Sandy beach wrack as a resource: nutrient quality explains amphipod preferences but not temporal variation

Sandy beach invertebrates depend largely upon allochthonous inputs of macrophytes and the oceanographic processes that deliver them as wrack. While part of this wrack is returned to the sea by the tides, a large portion remains, ages, and decomposes on the supralittoral, becoming colonized and used by macrofaunal organisms. This study quantified faunal colonization in relation to change in wrack nutritional quality, measured here as protein content. We prepared, deployed, and followed for 29 days a series of experimental wrack patches of two of the most common species in the wrack of Atlantic Canada sandy beaches: the rockweed Fucus serratus and the eelgrass Zostera marina. On a weekly basis, we measured the density of the macrofauna with a focus on the most abundant species, the talitroid amphipod Americorchestia longicornis, in both types of wrack and in bare sediments, while simultaneously measuring macrophyte protein contents. Our results show that rockweed patches concentrated higher densities of amphipods compared to eelgrass and bare sediments, a result that was matched by protein differences (higher concentrations in rockweed than eelgrass). However, while protein contents gradually declined over time in both macrophytes, the abundance of amphipods increased. Hence, while nutritional quality may explain between-wrack and wrack-bare sediment differences in faunal colonization, it cannot explain the counterintuitive temporal pattern recorded here. The latter result points to the often-ignored role played by the physical characteristics of the wrack and the conditions they create while decomposing over sandy beaches.