Mild in situ warming altered community structure, increased growth, and decreased survival in New Zealand subtidal encrusting ascidian and bryozoan assemblages

Abstract

To better understand the ecological consequences of ocean warming on marine ecosystems, experimental manipulation of ecologically relevant parameters is essential. Here, we employed short-term (3 months) in situ heated settlement panel experiments over four seasons to assess the effects of warming on shallow-water (3–5 m depth) encrusting invertebrate communities in a temperate marine ecosystem in New Zealand. Moderate warming (+ 1 °C above ambient) was generally associated with increases in growth, while the effects of greater warming (+ 2 °C above ambient) were more variable and seasonally dependent. Growth rates under a heatwave treatment were often reduced relative to the other heated treatments, with little difference from ambient. Notably, a non-native colonial ascidian, Lissoclinum perforatum emerged as the dominant species in warmed treatments, suggesting its potential to dominate future encrusting communities in this region, even after short-term warming events. For many taxa, increased growth rates did not equate with higher survival probabilities, with some species exhibiting decreased survival even under + 1 °C of warming, despite enhanced growth. Spatial competition within these encrusting communities also shifted in response to warming, with reduced density and complexity observed under + 2 °C of warming. Our experiment shows that even moderate warming, at temperatures already being experienced in marine ecosystems, can substantially alter growth and survival within these assemblages over short time periods and, by extension, benthic community processes and composition. Our findings highlight shifts in spatial competition dynamics that are anticipated to intensify with further ocean warming.

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