Earth Science Quiz: Marine Ecosystems

What this quiz covers

This quiz focuses on Marine Ecosystems, giving you a quick way to practice the rules, question types, and explanations that matter most for Earth Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. The presence of salt-tolerant terrestrial plants along the banks.
2. The constant cleansing effect of strong tidal currents that flush out sediments.
3. The shallow average depth of the water column and high nutrient replenishment from rivers. (correct answer)
4. The high water temperatures that accelerate the metabolic rates of phytoplankton.

Explanation: The paradox of high productivity in turbid estuaries is resolved by two main factors. First, estuaries are typically shallow, meaning that a significant portion of the water column remains within the euphotic zone despite high turbidity. Second, and most importantly, rivers provide a continuous and abundant supply of nutrients. This constant replenishment ensures that producers are rarely nutrient-limited and can grow rapidly whenever light is sufficient, leading to high overall productivity.

Question 2

1. surface convergence and downwelling, which creates a strong, permanent thermocline that isolates surface waters from deep nutrients. (correct answer)
2. a lack of iron deposition from atmospheric dust, which is the primary limiting nutrient in these warm-water systems.
3. intense grazing pressure from microzooplankton that consumes phytoplankton as rapidly as it can grow.
4. strong geostrophic currents that rapidly disperse phytoplankton, preventing the accumulation of biomass required for a bloom.

Explanation: Subtropical gyres are characterized by converging surface waters that lead to downwelling. This process creates a very stable and deep thermocline. This strong stratification acts as a barrier, preventing the upward mixing of nutrient-rich deep water into the sunlit euphotic zone. As a result, surface waters are permanently nutrient-depleted (specifically in nitrogen), leading to very low primary productivity.

Question 3

1. 2,500 kg (correct answer)
2. 25,000 kg
3. 2.5 x 10^5 kg
4. 2.5 x 10^4 kg

Explanation: This requires a three-step calculation based on a 10% (or 0.10) trophic efficiency:

1. Producers (Phytoplankton): 2.5 x 10^6 kg
2. Primary Consumers (Zooplankton): (2.5 x $10^6$ kg) * 0.10 = 2.5 x 10^5 kg
3. Secondary Consumers (Small Fish): (2.5 x $10^5$ kg) * 0.10 = 2.5 x 10^4 kg
4. Tertiary Consumers (Larger Fish): (2.5 x $10^4$ kg) * 0.10 = 2.5 x 10^3 kg, which is 2,500 kg.

Question 4

1. Satellite data consistently shows low chlorophyll concentrations in the region despite the presence of strong upwelling currents.
2. Shipboard measurements confirm that surface waters in the area have high concentrations of nitrate and phosphate but very low concentrations of dissolved iron.
3. Adding iron sulfate to a large patch of the ocean surface results in a significant increase in chlorophyll concentration relative to an untreated control patch. (correct answer)
4. Genetic analysis of the phytoplankton community reveals the presence of species known to have high iron requirements.

Explanation: While options A, B, and D provide correlational evidence that is consistent with the hypothesis, they do not prove causation. The strongest evidence comes from a direct manipulative experiment. The large-scale iron fertilization experiment (option C) directly tests the hypothesis by adding the proposed limiting factor and observing the ecosystem's response. A resulting phytoplankton bloom demonstrates that iron was indeed the factor limiting growth.

Question 5

1. total phytoplankton respiration within the mixed layer exceeds the total nutrient supply from below.
2. the total amount of photosynthesis integrated over the entire mixed layer equals the total amount of respiration integrated over the same layer. (correct answer)
3. the light intensity is precisely 1% of its surface value, marking the bottom of the euphotic zone.
4. the grazing rate of zooplankton on phytoplankton equals the phytoplankton growth rate throughout the mixed layer.

Explanation: The critical depth concept, developed by Harald Sverdrup, addresses the net productivity of the entire water column. While individual cells mixed below the compensation depth are losing carbon (respiration > photosynthesis), a bloom can be sustained as long as the net carbon gain of the cells in the upper part of the mixed layer is greater than the net carbon loss of the cells in the lower part. The critical depth is the point where these two integrated values are equal (total production = total respiration). For a bloom to occur, the mixed layer must be shallower than this critical depth.

Question 6

1. GPP = The final oxygen in the light bottle - the final oxygen in the dark bottle.
2. GPP = The final oxygen in the light bottle - the initial oxygen concentration.
3. GPP = The initial oxygen concentration - the final oxygen in the dark bottle.
4. GPP = The net oxygen change in the light bottle + the amount of oxygen consumed in the dark bottle. (correct answer)

Explanation: Gross Primary Production (GPP) is the total amount of photosynthesis. Net Community Production (NCP) is what's left after all organisms in the bottle have respired (GPP - R). The dark bottle measures only respiration (R) as a loss of oxygen. To find the total amount of oxygen produced by photosynthesis (GPP), one must add the oxygen that was consumed by respiration back to the net amount that accumulated. Therefore, GPP = NCP + R, which is calculated as the oxygen change in the light bottle plus the oxygen loss in the dark bottle.

Question 7

1. An increase in water clarity, leading to the rapid expansion of benthic kelp forests and seagrass beds.
2. A significant decrease in phytoplankton biomass, followed by a subsequent collapse in zooplankton and small fish populations. (correct answer)
3. A shift in the dominant primary producers from large diatoms to smaller, nitrogen-fixing cyanobacteria.
4. An increase in the concentration of dissolved oxygen in the surface waters due to reduced bacterial respiration.

Explanation: Coastal upwelling brings cold, nutrient-rich deep water to the sunlit surface, fueling high primary productivity. If the winds driving this process stop, the nutrient supply is cut off. This would lead to a rapid crash in the phytoplankton population (the base of the food web), which in turn would cause a decline in the populations of zooplankton and fish that depend on them.

Question 8

1. An expansion of oxygen minimum zones due to increased respiration at depth and a reduction in atmospheric CO2. (correct answer)
2. A decrease in surface nutrient concentrations leading to a long-term decline in global ocean productivity.
3. An increase in deep-ocean oxygen levels due to the transport of oxygen-rich surface water to the abyss.
4. A rise in the pH of deep ocean waters and a corresponding deepening of the carbonate compensation depth (CCD).

Explanation: A more efficient biological pump means more organic matter sinks from the surface. As this matter is decomposed by aerobic bacteria at depth, they consume dissolved oxygen, leading to the creation or expansion of oxygen minimum zones. Concurrently, by sequestering carbon in the deep ocean, the pump reduces the amount of CO2 in the surface ocean that can exchange with the atmosphere, thus leading to a reduction in atmospheric CO2 concentrations.

Question 9

1. Coccolithophores
2. Cyanobacteria
3. Dinoflagellates
4. Diatoms (correct answer)

Explanation: Different phytoplankton groups have different nutrient requirements for building their cellular structures. Diatoms are unique among the major groups in that they construct intricate cell walls, called frustules, out of silica (hydrated silicon dioxide). Therefore, the near-complete depletion of dissolved silicate from the water column is a clear indicator that the bloom was composed primarily of diatoms, which consumed the available silicate to build their frustules.

Question 10

1. bottom-up control, where nutrient availability dictates ecosystem structure.
2. a trophic cascade, where the impact of a top predator influences multiple lower trophic levels. (correct answer)
3. interspecific competition between sea otters and sea urchins for kelp.
4. a regime shift caused by eutrophication and subsequent hypoxia.

Explanation: This scenario describes a trophic cascade, an ecological phenomenon triggered by the addition or removal of top predators which results in dramatic, reciprocal changes in the relative populations of predator and prey through a food chain. The removal of the top predator (sea otters) caused a population increase in the primary consumer (sea urchins), which then led to a decrease in the primary producer (kelp). This is a top-down control mechanism.

Question 11

1. the exceptionally deep photic zone in clear open-ocean waters, which supports a large volume of producers.
2. the immense total surface area of the open ocean, which multiplies to a large total production despite the low rate. (correct answer)
3. the presence of highly efficient nitrogen-fixing bacteria that fertilize the open ocean surface.
4. the historically significant underestimation of picoplankton, which are the true drivers of open ocean productivity.

Explanation: This is a matter of rate versus total area. While the rate of production (e.g., in grams of carbon per square meter per year) is very low in the nutrient-poor open ocean, this biome covers roughly 70% of the Earth's surface. When this low rate is integrated over such a vast area, the total amount of carbon fixed is enormous, rivaling that of all terrestrial ecosystems combined.

Question 12

1. The upwelling of ancient, anoxic deep water which has not been in contact with the atmosphere for centuries.
2. The aggregation of large fish and zooplankton at these depths, whose collective respiration consumes the available oxygen.
3. A strong pycnocline that completely prevents the downward diffusion of atmospheric oxygen from the surface.
4. The aerobic decomposition of large quantities of sinking organic matter exported from the productive surface waters. (correct answer)

Explanation: High surface productivity generates a large amount of organic matter (dead phytoplankton, fecal pellets, etc.) that sinks into the water column. As this organic matter sinks, it is decomposed by aerobic bacteria. This decomposition process consumes large amounts of dissolved oxygen. The water at intermediate depths is often isolated from the surface by stratification, so the consumed oxygen is not quickly replenished, resulting in an Oxygen Minimum Zone.

Question 13

1. excess nutrients; reduces light penetration needed for photosynthesis
2. abnormally high water temperatures; inhibits the coral's ability to build its calcium carbonate skeleton (correct answer)
3. viral infections in the coral polyp; dissolves the existing coral skeleton
4. sedimentation from coastal runoff; smothers the coral and prevents feeding

Explanation: This question tests the distinction between two major threats to corals. Coral bleaching is a physiological stress response, primarily caused by sustained periods of anomalously warm water, which causes the symbiotic relationship with zooxanthellae to break down. Ocean acidification, caused by the ocean absorbing atmospheric CO2, lowers the pH and reduces carbonate ion concentration, making it more difficult for corals and other calcifying organisms to build their skeletons and shells.

Question 14

1. 2.5
2. 3.0
3. 3.5 (correct answer)
4. 4.0

Explanation: First, determine the trophic levels of the prey: Herbivorous copepods (primary consumers) are at Trophic Level (TL) 2. Anchovies that eat copepods (secondary consumers) are at Trophic Level (TL) 3. The fish's trophic level is 1 plus the weighted average of its preys' trophic levels. Average prey TL = (0.50 * TL_copepod) + (0.50 * TL_anchovy) = (0.50 * 2) + (0.50 * 3) = 1.0 + 1.5 = 2.5. The fish's TL is 1 + 2.5 = 3.5.

Question 15

1. localized nutrient release from sinking whale carcasses creating temporary 'oases' of productivity on the sea floor.
2. phytoplankton actively swimming and aggregating in response to chemical cues from potential mates.
3. the transport, mixing, and concentration of plankton populations by physical ocean processes like currents, fronts, and eddies. (correct answer)
4. patterns of intense, localized grazing by large schools of fish and zooplankton swarms.

Explanation: While biological factors (growth, grazing) are crucial, the large-scale spatial patterns of phytoplankton seen from space are predominantly shaped by physical oceanography. Ocean currents, eddies (swirls), and fronts (boundaries between water masses) act to transport and concentrate plankton. These physical processes create the complex and dynamic patterns of 'patchiness' by controlling the distribution of both the phytoplankton themselves and the nutrients they need to grow.

Question 16

1. surface convergence and downwelling, which creates a strong, permanent thermocline that isolates surface waters from deep nutrients. (correct answer)
2. a lack of iron deposition from atmospheric dust, which is the primary limiting nutrient in these warm-water systems.
3. intense grazing pressure from microzooplankton that consumes phytoplankton as rapidly as it can grow.
4. strong geostrophic currents that rapidly disperse phytoplankton, preventing the accumulation of biomass required for a bloom.

Explanation: Subtropical gyres are characterized by converging surface waters that lead to downwelling. This process creates a very stable and deep thermocline. This strong stratification acts as a barrier, preventing the upward mixing of nutrient-rich deep water into the sunlit euphotic zone. As a result, surface waters are permanently nutrient-depleted (specifically in nitrogen), leading to very low primary productivity.

Question 17

1. 2,500 kg (correct answer)
2. 25,000 kg
3. 2.5 x 10^5 kg
4. 2.5 x 10^4 kg

Explanation: This requires a three-step calculation based on a 10% (or 0.10) trophic efficiency:

1. Producers (Phytoplankton): 2.5 x 10^6 kg
2. Primary Consumers (Zooplankton): (2.5 x $10^6$ kg) * 0.10 = 2.5 x 10^5 kg
3. Secondary Consumers (Small Fish): (2.5 x $10^5$ kg) * 0.10 = 2.5 x 10^4 kg
4. Tertiary Consumers (Larger Fish): (2.5 x $10^4$ kg) * 0.10 = 2.5 x 10^3 kg, which is 2,500 kg.

Question 18

1. utilize heat energy from the vent fluids to synthesize organic molecules.
2. are adapted to extreme pressures but otherwise function like photosynthetic algae.
3. derive energy by oxidizing reduced chemical compounds like hydrogen sulfide. (correct answer)
4. fix nitrogen gas into ammonia, which is then consumed by other organisms.

Explanation: The primary producers at hydrothermal vents are chemosynthetic bacteria and archaea. In the complete darkness of the deep sea, they cannot perform photosynthesis. Instead, they harness chemical energy released from the oxidation of reduced inorganic compounds, such as hydrogen sulfide (H2S), which are abundant in the vent fluids. This process, called chemosynthesis, allows them to fix carbon and form the base of the vent ecosystem.

Question 19

1. The freshwater runoff lowered the salinity of the surface layer, which directly triggered the rapid division of dormant algal cysts.
2. The influx of terrestrial nutrients (nitrates, phosphates) fueled explosive algal growth in a stable, sunlit surface layer created by the calm weather. (correct answer)
3. The calm weather prevented winds from dispersing the algae, allowing them to become concentrated enough to be visible as a 'red tide'.
4. The sunny conditions increased water temperature, which allowed the warm-water dinoflagellate species to outcompete other phytoplankton.

Explanation: Harmful algal blooms are typically triggered by a combination of factors. Agricultural runoff provides a massive pulse of limiting nutrients (eutrophication). Calm, sunny weather promotes thermal stratification, creating a stable surface layer that keeps the phytoplankton in the sunlit zone, and provides the light energy for photosynthesis. This combination of abundant nutrients and optimal light/stability conditions allows for exponential growth and the formation of a bloom.

Question 20

1. Coccolithophores
2. Cyanobacteria
3. Dinoflagellates
4. Diatoms (correct answer)

Explanation: Different phytoplankton groups have different nutrient requirements for building their cellular structures. Diatoms are unique among the major groups in that they construct intricate cell walls, called frustules, out of silica (hydrated silicon dioxide). Therefore, the near-complete depletion of dissolved silicate from the water column is a clear indicator that the bloom was composed primarily of diatoms, which consumed the available silicate to build their frustules.