Low light exacerbates effects of marine heatwaves on seaweeds

Marine heatwaves (MHWs) are becoming more frequent and intense. At the same time, diverse human activities are causing coastal darkening: increased turbidity and reduced light at the seafloor. Since both temperature and light are critical to the physiological and ecological performance of seaweeds—important habitat-formers and primary producers—these co-occurring stressors could have knock-on effects in coastal ecosystems. We tested the effects of MHWs under different irradiance levels on 3 brown seaweeds: the kelp Ecklonia radiata and 2 common understorey species, Zonaria turneriana and Lobophora variegata. The seaweeds were subjected to 15 d MHWs of different maximum intensities (25° and 28°C) at 2 reduced light levels (~40 and ~10 µmol m -2 s -1 ). Effects on cover of bleached tissue, biomass, maximum quantum yield and pigment concentrations were assessed. Temperature was the main cause of observed changes in seaweed condition, with strong negative effects of the most intense MHW (28°C) especially for tissue bleaching and maximum quantum yield. Low light had a minor effect at 19°C (background temperature) and in the moderate MHW treatment (25°C). However, at extreme temperatures (28°C), low light enhanced the negative effects on virtually all seaweed condition metrics. For all species, the negative effects of the most severe MHW persisted during the recovery period, during which temperatures were returned to background conditions. The compounding effect of high temperature and reduced light highlights the need to assess the interactive nature of multiple stressors with respect to the emerging threat of MHWs to reef ecosystems.