BIOL 450 - 9 - Reefs, Kelp Forests, and Seagrass Beds

sea grasses are..

marine angiosperms (flowering plants), that are confined to very shallow water

sea grasses extend mainly by..

spreading through underground stems called rhizomes that grow in soft sediment (like sand or mud) below the water

sea grass are found..

throughout tropical and temperate oceans

sea grass grow best in...

very shallow water, lots of sunlight, and gentle water currents

ecology of sea grass

-produce a lot of food through photosynthesis, supporting a wide variety of animals.

-slow down the movement of water, which can be helpful for some animals and plants living there.

-create a barrier that makes it harder for predators like crabs and fish to enter from the sides, providing a safe area for other creatures.

-can help in the growth and increase the numbers of certain animals that feed on particles suspended in the water near the edge of the bed. nutrients from phytoplankton may not enter deep into the bed

grazing (feeding behaviour) & community structure

-in temperate areas, grazing on a Zostera marina (eel grass) is minimal.

-In tropics, sea grass beds have multiple species that are grazed differently because they vary in toughness and cellulose content.

-Green turtles graze on sea grasses by nipping the tips of the leaves. This grazing encourages the growth of new, softer, and more digestible grass.

-Even tough sea grasses are grazed by animals like turtles, urchins, and dugongs.
Green turtles have extended hindguts with intestinal microflora, digesting cellulose

Sea Grasses - Decline

-Sea grasses are very sensitive to eutrophication, (excess of nutrients like nitrogen and phosphorus causes excessive algae growth (phytoplankton - can block sunlight, leading to a decline in sea grass beds)). reductions in eel grass beds in North America.


-Overfishing can lead to a decrease in the number of grazing and overgrowth of epiphytes (plants that grow on sea grass) that can smother and harm the sea grasses

-dredging and boat traffic can decline of sea grasses

-Diseases, particularly fungal infections. there was a fungus-caused epidemic in eelgrass during the 1930s, other fungi now cause sporadic (irregularly) diseases in tropical sea grasses

kelp forests

-rocky reef habitat/complex found in cooler coastal waters with high nutrients

-rocky reefs are often dominated in shallow waters by kelps and seaweeks and by animals that live on the surfaces of rocks (epifaunal animals)

-shift in the types of organisms that dominate different parts of the kelp forest.
-Shallow areas are mainly covered by rapidly growing seaweeds
-deeper areas are dominated by epifaunal animals.

more on rocky reefs

-have lots of algae and invertebrates, especially colonies

-often patchy, with areas dominated by rich groups of invertebrates and red algae that form turf-like structures

-may be limited space for animals to live on the rock walls, suggesting that competition for space is important for these organisms.

more on kelp forests

-dominated by brown seaweeds in Laminariales

-found in clear, shallow waters, nutrient-rich, have temperatures below 20°C, exposed to the open sea.

-Laminarian seaweeds can grow quickly, Macrocystis kelp - 60 cm/day.

-forests can be 10-20 m high or around 1 m high (Macrocystis can grow up to 50 m)

kelp forests & urchins herbivory

herbivory - herbivorous sea urchins

carnivory - sea otter (Enhydra lutris) can regulate urchin pops.

result: trophic cascade;

add otters - have reduction of urchins and increase kelp abundance

reduce otters - kelp grazed down by urchins

-recent history: otters hunted to near extinction - recovery has strong impacts on urchin/kelp balance

-In lower-latitude California kelp forests, larger diversity of predators beyond sea otters exerts top-down effects

kelp forest community structure

-effects of storm: remove kelp

-el nino: storms + warm water = kelp mortality

-cali kelp forests:

-storms remove kelp, urchins roam, and inhibit kelp colonization and growth: barrens
-if kelp groth is rich, urcins stay in crevices and capture drift algae

-leads to two alternating states: barrens & kelp forest

geological importance of coral reefs

often massive physical structures

biological importance of coral reefs

biological structure, high diversity

economic importance of coral reefs

shoreline protection, harbours, fishing, tourism

coral reefs

-compacted and cemented skeletons and sediments of organisms that live in one place (sedentary organisms).

-constructional, wave- resistant features

-built up by corals, coralline algae, sponges, and other organisms, but also cemented together

-reef-building corals belong to the Scleractinia, have a partnership/endosymbiotic algae:zooxanthellae, helps them grow faster due to high rates of calcification.

-topographically complex, meaning they have varied shapes, structures, and levels, providing diverse habitats for marine life.

reef-building (hermatypic) corals

-belong to phylum Cnidaria, Classs Anthozoa, Order Scleractinia

-secrete skeletons of calcium carbonate

-colonies of many similar polyps (small, tentacle-like organisms)

-can be divided intro branching and massive forms

-have abundant endosymbiotic zooxanthellae

zooxanthellae

-type of dinoflagellate, (single-celled organisms found in marine)

-once considered as one species: Symbiodinium microadriaticum.

-identified at least 10 distinct taxa (varieties or types) of zooxanthellae with significant genetic differences between them.

-found in organisms such as anemones, reef-building corals (hermatypic corals), soft corals (octocorals), giant clams (bivalve Tridacna), ciliophora (Euplotes)
.
-found within the tissues of the coral, endodermal (inner tissue layer). concentrated in tentacles

hermatypic corals

-reef framework building

-have many zooxanthellae

-high calcification (good at turning dissolved calcium & carbonate ions in seawater into hard skeleton, helping them grow/build coral reefs)

ahermatypic corals

-not framework builders

-low calcification

coral growth

-branching: grow in linear dimension rapidly 10 cm per year

-massive: produce lots of calcium carbonate but grow more slowly in linear dimensions, ab 1 cm per year

Corals - Biodiversity and Form Diversity

-identified based on their appearance (morphology).

-problem: can change their shape in response to factors (water movement, light levels, competition with other species)

-problem: can look very similar even if they are different species

-DNA sequencing is used to identify coral species more accurately.

Zooxanthellae - Bleaching?

-Bleaching -expulsion of zooxanthellae.

-cause: stress (temperature, disease)

-poorly understood - zooxanthellae cells appear to die & are expelled

-test among mechanisms with flurochromes; support for cell death under temp stress

Coral Reefs - Limiting Factors

-Warm sea temp (current problem of global sea surface temperature rise)

-High light (symbiosis with algae)

-Open marine salinities

-Low turbidity - coral reefs do poorly in near- continent areas with suspended sediment

-Strong sea water currents, wave action

-Reef growth a balance between growth and bioerosion

-Reef growth must respond to rises and falls of sea level

-pH? Increasing ocean acidity a problem?

coastal reefs

wide variety of reefs from massive structues (Great Barrier Reef) to small patches (Eilat, Israel)

atolls

horseshoe or ring-shaped island chain of islands atop a sea mount

depth zonation on reefs

-reefs dominated by diff coral species at diff depths

-may be controlled by factors similar to rocky shores (not so well known), possible relationship to changing light conditions

competition

shading, overgrowth, interspecific digestion, sweeper tentacles, allelopathy(?)

predation and grazing

some common coral predators (e.g., crown-of-thorns starfish), grazers (e.g., surgeon fish, parrotfish, urchins)

disturbances

e.g., storms, hurricanes, cyclones, earthquakes

larval recruitment

mass spawning, question of currents and recruitment of larvae

disease

spread by currents, can cause mass mortality of some species (e.g., common black sea urchin Diadema antillarum in 1980s)

coral mortality

-catastrophic:

-hurricanes, typhoons, earthquakes
-sea star
-el nino - influx of nutrient poor warm water
-coral diseases: black or white band disease
-coral bleaching

Interspecific Competition

Scolymia lacera -Scolymia cubensis: bare zone established after mesentarial filaments extruded through polyp wall

more on predation and grazing

-Pacific Ocean: Crown-of- thorns starfish Acanthaster planci feeds on corals

-Outbreaks all over Indo-Pacific starting in 1960s

formaly rare, changed behaviour, started herding instead of being dispersed, and they switched from feeding at night (nocturnal) to feeding during the day (diurnal).