L10- Intertidal Rocky Shore Ecology and Marine Elemental Cycles

Elemental Cycles in Marine Ecosystems

Importance of Elemental Cycles
- There is a strong interest in understanding how organisms interact with the abiotic environment and how they obtain limiting resources.
- Organisms are fundamental to transferring energy and matter around systems, making knowledge of the periodic table essential for ecological studies.
- The primary cycles of interest for life are Carbon ( $C$ ), Nitrogen ( $N$ ), Phosphorus ( $P$ ), and Oxygen ( $O$ ).
The Carbon Cycle
- Atmospheric-Oceanic Equilibrium: $CO_2$ in the atmosphere dissolves into the ocean. This is an equilibrium process: as more $CO_2$ is added to the atmosphere (e.g., via burning fossil fuels), more is pushed into the ocean.
- Climate Change Context: The ocean acts as a massive thermal and chemical sink. Without the ocean absorbing surplus $CO_2$ , atmospheric temperatures would be significantly higher (the speaker uses the metaphor of "literally frying").
- Ocean Acidification: When $CO_2$ dissolves in sea water, it undergoes chemical reactions with water ( $H_2O$ ) that release hydrogen ions ( $H^+$ ). An increase in $H^+$ concentration defines acidity. The speaker notes it makes the water "less alkaline," which is functionally more acidic.
- Biological Pathways: Carbon is taken up by primary producers and enters the food chain. It is eventually released back into the ocean through respiration.
- Sedimentary Carbon: Dead organic matter and minerals sink to the bottom. For example, organisms that build calcium carbonate ( $CaCO_3$ ) skeletons contribute to the burial of carbon in the sediment.
The Phosphorus Cycle
- Contrast with Carbon: Unlike carbon, which has strong atmospheric interactions, phosphorus flux is primarily through rivers and terrestrial inputs.
- Precipitation and Sedimentation: Phosphorus tends to precipitate. It is taken up from its dissolved form by primary producers, passed through the food web and detritus systems, and incorporated into the sediment when organisms die.
- Unidirectional Pathway: The cycle is generally unidirectional (moving from rivers and atmosphere into the sediment) with very little flux returning from the sediment.
- Geological Returns: The only significant way phosphorus returns to the system for use by terrestrial or freshwater organisms is through tectonic activity, such as earthquakes and volcanic eruptions.
The Nitrogen Cycle
- Atmospheric Composition: Nitrogen ( $N_2$ ) is an inert gas making up approximately $78\%$ to $79\%$ of the atmosphere.
- Equilibrium: Similar to carbon, dissolved nitrogen exists in equilibrium between the ocean and atmosphere.
- Nitrogen Fixation: Because $N_2$ is inert, most organisms cannot use it directly. Specific nitro-fixing bacteria are essential for converting inorganic nitrogen into organic components that can enter the food chain.
- Chemical Forms: There is a complex interaction between ammonia ( $NH_3$ ), nitrates ( $NO_3^-$ ), and nitrites ( $NO_2^-$ ). The specific form depends on the amount of oxygen associated with the nitrogen.
- Scientific Debate: While textbooks often show no net movement of nitrogen into the sediment (focusing on the ocean-atmosphere interaction), some researchers argue that certain organic nitrogen-containing molecules are difficult to break down and may be stored long-term in the sediment.

The Physics and Predictability of Tides

Gravitational Basis of Tides
- Tides are based on gravitational forces between massive objects. Following Newton's Law of Universal Gravitation:
- $F = G \frac{m_1 m_2}{r^2}$
- Where:
  - $F$ is the gravitational force.
  - $m_1$ and $m_2$ are the masses of the objects.
  - $r$ is the distance between the centers of the masses.
  - $G$ is the gravitational constant.
- Application to Earth: The Earth and Moon attract each other. Because water is fluid, the Moon's pull creates a "bulge" of water on the side facing the Moon. There is also an opposite force on the other side of the planet.
Tidal Cycles
- Rotation: As the Earth spins, a specific point on the surface passes through the bulges. In a flat-earth scenario (ocean planet with no landmasses), we would experience a high tide, then a low tide 6 hours later, another high tide 6 hours after that, and so on.
- Lunar Day: Because the Moon is also moving in an orbit that takes 28 days to complete, it takes slightly longer for the Earth to re-align with the Moon. A full tidal cycle (two lows and two highs) takes approximately $24\text{ hours and }50\text{ minutes}$ .
- Predictability: Because planetary movements are highly consistent, tides are predictable years in advance, allowing for precise planning of field work.
Solar Influence and Oscillations
- The Sun is massively heavy and, despite its distance, modifies tidal patterns.
- Spring Tides: Every 14 days, when the Sun and Moon are aligned, their combined gravitational pull creates extra-strong high tides.
- Neap Tides: When the Sun and Moon are at right angles to each other relative to Earth, their pulls partially cancel out, resulting in lower tidal ranges.
- Highest Astronomical Tides (HAT): Occur when multiple planets (e.g., Venus and Neptune) align, creating exceptionally high tides.
Topographic Modifications
- Earth is not a flat ocean planet; it has mountains, channels, and inlets that modify tides.
- Tidal Types:
  - Semidiurnal: Two similar high and low tides every 24 hours (most common, standard in New Zealand).
  - Mixed Semidiurnal: Two highs and two lows of different heights.
  - Diurnal: Only one peak per 24-hour cycle.
- Local Examples:
  - The Baltic Sea: Due to narrow straits and topography, it has almost no tides (approx. $20\,cm$ ), often masked by the wake of ferries.
  - Bay of Fundy, Canada: A classic extreme example where the funnel-like topography creates a tidal amplitude of nearly $15\,m$ .
  - New Zealand: Ranges from $1\,m$ to nearly $4\,m$ (e.g., in the Cook Strait).

Biological Stressors and Adaptations in the Intertidal Zone

Major Stress Factors
- Desiccation: The single biggest stressor; marine organisms must avoid drying out when the tide is low.
- Temperature Fluctuations: Water has a high heat capacity and resists temperature changes. Air heats up quickly, meaning organisms are exposed to extreme heat on sunny days.
- Light Stress: Excessive sunlight can damage tissues or overwhelm photosynthesis in plants.
Morphological Adaptations
- Operculum: Snails like Brunella have a hard plate that seals their shell. This prevents water loss and protects against predation during low tide.
- Shell Features: Certain organisms have white shells or ridges to reflect heat rather than absorb it.
Behavioral Adaptations
- Cracks and Crevices: Organisms cluster in moist, dark, damp areas to avoid heat and drying out. This also provides protection from bird predators.
- Tide Pools: Depressions in rocks that retain water during low tide, acting as mini-subtidal systems. Snails like Snakeskin Chitons stay in these pools and move out to graze only when the tide returns.
Physiological Adaptations
- Heat Shock Proteins: Organisms produce proteins like $ASP\,70$ when stressed by heat. These proteins prevent other essential cellular proteins from breaking down at high temperatures.
- Desiccation Tolerance: Some seaweeds, such as Hormosira banksii, can survive being physically dried out and recover once submerged.
Habitat Modification
- Groups of organisms can modify their own microclimate. A "forest" of seaweed retains dampness and reduces evaporation compared to a single isolated plant.
- Small indentations in rocks can help a "baby" seaweed survive; as it grows, it creates a moist environment for more individuals to settle, leading to a self-sustaining population.

Wave Physics and Biomechanical Adaptations

Wave Dynamics
- Waves carry energy, not matter; molecules move up and down in a circular motion. As waves approach the shore, the bottom creates friction, making the crest move faster than the trough until the wave breaks.
- Wave Refraction: Even if wind comes from an angle, waves tend to hit a straight coast parallel. This is because the part of the wave hitting shallow water first slows down, causing the rest of the wave to "pivot" toward the shore.
- Exposure Gradients: Wave energy concentrates on headlands (exposed sites) due to refraction and is dissipated in bays (sheltered sites).
Biomechanical Adaptation to Wave Force
- Size: Being small reduces the surface area exposed to moving water.
- Flexibility: Species like Postelsia or Esonia (from the U.S.) bend over and lie flat with the flow rather than resisting it.
- Drag Force Equation:
- $F_d = \frac{1}{2} \rho v^2 C_d A$
- In a simplified ecological context, the drag depends on:
  - Density of seawater ( $\rho$ ): Constant (approx. $1$ ).
  - Drag Coefficient ( $C_d$ ): How streamlined the organism is.
  - Velocity ( $v$ ): The speed of the water.
  - Platform Area ( $A$ ): The area exposed to the force.
Measuring Attachment Strength
- Break Force ( $F_{break}$ ): The force required to dislodge an organism from the substrate, measured in Newtons ( $Newton = kg / 10$ ).
- Safe Populations: Research indicates that organisms on hard granite are safer than those on soft limestone. Small kelp are often safer than large kelp during storms.
- The Bull Kelp (Durvillaea): This New Zealand species is the strongest seaweed recorded. Researchers have used industrial tractors and chains to pull them off reefs. They are so strong that they sometimes rip the rock substrate off the reef rather than detaching from it.

Ecological Theories: Competition and Zonation

The Battle for Space
- Space is the primary limiting resource in rocky intertidal systems. Gaps created by storms or predators are quickly filled.
- R-Strategists: Produce many offspring/spores and rely on luck to find and colonize gaps quickly.
- K-Strategists: Competitive species that grow large and attempt to hold onto space for long periods.
Zonation Patterns
- Zonation is the orderly replacement of one community by another along a stress gradient (e.g., altitude on a mountain or depth in the ocean).
- Scale: While mountain zonation takes hours to observe, intertidal zonation (from water to dry land) can be seen across just 4 or 5 meters.
- Universal Concept: First popularized by Stephenson and Stephenson in the 1940s, who found similar patterns globally. Lewis (1969) later refined this by showing that zonation patterns shift based on wave exposure (e.g., wave splash extends the biological zones higher up the shore at exposed sites).
Classical Ecological Experiments
- Joseph Connell (1950s): Conducted seminal experiments with two barnacle species.
  - Upper Limit: Defined by abiotic stress (desiccation). Species cannot survive higher up because they dry out.
  - Lower Limit: Defined by biotic interactions (competition and predation). Smaller barnacles were outcompeted by larger ones, and larger barnacles were limited by predators like sea stars.
- Fundamental vs. Realized Niche: A species' fundamental niche is where it could live based on physiology; its realized niche is where it actually lives due to competition and predation.

Questions & Discussion

Question Regarding Ocean Acidification: Does $CO_2$ make the ocean more acidic?
- Response: Yes. The $CO_2$ reacts with water to release hydrogen ions. Hydrogen is acidity. Therefore, more $CO_2$ makes the ocean more acidic (or less alkaline).
Question Regarding Ridges and Heat: How do ridges on a shell help with heat?
- Response: Ridges and white coloration help reflect heat rather than absorb it. The speaker noted they would check the specific physical explanation for the next session.
Quiz Content: The upcoming quiz consists of 20 multiple-choice questions. It is open-book and covers all material up to the current week. It prioritizes logic and deduction. It will be open for two days.
Field Trip: The field trip will involve visiting a rocky shore and an estuary/mudflat to see these zonation patterns and intertidal organisms (e.g., Durvillaea) in person.