Demography, Biomass Production and Effects of Harvesting Giant Kelp Macrocystis pyrifera (Linnaeus) in Southern New Zealand.

This study examined the demography of giant kelp Macrocystis pyrifera (Linnaeus) and its interactions with understorey algae and invertebrates in southern New Zealand over two and a half years. Most of the study was done at two sites within Akaroa Harbour (Banks Peninsula) but ancillary sites at Tory Channel (Marlborough Sounds) were used for parts of the study. The kelp forests within Akaroa Harbour were generally highly productive, with a high annual turnover of giant kelp. Macrocystis plants were mostly annual and rarely reached ages greater than 12 months. Peak recruitment occurred in spring (November) during 1995-97, but lesser recruitment episodes occurred throughout the year. The maximum growth rates of Macrocystis fronds were comparable to rates reported elsewhere in southern hemisphere populations (22 mm - 24.5 mmlday), but considerably lower than those in northern hemisphere populations. The major experiment incorporated in the study tested the effects of the Macrocystis canopy and the understorey canopy of the stipitate laminarian Ecklonia radiata on macroalgae and invertebrates. The experiment was structured so that the effects of clearances at different times could be determined. One impetus for this experiment was the need to address issues relating to the commercial harvesting of giant kelp, its sustainability and its effects on other species. The effects of canopy removals on understorey algae, mostly juvenile Macrocystis, Ecklonia and Carpophyllum spp, were highly dependent on the timing of canopy removals and the combinations of canopies removed. For example, winter harvests of the Macrocystis canopy alone enhanced the survival of post-settlement Macrocystis recruits, but had little effect on Ecklonia recruitment. However, when both Macrocystis and Ecklonia canopies were removed in spring, there was heavy recruitment of Ecklonia that grew to dominate the understorey. Strong inter and intraspecific interactions from the Macrocystis surface canopy appeared to have been reduced by physical factors including water turbidity, sedimentation and the deterioration of the surface canopy during summer. These physical factors were not as limiting in Tory Channel. Fine scale extrinsic factor effects including nutrients, light and grazing on the early life history of Macrocystis were investigated in small experiments. Results suggest that recruitment may be nutrient limited even at moderately low temperatures, and that small herbivorous gastropods are an important source of mortality in the early life stages of Macrocystis. Culturing and transplantation cultivation techniques were also examined as a means of supplementing algal supplies. Macrocystis was cultured successfully through its life cycle onto culture ropes, but generally failed to produce visible sporophytes when placed in the field. Cultured plants did grow in Tory Channel, however. Juvenile plants transplanted to ropes for on-farm cultivation showed little growth during summer, but the addition of nutrients significantly enhanced growth rates of these plants during warmer months when natural nutrient levels were low. Increased growth rates at the onset of winter and with the addition of nutrients during summer confirmed that low nutrient levels during summer are growth limiting. Akaroa Harbour kelp forests exhibited considerable variation in Macrocystis canopy biomass through time. For example, the 32,000 m2 kelp forest at Wainui had a biomass of 144 t in October 1995, which then decreased to 21 t in October 1996. Canopies tended to deteriorate during summer. Thus, at Ohinepaka Bay kelp forest had a biomass of 31 t during winter 1997, which decreased to 0.06 t the following summer. The greatest reduction in biomass, however, coincided with a period of hugely increased sediment, which smothered blades in the sea-surface canopy, covered the substratum, and prevented successful recruitment of kelp for over a year. Nutrient depletion was one of several factors thought to cause the summer deterioration of the Macrocystis sea-surface canopy, which has important ramifications for the commercial harvesting of Macrocystis pyrifera in summer. Management considerations and options are discussed in relation the commercial harvesting of Macrocystis in New Zealand. The major conclusion of this study is that although Macrocystis was able to form dense surface canopies during winter its ability to dominate kelp forests was constrained by physical factors, especially sedimentation, high turbidity, nutrients, and storms. The lack of strong interactions between Macrocystis and Ecklonia are also largely a result of their different life history characteristics. Overall, there appear to be no significant negative flow-on effects resulting from kelp harvesting and it appears that Macrocystis can be harvested sustainably