What is the most significant challenge for organisms that live in estuaries

The most significant challenge is change in salinity.

Explanation:

Estuaries are apparently stressful for some organis, the estuaries are very dynamics and they are ecosystems with an abrupt salinity change because  seawater and fresh water meet.

 The changes in salinity cause internal osmotic stresses. Benthic organisms for example use a number of strategies like modification of physical systems (oysters close their shells in low salinity water).

image result for what happens whenever a net force acts on an object

if an object has a net force acting on it, it will accelerate. the object will speed up, slow down or change direction. an unbalanced force (net force) acting on an object changes its speed and/or direction of motion. an unbalanced force is an unopposed force that causes a change in motion.

i'm sorry, but can you list the answers? i cannot with something i don't know the answers to

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chris