L11- Intertidal Sediments and Estuarine Ecosystems

Overview of Intertidal Sediments and Estuaries

• Introduction to Habitat Types

  • The lecture focuses on intertidal habitats composed of sand and sediments, contrasting them with the rocky intertidal zones previously discussed.

  • Intertidal Zone Definition: A narrow band of the coastline affected by the gravitational pull of the moon and sun. It is emergent (exposed to air) at low tide and covered by water at high tide.

  • Spatial Extent: The zone is extremely narrow on a global scale—often just meters wide, reaching a few hundred meters or up to $1\,km$ in specific locations. It is the primary site of human interaction with the ocean.

• Introduction to Estuaries

  • Definition: An estuary is the area where a river meets the sea, resulting in the mixing of fresh and salt water.

  • Scope of Content: The lecture covers sedimentary systems (Chapter $11.2$) and estuaries (Chapter $12$), excluding seagrasses and mangroves which are scheduled for later discussion.

• Seagrasses and Mangroves (Clarification)

  • Seagrasses: These are angiosperms (flowering plants) with true roots and rhizomes, distinguishing them from seaweeds (algae). They are common in sand and mud because their roots can anchor in soft substrate.

  • Mangroves: Also angiosperms; essentially terrestrial trees adapted to tropical marine environments.

Sediment Classification and Physical Properties

• The Nature of Shifting Sediments

  • Unlike rocky shores where organisms live on a fixed substrate, sedimentary shores consist of loose particles.

  • The Rack Line: Seaweed (e.g., cast-off pieces from rocky reefs) often deposits at the high tide mark. This is called "beach rack" and represents a transfer of nutrients from the ocean to the shore.

• Grain Size Classification System

  • Clay: Particles smaller than $0.004\,mm$.

  • Silt: Particles larger than clay but smaller than $0.062\,mm$.

  • Sand: Particles between $0.062\,mm$ and $2\,mm$.

  • Gravel: Particles larger than $2\,mm$.

  • Mud Definition: In marine biology, mud is defined as the combination of clay and silt (all particles smaller than $0.062\,mm$).

  • Larger Substrates: Above gravel are pebbles, rocks, and boulders. Boulders are termed "consolidated reef" once they are too heavy to be easily turned over.

• Environmental Relationship with Grain Size

  • Wave-Exposed Coastlines: Characterized by sand, gravel, or pebbles. High wave energy keeps fine particles (clay/silt) suspended in the water column, preventing settlement. Example: Brighton Beach (sand) and Kaikoura (gravel/pebbles).

  • Protected/Sheltered Sites: Characterized by mud. In areas like estuaries with low wave energy and slow-flowing currents, fine particles can settle out of the water.

• Biogeochemical Implications

  • Drainage: Sandy shores drain quickly due to large spaces between particles, leading to high oxygen penetration.

  • Anoxia: Muddy sediments are compact; water does not drain well, leading to puddles and rapid oxygen depletion. This results in anoxic conditions (low oxygen) just below the surface.

Biology of Sedimentary Organisms

• Infauna and Epifauna

  • Infauna: Animals that live inside or burrowed within the sediment (e.g., bivalves, worms). This is the dominant life form in soft-bottom systems.

  • Epifauna: Animals that live on top of the sediment surface. These are rare on sandy/muddy shores due to high predation risk from birds (at low tide) and fish (at high tide).

• Adaptations for Burrowing

  • Bivalves: Use a vascular foot that changes shape and fluid pressure to anchor and pull the animal down.

  • Polychaete Worms: Segmented bodies allow for vascular changes to burrow efficiently.

• Feeding Mechanisms

  • Suspension/Filter Feeders: Organisms that filter organic material and phytoplankton from the water column. Bivalves often use siphons to pull water in, filter it, and squirt clean water out. Sponges can filter even finer particles, such as bacteria.

  • Deposit Feeders (Sand Eaters): Organisms that ingest sediment to digest the organic film (detritus, bacteria, diatoms) on the particles. Example: The sand worm creates "casts" of clean, processed sand on the surface.

• Predation and Defense

  • Predatory Infauna: Worms or snails that burrow to find prey.

  • Sea Cucumbers: Known as the "vacuum cleaners of the sea floor." They live on the surface (epifauna) but avoid predation through:

    • Chemical Defenses: Bad taste/toxins.

    • Physical Defenses: Sticky, nasty white filaments designed to cover predators.

    • Evisceration: Spitting out their own guts to distract predators, then regenerating them.

Zonation and Food Webs

• Relative Lack of Zonation

  • Unlike rocky shores (which show clear bands of organisms driven by desiccation and wave exposure), sand flats lack obvious zonation. The sediment remains damp, providing a buffer against desiccation, so organisms are more scattered.

• Soft-Bottom Food Web Components

  • Primary Producers: Phytoplankton in the water and benthic diatoms on the sediment. Benthic diatoms can make the mud look green, yellow, or greasy.

  • Consumers: Deposit feeders (eating diatoms and detritus) and suspension feeders (eating plankton).

  • Top Predators: Birds (intertidal) and fish (subtidal/high tide).

Estuarine Geomorphology and Formation

• Human Significance: Estuaries were the "birth of civilization" due to access to fresh water, marine food, and transport routes. Most major cities (e.g., Auckland) are built around estuaries.

• Four Geomorphological Types

  1. Tectonic Estuaries: Formed by faulting or volcanic activity. Example: Banks Peninsula (Akaroa and Lyttelton Harbours), formed by volcanic domes approximately $10,000,000$ years ago and later flooded by sea-level rise.

  2. Fjords: Carved by glacial activity. Typically U-shaped and deep, with a "sill" (mound of boulders) at the mouth deposited by the retreating glacier. Found in Fiordland, New Zealand, and Norway.

  3. Drowned River Valleys: Most common type globally. Formed when sea levels rose approximately $100\,m$ after the last ice age, flooding existing river valleys. Example: Marlborough Sounds.

  4. Bar-Built Estuaries (Lagoons): Formed when longshore currents deposit sand spits or barriers that semi-enclose an area. These are geologically young (evolve over hundreds rather than thousands of years). Example: Avon-Heathcote Estuary (South Shore Spit).

Estuarine Salinity Profiles

• Defining Gradient: Salinity is the primary driver of estuarine ecology. Freshwater is nearly $0\,PSU$; ocean water is approximately $35\,PSU$.

• Salt Wedge Estuary: Occurs where river flow is much stronger than tidal mixing. Fresh water (less dense) floats on top of the heavier salt water, creating a wedge shape. The boundary lines of equal salinity are called "isohalines."

• Vertically Mixed Estuary: Occurs when tidal mixing or wave action is stronger than river flow, resulting in uniform salinity from the surface to the bottom.

• Inverse/Reversed Estuary: Found in hot, arid regions with little freshwater input. Evaporation makes the water hyper-saline (higher than $35\,PSU$). Example: The Dead Sea.

New Zealand Estuaries: Characteristics and Threats

• Prevalence: Over $300$ estuaries identified (more with satellite data). Most are small, bar-built, and well-mixed.

• Anthropogenic Impact: Though only $6$ New Zealand estuaries have over $80,000$ people living nearby, most are heavily affected by farming. Agriculture introduces excess nutrients and sediment into river systems, which then accumulate in estuaries.

• Key Stressors:

  • Salinity: Constant fluctuation.

  • Nutrients: High levels lead to blooms of seaweeds (e.g., Ulva).

  • Sedimentation: Risk of burial for organisms.

  • Temperature: Shallow, protected waters fluctuate wildly (e.g., $5$ to $25^\circ C$) compared to the stable open ocean ($12$ to $19^\circ C$).

Osmoregulation and Adaptation

• Osmoconformers: Simple organisms (e.g., some worms) whose internal salinity changes with the environment. They have physiological mechanisms to tolerate broad salinity ranges.

• Osmoregulators: Higher organisms (e.g., fish) that expend energy to maintain constant internal blood salinity (typically around $10\,PSU$) regardless of external conditions.

• Classification based on Salinity Tolerance:

  • Stenohaline: Organisms with a narrow tolerance (e.g., most marine fish and freshwater species).

  • Euryhaline: Organisms that tolerate a wide range of salinities (common for marine species moving into estuaries).

  • True Brackish Water Species: Rare species that thrive specifically in intermediate salinities (around $10\,PSU$).

Case Study: Salt Marshes and Trophic Cascades

• Salt Marsh Characteristics: Domesticated by terrestrial angiosperms that can tolerate salt. They are highly productive ($2,000\,g/m^2$) and act as carbon sinks ("blue carbon").

• Trophic Cascade (Brian Silliman Study):

  • The Paradigm: It was previously believed that marshes were controlled "bottom-up" and that snails were detritivores (eating dead grass).

  • The Experiment: In Virginia, Silliman used cages to exclude or add snails. He found snails were "fungal farmers." They rasp holes in live grass, allowing fungi to infect the plant, which the snails then eat.

  • The Result: Without predators (Blue Crabs and Terrapins) to keep snails in check, the snails can completely denude a marsh. This is a "top-down" trophic cascade.

Habitat Complexity in Estuaries

• The Importance of Structure: Mudflats are naturally simple. Adding three-dimensional structure increases biodiversity significantly.

• Structural Anchors:

  • Seaweeds: Provide habitat for epifauna on mudflats.

  • Oyster Reefs: Historically over-harvested; currently the focus of restoration efforts to provide habitat.

  • Shelled Organisms: Clams and dead shells provide similar structural benefits to oysters on a smaller scale.

Questions & Discussion

• Student Question: If a clam is digging and spraying out water, is that "clean" water?

  • Response: Generally, yes. It has been filtered for organic material and phytoplankton, though very fine particles like bacteria may still pass through depending on the organism's filter size.

• Student Question: Do they use siphons for digging?

  • Response: No, siphons are primarily for sensing and feeding (water exchange). Digging is performed by the muscular foot.

• Student Question: What did sea cucumber taste like?

  • Response: One student noted it was "crunchy" but only because it was cooked on a fire; the lecturer expected a "chewy" texture. It is often used in agriculture and prepared with many spices.

• Discussion on Logistics: The upcoming lab involves health and safety forms and drawing organisms. Students are permitted to take photos and complete drawings later.