Decadal changes in sea surface temperature, wave forces and intertidal structure in New Zealand

Predictions of the effects of climate change in the coastal zone require an understanding of the relationships between environmental and biotic variables. These are often highly complex and uncertain because of the many ways marine biota interact with each other under different environmental conditions. We use data collected over the past several decades to de termine changes in the key environmental variables and area-specific changes in the dominant habitat-forming macroalgae. Sea surface temperature (SST) data and wave heights from ERA-Interim reanalysis were analysed for 3 areas of the east coast of New Zealand’s South Island to detect trends over the past 30 yr. We then used detailed benthic survey data acquired quarterly or half-yearly in 2 tidal zones at the same coastal areas from 1994 onwards. There were significant increases in the mean SST at 2 of the 3 areas, with average increases of 0.16°C per decade over 3 decades. Maximum SST did not increase, but the minimum seawater temperatures did, by up to 0.34°C per decade. Mean significant wave height also increased over this period by 0.06 m per decade, and maximum wave height by up to 0.3 m per decade at 2 of the 3 areas. Boosted regression tree analysis was used to determine any consistent patterns between physical variables and benthic algal cover. Generally, air temperature and the Southern Oscillation Index (SOI) were the most influential variables on cover of fucoid macroalgae. SST and wave height were also important but less influential. Fucoid cover increased with maximum air temperature beyond ca. 22 to 24°C, and cover decreased during La Niña periods at the most northern site, but increased during La Niña periods at the most southern site. The relative contributions of SST and wave height variables to the models were area- and tidal zone-specific. Overall, this study showed highly variable effects of a changing climate on an ecologically important habitat-former, highlighting the problems of dealing with ecological and climate variables that operate at differing spatial and temporal scales. We discuss this with respect to community structure and dynamics.