OCEAN 100
LAB 9 INVESTIGATION: MARINE ECOSYSTEMS  | NAME: Sabrina Ruiz
SECTION:MARINE ECOSYSTEMS
An ecosystem consists of a group of living organisms, the physical environment in which they live, and an energy
source (e.g., sunlight in photosynthesis-based ecosystems). The largest ecosystem can be considered to be the earth as a whole,
but the planet may be subdivided into terrestrial and marine ecosystems, and each of these may be further subdivided, often on
the basis of environmental conditions (e.g., depth, temperature, etc.). Within most ecosystems, we find organisms that produce
their own food (known as primary producers or autotrophs), organisms that consume other primary producers to produce their
own food (known as primary consumers, such as herbivores, or secondary consumers, such as carnivores, or more simply and
generally as heterotrophs), and organisms that decompose dead organic matter and waste (known as decomposers; generally
fungi and bacteria). Ecosystems have two fundamental properties (Figure 1). The first property is that energy transfer through
living organisms is one-way: Energy is received from the sun, transformed into organic material by primary producers, and
captured by various consumers and decomposers through feeding. Energy within living creatures is eventually lost to the
environment as heat. Depending on the complexity of the ecosystem, this one-way transfer of energy may be represented by a
trophic chain or trophic web. The second property is that nutrients, those elements necessary for life’s metabolic processes, are
recycled many times within most ecosystems. Decomposers play a crucial role in this recycling through their feeding upon
dead organic matter, which releasing nutrients back to the environment where they can be used for new primary production
from newly captured energy.
TROPHIC PYRAMIDS AND WEBS: EXAMPLES FROM THE ANTARCTIC OCEAN
The trophic relationships among the major organisms within the Antarctic Ocean can be summarized as a trophic web
(Figure 2). Microplankton, bacteria, and macroalgae are the primary producers upon which all organisms ultimately depend,
and this role is graphically depicted in two ways: Through arrows indicating the direction of energy flow via feeding and
through their grouping into a first trophic tier or level. As indicated by the arrow, these primary producers are consumed by the
primary consumers (i.e., copepods, krill, protozoans, benthic invertebrates) in the second tier, which in turn are consumed by
organisms on the third and higher trophic tiers. Note that a given organism can show multiple trophic strategies. For example,
krill act as primary consumers when feeding upon primary producers (e.g., microplankton) and as secondary consumers when
feeding on primary consumers (e.g., protozoans). Thus, the assignment of a given organism to a specific trophic tier is typically
based on its average trophic strategy (i.e., krill predominantly feed upon microplankton).
This averaging approach to assigning trophic levels becomes increasing important at higher trophic levels, where a
greater diversity of organisms feed on a greater variety of lower trophic levels. For example, leopard seals may be assigned to
the fourth trophic level when feeding upon pelagic fish or squid or the fifth trophic level when feeding upon penguins.
However, leopard seals are typically assigned to the fifth trophic level because they mainly feed on penguins (fourth trophic
level) which mainly feed on squid (third trophic level) which mainly feed on krill (second trophic level) which mainly feed on
microplankton (first trophic level) — wheew! Similarly, the killer whale technically belongs to the fifth trophic level when
feeding upon penguins and to the sixth trophic level when feeding on leopard seals. However, killer whales mainly feed on
leopard seals and are therefore assigned to the sixth trophic level. Note that some organisms may shift their trophic position
during their lifetime; for example, as individuals of a fish species grow larger, they may start feeding on organisms in higher
trophic levels, leading to a shift in their own assigned trophic level.
The actual population size of a given organism can be limited by the trophic level below it or by the trophic level
above it. “Bottom-up” control is when population size is limited by food supply (e.g., not enough food is available for the
population to increase beyond a certain abundance). “Top-down” control is when population size is limited by predator
abundance (e.g., population has enough food to grow larger, but it is kept smaller by predators).
ECOLOGICAL EFFICIENCY, BIOACCUMULATION, AND BIOMAGNIFICATION
Recall that energy contained within an ecosystem is not recycled, but is transferred one-way through successively
higher trophic levels. Within all organisms and any given trophic level, most of the obtained energy is used for respiration and
metabolism and is ultimately lost as heat; only a small portion of the energy is transformed into biomass through growth. In
addition, not all organisms (and therefore all biomass) in a lower trophic level will be consumed by organisms in the next
higher trophic level; some will die of natural causes. Such loss of energy within and between trophic levels can be examined in
terms of ecological efficiency, which describes the percentage of biomass in one trophic level that is converted into biomass in
the next higher trophic level. Ecological efficiency averages around 10% in the ocean. For example, in a simple food chain
with 10% ecological efficiency and consisting of diatoms (primary producers), krill (primary consumers), and blue whales
(secondary consumers), one would predict that ~100 grams of diatoms would be required to support ~10 grams of krill, and
~10 grams of krill would be required to support ~1 gram of blue whale. Appreciate that the average blue whale weighs ~100
metric tons (= 100,000 kg = 100,000,000 g = 108 g), which in turn would require a billion grams of krill (i.e., 1,000,000,000 g