Bull kelp in a warmer world

Kelps are the ‘forests’ of the sea, and therefore often referred to as ‘foundation species’1,2. Like their terrestrial counterparts, these large seaweed modify environmental conditions, increase biodiversity and provide habitat for cultural and economic important species, like fish, shellfish and marine mammals3,4. In New Zealand, bull kelp (Durvillaea spp., Rimurapa) is one of the most important types of kelp (Fig. 1). Bull kelps are represented by at least 3 co-existing species on the main islands of New Zealand, D. antartica, D. willana and D. poha, where the two latter only exist in New Zealand5. Bull kelps are amongst the largest seaweed worldwide, living up to 10 year and reaching 10 m length and 70 kg weight. These massive iconic seaweed epitomizes rugged wave-exposed rocky coastlines, as they provide critical ecological functions, increase biodiversity and provide habitat to seals, pāua, butterfish and other important animals6-8 (Fig. 1). Bull kelp also have immense cultural values, as coastal iwi over centuries have used bull kelp forest as hunting grounds, for food, and to make products, like pōhā for food storage of, for example, muttonbirds9. Today, bull kelps are threatened by a range of natural and anthropogenic stressors10. For example, we recently reported massive loss of intertidal bull kelp along 120 km coastline, from Oaro to Cape Campbell, following a cataclysmic seismic uplift from the November 14 2016 Kaikōura earthquake (Fig. 2, Thomsen et al, unpublished data). In addition, bull kelp is threatened by enhanced sedimentation following centuries of urbanization and forest clearances, invasive species, and by ‘creeping’ global warming4,10.

For example, we used climate models, conservative IPCC temperature predictions and distribution data, to estimate that the Australian Durvillaea potarum will reduce its current range from 4,126 km to only 295 km by year 210011,12 (and become extinct soon after). Similarly, as temperature slowly rises, Durvillaea species in New Zealand are also expected to shift their distributional ranges southward. However, species range-predictions are currently only based on ‘creeping climate models’, and does not incorporate short term temperature anomalies such as ‘marine heat waves’ (MHW)10,13,14. We have previously documented how a MHW caused region-wide extinction of the kelp Ecklonia radiata in Australia, with dramatic cascading impact on all the plants and animals that typically depend on Ecklonia, along >100 km coastline13,14. This extinction occurred along Ecklonias’ northern (subtropical) range limits, a temperature zone where Ecklonia was expected to be vulnerable to high temperature. By contrast, bull kelps in New Zealand are expected to be robust to temperature fluctuations, because these waters, particular on the South Island, are ‘cold’ by comparisons. From November 2017 to February 2018, New Zealand experienced the warmest year on record, as the ‘Tasman Sea 2017/18 MHW’ covered much of the South Island15, with temperature exceeding 24C in Lyttelton harbor (Fig. 2). We therefore impromptu revisited sites with known bull kelp near Christchurch, only to discover that many of these were severely impacted with several reefs experiencing total annihilation (Thomsen, unpubl. data, Fig. 2). Importantly, these initial findings provides evidence that even ‘safe taxa’ from ‘safe regions’ may also to susceptible to MHWs, particularly when MHWs are super-imposed on climate changes and El Niño-Southern Oscillation temperature fluctuations15. Clearly, our very recent observations from February-March 2017 (Thomsen et al. unpubl.) should be verified and expanded upon with much more rigorous and systematic data collections to better understand how bull kelp, and other marine foundation species, will respond to future MHWs in Canterbury, New Zealand, and, more generally, worldwide.

We now do research, supported by Brian Mason, that aim to quantify

• the extent of bull kelp loss following the Tasman Sea 2017/18 MHW,

• the extent of surviving healthy bull kelp, and

• bull kelp resistance and ability to recovery from MHWs and co-occurring stressors.

Figure 1: Healthy bull kelps. A. Extensive stands of Durvillaea poha. B. Durvillaea. antartica. C. Durvillaea willana. D. Healthy bull kelp holdfast with attached intact frond and unique biodiversity hiding within the holdfast’s crack and crevices, E. pāua hiding from predators underneath bull kelp, F. Fur seal resting on bull kelp (photos M. Thomsen).

Figure 2: Stressed bull kelp. A. The hottest summer in NZ recorded history in the news. B. Water temperature exceeding 24 C in Lyttelton harbor during the Tasman Sea 2017/18 marine heat wave (MHW, red circles). C. Close-up of lost bull kelp following seismic uplift after the Kaikōura earthquake; these ‘ghost-holdfasts’ provided evidence for many months about past bull kelp stands. D. Healthy bull kelp bed in Lyttelton harbor prior to the MHW in November 2017. E. the same bull kelp stand was extinct after the MHW in March 2017. F. Close-up of dying bull kelp following the MHW, note the similarity to image C (photos M. Thomsen).
This project is funded by the Brian Mason Scientific & Technical Trust (http://brianmasontrust.org/)


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