Identify which of the types of interaction shown in the key occur between crabs and seagrass.

Describe how the sea otter indirectly affects seagrass.

Explain how nutrients can have a positive or negative bottom-up effect on seagrass.

a. indirect 

b. negative «effect» 

c. top-down

a. sea otters have a positive «indirect» effect as sea otters feed on crabs that feed on isopods that feed on algae 

b. less crabs means more isopods so less algae 

c. less algae means more seagrass
OR
less competition between algae and seagrass

a. the positive «bottom up» effect is due to availability of nutrients 

b. «nutrients» increase abundance/plant growth rates 

c. the negative «bottom-up» effect as excess of nutrients causes a bloom in algae growth/eutrophication/competition 

d. limited nutrients have negative effect on seagrass «growth» 

e. «excess» algae cause seagrass to die/decompose/replenish nutrients

Do not accept answers referring to top down effects.