conditions but support less productive food webs.

Consequences for marine ecosystems:

• Regional productivity patterns change, with some areas becoming less productive and others potentially more so.
• Species distributions shift as habitats become more or less suitable.
• Mismatches can develop between when and where prey is available and when and where predators need it, disrupting food web relationships.

Marine species distribution changes

As ocean temperatures rise, marine species are on the move. These shifts are already well-documented and are reshaping ecosystems worldwide.

Shifts in geographical ranges:

• Many species are migrating poleward to track cooler water temperatures. Studies show marine species are shifting toward the poles at an average rate of about 72 km per decade, much faster than terrestrial species.
• Suitable habitats expand in some regions and contract in others, depending on local changes in temperature, salinity, and oxygen levels.
• New species arriving in a region create novel interactions: unfamiliar competitors, predators, and prey that can destabilize existing communities.

Changes in phenology and life cycles:

• Seasonal events like spawning, migration, and plankton blooms are shifting earlier in the year as waters warm sooner.
• This creates phenological mismatches: for example, fish larvae may hatch before or after the plankton bloom they depend on for food.
• These timing disruptions can cascade through food webs, affecting predator-prey relationships and nutrient cycling.

Impacts on migratory species:

• Migration routes and timing shift as ocean currents and temperature gradients change.
• Key stopover sites and feeding grounds may degrade or disappear entirely.
• Reproductive success can decline if species arrive at breeding grounds at the wrong time or in poor condition.

Abundance and population dynamics:

• The balance between mortality and recruitment (new individuals entering the population) shifts as conditions change.
• Altered competition and predator-prey dynamics reshape community structure.
• Species that can't adapt or relocate fast enough face population declines or local extinction. Slow-moving, habitat-specialist, and cold-water species are especially at risk.