ENSO and Marine Climate Change Lecture Notes

Definition of ENSO: The El Niño Southern Oscillation refers to natural climate patterns that significantly affect global rainfall, temperature, and wind.
Sea Surface Temperature (SST) Links: The transcript identifies specific historical El Niño events linked to significant changes in SST:
- $1972/73$
- $1982/83$
- $1997/98$
- $2015/16$
Coriolis and Ekman Flow: These physical oceanographic processes drive upwelling zones:
- Coastal Upwelling: Occurs near shorelines.
- Equatorial Upwelling: Occurs along the equator.
Epipelagic Food Webs: Production in these regions is heavily influenced by upwelling, which brings nutrient-rich water to the surface (referencing Figs. $15.30$ and $15.31$ ).

Greenhouse Effect and Carbon Dioxide ( $CO_2$ ): Increasing atmospheric concentrations of greenhouse gases like $CO_2$ and methane trap heat.
Atmospheric $CO_2$ Concentrations: Currently sitting at approximately $430\,\text{ppm}$ . This value represents the highest level the Earth has experienced in an extremely long time.
Ocean Chemical Equilibrium: When $CO_2$ dissolves into seawater, it reacts with water following the equation:
- $CO_2 + H_2O \rightleftharpoons HCO_3^- + H^+$
Resulting Ocean Changes:
- Temperature: Increasing ( $\uparrow$ ).
- $CO_2$ Concentration: Increasing ( $\uparrow$ ).
- Bicarbonate ( $HCO_3^-$ ): Increasing ( $\uparrow$ ).
- pH Levels: Decreasing ( $\downarrow$ ).
Ocean Acidification Specifics:
- The pH of the ocean has dropped by $0.1$ units in just $20$ years, making the water less alkaline.
- There is a shift in the carbonate system: Bicarbonate ( $HCO_3^-$ ) and Hydrogen ions ( $H^+$ ) are increasing, while Carbonate ( $CO_3^{2-}$ ) is decreasing.

Aragonite Dissolution: Aragonite is a form of Calcium Carbonate ( $CaCO_3$ ) essential for building shells and skeletons. Increased $CO_2$ leads to less carbonate uptake and the dissolution of aragonite, stressing calcifying organisms.
Shift in Resource Availability ( $HCO_3^-$ ):
- Bicarbonate ( $HCO_3^-$ ) serves as a resource for "weedy" marine plants (e.g., turf algae).
- These weedy species are often Carbon-limited ( $C$ -limited) and can grow significantly faster under increased $CO_2$ conditions.
- Resulting Imbalance: This facilitates turf algae at the expense of habitat-forming organisms like Corals and Kelps ( $Connell\,et\,al.\,2013$ ).

Nature of Warming: Temperature rise is described as the most fundamental and pervasive physical change, affecting every biological system from biochemical reactions to the entire biosphere.
Spatial Variability: Warming is not constant in space or time. There is distinct spatial variability in warming over the last $100$ years between land and oceans.
Shelford's Law of Tolerance: This principle explains how species have specific environmental ranges they can tolerate.
Species on the Move (Range Shifts):
- Species are moving poleward to escape warming temperatures.
- A study of temperate seaweeds (comparing herbarium collections from $1940\text{--}1960$ to $1990\text{--}2009$ ) showed shifts in the northern-most records in Australia/New Zealand ranging from $0.46^\circ\text{S}$ to $1.92^\circ\text{S}$ .
- Mobile species (swimmers) tend to respond to these changes faster than sessile ones.

Defining "The New Kid on the Block": While gradual global warming is occurring, episodic short-term marine heatwaves are becoming stronger, longer, and more frequent.
Technical Definition: A Marine Heatwave is defined as a Sea Surface Temperature (SST) that is at or above the $90^{\text{th}}$ percentile of a baseline climate (e.g., a $30$ -year average) for a duration of at least $5$ days ( $Hobday\,et\,al.\,2018$ ).
Metrics of MHWs:
- Duration: The number of days the event lasts.
- Maximum Intensity ( $I_{\text{max}}$ ).
- Cumulative Intensity: Calculated as $\text{Cumulative Intensity} = \sum (\text{Day} \times \text{Intensity})$ .

Magnitude: This was the highest temperature on record in over $140$ years.
Habitat Context: Western Australia hosts a mega-diverse flora and fauna, including over $1,000$ species of seaweeds. The ecosystem was originally dominated by massive forests of Ecklonia (kelp).
Productivity: Ecklonia kelp forests are up to $15$ times more productive than Australia's wheat fields.
Impact on Kelp Forests:
- Ecklonia is a cool-water adapted species.
- The heatwave caused an extinction of Ecklonia along $100\,\text{km}$ of coastline.
- Kalbarri: At this location, the kelp forest collapsed and was replaced by turf algae ( $Wernberg\,et\,al.\,2013$ , $2016$ ).
Tropicalisation and Ecosystem Shifts:
- The loss of kelp led to "Tropicalisation," with a proliferation of warm-adapted tropical fish (a $400\%$ increase in herbivores), corals (<\,6\,\text{cm}), and mobile invertebrates.
- Regime Shifts and Hysteresis: Even after the heatwave ended, the kelp did not re-establish. The ecosystem entered an Alternative Stable State dominated by tropical herbivores that prevent kelp recovery.

Focus Species: Bull kelp (Durvillea).
- Physical Characteristics: Can grow up to $10\,\text{m}$ and weigh $70\,\text{kg}$ .
- Ecological Role: Controls habitat through whiplash (sweeping the rock surface) and shading; supports and provides high biodiversity and productivity.
Temperature Extreme: In Lyttelton Harbour, temperatures exceeded $23^\circ C$ .
Impact:
- Pile Bay (an exposed site) showed "ghost holdfasts" in November $2017$ , where the kelp had died off.
- By March $2018$ , there was regional extinction in the area.
- Durvillea is considered the "mining canary bird" (canary in a coal mine) for climate change impacts in New Zealand.

Variability: Biological impacts are highly variable based on taxa, life stage, life-history, and heat tolerance.
Greenhouse Gases: Increasing due to fossil fuel burning and methane emissions.
Acidification: Facilitates weedy seaweeds while stressing calcifying organisms.
Excluded Topics for Further Study:
- Sea level rise (caused by water expansion and ice melting).
- Increased frequency and strength of storms.
- Interactions with other stressors such as coastal darkening.
- Economic impacts of ecosystem loss.
- Short-term vs. long-term mitigation and solutions.