Warming Effects on Two Autogenic Engineers (zostera Capensis and Gracilaria Gracilis): Consequences for Macrofaunal Assemblages and Benthic Heterogeneity in Intertidal Sandflat Ecosystems

Ecosystem engineering by organisms is a central determinant of multifunctionality in marine sedimentary systems. However, global warming can alter this multifunctionality, by modifying engineer co-existence and the diversity of functional traits present in a landscape. This is mediated by warming favoring retention of traits of tolerant engineers over sensitive ones and functional characteristics of residents nested within engineered patches. This study aims to test the influence of warming on co-existence of two sympatric autogenic engineers (seagrass Zostera capensis and red macroalga Gracilaria gracilis) that co-occur in intertidal sandflats of Langebaan Lagoon, on the west coast of South Africa. Findings from mesocosm experiments indicate that increasing temperature to maximum levels recorded in situ is linked to greater seagrass senescence (80–93%) and epiphytic fouling (80–86%) relative to low temperatures. Warming was associated with greater biomass of G. gracilis (30–45%) relative to controls, despite it incurring greater fouling levels (55–60%) and potentially light reduction. In situ comparisons of functional traits and community metrics of macrofauna inhabiting beds of Zostera and Gracilaria and unvegetated sandflats, indicated unique assemblages in Zostera habitats and a dominance of surface-dwelling fauna. Macrofaunal abundance was reduced in Gracilaria beds (72%) and unvegetated sandflats (47%), which were dominated by burrowing infauna. Findings indicate that warming in temperate coastal systems may lead to seagrass declines, but increased cover of G. gracilis. This shift would likely lower ecosystem heterogeneity, given the faunal similarity between unvegetated sandflats and Gracilaria beds that was recorded, while reducing habitat for critically endangered specialist seagrass inhabitants.