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The modification of toheroa habitat by streams on Ripiro Beach A thesis submitted in partial fulfilment of the requirements for the degree of Master of Science (Research) in Environmental Science at The University of Waikato by JANE COPE 2018 ―We leave something of ourselves behind when we leave a place, we stay there, even though we go away. And there are things in us that we can find again only by going back there‖ – Pascal Mercier, Night train to London i Abstract Habitat modification and loss are key factors driving the global extinction and displacement of species. The scale and consequences of habitat loss are relatively well understood in terrestrial environments, but in marine ecosystems, and particularly soft sediment ecosystems, this is not the case. The characteristics which determine the suitability of soft sediment habitats are often subtle, due to the apparent homogeneity of sandy environments. This can make it difficult to detect habitat change in the first place let alone understand ecological consequences. Subtle differences in habitat quality are even harder to detect on dynamic surf beaches which are controlled by the interactions between the wave climate and the beach sands, and experience frequent and large changes. The exposed surf beaches of northern New Zealand, are home to toheroa (Paphies ventricosa), an endemic surf clam and New Zealand‘s most protected shell fish species. Over the course of the 20th century, toheroa were harvested to the point of collapse resulting in the closure of an important cultural, recreational and commercial fishery. Despite more than 40 years of protection, toheroa populations have failed to recover with populations in most locations continuing to decline. In northern New Zealand toheroa distribution is strongly associated with the small streams which flow across the toheroa beaches. A reduction in the flow of fresh water to the beach via these streams has been suggested as one possible factor which may be preventing the recovery of toheroa. However, little is known about the characteristics of the habitat, and the mechanism driving this toheroa-stream relationship has not received specific attention. The objective of this research was to better understand the effect of streams on beach habitat in the hope of determining the drivers of toheroa distribution. Specifically, I investigated how sediment type, topography, water table depth and sediment temperatures varied across exposed intertidal sediments adjacent to and away from streams. Sampling took place at four sites along Ripiro Beach during the summer, the season when toheroa are most susceptible to thermal stress and streams are thought to provide some protection from dissipation and heat stress. Grainsize distributions from the sampled sediment cores were processed using a Malvern Mastersizer 3000 laser ii diffractometer (Malvern, UK). The surface elevation and sediment temperature profiles of the subaerial beach was measured using a theodolite CDN pro accurate waterproof thermometers (Model DTW450L). Temperatures were measured at five sediment depths (2cm, 4cm, 7.5cm, 10.5cm, 20.5cm) throughout the sampling period. The effect of the streams on the grain size distribution of intertidal sediments appeared to be limited to the high tide, where the proportion of fine sediments was highest in stream adjacent sediments. Greater variations in grain size occurred along the beach, but the greatest difference in sediment sizes occurred when contrasting current day beach grain size distributions with sediment data collected in 1974. Beach sediments from 2017 contained 26661% time more medium sediment indicating the beach is now much coaster than it was 44 years ago. However, as no time series is available for sediment composition at Ripiro it is uncertain whether this change in beach make up is indicative of year-to-year variation or a long term trend. Beach topography varied consistently in the vicinity of streams. The elevation and slope of the beach face was lowest in the path of streams. Along the upper beach, low elevation in the path of the stream lead to the formation of deep bowl like features. As a result of this topography, adjacent to the stream, the water table remained close to the sediment surface throughout the sampling period (< 20 cm), in contrast to point‘s away from the stream where the water table was below the sampling depth. The hypothesis that streams would provide a thermal refuge for toheroa was one of the key questions of this thesis. What I found was that stream driven difference in sediment temperatures were only apparent in the upper intestinal, the area occupied by juvenile toheroa when they first recruit to the beach. Temperatures away from streams were significantly higher (> 3.5 °C) than stream adjacent sediments. However, in the middle intestinal, the area occupied by adult toheroa, the seep face exerted a greater influence on sediment temperatures than the stream and temperatures were no different regardless of stream proximity. The findings of this study suggest that the toheroa-stream relationship is not a direct response to the presence of freshwater, but rather is driven by the modification of the intertidal sediments by the streams. The increased erosion in these areas reduces iii the distance between the sediment surface and the water table, increasing the moisture content of the sediments. Additionally, the formation of the basin like depressions in the upper intertidal may act to aggregate spat and juvenile toheroa through the effect of this topography on the beach swash regime. At mid-tide areas adjacent to streams, the depth of the water table was such that adult toheroa are likely to be either fully or partially immersed throughout the low tide. Away from streams, the water table is beyond the burrowing depth of toheroa. For both juveniles and adults, stream associated areas may provide important protection against desiccation, through increasing moisture content of the sediments and for juveniles through reduced sediment temperatures. Evidently, streams do modify beach habitat in a way ‏that makes it inhabitable for toheroa. Just not in the way that was anticipated. This study has increased our knowledge of beach characteristics that provide good toheroa habitat. Further research is required to test these hypotheses and better understand the effects of climate and terrestrial land use on beach state and therefore the suitability of the beach for toheroa. iv Acknowledgements Many thanks to my supervisors Phil Ross, Willem de Lange, Conrad Pilditch, Shade Smith and James Williams for your input, support and encouragement during this thesis and in the years leading up to it. Phil, a baptism by fire for sure! Thank you for the hours of investment and mentoring and the final drag to the finishing line. I‘m not certain if you chose well, or if it was me, but I do know that there is nothing else that I would rather have worked on. The charismatic appeal of clams and toheroa in particular, is severely underrated To those who made this project about more than just science and clams. This includes kaitiaki, iwi and the many locals from the Ripiro Beach region (and a Southlander). Special thanks go to Barry and Robyn Searle, and Jim Te Tui. Barry and Robyn, your support, encouragement and friendship has been greatly valued, as has your passion and lifelong commitment to toheroa and the wider environment. The responsibility of kaitiakitanga is now one I share with you. A wonderful gift in its own right. Thanks too to the wonderful families that opened their homes to me during extended stays up north. I‘d also like to thank Katrin Berkenbusch for the opportunity to be part of the Southland toheroa survey. #TeamToheroa – special shout out. May the epic roadies and long days on the beach last forever. Jadern, Felicia, Ming – best science and travel buddies one could wish for. You guys have been incredible. Thanks too to Nikki and Ryan for their help during my data collection. To the wonderful folk at the coastal marine field station. Academics, staff, students, my own cohort. You are like family, and by that, I mean I did not get to choose you… but I wouldn‘t want it any other way. Most of the time anyway. To my cohort with the luxury of time, over the next six months of your write up, I will make sure you know every time I‘m out sailing, skiing, diving, surfing, holidaying, fishing, doing nothing. Seriously though, thank you for everything. It‘s been an epic five years. Special mention here to Chris, thank you for everything. v For everyone who has helped with this research in some capacity, thank you. I hope your minds have been broadened. In no particular order: Chris, Kim, Phil, Jax, Vanessa, Joey, Fenna, Vanessa, Ryan, Helen, Soli, Alice, Dave, Dan, Carlos, Holly, Kirsty, Kirsten, Nikki, Felicia, Jarderne, To my family, I suspect you will all be prouder of this accomplishment than I‘ll ever be. I guess being a scientist makes it officially acceptable for me to wear socks with my crocs.