Demography and biochemistry of Ecklonia radiata (Laminariales) in north-eastern New Zealand

Abstract

Monospecific stands of the laminarian alga Ecklonia radiata undergo both large - and small-scale synchronous mortality (dieback) in north-eastern New Zealand. Large-scale mortalities occurred in even-aged, deep-water stands (> 12 m depth), associated with strong infrequent El Niño periods and intense shading by phytoplankton blooms. Deep-water sporophytes were found to contain lower concentrations of mannitol and phlorotannins compared to shallow-water sporophytes, factors considered to be important in terms of susceptibility to dieback mediators, particularly low light. Dieback spread into mid-depth areas of one assemblage, consistent with the involvement of a contagious agent. S hallow-water E. radiata patches were unaffected by shading and the absence of dieback was considered to be due to high phlorotannin content, low plant density, and/or heterogeneous age-structures. Small-scale synchronous mortality was connected to stipe bleaching and grazing by the lysianassid amphipod Orchomenella aahu. The grazing was variable among sites and between depths through time and accelerated the dieback processes by 12-18 months. The grazing was linked to sporophyte phlorotannin levels, as the stipe meristoderm tissue of bleached sporophytes was one-third (~ 4% dry mass) that of ungrazed mature and juvenile sporophytes, and in feeding assays tissues with high phlorotannin concentrations were consumed the least. At one site, O. aahu destructively grazed sporophytes previously damaged by wave surge. On the whole, E. radiata was resilient to both small- and large-scale disturbances, however, because both large- and small-scale disturbance processes differed in space and time; E. radiate assemblage structure was highly variable over small to intermediate spatial scales (10-1000s m). This was also true for the chemical composition of a range of sporophyte stages. In a small-scale dieback event during 2001 (unrelated to phytoplankton shading and stipe-bleaching/amphipod grazing), the dieback process was evaluated at a biochemical level. Dieback sporophytes showed a decrease in chlorophyll a and the primary photosynthate mannitol and an increase in the amino acid phenylalanine and phlorotannins, the latter suggesting an inductive response to the dieback agent. Tissue biochemistry also differed between sub-canopy juvenile sporophytes (shaded) and those growing in the open. These biochemical traits are considered useful for assessing sporophyte health.