IB Marine Science Standard Level (SL) Practice Exam

This question tests how salinity varies with depth in different estuary mixing regimes. In a partially mixed estuary, vertical mixing is not strong enough to erase the freshwater–saltwater differences, so a distinct vertical gradient forms. Freshwater remains near the surface while more saline water is found deeper, leading to salinity increasing with depth. That pattern matches the description of a partially mixed estuary: a halocline exists, and there is some stratification because vertical mixing is limited. The other patterns describe different setups—well-mixed estuaries have uniform salinity throughout, salt-wedge estuaries feature a sharp intrusion of saltwater along the bottom with little vertical mixing, and complete vertical mixing would erase any salinity difference, yielding uniform salinity.