Question

throughout the 1900s, many large dams were constructed along river systems throughout the united states. these dams have disrupted the flow of fresh water and sediments into coastal ecosystems. such disruptions have led to ecosystem disturbances and biodiversity loss.
many dams no longer in use are being removed from river systems in an effort to counteract the biodiversity losses they had caused. to evaluate how effectively dam removal reverses biodiversity loss, scientists monitored biological and physiological parameters at 16 sites in a coastal ecosystem in the years following the removal of two dams. these dams were located on the same river that emptied into the pacific ocean. the removal of these two dams occurred from 2011 to 2012.
following the removal of the dams, the scientists recorded an influx of sediment into the coastal ecosystem at the mouth of the river. this influx persisted through the year 2014, the end of the study period. over this period, four of the offshore sites were covered by between 10 cm and 13 m of sediment, while the remaining 12 sites had no discernible change in sedimentation along the seafloor. all sites exhibited an increase in the turbidity or cloudiness, of the water within the first year after dam removal. sites at the mouth of the river exhibited an increase in turbidity that continued through 2014.
each year, the researchers recorded the median number of algae and invertebrate species at sites in which sediment deposition had and had not changed. the table below shows these results.

year	deposition increased		deposition not changed
	algae species	invertebrate species	algae species	invertebrate species
2010	8	14	8	16
2012	2	13	2	17
2013	1	10	1	27
2014	0.5	8	1	23

based on the findings above, which of the following strategies would be most likely to improve biodiversity recovery following dam removal?
a. new species of algae should be added to the mouth of a river a year after dam removal in order to increase the biodiversity of algae in the downriver ecosystem
b. sediment that has built up behind a dam should be cleared out before the dam is removed in order to decrease the risk that sediment in the water will block light from reaching algae
c. sediments should be deposited along the seafloor in the year prior to dam removal in order to increase the biodiversity of invertebrates at sites along the coast
d. a smaller dam should be installed at the mouth of a river in which a dam is to be removed in order to decrease the likelihood of sediment deposition in coastal ecosystems
3 of 20 answered
session timer: 4:42
session score: 67% (2/3)

Explanation:

The data shows that increased sediment deposition is associated with a decline in the number of algae and invertebrate species. Clearing out the sediment that has built - up behind a dam before removal would reduce the amount of sediment entering the water after dam removal. This would minimize the risk of sediment blocking light from reaching algae, which is crucial for their survival and thus for biodiversity recovery. Adding new algae species may not address the root cause of biodiversity decline related to sediment issues. Depositing more sediments before dam removal would likely worsen the situation, and installing a smaller dam may not effectively deal with sediment - related biodiversity problems.

Answer:

B. Sediment that has built up behind a dam should be cleared out before the dam is removed in order to decrease the risk that sediment in the water will block light from reaching algae.