Kelp forests, grass beds, and coral reefs are the planet's three most productive subtidal benthic habitats

Despite obvious differences, all three communities share several important ecological features:

High levels of benthic productivity

Structural complexity (generated by the primary producers)

Arising from these first two features: all support high levels of biological diversity

 

These nearshore communities are dominated by a few species of fast growing brown seaweed [though found worldwide, they are dominant ecological features only in colder (<20°C) waters].

Primary producers

Kelp

Because kelps are attached to the bottom, yet require sunlight, kelp forests occur only in shallow (5 - 20 m) regions

They are common in zones of upwelling where nutrients are not limiting, and high growth (up to 30 cm/day) is possible for species such as Macrocystis under ideal conditions.

Other important species of kelp include: Nereocystis along our coast, and the shorter Laminaria along the northeast coast of the United States.

Most kelps have complex life cycles that include spores and vegetative reproduction... thus, there is potential for dispersal by a variety of methods.

Many other species of seaweed (often adapted to low light levels) live beneath the kelp canopy

Primary consumers

Together, these algae provide food for many grazing invertebrates (mollusks, arthropods, and echinoderms... also, previously, the Stellar's sea cow along the north Pacific rim.

Urchins are particularly important, because their scouring of the benthos can eliminate recently settled organisms.

Population booms of urchins can eliminate kelp forests, creating urchin barrens

Particles of decomposing kelp are also important for suspension feeders, such as mussels.

Secondary consumers

Carnivorous species such as sea stars, snails, lobsters, fish, and the sea otter prey on various invertebrates.

Otters can be a "keystone" species, and population fluctuations resulting from hunting and conservation efforts have obvious effects on kelp forest diversity.

Kelp forests are also influences by physical disturbance: big storms and el Nino events both diminish local kelp density, but may also promote dispersal

Together, predation, grazing pressure, disturbance, and competition for light all influence patterns of kelp forest community structure and diversity.

A central theme in all environments (terrestrial, as well as marine): what determines spatial and temporal patterns of diversity?

Thus far, we have focused on the physical environment.... now consider how biological interactions may influence patterns of marine diversity.

Inter-specific competition and predation/herbivory/parasitism coupled with environmental factors often play important roles in structuring marine communities.

Competition and predation

The competitive exclusion principle suggests that there should be little diversity: how it it maintained?

Remember, food and space can both be important limiting resources for marine organisms.

Niche partitioning

Character displacement

Predation/herbivory/parasitism

Intense patterns of exploitation can reduce the abundance of certain species: sponges,

  Keystone predation

Classic example: Pisaster seastar in intertidal habitats prey preferentially on mussels, allowing others

Another "local" example: Sea otters, urchins, and kelp. 

Disturbance

As in terrestrial habitats, community succession following disturbance is commonly observed.

Intertidal habitats offer opportunities to examine physical and biological interactions

Intertidal habitats are characterized by periodic (predictable) exposure to air during low tide.

Marine animals within these habitats must cope with desiccation, heat stress, and exposure to terrestrial predators

As mentioned in past lectures, strategies include: "Clamming up", evaporative cooling (increases the risk of desiccation) and moving to cracks or tide pools

Strategies for coping with wave stress/shock (abrasion, pressure changes, drag) include flexibility, low profiles, hard shells, and strong muscles or hold fasts for attachment

Because of the constant and predictable fluctuation in exposure to water along a vertical gradient, zonation is common in all tideland habitats

Zonation is often organized by:

Physical limitations/tolerances at upper ranges of distribution

Larval preferences (cracks and crustose coraline red algae often induce settlement)

Competition for space: overgrowing and undercutting

Predation: Immersion times often limit the effectiveness of predators

 

Several temperate tideland habitats are of interest to marine biologists, including: the rocky intertidal, estuaries, and Spartina salt marshes.

Rocky intertidal regions are perhaps the best studied

Some important lessons from intertidal studies:

Keystone predation

Complex food webs are well studied in intertidal regions

Indirect interactions within intertidal food webs can influence distribution and abundance

Cthamalus have an inducible morphological defense against predation by Acanthina

 

Estuaries and tide flats

These large expanses of relatively loose sediment are characterized by extensive flats and a network of drainage channels.

Because of their flat topography, estuaries are relatively ephemeral habitats (geologically), strongly influenced by relatively small changes in sea level.

In the short-term, they represent extremely rich, productive habitats: combining high levels of nutrient input with shallow, relatively protected marine habitats.

A gradient of salinity occurs in most estuaries: this gradient shifts with the tides.

Some species (e.g. crustaceans) have adapted to low salinities (10-15 o/oo) while others (e.g., many echinoderms) have not.

A critical salinity range of 3-8 o/oo limits exchange between fresh and marine spp. Salty enough to inhibit purely freshwater spp, but too dilute for marine adapted spp.

With time to adjust (acclimate), some spp (e.g. salmon) can make the transition from salt to fresh or vice versa.

Lots of work on physiology to understand how these tolerances change.

Large estuaries can be important nursery area, retaining larvae while they develop. Some species show larval adaptations to remain within estuaries (e.g. staying close to the bottom during outgoing tides)

Many estuary residents are adapted to life on soft bottoms rather than hard substrate

Estuary habitats are extremely important coastal wet lands

They are threatened by development, pollution, and agricultural practices that divert water flow.

Spartina/salt marshes

Spartina grasses are salt tolerant species that bind sediment with their rhizomes (like roots).

Rhizomes allow uptake of nutrients and vegetative reproduction.

The presence of grasses builds substrate... and where they have been introduced, their spread threatens native spp.

Spartina salt marshes show distinct patterns of zonation that reflect levels of competition and physiological tolerances