What can kelp loss processes and beach-cast patterns tell us about sandy beach management?

Abstract

This project set out to assess the ecological role of beach cast kelp—which tends to be routinely removed from recreational beaches—on the beach surface of selected sandy shores in the City of Cape Town municipal area. Particularly, we were interested in understanding the role that kelp wracks may provide as a nutritional source to the beach consumer biota after it had been mechanically and biologically transformed to become particulate organic matter (POM) and incorporated in the beach sediment. Our working null hypothesis is that there is no difference in the amount of POM measured in the sediments of cleared vs. non-cleared urban beaches. The outcome of the study is that kelp removal does in fact not alter the POM matter content of beaches in areas where kelp wrack is cleared (i.e. we could not reject the null hypothesis). We suggest that this is due to the highly modified nature of the beach systems that we studied: i) the overriding influence of the anthropogenic (engineering) modifications to the beach systems override any influence that might be caused by altered inputs of kelpderived POM; and ii) an array of additional impacts on the managed beaches may add additional confounding influences that distort the effect due to the main driver that our study intended to find. This research links to a topical environmental management issue that the CoCT faces, and supports on an ecological basis their decision to actively manage the beaches through kelp cast removal programmes. The research also added a significant step forward to the ongoing work that the UWC Biodiversity and Conservation Biology Department’s Kelp Research Group has done around the ecological functioning of kelp beds in the Western Cape region, and adds an new dimension to our current focus, climate change. Since the consequences of climate change will be especially noticeable in urban settings such as the CoCT, we would very much like to see future research efforts merging the City’s climate mitigation and adaptation strategies with our studies on the ecological role that kelps may contribute towards the well-being of our urban coastline.