Fundamentals of Marine Biology

Experimental removal of urchins: 1-year later: several species of kelps

Nereocystis is an upper canopy species, with fronds at the surface and it is often annual and dies back each year. Laminaria gradually shades out other seaweeds and prevent annual species. It becomes dominant if no dense urchin populations are present.

Succession in an Alaskan kelp forest. Light competition leading to dominance by Laminaria. Other organisms are important in controlling kelp forests. e.g. Pycnopodia heliantoides and several species of crabs feed on a variety of species, with a preference on sea urchins. Multiple predators increase the spectrum of effectiveness on multiple grazers.

Carnivore diversity is:

• negatively correlated with herbivore abundance
• positively correlated with kelp biomass

No single carnivore species affects all prey species → Carnivore diversity itself provides a deterrent to herbivory (trait-mediated indirect effect)

Resistance to Herbivory:

• Enormous Macrocystis

pyrifera and Nereocystis luetkeana have the large and vulnerable fronds that float at the surface and sea urchin cannot reach this height

• synthesize sulfuric acid, which can erode the Aristotle's lantern of sea urchin the Desmarestia coverage
• Agarum fimbriatum has high concentrations of polyphenols which discourage urchins

Bryozoans seem to be able to settle and live on this seaweed and the coverage further defends the kelp against grazing.

There is a depth pattern of chemical defense:

• Shallow species are less chemically defended, because urchin grazing is kept low by otters
• Deeper species, out of the diving capabilities of otters, developed a more powerful chemical defense.

Benthic environments: subtidal rocky reef

Subtidal rocky reefs host communities dominated by invertebrates and algae and are often dominated by colonial invertebrates. In tropical environments shallow rocky reefs are usually dominated by corals and other large calcareous organisms. In temperate and high latitudes,

they are dominated by a variety of plants, algae and animals that do not secrete CaCO3 at the same rates as in the tropics.

Intertidal rocky reefs are dominated by animals occurring as individuals

Subtidal rocky reefs are dominated by colonial animals

The low levels of colonial organisms in the intertidal may relate to the physiological limitations they have:

• Not able to close off soft tissues from temperature stress and desiccation
• Most organisms are not adapted to strong salinity variations
• In the first meter or so, in many localities temperature and salinity greatly vary seasonally (e.g. New Zealand fjords and Russian Pacific coast). Organisms occurring as individuals are not often dominant in subtidal reefs:

Differences between individual and modular organisms.

Modular organisms: interconnected and usually identical units (polyps, zooids, etc).

Modular organisms spread along a surface:

• More impervious to predation and competition
• Predators normally

Take only a part of the colony, and the colony grows back (predation over an individual normally lead to the death of the individual)

Rocky reefs are often patchy: rock dominated by rich invertebrate assemblages / turf-forming calcareous red algae.

Patches with rich invertebrate assemblages:

• Often on vertical rock walls- Space is very limited: inter- and intra-specific competition
• Species with different life histories and morphologies compete for the same space

Patches with red algae:

• Invertebrates are found in this patches but are rarer that on other patches
• On more horizontal surfaces, seaweeds dominate (controlled by grazing). The alternation of patches seems to be regulated by different processes.

Physical and biological disturbances select for the algal patches

For example:

• Sea urchins are associated with calcareous algae (more resistant than more delicate invertebrates)
• Landslides facilitate algal turfs

For example in New Zealand:

• Mussels dominate a surface low-salinity layer

(0-5 m). They are physiologically more tolerant that deeper invertebrates and the pycnocline may be a barrier for larval dispersal

• In deeper water, large patches of invertebrates are interspersed with low-diversity patches of algae. The algal patches are maintained by landslides, which are quite localized
• The animal patches are relatively closed environments: larvae from a patch mostly settle within the patch (patches dominated by animals with short-lived larvae)
• Self-seeding patches

Competitive interactions within rocky reef animal patches are still poorly understood. At the edges: animal patches grow into empty spaces (or space dominated by algae). In the middle: patches are crowded → almost all the space is occupied but space is constantly lost by a species and gained by another. Competitive success is related to the ability to overgrow and resist overgrowing by neighbors: Space occupation is linked with competitive ability of displacing others or holding out

againstincursions.Rapidly overgrowing species not good at resisting overgrowth VS Slow-growing and more resistant(production of toxic secondary metabolites, morphological plasticity).Also the priority effect can lead to great differences in patterns of dominance. Most of the invertebrates inthese patches are passive suspension feeders: their position in relation to the flux of food is important.Competitive interactions are also mediated by predation and grazing: all processes that liberate space can leadto strong shifts in community dominance.

Example:

1. Sea Urchins they are abundant in rocky reefs, more on horizontal surfaces (less current).
   • If colonial invertebrates arrive first, urchins may have no effect
   • If invertebrates die back, urchins move in exploiting newly settled seaweeds → this can favor a shift towards calcareous algae patches 392.
2. Invasion of the nudibranch Tritonia plebeia in Massachussets (from Europe)Normal condition: reefs dominated by octocorals, ascidians,

sponges, amphipod (tubes), and fleshy crustosealgae. Invasion: T. plebiea fed on octocorals (odor and visual detection) similar to their native diet, nearlyeliminating them. Open space colonized by urchins, which also moved to vertical walls to eliminate theremaining colonies→ invasion + urchin explosion = phase shift in dominance that was not reversed even when the nudibranchdisappeared.

Reorganization of a rocky subtidal ecosystem: an example

Trophic level disfunction: removal of crucial parts of the trophic cascade:

• Cods and other fishes functioned as top carnivores in the past, keeping crabs and lobsters in check. Also urchins were probably low in numbers: dominance of kelp
• Starting from the 40s, lobster population increased (cod fished → relaxation of fish predation). Fromthe 60s to the 70s stocks were partially exploited by non US fishery.

After the extension of US fishery zone to offshore, the cod continued to decline. In a few decades, a toppredator was

deposit calcium carbonate and create the tridimensional complexity of coral reefs. They host incredible diversity. They have a symbiosis with unicellular algae called zooxanthellae. They show different in temperature tolerance. The symbiosis cause algae are photosynthetic organisms and provide food to corals. Coral reef are an important hot spot of biodiversity. They are located to tropical latitude where there is an optimal range of temperature, optimal amount of nutrients and clear water. They are exposed to a different threat: Bleaching, disease outbreaks, predator outbreaks, algal overgrowth, overfishing, sedimentation, pollution, direct anthropogenic stressors, etc

Benthic environments: polar regions ice can cover up to 13% of the Earth's surface (polar circles + ephemeral in other regions - e.g. Baltic, Caspian, etc.). Relatively few oceanographic expeditions and research outpost in polar waters → Knowledge is still little

Polar oceans are

1. Ice crystals float on the surface and aggregate: grease ice.
2. Salt is not included in the ice-crystals structure: brine solution→spongy structures (pores and channels).
3. At this moment organisms can be inoculated into ice (e.g. plankton).
4. Ice crystals accumulate and form aggregated disks (5-10 cm): ice pancakes, that later become super pancakes (several meters across).
5. After a few days they freeze together and form a closed ice cover, that continues to increase in size by congelation.

However, ice floes are continuously moved by currents, winds, and waves.

There are great differences between the Antarctic and Arctic Oceans:

• Area: 36 million km² vs 15 million km².
• Frozen area: 20 million km² (4 in summer) vs 14 million km² (7 in summer).
• Arctic: closed sea. Antarctic: open - exchange with all other oceans.
• Ice in the Arctic lasts longer → Generally thicker (3.5 m vs 1.5 m in the Antarctic).
• Arctic is very shallow (average 1800m).
• Arctic has a considerable freshwater input (ice with pollutants, sediment, allochthonous).

material transported and release