Factors influencing the establishment and phenology of the invasive kelp Undaria pinnatifida in northern New Zealand

Abstract

Macroalgal invasions can change the structure and function of benthic habitat, dominating space, altering primary productivity, nutrient cycles, and community composition. This is a serious concern for receiving environments. A major determinant of the geographic distribution of macroalgal species is water temperature. Investigating the population ecology, genetic variation and factors controlling the density and spread of invasive macroalgae towards the extent of their thermal distribution limits can therefore provide important insights into the potential range and ecological impacts both now and with future climate change. The kelp Undaria pinnatifida is a prolific and cosmopolitan invasive species. Undaria has invaded countries around the world across a variety of temperature regimes. Undaria displays a variable phenology at introduced locations; it is a winter annual in its native range, but sporophyte presence can vary from annual to year-round across its invasive range. Information is lacking on how Undaria phenology and impacts manifest under warm-temperate conditions. This work studies Undaria in northern New Zealand, one of the warmest-water populations in the world. I investigated how water temperature influenced the phenology and distribution of Undaria. In addition I examined some attributes of the invasion process for warm-water populations; I looked at the role mussel aquaculture plays in influencing the spread of Undaria to native habitats, how disturbance might facilitate the spread of Undaria to subtidal reefs, and the genetic composition of populations in northern New Zealand. Monitoring over three growth seasons revealed how Undaria in northern New Zealand displayed an annual growth cycle. Recruitment occurred when temperatures dropped below ~15 oC and density was minimal at temperatures above 20 oC. Sporophyte size and growth rates were comparable to those of cooler water populations but the seasonal cycle was compressed into a shorter timeframe. Undaria was found to proliferate on mussel farms and low numbers were found on reefs adjacent to farms, in shallow habitats lacking native macroalgal canopies. Undaria was rare in deeper habitats dominated by native macroalgal canopies, experimental canopy removal did not facilitate the spread of Undaria into these habitats at the site examined. This suggests that native canopy-forming species are not the sole factor prohibiting the invasion of Undaria to subtidal reef sites in this region. Genetic analysis of Undaria from all known populations in the North Island revealed very low genetic diversity in northern New Zealand. The presence of a single haplotype in northeastern New Zealand may indicate a particularly warm-tolerant strain and suggests thermal preadaptation may influence invasion success. An investigation into the global distribution and associated phenology of Undaria, and how this related to water temperature regimes, demonstrated how Undaria only occurred where winter temperatures dropped below ~15 oC and that persistent year-round populations were only present at sites where summer temperatures did not exceed 20 oC. This analysis also revealed that there are extensive areas of the world where Undaria does not occur that have temperature regimes suitable for Undaria colonisation. Such areas are potentially at risk from future invasion based on current temperature regimes. These thermal limits for Undaria indicate that populations in northern New Zealand are towards the limit of its distribution, with winter temperatures close to 15 oC, Undaria is unable to withstand warmer winter temperatures. Ocean warming is likely to restrict the potential geographic distribution of Undaria, particularly where winter temperatures exceed 15 oC. Increases in summer temperatures, which prolong the period during the year when temperatures exceed 20 oC, will reduce the annual persistence of Undaria. A limited annual presence of sporophytes at warmer locations may reduce the ecological impacts of Undaria invasion as compared to cooler water locations where sporophytes persist for a longer portion of the year.