Arsenal manager Mikel Arteta said he was "concerned" by his side's performance as they handed the initiative in the race for Champions League qualification to rivals Tottenham with a deserved defeat by Brighton at Emirates Stadium.
Brighton - seven games without a victory before kick-off - frustrated the home side with their organisation and efficient forward play in a game where Arsenal failed to look threatening until they mustered a desperate late push.
Leandro Trossard's curling first-half finish - after a fine assist by the impressive Enock Mwepu - gave Graham Potter's side something to build on.