Marine Ecosystems: Seagrasses, Kelp Forests, Coral Reefs, and Deep-Sea Habitats

Seagrasses

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

Rhizome systems

Subsurface systems that allow seagrasses to extend within soft sediment.

Habitat

Seagrasses are found throughout tropical and temperate oceans.

Optimal growth conditions

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

Impact of stagnant water

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

Impact of strong current

Currents greater than 0.5 m/s adversely impact seagrasses, possibly due to shear stress and sediment erosion.

Flower characteristics

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

Pollination method

Seagrasses 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.

Ecological role of seagrass beds

High primary production, support diverse animal species, reduce current flow, and provide refuge for prey.

Flow and grass density relationship

No grass allows flow to carry larval patches with no settlement; low density reduces flow allowing larvae to settle throughout the grass bed.

High density grass impact

High density causes strong flow reduction, with larvae settling on the periphery but not penetrating far into the grass bed.

Grazing variability

Grazing on seagrasses is minimal in temperate zones; unclear reasons include poor nutrition and lack of suitable grazers.

Tropical grazing

In tropics, seagrass beds are grazed differentially due to varying toughness and cellulose content.

Grazers of tough grasses

Turtles, urchins, and dugongs graze even tough grasses.

Green turtles

Chelonia mydas grazes on Thalassia, nipping leaf tips to encourage new growth that is easier to digest.

Urchin grazing behavior

Urchins graze tropical seagrasses at night after hiding during the day, creating a clearing or halo around patch reefs.

Nutritional adaptation

Green turtles have extended hindguts with intestinal microflora to digest cellulose.

Dugongs

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

Eutrophication

A process where phytoplankton shade sea grasses, leading to strong reductions of eelgrass beds.

Chesapeake Bay

An area that has completely lost its eelgrass beds, although restoration efforts have led to some local recoveries.

Overfishing

May result in reduced grazing and overgrowth of epiphytes, which smother sea grasses.

Suspension feeding bivalves

Restoration of these should greatly reduce phytoplankton loads and further benefit sea grasses.

Dredging and boat traffic

Causes decline of sea grasses.

Eelgrass epidemic

A disease caused by fungus in the 1930s that affected eelgrass, with recovery but other fungi now causing sporadic diseases in tropical sea grasses.

Kelp forests

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

Growth rates of laminarian seaweeds

Generally high, often of the order of centimeters per day.

Kelp forest height

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.

Suspension feeders

Common in kelp forests, especially among sessile benthic species living on hard substrata.

Trophic cascades

Interactions in ecosystems where changes in the population of one species can influence the populations of other species.

Herbivory

Herbivorous sea urchins that feed on kelp.

Carnivory

Sea otter (Enhydra lutris) can regulate urchin populations.

Otter recovery

Their recovery has strong impacts on the balance between urchins and kelp.

Barrens vs Kelp forests

Two alternating states influenced by storms and herbivory.

Effect of storms

Storms can remove kelp, leading to barrens.

El Niño

Storms combined with warm water can lead to kelp mortality.

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

Otters cannot dive deep enough to remove urchins, leading to their absence in tropical regions.

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

Includes suspended phytoplankton (e.g., bivalves, polychaetes).

Food supply for deposit feeders

Includes microalgae and bacteria.

Decomposing organic matter

Includes phytodetritus and decomposing seaweeds.

Spatially variable inputs

Patchy occurrence of the green macroalga Ulva leads to spatially patchy inputs of particulate organic matter.

Anoxic sediments

Can occur in patches.

Polychaetes colonizing patches

They aerate the sediment.

Mobile snails

Congregate on patches and reduce polychaete densities.

Sediment interactions - competition

Includes spatial interference and chemical interference (allelopathy).

Bromophenol poisons

An example of chemical interference.

Food exploitation

Involves feeding rapidly on detritus and microalgae.

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.

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.

Asexual spread of grasses

Occurs generally from the rhizomes, trapping more sediment.

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.

Aerenchyma tissue

Air-filled cavities allowing gas exchange from leaves to roots.

Bioturbation

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

Grazing on Spartina

The act of feeding on Spartina, which appears to be very low due to the tough leaves rich in cellulose and silica.

Grazing by invertebrates

Relatively slight grazing on Spartina, with possible exceptions.

Seed set failures

Frequent failures of seed set due to grazing on flowers.

Littorina irrorata

A snail that does damage while feeding on fungi in southerly areas.

Trophic cascade

A phenomenon where crabs and other predators of snails facilitate growth of Spartina by reducing snail densities.

Mature marshes

Older, mature salt marshes that consist of meadows with interspersed tidal creeks.

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 exhibited by salt marshes, showing distinct zones from low to high intertidal.

Salt marsh species

Species such as S. alterniflora, S. patens, and Juncus gerardi found in salt marshes.

Mangrove forests

Forests that replace salt marshes toward the tropics in warmer waters.

Mangrove roots

Broad roots that project into air to acquire oxygen, but only extend to shallow depths in anoxic soils.

Mangrove seeds

Seeds that develop directly on the tree and then fall off or are carried by water.

Salt glands

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

Particulate organic matter

High supply of organic matter, especially falling leaves, which subsidize animal growth in mangroves.

Coastal erosion

The process that mangroves help stabilize by preventing the loss of coastal intertidal soils.

Water quality maintenance

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

Estuaries

Coastal zones where seawater meets freshwater from a river.

Estuarine flow

The movement of low density river water downstream coming into contact with saline seawater.

Stratified estuary

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

Vertically homogenous estuary

Vigorous tidal mixing in relatively small estuaries homogenizes the vertical salinity gradient.

Salinity changes

At low tide, river input dominates salinity. At high tide, a rush of seawater inward dominates the salinity.

Geologically ephemeral but biologically rich

Even slight changes in sea level can drain or fill in estuaries (e.g. ice ages).

Estuarine environment

Intimately related to watershed, which helps determine water and nutrient input and flow regime.

Biodiversity and salinity

Biodiversity declines with decreasing salinity, especially in the critical salinity range of 3-8 o/oo.

Zonation

Universal feature of rocky shores, also true of soft sediments but not as distinct.

Black lichen zone

The highest vertical zone in the general pattern of zonation.

Periwinkle zone

A zone often with sparse barnacles, located below the black lichen zone.