Whole community estimates of macroalgal pigment concentration within two southern New Zealand kelp forests1.

Light availability is a fundamental factor that controls the productivity and distribution of macroalgae and is highly variable, both spatially and temporally, in subtidal coastal systems. Our comprehension of how macroalgae respond to such variability is a significant knowledge gap that limits our understanding of how light influences the structure and productivity of these environments. Here, we examined the pigment characteristics of individual species, and for the first time the whole community, within one low-light, and one high-light kelp-forest system in southern New Zealand. The aim was to quantify the range of pigmentation seen within the two kelp-forests which differed in irradiance regime. Light availability was 33% and 64% greater at the high-light compared to the low-light site at 2 and 10 m depth, respectively. Results suggested Phaeophyceae species at deeper depths in the low-light site may be living at the edge of their photosynthetic ability and pigment synthesis appeared significantly restricted. Even with greater investment in the pigment fucoxanthin, biomass of Phaeophyceae species was significantly lower in the low-light site. Highly pigmented Rhodophyceae species made a greater proportional contribution to community biomass within the low-light site where they likely possessed a photosynthetic advantage. This work helps explain discrepancies in community structure between the two study sites and explores the complex relationship between irradiance and photoacclimation. The comparison of community pigment concentration holds potential as a tool for assessing the relative degree of photoacclimation occurring between sites and provides a proxy of photosynthetic cost under a specific light regime.

A kelp with integrity: Macrocystis pyrifera prioritises tissue maintenance in response to nitrogen fertilisation.

Our understanding of the response of vascular, terrestrial plants to nitrogen (N) addition is advanced and provides the foundation for modern agriculture. In comparison, information on responses of marine macroalgae to increased nitrogen is far less developed. We investigated how in situ pulses of nitrate (NO3 (-)) affected the growth and N physiology of Macrocystis pyrifera by adding N using potassium nitrate dissolution blocks during a period of low seawater N concentration. Multiple parameters (e.g. growth, pigments, soluble NO3 (-)) were measured in distinct tissues throughout entire fronds (apical meristem, stipe, adult blade, mature blade, sporophyll, and holdfast). Unexpectedly, N fertilisation did not enhance elongation rates within the frond, but instead thickness (biomass per unit area) increased in adult blades. Increased blade thickness may have enhanced tissue integrity as fertilised kelp had lower rates of blade erosion. Tissue chemistry also responded to enrichment; pigmentation, soluble NO3 (-), and % N were higher throughout fertilised fronds. Labelled (15)N traced N uptake and translocation from N sources in the kelp canopy to sinks in the holdfast, 10 m below. This is the first evidence of long-distance (>1 m) transport of N in macroalgae. Patterns in physiological parameters suggest that M. pyrifera displays functional differentiation between canopy and basal tissues that may aid in nutrient-tolerance strategies, similar to those seen in higher plants and unlike those seen in more simple algae (i.e. non-kelps). This study highlights how little we know about N additions and N-use strategies within kelp compared to the wealth of literature available for higher plants.