Western Australian Marine Ecosystems and Currents Overview

What are the main features of the Leeuwin Current?

Broad (80-200km) and shallow (50-200m), relatively warm (>24°C in the north to 22°C in the south), low salinity (<35ppt), flows north to south along WA's coastline, nutrient poor, bends and spins up eddies, speeds of 1-2 knots, and becomes cooler, faster, and more saline as it moves south.

What is the source of the Leeuwin Current?

Warmer, low salinity water flows through the Indonesian archipelago from the Pacific to the Indian Ocean, resulting in lower density water between Australia and Indonesia.

How does the density difference affect the Leeuwin Current?

It results in a change in sea level of about 0.5m along the WA coast, which is the driving force of the Leeuwin Current.

What seasonal variability affects the Leeuwin Current?

From October to March (summer), southerly winds strengthen, causing the current to slow down. From April to August (winter), the winds weaken, allowing the current to strengthen.

What interannual variability influences the Leeuwin Current?

It is driven by a geopotential (density) gradient, with changes in density in NW WA affecting the current's strength, influenced by the El Niño Southern Oscillation.

What happens to the Leeuwin Current during El Niño conditions?

Trade winds in the Pacific relax, pushing less water to NW WA, resulting in lower steric heights and a decrease in the strength of the Leeuwin Current.

What is the effect of the Leeuwin Current on Western Australian ecosystems?

The Ekman transport of the poleward flowing current causes downwelling of coastal waters, leading to oligotrophic conditions and reduced total biomass compared to nutrient-rich equatorward flows.

How does the Leeuwin Current affect the migration and dispersal of marine species?

It allows tropical corals and organisms to exist as far south as Rottnest Island, with warmer temperatures supporting their survival and influencing the recruitment of species like WA Rock Lobster and Australian salmon.

What is the climate impact of the Leeuwin Current on Western Australia?

It increases rainfall in the southwest by cutting off cold-water currents, resulting in higher rainfall amounts compared to other regions at similar latitudes.

What are the main features of the West Australian Current?

Cold (20-22°C), surface current, relatively high salinity (>35.9ppt), nutrient poor compared to eastern boundary currents, located further offshore, and speeds of 0.5 knots.

What is the source of the West Australian Current?

It is part of the counterclockwise Indian Ocean Gyre, predominantly controlled by trade winds.

How do ocean gyres influence the West Australian Current?

Gyres are influenced by the rotation of the Earth, causing deviation to the left in the southern hemisphere, and are created by a balance of winds, pressure gradients, and the Coriolis effect.

What is the Sverdrup balance in relation to ocean gyres?

It is the balance between winds, pressure gradients, and the Coriolis effect that creates ocean gyres.

How does the temperature of the West Australian Current compare to the Leeuwin Current?

The West Australian Current is generally colder (20-22°C) compared to the warmer Leeuwin Current.

What is the nutrient status of the West Australian Current?

It is relatively nutrient poor compared to other eastern boundary currents.

What is the speed of the West Australian Current?

The speed of the West Australian Current is approximately 0.5 knots.

How does the location of the West Australian Current differ from the Leeuwin Current?

The West Australian Current is located further offshore compared to the Leeuwin Current.

What is the relationship between the Leeuwin Current and the recruitment of WA Rock Lobster?

There is a clear link between the settlement of WA Rock Lobster larvae and the interannual variability of the Leeuwin Current; a stronger current leads to more rock lobster.

What is the impact of the Leeuwin Current on nutrient availability?

The current's oligotrophic conditions lead to lower total biomass due to the lack of nutrients compared to other western continental boundaries.

What role do eddies play in the Leeuwin Current?

Eddies cause mixing along the current's path, particularly at locations like Shark Bay, Abrolhos Islands, and Rottnest Island.

What is the significance of the temperature gradient in the Leeuwin Current?

As the current moves south, it becomes cooler, faster, and more saline, influencing marine ecosystems along the WA coast.

What are the Trade Winds and where are they located?

Winds that blow from East to West near the tropics.

What are the Westerlies and where are they located?

Winds that blow from West to East in the mid latitudes, further away from the equator.

How do winds affect ocean currents?

Winds push on the surface of the ocean, creating currents that form the top and bottom of ocean gyres.

What is the Coriolis effect?

A phenomenon that causes moving objects, like ocean currents, to be deflected to the left in the southern hemisphere and to the right in the northern hemisphere due to Earth's rotation.

What are eastern and western boundary currents?

Currents that complete ocean gyres, formed by the deflection of surface currents.

What is the WA current?

An eastern boundary current of the Indian Ocean Gyre.

How does the WA current influence nutrient levels in WA ecosystems?

It causes upwelling of cold, nutrient-rich waters from the deep, increasing primary productivity, although WA remains relatively nutrient poor compared to other regions.

What role does the Leeuwin current play in relation to the WA current?

The Leeuwin current cuts the WA current off from the coast, causing downwelling and contributing to lower nutrient levels.

How does the WA current affect the dispersal of organisms?

It brings zooplankton from the southern ocean to the WA coastline and supports temperate species, creating high biodiversity.

What is the South Equatorial Current (SEC)?

A westward flowing current formed by water from the Pacific Ocean via the Indonesian Throughflow (ITF).

What are the characteristics of the SEC?

It is warm, nutrient poor, and has low salinity due to freshwater inputs from high rainfall in Indonesia.

What causes the Indonesian Throughflow (ITF)?

A buildup of water on the incoming side due to winds and ocean currents, resulting in a higher sea level.

How does the strength of the SEC relate to the ITF?

The strength of the SEC is determined by the flow and strength of the ITF.

What is the impact of the ITF on primary productivity in the North-west Marine Region?

It suppresses upwelling of deeper nutrient-rich waters, forcing higher primary productivity to occur at depths associated with the thermocline.

What happens to the thermocline when the ITF is weaker?

The thermocline lifts, bringing deeper, nutrient-rich waters into the photic zone, which increases productivity.

What is the main effect of the SEC and ITF on WA marine ecosystems?

They indirectly affect the strength of the Leeuwin Current, which is crucial for the ecosystem.

What would happen to the Leeuwin Current without the SEC and ITF?

Without them, the Leeuwin Current would not exist, significantly altering the ecosystem.

What is the depth range of the thermocline in the North-west Marine Region?

Generally at 70-100 meters depth.

How does the WA current support primary productivity?

By upwelling cold, nutrient-rich waters that enhance phytoplankton growth.

Why is the WA coastline considered relatively nutrient poor?

Due to the influence of the Leeuwin current, which causes downwelling and limits nutrient availability.

What unique feature does the combination of WA current and Leeuwin current create?

A unique blend of temperate and tropical species, resulting in high biodiversity in WA ecosystems.

What is productivity in marine ecosystems?

The rate at which energy is added to the bodies of a group of organisms in the form of biomass.

How is biomass defined in an ecosystem?

The total combined weight of all plant and animal matter within an ecosystem at any given time, often measured in kg/m².

What is energy transfer in marine ecosystems?

The conversion of the sun's energy into chemical energy for movement, growth, reproduction, and repair, transferred within a food chain.

What role do autotrophs play in marine food webs?

They are primary producers that mainly produce energy through photosynthesis.

What are the main types of autotrophs in marine ecosystems?

Photoautotrophs (e.g., seagrasses, algae, coral, phytoplankton) and chemoautotrophs (e.g., nitrogen-fixing bacteria near deep sea vents).

What are heterotrophs in marine ecosystems?

Organisms that obtain their energy solely from the respiration of organic matter.

Who are the primary consumers in marine food webs?

Organisms that consume primary producers, such as zooplankton, bacteria, and filter-feeding animals.

What are examples of second-level consumers in marine ecosystems?

Carnivorous plankton, squid, pelagic fish, and baleen whales.

Name some third, fourth, and fifth level consumers in marine food webs.

Sperm whales, penguins, marine birds, orcas, and seals.

What does the 10% Rule in energy transfer state?

Only 10% of the energy intake is used to build the body; the rest is lost as waste or used for maintenance.

What formula expresses the energy transfer in food webs?

C = P + R + F + U, where C is energy consumed, R is energy used in respiration, F is energy in faeces, U is energy in urine, and P is energy for new biomass production.

What factors affect the efficiency of energy transfer between trophic levels?

How much food is eaten, ease of digestion and nutrient assimilation, energy used for movement, and energy lost in metabolic waste.

Why do consumers generally find it easier to assimilate energy from animals than from producers?

Because the energy in other animals is generally easier to digest and assimilate.

What is the difference between ectothermic and endothermic organisms?

Ectothermic organisms maintain body temperature by exchanging heat with their environment, while endothermic organisms generate heat through metabolic processes.

How does the ocean sunfish exemplify an ectothermic organism?

It does not use energy in respiration to maintain a body temperature higher than the surrounding water.

What is the energy transfer efficiency from sunfish to small sharks compared to tuna?

The efficiency of energy transfer is higher from sunfish because tuna expend more energy in respiration to maintain body temperature.

What are the primary producers in the ocean system?

Phytoplankton, which contribute the highest to carbon sequestration and primary production.

What are some examples of primary producers in marine ecosystems?

Seagrasses, algae (seaweed, kelp), coral, and phytoplankton.

What is the significance of the energy of production (P) in food webs?

It is the energy available to pass on to the next trophic level.

What is the role of small sharks in the marine food web?

They feed on both the sunfish and the tuna, but energy transfer is more efficient from the sunfish.

What happens to the majority of organic matter ingested by heterotrophic organisms?

It is respired back to dissolved inorganic carbon and nutrients.

What is the primary function of primary consumers in marine ecosystems?

To consume primary producers and obtain energy.

What types of organisms are typically found at the top of marine food webs?

Higher-level consumers such as sperm whales, orcas, and seals.

What is the efficiency of energy transfer from ectothermic organisms?

Ranges from 5% to 15%.

What is the efficiency of energy transfer from endothermic organisms?

Ranges from 1% to 5%.

How does the efficiency of transfer between trophic levels affect ecosystems?

Higher efficiency allows for more trophic levels to be supported in the ecosystem.

What characterizes typical biomass pyramids in food webs?

A reduction in biomass of roughly 90% with each trophic level.

What type of ecosystems exhibit 'normal' biomass pyramids?

Nutrient-rich ecosystems with a plant or macroalgae base.

What is a unique characteristic of biomass pyramids in ultra nutrient poor oceanic ecosystems?

The producer biomass (phytoplankton) can be lower than the biomass of higher trophic levels at a given moment.

Why can phytoplankton lead to an unusual biomass pyramid shape?

Due to their high rate of reproduction and short life cycle.

How can gigantic secondary consumers like baleen whales affect biomass pyramids?

They can cause the largest layer to be in the middle of the pyramid.

What effect do large generalist predators like seals have on biomass pyramids?

They may cause top-heavy pyramids by feeding at many trophic levels.

What triggers a trophic cascade?

The addition or removal of a significant number of organisms at a particular trophic level.

What are the consequences of a trophic cascade?

Reciprocal changes in predator and prey populations, leading to dramatic changes in ecosystem structure and nutrient cycling.

What are plankton?

Organisms drifting or floating in water that lack the ability to swim against a current.

What factors are important to consider when studying phytoplankton and zooplankton?

Identification, life cycles, interactions, seasonal patterns, importance to fish stocks, and measurement.

What is the primary difference between phytoplankton and zooplankton?

Phytoplankton are photosynthetic organisms, while zooplankton are heterotrophic animals.

What are phytoplankton and what do they require for growth?

Phytoplankton are photosynthetic organisms that grow in the presence of sunlight and nutrients such as nitrogen and phosphorus.

What role do phytoplankton play in the marine food web?

They serve as the base of the food web, similar to land plants.

What is the photic zone and why is it important for seagrasses and kelp?

The photic zone is the top 100 meters of the ocean where light is available for photosynthesis, crucial for seagrasses and kelp that are anchored to the ocean floor.

How do phytoplankton's single-celled structure benefit their growth?

Their single-celled structure allows light to penetrate their cells and nutrients to be directly absorbed through their membranes, enabling rapid growth.

What are the two major groups of diatoms?

Pennate (long) and centric (round) diatoms.

What distinguishes dinoflagellates from other phytoplankton?

Dinoflagellates have two flagella for movement and can be both photosynthetic and heterotrophic.

What are cyanobacteria and why are they significant?

Cyanobacteria, known as blue-green algae, can fix atmospheric nitrogen into nitrate, making them important for nutrient cycling.

What are holoplankton and give examples?

Holoplankton are organisms that spend their entire life cycle as plankton, including copepods, krill, and ctenophores.

What are meroplankton and how do they differ from holoplankton?

Meroplankton are organisms that are planktonic for only part of their life cycle, often larval stages of larger organisms.

What is the typical lifecycle duration for copepods?

Copepods have a lifecycle from egg to adult that lasts 2-6 weeks, depending on temperature and food availability.

What is the larval duration of WA Rock Lobster and what are their adaptations?

The larval duration can be several years, and they are adapted to floating with a thin, flat, transparent body.

What is the typical food web involving plankton?

Particulate and dissolved organic matter is consumed by phytoplankton, bacteria, and ciliates, which are then grazed on by zooplankton, leading up to fish and larger predators.

How does body size affect the classification of plankton?

Body size is used for classification, with categories including megaplankton, macroplankton, mesoplankton, microplankton, nanoplankton, and picoplankton.

What are the size ranges for megaplankton and macroplankton?

Megaplankton are 20cm and larger, while macroplankton range from 2-20cm.

What are the ecological factors that define plankton habitats?

Plankton habitats are defined largely by temperature, salinity, and nutrient availability (nitrate and phosphate).

What is the significance of the microbial loop in the marine food web?

A significant portion of phytoplankton production is cycled by the microbial community before becoming available to consumers.

What adaptations do meroplanktonic larvae have for dispersal?

Meroplanktonic larvae often bear little resemblance to their adult forms and drift for days or weeks before metamorphosing.

What is the role of mixotrophic ciliates in the food web?

Mixotrophic ciliates contain chloroplasts from ingested phytoplankton and also consume other organisms.

How do zooplankton typically move in the water?

Most zooplankton can swim but not strongly enough to progress against a current.

What are the main components of zooplankton?

Zooplankton includes small multicellular animals, jellyfish, and is dominated by crustaceans.

What is the significance of the diurnal migrations of dinoflagellates?

Many dinoflagellates migrate up and down the water column daily, which can affect their exposure to light and predation.