(18) Seagrass Beds Subtidal Rocky Reefs

Seagrass Beds

• Definition: Seagrass beds are underwater areas dominated by submerged angiosperms.

• Location: Found widely in shallow soft sediments, typically at depths of 3-5 meters.

• Habitat Complexity: They increase habitat complexity by providing vertical structure which is crucial for various marine species.

• Biodiversity: Serve as food and shelter for benthic invertebrates.

Seagrass Beds and Water Motion

• Impact of Water Motion: Seagrass beds are influenced by water motion, leading to different morphological forms:

  • High Water Motion: Results in the formation of mounds.

  • Low Water Motion: Leads to flat, extensive carpets of seagrass.

• Wave Action Damping: Seagrass beds can significantly dampen wave action, with waves being almost entirely cancelled out just 1 meter into a seagrass bed.

Sedimentation in Seagrass Beds

• Sediment Accumulation: When water motion slows, particles including larvae and sediments accumulate within seagrass beds.

• Current Regime Influence:

  • Strong Currents: Can wash out detritus.

  • Weak Currents: Allow detritus to accumulate, affecting overall ecosystem health.

Seagrass Productivity

• Microbial Activity: High rates of microbial metabolism and nutrient regeneration contribute to organic matter accumulation.

• Biodiversity: High productivity supports many associated animal and algal species, including epiphytes, especially those with long blade life cycles.

• Ecosystem Impact: Epiphytes can shade seagrasses which increases the risk of blade detachment.

Infaunal Suspension Feeders

• Increased Abundance: There is an increase in the abundance and growth of infaunal suspension feeders within seagrass beds due to:

  • Enhanced Food Supply: More suspended food available.

  • Predation Protection: Presence of seagrasses helps protect them from predators such as large snails and predatory fish.

Grazing on Seagrasses

• Minimal Grazing: Grazing on Zostera marina is minimal, primarily from geese.

• Other Species: Thalassia testudinum experiences grazing from:

  • Sea urchins, which create halos around reefs.

  • Green turtles (Chelonia mydas), herbivorous fish, and marine mammals such as dugongs and manatees are also significant grazers.

Dugong Grazing

• Impact of Grazing: Intense grazing by dugong herds can lead to extensive areas being denuded of seagrass blades.

• Regeneration: Seagrass can rapidly regrow through rhizomes, allowing the ecosystem to renew itself.

Export of Organic Matter

• Organic Matter Flow: The movement of organic matter out of seagrass beds involves:

  • Export of Leaves: Whole leaves can be exported by waterfowl like dabblers and Black Brant.

  • Detritus and Fecal Matter: Organic detritus and leaf material contribute to energy flow within the marine ecosystem.

  • Decay Process: Cycling of nutrients through decay of detritus and dissolved organic matter into deeper marine environments, potentially serving as energy sources for abyssal benthos.

Loss of Seagrass Beds

• Wasting Disease: In the 1930s, a microbe called Labyrinthula caused significant loss of seagrass beds, leading to changes in sediment dynamics and slope, often resulting in replacement by algae and suspension feeders.

Seagrasses and Seaweeds

• Interactions: Seaweeds often grow among seagrass beds.

• Nutrient Addition Effects: Nutrient boosts can lead to the proliferation of seaweeds and harmful growth of epiphytes, negatively affecting seagrass health.

• Impact from Suspension Feeder Loss: The loss of suspension feeders can increase local phytoplankton, which may favor seaweeds over seagrasses, altering the ecosystem dynamics.

Propeller Scarring

• Impact of Boat Traffic: Areas of high-density, moderate-density, and low-density scars from propeller damage can significantly affect seagrass ecosystems.

Subtidal Rocky Reefs

• Habitat Definition: Subtidal rocky reefs are hard-bottom communities containing various microhabitats.

• Feeding Mechanism: Many attached organisms are suspension feeders, relying on detritus carried by currents.

• Habitat for Species: Crevices and spaces within the reefs provide refuge from strong currents and predators, critical for species like lobsters.

Rock Reef Communities

• Species Dominance Change: As depth increases, there is a shift from predominance of seaweeds or kelps to various invertebrates, mostly sessile suspension feeders (sponges, bryozoans) and mobile predators (sea stars, snails).

• Diversity: High species diversity is maintained despite the two-dimensional nature of the habitat.

Depth Transects across Rocky Substrates

• Species Cover: Varying percent cover of kelp, macroalgae, and invertebrates observed with increasing depth (0 to 30m) shows diverse community structure.

Rocky Substrate - Depth Transect

• Specific Organisms: The rocky face in temperate zones hosts various species including kelp, red algae, sponges, and a wide range of invertebrates, displaying high biodiversity.

Rocky Faces - Tropics

• Tropical Biodiversity: Tropical rocky faces exhibit an array of organisms including corals, sponges, sea urchins, and various anemones, showcasing rich and diverse marine life.

Competition for Space

• Space Dominance: Organisms in rocky subtidal zones compete for space, often leading to a dominance of colonial organisms which may be more resistant to predation due to their ability to spread.

High Diversity in Subtidal Rocky Reefs

• Patch Mosaic: Subtidal rocky reefs are often characterized by patches of various ages and species diversity, contributed by both intraspecific and interspecific competition.

Seagrass Beds

• Definition: Seagrass beds are underwater areas dominated by submerged angiosperms, playing a vital role in marine ecosystems by providing habitat and food sources.

• Location: Typically found in shallow soft sediments, especially at depths ranging from 3 to 5 meters where sunlight can penetrate for photosynthesis.

• Habitat Complexity: Seagrass beds significantly enhance habitat complexity, providing vertical structure that is essential for various marine species, including fish and invertebrates.

• Biodiversity: These habitats serve as crucial food and shelter for benthic invertebrates, contributing to high marine biodiversity and facilitating nutrient cycling within ecosystems.

Seagrass Beds and Water Motion

• Impact of Water Motion: Water motion affects the morphology and health of seagrass beds in different ways:

  • High Water Motion: Leads to the formation of mounds due to sediment accumulation and dislodging, impacting the seagrass structure and light availability.

  • Low Water Motion: Results in extensive, flat carpets of seagrass that can support diverse marine life, enhancing habitat quality.

• Wave Action Damping: Seagrass beds can substantially dampen wave action; it has been observed that waves can be nearly entirely cancelled out just 1 meter into a seagrass bed, providing a calmer environment for marine organisms.

Sedimentation in Seagrass Beds

• Sediment Accumulation: When water motion is reduced, sediments and organic particles, including larvae, accumulate within seagrass beds, promoting biodiversity and providing nutrients for growth.

• Current Regime Influence:

  • Strong Currents: Can wash out detrital materials, leading to nutrient depletion in the seagrass habitat.

  • Weak Currents: Favor detrital accumulation, which is vital for the overall health of the ecosystem and supports a food web.

Seagrass Productivity

• Microbial Activity: High levels of microbial metabolism within seagrass beds contribute to nutrient regeneration and organic matter accumulation, enhancing the productivity of the beds.

• Biodiversity: Elevated productivity levels provide habitat and food for various associated animal and algal species, including epiphytes. The ecosystem supports organisms with varying lifecycles and growth strategies.

• Ecosystem Impact: Epiphytic algae may shade seagrasses, increasing the risk of blade detachment which can negatively affect light availability and growth.

Infaunal Suspension Feeders

• Increased Abundance: Seagrass beds experience a rise in infaunal suspension feeders due to:

  • Enhanced Food Supply: Seagrass beds provide a rich source of suspended organic matter for filter feeders.

  • Predation Protection: The complex structure of seagrasses provides crucial refuge from predators, which encourages growth and population stability among these organisms.

Grazing on Seagrasses

• Minimal Grazing: Grazing pressure on Zostera marina is typically low, with geese being the primary grazers that do not lead to significant ecological impacts.

• Other Species: Thalassia testudinum faces grazing from several groups:

  • Sea Urchins: Contribute to reef dynamics by creating halos through their grazing actions.

  • Green Turtles (Chelonia mydas): Play a significant role in seagrass dynamics as herbivores, influencing the growth and health of seagrass beds along with herbivorous fish and marine mammals such as dugongs and manatees.

Dugong Grazing

• Impact of Grazing: Significant grazing by dugong (Dugong dugon) herds can lead to denuded areas devoid of seagrass, affecting the structure and function of marine habitats.

• Regeneration: Despite intense grazing, seagrass can regenerate rapidly through rhizomes, allowing for ecosystem stability and renewal following disturbances.

Export of Organic Matter

• Organic Matter Flow: The export of organic matter from seagrass beds is critical in maintaining marine ecosystem productivity:

  • Export of Leaves: Whole leaves can be transported by waterfowl such as dabblers and Black Brant.

  • Detritus and Fecal Matter: Organic detritus and leaf material contribute to energy flow in the marine ecosystem, acting as food sources for various organisms.

  • Decay Process: Nutrients are cycled through the breakdown of detritus and dissolved organic matter, providing energy sources for deeper marine environments and abyssal benthos.

Loss of Seagrass Beds

• Wasting Disease: The 1930s saw significant losses of seagrass beds due to a devastating microbe called Labyrinthula, which altered sediment dynamics and slope, often resulting in replacement by algae and suspension feeders, leading to long-term ecosystem changes.

Seagrasses and Seaweeds

• Interactions: Seaweeds frequently coexist with seagrass beds, leading to complex ecological interactions.

• Nutrient Addition Effects: Enhanced nutrient availability can lead to excessive seaweed growth and harmful epiphyte proliferation, negatively impacting seagrass health and productivity.

• Impact from Suspension Feeder Loss: The loss of suspension feeders can lead to increased phytoplankton levels, further favoring seaweed growth over seagrasses, thus shifting ecosystem dynamics.

Propeller Scarring

• Impact of Boat Traffic: Areas impacted by boat traffic exhibit scars of varying density—high, moderate, and low—which significantly affect seagrass ecosystems and can lead to habitat degradation over time.

Subtidal Rocky Reefs

• Habitat Definition: Subtidal rocky reefs are characterized as hard-bottom communities that harbour diverse microhabitats critical for marine life.

• Feeding Mechanism: Many attached organisms are suspension feeders that depend on detritus carried by currents, highlighting the interplay between reef structure and nutrient availability.

• Habitat for Species: Interstitial spaces within rocky reefs provide essential refuge from strong currents and predation for various species such as lobsters.

Rock Reef Communities

• Species Dominance Change: Observations show a shift from dominant seaweeds or kelps to various invertebrates, including sessile suspension feeders (e.g., sponges, bryozoans) and mobile predators (e.g., sea stars, snails) with increasing depth.

• Diversity: Despite its complexity, high species diversity is often maintained in subtidal rocky reef environments.

Depth Transects across Rocky Substrates

• Species Cover: Percent cover data from various transects of kelp, macroalgae, and invertebrates show how community structure varies with changes in depth (0 to 30 meters), reflecting diverse species interactions and adaptations.

Rocky Substrate - Depth Transect

• Specific Organisms: In temperate zones, rocky faces host diverse species including kelp, red algae, sponges, and a vast array of invertebrates, underscoring high biodiversity within these ecosystems.

Rocky Faces - Tropics

• Tropical Biodiversity: Tropical rocky faces are home to an array of organisms such as corals, sponges, sea urchins, and various anemones, demonstrating the rich and diverse marine life that characterizes these habitats.

Competition for Space

• Space Dominance: In rocky subtidal zones, organisms compete intensely for limited space, often leading to a dominance of colonial organisms which can spread more successfully and become more resistant to predation.

High Diversity in Subtidal Rocky Reefs

• Patch Mosaic: Subtidal rocky reefs are characterized by patches of differing ages and species diversity, shaped by both intraspecific and interspecific competition, contributing to ecosystem resilience and functionality.