The brackish waters along the Hokkaido coast contain a mixture of plant material of both marine and terrestrial origin that provide food and habitat sources for many macroinvertebrates. Field and laboratory experiments were used to investigate the amount of breakdown of seaweed (kelp) and terrestrial oak leaves (green, senescent, and stream-water soaked (‘conditioned’)). In addition, the consumption and growth rates of Anisogammarus pugettensis were compared for the different leaf treatments. The breakdown of kelp was greater than that of terrestrial leaves. Among the three types of oak leaves, the amount of breakdown of green leaves was smallest and the breakdown of conditioned senescent oak leaves was 1.5 times greater than that of senescent leaves. Conditioning of leaves in fresh water enhances leaf decomposition in brackish water. The growth rates of A. pugettensis were similar and fastest when fed seaweed and a seaweed–leaf mixture (0.035 ± 0.009 and 0.043 ± 0.013 mg mg^–1 day^–1, respectively). The rapid colonisation of oak, despite its low nutritional value, suggests that leaves are important as a habitat, rather than as a food resource. Availability of both seaweed and terrestrial leaves as food resources and habitat would yield benefits for seawater-inhabiting macroinvertebrates.