Fiveable
♻️AP Environmental Science
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♻️AP Environmental Science

FRQ 1 – Experimental Design
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Unit 1: The Living World: Ecosystems
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Practice FRQ 1 of 221/22
1. A freshwater lake ecosystem contains phytoplankton (photosynthetic algae), zooplankton (small herbivorous animals), small fish, and large predatory fish. Energy flows through this ecosystem as organisms consume those at lower trophic levels. The productivity of the ecosystem depends on the availability of limiting nutrients, particularly nitrogen and phosphorus.
A. Describe one process in the nitrogen cycle by which atmospheric nitrogen (N2\text{N}_2N2​) is converted into a form usable by phytoplankton.
B. Explain why phytoplankton biomass is highest near the surface of the lake despite the low nutrient concentrations at shallow depths, as shown in Figures 1 and 2.

Figure 1. Dissolved Nutrient Concentrations at Different Depths in a Freshwater Lake (Nitrogen and Phosphorus)

A clean, two-line scientific plot showing dissolved nutrient concentration versus depth. Axes (all text printed clearly): - Horizontal axis label: "Depth (m)". - Horizontal axis range: from 0 to 40. - Horizontal axis tick marks and printed numbers: 0, 10, 20, 30, 40 (tick interval = 10 m). - Vertical axis label: "Nutrient concentration (mg/L)". - Vertical axis range: from 0.0 to 2.0. - Vertical axis tick marks and printed numbers: 0.0, 0.5, 1.0, 1.5, 2.0 (tick interval = 0.5 mg/L). - The origin is the bottom-left corner where Depth = 0 and Nutrient concentration = 0.0, and the number "0" is printed at this intersection. - Arrows on the positive ends of both axes (right end of the depth axis and top end of the concentration axis). Legend (visible text): - A small legend inside the plotting area in the upper-right corner. - Legend entries: "Nitrogen" and "Phosphorus" with matching line styles. Data and line construction (force exact numeric fidelity): - Plot EXACTLY five measured depths: 0 m, 10 m, 20 m, 30 m, 40 m. - At each of these five depths, place a clearly visible closed circular marker on each nutrient line. - Connect markers using perfectly straight line segments (no smoothing/curving), so the graph is piecewise-linear between the measured depths. Nitrogen series (must be higher than phosphorus at every shown depth): - At depth 0 m, the nitrogen marker sits exactly at 0.2 mg/L. - At depth 10 m, the nitrogen marker sits exactly at 0.5 mg/L. - At depth 20 m, the nitrogen marker sits exactly at 0.9 mg/L (this intersection must be easy to read because Part C asks for nitrogen at 20 m). - At depth 30 m, the nitrogen marker sits exactly at 1.4 mg/L. - At depth 40 m, the nitrogen marker sits exactly at 1.8 mg/L. - Visual behavior: a monotonic increase with depth, drawn as four straight rising segments. Phosphorus series: - At depth 0 m, the phosphorus marker sits exactly at 0.1 mg/L. - At depth 10 m, the phosphorus marker sits exactly at 0.3 mg/L. - At depth 20 m, the phosphorus marker sits exactly at 0.5 mg/L. - At depth 30 m, the phosphorus marker sits exactly at 0.7 mg/L. - At depth 40 m, the phosphorus marker sits exactly at 0.9 mg/L. - Visual behavior: a monotonic increase with depth, drawn as four straight rising segments. Relative placement constraints to eliminate ambiguity: - At every one of the five depth ticks, the nitrogen marker is vertically above the phosphorus marker. - At the left edge (depth 0), both series are close to the bottom of the y-range, with nitrogen at 0.2 and phosphorus at 0.1. - At the right edge (depth 40), the nitrogen marker is near the top of the y-range at 1.8, while phosphorus remains below the midline at 0.9. Styling (to keep the figure readable): - Nitrogen line: solid, dark (black or dark blue), medium thickness. - Phosphorus line: dashed, dark gray or dark green, medium thickness. - No gridlines. - No extra title text beyond the caption; only axes labels, tick numbers, and the legend appear as text.
C. Based on the data in Figure 1, identify the nitrogen concentration at a depth of 20 meters.

Figure 2. Phytoplankton Biomass at Different Depths in a Freshwater Lake

A single-line scientific plot showing phytoplankton biomass versus depth. Axes (all text printed clearly): - Horizontal axis label: "Depth (m)". - Horizontal axis range: from 0 to 40. - Horizontal axis tick marks and printed numbers: 0, 5, 10, 20, 30, 40 (tick marks at exactly these six depths; this includes 5 m because the maximum occurs there). - Vertical axis label: "Phytoplankton biomass (mg/L)". - Vertical axis range: from 0 to 120. - Vertical axis tick marks and printed numbers: 0, 20, 40, 60, 80, 100, 120 (tick interval = 20 mg/L). - The origin is the bottom-left corner where Depth = 0 and Biomass = 0, and the number "0" is printed at this intersection. - Arrows on the positive ends of both axes (right end of the depth axis and top end of the biomass axis). Data and line construction (force exact numeric fidelity): - Plot EXACTLY six measured depths: 0 m, 5 m, 10 m, 20 m, 30 m, 40 m. - At each of these six depths, draw a clearly visible closed circular marker. - Connect markers using perfectly straight line segments (no smoothing/curving), creating a piecewise-linear profile. Exact biomass values at each measured depth: - At depth 0 m, the marker sits exactly at 95 mg/L. - At depth 5 m, the marker sits exactly at 110 mg/L. - At depth 10 m, the marker sits exactly at 85 mg/L. - At depth 20 m, the marker sits exactly at 52 mg/L. - At depth 30 m, the marker sits exactly at 18 mg/L. - At depth 40 m, the marker sits exactly at 3 mg/L. Curve shape description (using the piecewise-linear constraint): - Segment from 0 m to 5 m: straight line rising from 95 to 110. - The graph reaches its single highest point (global maximum) exactly at the 5 m tick, at 110 mg/L. This maximum is a sharp corner (because straight segments meet) rather than a rounded peak. - Segment from 5 m to 10 m: straight line falling from 110 to 85. - Segment from 10 m to 20 m: straight line falling from 85 to 52. - Segment from 20 m to 30 m: straight line falling from 52 to 18. - Segment from 30 m to 40 m: straight line falling from 18 to 3. Relationship constraint for Part D (below 5 m): - For all plotted depths deeper than 5 m (10, 20, 30, 40), each successive marker must be lower than the previous one, producing a strictly decreasing trend with increasing depth below 5 m. Styling (to keep the figure readable): - Single biomass line: solid black, medium thickness. - No gridlines. - No legend needed (only one line). - No extra title text beyond the caption; only axes labels and tick numbers appear as text.
D. Based on the data in Figure 2, describe the relationship between depth and phytoplankton biomass below 5 meters.
E. Describe how much biomass of large predatory fish could be supported by the phytoplankton, assuming 10% energy transfer efficiency between each trophic level. Show all work.
F.
i. Identify the independent variable in the students' investigation.
ii. Identify one variable that was held constant in the experimental design.
Lake ZoneSpecies 1Species 2Species 3Species 4Species 5Species 6
High nutrient zoneXX
Low nutrient zoneXXXXXX
G.
i. Explain why the low nutrient zone would be more resilient to environmental disturbances, such as temperature changes or disease outbreaks, than the high nutrient zone would.
ii. Explain one reason why excess phosphorus from agricultural runoff could lead to decreased dissolved oxygen concentrations in the lake, negatively affecting fish populations.
H. Describe one environmental consequence of deforestation in the watershed surrounding the lake on the phosphorus cycle.
Pep