Lecture Notes: Seagrasses, Mangroves, and Seaweeds

Overview of Upcoming Lectures

  • The next lectures will cover seagrasses, mangroves, and seaweeds.
  • Coral reefs are a separate subject, to be discussed by Richard Toews.

Size Categories

  • Small: Phytoplankton (mostly single-celled, some invisible with a light microscope).
  • Benthic microalgae (grow adhered to sand grains in shallow coastal waters, not covered in detail).
  • Intermediate: Seagrasses and seaweeds (attached to the sea floor).
  • Some unattached seaweeds exist in the water column.
  • Large: Mangroves (large trees).

Nutrient Sources

  • Most marine plants derive nutrients from the water column.
  • Seagrasses: Take nutrients from both the water column and the sediment via roots.
  • Seaweeds: Mainly from the water column, but some tropical species have rhizoids that can take up nutrients from the sediment.
  • Mangroves: Take up nutrients from the sediment.

Foundation Species

  • Definition: Foundation species are dominant species forming the structure of a community, supporting associated species.
  • Examples: Coral reefs, kelp forests, seagrass meadows, and mangroves.
  • Impact: Affect communities primarily through their presence as structures that other organisms inhabit.
  • Importance: Loss of foundation species can lead to the loss of associated species.
  • Decline: Seagrasses, mangroves and seaweeds are decreasing in their footprints.

Seagrasses

  • Rooted plants with rhizomes.
  • Vascular flowering plants (angiosperms) that originated on land and invaded the water.
  • Belong to the Alismatales family (like water plantains).
  • Diverged approximately 5,000,000 years ago.
  • Coverage: Approximately 160,000 \text{ km}^2.
  • Distribution: Mainly subtidal, but some are intertidal.
  • Habitat: Shallow coastal waters and estuaries with sand or mudflats.
  • Size range: From 2-3 \text{ cm} (small species) to 4 \text{ meters} (Zostera caulescens).
  • Importance:
    • Nursery areas for commercial fisheries (North America).
    • Obstetric nurseries in New Zealand.
    • Coastal protection by stabilizing sediment.
    • Carbon storage.
    • Water filtration.
  • Conservation: Important for species like dugongs, green turtles, and manatees.
  • Threats:
    • Eutrophication (indirect effects).
    • Sedimentation.
    • Marine heat waves (e.g., Shark Bay, Western Australia, lost nearly 700 \text{ km}^2 in 2010-2011).
  • Reproduction:
    • Sexual reproduction: Pollen release (usually separate male and female plants).
    • Tropical seagrasses: Release pollen mixed with mucilage, attracting invertebrates for pollination.
    • Asexual reproduction: Primary mode of spread via rhizomes, forming clones.
    • Example: A single individual can be 80 \text{ km} long
  • Adaptations:
    • Air enchyma: Gaps in tissues that transport oxygen from leaves to rhizomes and roots, enabling survival in waterlogged sediment.
  • Problems:
    • Disappearing at a rate of 10 \text{ km}^2 per year.
    • Sensitivity to light: Require at least 11\% of surface light (compared to kelps at 1\%).
    • Eutrophication: Filamentous algae block light.
    • Sedimentation: Increased sediment scatters light.

Wakalibu Bay Example (Massachusetts, USA)

  • Cause: Eutrophication due to leaking septic tanks.
  • Process:
    • Increased nitrogen loading.
    • Growth of microalgae and macroalgae, increasing turbidity.
    • Loss of seagrasses.
    • Sediment erosion and resuspension.
    • Anoxic conditions due to seaweed decomposition, killing grazers.
  • Outcome: Shift from seagrass dominance to macroalgal dominance.

Tauranga Harbor Example (New Zealand)

  • Cause: Increased sedimentation due to changes in land use and geology.
  • Process: Seagrasses decline, mangroves expand.
  • New Zealand Seagrasses:
    • Mainly Zostera muelleri.
    • Mostly intertidal and perennial.
    • Approximately 44 \text{ km}^2 in New Zealand.
    • Subtidal populations have been lost.

Mangroves

  • Intertidal trees and shrubs.
  • Forests are called mangals.
  • Taxonomy: Approximately 55 species in 19 genera (subject to change).
  • Distribution: Tropics and subtropics.
  • Southern limit: New Zealand.
  • Coverage: Approximately 150,000 \text{ km}^2.
  • Carbon storage: Significant carbon storage in soil.
  • Types: High-density (Indo-Pacific) and Low-density mangroves.

Carbon Storage

  • Importance: Act as carbon sinks, storing carbon in soil.
  • Comparison: Mangroves store a significant amount of carbon compared to other ecosystems.
  • Land Clearance: Loss of about 30 \text{ cm} (topsoil) is expected during land clearance, however, the amount of stored should be preserved.

Threats and Issues

  • Loss: Up to 50\% lost in the last 60 years in the tropics mainly for land clearance, development, and aquaculture.
  • Recovery: Can recover within approximately 15 years.
  • Lifespan: Similar to humans.

Adaptations and Environment

  • Limitation: Frequently suffer from nutrient limitation due to saline, anoxic, acidic, organic-rich soils.
  • Evergreen: Mostly evergreen with a leaf lifespan of approximately 16 months.
  • Sclerophyll: Exhibit sclerophylly (tough leaves with stem cell orbs).
  • Water loss: Can lose a lot of water before wilting.
  • Nutrient retention: Retain most nutrients in leaves before shedding.
  • High photosynthetic nutrient use efficiency.
  • Anaerobic microbial activity.
  • Halophytes: Tolerate high salt concentrations and flooding.
  • Pneumatophores: Roots that stick up from sediment to absorb oxygen.
  • Air enchyma: Spaces in tissues for oxygen diffusion.
  • Salinity management:
    • Salt excretion via glands on leaves.
    • Osmoregulation: Production of compatible solutes (e.g., glycine betaine, mannitol, proline, sucrose) to maintain water potential.
  • Seeds germinate on trees and drop off as large propagules.

Mangrove Expansion

  • Distribution: Expanding southward, likely due to increased sedimentation.
  • Colonization: Example of the further Thames, with colonization starting in the 1950s.
  • Thames Forest: This mangrove forest reached 1 kilometer wide by 2007.
  • Example stands from 02/2007:
    • Nearest the land, planted in 1980s.
    • Further inland, planted in 1990s.

El Nino

  • Cause of Wind: Winds come from southwest due to El Nino.
  • The condition required for recruitment of mangrove seeding is calm water (i.e., less wind and less waves).

Seaweeds

  • Diverse group (polyphyletic).
  • Origin: Red algae are the oldest (over one billion years ago), brown algae are more recent.
  • Taxonomy: Diverse.
  • Habitat: Mainly benthic (attached to a substrate).
  • Coverage: Approximately 3.5 million \text{km}^2.
  • Importance: Ecologically important, commercially important (food source in Asia).
    • Examples: Nori (Porphyra), kombu, and wakame.

Three Groups of Seaweeds

Red Algae

  • Pigment: Colour coded by pigment.
  • Polysaccharides: Produce cellular polysaccharides like agar and carrageenans.
  • Endemic Algae: Refisia, endemic North Island genus. South Island equivalent - Zairelii
  • Giant Celled Algae: unique properties
    • Single cell
    • Can regenerate (knows to have a top and bottom) and completely regenerates.
    • Contains hundreds of nuclei per cell.

Green Algae

  • Similar characteristics to terrestrial plants such as chlorophyll a and b, sucrose, and starch.
  • Tropical Reefs: Sand is produced from calcifying green algae.
  • Taxifolia: Problematic and causes problems. Currently causing problems at Great Barrier Island.
  • Ulva: Causes its own distinct problems over time.
  • Other examples of pollution
    • Tauranga citizens aggravated by green stuff (Ulva) is due to the southern oscillation and is not to do with pollution.
    • Blue Volva - largest in the world! Related to nutrients.

Brown Algae

  • Most recent innovation (~150,000,000 years ago).
  • Pigment: Have same accessory pigment as diatoms (fucoxanthin).
  • Gigantic: Biggest seaweeds (e.g. Macrocystis pyrifera). Can grow up to 60meters in length!
  • Brown Algae outbreaks - pacific islands, caused by Turbinaria ornate.
  • Forereef: On forereef, short and stubby due to wave action.
  • Backreef: Away from wave action it grows bigger and grows old - the brittle stocks lead to breakage in high quantites and the floatation makes the algae a big problem.