Marine Ecosystems: Seagrasses, Kelp Forests, Salt Marshes, Mangroves, Coral Reefs

Seagrasses

Marine angiosperms, or flowering plants, confined to very shallow water that require light for photosynthesis.

Rhizome systems

Subsurface systems that extend mainly within soft sediment, similar to Spartina.

Habitat

Found throughout tropical and temperate oceans.

Optimal growth conditions

Grow best in very shallow water, high light, and modest current flow.

Effects of stagnant water

Too stagnant water does not deliver enough nutrients and causes turbidity due to plankton density.

Effects of strong current

Currents greater than 0.5 m/s adversely impact sea grasses, possibly due to shear stress of leaves and erosion of sediment.

Flower characteristics

Lack ornate flowers typical of terrestrial angiosperms because sexual reproduction is secondary to asexual reproduction.

Pollen dispersal

Sea grasses rely on water dispersal of pollen rather than pollinators.

Zostera marina

Eel grass that dominates in temperate zones.

Thalassia testudinum

Turtle grass found in tropical regions.

Primary production

Sea grasses have high primary production and support a diverse group of animal species.

Current flow reduction

Sea grass beds reduce current flow.

Refuge for prey

Sea grass beds deter the entry of crab predators and larger fish.

Infaunal suspension feeders

May enhance growth and abundance near the edge of grass beds.

Flow and grass density relationship

No grass allows flow to carry larval patches across sediment with no settlement.

Low density grasses

Cause a reduction in flow, allowing larvae to settle throughout the grass bed.

High density grasses

Cause strong flow reduction, with larvae settling on the periphery of the grass bed.

Grazing variability

Grazing on sea grasses is variable; in temperate zones, grazing on Zostera marina is minimal.

Tropical grazing

In tropics, sea grass beds are comprised of several species grazed differentially due to toughness and cellulose content.

Green turtles

Grazes on Thalassia, nipping leaf tips to encourage new growth that is easier to digest.

Urchins grazing behavior

Urchins graze tropical sea grasses at night after retreating to patch reefs during the day.

Halo effect

Grazing by urchins causes a clearing, or halo, around patch reefs.

Dugongs

Related to the manatee, they graze out eelgrass which encourages growth of Halophila ovalis, a more nutritious and faster growing grass.

Decline of seagrasses

Sea grasses are vulnerable to eutrophication, where phytoplankton shade sea grasses, leading to strong reductions of eel grass beds.

Eelgrass beds in North America

Chesapeake Bay has completely lost its eel grass beds, although restoration efforts have led to some local recoveries.

Overfishing

Possible that overfishing results in reduced grazing and overgrowth of epiphytes, which smothers sea grasses.

Restoration of suspension feeding bivalves

Should greatly reduce phytoplankton loads and further benefit sea grasses.

Dredging and boat traffic

Causes decline of sea grasses.

Eelgrass epidemic in the 1930s

A disease caused by fungus that led to a significant decline in eelgrass, with recovery occurring later.

Kelp forests

Dominated by brown seaweeds in the Laminariales, found in clear, shallow water, nutrient-rich and usually below 20°C.

Growth rates of laminarian seaweeds

Generally have high growth rates, often of the order of centimeters/day.

Height of kelp forests

Forests can be 10-20 m high or only a meter in height.

Diversity in kelp forests

Kelp forests have many species of seaweeds, even if sometimes dominated by one species.

Community structure in kelp forests

Includes trophic cascades involving herbivory and carnivory.

Herbivory in kelp forests

Herbivorous sea urchins play a significant role.

Carnivory in kelp forests

Sea otter (Enhydra lutris) can regulate urchin populations.

Trophic cascade

Adding otters reduces urchins and increases kelp abundance; reducing otters leads to kelp being grazed down by abundant urchins.

Impact of otters on kelp forests

Otters hunted to near extinction; their recovery has strong impacts on urchin/kelp balance.

Barrens vs Kelp forests

Effect of storms can remove kelp, leading to barrens.

El Niño effect on kelp

Storms combined with warm water lead to kelp mortality.

California kelp forests

Storms remove kelp, allowing urchins to roam and inhibit kelp colonization and growth.

Grazing in kelp forests

Most species of kelps rely on fast growth rates to overcome intense herbivory.

Desmarestia

A brown alga that produces sulfuric acid, eroding the Aristotle's lantern of urchins.

Geographic patterns in kelp defenses

Australasian kelps are better defended than North Pacific kelps, with 5-6 times the concentration of deterrent compounds.

Top predators in deep water

Lack of top predator in deep water means otters cannot dive deep enough to remove urchins.

Soft sediment intertidal

Zonation is not as apparent as it is in the rocky intertidal due to the 3-dimensional structure of the sediments.

Vertical gradients in heat/desiccation

Not as strong due to the water retention in the porous sediments.

Higher intertidal species

Burrow more deeply.

Space as a limiting resource

In soft sediments, vertical stratification of organisms occurs.

Deep dwelling clams

Are unaffected by surface dwelling clams, but depressed by other deep swelling clams.

Polychaetes

Some produce chemicals that can deter recruitment of other invertebrates.

Food supply for suspension feeders

Suspended phytoplankton (e.g., bivalves, polychaetes).

Food supply for deposit feeders

Microalgae and bacteria.

Decomposing organic matter

Includes phytodetritus and decomposing seaweeds.

Patchy occurrence of Ulva

Leads to spatially patchy inputs of particulate organic matter.

Anoxic sediments

Can occur in patches.

Polychaetes colonization

They colonize patches and aerate the sediment.

Mobile snails

Congregate on patches, reducing polychaete densities.

Sediment interactions - competition

Includes spatial interference and chemical interference (allelopathy).

Bromophenol poisons

Example of chemical interference.

Seasonal predation

Migrating birds/fishes can devastate invertebrates.

Caging experiments

Demonstrate that areas with ropes (preventing birds) had normal benthic invertebrate density that remained unchanged throughout the season.

Salt marshes

Are accretionary environments.

Colonization of sediment by salt marsh plants

Is followed by trapping of fine particles and accretion of sediments.

Marsh Spartina spp.

Plants spread by means of a rhizome system - interconnected and often consist of broad stands of the same genotype.

Ecosystem engineers

Cordgrasses in Spartina salt marshes bind fine sediment and cause the buildup of meadows above low water.

Formation of organic peat

Results from the buildup of sediment colonized by other species at higher levels.

Bare intertidal sediment

Is colonized by patches of Spartina grass.

Grass retarding flow of water

Allows sediment to begin to accumulate.

Sediment accretion

Gradually occurs as grasses extend/spread (generally asexually from the rhizomes).

Higher marsh development

Is dominated by terrestrial plants.

Anoxic sediment in marshes

Can be seen by digging into the sediment.

Anaerobes

Microorganisms that reduce sulfates when O2 is absent, producing sulfides that react with iron, turning sediments black.

Aerenchyma tissue

Air-filled cavities in plants that allow gas exchange from leaves to roots.

Bioturbation

The process by which burrowers, such as fiddler crabs, aerate the soil and enhance the growth of Spartina.

Grazing on Spartina

Generally low, with invertebrates showing relatively slight grazing, possibly due to tough leaves rich in cellulose and silica.

Grazing on flowers

May be far greater than grazing on leaves, resulting in frequent failures of seed set.

Mature marshes

Often consist of tidal creeks interspersed with meadows.

Tidal creeks

Creeks that fill with saltwater at high tide and drain at low tide, supporting large populations of invertebrates.

Vertical zonation

The intertidal phenomenon where salt marshes exhibit distinct zones from low to high intertidal.

Spartina alterniflora

A lower intertidal species that is more salt tolerant than high intertidal forms.

Mangroves

Forests that replace salt marshes in warmer waters, characterized by broad roots and adaptations to high salinity.

Salt glands

Structures in mangroves that excrete salts from the cytosol to the leaf surfaces.

Particulate organic matter

High supply in mangroves, especially from falling leaves, which subsidizes animal growth.

Coastal erosion

A process that mangroves help prevent by stabilizing coastal intertidal soils.

Water quality maintenance

The ability of mangroves to trap sediments and take up excess nutrients from the water.

Zonation of mangrove species

Occurs due to differences in salt tolerance and ability to withstand seed predation.

Estuaries

Coastal zones where seawater meets freshwater from a river.

Estuarine flow

The movement where low density river water flows downstream and comes into contact with saline seawater.

Highly stratified estuary

An estuary where a layer of freshwater flows at the surface toward the sea, with denser oceanic water flowing underneath.

Moderately stratified estuary

An estuary where moderate winds and tidal action cause mixing at all depths, increasing salinity seaward.

Vertically homogenous estuary

An estuary where vigorous tidal mixing homogenizes the vertical salinity gradient, changing with the tide.

Salinity

The concentration of salt in water, which changes in estuaries depending on the tide.

Low Tide

A condition when river input dominates salinity in the estuary.

High Tide

A condition when a rush of seawater inward dominates salinity in the estuary.

Geologically ephemeral

Refers to estuaries that can drain or fill with slight changes in sea level, such as during ice ages.

Watershed

An area of land that drains into a particular water body, influencing water and nutrient input and flow regime in estuaries.

Biodiversity

The variety of life in a particular habitat, which declines with decreasing salinity, especially in the critical salinity range of 3-8 o/oo.

Zonation

A universal feature of rocky shores, also applicable to soft sediments, indicating distinct vertical zones.

Black Lichen Zone

The highest vertical zone in the intertidal area characterized by the presence of black lichen.