Why can shallow barrier island lagoons be colder than the open ocean during winter?

Barrier island lagoons are typically shallow, which causes their water temperatures to fluctuate more significantly than those of the open ocean. Because of this limited depth, the water in these lagoons closely approximates the ambient air temperature.1

During the winter, when cold air masses move off the continents, the shallow water in a lagoon loses heat rapidly to the atmosphere. In contrast, the open ocean has a much greater volume and a higher heat capacity, allowing it to retain heat more effectively and resist rapid temperature drops. Consequently, the lagoon waters become colder than the adjacent open marine environment during the winter months.12