AP Environmental Science FRQ Practice

♻️AP Environmental Science

Guided Practice

Practice FRQ 1 of 221/22

1. Scientists studied two temperate freshwater lakes to investigate how nutrient availability affects ecosystem productivity and food web structure. Lake A is oligotrophic with low nutrient concentrations, while Lake B is mesotrophic with moderate nutrient concentrations. Both lakes are located in the same climate region and have similar physical characteristics including depth, surface area, and temperature profiles.

A. Describe the process of nitrogen fixation and identify one organism that performs this process in aquatic ecosystems.

B. Explain how phosphorus availability can limit primary productivity in freshwater lake ecosystems even when nitrogen is abundant.

Figure 1. Nitrogen and phosphorus concentrations (mg/L) in two temperate freshwater lakes (Lake A oligotrophic; Lake B mesotrophic). Each lake displays two adjacent bars: total nitrogen and total phosphorus; vertical error bars show ±1 standard error.

Grouped bar graph titled exactly: "Nitrogen and Phosphorus Concentrations in Lake A and Lake B".

Axes and scale (must be exact):
- X-axis label: "Lake".
- X-axis has exactly two category groups, left to right: "Lake A" then "Lake B".
- X-axis tick marks: one centered tick under each group label (one under "Lake A", one under "Lake B"). No other x-axis tick labels.
- Y-axis label: "Nutrient Concentration (mg/L)".
- Y-axis range: from 0.00 to 0.50.
- Y-axis tick marks and labels at exactly: 0.00, 0.10, 0.20, 0.30, 0.40, 0.50.
- The origin is shown at the bottom-left of the plotting area and the y-axis tick label "0.00" is present at the baseline.
- No arrows on the positive ends of either axis.
- Optional faint horizontal gridlines aligned with each y-axis tick value (0.10 increments); if gridlines are shown, they are light gray and do not obscure bars.

Bar layout (must be exact and unambiguous):
- Each lake group contains exactly two vertical bars placed side-by-side with a small gap between them.
- All bars share the same width (uniform bar width) and the same black outline (solid black outline, medium stroke thickness). Bars are fully opaque (no transparency).
- Within each lake group: the left bar is Nitrogen; the right bar is Phosphorus.

Color/legend (must be exact):
- Nitrogen bars: solid dark blue fill.
- Phosphorus bars: solid light blue fill.
- A legend is placed inside the plotting area in the upper-right corner showing two color keys labeled exactly: "Nitrogen" (dark blue) and "Phosphorus" (light blue).

Exact bar heights and exact error bars (all error bars must be non-zero):
- Lake A, Nitrogen (dark blue, left bar in Lake A group): bar height = 0.12 mg/L. Error bar is centered on the bar top and extends from 0.11 to 0.13 mg/L (±0.01). Error bar is a thin black vertical line with horizontal caps.
- Lake A, Phosphorus (light blue, right bar in Lake A group): bar height = 0.008 mg/L. Error bar extends from 0.007 to 0.009 mg/L (±0.001).
- Lake B, Nitrogen (dark blue, left bar in Lake B group): bar height = 0.45 mg/L. Error bar extends from 0.41 to 0.49 mg/L (±0.04).
- Lake B, Phosphorus (light blue, right bar in Lake B group): bar height = 0.022 mg/L. Error bar extends from 0.020 to 0.024 mg/L (±0.002).
- Error bar cap width: each cap spans roughly one-half of the corresponding bar’s width (caps clearly visible and symmetric).

Text rules for readability:
- All tick labels use two decimal places on the y-axis (including 0.00).
- No numerical value labels are printed above the bars (values are read from the axis).

C. Based on the data in Figure 1, identify the phosphorus concentration in Lake B.

Figure 2. Biomass (g/m²) at four trophic levels in Lake A versus Lake B. Each trophic level displays two adjacent bars (Lake A and Lake B); vertical error bars show ±1 standard error.

$Grouped bar graph titled exactly: "Biomass at Different Trophic Levels in Lake A and Lake B". Axes and scale (must be exact): - X-axis label: "Trophic Level". - X-axis shows exactly four category groups, left to right in this exact order: "Primary Producers", "Primary Consumers", "Secondary Consumers", "Tertiary Consumers". - X-axis tick marks: one centered tick directly beneath each trophic-level label; no additional x-axis tick labels. - Y-axis label: "Biomass (g/m²)". - Y-axis range: from 0 to 180. - Y-axis tick marks and labels at exactly: 0, 20, 40, 60, 80, 100, 120, 140, 160, 180. - The origin is shown at the bottom-left of the plotting area and the y-axis tick label "0" is present at the baseline. - No arrows on the positive ends of either axis. - Optional faint horizontal gridlines at every 20 g/m²; if shown, they are light gray and behind the bars. Bar layout (must be exact and unambiguous): - Each trophic-level group contains exactly two vertical bars placed side-by-side with a small gap between them. - All bars share the same width (uniform bar width) and the same black outline (solid black outline, medium stroke thickness). Bars are fully opaque. - Within each trophic-level group: the left bar is Lake A; the right bar is Lake B. Color/legend (must be exact): - Lake A bars: solid green fill. - Lake B bars: solid blue fill. - Legend inside the plotting area in the upper-right corner with two labeled color keys: "Lake A" (green) and "Lake B" (blue). Exact bar heights and exact error bars (all error bars must be non-zero): Primary Producers group: - Lake A (green, left bar): bar height = 85 g/m². Error bar extends from 77 to 93 g/m² (±8). - Lake B (blue, right bar): bar height = 165 g/m². Error bar extends from 150 to 180 g/m² (±15). Primary Consumers group: - Lake A (green, left bar): bar height = 12 g/m². Error bar extends from 11 to 13 g/m² (±1). - Lake B (blue, right bar): bar height = 28 g/m². Error bar extends from 25 to 31 g/m² (±3). Secondary Consumers group: - Lake A (green, left bar): bar height = 2.5 g/m². Error bar extends from 2.2 to 2.8 g/m² (±0.3). - Lake B (blue, right bar): bar height = 5.8 g/m². Error bar extends from 5.1 to 6.5 g/m² (±0.7). Tertiary Consumers group: - Lake A (green, left bar): bar height = 0.4 g/m². Error bar extends from 0.35 to 0.45 g/m² (±0.05). - Lake B (blue, right bar): bar height = 0.9 g/m². Error bar extends from 0.80 to 1.00 g/m² (±0.10). - Error bar cap width: each cap spans roughly one-half of the corresponding bar’s width (caps clearly visible and symmetric). Text rules for readability: - No numerical value labels are printed above the bars; values are read from the y-axis. - Category labels are fully spelled as shown and aligned under their groups; labels may be line-broken only if needed to prevent overlap (e.g., "Primary\nProducers"), but wording must be identical.$

D. Based on the data in Figure 2, describe the relationship between nutrient availability and biomass at the primary producer trophic level.

E. Scientists hypothesized that Lake B would support greater biomass at the primary producer level than Lake A due to higher nutrient availability. Describe one way that the data in Figure 2 support this hypothesis.

A group of students designed an investigation to test how phosphorus availability affects algal growth in freshwater ecosystems. They collected water samples from an oligotrophic lake and divided the water equally into 20 identical glass containers. They added different concentrations of phosphorus to the containers: 5 containers received 0 mg/L (control), 5 received 0.01 mg/L, 5 received 0.02 mg/L, and 5 received 0.03 mg/L. All containers were placed under identical light conditions at 20°C for 14 days. Students measured algal biomass using chlorophyll-a concentration at the end of the experiment.

i. Identify the independent variable in the students' investigation.

ii. Describe one reason why the students used multiple containers (replicates) for each phosphorus concentration rather than using only one container per concentration.

The students' investigation showed that algal biomass increased with phosphorus concentration up to 0.02 mg/L, but showed no additional increase at 0.03 mg/L.

i. Explain why algal biomass did not continue to increase beyond 0.02 mg/L phosphorus concentration, even though more phosphorus was available.

ii. Explain how the results of the investigation would likely differ if the students had conducted the experiment using water from Lake B instead of water from an oligotrophic lake.

Oligotrophic lakes like Lake A typically have high dissolved oxygen concentrations throughout the water column, while eutrophic lakes often experience hypoxic (low oxygen) conditions in deeper waters during summer months.

H. Describe one ecological consequence of hypoxic conditions on fish populations in the hypolimnion (bottom layer) of a eutrophic lake during thermal stratification.