What do we mean by a terrestrial and aquatic ecosystem?

• Simple answer: Land vs water

• However the boundaries are not clear cut

• Systems can have some characteristics of both

  • Marshes
  • Coastal ecosystems
  • Rivers and streams
  • Shallow lakes

Rooted vs unrooted autotrophs (macrophytes and plankton)

• One clear distinction involves the form of the primary producers
• Marine ecosystems: Almost all unrooted (phytoplankton)
• Terrestrial ecosytems: Almost all rooted (vascular plants)
• Rivers, streams, lakes, saltmarshes; Mixture of both

Why does this matter?

• The trophic webs formed when autotrophs are rooted vs unrooted differ (Sayer, Davidson, and Jones 2010)
• Unrooted autotrophs (plankton) tend to be very small
• Rooted autotrophs (vascular plants) can be very large
• Primary productivity accumulates in different ways

A rather poor, simplistic view

• The internet (and school textbook) type figures don’t provide a very complete picture

A more sophisticated view

(Shurin, Gruner, and Hillebrand 2005) (Chase 2000)

What are the key differences?

• The autotroph compartment in terrestrial systems tends to be larger

  • More primary productivity locked up in plants (Roy and Saugier 2001)

• The herbivore (primary consumer) compartment in terrestrial systems tends to be comparatively smaller

  • Less direct consumption of the primary productivity (Kareiva 1983)

Why is the ocean blue?

Chlorophyll drives net primary productivity

• NPP (net primary productivity) almost all the result of photosynthesis
• The most productive areas of terrestrial ecosystems are the greenest
• NPP in terrestrial systems limited by water availability
• Where water is available on land most incoming light is intercepted by leaves.
• However, only a tiny fraction of the sea is photosynthesising (most is just watery space between the chloroplasts)

What does primary productivity rely on?

• Sunlight

• Carbon dioxide

• Water

• Nitrogen

• Phosphorous

• Potassium

• Other trace elements (including iron in seawater)

What is similar in availability?

• Sunlight

• Carbon dioxide

• Water?

  • Generally present on land, but can be in short supply

What differs in availability?

• Net primary productivity in aquatic systems often limited by nutrients

• Net primary productivity in terrestrial systems can be limited by water availability

• Sunlight availability varies in space and time for both systems

Where is nutrient limitation greatest?

• Nitrogen, phosphates (and iron) limit phytoplankton growth in marine ecosystems

• Inflow from the land adds nutrients

• Volcanic activity adds some nutrients (Weis 2015)

• Ocean currents can redistribute nutrients (Joseph 2014)

Does temperature limit productivity?

• Not much.
• Photosynthesis tends to stop below 6C
• Temperate terrestrial ecosystems highly productive (as water is usually available)
• High latitude oceans highly productive (as nutrients usually available)
• Tropical systems tend to be more diverse, but less productive

Example for phytoplankton

(Fernández-González et al. 2022)

Freshwater ecosystems have higher NPP than marine ecosystems

Eutrophic vs oligotrophic

• Lakes can be classified as eutrophic, mesotrophic or oligotrophic
• Very high nutrient concentrations can be very problematic
• Autotrophs also use oxygen, which can lead to fish deaths

Differences in nutritive quality of primary production

• Sometimes referred to as nutrient stoichiometry

• Phytoplankton tend to be a rich source of food

  • Phytoplankton lack structural and connective tissue

  • Most of the fixed carbon in phytoplankton can be digested and passed on as energy

• Terrestrial vegetation, particularly trees, is a poor food source

  • Lignin virtually indigestible for most higher organisms

  • Cellulose very hard to digest

  • Terrestrial herbivores must break down large amounts of cellulose to get at cellular content

Much of the terrestrial NPP passed to decomposers

• Structural material forms “detritus” in terrestrial ecosystems

• Mainly decomposed in the soil

• Fungi are very important (no truly marine fungi)

Aquatic NPP mainly passes on through zooplankton

• Zooplankton present in both freshwater and marine ecosystems

• Large variety of different organisms (including unicellular)

• Small zooplankton feed directly on phytoplankton

Formalising the differences

1. Size structure
2. Growth rate
3. Nutrient stoichiometry (digestibility)

Size structure

• Aquatic food webs more strongly size-structured than terrestrial (particularly pelagic)

• Positive correlations between body size and trophic position.

  • Small at the bottom, large at the top in aqautic systems
  • Less clearcut in terrestrial ecosystems. Herbivores may be larger than carnivores.

• Benthic food webs involving macrophytes may be closer to terrestrial wood webs

Growth rate

• Producers fix carbon at similar absolute rates (although depends on nutrient and water availability)
• Less material is stored in living biomass in phytoplankton communities than in forests or grasslands
• Primary producers replace tissues at a faster rate in water than on land.
• Aquatic macrophytes have higher growth rates than terrestrial plants (water never limited)

Nutrient stoichiometry

• Phytoplankton are composed almost entirely of nutrient-rich (high N and P) photosynthetic material.
• Consumers have high demands for N and P relative to supply from the primary producers
• Terrestrial herbivores face a major potential nutritional deficit.
• Aquatic primary consumers main challenge is finding food, not digesting it

One very key difference

• Herbivorous zooplankton in lakes remove a three to four times greater proportion of primary productivity than grazers in terrestrial systems
• The efficiencies of herbivores removing plant material and converting it to their own biomass are greatest in lake (and marine) plankton, lowest in forests and intermediate in grasslands.
• Differences in the plant–herbivore link rather than the overall supply of energy govern trophic structure variation

Trophic complexity

• Terrestrial foodwebs apparently have only three functional trophic levels (plants,herbivores and primary predators),
• Pelagic zones of lakes and seas have piscivorous fishes that occupy a fourth trophic level (plus large marine mammals and very large fish that prey on them making a fifth level)
• However, this is an over simplification that ignores detritus and decomposition

Bottom up control vs top down control

• Both systems display a mixture of both

• Mid trophic level consumers in aquatic systems are controlled more from the top down than in terrestrial systems

  • Fish produce thousands of fry, very few reach maturity due to predation (top down)
  • Terrestrial herbivores produce fewer young, many don’t reach maturity due to shortage of food (bottom up).

• However net primary productivity limits total biomass in both systems

  • NPP in terrestrial systems largely limited by water availability
  • NPP in aquatic systems largely limited by nutrient availability

Consequences for human exploitation of ecosystems

• In terrestrial ecosystems (either artificial or natural) humans consume primary producers (plants) and primary consumers (herbivores)
• In aquatic ecosystems humans generally consume organisms at higher trophic levels (predatory fish, large crustaceans)

ChatGTPs answer (mainly irrelevant!)

What are the key differences between terrestrial and aquatic ecosystems?

🗒️ Answer

Terrestrial and aquatic ecosystems are two fundamental types of ecosystems that support diverse communities of organisms. They differ significantly in their physical and chemical environments, leading to distinct adaptations and interactions among the organisms that inhabit them. Here’s a summary of the key differences between terrestrial and aquatic ecosystems:

1. Medium:

• Terrestrial ecosystems: Occur on land, characterized by soil, air, and varying levels of moisture.

• Aquatic ecosystems: Found in water bodies, including oceans, lakes, rivers, and wetlands.

2. Buoyancy:

• Terrestrial ecosystems: Organisms must support their own weight against gravity.

• Aquatic ecosystems: Buoyancy provided by water reduces the need for heavy support structures.

3. Gas Exchange:

• Terrestrial ecosystems: Organisms primarily rely on diffusion of oxygen and carbon dioxide through air.

• Aquatic ecosystems: Organisms extract oxygen from water through gills or specialized adaptations.

4. Temperature Regulation:

• Terrestrial ecosystems: Organisms face greater temperature fluctuations due to exposure to air and sunlight.

• Aquatic ecosystems: Water has a high heat capacity, moderating temperature fluctuations.

5. Light Availability:

• Terrestrial ecosystems: Light availability varies with latitude, season, and vegetation cover.

• Aquatic ecosystems: Light availability decreases with depth, limiting photosynthesis in deeper waters.

6. Salinity:

• Terrestrial ecosystems: Salinity is generally low, except in some arid regions.

• Aquatic ecosystems: Salinity varies from freshwater to saltwater environments, affecting organism distribution.

7. Productivity:

• Terrestrial ecosystems: Primary productivity is generally lower than in aquatic ecosystems due to limited nutrient availability.

• Aquatic ecosystems: Nutrient-rich water supports higher rates of primary productivity, forming the base of aquatic food webs.

These fundamental differences shape the adaptations and interactions of organisms in terrestrial and aquatic ecosystems, leading to the rich biodiversity and ecological processes observed in these diverse environments.

🌐 Sources

1. Terrestrial and Aquatic Ecosystems | principlesof-ecology: https://emmabriggs.wixsite.com/principlesof-ecology/terrestrial-and-aquatic-ecosystems

2. All wet or dried up? Real differences between aquatic and terrestrial biomes: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1560001/