Comparison of recruitment tile materials for monitoring coralline algae responses to a changing climate

Settlement plates are widely used in ecological studies as a simple and effective way to assess recruitment and growth of sessile benthic communities. Plate material and surface complexity, positioning, size, attachment method, placement within the reef and duration of deployment can all influence the communities that recruit. Using different plate types can affect study reproducibility and skew results, yet no standardised guidelines exist for monitoring crustose coralline algae (CCA), a key bioindicator of changing seawater carbonate chemistry. Here, we compare 6 experimental tile materials (including plastic, ceramic and glass tiles) at 2 orientations (horizontal and vertical) for CCA community recruitment and calcification across 3 habitats (back reef, fore reef crest and reef slope). CCA were 20% more abundant, calcified faster and were twice as speciose on fore reef habitats. Community composition also varied with tile material and orientation. Vertically oriented surfaces and plastic and limestone tiles produced the closest approximation of real-world communities. Horizontal surfaces appeared to encourage coralline recruitment, particularly in light-limited environments, but also attracted greater herbivory. PVC tiles were the optimal approach when all factors were considered, as they generated a percent cover representative of adjacent reef communities and produced more consistent calcification estimates (across space and time), as well as being cheaper and easier to work with. Selection of plate type and deployment methods will ultimately depend on the experimental question; here we describe the benefits and costs of each material and/or orientation and suggest the adoption of PVC tiles as an optimal overall approach for monitoring CCA.