This dissertation evaluated the typical spatial patterns of macroalgal communities in the Gulf of Mexico (GoMx), the typical seasonal patterns of macroalgal communities in offshore Louisiana, the population genetic patterns of a common macroalgal species widespread in the GoMx, and whether such community/population patterns changed after the Deepwater Horizon oil spill (DWH). To accomplish these goals, the dissertation was structured into three main research projects, each presented as a chapter. The most remarkable results showed that each of the three GoMx regions (Campeche Banks, Offshore Florida, and Offshore Louisiana) harbor distinct macroalgal communities shaped by regional differences in nutrients associated with river discharges from Louisiana, Alabama, and West Florida; likewise, each region harbored distinct populations of the red alga Botryocladia occidentalis that are likely shaped by their differential exposure to haplotypes immigrating from the Caribbean via the loop current. This dissertation also elucidated the typical seasonal patterns of the macroalgal community from offshore Louisiana and described important changes, at the community and population level, that may be associated with the DWH. Nevertheless, future assessments of community structure or haplotype frequencies will be necessary to confirm whether these changes are permanent or simply represent a temporary response to disturbance. Importantly, these changes may not necessarily be a consequence of crude oil contamination but could be caused by other DWH factors such as the release of Corexit oil dispersant or the closure of fisheries, for a prolonged period, in the vicinity of the DWH disaster. Likewise, it is also possible that these population changes were driven by factors unrelated to the DWH.