Lake Cayuga in upstate New York is about 61 km long and 3 km wide, with an average
depth of 132 m. Strong winds occasionally push the warm surface water away from the
shore, which allows the upwelling of the cooler deep water. Why would this upwelling be
beneficial for the ecosystem in the lake?

Hello there, I see that this is an Environmental Science question.

As strong winds push the warm surface water away from the shore, it allows the upwelling of the cooler deep water. This upwelling is beneficial because as cooler water emerges, it brings the surface water's temperature down and thus allows more animal species that are not comfortable in warmer water to thrive. Some species are comfortable in warmer waters, others are not. Random thing to say, but remember that aquatic species are always more comfortable in cold waters, as dissolved oxygen increases if the temperature is low.  

halogen, any of the six nonmetallic elements that constitute group 17 (group viia) of the periodic table. the halogen elements are fluorine (f), chlorine (cl), bromine (br), iodine (i), astatine (at), and tennessine (ts).