Mesograzer Interactions with a Unique Strain of Irish Moss Chondrus Crispus: Colonization, Feeding, and Algal Condition-Related Effects

Marine macroalgae are exposed to multiple sources of stress. As a result, perennial macroalga habitats have become depleted in many coastlines. Here, we investigated the role of mesograzers in the sharp decline of a unique strain of Chondrus crispus (the giant Irish moss) found solely in a lagoon in Atlantic Canada. This study was prompted by damage resembling grazing scars that appeared on the fronds as the population declined, for which no grazer had been identified. We identified potential grazers of the seaweed by deploying 4 types of experimental clumps of giant Irish moss and sampling the epifauna that colonized them. Laboratory assays were then run with an abundant species, the amphipod Gammarus oceanicus, to measure feeding rates and test whether this mesograzer is capable of consuming the alga and creating measurable damage. G. oceanicus readily consumed the Irish moss at a grazing rate of 5.24 mg amphipod-1 d-1 and created deep lateral grazing wounds similar to those observed in the field. An additional experiment was conducted to assess whether a co-acting stressor in the lagoon, the accumulation of fine sediments, could explain the appearance and spatially patchy distribution of the damage in the population. Giant Irish moss fronds that had been buried under sediment lost twice as much biomass as those that had not. These results suggest that grazer activity and declining conditions in the lagoon have a negative and additive effect on this unique strain of Irish moss, with clear implications for its restoration.