What were some effects the transatlantic slave trade had?

One effect was felt directly in Africa—the European need for chattel slavery made the slave trade grow exponentially.  African tribes searched for more and more slaves and became dependent on European trade goods.  This would eventually impoverish Africa, making it easier to colonize in the nineteenth century.  While slavery had existed in Africa well before the time of Europeans, Europeans made slavery generational.  

African slavery also made large-scale agriculture in the Americas practical.  Native...

One effect was felt directly in Africa—the European need for chattel slavery made the slave trade grow exponentially.  African tribes searched for more and more slaves and became dependent on European trade goods.  This would eventually impoverish Africa, making it easier to colonize in the nineteenth century.  While slavery had existed in Africa well before the time of Europeans, Europeans made slavery generational.  

African slavery also made large-scale agriculture in the Americas practical.  Native Americans were quick to run away from their European masters, and English indentured servants often died of mosquito-borne diseases.  The Europeans used Africans because they were not familiar with the territory and, therefore, were less of a flight risk.  Also, Africans were less likely to catch diseases like malaria.  African labor made the sugarcane and tobacco fields possible, thus making a small number of whites very wealthy and enriching the imperial powers who owned the colonies.  

Finally, the transatlantic slave trade enriched many port cities.  Before the Revolutionary War, New York and Boston were common ports for slave catchers to stop and sell their cargo.  This increased business around the cities, as the trade needed auctioneers to sell the slaves.  There was also a need for taverns and boardinghouses for people coming from out of town to buy slaves.  In the early days of America, the slave trade was a major part of the national economy, though some were complaining about the social issues that slavery raised even back then.